Analysis of a dryline-like feature in northern Germany detected by … · 2018-08-05 · Meteorologische Zeitschrift, Vol. 20 , No. 4, 409-421 (August 2011) Article c by Gebru¨der

Meteorologische Zeitschrift Vol 20 No 4 409-421 (August 2011) Articleccopy by Gebruder Borntraeger 2011

Analysis of a dryline-like feature in northern Germanydetected by ground-based microwave profiling

DIETRICH SPANKUCH1 lowast JURGEN GULDNER2 HANS STEINHAGEN and MICHAEL BENDER3

1Leibniz-Soziatet der Wissenschaften zu Berlin eV Germany2Deutscher Wetterdienst Meteorologisches Observatorium Lindenberg ndash Richard-Aszligmann-ObservatoriumLindenberg Germany3Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum GFZ Germany

(Manuscript received July 22 2010 in revised form April 18 2011 accepted April 18 2011)

Abstract

Two dryline-like humidity drops without considerable temperature change were detected by theground-based microwave radiometer profiler (MWRP) at the Richard-Aszligmann-Observatory Lindenberg(5221N 1412E) on April 28 2007 The detailed analysis of these two events includes cloud radar andradar wind profiler measurements at the site as well as data from the surface synoptic network and from in-tegrated water vapour (IWV) maps derived from GPS The first more pronounced humidity drop is part of aroughly 200 km long line that meets the criterion of a classical dryline or dewpoint front namely of a mois-

ture gradient larger 35 g mminus3 per 100 km This dewpoint front is ahead of an approaching cold front and iscaused by strong downdraft induced by low tropospheric wind shear due to weakening of a midtropospherichigh over Germany It consisted in particular in two kernels of variable size depending on their stage Thefate of the kernels ndash migration speed unification and divorce ndash is described in detail Their lifetime was a bitmore than 9 hours The second humidity drop at the site was observed after the passage of the cold front andwas caused by dry advection behind the front Both events are predicted by the numerical weather predictionmodel COSMO-EU of the German Weather Service to some extent

Zusammenfassung

Am 28 April 2007 wurden mit dem Microwave Radiometer Profiler (MWRP) am Richard-Aszligmann-Observatorium Lindenberg (5221N 1412E) zwei ausgepragte Feuchteruckgange in der Grenzschichtohne nennenswerte Temperaturanderungen festgestellt Solche Erscheinungen treten als Taupunktsfrontenoder Drylines haufig im mittleren Westen der USA auf Eine detaillierte Analyse der Feuchteruckgangewurde unter Nutzung von Wolkenradar- und Windprofilermessungen vor Ort sowie von Daten des synop-tischen Beobachtungsmessnetzes und Karten der Verteilung des integralen Wasserdampfgehalts abgeleitetaus GPS-Daten vorgenommen Der erste intensivere Feuchteruckgang gehort zu einer etwa 200 km lan-gen Taupunktsfront die sich vor einer heranruckenden Kaltfront im Ergebnis stark absinkender trockenerLuft gebildet hat Die Absinkbewegung wird verursacht durch niedertropospharische Windscherung alsFolge eines uber Deutschland liegenden sich auflosenden Hohenhochs Die prafrontale Dryline besteht imWesentlichen aus zwei Kernen deren Groszlige von deren Entwicklungsstand abhangt Das Schicksal dieserKerne ndash Wanderung Wanderungsgeschwindigkeit Vereinigung und Trennung ndash wird ausfuhrlich dargestelltDer prafrontale Charakter und die Starke des Feuchtegradienten der Erscheinung entsprechen den Merkmaleneiner Dryline Der zweite Feuchteruckgang wurde nach dem Durchgang der Kaltfront beobachtet und ist dasErgebnis von Advektion trockener Luft nach der Kaltfront Beide Vorkommnisse wurden in gewissem Graddurch das numerische Vorhersagemodell COSMO-EU prognostiziert

1 Introduction

The continuous monitoring of the atmospheric bound-ary layer (ABL) is vital for many meteorological appli-cations Microwave radiation emitted by the atmospherecontains information on temperature water vapour andliquid cloud water (WESTWATER et al 2005) Ground-based microwave radiometer profiler (MWPR) providevertical thermodynamic profiles up to 10 km in bothclear and cloudy conditions with high temporal reso-lution and sufficient accuracy in the lower troposphere(GULDNER and SPANKUCH 2001) and are thereforewell suited for ABL monitoring (KNUPP et al 2009

lowastCorresponding author Dietrich Spankuch Schmerberger Weg 72 14548

Schwielowsee OT Caputh Germany e-mail dietrichspaenkuchwebde

GULDNER et al 2009) Combined with other remotesensing measurements MWRP observations provideessential information for a multitude of applications(LOHNERT et al 2004 HEWISON 2007 ILLING-WORTH et al 2007 TURNER 2007) Quasi continuousmonitoring of the temperature-humidity structure of thelower atmosphere has been made at the Richard AszligmannObservatory Lindenberg (5222N 1412E) of the Ger-man Weather Service since November 1998 by meansof the 12 channel Radiometrics profiler TPWVP 3000(SOLHEIM et al 1998 WARE et al 2003) This morethan 11 years long series is probably the longest series ofthis kind worldwide demonstrating convincingly the ca-pability and feasibility of ground-based microwave pro-filing

0941-294820110222 $ 585

DOI 1011270941-294820110222 ccopy Gebruder Borntraeger Stuttgart 2011

410 D Spankuch et al Analysis of a dryline-like feature Meteorol Z 20 2011

In this paper we describe the passage of two dew-point drops as particular mesoscale features similar to adryline traced by MWRP in April 2007 at Lindenbergthat has not been reported so far in Central Europe Ad-ditional local information is used for the analysis fromthe 1290 MHz boundary layer radar wind profiler theKa-band cloud radar and surface instrumentation at thesite Maps of absolute humidity at the surface and of in-tegrated water vapour (IWV) from the German GPS net-work show the development and dimension of this phe-nomenon in space and time

