Assessment of the dehydration-greenhouse feedback over the Arctic during February 1990

INTERNATIONAL JOURNAL OF CLIMATOLOGYInt. J. Climatol. 27: 1047–1058 (2007)Published online 29 December 2006 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/joc.1455

Assessment of the dehydration-greenhouse feedback over theArctic during February 1990

Eric Girard* and Alexandru StefanofDepartment of Earth and Atmospheric Sciences, University of Quebec at Montreal, Montreal (Quebec), Canada

Abstract:

The effect of pollution-derived sulfuric acid aerosols on the aerosol-cloud-radiation interactions is investigated over theArctic for February 1990. Observations suggest that acidic aerosols can decrease the heterogeneous nucleation rate ofice crystals and lower the homogeneous freezing temperature of haze droplets. On the basis of these observations,we hypothesize that the cloud thermodynamic phase is modified in polluted air mass (Arctic haze). Cloud ice numberconcentration is reduced, thus promoting further ice crystal growth by the Bergeron–Findeisen process. Hence, ice crystalsreach larger sizes and low-levkel ice crystal precipitation from mixed-phase clouds increases. Enhanced dehydration of thelower troposphere contributes to decreased water vapour greenhouse effect and cools the surface. A positive feedback iscreated between surface cooling and air dehydration, accelerating cold air production. This process is referred to as thedehydration-greenhouse feedback (DGF).

Simulations performed using an Arctic regional climate model for February 1990 are used to assess the potential effect ofthe DGF on the Arctic climate during February. Results show that the DGF has an important effect on cloud, atmosphericdehydration, and temperature over the Central and Eurasian Arctic, which is the coldest part of the Arctic. Cloud ice issignificantly reduced and the total atmospheric water path is decreased by as much as 12%. This results in a surface coolingranging between 0 and −3 K. Moreover, the lower tropospheric cooling over the Eurasian and Central Arctic strengthensthe atmospheric circulation at the upper level, thus increasing the aerosol transport from the mid-latitudes and enhancingthe DGF. Over warmer areas, the increased aerosol concentration (caused by the DGF) leads to longer cloud lifetime,which contributes to warm these areas. Copyright 2006 Royal Meteorological Society

KEY WORDS Arctic climate; Arctic haze; aerosols; clouds; ice nuclei

Received 24 July 2006; Revised 26 September 2006; Accepted 7 October 2006

INTRODUCTION

According to the IPCC (2001), the Arctic and Antarc-tic are likely to experience the largest warming on Earthowing to increasing greenhouse effect produced by man-made release of radioactive gases. Observations coveringthe second half of the last century indicate a warmingtrend over the Arctic during spring, summer and autumn(Rigor et al., 2000). However, during winter, althoughthe mean temperature has increased over the sub-Arcticlatitudes, the Central Arctic has experienced no statisti-cally significant temperature trend during the second halfof the twentieth century (Rigor et al., 2000; Kahl et al.,1993). This is not in agreement with global climate mod-els (GCM), which predict a warming trend over the Arcticduring winter.

Blanchet and Girard (1994) have formulated a processby which the Arctic might cool during winter becauseof the acidification of aerosols. This process is not

* Correspondence to: Eric Girard, Department of Earth and Atmo-spheric Sciences, University of Quebec at Montreal, PO Box 8888,Station “Downtown”, Montreal, Quebec, H3C 3P8, Canada.E-mail: [email protected]

accounted for in climate models and could explain inpart the absence of statistically significant surface tem-perature trends over the Central Arctic during winter.The process, namely, the dehydration-greenhouse feed-back (DGF), may be summarized as follows: Anthro-pogenic and biogenic activities produce sulfuric acid, anorthward extension of the mid-latitude acid precipitationproblem, that coats most of existing aerosols (e.g. Bigg,1980). During late winter and spring, large concentrationsof anthropogenic aerosols are transported to the Arcticfrom the mid-latitudes (Barrie et al., 1989; Shaw, 1995).This phenomenon is referred to as Arctic haze. This isa well-known phenomenon, which has been observed inmany field studies over the last 30 years (Schnell, 1984;Yli-Tuomi et al., 2003). Arctic haze aerosols are mostlyconfined to the accumulation mode and are composed byan internal mixing of soot, sulfate, and organics (Bar-rie, 1986; Barrie and Barrie, 1990; Yamanouchi et al.,2005). Most of the aerosols in the accumulation modeare covered by a thin film of sulfuric acid (Bigg, 1980).The increased concentration of sulfuric acid decreasesthe saturation vapour pressure above aerosol and lowersthe homogeneous freezing point (e.g. Cziczo and Abbatt,

