Soil Moisture–Atmosphere Interactions during the 2003 European Summer Heat …vidale/papers/Fischer_heat_waves... · 2009-02-20 · the term “heat wave” is often defined as

Soil Moisture–Atmosphere Interactions during the 2003 European Summer Heat Wave

E. M. FISCHER AND S. I. SENEVIRATNE

Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland

P. L. VIDALE

Centre for Global Atmospheric Modelling, Department of Meteorology, University of Reading, Reading, United Kingdom

D. LÜTHI AND C. SCHÄR

Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland

(Manuscript received 15 August 2006, in final form 12 February 2007)

ABSTRACT

The role of land surface–related processes and feedbacks during the record-breaking 2003 Europeansummer heat wave is explored with a regional climate model. All simulations are driven by lateral boundaryconditions and sea surface temperatures from the ECMWF operational analysis and 40-yr ECMWF Re-Analysis (ERA-40), thereby prescribing the large-scale circulation. In particular, the contribution of soilmoisture anomalies and their interactions with the atmosphere through latent and sensible heat fluxes isinvestigated. Sensitivity experiments are performed by perturbing spring soil moisture in order to determineits influence on the formation of the heat wave. A multiyear regional climate simulation for 1970–2000 usinga fixed model setup is used as the reference period.

A large precipitation deficit together with early vegetation green-up and strong positive radiative anoma-lies in the months preceding the extreme summer event contributed to an early and rapid loss of soilmoisture, which exceeded the multiyear average by far. The exceptionally high temperature anomalies,most pronounced in June and August 2003, were initiated by persistent anticyclonic circulation anomaliesthat enabled a dominance of the local heat balance. In this experiment the hottest phase in early August isrealistically simulated despite the absence of an anomaly in total surface net radiation. This indicates animportant role of the partitioning of net radiation in latent and sensible heat fluxes, which is to a large extentcontrolled by soil moisture. The lack of soil moisture strongly reduced latent cooling and thereby amplifiedthe surface temperature anomalies.

The evaluation of the experiments with perturbed spring soil moisture shows that this quantity is animportant parameter for the evolution of European heat waves. Simulations indicate that without soilmoisture anomalies the summer heat anomalies could have been reduced by around 40% in some regions.Moreover, drought conditions are revealed to influence the tropospheric circulation by producing a surfaceheat low and enhanced ridging in the midtroposphere. This suggests a positive feedback mechanism be-tween soil moisture, continental-scale circulation, and temperature.

1. Introduction

A record-breaking heat wave affected the Europeancontinent in summer 2003. With mean summer [June–August (JJA)] temperatures exceeding the 1961–90mean by about 3°C over large areas and by over 5°Cregionally (Schär et al. 2004, hereafter S04), it was very

likely the hottest European summer over the past 500yr (Luterbacher et al. 2004). In central and southernEurope the socioeconomic impact of this extraordinaryheat wave was disastrous. Estimates by the WorldHealth Organization (WHO) and the Earth Policy In-stitute indicate a statistical excess over the mean mor-tality of between 22 000 and 35 000 heat-related deathsacross Europe (Larsen 2003; Vandentorren et al. 2004;Hémon and Jougla 2004; Koppe and Jendritzky 2004;Schär and Jendritzky 2004). During the maximum heatwave, in the first 2 weeks of August, the mortality ratein France increased by 54% (Hémon and Jougla 2004).

Corresponding author address: E. M. Fischer, Institute for At-mospheric and Climate Science, ETH Zurich, Universitätsstrasse16, Zurich 8092, Switzerland.E-mail: [email protected]

15 OCTOBER 2007 F I S C H E R E T A L . 5081

DOI: 10.1175/JCLI4288.1

© 2007 American Meteorological Society

JCLI4288

The financial loss due to crop failure over southern,central, and eastern Europe is estimated at $12.3 billion(Heck et al. 2004). Forest fires in Portugal alone re-sulted in total damage costing $1.6 billion (Heck et al.2004). In addition to these socioeconomic impacts, the2003 heat wave was associated with a mass loss of theAlpine glaciers of 5%–10% (Zemp et al. 2005). Thehigh temperatures led to excessive near-surface ozoneconcentrations with major impacts upon the continen-tal-scale air quality (Solberg et al. 2005; Ordonez et al.2005).

Temperature anomalies as observed in JJA 2003 arestatistically highly unlikely, even when the observedwarming is taken into account (S04). Schönwiese et al.(2004), using a statistical time series analysis on Ger-man surface temperature series, reveal an increasingprobability of hot summers taking place along with awarming trend observed especially within the recentdecades (Klein Tank et al. 2005). Stott et al. (2004)point out that this increasing probability of anoma-lously warm summers can partly be attributed to pasthuman influence on the climate system. Several modelstudies suggest that events such as the 2003 summerheat wave will become more frequent, more intense,and longer lasting in the future (S04; Beniston 2004;Meehl and Tebaldi 2004; Vidale et al. 2007). Severalstudies have suggested that the projected changes insummer climate strongly rely on soil moisture–atmo-sphere interactions (Seneviratne et al. 2006b; Rowell2005; Rowell and Jones 2006; Vidale et al. 2007).

Heat waves are generally associated with specificlarge-scale anticyclonic atmospheric circulation pat-terns (Black et al. 2004). These patterns are often char-acterized by quasi-stationary 500-hPa height anomaliesthat dynamically produce subsidence, clear skies, lightwinds, warm-air advection, and prolonged hot condi-tions at the surface (Kunkel et al. 1996; Palecki et al.2001; Xoplaki et al. 2003; Meehl and Tebaldi 2004). Thesurface temperature response to such circulationanomalies is amplified by a positive feedback due tosuppressed evapotranspiration owing to the lack of soilmoisture (S04; Hartmann 1994; Lakshmi et al. 2004;Ferranti and Viterbo 2006; Seneviratne et al. 2006b).

Several studies have analyzed the mechanisms thatcontributed to the formation of previous summer heatwaves. Cassou et al. (2005) demonstrate that anoma-lous tropical Atlantic heating may significantly favorthe formation of Rossby wave trains and atmosphericblocking conditions as in 2003. Vautard et al. (2006)show that rainfall deficiencies in the preceding winterMediterranean precipitation have a discernable effecton the frequency of heat waves through northwardtransport of latent heat fluxes. Della-Marta et al. (2006)

suggest that preceding winter North Atlantic SSTs andJanuary–May Mediterranean precipitation (and thusspring soil moisture anomalies) may provide predictiveskill for summer heat waves. On a multidecadal timescale, the variability of European summer temperatureshas been attributed to basin-scale changes in the At-lantic Ocean, associated with the Atlantic multidecadaloscillation (Sutton and Hodson 2005).

In the specific case of the 2003 heat wave, the pre-ceding spring was anomalously warm throughout cen-tral and western Europe. This warm anomaly was ac-companied by a persistent precipitation deficit (Fink etal. 2004), starting as early as February and lasting untilthe end of summer 2003. Using satellite imagery andmeteorological station data, Zaitchik et al. (2006) dem-onstrate that the early vegetation green-up due tospringtime warmth, together with the lack of precipita-tion, resulted in an early season soil moisture deficit.Black et al. (2004) and Zaitchik et al. (2006) point outthat the first extreme temperature anomaly in June wasmainly the result of the persistent anticyclonic circula-tion anomaly. During the entire month of June a char-acteristic wave pattern of 500-hPa height anomalies fea-turing deep troughs over the eastern Atlantic and west-ern Russia, and ridges over Europe and central Russia,was observed (Ferranti and Viterbo 2006). The anoma-lously clear skies and the extremely strong radiativeanomalies in June contributed to further loss of soilmoisture. In July, few Atlantic frontal systems pen-etrated as a result of a blocking pattern over Scandina-via and a pronounced split-flow configuration farthersouth (Ferranti and Viterbo 2006), while temperatureanomalies were somewhat less extreme (�2°C abovethe climatological mean). Temperature anomalies cul-minated in a maximum heat wave over France between2 and 12 August. This episode was associated with anabnormally positive phase of the summer Northern An-nular Mode (NAM) index (Ogi et al. 2005) and unusu-ally low upper-level ozone concentrations (Orsolini andNikulin 2006). Black et al. (2004) suggest that the ex-ceptionally high temperatures in late summer 2003were mainly the consequence of the atmospheric circu-lation enabling a dominance of the local heat balanceover Europe. The severe late-summer drought contrib-uted to strongly reduced latent heat flux and positivesensible heat anomalies (Black et al. 2004; Zaitchik etal. 2006).

In addition to land surface temperatures, strongmean Mediterranean SST anomalies of about 2.1°Cwere observed during JJA 2003 (Jung et al. 2006;Grazzini and Viterbo 2003). While SST anomalies ex-ceeded the long-term standard deviation by more thana factor of 3 in the western part, they were far smaller

5082 J O U R N A L O F C L I M A T E VOLUME 20

in the southeastern part of the Mediterranean. How-ever, Jung et al. (2006) and Ferranti and Viterbo (2006)suggest that the enhanced SSTs in the Mediterraneanhad a marginal influence on the midtropospheric dy-namical circulation in Europe and rather followed thetropospheric temperature signal. Black and Sutton(2006) challenge these results, suggesting that the SSTanomalies in both the Indian Ocean and in the Medi-terranean Sea had a significant influence on the 2003heat wave. They point out that the Mediterranean con-tributed most strongly to the early part of the heat waveand the Indian Ocean enabled the positive temperatureanomalies to persist into August.