2 Data sets

21 Microwave profiler

The MWRP measures atmospheric brightness temper-atures at five frequencies from 22 to 30 GHz for wa-ter vapour profiling and at seven channels from 51 to59 GHz for temperature profiling respectively The dataare provided with a temporal resolution of about oneminute and are capable to resolve mesoscale featuresremaining often undetected by radiosondes Profiles oftemperature water vapour density and liquid water areretrieved operationally using a neural network (NN) andan observation-based regression method (REGobs) in-cluding additionally zenith infrared and surface temper-ature humidity and pressure sensors The NN is trainedon the basis of radiative transfer calculations using his-torical radiosonde soundings and REGobs applies si-multaneous observations (brightness temperature) of theMWRP and radiosondes from the past to calculate a re-gression operator Neural networks are non-linear meth-ods which provide profiles with a slightly higher verticalresolution The REGobs operator is not affected by errorsin the absorption model or instrumental bias system-atic deviations are considerably reduced In spring thecharacteristic rms error of differences between MWRPretrievals and radiosondes is less than 1 K for tempera-ture up to 1 km height and about 15 K between 1 and3 km height For vapour density the error increases upto 1 g mminus3 at 1 km and declines upward due to thelower amount of water vapour at these levels (GULD-NER and LEPS 2005) Nevertheless ground-based mi-crowave profiling enables monitoring of boundary layerprocesses during nearly all weather conditions in unat-tended mode For this study the operationally archived10-minutes mean values of NN retrievals were usedThere was no reason to use REGobs retrievals

22 Boundary Layer Radar Wind Profiler

Measurements of the mean horizontal wind were per-formed with a 129 GHz pulsed Doppler beam swing-ing Radar Wind Profiler (ENGELBART et al 1996) Thesystem was operated in a single resolution wind modeusing a 5-beam configuration (four tilted beams North

South West East with 75 elevation and one verticalbeam) with a pulse width of 700 ns (corresponding toa radial resolution of 100 m) The received signal wasslightly oversampled in range with a 500 ns gate spac-ing (corresponding to a vertical range of 714 m) Thetransmitted pulse propagates along the different antennadirections which are sequentially switched one after theother A small fraction of the transmitted energy is scat-tered back to the antenna The two main echoing mecha-nisms for L-band Radar Wind Profilers are scattering atinhomogeneities of the refractive index (Bragg scatter-ing) or at precipitation particles (Rayleigh scattering)After signal amplification demodulation and AD con-version the Doppler spectrum of the receiving signaltime series is estimated using standard Fast Fourier tech-niques followed by the computation of the first threemoments of the maximum energy peak signal inten-sity (0 moment) radial velocity (1 moment) and spec-tral width (2 moment) A consensus avaraging of the ra-dial wind estimates over 30 minutes discriminates atmo-spheric signals from noise peak estimates and the re-sulting mean radial velocities are finally combined toobtain the three-dimensional wind vector As usual thisassumes horizontal homogeneity of the wind field in thearea spanned by the oblique beams over the averagingtime period

23 Cloud Radar

The Ka-band cloud radar MIRA 36 is a pulsed Dopplerradar operating at a frequency of 355 GHz to mea-sure vertical profiles of reflectivity Doppler velocityas well as spectral width of meteorological targets ina height range between 150 m and 15 km (GORSDORF

and HANDWERKER 2006) The cloud radar transmitspulses of a length of 200 ns (corresponding to 30 m verti-cal resolution) and receives the signals backscattered bymeteorological targets like cloud droplets drizzle do-plets and ice crystals but also by other particles likeinsects The signal processing contains a Fast Fouriertransformation with 256 FFT points and an averag-ing of 200 spectra (corresponding to an averaging timeof 10 s) Normally the measuring quantities are usedto determine cloud characteristics like cloud base andcloud top as well as droplet distribution or liquid wa-ter content In our case we use the cloud radar to de-termine the vertical velocity in the boundary layer al-though no clouds exist In this case insects serve as tar-gets assuming that they move passively with the windHowever the assumption of passively moving insectsin the wind field is not quite correct Using this non-correct assumption causes some bias in vertical veloc-ity on the order of ndash 02 to ndash 05 m sminus1 for updrafts thatis a reduced updraft and much smaller amounts fordowndrafts as shown by several studies using differentmicrowave frequencies namely 915 MHz windprofilerdata (ANGEVINE 1997) 1238 MHz windprofiler and C-band Doppler radar radar measurements (LOTHON et al

Meteorol Z 20 2011 D Spankuch et al Analysis of a dryline-like feature 411

Figure 1 The daily course of temperature (top panel) vapour density (central panel) and relative humidity (lower panel) at Lindenberg

on April 28 2007 from MWP measurements (left panels) and corresponding to the NWP model COSMO-EU (right panels) The MWRP-

derived IWV in cm is given as dashed red line in the central panels The modeled IWV is given as white dashed line in the right central

panel Isolines are given in steps of 2 K for temperature 1 g mminus3 for vapour density and 10 for relative humidity respectively

2009) and 95 GHz Doppler radar studies (GEERTS andMIAO 2005) This bias is not relevant for our studyhowever Due to the small measuring volume (05 an-tenna beam width corresponding to about 10 m hori-zontal range at 1 km) and the high velocity resolution(008 m sminus1) the radar is able to measure the small scalestructure of the vertical velocity field

24 GPS derived integrated water vapour

The GPS microwave signals are modified on their paththrough Earthrsquos atmosphere leading to an additional sig-nal path delay as compared with vacuum propagationSpecial GPS processing techniques were developed toseparate the path delays due to the neutral atmosphereand especially due to the water vapour and to estimatethe integrated water vapour (IWV) above each individ-ual GPS station (BEVIS et al 1992) Since the begin-


Figure 2 Skew T-log p plots at Lindenberg 0600(left) 1200(centre) and 1800(right) Note the very shallow near-surface superadiabatic

layer of 20 m depth in the 1200 ascent Reproduced by kind permission of the University of Wyoming Department of Atmospheric Science

Table 1 Hourly weather observations at Lindenberg on April 28

2007 (extract)

Time T RH Pressure Cloud observation

(UTC) (C) () (hPa) octa type height(m)

10 239 39 10209 2 Ci fib 9000

11 251 34 10204 lt1 Cu tra 1000

lt1 Ci fib 9000

12 262 27 10197 1 Cu hum tra 1500

13 266 20 10192

14 264 23 10186 1 Cu med tra 1200

3 Ci-Cs fib 6000

15 256 34 10180 3 Cu con med tra 9000

2 Ci fib 8000

16 250 39 10177 2 Ci fib 9000

17 230 44 10177 5 Cu con med tra 2000

2 Ac str tr 3600

18 208 35 10180 3 Cu con med tra 2000

1 Ac tr 3000

2 Ci fib 8000

19 183 62 10190 1 Cu con med 2000

2 Ac str tr 3150

2 Ci fib 8000

ning of the last decade GPS-based IWV observationsfrom large nationwide networks have been available onan operational basis (WARE et al 2000) The GermanResearch Center for Geosciences (GFZ) processes thedata of about 350 German GPS stations operationallyin near real-time and provides IWV observations with atemporal resolution of 15 minutes (DICK et al 2001)IWV maps obtained from dense networks of GPS re-ceivers give a detailed view on the spatiotemporal varia-tion of water vapour