Copyright 2006 Royal Meteorological Society

1048 E. GIRARD AND A. STEFANOF

1999). Moreover, observations taken during field exper-iments suggest that high concentration of acidic aerosolsmay decrease significantly the heterogeneous ice nucle-ation rate by reducing the ice nuclei (IN) concentration(Radke et al., 1976; Borys, 1983,1989; Bigg, 1996). Yet,some aerosols present in Arctic haze, like soot and min-eral dust, are known to be good IN (Pruppacher and Klett,1997; Gorbunov et al., 2001). Borys (1989) has hypoth-esized that the IN decrease in Arctic haze may be causedby sulfuric acid coating on existing IN, thus hamperingthe ice nucleation by contact, condensation–freezing, anddeposition modes. More recently, Archuleta et al. (2005)have shown that the decrease of ice nucleation at tem-peratures below −40 °C by immersion and condensationmodes because of sulfuric acid coating is variable anddepends on the IN chemical composition. Other labo-ratory experiments performed at temperatures rangingbetween −10 and −40 °C also show that the heteroge-neous freezing temperature initiated by the immersion ofvarious mineral dust particles decreases as the percentageby weight of sulfuric acid in the particle increases (Ettneret al., 2004). Combined with Borys field measurements,which were obtained at −15 °C and −25 °C, these resultssuggest that sulfuric acid coating on aerosols could havea significant effect on ice nucleation over a wide rangeof temperatures. However, further research is needed tofully explain and quantify this phenomenon.

According to the DGF hypothesis, at cold tempera-tures, sulfuric acid coating on aerosols inhibits ice crys-tal production and favours the formation of fewer andlarger ice crystals instead of many smaller ones. In effect,it enhances the formation of low-level precipitating icecrystals at the expense of more persistent stratus orice fog. Single-column model simulations (Girard andBlanchet, 2001a,b; Girard et al., 2005) show an increaseof the water flux from the lower atmosphere to the sur-face. The dehydration of the lower atmosphere decreasesthe greenhouse effect due to the strong effect of watervapour, primarily in the broad rotation band. The reducedgreenhouse effect further promotes dehydration and thecooling cycle (Blanchet and Girard, 1995; Curry et al.,1995). The process also affects the mid-latitude regionsby increasing the production of available potential energythat feeds the intensity of our winter storms. A feed-back is established between the enhanced circulation andthe transport of anthropogenic aerosols from the mid-latitudes.

Girard et al. (2005) have used a single-column modelto simulate four cold seasons (from November to May)at Alert, Canada. They have compared two aerosolscenarios: a baseline case in which aerosols are assumedto be of natural origin and an acidic aerosol scenario inwhich the IN concentration is reduced accordingly to theobserved sulfuric acid concentration. The homogeneousfreezing temperature of haze particles is also reduced as afunction of sulfuric acid concentration. Results show thatthe alteration of ice nucleation by acidic aerosols leads toan averaged cooling of 2 °C near the surface. An analysisof aerosol and weather observations, which were taken

during the same years at Alert, have shown that acidicaerosols favour low-level ice crystals precipitating to thesurface, a common phenomenon in the Arctic duringwinter (Curry et al., 1990). However, the time and spatialresolution of the aerosol dataset (aerosols were measuredonly once a week near the surface) were not high enoughto unravel this potential link between acidic aerosols andprecipitating ice crystals.