The overall goal of this study is to improve our un-derstanding on the land surface–related processes andfeedbacks that led to the extreme anomalies during sum-mer 2003 by means of simulations and observational data.We perform a set of regional climate simulations for theyear 2003 using the European Centre for Medium-Range Weather Forecasts (ECMWF) operationalanalysis as lateral boundary conditions. Thereby we en-sure that all simulations are driven with a realistic rep-resentation of the large-scale atmospheric circulation.

The first specific focus of our study is to establish theextent to which such a modeling framework is able toreplicate an extreme event such as summer 2003. Tothis end, the simulated fields are compared to a 31-yr-long model simulation (1970–2000). The 2003 anoma-lies are carefully validated with independent observa-tional temperature and precipitation fields as well asdiagnostic estimates of terrestrial water storage varia-tions (Seneviratne et al. 2004; Hirschi et al. 2006b) inthe months prior to and during the heat wave. Note thatthe term “heat wave” is often defined as a number ofconsecutive days with temperatures exceeding somethreshold. In this sense, summer 2003 was associatedwith at least three distinct heat waves. However, in thecurrent study we use the term heat wave in a seasonalsense, while addressing the temporal variations of thesignal in considerable detail.

The second focus of this study is to assess the role ofsoil moisture–atmosphere interactions as a key feed-back mechanism for the 2003 summer heat wave. Wefind that the soil moisture content was relatively high inthe beginning of the year 2003 due to anomalously highprecipitation in summer and autumn 2002 over centraland eastern Europe (e.g., the flooding event in August2002; see Rudolf et al. 2003; Christensen and Chris-tensen 2003). However, the persistent precipitationdeficit and the excess in total net radiation in late win-ter and spring 2003 contributed to an early and rapidsoil drying. To investigate the effect of this anomaloussoil moisture depletion upon the subsequent summer

drought, we perform a sensitivity experiment, whichincludes 10 simulations with perturbed spring soil mois-ture. Soil moisture is artificially increased and de-creased on 1 April to identify the sensitivity to differentsoil moisture conditions given the same large-scale at-mospheric circulation. The difference of the controland the wettest simulation, which produces approxi-mately long-term mean late summer soil moisture con-ditions, provides of a rough estimate on the soil mois-ture contribution to the heat wave. We analyze the re-sponse of temperature, precipitation, and atmosphericcirculation to different soil moisture conditions, as wellas the related anomalies in the water and surface en-ergy budget.

An earlier study by Ferranti and Viterbo (2006) ana-lyzed the influence of different soil moisture initializa-tions in June 2003 upon the seasonal forecast of thesubsequent three summer months. They found a signifi-cant atmospheric response to substantial soil moistureperturbations during 2–3 months. Note that in the Fer-ranti and Viterbo (2006) framework, there is no guar-antee that the atmospheric circulation is close to itsobserved evolution. Indeed, their simulations did notreplicate the characteristic 2003 circulation anomalies,whereas our study addresses the soil moisture–atmo-sphere feedback processes using a best estimate of theobserved continental-scale circulation (ECMWF opera-tional analysis). In contrast to Ferranti and Viterbo(2006), we prescribe the circulation at the lateralboundaries and thereby ensure a realistic representa-tion of the observed persistent circulation anomalies.This allows us to isolate the response to different soilmoisture conditions given identical lateral boundaryconditions. Moreover, in our simulations the effect ofthe imposed soil moisture perturbation persists sub-stantially longer than in their experiment (see section 4a).

The paper is organized as follows. We start by pre-senting the numerical model and its application in theexperiments, as well as the employed observationaldatasets in section 2. Section 3 details the validation ofthe control simulation. The results of the sensitivity ex-periments are presented in section 4 with emphasis onsoil moisture, temperature and precipitation response,circulation anomalies, and surface energy budgets. Themain conclusions are discussed in section 5.

2. CHRM climate model and experimental setup

a. Model details and origin

The Climate High-Resolution Model (CHRM) ver-sion 2.3 (Vidale et al. 2003) is a state-of-the-art regionalclimate model (RCM) using a regular latitude–longi-tude grid (0.5° � 0.5°) with a rotated pole and a hybrid


sigma pressure coordinate with 20 vertical levels and 3active soil layers (total depth of 1.7 m). The CHRM isthe climate version of the former mesoscale weatherforecasting model of the German and Swiss Meteoro-logical Services known as the High-Resolution Model(HRM) or, formerly, the Europa-Modell (EM; Majew-ski 1991; Majewski and Schrodin 1994). The model hasbeen modified by Lüthi et al. (1996) for application asa regional climate model. Its physical package includesa mass flux scheme for convection (Tiedtke 1989),Kessler-type microphysics (Kessler 1969; Lin et al.1983), a land surface scheme (Dickinson 1984), and asoil thermal model (Jacobsen and Heise 1982). The soilmoisture storage capacity had to be increased to im-prove the simulation of land surface balances of heatand water in a realistic and sustainable fashion on timescales longer than in standard NWP (see Vidale et al.2003). The soil moisture evolves freely after initializa-tion of the RCM and is never corrected or nudged inthe course of the simulation. Vertical diffusion and tur-bulent fluxes in the atmosphere are parameterized by aflux gradient approach of the Beljaars and Viterbo(1998) type in the surface layer and Mellor and Yamada(1974) type in the boundary layer. The CHRM modelalso includes a radiative transfer scheme developed byRitter and Geleyn (1992). The CHRM has been vali-dated regarding its ability to represent natural variabil-ity on different time scales (Vidale et al. 2003). Notethat the model has not been tuned for an optimal rep-

resentation of the 2003 heat wave. Earlier versions ofthe model have previously been used in a series of sen-sitivity and process studies about the role of soil mois-ture–precipitation feedbacks (Schär et al. 1999) and theinfluence of vegetation changes on the European cli-mate (Heck et al. 2001). More recently the currentmodel version has been used in climate scenario simu-lations (S04; Vidale et al. 2007; Seneviratne et al.2006b).

b. Experimental design

The computational domain used in this study coversEurope and the northeastern Atlantic with a horizontalresolution of about 56 km (see map in Fig. 1a). Theexperiment includes 16 simulations, which are detailedin the following subsections (see also schematic inFig. 1b).

1) CTL, INIT, AND CLIM

The conducted experiment includes a control simu-lation (CTL), an ensemble of four simulations withslightly perturbed initial conditions (INIT), and a cli-matological reference simulation covering the whole40-yr ECMWF Re-Analysis (ERA-40) period (1958–2001). The CTL and INIT simulations (2002–03) aredriven by assimilated lateral boundary conditions andsea surface temperatures from the ECMWF opera-tional analysis (TL511) using the relaxation boundary

FIG. 1. (a) Map of the model domain used in all simulations. The land portion within the bluesquare is used for the analysis of area-averaged soil moisture, temperature, precipitation, and thesurface energy budget. The red triangles mark the location of the two CARBOEUROPE sta-tions, Le Bray (southwestern France) and Hesse (northeastern France). The French basin (en-compassing the Rhone, Loire, Seine, and Garonne catchments) used for the validation of theterrestrial water storage cycle is highlighted in green. (b) Schematics of the sensitivity experiments.The lateral boundary conditions for the different periods are indicated at the top of the panel. On1 Apr, the soil moisture of the sensitivity experiments is reduced/increased by up to �50%.


Fig 1 live 4/C

technique (Davies 1976). The CTL and INIT simula-tions are initialized on 5 consecutive dates, on 3 Janu-ary 2002 (CTL) and on 1, 2, 4, and 5 January 2002(INIT), respectively, and use virtually identical initialsoil conditions.

As a reference period we use the 31-yr period (1970–2000) out of the long-term 44-yr CHRM simulationdriven and initialized with ERA-40 boundary condi-tions. The simulated soil moisture fields at the end ofthis run (31 December 2001) are used for the soil mois-ture initialization of the CTL and the INIT simulations.This provides the best estimate for the soil moistureinitialization field in January 2002 and ensures that it iswithin a typical interannual range of the CHRM soilmoisture equilibrium.

2) DRY AND WET SENSITIVITY EXPERIMENTS

In addition to the CTL, INIT, and CLIM simulations,10 sensitivity simulations with perturbed spring soilmoisture conditions (DRY, WET; see Fig. 1b) are per-formed in this study. The simulations are initialized on1 April 2003, with the corresponding fields from theCTL simulation but perturbed soil moisture fields. Theinitialization fields are altered by increasing or decreas-ing the soil moisture by 10%, 15%, 20%, 25%, or 50%in the total soil depth at every grid box over the modeldomain. In the WET experiments, the soil moisturereached the water holding capacity at some grid boxesafter the perturbation. In this case the additional soilwater went instantaneously into runoff and was re-moved from the grid boxes. After the perturbation, thesoil moisture evolves freely until the simulations end inDecember 2003. All DRY and WET simulations aredriven with the same lateral boundary conditions as theCTL and INIT simulations. The comparison of theseperturbed simulations against CTL allows isolating theeffect of the soil moisture on the 2003 summer climateover Europe.