Several validation studies comparing IWV obtainedby GPS by microwave radiometer and by radiosondeshowed mean differences of approximately 1 to 15 mm(GULDNER 2001 GENDT et al 2004 MARTIN et al2005) However when comparing GPS-IWV with ra-diometer observations it should be kept in mind that sev-

eral GPS observations along different satellite receiveraxes are combined to estimate the IWV The GPS-IWVis therefore an average over a radius of about 15 kmaround each GPS station and a period of 15 minutesSpatial IWV variations of 10 were found on this spacescale at midlatitudes by KNEIFEL et al (2009) The tem-poral variations might therefore be somewhat smoothedand delayed as compared with more pointlike observa-tions provided by water vapour radiometers

3 MWRP records and supportingobservations at the site

On 28 April 2007 a dryline-like feature was observedby the MWRP as shown in the left panels of Fig 1presenting the course of temperature (top) vapour den-sity (centre) and relative humidity (bottom) on that dayThe right panels give the corresponding model outputsof the numerical weather prediction model COSMO-EUof the German Weather Service (STEPPELER et al2003) There are two sharp drops of vapour densitywithout corresponding temperature drops as it is ob-served at drylines or dewpoint fronts well known fea-tures in particular in the Southern Great Plains of theUnited States where moist and hot air from the Gulf ofMexico meets dry and hot air coming down from theRocky Mountains Drylines were also observed in north-ern India as a spatial stationar feature during the entiremonsoon season (WESTON 1972 CHIAO and BARROS2007) and in the northern part of the Australian conti-nent as a regular nocturnal feature through the springand summer (ARNUP and REEDER 2007) Drylinesare meso-gamma scale features in width (2-20 km) witha length of 500 to 1000 km (ZIEGLER et al 1997)They are known as favoured zones of severe weather(RHEA 1966 ZIEGLER and RASMUSSEN 1998) Theoccurence of two humidity drops suggests even the pas-sage of a double dryline (HANE et al 1993 CRAW-FORD and BLUESTEIN 1997 DEMOZ et al 2006)


Figure 3 Vertical wind from the Ka-band cloud radar (left panel) from 0800 to 1200 UTC (top) and from 1200 to 1600 UTC (bottom) and

horizontal (right upper panel) and vertical wind (right lower panel) from 1000 to 1700 UTC from the Boundary Layer Wind Profiler 30 min

averages

The first humidity drop starts at about 1000 UTC(thereafter all times UTC) at the surface and is distinctlyseen up to about 1 km At about 1200 humidity decreasesets in too above that level drying the whole lower tropo-sphere The culmination of this drying event takes placebetween 1300 and 1400 The relative humidity dropsto 17 at the surface between 1303 and 1307 and isonly slightly above 70 between 15 and 2 km heightSignificant humidity increase begins at all levels shortlybefore 1400 with 3 g mminus3 within 1 hour in the lowest800 m accompanied by cloud formation A few singlefair weather cumulus clouds were observed just before1100 at 1000 to 1500 m height which were not caughtby the MWRP and the ceilometer and disappeared af-ter 1200 (see Table 1 for details of the weather obser-vations) The hourly visual cloud observations are inour case more informative than the ceilometer measure-

ments due to the small amount of cloudiness at that dayas the weather observers see the whole sky and not onlythe zenith Cumulus clouds are formed again at about1500 at 2000 m with its largest amount of 5 octas twohours later indicating the passage of a frontal line at thattime

The second humidity drop of less intensity than thefirst one starts shortly after 1700 quasisimultaneously upto 2 km or even more and lasts for close upon one hourThere is a quasicontinuous moisture decline at greaterheights till about 2230 when a further sharper humid-ity decrease is observed in the whole boundary layerBoth humidity drops were not caught by the radiosondeascents made at this site every 6 hours as shown inFig 2 No pecularities are found in the temperaturecourse (Fig 1 top left) Only a marginal temperatureincrease of about 05 K above 2 km was retrieved dur-


Figure 4 Surface weather chart on April 28 2007 at 1200 UTC

(Berliner Wetterkarte) Isobars are drawn in steps of 1hPa Wind

vectors follow the convention with full barb less than or equal

5 m sminus1 and half barb equal or less than 25 m sminus1 respectively

Notations on the left are northern latitude on the top are eastern

longitude respectively The Lindenberg site is indicated by a star

ing the first humidity drop A small temperature de-crease is observed simultaneously with the second dropabove about 300 m which strengthens with increasingheight but there is no discernible deviation in the de-creasing temperature course at the surface during thisevent Both humidity drops are reflected by the nonhy-drostatic NWP model COSMO-EU to some extent butless pronounced than observed and shifted in time how-ever (Fig 1 right central panel) The first drop occursat the surface much stronger and about 2 hrs later thanpredicted The model gives indeed humidity decrease at1200 above 1 km but further increase below that altitudeThe resulting modelled IWV drawn as white dashed linein the central right panel of Fig 1 shows thus only amarginal dip at that time in contrast to the observationAnother and probably more plausible interpretation ofthe model output is the following The drying event aloftis exactly predicted in time but too weak to penetrateinto the boundary layer up to the ground The dryingin the lowest hundred meters between 0900 and 1200is due to evapotranspiration with moisture transport tohigher levels explaining the steady modeled moisture in-crease at eg 500 m until about 1300 The temperaturecourse is also by and large correctly given by the modelbut with a reduced daily amplitude however The realsurface temperature is more than 2 K lower during early

morning between 0300 and 0500 and higher by morethan 2 K in maximum from 1000 and 1600 The modelunderestimates the temperature lapse rate distinctly ex-cept for the first four to five hours of the day The devi-ations of temperature and vapour density between mea-surement and model output are enlarged in the field ofrelative humidity with too low values particularly afterthe passage of the first dew point drop (Fig 1 bottom)

The development of the ABL during the day asshown in Figs1 and 2 is typical for sunny summerdays with a stable boundary layer (SBL) in the morn-ing caused by radiative cooling near the ground at nightand subsequent transition to a convective boundary layer(CBL) driven by surface heating during daytime TheSBL had a depth of about 300 m at 0600 with a weakstable low tropospheric layer overhead The 0600 ascent(Fig 2 left panel) shows a remarkable sharp moisturedrop within 250 to 300 m from 35 g kgminus1 at 765 hPa to028 g kgminus1 at 740 hPa The transition from SBL to CBLtook place within about one hour between 0700 and0800 according to the course of potential temperature θ(not given) Fig 3 left panels showing the registrationsof the 10s vertical velocity from the Ka-band cloud radaris a good demonstration of the mixing process withinthe CBL The rising thermals only marginally tilted ifat all have in general a diameter of the order of 100 mand a depth of about 500 to 800 m with the exceptionof the thermal plume between 1000 and 1025 that had adiameter of about 4 km between about 1 and 15 km al-titude The CBL reached its final depth of about 1900 mat 1100 The 1200 ascent (Fig 2 centre) shows a CBL ofroughly constant θ of 296 K bounded by an extremelyshallow superadiabatic layer of only about 20 m at thebottom where θ dropped by more than 2 K This shal-low superadiabatic layer is a challenge to every numeri-cal model The CBL was capped by a slight inversion of2 K in θ Moist air was advected above the CBL between790 and 750 hPa in an about 400 m thick layer Moisten-ing of the CBL by 2 g kgminus1 and more was observed till1800