A natural extension to the investigation of Girard et al.(2005) consists in assessing the magnitude of the DGFover the whole Arctic basin and the impact of the Arcticcooling on the large-scale atmospheric circulation. Thisresearch seeks to better evaluate the potential effect ofthe DGF and give insights into areas in which this effectcould possibly be observed in future field experiments.In this paper, the DGF is investigated with a 3D regionalclimate model, which simulates the aerosol transportover a pan-Arctic domain. In the next section, a briefdescription of the model is made. The experimentaldesign and results are presented in Sections ‘Design ofthe Experiment’ and ‘Results’.

MODEL DESCRIPTION

The Northern Aerosol Regional Climate Model(NARCM) is a limited-area non-hydrostatic climatemodel with a dynamic size-distributed aerosol scheme,the Canadian Aerosol Module (CAM). The numericalformulation of NARCM is derived from the CanadianRegional Climate Model (CRCM) (Caya and Laprise,1999). It is based on the fully elastic, non-hydrostaticEuler field equations solved with a state-of-the-artsemi-implicit and semi-Lagrangian (SISL) marchingalgorithms adequate for computing atmospheric flowat all spatial scales. CRCM atmospheric variables arediscretized on an Arakawa C-type staggered grid on apolar-stereographic projection in the horizontal and Gal-Chen terrain-following scaled height coordinate in thevertical (Gal-Chen and Somerville, 1975).

NARCM simulates aerosol explicitly with 12 size binsfrom 0.005 to 20.48 µm (Gong et al., 2003). The aerosolmodule CAM is a size-segregated multi-component algo-rithm that treats five major types of aerosols: sea-salt,sulfate, black carbon, organic carbon and soil dust. Itincludes major aerosol processes in the atmosphere: pro-duction, growth, coagulation, nucleation, condensation,dry deposition, below-cloud scavenging, activation, acloud module with explicit microphysical processes totreat aerosol-cloud interactions, and chemical transfor-mation of sulfur species in clear air and in clouds. Eachof these aerosol processes is strongly dependent on par-ticle size, thus requiring an explicit representation ofthe size distribution. Although still demanding in com-puter resources, the numerical solution is optimized toefficiently solve the complicated size-segregated multi-component aerosol system and make it feasible to beincluded in global and regional models. Application ofCAM in the Canadian GCM has resulted in a very rea-sonable global sea-salt and sulfate aerosol distribution

Copyright 2006 Royal Meteorological Society Int. J. Climatol. 27: 1047–1058 (2007)DOI: 10.1002/joc

ASSESSMENT OF THE DEHYDRATION-GREENHOUSE FEEDBACK 1049

compared with observations. Previous experiments haveshown that CAM is able to predict relatively well aerosolsize and concentration in marine and continental area(Covert et al., 1996; Quinn et al., 1996; Gong et al.,1997a,b). With proper anthropogenic emissions surround-ing the Arctic, CAM is able to predict the spatial andtemporal aerosol compositions and size distributions inthe Arctic atmosphere.

Surface processes are simulated with the CanadianLand Surface Scheme (CLASS). The microphysicsscheme is from Lohmann and Roeckner (1996). Icenucleation by contact and immersion freezing modesare parameterized following Levkov et al. (1992). Watervapour deposition is permitted only on pre-existing icecrystals, that is ice nucleation by deposition is notincluded. The heterogeneous ice nucleation has beenslightly modified for our experiment as explained inthe next section. The other physical parameterizationsare from the Canadian GCM (McFarlane et al., 1992).NARCM can be implemented at a range of grid res-olutions (generally from 10 to 100 km) and a varietyof domains. The model has 22 Gal-Chen vertical levelswith high vertical resolution (50 m) in the lower tro-posphere. The vertical resolution gradually decreases to1750 m at 500 hPa and remains constant from this pointto 34 120 m.