Most analyses have been performed for regional av-erages over nine European subdomains. However, inorder to keep this paper concise, we decided to presentregionally averaged analyses for the subdomain ofFrance only (outlined in Fig. 1a). This region was se-lected because of its high anomalies during the peakepisode in early August and the availability of terres-trial water storage estimates and observational data forvalidation.

c. Observational data

1) GISTEMP

In this study, the National Aeronautics and SpaceAdministration Goddard Institute for Space Stud-

ies (NASA GISS) Surface Temperature Analysis(GISTEMP; Hansen et al. 1999, 2001) is used to vali-date the simulated 2003 temperature anomalies com-pared to the climatology. GISTEMP is a global analysisof surface temperature using observational station dataas input data. The analysis includes land surface and seasurface temperature (1° � 1°) as anomalies with respectto a user-defined reference period, in our case 1970–2000.

2) GPCC

We use the Full Data Reanalysis Product (Rudolf etal. 1994, 2005) provided by the Global PrecipitationClimatology Centre (GPCC) to validate simulated pre-cipitation anomalies. The global dataset (resolution0.5° � 0.5°) covers the period 1951–2004 and is basedon quality-controlled in situ observations from a largenumber of rain gauge stations.

3) BSWB

The diagnostic basin-scale water balance (BSWB)approach (Seneviratne et al. 2004) allows one to assessmonthly changes in terrestrial water storage (TWS) forlarge-scale catchments (�105�106 km2) using estimatesof atmospheric moisture flux convergence and changesin atmospheric moisture content from reanalysis data,and streamflow measurements within the area. TWSencompasses soil moisture, groundwater, snow, andsurface water. Here we use estimates derived usingmoisture flux convergence from ERA-40 for 1970–2000(Hirschi et al. 2006a) and from the ECMWF opera-tional analysis for 2003 (Hirschi et al. 2006b), respec-tively. The discharge data are based on runoff measure-ments retrieved from the Global Runoff Data Centre(available online at http://grdc.bafg.de/). Seneviratne etal. (2004) and Hirschi et al. (2006a) demonstrate suc-cessful validation of this method for various river ba-sins.

3. Validation of CTL simulation

Here we present a validation of the CTL simulationfor 2003. The CHRM has been extensively validatedregarding its ability to represent temperature variationson annual as well as on interannual time scales (Vidaleet al. 2003, 2007). However, the representation of anextreme event such as summer 2003 is an additionalchallenge for a climate model. Here, we focus on thevalidation of surface temperature (seasonal and dailytime scale), precipitation, and soil moisture.

a. Daily and seasonal temperature

Figure 2 displays the summer (JJA) 2003 tempera-ture anomaly with respect to the reference period 1970–


2000 represented by the GISTEMP dataset (Fig. 2a)and the CHRM CTL simulation (CTL � CLIM; Fig.2b). The overall anomaly patterns compare well, de-spite a weak underestimation of the spatial extent ofthe warm anomaly over central Europe. The amplitudeof the warm anomalies is in reasonable agreement ex-cept for the maximum heat anomaly over northernFrance, where it is slightly underestimated. On amonthly time scale (not shown), the agreement is bestin June, whereas in July the strong anomaly over Scan-dinavia is somewhat underestimated. During the maxi-mum heat wave in August the heat anomalies overFrance, the Alps, and northern Italy are underesti-mated by about 1°C in CTL. Apart from the weak un-derestimation of the extreme July and August tempera-tures, the monthly anomalies are well captured. Vali-

dation of the perturbed simulations reveals that theobserved central European temperature extremes inJuly and August are better captured in the runs withspring soil moisture reduced by 10% (DRY10) and15% (DRY15). This may indicate that the water stressand the related flux anomalies are somewhat underes-timated in the CTL simulation. Good agreement betweensimulated (CTL � CLIM) and observed (GISTEMP)temperature anomalies is found in all months prior tothe summer heat wave, from January to May, with bi-ases similar to or smaller than in June (not shown).

The simulated temperature is also consistent with ob-servations on shorter time scales. Figures 2c,d displaysimulated daily mean temperatures (CTL; inverse dis-tance weighted average of the four nearest grid boxes)compared against observations at two CARBOEUROPE

FIG. 2. Validation of surface temperature. Summer (JJA) temperature anomalies 2003 w.r.t.1970–2000 (K) represented by (a) GISTEMP analysis and (b) the difference between CTL andCLIM simulations. (c),(d) Observed daily mean temperature (thin blue lines) and simulatedtemperature (CTL) (thin red line) at Le Bray and Hesse, respectively. The thick lines show an11-day running mean of the corresponding datasets.


Fig 2 live 4/C

stations in Le Bray (western France, coniferous forest)and in Hesse (northeastern France, hardwood forest;see map in Fig. 1a). At both stations, simulated dailytemperature variations are in good agreement with themeasurements. While the timing compares very well,the amplitude of the short-term variations is somewhatunderestimated. This is mainly due to the smoothingeffect of the averaging over grid boxes, which reducesthe variability with respect to a single measurementstation. For Hesse, the 11-day running average (thickline) shows local underestimation (around 3°C) of themaximum heat wave episode in the first half of August.The maximum warm anomalies in Le Bray are wellcaptured.

b. Precipitation anomaly fields

European precipitation was below normal during thespring and summer of 2003 (Black et al. 2004; Zaitchiket al. 2006). We validate the model’s representation ofthe pronounced precipitation deficit using GPCC ob-servational data [see section 2c(2)]. Figure 3 displaysthe 2003 summer (JJA) precipitation anomaly relativeto 1970–2000 for GPCC and CTL. There is good agree-ment on the overall anomaly pattern that showsstrongly reduced summer precipitation over central Eu-rope and anomalously wet conditions over northeasternEurope. Over Germany and eastern Europe, the lack ofprecipitation is somewhat underestimated by themodel.

Note that during spring 2003 equally pronouncedprecipitation deficiencies are observed over France, theAlpine region, and southeastern Europe (not shown).This negative precipitation anomaly pattern is rela-tively well captured by the CTL simulation. Some partsof the Alpine region experienced persistent negativeprecipitation anomalies of 2 mm day�1 averaged overwinter, spring, and summer 2003. The spring precipita-tion deficit has contributed to a strong soil moisturedepletion, which will be addressed in the next section.

c. Soil moisture cycle and terrestrial water storagevariations

The realistic representation of the seasonal soil mois-ture evolution is crucial for a simulation of the 2003summer heat wave, as it strongly affects the partitioningof the surface energy into sensible and latent turbulentfluxes. In observational terms this involves large uncer-tainties due to the lack of a dense soil moisture obser-vational network allowing for a spatially representativevalidation. Hence, we compare our simulations with ba-sin-scale water balance [see section 2c(3)] estimates forthe French catchments of Rhone, Loire, Seine, and Ga-ronne (total area: 335 450 km2; outlined in Fig. 1a).

The BSWB provides monthly rates of change of ter-restrial water storage (�TWS/�t), which are comparedagainst monthly changes of simulated snow and soilmoisture aggregates. Figure 4a depicts the monthlyTWS variations in 2003 (CTL, red; BSWB, blue) and

FIG. 3. Summer precipitation anomaly 2003 w.r.t. 1970–2000 represented by (a) the GPCCprecipitation analyses and (b) the difference between CTL and CLIM simulation (mm day�1).


Fig 3 live 4/C

the climatological mean (CLIM, solid line; BSWB,dashed line). An estimate of the time-integrated TWSanomalies is shown in Fig. 4b. Note that the BSWBprovides no exact information on the absolute TWSvalues in 2003 with respect to the mean climatology butrather on the amplitude and phase of the annual cycle.

The climatological mean TWS variations (Fig. 4a)and time-integrated TWS anomalies (Fig. 4b) comparereasonably well between the simulations and theBSWB data. The TWS increase is somewhat overesti-mated in winter and underestimated in autumn. Thisresults in a slight time shift and a weak overestimationof the climatological TWS cycle by the model (Fig. 4b).The interannual variability is underestimated by themodel. This systematic deficiency has been demon-strated to be a general problem for a wide range ofRCMs (Hirschi et al. 2007).

In 2003 the largest differences between the CTLsimulation and BSWB are found in the rates of changein January and October. However, both datasets agreeon anomalously strong TWS depletion in February andMarch 2003. As indicated above, the rapid decrease insoil moisture is associated with persistent negative pre-cipitation anomalies (section 3b). This results instrongly reduced TWS from spring to autumn (Fig. 4b).The TWS variations (Fig. 4a) reveal only weak reduc-tions in late spring and around average variations inlate summer, both according to BSWB and CTL.Hence, after an anomalous depletion in early spring2003 the TWS remained at a constant anomaly belowthe climatological mean until autumn (Fig. 4b). In thefollowing section, we will assess the influence of thesesoil moisture anomalies in spring 2003 on the evolutionof surface variables in the following months.