The onset of humidity decrease at about 1000 iscaused by downdraft of very dry air of about 5 above2 km corresponding to the radiosonde ascents of 0600and 1200 at the site (Fig 2) that is at that time when theboundary layer growth is rapid MAHRT (1976) statedrapid boundary layer growth and rapid downward en-trainment of dry air in case of weak stratification of theoverlying free flow The cloud radar data (Fig 3 up-per left panel) show that downdraft intervals with veloc-ities around -1 m sminus1 become superior to updraft inter-vals after about 1025 Strong downdraft with to about-5 m sminus1 is observed after 1320 for about 15 min ade-quate to about 25 km horizontal extent (Fig 3 lowerpanels) when the MWRP indicated the first significanthumidity drop The strongest downdraft is found be-tween about 500 and 1500 m This downdraft maxi-mum is simultaneously connected with abating horizon-


tal winds of about 25 m sminus1 in this height region anda wind shift from NW to N as shown by the 1290 MHzradar windprofiler data (Fig 3 upper right panel) Com-bining both information from the cloud radar and theradar wind profiler the real wind blowed down a gen-tle SE slope of about 10 to the horizontal with a speedof about 6 m sminus1 before the onset of the strong down-draft With the onset of the strong downdraft the windspeed did not change very much but blowed down asteep southward looking slope of 65 to the horizontalbetween 500 and 1300 m and of 45 below that altitudeUpdraft plumes suggesting gravity waves by their reg-ular time spacing set in at about 1330 producing theclouds observed around 1400 as mentioned already Onepossible explanation of increasing downdraft with si-multaneous abating horizontal winds in flat terrain is theexistence of a mesoscale low that sucks the surroundingair The surface weather map (Fig 4) does not show sucha low but indicates a slight convergence region near thesite probably as a result of the downdraft accompaniedby some lowtropospheric wind shear General pressuredecline starts slowly at the site after 0600 with 01 to02 hPa per hour when 10218 hPa were observed Al-though the inter-hourly pressure fall intensified to 05 -07 hPa between 1100 and 1600 resulting in 10177 hPaas minimum no distinct dip is found in the general de-cline however (see Table1) Pressure rise sets in at 1650quasisimultaneously with the second humidity drop in-dicating the passage of a frontal system

No remarkable pecularities in wind shift are observedduring the onset of the first humidity drop at about 1000but there is a slight and steady increase of 1 m sminus1 perhour in wind speed at the surface from 1 m sminus1 at 0900 to4 m sminus1 at 1200 followed by similar decrease till 1600The wind direction went back by 50 from 350 to 300

between 1200 and 1300 indicating cold advection aheadof the front There are also no noteworthy pecularitiesduring the second humidity drop except the fresheningof the wind in the lowest 2 km and the reversal of thepressure tendency as already mentioned

4 Weather situation

A ridge of high pressure extending from the Azoresto the Ukraine dominates the weather in Germany onApril 28 2007 with temperatures much above the cli-matological norm (24C around noon except in moun-tain regions) large scale downdraft and mostly cloud-less sky The surface analysis at 1200 of the BerlinerWetterkarte (Fig 4) shows a cold front over NorthernGermany roughly at 53N about 100 km north of thesite as part of a strong low pressure system at the Pe-chora mouth in the northeastern corner of Europe Thecold front replaces continental tropical air by maritimepolar air where isolated weak showers with marginalmeasurable precipitation were observed over southernBrandenburg and Saxonia in the evening and some more

Figure 5 Channel 3 (459 to 479 nm) MODIS image of the AQUA

satellite of April 28 2007 at 1208 UTC The Lindenberg site is

indicated by a rhomb The white arrows point to the swath of

clear skies in front of the cold front seen as the grey band to the

north Drawn in are the 50N latitude circle and the 10E and 20E

longitude circles This high resolution image was gratefully prepared

by NEODAASUniversity of Dundee

rain in southern Poland The front line is well seen inthe channel 3 (459 to 479 nm) image of the ModerateResolution Image Spectrometer (MODIS) of the AQUAsatellite at 1208 as a diffuse somewhat structured bandof moderate reflectivity of about 65 to 100 km widthstretching from Pomerania across the North GermanLowland until the German Bay (Fig 5) The band lo-cated north of the site indicates enhanced aerosol opti-cal depth andor the existence of marginal partial cloudi-ness of subpixel size less than 250 m South to the front-line there is east of about 9E longitude a roughly 40 kmtimes 300 km wide dark swath of cloudfree and aerosolpoor atmosphere indicated in Fig 5 by white arrowswith many peninsulas of moderate reflectivity withinThis dark swath is the region of enhanced downdraftin front of the cold front The pattern described is notseen in the AVHRR images at 1033 and only sligthlydetectable in the AVHRR channel 1 (580 to 680 nm) im-age at 1323 UTC and in the Meteosat IR channel at 1200(Berliner Wetterkarte 56 April 2007) There is north tothe diffuse band of moderate reflectivity another darkregion indicating clear sky behind the front that extendsbeyond the coastline of the Baltic Sea The developmentin the midtroposphere on April 28 is displayed in Fig6 taken from httpwwwwetter3deArchiv where manyother maps of several meteorological parameters at ev-ery six hours are compiled Given are the 500 hPa geopo-tential height in black lines the 500ndash1000 hPa thickness


Figure 6 500 hPa geopotential height (black lines) 500ndash1000 hPA

thickness (grey shading) and surface pressure (white lines) on

April 28 2007 for 0000 UTC (top) and 1800 UTC (bottom) Kind

permission of wetter3de

in grey shading and the surface pressure field in whitelines for 0000 (top) and 1800 (bottom) Central Europehad been dominated for several days by the midtropo-spheric high that laid at 0000 above Germany with morethan 574 gpdm Eighteen hours later the high was dis-solved and a high pressure ridge where the midtropo-spheric high was imbedded in had extended to the northbeyond Iceland The degration of the midtropospherichigh was due to outflow of dry air at its edges that causedthe observed downdrafts at the site This outflow was ac-companied by some windshear as seen in Fig 3 Therewas no significant thickness advection during that time