DESIGN OF THE EXPERIMENT

The month of February 1990 is simulated. The choice ofthe month of February is motivated by two factors thatfavour the DGF: cold temperature and high concentrationof acidic aerosols (Sirois and Barrie, 1999). 1D simula-tions have also shown that the DGF is the strongest inFebruary (Girard et al., 2005). Two aerosol scenarios areconsidered. In the first scenario (hereafter scenario A),it is assumed that sulfuric acid aerosols have no effectwhatsoever on the ice nucleation process. In the secondaerosol scenario (hereafter scenario B), the IN concen-tration is decreased when sulfate aerosol concentrationreaches values above the monthly mean concentration ofsulfate over the domain. The decrease of IN concentra-tion is based on Borys (1989) observation and is given byan exponential function, which depends on sulfate con-centration following Girard et al. (2005) as follows:

Fr = 10−[B×m(SO4)] (1)

where Fr is the IN reduction factor, m(SO4) is the sulfateconcentration and B is a constant chosen so that Fr =0.001 when the maximum sulfate concentration duringthe simulation is reached. In scenario B, Fr is multipliedby the IN concentration. Both immersion and contactnuclei are reduced in the experiment. Although theexponential dependence of Fr on sulfate concentrationis subjective (as explained in Girard et al. (2005)), thisfunction has the advantage of substantially decreasingIN concentration only when Arctic haze occurs. At

low aerosol concentrations (non-haze), small variationsof sulfate concentration have little impact on the INwhen using the equation above. For most common hazeevents, sulfate concentrations range from 2 to 4 µg m−3

(moderate haze events). This leads to a decrease of IN bya factor of 4 to 15, respectively. In cases of major Arctichaze events (10 µg m−3), the IN concentration decreasedby three orders of magnitude, which agrees with Borysobservations.

To investigate a climate signal such as the DGF, onemust account for the large internal variability characteriz-ing the Arctic climate, particularly during winter (Rinkeet al., 2004; Girard and Bekcic, 2005). Hence, a largenumber of simulations are often necessary to distinguishthe investigated climate signal from the model internalvariability. Each simulation within an ensemble is ini-tialized with different initial conditions following Rinkeand Dethloff (2000). In our experiments, 12 simulationsof each aerosol scenarios were necessary to get a statis-tically significant area reasonably large over the Arctic.Results presented in the next section always representthe ensemble mean of either aerosol scenario A or B andshadowed areas indicate that results are statistically sig-nificant with a confidence level of 95%. The statisticaltest is described in Girard and Bekcic (2005).

The domain size is 6800 km by 6800 km and includesmost of Europe, Northern Canada, Siberia, and the NorthAtlantic Ocean. The domain is centred at 360 °E and79 °N. The slight eastward location of the domain is acompromise between the need to include source area ofaerosols and the computational cost of the model. Resultsare analysed on a sub-domain of 3900 km by 3900 kmto eliminate the influence of the sponge of the domainboundaries.

Initial and boundary conditions for atmospheric fieldsare provided by the National Centre for EnvironmentalPrediction (NCEP) analyses on a 2.5° by 2.5° longi-tude/latitude grid. Monthly mean sea-surface tempera-ture and sea-ice concentration are from the AtmosphericModel Intercomparison Project (AMIPII) reanalysis on a1° by 1° grid. These values are interpolated on the grid(100 km horizontal resolution) used for simulations.

In this experiment, sulfate and sea-salt aerosols areconsidered. Emission of sea-salt aerosols is calculatedfollowing a function developed by Gong et al. (1997a).Natural sources of sulfate (DMS and H2S) are providedby dataset of Bates et al. (1992) for DMS and by Kettleet al. (1999) for H2S. Anthropogenic emission of sulfurdioxide and sulfate (SO2 and SO4

−2) is from the GlobalEmission Inventory Activity (GEIA) 1985 (Benkovitzet al., 1996). Seasonal emission rate on two levels isconsidered in the dataset. Initially, the aerosol concentra-tion is set to 0 everywhere in the domain. At the lateralboundaries, the aerosol concentration is maintained to 0during the whole simulation. The model generates its ownaerosol and simulates the transport, source and sink ofaerosols. The aerosol spatial distribution and concentra-tion reach background observed value within 1 month,which is the spin-up time in our experiment.