4. Sensitivity experiment

a. Soil moisture evolution

In this section we analyze the effect of the spring soilmoisture perturbations applied in the DRY and WETsimulations on the subsequent soil moisture evolution.The soil saturation at the time of the perturbation (1April 2003) is shown in Fig. 5a in terms of the soilmoisture index (SMI; two uppermost soil layers with10- and 25-cm depths, respectively). The soil saturationis found to be relatively high except for mountainousregions in the Alps and Norway.

The soil moisture evolution before and after the per-turbation is shown in Fig. 5b for the example of thesubdomain France (FR; outlined in Fig. 1b). The DRYand WET experiments are not symmetric, since at somegrid boxes the water holding capacity is reached afterthe soil moisture increase in the WET experiments. Inthese cases the soil moisture was instantaneously re-moved from the grid box through runoff.

The differences due to the slightly differing atmo-spheric initial conditions between the CTL and the fourINIT members are revealed to be very small (maximumJJA range � 2.4 mm) compared to the interannual vari-ability of CLIM (�JJA � 31 mm). In early 2003, thesimulated soil moisture content (CTL and INIT) is rela-tively high due to anomalous precipitation amounts insummer and autumn 2002.

Independent of the soil moisture content in April, allperturbed and nonperturbed simulations show dra-matic reductions of soil moisture from April to lateAugust. This highlights the important role of the strongradiative anomalies, the lack of precipitation, and earlyactivation of vegetation, which resulted in a strong des-

FIG. 4. (a) Monthly terrestrial water storage (TWS) variations averaged over the French basin(encompassing the Rhone, Loire, Seine, and Garonne catchments) diagnosed with the BSWBapproach (see section 2c) for 2003 (blue line) for 1973–2000 (dashed black line; �1 std devmarked; light gray shading). Corresponding values (sum of soil moisture and snow depth) for theCTL simulation for 2003 (red line) and 1970–2000 (CLIM; mean marked by solid black line; darkgray shading), (b) Same as (a), but absolute anomalies of TWS relative to the climatologicalannual mean.


Fig 4 live 4/C

iccation of the soil. In August 2003 the soils in all re-gions except for northeastern Europe and Scandinaviaare anomalously dry in the CTL simulation (Fig. 5c)and very dry over almost the entire domain in DRY25(Fig. 5d). Even in WET25 (Fig. 5e) and WET50 (notshown), the soil moisture values over parts of westernand central Europe drop slightly below the long-termmean. Hence these two wet simulations represent a hy-pothetical 2003 climate with summer soil moisture con-ditions close to the climatological mean over France(Fig. 5b), as well as over central Europe and east to-ward Poland and Hungary (Fig. 5e). Note that the lat-eral boundary conditions are identical for CTL, WET,

and DRY simulations. The difference between theWET25/WET50 and the CTL simulation provides arough quantification of the soil moisture effect uponthe 2003 summer heat wave for France as well as cen-tral Europe.

Over all regions, the imposed soil moisture anoma-lies in the DRY and WET simulations decrease con-tinuously over time and approach the CTL in a quasi-exponential fashion. The characteristic half-life periodvaries between 3 and 8 months depending on the hy-draulic conductivity of the soil and the saturation at thetime of the perturbation. In FR (Fig. 5a), the distur-bance is halved within the first 3.0–3.5 months (until

FIG. 5. (a) Soil moisture index (two uppermost soil layers with 10- and 25-cm depth, respec-tively) in the CTL simulation at the time step of perturbation (1 Apr 2003). (b) Semimonthly 2003soil moisture depth averaged over France for the CTL simulation (dark green line), the four INITmembers (light green lines), and the DRY (yellow to red lines) and the WET (bluish lines)simulations. The black line indicates the mean values for the period 1970–2000 (CLIM) and thegray shading indicates the typical interannual range (�1 std dev). Each tick mark on the abscissarepresents the average over half of a month (e.g., the 1–15 Jan and 16–31 Jan). (c)–(e) Aug 2003soil moisture content in the CTL, DRY25, and WET25 simulations, respectively, divided by theclimatological mean (CLIM, 1970–2000) for the corresponding month.


Fig 5 live 4/C

mid July) of the WET runs and within 4 months (untilAugust) of the DRY runs, respectively. The decay ofthe imposed perturbation is slowest and soil moisturememory longest in the simulations with small perturba-tions. This compares well with previous studies (Kosterand Suarez 2001; Seneviratne et al. 2006a) suggestingthat the sensitivity of runoff with respect to soil mois-ture increases with higher soil moisture content, andthe sensitivity of evapotranspiration with respect to soilmoisture increases with lower soil moisture contents.Therefore, the longest soil moisture memory is ex-pected at intermediate soil wetness values (Seneviratneet al. 2006a).

b. Temperature response

The imposed spring soil moisture perturbations sub-stantially affect surface temperature during the subse-quent months. To quantify this effect, surface tempera-tures in the perturbed, unperturbed, and climatologicalsimulations are compared. Figure 6a shows semi-monthly temperature averaged over the subdomain FRfor 2003 (colored lines) and the mean model climatol-ogy (1970–2000). In general all perturbed and unper-turbed simulations follow the same path in 2003, show-ing the characteristic warm anomalies in June and the

first half of August. The summer temperatures corre-late quasi-linearly (gradient �0.015 K mm�1) withthe absolute soil moisture amount in summer or in thepreceding spring (Fig. 6c). The range of this effect inour simulations is relatively large and amounts toaround 2.5 K, which corresponds to more than 2 stan-dard deviations of interannual summer temperaturevariability.

Note that the driest simulation (DRY50) is an excep-tion, since the additional warming with respect to theDRY25 simulation is relatively small. Due to the ex-tensive perturbation the soil saturation is close to thewilting point in these two simulations. Hence an addi-tional soil moisture reduction does not further affectthe latent cooling and surface temperature.

The spatial temperature anomaly patterns of theDRY25 and WET25 simulations with respect to theunperturbed CTL simulation are displayed in Fig. 7.The reduction of spring soil moisture by 25% results ina strong summer (JJA) warm anomaly of about 2°Cover a zonal band covering land regions between theMediterranean and the North Sea and the Atlantic andthe Black Sea. The effect is much weaker over the Ibe-rian Peninsula and northern Europe. The anomalyDRY25 � CTL is caused solely by reduced spring soil

FIG. 6. (a) Semimonthly 2003 temperature at 2 m averaged over France (conventions as in Fig.5b). (b) Same as (a), but for precipitation in mm day�1. (c) Scatterplot between JJA 2003 surfacetemperature and soil moisture in the spring (April–May, triangles) and subsequent summer (JJA,circles) for all simulations. (d) Same as (c), but for precipitation and soil moisture.


Fig 6 live 4/C

moisture and the subsequent feedback processes andcorresponds to about 30%–50% of the 2003 warmanomalies (CTL � CLIM) over France and large partsof central Europe (Fig. 2b). Hence, if soil moisture hadalready been at low levels in early 2003, the anomalyDRY25 � CTL would have superimposed upon theCTL � CLIM, leading to a strongly enhanced heatanomaly. The soil moisture effect on surface tempera-ture occurs mainly through reduced latent cooling andto a lesser extent through a positive circulation feed-back to be discussed in section 4d.

Increased spring soil moisture (WET25 � CTL) pro-duces an opposite summer temperature signal and re-sults in a reduction of the summer temperatureanomaly (Fig. 7b). Again the strongest anomalies arefound in a zonal band between the Mediterranean andthe North Sea. Over large areas, the spring soil mois-ture increase in WET25 implies negative temperaturedepartures of around �1.5°C with respect to CTL(WET50 � CTL; even more than �2°C over some re-gions; not shown). This signal represents a substantialportion of the 2003 summer temperature anomaly inthe unperturbed simulation (CTL � CLIM). Thesefindings suggest that 2003 summer temperatures overFrance and parts of central Europe would have beensubstantially cooler (by up to 2°C) given the samelarge-scale circulation pattern but climatological meansummer soil moisture. Note that this finding may bemodel dependent and that the soil moisture anomaly

has only been validated over France. Over northernEurope there are virtually no effects of the soil mois-ture increases.

These regional differences of the response are mainlyrelated to the different sensitivities of the latent heatflux to changes in soil moisture. To analyze these sen-sitivities we have calculated the percentage of net ra-diation going into latent heat flux for different simula-tions and different regions (not shown). This analysisrevealed different regional sensitivities depending onthe soil saturation. The sensitivity is low at dry (nearwilting point) and at wet (near field capacity) soil mois-ture contents and strong at the intermediate contents.Over Scandinavia evaporation is not limited due to thehigh soil saturation. France and central–eastern Europeshow high sensitivities for all simulations with interme-diate soil moisture values. The two driest simulations(DRY25 and DRY50) show weaker sensitivity to fur-ther drying, since the maximum limitation is reached atmany grid points (wilting point). Likewise the sensitiv-ity is very weak over the generally dry Iberian Penin-sula, explaining the small temperature response to soilmoisture perturbations.