5 Development of the features in spaceand time

There are a couple of questions that cannot be answeredby local measurements like origin development mi-gration and lifetime of the features described It needsadditional information of meteorological parameters in

space and time to give adequate answers Two differentdata sources are capable of useful information meteo-rological parameters at the surface available every hourfrom the regular meteorological network and IWV mapsderived from GPS data every 15 min There were no fur-ther suitable information from satellite images that couldbe added to the MODIS image (Fig 5) Surface weatheris per se more influenced by the underlying boundaryin particular in mountain regions than integral atmo-spheric parameters like IWV Nevertheless the down-draft events detected by screening of the routinely madeMWRP records should clearly be seen in the surfacemaps too The field of absolute humidity at the surfaceproved to be the most informative data source Fig 7gives a set of maps of this parameter beginning with thefield at 1000 (Fig 7a) when the drying at the site in-dicated by a cross in the Figure sets in At this timethe site was located in a ridge of enhanced vapour den-sity of slightly more than 8 g mminus3 A streak of reducedwater vapour with amounts less than 7 g mminus3 is seennorthwest of the site with two pronounced kernels oflocal vapour minima of less than 5 g mminus3 in a distanceof roughly 140 km from each other This streak corre-sponds with the dark swath of the MODIS image (Fig5) It indicates the downdraft zone in front of the ap-proaching cold front The sequence of the maps of Fig7 shows that one of the kernels of this downdraft zone isequivalent to the first humidity drop Both kernels unitedafter 1100 (not shown) and formed a narrow stripe ofabout 70 km length in west-east direction about 70 kmWNW of Lindenberg at 1200 (Fig 7b) The stripe di-vided again after 1200 into two kernels one of them atthe site the other more intense one about 140 km apartfrom and west of Lindenberg at 1300 (Fig 7c) Bothkernels approach each other by migration of the westernone in southeast direction and of the eastern one at Lin-denberg in southwest direction between 1300 and 1400the time of the culmination of the first downdraft Theyare at 1400 UTC located at 522N about 70 km apartat 125E and 135E southwest of Lindenberg (Fig7d) Both kernels weakend during the next hour between1400 and 1500 (Fig 7e) the western kernel moved east-ward to the position of the eastern kernel and the weakerone to the southeast both with a speed of about 70 kmper hour Further weakening at least of the western ker-nel (the other kernel moved out of the map section) tookplace with reduced velocity of about 40 km per hour indirection southeast between 1500 and 1600 (not shown)followed by strengthening again with unchanged speedin the same direction during the following hour so thatthe downdraft cell was located about 60 km SSE fromthe site at 1700 (Fig 7f) The kernel deceased between1800 and 1900 slightly south of the 1700 position

The origin of the downdraft cells could be fixed tooThe western cell that developed to the stronger oneduring the day is seen in the 0900 map (not shown)as a region of slightly reduced water vapour as the


(a) (b)

(c) (d)

(e) (f)

Figure 7 Maps of near-surface distributions of absolute humidity on 28 April 2007at different times Upper row 1000 UTC (a) 1200 UTC

(b) central row 1300 UTC (c) 1400 UTC (d) and lower row 1500 UTC (e) 1700 UTC (f) The site is marked by a cross Isolines are given

in steps of 1 g mminus3


(a) (b)

(c) (d)

Figure 8 Maps of Integrated Water Vapour at 1007(a) 1307(b) 1507(c) and 1707(d) UTC Indicated are the GPS stations The Lindenberg

site is marked by a star Isolines ae given in IWV steps of 2 kg mminus2 equivalent to 2 mm ppw

same position as at 1000 The other cell also at thesame position as one hour later was well developedalready at that time with amounts less than 7 g mminus3 Thelifetime of the downdraft cells was thus somewhat morethan 9 hours The place of death of the western cell isabout 180 km southeast of its place of birth the totalcovered distance is larger however due to its partlynonrectilinear movement The cells were nearly fixedduring the first and last hour of their life The maximumvelocity was about 70 km hrminus1 during their climax

Fig 7 is also used to investigate the history of thesecond humidity drop It approaches the site at 1700

as the remnant of the postfrontal dry zone that ex-tended across northern Germany during the morninghours from Schleswig-Holstein to the Uckermark north-east of Berlin This dry zone is located at the easternside of the deceasing high at 500 hPa (Fig 6) and is thuscaused by the outflow of dry and cold air coming fromthis high The sequence of the maps of Fig 7 showsclearly the weakening of this zone and its disintegrationinto single cells during the day due to undermined out-flow from the weakened high There is only one cell leftfrom this drying zone at 1700 This cell was nearly sta-tionary for about three hours between 1200 and 1500


and moved afterwards southward with a speed of about40 km per hour

The migration of the patterns in the surface humid-ity field excludes local causes and suggests downdraftslike observed at Lindenberg as plausible cause All hu-midity minima of the surface humidity field were with-out exception not accompanied by temperature changesThe decline in surface humidity at the site down to 17 was also not the strongest one The absolute minimumwas observed with 14 relative humidity 150 km to thewest at 1300

The downdraft cells are also detectable in the maps ofIWV in particular at 1307 (Fig 8b) where two seperatecells with minima less than 12 mm are seen at Linden-berg and somewhat west of the site Both cells were alsofirst seen like in the surface vapour density map at 0907(not shown) The features in the IWV maps are generallyless pronounced than in the surface humidity maps how-ever On the other hand the position of the front at thesurface is more clearly definable in the IWV map than bythe classical meteorological parameters as the IWV gra-dient is significantly stronger than the temperature gra-dient and the wind field too weak to determine a distinctwind shift The marginal cloudiness in the region givingno information was mentioned already As fronts are of-ten not well-defined features (MASS 1991 SANDERS

and DOSWELL 1995 SANDERS 1999) the capabilityof IWV in improving frontal analysis is worthwhile forfurther studies

The belt of enhanced IWV of more than 18 mm withthree more or less pronounced maxima above 20 mmcorresponds well with the diffuse band of enhanced op-tical depth of the MODIS image (Fig 5) The maximumIWV gradient defines the position of the frontline Lesspronounced than in the MODIS image is the dark beltof low optical depth in front of the frontline howeverThere is a general decrease of IWV from north to southfrom amounts of about 20 mm in maximum to partly lessthan 10 mm in central and southern Germany The IWVmaps show like the surface maps many mesoscale fea-tures Their analysis and discussion is however beyondthe scope of this paper and needs further research