RESULTS

Figure 1 shows the differences between the simulated(natural aerosol scenario) and analysed mean sea-levelpressure and geopotential height at 500 hPa for the monthof February 1990. The model reproduces reasonably wellthe main features of the MSLP characterized by a lowpressure over Iceland and an anticyclone over Siberia.Their location in the simulations differs slightly from theanalyses leading to localized differences up to 10 hPa.The largest MSLP difference occurs over the easternBarents Sea and western Kara Sea where the pressuretrough is too deep in the simulations when comparedto NCEP analysis. The geopotential height at 500 hPais also qualitatively well reproduced in both scenarios.However, the simulated low at 500 hPa is too deepby 120 m over the Greenland Sea and 100 m over theBarents Sea when compared to the observation analysis.

Differences between the observation analysis and sim-ulations for these two variables are localized over areaswhere strong variations of sea-ice cover were observedduring February 1990. Parkinson et al. (2001) haveshown that sea-ice variation can lead to important errorsin the simulated surface air temperature. According tosatellite observations (EOS-AMSR-E), sea-ice cover hasoscillated by up to 20% around its monthly mean valueduring February 1990 over the Barents and Kara Seas.This large variation has led to an overall underestimationof ice cover by the model, which considers a monthlymean sea-ice concentration value. As a result, large sim-ulated values of sensible heat flux at the surface warmthe lower atmosphere (not shown) and enhance surfacecyclogenesis, thus contributing to the simulated deepertrough over the Kara Sea when compared to observa-tion analysis. The presence of this trough strengthens thenorthern component of the winds near the surface and

in the free atmosphere over the Barents Sea, thus sig-nificantly cooling this area (not shown) and lowering thegeopotential heights at 500 hPa. Although differences canbe locally large, they have little impact on large-scale cir-culation and aerosol transport. The model simulation isacceptable in the context of this sensitivity study.

Wintertime large-scale atmospheric circulation pro-motes the transport of anthropogenic aerosols from pol-luted areas north of the polar front to the Arctic. Thepresence of the Icelandic low and the Siberian high isparticularly favourable to the transport of pollution emit-ted over Eurasia. Observations typically show a ridge ofanthropogenic aerosols stretching from polluted areas ofNorthern Europe and Western Russia to the Central Arc-tic and Northern Canada (Shaw, 1995). Figure 2 showsthat both aerosol scenarios reproduce this pattern for sul-fate aerosols. The sulfate vertical path is much largerin scenario B (acid aerosol scenario) when comparedto scenario A (natural aerosol scenario) with a spatiallyaveraged relative difference of 44%. Differences are thelargest over areas where sulfate concentration is high, thatis over Northern Russia, and Central and Canadian Arc-tic. We believe that this major difference between bothscenarios originates from the complex positive feedback,which is established between the aerosol-cloud-radiationinteractions and atmospheric circulation. This feedbackmechanism will be explained at the end of the section.

Figure 3 shows the February mean liquid and ice waterpath differences between aerosol scenarios B and A. Asexpected, in aerosol scenario B, the ice water pathdecreases the most where sulfate concentration is large,that is over Northern Russia, Northern Europe, andCentral Arctic. The decrease of the IN concentrationis larger over polluted areas of Northern Europe andleads to the formation of clouds containing more liquid

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Figure 2. February mean sulfate vertical path (mg m−2) for (a) aerosol scenario A, (b) aerosol scenario B, and (c) scenario B minus scenario A.Areas where the differences are statistically significant (95% confidence level) are shadowed

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Figure 3. February mean difference between aerosol scenarios B and A for (a) liquid water path (g m−2) and (b) ice water path (g m−2). Areaswhere the differences are statistically significant (95% confidence level) are shadowed



droplets and less ice crystals. Over the coldest part ofthe domain, which is over the Eurasian and Central Arc-tic, the decrease of ice water path is not compensatedby an increase of liquid water path. This is because ofthe exponential variation of IN concentration with tem-perature. Indeed, over colder regions, IN concentrationis much higher when compared to warmer regions. Asa result, the IN decrease caused by sulfuric acid overwarmer areas leads to IN concentration approaching 0 l−1