In a recent study, Ferranti and Viterbo (2006) ana-lyzed the effect of soil moisture initial conditions on2003 summer temperatures. They initialized the ECMWFatmospheric model with different May soil moistureconditions and performed seasonal forecasts for sum-mer 2003. They did not prescribe the large-scale circu-

FIG. 7. Summer 2003 temperature anomaly due to spring soil moisture perturbation in (a)DRY25 � CTL and (b) WET25 � CTL.


Fig 7 live 4/C

lation but nevertheless found a significant temperatureresponse, which shows the same overall anomaly pat-tern as in our simulations, albeit shifted somewhat tothe south with maximum amplitude over the Mediter-ranean and the Iberian Peninsula.

c. Precipitation response

Previous studies (e.g., Rowntree and Bolton 1983;Schär et al. 1999) have shown that European summerprecipitation is sensitive to soil moisture conditions.Schär et al. (1999) have identified in numerical experi-ments that the effect of soil moisture conditions on pre-cipitation is in relative terms strongest in a belt stretch-ing from the Atlantic to the Black Sea to the north ofthe Mediterranean.

Here, we present a brief analysis (confined to thesubdomain FR) of the soil moisture–precipitation feed-back. The spring soil moisture perturbations are re-vealed to have a substantial effect on precipitation dur-ing the subsequent months until the end of summerover France (Fig. 6b) as well as over the rest of thedomain except for northeastern Europe and northernScandinavia (not shown). Generally lower spring soilmoisture leads to lower summer precipitation.

The precipitation response to the soil moisture per-turbation is less continuous than for temperature. OverFrance the soil moisture–precipitation relationship ap-pears relatively nonlinear, particularly in the DRYsimulations (Fig. 6d). In the two driest simulationsevaporation is strongly limited, and a further drying hasonly a minor effect on precipitation. A similar relation-ship applies to all other analyzed western, central Eu-ropean regions and the Iberian Peninsula (not shown).Over northern Europe, soil moisture is close to satura-tion so that evaporation and precipitation are not af-fected (see also Rowell and Jones 2006).

The soil moisture effect on precipitation mainly oc-curs through a limitation of summer evaporation, whichreduces the convective activity. Schär et al. (1999)demonstrated by means of detailed budget analysesthat the feedback cannot be interpreted in terms ofprecipitation recycling, since recycling is far too ineffi-cient on the spatial scales that matter in the Europeancontext. They suggested that the surplus of precipita-tion over wet soils is derived primarily from atmo-spheric advection and is triggered by more efficientconvective precipitation processes over wet soils. Herewe do not perform budget analyses to distinguish be-tween advected and recycled precipitation as well aslocal and nonlocal soil moisture effects. However, weexpect the same mechanisms to apply as described inSchär et al. (1999).

d. Circulation response

The persistent strong anticyclonic circulationanomaly over central Europe played a crucial role dur-ing the 2003 summer heat wave (Black et al. 2004; Ogiet al. 2005). The 500-hPa height anomaly (Fig. 8a) iswell captured by the CTL simulation and compares wellwith the National Centers for Environmental Predic-tion (NCEP) reanalysis and ECMWF operationalanalysis (Grazzini et al. 2003). The 500-hPa anomalyfields (CTL–CLIM) show a characteristic wave patternwith negative anomalies over the eastern North Atlan-tic and western Russia and pronounced positive anoma-lies northwest of Scandinavia and central Europe. The1000-hPa height anomaly (Fig. 8d) is much weaker. Inparticular, over central Europe the positive anomaly ishardly detectable at the surface (10 m). This is ingood agreement with ECMWF operational analysis andNCEP reanalysis (not shown). We suggest that the rela-tively weak 1000-hPa height anomaly is partly due tothe strong surface heating causing a weak surface heatlow as described below.

The analysis of the sensitivity experiment reveals thatsoil moisture affects geopotential height from the sur-face to the upper troposphere. Figure 8 shows the 500-(Figs. 8b,c) and the 1000-hPa (Figs. 8e,f) geopotentialheight anomaly caused by the spring soil moisture re-duction (DRY25–CTL; Figs. 8b,e) and increase(WET25 � CTL; Figs. 8c,f). In DRY25 the 1000-hPaheight is found to be substantially reduced by up to 12m. The shape of the anomaly corresponds to the asso-ciated surface warm anomaly depicted in Fig. 7a, point-ing at the presence of a heat low mechanism. The sameeffect as in DRY25, but opposite in sign, causes a posi-tive 1000-hPa height departure in the WET25 simula-tion (Fig. 8f).

The effect of soil moisture on 500-hPa geopotentialheight is reverse and of similar amplitude. The dryanomaly in DRY25 forces a positive 500-hPa heightanomaly aloft, situated roughly above the same regioncovered by the surface heat low. The amplitude of thispositive height departure (DRY25 � CTL), induced bythe soil moisture perturbations, corresponds to about10%–15% of the total CTL � CLIM anomaly.

Qualitatively consistent results are found for theother sensitivity experiments. These findings suggestthat a moderate soil moisture reduction may enhance apositive height anomaly at upper levels and thus makeit more persistent. Through this mechanism a dry soilanomaly should positively feed back on itself by forcinga strengthening of the anticyclonic circulation and anassociated slight northward shift of the storm tracks.

The height anomaly DRY25 � CTL has its maximum


slightly east of the CTL � CLIM anomaly. Hence, thedry anomaly adds a slight eastward component and mayimply a weak change in advection to generally warmerair masses originating from a more eastern sector (i.e.,the Mediterranean and continental northern Africa).The vertical height anomaly profile (not shown) overFR shows that the positive height anomaly is increasingin the upper troposphere, reaching a maximumanomaly of 20 m at 250 hPa. This corresponds to thelevel of the strongest 2003 height anomalies (NCEP andECMWF; not shown). A comparable response of the

500- and 1000-hPa geopotential height to dry soils hasbeen observed in the study Ferranti and Viterbo (2006).Previously, Oglesby and Erickson (1989) and Pal andEltahir (2003) performed soil moisture sensitivity ex-periments over North America using different climatemodels and different experimental setups. Both studiesfound a heat low at the surface and an enhanced posi-tive height anomaly aloft due to substantially reducedsoil moisture. The increased 1000–500-hPa thickness isdirectly linked to higher tropospheric air temperatureand an expanded atmospheric column in this layer. The

FIG. 8. Summer 2003 geopotential height anomalies CTL � CLIM at (a) 500 and (d) 1000 hPa, respectively, w.r.t. 1970–2000 mean.(b), (e) Same as (a), (d), but for DRY25 � CTL spring soil moisture perturbation. (c), (f) Same as (b), (e), but for WET25 � CTL springsoil moisture perturbation. Note that the scales are different in (a), (d) and (b)–(f).


Fig 8 live 4/C

tropospheric temperature increase due to drought con-ditions is strongly influenced by the enhanced sensibleheat flux (reduced latent cooling). This will be dis-cussed in the following subsection. Moreover, the re-duced latent heat flux in the DRY simulation impliesless diabatic (condensational) heating at midtropo-spheric levels. It is possible that this enhances subsid-ence through the mechanism described in Xue (1996).

e. Surface energy balance

Black et al. (2004) have shown that during the maxi-mum heat wave from 6–12 August, air was essentiallytrapped within a strong anticyclonic block over north-ern France. Their analysis showed that air parcels trav-eled only very short distances, and they concluded that

the regional heat budget must have enabled the ex-tremely high temperatures to occur. To investigate therole of the surface energy budget we compare the simu-lated radiative and turbulent fluxes in 2003 (CTL)against the model climatology (CLIM). In a second stepthe effect of the soil moisture perturbations on the sur-face energy budget is evaluated by analyzing the DRYand WET simulations.

In the case of the sensible heat flux, diurnal fluxes arediscussed separately; since daily averages are found torepresent sums of reverse day and night signals, therebyobscuring important processes. Figures 9a–f display, re-spectively, semimonthly shortwave, longwave, and totalnet radiation, vertically integrated absolute water vaporas well as latent and sensible heat fluxes averaged over

FIG. 9. Surface energy balance averaged over the land portion of domain FR (see Fig. 1a,conventions as in Fig. 5b): (a) Semimonthly 2003 surface shortwave net radiation, (b) longwavenet radiation, (c) total net radiation, (d) total water vapor column, (e) latent heat flux, and (f)sensible heat flux.


Fig 9 live 4/C

FR for CTL, INIT, DRY, and WET simulations andthe model climatology (CLIM). Note that for simplifi-cation positive-signed sensible and latent heat fluxesare defined to be directed upward.