6 Discussion

The combined analysis of ground-based microwavewindprofiler and cloud radar measurements at the siteand of data of the surface meteorological network andof the national GPS network show the different natureof two dryline-like humidity drops observed on the sameday in April 2007 in Lindenberg The first humidity dropis induced by strong downdraft and is the result of strongmixing of thermal plumes from below and dry air en-trained from above Slight convergence near the surfacewith lowtropospheric windshear advects cold dry airahead of the front The second humidity drop is causedby replacement of continental tropical air by maritime

polar air of nearly the same temperature when it ar-rived at the site due to strong solar insolation during theday The second event is thus not caused by subsidencerather than by dry advection behind the front

The downdraft that caused the first humidity drophad a slope of 45 to 65 to the horizontal at maximumat the site with a speed of 7 m sminus1 and travelled byand large parallel to and about 40 km in front of thefrontline with variable speed It meets the criterion of adryline with its gradient of specific humidity larger than3 g kgminus1 per 100 km (HOCH and MARKOWSKI 2005)equivalent to about 35 g mminus3 per 100 km of vapourdensity or larger than 75 mm per 100 km of IWV whenthe well-known relation IWV=216 ρs is applied withρs the vapour density at the surface in g m minus3 (egMOLLER and BULLRICH (1961)) It meets the conditionof a line only partly however Although there seems toexist a line from time to time eg at 1400 and 1500(Fig 7) the humidity-gradient-criterion is not fulfilledfor a great part of this feature The greatest length of theline meeting the criterion of the humidity gradient wasestimated to be a bit more than 200 km compared to the500 to 1000 km length of the drylines in the Great PlainsThe smaller length of the line in northern Germany isnot surprising however considering the differences innature and scale of the two landscapes Most of thetime the humidity-gradient-criterion was met only bythe two prefrontal kernels within the zone of reducedmoisture and not by the whole zone The maximumsize of one cell was found to about 70 km x 38 km at1400 The horizontal dimension of the downdraft plumein Lindenberg is estimated to 18 to 32 km in north-south direction using the measured wind velocity of5 m secminus1 at the site Prefrontal features associated withcold fronts are not uncommon An excellent overviewof such features and their responsible mechanisms isgiven in the review paper of SCHULTZ (2005) Thesize of the second humidity drop at 1700 that alsomeet the humidity-gradient-criterion was about 40 kmin meridional direction in agreement with the estimatemade by the local wind measurements and about 230 kmin west-east direction according to Fig 7

Drylines are regular seasonal features in low lati-tude regions of the world above sloped terrain like inthe Great Plains of the United States and in India orabove flat terrain like in northern Australia where theydevelop at the boundary between tropical maritime andcontinental air masses over the heated continent froma sea-breaze circulation Both preconditions ndash slopedterrain or sea-breaze circulation ndash are not given in ourcase There is weak cyclonal circulation near the surfacearound the site with weak winds ahead the front (Fig 4)The wind profiler data (Fig 3 upper right panel) showthat the wind did not change with height very muchexcept at around 1300 but the wind direction tendedto back with height suggestive of cold advection bothahead and behind the front due to low-level wind shear


SUN and WU (1992) investigating dryline formationduring fair weather found dryline strength most sensitiveto low-level wind shear large-scale terrain slope andvolumetric soil moisture gradients in that order Thereare many lakes and forests around the site Significantmoisture gradients at the surface are thus per se givenThe occurrence of kernels are thus plausible as well astheir partly irregular migration

The predominance of local kernels or cells withinthe prefrontal drying zone suggests the term dry cellor dry kernel It is justified from our point of view toconsider the two dry cells found in the surface humidityfield as well as in the IWV field as individuals becausethey move partly independent from each other They ap-proach each other unify for a certain time and sepa-rate in their lifetime of a bit more than 9 hours Detailedstructures in the horizontal along-line direction of dry-lines in the mesoscale in particular horizontal convec-tive rolls in more or less regular spacing (BLUESTEIN

et al 1988) are common and have been extensivelyinvestigated by sophisticated field experiments in theGreat Plains (HANE et al 1993 ATKINS et al 1998DEMOZ et al 2006)

It is worthwhile mentioning that the drying event waspredicted to some extent by the numerical weather pre-diction model (Fig 1) All in all the term dryline-like isfrom our point of view the adequate description of thephenomenon discussed rather than the term dryline asthe first dry zone consisted in particular in the two down-draft kernels or cells that caused the dew-point dropsThese cells were more or less independent individuals

7 Conclusions

The detailed analysis of two humidity drop events with-out corresponding temperature changes detected by theground-based microwave radiometer profiler at Linden-berg in the Northern German Lowlands showed thatthe first and more intense drop was caused by severedowndraft in front of a cold front Measurements of thesynoptic surface network and GPS derived IWV datawere indispensable analysis tools to show the prefrontaldowndraft region as consisting in particular of two pro-nounced mesoscale downdraft kernels of variable sizeand shape with a lifetime of a bit more than 9 hoursThe second humidity drop in the afternoon was causedby advection of dry air behind the front with nearly thesame temperature as the previous air The phenomenondescribed has many features of a dryline and is so far aswe know the first one observed in Central Europe Thisstudy shows convincingly the role ground-based passivemicrowave profiling can play in concert with other so-phisticated remote sensing systems in operational mon-itoring of mesoscale ABL processes Its all weather fea-sibility is particular attractive for such studies

Acknowledgments

We would like to thank Ulrich Gorsdorf Volker Lehmann(both at the Richard-Aszligmann-Observatory Lindenberg)and Werner Wehry (Free University Berlin) for their in-put and discussions as well as Jorg Liebing (RegionaleMessnetzgruppe Potsdam des DWD) for providing thedata of the surface network The authors acknowledgethe constructive comments of two anonymous refereesWe appreciate the support of NEODAASUniversity ofDundee for providing the MODIS image (Fig 5) Thefirst author is indebted to the German Weather Servicefor support

References

ANGEVINE WM 1997 Errors in mean vertical velocitiesmeasured by boundary layer wind profilers ndash J AtmosOceanic Technol 14 265ndash269

ARNUP SH MJ REEDER 2007 The diurnal and seasonalvariation of the northern Australian dryline ndash Mon WeaRev 135 2995ndash3008

ATKINS NT RM WAKIMOTO C L ZIEGLER 1998Observations of the finescale structure of a dryline duringVORTEX 95 ndash Mon Wea Rev 126 525ndash550

BEVIS M S BUSINGER T A HERRING C ROCKENRA ANTHES RH WARE 1992 GPS meteorology Re-mote sensing of atmospheric water vapor using the globalpositioning system ndash J Geophys Res 97 15787ndash15801