while IN concentration is still well above 0 l−1 under thesame IN reduction over cold areas. Therefore, over warmareas, heterogeneous ice nucleation is almost reduced to 0and only cloud droplet can nucleate. Under these circum-stances, the ice crystal concentration is not large enoughto absorb excess water vapour and cloud saturation withrespect to water approaches 1. As a result, cloud coverincreases in scenario B over warmer regions as seen onFigure 4. Over colder areas, ice nucleation is reduced butstill effective. This process contributes to enhance theBergeron–Findeisen process (less ice crystals are avail-able to absorb the excess water vapour produced by theevaporating droplets) and promote ice crystal growth andprecipitation. As a result, cloud cover decreases in aerosolscenario B (see Figure 4).

Figure 5 shows the monthly mean precipitation ratedifference between both aerosol scenarios. Because ofa more efficient Bergeron–Findeisen process, colderregions (that is the Eurasian and Central Arctic) are char-acterized by an enhanced precipitation rate in aerosolscenario B when compared to aerosol scenario A. Onthe other hand, precipitation is reduced over the warmerregions of Northern Europe and the North Atlantic due

to enhanced cloud liquid water and reduced cloud ice.Again, the pattern of enhanced precipitation over coldregions is well correlated with high sulfate concentra-tion. According to these results, it seems that the DGFhas little effect when temperature gets warmer. This wasexpected since IN concentration is very low at tempera-tures greater than −15 °C. As a result, the IN reductionby aerosol acidification has limited effects. However, asshown in Figure 1, the sulfate concentration over North-ern Russia and Europe is increased in aerosol scenario Bbecause of a complex feedback between the Arctic tropo-spheric cooling due to DGF and mid-latitude circulation(discussed later). In turn, the increased concentration ofsulfate over warmer areas results in clouds with smallercloud droplets in larger concentration, thus reducing pre-cipitation. This effect is referred to as the second indirectradiative effect (SIRE) of aerosols by the IPCC (2001).However, in this case, the SIRE is not initiated only byan increase of aerosol concentration but also by the INreduction, which contributes to maintain clouds in liquidphase and eliminate the loss of water droplets by hetero-geneous freezing.

To quantify the atmospheric dehydration, we intro-duce the dehydration efficiency (ED), which is definedas the ratio between daily averaged precipitation amountand total atmospheric water (including water vapour,cloud ice and cloud water). Figure 6 shows the dehydra-tion efficiency difference between both aerosol scenar-ios for February. Clearly, the dehydration of the atmo-sphere is much larger for aerosol scenario B over thecold regions of the Arctic, whereas the opposite is truefor the warmer regions of Northern Europe and the

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Figure 4. February mean difference between aerosol scenarios B and A for cloud cover. Areas where the differences are statistically significant(95% confidence level) are shadowed



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Figure 6. February mean difference between aerosol scenarios B and A for the dehydration efficiency (ED) (%). Areas where the differences arestatistically significant (95% confidence level) are shadowed

North Atlantic. ED differences reach a maximum of 0.12over areas characterized with large sulfate concentra-tions (see Figure 2). Over warmer regions of NorthernEurasia, negative values of the same order are obtained.

Over cold regions, the DGF dominates and dehydratesthe lower atmosphere while over warmer regions, theSIRE dominates and decreases the dehydration of theatmosphere resulting in lower values of ED.



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Figure 7. Dehydration efficiency difference (grey) and sulfate vertical path difference (black) between aerosol scenarios B and A as a functionof surface air temperature. Each point represents a daily averaged value for one grid point. Grey and black lines are regression fits to the results

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Figure 8. February mean difference between aerosol scenarios B and A for downwelling infrared radiation flux at the surface (W m−2). Areaswhere the differences are statistically significant (95% confidence level) are shadowed

Figure 7 shows the daily ED anomaly (an anomalyis defined as the difference between aerosol scenariosB and A) and the sulfate anomaly as a function ofsurface air temperature for each grid point. Surface airtemperature is assumed here to be representative ofcloud base temperature since clouds over sea ice in thesimulations are confined within the first 1000 m abovethe surface most of the time. The ED anomaly is positivebetween 240 K to about 257 K and becomes negativefor higher temperatures. The sulfate anomaly is positivefor most temperatures up to 270 K. According to these

results, the DGF dominates over the SIRE when thetemperature is below the threshold temperature of 257 K.At warmer temperatures, even though the sulfate anomalyis positive, the SIRE dominates over DGF with negativeED anomalies.