1) RADIATIVE AND TURBULENT FLUX ANOMALIES

IN 2003

In good agreement with Black et al. (2004) and Fer-ranti and Viterbo (2006), surface shortwave net radia-tion in the CTL simulation is found to be anomalouslypositive throughout the period of mid-February to mid-August 2003. On average the climatological mean(CLIM) is exceeded by 17 W m�2 during these 6months. Maximum anomalies are reached in late Marchand June (around �30 W m�2). Low integrated liquidcloud water contents (not shown) during these twoperiods point to anomalous clear-sky conditions. Fer-ranti and Viterbo (2006) show that cloudiness in theECMWF analysis was reduced during the whole periodbetween spring and early summer 2003. The hottestperiod over FR (in early August), however, was asso-ciated with somewhat smaller shortwave net radiationdepartures than the June and late March periods.

Outgoing longwave radiation over FR was anoma-lously strong from mid-February to August and again inlate September due to the high surface temperatures.The average departure (February–August) of the CTLsimulation from climatology (CLIM) amounts to 11 Wm�2. This is in good agreement with Moderate Reso-lution Imaging Spectroradiometer (MODIS) satellitemeasurements, which recorded an average summeranomaly of 12.6 W m�2 between April and Septemberover France (Zaitchik et al. 2006). Interestingly, thestrongest longwave net radiation anomalies are notfound during the warmest periods in June and earlyAugust, but in late March. This is due to the compen-sating effect of anomalously strong longwave down-ward radiation (not shown), with values of 20 W m�2

(CTL � CLIM) during June and early August. Thisincrease of downward radiation is caused by high ab-solute atmospheric water vapor content (Fig. 9d) andhigh tropospheric temperatures. Zaitchik et al. (2006)suggest that the reduction of longwave downwardfluxes due to reduced cloudiness (not shown) was over-powered by the increase due to higher humidity andtemperature.

The total net radiation (Fig. 9c) over FR was anoma-lously high (CTL � CLIM) between March and earlyJuly, when the shortwave downward dominated overthe longwave upward radiation. We suggest that thisextremely persistent total net radiation excess duringall spring and early summer months strongly contrib-uted to the depletion of soil moisture. Interestingly,

however, simulated total net radiation returned to cli-matological mean conditions in late July and during thehottest phase in early August. This finding is in agree-ment with in situ radiation measurements at the Base-line Surface Radiation Network (BSRN) station in Pay-erne, Switzerland (Ohmura et al. 1998; not shown),which is the only BSRN station that is available for theperiod and region under consideration. A comparisonwith net radiation at three CARBOEUROPE stationsin France reveals large local differences. While thedaily net radiation during the first half of August 2003was anomalously high in Le Bray, it was around thelong-term average in Puechabon, France, and below theclimatological mean in Hesse (not shown).

Ferranti and Viterbo (2006) and Black et al. (2004)identified positive total net radiation anomalies in theECMWF operational analysis during August. They dis-cuss some uncertainties due to the representation oflow-level clouds (Ferranti and Viterbo 2006). Similarly,it is possible that our simulation underestimates theshortwave radiation at the surface due to the use of anold aerosol climatology that overestimates clear skyshortwave absorption (Hohenegger and Vidale 2005).

While the exact net radiation anomalies during earlyAugust involve uncertainties, it is interesting that in oursimulations the hottest phase during the first two weeksof August is well represented despite the absence of ananomaly in total net radiation (Fig. 9c). This counter-intuitive result indicates that the extreme August tem-peratures may be the result of the partitioning of thenet radiation in latent and sensible heat flux, and notdirectly inflicted by net radiation anomalies.

The latent heat flux (Fig. 9e) in late summer 2003 wasstrongly constrained by the lack of soil moisture. Inagreement with the analysis by Ferranti and Viterbo(2006), the CTL simulation shows latent heat flux val-ues around the long-term average until mid-June. How-ever, in the following months until October, the lack ofsoil moisture led to a dramatic reduction of latent heatflux. Averaged over JJA, the simulated latent fluxanomaly (CTL � CLIM) over FR amounts to 11 Wm�2. This anomaly is somewhat stronger than thelarger-scale European average anomaly derived fromthe ECMWF analysis (Black et al. 2004). However, theanomaly varies strongly in space and is highly depen-dent on the representation of the soil moisture content.Note that in the CHRM the vegetation cover is pre-scribed by a seasonally varying climatological mean leafarea index (LAI). Thus the vegetation cover is not di-rectly affected by the dry conditions. However, throughthe dependence of the stomatal resistance on the soilmoisture content and temperature, the transpirationrate may still be strongly sensitive to perturbations.


The simulated deficit of the latent heat flux in latesummer 2003 is compensated by increased sensible heatflux (Fig. 9f). The respective anomaly amounts to 17 Wm�2 in JJA (CTL � CLIM). This signal is found to bethe sum of two opposite effects during day and night(not shown). During daytime the upward sensible heatflux is strongly enhanced, whereas during nighttime thedownward directed sensible heat flux is enhanced.Analysis of the vertical temperature profile indicatesthat a nighttime temperature inversion develops in theboundary layer, resulting in downward sensible heatflux during the night and reduced upward sensible heatflux in the early morning. The simulated temperatureinversion develops due to nighttime radiative cooling.In our simulation this phenomenon may be overesti-mated during the first half of August, when it contrib-utes to unrealistically small daily mean sensible heatflux departures in contrast to previous studies by Blacket al. (2004) and Zaitchik et al. (2006).

2) EFFECT OF SOIL MOISTURE PERTURBATIONS ON

THE SURFACE ENERGY BALANCE

Here we evaluate the sensitivity of the differentfluxes to soil moisture perturbations by comparing theDRY and WET simulations (red to blue lines in Fig. 9).Shortwave net radiation in the DRY simulations isslightly increased (Fig. 9a) mainly through reduced liq-uid cloud water content. The shortwave response cor-responds to roughly half of the soil moisture effect onlongwave net radiation (Fig. 9b), which is directly re-lated to the surface temperature. Hence, the longwaveeffect dominates over the shortwave net radiation ef-fect, which means that total net radiation in summer isgenerally lower in the DRY than in the WET simula-tions. Longwave downward radiation on the other handseems to be indifferent to soil moisture perturbation(not shown). We expect this to be the consequence oftwo compensating effects: reduction due to lower atmo-spheric water vapor in the DRY simulations, which iscompensated for by the increase due to the higher tro-pospheric temperatures.

The soil moisture effect on the turbulent fluxes isvery pronounced. Both the latent and sensible heatfluxes are highly sensitive to the soil moisture condi-tions. A spring soil moisture reduction of 10%(DRY10) produces latent heat flux departures from theCTL simulation of 10 W m�2. Over FR, this roughlycorresponds to the anomaly CTL � CLIM. This indi-cates that a moderate additional spring soil moisturereduction may substantially enhance the latent heatanomaly in the subsequent summer. Note that theDRY10 simulation produced more realistic maximumtemperature anomalies than the CTL simulation (sec-

tion 3a). The reverse effect due to wet perturbationsmay produce latent heat flux values around the clima-tological mean despite the presence of strong anticy-clonic forcing. This strong sensitivity of latent heatfluxes to soil moisture is the main factor causing thestrong surface temperature differences between theperturbed simulations and substantially contributed tothe temperature extremes in late summer 2003.

5. Conclusions

Key processes and feedback mechanisms contribut-ing to the intensity and persistence of the 2003 Euro-pean summer heat wave have been analyzed by meansof regional climate simulations. Through careful vali-dation these simulations are found to provide a credibleand coherent picture of the 2003 summer heat wave.From our analysis, it is evident that both the anomalousatmospheric circulation during the summer itself as wellas the anomalously dry continental-scale soils haveplayed important roles.

The dry soil moisture conditions resulted from acombination of factors. Soil drying began early in theyear with a persistent precipitation deficit. Later, theloss of soil moisture accelerated in response to earlyvegetation activation in the months preceding the ex-treme summer event. Moreover, the persistent excessof shortwave and total net radiation due to exceptionalclear sky conditions further amplified the dryingthrough evapotranspiration. After the strong reduction,soil moisture remained at exceptionally dry conditionsthroughout summer and autumn 2003.

The prime objective of our study was to assess therole of spring soil moisture anomalies in the subsequentextreme summer. We find a string of evidence suggest-ing that the dry soil conditions and ensuing soil mois-ture dynamics were a key in the sequence of events thatled to the record-breaking summer of 2003.

• In our control integration, the hottest phase of thesummer in early August took place despite the ab-sence of an anomaly in total net radiation. Thus, thepartitioning of net radiation in latent and sensibleheat fluxes, which are to a large extent controlled bysoil moisture, has strongly contributed to the extremeAugust temperatures. The lack of soil moisture re-sulted in strongly reduced evapotranspiration and la-tent cooling, which was compensated for by enhancedsensible heat flux. This mechanism amplified the sur-face temperature anomalies, particularly during Au-gust 2003.

• Comparison against our 30-yr (reanalysis driven)model climatology demonstrates that the soil mois-


ture depletion over central Europe and France inearly spring 2003 exceeded the multiyear average byfar. Subsequently, soil moisture remained at ex-tremely low levels until the end of the summer.