BLUESTEIN HB EW MCCAUL JR G P BYRD GRWOODALL 1988 Mobile sounding observations of atornadic storm near the dryline The Canadian Texas stormof 7 May 1986 ndash Mon Wea Rev 116 1790ndash1804

CHIAO S AP BARROS 2007 A numerical study of thehydrometeorological dryline in northwest India during themonsoon ndash J Met Soc Japan 85A 337ndash361

CRAWFORD TM HB BLUESTEIN 1997 Characteristicsof dryline passage ndash Mon Wea Rev 125 463ndash477

DEMOZ B and Co-authors 2006 The dryline on 22 may2002 during IHOP 2002 Convective scale measurementsat the profiling site ndash Mon Wea Rev 134 294ndash310

DICK G G GENDT C REIGBER 2001 First experiencewith near real-time water vapor estimation in a GermanGps network ndash J Atmos Solar-Terrest Phys 63 1295ndash1304

ENGELBART D H STEINHAGEN U GORSDORF J LIPP-MANN J NEISSER 1996 A 1290 MHz profiler withRASS for monitoring wind and temperature in the bound-ary layer ndash Beitr Phys Atmos 69 63ndash80

GEERTS B Q MIAO 2005 The use of millimeter DopplerRadar echoes to estimate vertical air velocities in the fair-weather convective boundary layer ndash J Atmos OceanicTechnol 22 225ndash246

GENDT G G DICK C REIGBER M TOMASSINI Y LIUM RAMATSCHI 2004 Near real time GPS water vapormonitoring for numerical weather prediction in Germanyndash J Meteor Soc Japan 82 361ndash370

GORSDORF U J HANDWERKER 2006 A 36 GHz highsensitivity cloud radar for continuous measurements ofcloud parameters ndash experiences of 2-years operation andsystem intercomparison ndash In Proceedings of the 7th Inter-national Symposium on Tropospheric Profiling Needs andTechnologies Boulder 12ndash16062006


GULDNER J 2001 Validation of integrated water vaporusing independent measurement techniques ndash Phys ChemEarth 26 427ndash431

GULDNER J J-P LEPS 2005 Analysis of CLIWA-NETintensive operation period data as part of the monitoringactivities at the German Meteorological Service site Lin-denberg ndash Atmos Res 75 151ndash166

GULDNER J D SPANKUCH 2001 Remote sensing of thethermodynamic state of the atmospheric boundary layer byground-based microwave radiometry ndash J Atmos OceanicTechnol 16 925ndash933

GULDNER J A CHRISTOPH D ENGELBART M EFERRARIO C HERET U LOHNERT F MADONNAD RUFFIEUX C WRENCH Y ZOLL 2009 Towardsthe comparability of microwave observations Results froma temporary profiler network during the WMO cam-paign LUAMI in November 2008 ndash In APITULEY AHWJ RUSSCHENBERG (Eds) Proceedings of the 8thInternational Symposium on Tropospheric Profiling ISBN978-90-6960-233-2 Delft The Netherlands October 2009

HANE CE CL ZIEGLER HB BLUESTEIN 1993 In-vestigation of the dryline and convective storms initiatedalong the dryline Field experiments during COPS-91 ndashBull Amer Meteor Soc 74 2133ndash2145

HEWISON T 2007 1D-VAR retrievals of temperature andhumidity profilesfrom a ground-based microwave radiome-ter ndash IEEE Trans Geosci Remote Sens 45 2163ndash2168

HOCH J P MARKOWSKI 2005 A climatology of spring-time dryline position in the US Great Plains region ndash JClimate 18 2132ndash2137

ILLINGWORTH A J AND CO-AUTHORS 2007 CLOUD-NET Continuous evaluation of cloud profiles in seven op-erational models using ground-based observations ndash BullAmer Meteor Soc 88 883ndash898

KNEIFEL S S CREWELL U LOHNERT J SCHWEEN2009 Investigating water vapor variability by ground-based microwave radiometry Evaluation using airborneobservations ndash IEEE Geosci Rem Sens Letters 6 157ndash161 DOI101109LGRS20082007659

KNUPP K R WARE D CIMINI F VANDENBERGHEJ VIVEKANANDAN ER WESTWATER TCOLEMAN2009 Ground-based passive microwave profiling duringdynamic weather conditions ndash J Atmos Oceanic Technol26 1057ndash1073

LOHNERT U S CREWELL C SIMMER 2004 An in-tegrated approach toward retrieving physically consistentprofiles of temperature humidity and cloud liquid water ndashJ Appl Meteor 43 1295ndash1307

LOTHON M B CAMPISTRON S JACOBI-KOALY F LO-HOU F GIRARD-ARDHUIN 2009 Comparison of radarreflectivity and vertical velocity observed with a scannableC-band radar and two UHF profilers in the lower tropo-sphere ndash J Atmos Oceanic Technol 19 899ndash910

MAHRT L 1976 Mixed layer moisture structure ndash MonWea Rev 104 1403ndash1407

MARTIN L C MATZLER TJ HEWISON D RUFFIEUX2005 Intercomparison of integrated water vapour measure-ments ndash Meteorol Z 15 57ndash64

MASS CF 1991 Synoptic frontal analysis Time for areassessment ndash Bull Amer Meteor Soc 72 348ndash363

MOLLER F K BULLRICH 1961 ThermodynamischeGrundlagen und Arbeitsmethoden der Aerologie ndash InHESSE W Handbuch der Aerologie Akademische Ver-lagsgesellschaft Geest amp Portig Leipzig34

RHEA JO 1966 A study of thunderstorm formation alongthe dryline ndash J Appl Meteor 5 58ndash63

SANDERS F 1999 A proposed method of surface mapanalysis ndash Mon Wea Rev 127 945ndash955

SANDERS F A C DOSWELL 1995 A case for detailedsurface analysis ndash Bull Amer Meteor Soc 76 505ndash521

SCHULTZ DM 2005 A review of cold fronts with pre-frontal troughs and wind shifts ndash Mon Wea Rev 1332449ndash2472

SOLHEIM F JR GODWIN ER WESTWATER Y HANSJ KEIHM K MARSH R WARE 1998 Radiometricprofiling of temperature water vapor and cloud liquid wa-ter using various inversion methods ndash Radio Sci 33 393ndash404

STEPPELER J G DOMS U SCHATTLER H BITZERA GASSMANN U DAMRATH G GREGORIC 2003Meso-gamma scale forecasts using the nonhydrostaticmodel LM ndash Meteor Atmos Phys 82 79ndash93

SUN WY CC WU 1992 Formation and diurnal variationof a dryline ndash J Atmos Sci 49 1606ndash1619