Figure 8 shows the downwelling infrared radiation fluxanomaly at the surface. Over most of the Arctic, thedownwelling infrared radiation anomalies are negativereaching values as high as −12 W m−2. This negativeanomaly is associated with the ED positive anomalyobtained over the same area. The atmosphere contains



less total water in aerosol scenario B and, consequently,it emits less infrared radiation towards the surface. Asexpected, the surface air temperature anomaly is negativewith values up to −3 K (see Figure 9(a)). Over NorthernEurasia and the North Atlantic, the downwelling infraredradiation and temperature anomalies are both positivebecause of increased amount of total atmospheric waterover these areas. Figure 9(b) shows that the temperatureanomaly is the largest at the surface and decreases withheight. Aerosols in the Arctic are carried over long dis-tances on isentropic surface. Their vertical distribution isoften characterized by thin layers of high concentrations

generally confined between the surface and 3 km (Shaw,1995). This is why most of the dehydration and coolingin our experiment occurs in the lower troposphere.

In aerosol scenario B, colder tropospheric temperatureover the Arctic combined with warmer tropospheric tem-perature over Northern Europe and Siberia strengthensthe baroclinic zone, particularly over Northern Europeand the Laptev Sea. Figure 10 shows that the monthlymean wind velocity increased by up to 3 m s−1 overthe Laptev Sea at 500 hPa. This favours a more efficienttransport of aerosols from Europe as seen in Figure 1.Therefore, the DGF initiates a positive feedback loop

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Figure 9. February mean difference between aerosol scenarios B and A for (a) surface air temperature (K) over the domain (areas where thedifferences are statistically significant are shadowed) and (b) mean vertical profile of temperature averaged over grid points with sea-ice cover

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(a) (b)

Figure 10. February mean difference between aerosol scenarios B and A for 500 hPa wind (a) direction (black arrows for scenario A and grayarrows for scenario B) and (b) velocity (ms−1). Areas where the differences are statistically significant (95% confidence level) are shadowed

between the cooling of the Arctic and the atmospheric cir-culation at lower latitudes, which contributes to increasethe aerosol concentration over the domain. In turn, theincreased aerosol concentration increases cloud lifetime(SIRE) over Northern Europe and leads to a warmingover this area while it enhances further the DGF over theCentral Arctic, thus reducing even more the surface airtemperature. In aerosol scenario B, low IN concentrationalso contributes to maintain higher aerosol concentration.As shown by Blanchet and Girard (2001a) with a detailedmicrophysics model, the acidification of aerosols favourshigher frequency of small aerosol activations as opposedto low frequency of large aerosol activation episodes asseen in the low acidic case. Hence, the aerosol spectrumregenerates itself by coagulation much faster comparedto the natural aerosol scenario.

SUMMARY AND CONCLUSIONS

Previous field and laboratory experiments have shownthat acidic aerosols may significantly decrease the abil-ity of aerosols to nucleate ice crystals. In this research,we evaluate the effect of decreasing the IN concentrationon cloud formation and radiative budget of the Arcticfor February. The Arctic is characterized by episodesof high acidic concentration during winter (known asArctic haze). Blanchet and Girard (1994) have hypoth-esized that, through their ability to reduce IN concentra-tion, acidic aerosols can affect cloud microphysics andincrease precipitation originating from boundary layermixed-phase clouds. Enhanced precipitation dehydratesthe troposphere, thus reducing the greenhouse effect. Asa result, the surface air temperature decreases. If this pro-cess occurs over a long period of time, it could leadto a surface cooling detectable on monthly time scale.This process is known as the DGF. The DGF was first

evaluated over a single location (Alert) for four cold sea-sons by Girard et al. (2005). Results have shown that theDGF produces a substantial cooling in the lower tropo-sphere. In this research, we extend the previous study tothe whole Arctic for February, which is the month dur-ing which the DGF surface cooling was the largest in theGirard et al. (2005) investigation.