• Sensitivity experiments using perturbed spring soilmoisture suggest that given climatological mean soilmoisture conditions in summer and similar continen-tal-scale circulation, the 2003 JJA surface tempera-ture anomalies would have been reduced by around40%. Thus in absence of soil moisture feedbacks,summer 2003 would still have been warm, but itwould not have been such a devastating event as itturned out to be.

• In the hypothetical case of dry conditions presentalready in winter 2003, central Europe might haveexperienced even substantially warmer conditions.Our analysis clearly shows that this effect occursthrough reduced latent cooling, which is compen-sated for by enhanced sensible heat flux.

It is important to stress that the spring soil moistureconditions and the summer atmospheric circulationmay largely be seen as independent forcing factors.Nevertheless, our results suggest that soil moisture per-turbations can affect continental-scale circulation andthat there is a positive feedback between the two. Theresponse of the atmospheric circulation to the (soilmoisture induced) low-level heat source is that of a heatlow at lower levels and an anticyclonic ridging in theupper troposphere. In summer 2003, this has likely am-plified the (preexisting) anticyclonic circulation, whichin turn should positively feed back on surface tempera-ture and soil moisture conditions. According to our sen-sitivity experiments, the strength of this feedback mayexplain up to about 10 m in 500-hPa geopotentialheight. However, the strength of this estimate may besomewhat constrained by the lateral boundaries.

It should not be overlooked that several of the pro-cesses that participate in the soil moisture feedbacksare of small scale and, thus, must be parameterized atthe employed numerical resolution. The parameteriza-tion of these processes must be considered uncertain.As a result, the strength of the simulated feedbackmechanisms may be model dependent and should beinterpreted with caution. Despite these uncertainties,the simulated seasonal and daily temperatures and ter-restrial water storage variation show notable agreementwith the validation datasets. This provides confidencethat the simulations represent a realistic and consistentpicture of the 2003 summer heat wave.

We have demonstrated that soil moisture maystrongly amplify European temperature anomalies inan extreme summer such as in 2003. This result raises a

series of questions. First, it would be interesting to ana-lyze the role of the discussed feedbacks in a more typi-cal summer, and more generally, the contribution ofsoil moisture dynamics to the seasonal cycle and theinterannual variability. Second, it would be desirable tostudy the soil moisture effect in other previous extremeevents, such as the 1976 heat wave over Great Britainand northern France. This may add to our understand-ing of the processes governing heat waves, which—inresponse to climate change—are expected to becomemore frequent, more intense, and longer lasting in thefuture.

Acknowledgments. We appreciated the valuablecomments of the three anonymous reviewers that con-tributed to improving the manuscript. We thank MartinHirschi for making available the BSWB data. Manythanks to Paul Berbigier and André Granier for pro-viding the measurements from the CARBOEUROPEstations in Le Bray and Hesse. We wish to thank alsoMartin Wild for providing the BSRN radiation mea-surements from Payerne. This research was supportedthrough the NCCR Climate of the Swiss National Sci-ence Foundation.

REFERENCES

Beljaars, A. C. M., and P. Viterbo, 1998: The role of the boundarylayer in a numerical weather prediction model. Clear andCloudy Boundary Layers, A. A. M. Holstlag and P. G.Duynkerke, Eds., Royal Netherlands Academy of Arts andSciences, 287–304.

Beniston, M., 2004: The 2003 heat wave in Europe: A shape ofthings to come? An analysis based on Swiss climatologicaldata and model simulations. Geophys. Res. Lett., 31, L02202,doi:10.1029/2003GL018857.

Black, E., and R. Sutton, 2006: The influence of oceanic condi-tions on the hot European summer of 2003. Climate Dyn., 28,53–66.

——, M. Blackburn, G. Harrison, and J. Methven, 2004: Factorscontributing to the summer 2003 European heat wave.Weather, 59, 217–223.

Cassou, C., L. Terray, and A. S. Phillips, 2005: Tropical Atlanticinfluence on European heat waves. J. Climate, 18, 2805–2811.

Christensen, J. H., and O. B. Christensen, 2003: Severe summer-time flooding in Europe. Nature, 421, 805–806.

Davies, H., 1976: A lateral boundary formulation for multi-levelprediction models. Quart. J. Roy. Meteor. Soc., 102, 405–418.

Della-Marta, P. M., J. Luterbacher, H. V. Weissenfluh, E.Xoplaki, M. Brunet, and H. Wanner, 2006: Summer heatwaves over western Europe 1880–2003, their relationship tolarge scale forcings and predictability. Climate Dyn., 29, 251–275.

Dickinson, R. E., 1984: Modeling evapo-transpiration for thethree dimensional global climate models. Climate Processesand Climate Sensitivity, Geophys. Monogr., Vol. 29, Amer.Geophys. Union, 58–72.

Ferranti, L., and P. Viterbo, 2006: The European summer of 2003:


Sensitivity to soil water initial conditions. J. Climate, 19,3659–3680.

Fink, A., T. Brücher, A. Krüger, G. Leckebusch, J. Pinto, and U.Ulbrich, 2004: The 2003 European summer heat waves anddrought–synoptic diagnosis and impacts. Weather, 59, 209–216.

Grazzini, F., and P. Viterbo, 2003: Record-breaking warm seasurface temperature of the Mediterranean Sea. ECMWFNewsletter, No. 98, ECMWF, Reading, United Kingdom, 30–31.

——, L. Ferranti, F. Lalaurette, and F. Vitart, 2003: The excep-tional warm anomalies of summer 2003. ECMWF Newsletter,No. 99, ECMWF, Reading, United Kingdom, 2–8.

Hansen, J., R. Ruedy, J. Glascoe, and M. Sato, 1999: GISS analy-sis of surface temperature change. J. Geophys. Res., 104,30 997–31 022.

——, ——, M. Sato, M. Imhoff, W. Lawrence, D. Easterling, T.Peterson, and T. Karl, 2001: A closer look at United Statesand global surface temperature change. J. Geophys. Res., 106,23 947–23 963.

Hartmann, D. L., 1994: Global Physical Climatology. AcademicPress, 411 pp.

Heck, P., D. Lüthi, H. Wernli, and C. Schär, 2001: Climate impactsof European-scale anthropogenic vegetation changes: Astudy with a regional climate model. J. Geophys. Res., 106,7817–7835.

——, A. Zanetti, R. Enz, J. Green, and S. Suter, 2004: Naturalcatastrophes and man-made disasters in 2003. Sigma Rep.1/2004, Swiss Re Tech. Rep., Zurich, Switzerland, 10 pp.

Hémon, D., and E. Jougla, 2004: Surmortalité liée à la canicule daoût 2003. L’Institut National de la Santé et de la rechercheMédicale (INSERM), Paris, France, 76 pp.

Hirschi, M., S. Seneviratne, and C. Schär, 2006a: Seasonal varia-tions in terrestrial water storage for major midlatitude riverbasins. J. Hydrometeor., 7, 39–60.

——, P. Viterbo, and S. I. Seneviratne, 2006b: Basin-scale water-balance estimates of terrestrial water storage variations fromECMWF operational forecast analysis. Geophys. Res. Lett.,33, L21401, doi:10.1029/2006GL027659.

——, S. Seneviratne, S. Hagemann, and C. Schär, 2007: Analysisof seasonal terrestrial water storage variations in regionalclimate simulations over Europe. J. Geophys. Res., in press.

Hohenegger, C., and P. L. Vidale, 2005: Sensitivity of the Euro-pean climate to aerosol forcing as simulated with a regionalclimate model. J. Geophys. Res., 110, D06201, doi:10.1029/2004JD005335.

Jacobsen, I., and E. Heise, 1982: A new economic method for thecomputation of the surface temperature in numerical models.Contrib. Atmos. Phys., 55, 128–141.

Jung, T., L. Ferranti, and A. M. Tompkins, 2006: Response to thesummer of 2003 Mediterranean SST anomalies over Europeand Africa. J. Climate, 19, 5439–5454.

Kessler, E., 1969: On the Distribution and Continuity of WaterSubstance in Atmospheric Circulation. Meteor. Monogr., No.32, Amer. Meteor. Soc., 84 pp.

Klein Tank, A., G. Konnen, and F. Selten, 2005: Signals of an-thropogenic influence on European warming as seen in thetrend patterns of daily temperature variance. Int. J. Climatol.,25, 1–16.

Koppe, C., and G. Jendritzky, 2004: Die Auswirkungen der Hitze-wellen 2003 auf die Mortalitat in Baden-Wurttemberg. Ge-sundheitliche Auswirkungen der Hitzewelle im August 2003,

Sozialministerium Baden-Württemberg, Stuttgart, Germany,5–18.

Koster, R. D., and M. J. Suarez, 2001: Soil moisture memory inclimate models. J. Hydrometeor., 2, 558–570.

Kunkel, K. E., S. S. Changnon, B. Reike, and R. W. Arritt, 1996:The July 1995 heat wave in the Midwest: A climatic perspec-tive and critical weather factors. Bull. Amer. Meteor. Soc., 77,1507–1518.