TURNER D D 2007 Improved ground-based liquid wa-ter path retrievals using a combined infrared and mi-crowave approach ndash J Geophys Res 112 D15204doi1010292007JD008530

WARE RH DW FULKER SA STEIN DN ANDER-SON S K AVERY RD CLARK KK DROEGEMEIERJ P KUETTNER JB MINSTER S SOROOSHIAN 2000SuomiNet A real-time national GPS network for atmo-spheric research and education ndash Bull Amer Meteor Soc81 677ndash694

WARE R R CARPENTER J GULDNER J LILJEGRENT NEHRKORN F SOLHEIM F VANDENBERGHE 2003A multi-channel radiometric profiler of temperature hu-midity and cloud liquid ndash Radio Sci 38 8079doi1010292002RS002856

WESTON KJ 1972 The dry-line of northern India and itsrole in cumulonimbus convection ndash Quart J Roy MeteorSoc 98 519ndash531

WESTWATER ER S CREWELL C MATZLER 2005Surface-based microwave and millimeter wave radiomet-ric remote sensing of the troposphere a tutorial ndash IEEEGeosci Remote Sens Soc Newslett 134 16ndash33

ZIEGLER CL TJ LEE RA PIELKE SR 1997 Con-vective initiation at the dryline A modeling study ndash MonWea Rev 125 1001ndash1026

ZIEGLER CL EN RASMUSSEN 1998 The initiation ofmoist convection at the dryline Forecasting issues from acase study perspective ndash Wea Forecast 13 1106ndash1131


In this paper we describe the passage of two dew-point drops as particular mesoscale features similar to adryline traced by MWRP in April 2007 at Lindenbergthat has not been reported so far in Central Europe Ad-ditional local information is used for the analysis fromthe 1290 MHz boundary layer radar wind profiler theKa-band cloud radar and surface instrumentation at thesite Maps of absolute humidity at the surface and of in-tegrated water vapour (IWV) from the German GPS net-work show the development and dimension of this phe-nomenon in space and time

2 Data sets

21 Microwave profiler

The MWRP measures atmospheric brightness temper-atures at five frequencies from 22 to 30 GHz for wa-ter vapour profiling and at seven channels from 51 to59 GHz for temperature profiling respectively The dataare provided with a temporal resolution of about oneminute and are capable to resolve mesoscale featuresremaining often undetected by radiosondes Profiles oftemperature water vapour density and liquid water areretrieved operationally using a neural network (NN) andan observation-based regression method (REGobs) in-cluding additionally zenith infrared and surface temper-ature humidity and pressure sensors The NN is trainedon the basis of radiative transfer calculations using his-torical radiosonde soundings and REGobs applies si-multaneous observations (brightness temperature) of theMWRP and radiosondes from the past to calculate a re-gression operator Neural networks are non-linear meth-ods which provide profiles with a slightly higher verticalresolution The REGobs operator is not affected by errorsin the absorption model or instrumental bias system-atic deviations are considerably reduced In spring thecharacteristic rms error of differences between MWRPretrievals and radiosondes is less than 1 K for tempera-ture up to 1 km height and about 15 K between 1 and3 km height For vapour density the error increases upto 1 g mminus3 at 1 km and declines upward due to thelower amount of water vapour at these levels (GULD-NER and LEPS 2005) Nevertheless ground-based mi-crowave profiling enables monitoring of boundary layerprocesses during nearly all weather conditions in unat-tended mode For this study the operationally archived10-minutes mean values of NN retrievals were usedThere was no reason to use REGobs retrievals

22 Boundary Layer Radar Wind Profiler

Measurements of the mean horizontal wind were per-formed with a 129 GHz pulsed Doppler beam swing-ing Radar Wind Profiler (ENGELBART et al 1996) Thesystem was operated in a single resolution wind modeusing a 5-beam configuration (four tilted beams North

South West East with 75 elevation and one verticalbeam) with a pulse width of 700 ns (corresponding toa radial resolution of 100 m) The received signal wasslightly oversampled in range with a 500 ns gate spac-ing (corresponding to a vertical range of 714 m) Thetransmitted pulse propagates along the different antennadirections which are sequentially switched one after theother A small fraction of the transmitted energy is scat-tered back to the antenna The two main echoing mecha-nisms for L-band Radar Wind Profilers are scattering atinhomogeneities of the refractive index (Bragg scatter-ing) or at precipitation particles (Rayleigh scattering)After signal amplification demodulation and AD con-version the Doppler spectrum of the receiving signaltime series is estimated using standard Fast Fourier tech-niques followed by the computation of the first threemoments of the maximum energy peak signal inten-sity (0 moment) radial velocity (1 moment) and spec-tral width (2 moment) A consensus avaraging of the ra-dial wind estimates over 30 minutes discriminates atmo-spheric signals from noise peak estimates and the re-sulting mean radial velocities are finally combined toobtain the three-dimensional wind vector As usual thisassumes horizontal homogeneity of the wind field in thearea spanned by the oblique beams over the averagingtime period

23 Cloud Radar

The Ka-band cloud radar MIRA 36 is a pulsed Dopplerradar operating at a frequency of 355 GHz to mea-sure vertical profiles of reflectivity Doppler velocityas well as spectral width of meteorological targets ina height range between 150 m and 15 km (GORSDORF

and HANDWERKER 2006) The cloud radar transmitspulses of a length of 200 ns (corresponding to 30 m verti-cal resolution) and receives the signals backscattered bymeteorological targets like cloud droplets drizzle do-plets and ice crystals but also by other particles likeinsects The signal processing contains a Fast Fouriertransformation with 256 FFT points and an averag-ing of 200 spectra (corresponding to an averaging timeof 10 s) Normally the measuring quantities are usedto determine cloud characteristics like cloud base andcloud top as well as droplet distribution or liquid wa-ter content In our case we use the cloud radar to de-termine the vertical velocity in the boundary layer al-though no clouds exist In this case insects serve as tar-gets assuming that they move passively with the windHowever the assumption of passively moving insectsin the wind field is not quite correct Using this non-correct assumption causes some bias in vertical veloc-ity on the order of ndash 02 to ndash 05 m sminus1 for updrafts thatis a reduced updraft and much smaller amounts fordowndrafts as shown by several studies using differentmicrowave frequencies namely 915 MHz windprofilerdata (ANGEVINE 1997) 1238 MHz windprofiler and C-band Doppler radar radar measurements (LOTHON et al













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Documents

Analysis of a dryline-like feature in northern Germany detected by … · 2018-08-05 · Meteorologische Zeitschrift, Vol. 20 , No. 4, 409-421 (August 2011) Article c by Gebru¨der