Two aerosol scenarios are compared: a natural sce-nario in which the IN concentration is not altered and anacidic aerosol scenario in which the IN concentration isreduced as a function of the sulfate concentration follow-ing Girard et al. (2005). An ensemble of 12 simulationsis performed for each aerosol scenario to eliminate asmuch as possible the model internal variability from theinvestigated climate forcing. Results show that the acidi-fication of aerosols leads to clouds containing less ice andmore water. This effect is important over cold regions ofthe Central Arctic while it is much less important overwarmer regions such as Northern Europe. At warmertemperatures, the IN concentration is low. Therefore,reducing IN concentration (by acidic coating) at warmertemperatures has no significant effect since IN concentra-tion is low even without any reduction by acidic aerosols.Over colder areas, IN concentration is higher. There-fore, reducing IN concentration has a substantial effecton cloud phase and indirectly on precipitation. Indeed,at low temperatures, the IN reduction favours higher icecrystal growth rate by the Bergeron–Findeisen processsince the available water vapour (from evaporating clouddroplets) deposits on fewer ice crystals. Ice crystals arelarger in size and precipitate more efficiently, leading toan increased precipitation rate. Results indicate that thedehydration efficiency (defined as the ratio between dailyaveraged precipitation rate and total atmospheric water)increases by as much as 12% over the Central Arctic.

Even though the DGF process has no effect on warmerregions, results show that these regions are indirectly



affected by the DGF. The cooling of the Central Arcticchanges the large-scale circulation with an intensifica-tion of the winds at 500 hPa over the Kara and LaptevSeas. Since the aerosol transport from Europe is themost intense over these areas, the strengthened circu-lation leads to an increase of aerosol transport to theArctic. Therefore, the aerosol concentration is increasedover the whole domain. This has two different effectsdepending on temperature. Over cold areas of the Centraland Eurasian Arctic, the DGF is enhanced, whereas overwarmer areas of Northern Europe, the increased aerosolconcentration leads to longer cloud lifetime. Therefore, apositive feedback occurs between the DGF cooling overthe Central Arctic and the aerosol concentration.

This research has shown that the DGF could be impor-tant during February. It supports previous results ofGirard et al. (2005) and Blanchet and Girard (2001) whohave done similar experiments with box and column mod-els. The DGF is based on ice nucleation process andthe reduction of IN caused by the aerosol acidification.The IN reduction will have to be further investigated byadditional laboratory studies aimed specifically at mea-suring the effect of sulfuric acid coating on IN activityfor temperatures typical of the Arctic during winter. Thechoice of the reduction function made in this experimentwas based on limited field measurements (Borys, 1989)and assumed to be valid over the whole Arctic. The INreduction was also applied to both immersion and con-tact freezing modes. These assumptions will have to berefined in future experiments as soon as new laboratoryand field experiments become available. Besides, carefulanalysis of observations (in situ and satellite) will be nec-essary to directly observe the impact of decreasing IN andits effect on cloud phase and precipitation. This researchwill help identify areas where the DGF might be themost active. February 1990 was characterized by a highpositive North Atlantic Oscillation (NAO) index withenhanced atmospheric circulation in the North Atlanticand warmer than normal surface air temperature overNorthern Europe. This type of atmospheric circulationpattern favours aerosol transport to the Arctic during win-ter. Other simulations covering other winter months andother years will also have to be performed to better eval-uate the DGF importance under other atmospheric circu-lation regimes (e.g. negative NAO index). This researchshows that the DGF has the potential to counteract tosome extent the warming because of increased green-house gases over the Central Arctic during winter andshould therefore be investigated in future field, laboratoryand modelling experiments.

ACKNOWLEDGEMENT

The authors would like to thank the Canadian Foundationfor Climate and Atmospheric Sciences (CFCAS grantnumber GR-333) for financial support.

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Assessment of the dehydration-greenhouse feedback over the Arctic during February 1990