Lakshmi, V., T. Piechota, U. Narayan, and C. Tang, 2004: Soilmoisture as an indicator of weather extremes. Geophys. Res.Lett., 31, L11401, doi:10.1029/2004GL019930.

Larsen, J., 2003: Record heat wave in Europe takes 35,000 lives.Earth Policy Institute, 1 pp.

Lin, Y.-L., R. D. Farley, and H. D. Orville, 1983: Bulk parameter-ization of the snow field in a cloud model. J. Climate Appl.Meteor., 22, 1065–1092.

Luterbacher, J., D. Dietrich, E. Xoplaki, M. Grosjean, and H.Wanner, 2004: European seasonal and annual temperaturevariability, trends, and extremes since 1500. Science, 303,1499–1503.

Lüthi, D., A. Cress, H. Davies, C. Frei, and C. Schär, 1996: Inter-annual variability and regional climate simulations. Theor.Appl. Climatol., 53, 185–209.

Majewski, D., 1991: The Europa-Modell of the Deutscher Wet-terdienst. Proc. Seminar on Numerical Methods in Atmo-spheric Models, Vol. 2, Reading, United Kingdom, ECMWF,147–191.

——, and R. Schrodin, 1994: Short description of the Europa-Modell (EM) and Deutschland-Modell (DM) of the Deut-scher Wetterdienst (DWD). DWD Quarterly Bull., 78 pp.

Meehl, G. A., and C. Tebaldi, 2004: More intense, more frequent,and longer lasting heat waves in the 21st century. Science,305, 994–997.

Mellor, G. L., and T. Yamada, 1974: A hierarchy of turbulentclosure models for planetary boundary layers. J. Atmos. Sci.,31, 1791–1806.

Ogi, M., K. Yamazaki, and Y. Tachibana, 2005: The summernorthern annular mode and abnormal summer weather in2003. Geophys. Res. Lett., 32, L04706, doi:10.1029/2004GL021528.

Oglesby, R. J., and D. J. Erickson, 1989: Soil moisture and thepersistence of North American drought as simulated by theNCAR community climate model 1. J. Climate, 2, 1362–1380.

Ohmura, A., and Coauthors, 1998: Baseline Surface RadiationNetwork (BSRN/WRMC): New precision radiometry for cli-mate research. Bull. Amer. Meteor. Soc., 79, 2115–2136.

Ordonez, C., H. Mathis, M. Furger, S. Henne, C. Huglin, J. Stae-helin, and A. S. H. Prevot, 2005: Changes of daily surfaceozone maxima in Switzerland in all seasons from 1992 to 2002and discussion of summer 2003. Atmos. Chem. Phys., 5, 1187–1203.

Orsolini, Y., and G. Nikulin, 2006: A low-ozone episode duringthe European heat wave of August 2003. Quart. J. Roy. Me-teor. Soc., 132, 667–680.

Pal, J. S., and E. A. B. Eltahir, 2003: A feedback mechanism be-tween soil moisture distribution and storm tracks. Quart. J.Roy. Meteor. Soc., 129, 2279–2297.

Palecki, M. A., S. A. Changnon, and K. E. Kunkel, 2001: The na-ture and impacts of the July 1999 heat wave in the midwest-ern United States: Learning from the lessons of 1995. Bull.Amer. Meteor. Soc., 82, 1353–1367.

Ritter, B., and J. Geleyn, 1992: A comprehensive radiationscheme for numerical weather prediction models with poten-


tial applications in climate simulations. Mon. Wea. Rev., 120,303–325.

Rowell, D. P., 2005: A scenario of European climate change forthe late twenty-first century: Seasonal means and interannualvariability. Climate Dyn., 25, 837–849.

——, and R. G. Jones, 2006: Causes and uncertainty of futuresummer drying over Europe. Climate Dyn., 27, 281–299.

Rowntree, P. R., and J. A. Bolton, 1983: Simulations of the atmo-spheric response to soil moisture anomalies over Europe.Quart. J. Roy. Meteor. Soc., 109, 501–526.

Rudolf, B., H. Hauschild, W. Rueth, and U. Schneider, 1994: Ter-restrial precipitation analysis: Operational method and re-quired density of point measurements. Global Precipitationsand Climate Change, M. Desbois and F. Désalmand, Eds.,NATO ASI Series I, Vol. 26, Springer-Verlag, 173–186.

——, T. Fuchs, U. Schneider, and A. Meyer-Christoffer, cited2003: Introduction of the Global Precipitation ClimatologyCentre (GPCC). Deutscher Wetterdienst Publication. [Avail-able online at http:/www.dwd.de/en/FundE/Klima/KLIS/int/GPCC/Reports_Publications/QR/GPCC_Introduction.pdf.]

——, C. Beck, J. Grieser, and U. Schneider, 2005: Global precipi-tation analysis products. Global Precipitation ClimatologyCentre (GPCC), DWD Publication, 1–8. [Available online athttp://www.dwd.de/en/FundE/Klima/KLIS/int/GPCC/Reports_Publications/QR/GPCC-intro-products-2005.pdf.]

Schär, C., and G. Jendritzky, 2004: Hot news from summer 2003.Nature, 432, 559–560.

——, D. Lüthi, U. Beyerle, and E. Heise, 1999: The soil-precipitation feedback: A process study with a regional cli-mate model. J. Climate, 12, 722–741.

——, P. L. Vidale, D. Lüthi, C. Frei, C. Häberli, M. A. Liniger,and C. Appenzeller, 2004: The role of increasing temperaturevariability in European summer heat waves. Nature, 427,332–336.

Schönwiese, C., T. Staeger, and S. Trömel, 2004: The hot summer2003 in Germany. Some preliminary results of a statisticaltime series analysis. Meteor. Z., 13, 323–327.

Seneviratne, S. I., P. Viterbo, D. Lüthi, and C. Schär, 2004: Infer-ring changes in terrestrial water storage using ERA-40 re-analysis data: The Mississippi river basin. J. Climate, 17,2039–2057.

——, and Coauthors, 2006a: Soil moisture memory in AGCMsimulations: Analysis of Global Land–Atmosphere CouplingExperiment (GLACE) data. J. Hydrometeor., 7, 1090–1112.

——, D. Lüthi, M. Litschi, and C. Schär, 2006b: Land-atmospherecoupling and climate change in Europe. Nature, 443, 205–209.

Solberg, S., R. G. Derwent, O. Hov, J. Langner, and A. Lindskog,2005: European abatement of surface ozone in a global per-spective. Ambio, 34, 47–53.

Stott, P. A., D. A. Stone, and M. R. Allen, 2004: Human contri-bution to the European heat wave of 2003. Nature, 432, 610–614.

Sutton, R. T., and D. L. R. Hodson, 2005: Atlantic Ocean forcingof North American and European summer climate. Science,309, 115–118.

Tiedtke, M., 1989: A comprehensive mass flux scheme for cumu-lus parameterization in large-scale models. Mon. Wea. Rev.,117, 1779–1800.

Vandentorren, S., F. Suzan, S. Medina, M. Pascal, A. Maulpoix,J. C. Cohen, and M. Ledrans, 2004: Mortality in 13 Frenchcities during the August 2003 heat wave. Amer. J. PublicHealth, 94, 1518–1520.

Vautard, R., and Coauthors, 2007: Summertime European heatand drought waves induced by wintertime Mediterraneanrainfall deficit. Geophys. Res. Lett., 34, L07711, doi:10.1029/2006GL028001.

Vidale, P. L., D. Lüthi, C. Frei, S. I. Seneviratne, and C. Schär,2003: Predictability and uncertainty in a regional climatemodel. J. Geophys. Res. , 108, 4586, doi:10.1029/2002JD002810.

——, ——, R. Wegmann, and C. Schär, 2007: European summerclimate variability in a heterogeneous multi-model ensemble.Climatic Change, 81, 209–232.

Xoplaki, E., J. F. González-Rouco, J. Luterbacher, and H. Wan-ner, 2003: Mediterranean summer air temperature variabilityand its connection to the large-scale atmospheric circulationand SSTs. Climate Dyn., 20, 723–739.

Xue, Y., 1996: The impact of desertification in the Mongolian andthe Inner Mongolian grassland on the regional climate. J.Climate, 9, 2173–2189.

Zaitchik, B., A. K. Macalady, L. R. Bonneau, and R. B. Smith,2006: Europe’s 2003 heat wave: A satellite view of impactsand land-atmosphere feedbacks. Int. J. Climatol., 26, 743–769.

Zemp, M., R. Frauenfelder, W. Haeberli, and M. Hoelzle, 2005:Worldwide glacier mass balance measurements: Generaltrends and first results of the extraordinary year 2003 in Cen-tral Europe. Data Glaciol. Stud., 99, 3–12.


Documents

Soil Moisture–Atmosphere Interactions during the 2003 European Summer Heat …vidale/papers/Fischer_heat_waves... · 2009-02-20 · the term “heat wave” is often defined as