role of land surface - ecmwf

The role of the land surface in the climate system

Meteorological Training Course Lecture Series

ECMWF, 2002 1

The role of the land surface in the climatesystem

By Pedro Viterbo ([email protected])

European Centre for Medium-Range Weather Forecasts, Shinfield Park, Reading RG2 9AX, United Kingdom

Abstract

Therole of the landsurfacein theclimatesystemis illustrated,with a focuson recentexperienceat theEuropeanCentreforMedium-RangeWeatherForecasts(ECMWF). Global energy and water budgetsare discussedand comparedwith theircounterpartsover theocean,highlightingphysicalmechanismsresponsiblefor their differences.Time scalesassociatedwiththeglobalhydrologicalbudgetarepresented.Usingfield dataandmodelresults,soil moistureis shown to beresponsibleformodulatingthesurface-atmosphereinteractionat a continentalscale,on time scalesrangingfrom thediurnal to theseasonal.After a brief review of theimpactof landsurfaceon weather, threeECMWF casestudiesarepresentedwherea morerealisticrepresentationof landsurfacewascrucialfor theperformanceof theforecastsystem.They correspond,respectively, to theroleof soil moisturein determiningthepositionandintensityof theprecipitationmaximumin anextremeeventof mid-latitudessummer, the role of albedoof thesnow in thepresenceof forestsin springandtheeffect of soil waterfreezingasa thermalregulatorof thesurfacein coldclimates.Finally, theevolutionof thesystematicerrorsin theECMWFforecastsof nearsurfacetemperatureandhumidity is presentedover thelasttenyears;aclearsignatureof changesto therepresentationof landsurfaceprocesses (and other physical processes affecting the energy and water fluxes at the surface) can be found on that record.

Table of contents

1 . Introduction

2 . Surface energy and water budget

3 . Time scales and the role of soil moisture

3.1 Global time scales

3.2 The role of soil moisture

3.3 The interplay between the diurnal and the seasonal time scales

3.4 A schematic view of the role of land surface

4 . Impact of land surface on weather: A brief literature survey

5 . Examples from ECMWF recent experience

5.1 Soil moisture

5.2 Boreal forests

5.3 Soil water freezing: A regulator of cold climates

6 . Conclusions

1. INTRODUCTION

Theclimatesystemis influencedby thelandsurfaceatavarietyof timeandspatialscales.Firstly, theatmosphere


2 Meteorological Training Course Lecture Series

ECMWF, 2002

is in directcontactwith thesurface,whichactsasasourceor sink for theatmosphericenthalpy andmoisture(and

asasinkfor theatmosphericmomentum),via thesurfacesensibleheatflux andevaporation(andthesurfacestress).

Secondly, thesurfaceconditionsactasaregulatorfor importantfeedbackcyclesin theclimatesystem.Thirdly, the

partitioningof netradiationatthesurfaceinto sensibleandlatentheatfluxesdeterminesthesoil wetnessevolution,

whichactsasoneof theforcings– or, at thevery least,amodulator- of low frequency variability. In fact,afterthe

seasurfacetemperature,soil wetnessandsnow massarethemostimportant“memory” mechanismsfor timescales

rangingfrom weeksto seasons.Finally, thesurfaceenergy fluxesdetermineto a largeextent thesurfaceweather

variables,suchasscreen-level temperature,humidity, andwind speedand,to a lesserextent,low-level cloudiness

andprecipitation.Humankindlivesin thelowesttwo-metresof theatmosphereandis directly affectedby theat-

mospheric conditions at the near surface.

Thispaperillustratestheimpactof thelandsurfacein theclimatesystem,with afocusonexamplesbasedonrecent

experienceat theEuropeanCentrefor Medium-RangeWeatherForecasts(ECMWF).Theprimarypurposeof the

paperis to highlight typicalweatherregimes/ecosystemswherethelandsurfaceis of relevanceto theevolutionof

atmosphericvariables,ratherthanpresentinga systematicreview of themultifacetedresearchon surface-atmos-

phere interactions.

Theimportanceof landsurfacein numericalweatherprediction(NWP) stemsfrom a blendof practicalconsider-

ationsandbasicphysicalprinciples.Thosearenotverydifferentfrom thegeneralargumentspresentedin theopen-

ing paragraph,but two key issuesareof paramountimportancefor NWP. Firstandforemost,accurateforecastsof

nearsurfaceweatherparametersarerequestedby theNWPusers’community. Thequalityof suchproductsasthe

diurnal cycle of nearsurfaceair temperatureandhumidity, winds, low level cloudinessandprecipitationis to a

largeextentdeterminedby thephysicalrealismof themodelrepresentationof thesurface-atmosphereinteractions.

Secondly, remotesensingobservationsof theatmospherehaveincreasedimportanceto characterisethestateof the

atmosphereover land.Thesensorsthataresensitive to the lower tropospherecanonly beusedeffectively in the

presence of a good quality background field of skin temperature.

It is importantto emphasisethat,despitethe fact thatdeterministicforecastsof NWP do not extendbeyond two

weeks,themodellandsurfacecontrolsmuchlongertimescales.Thesoil variablesin themodelareinitialisedevery

6 hoursbasedon indirect, imperfectandvery sparseobservations(Douville et al. 2000).Dataassimilationcom-

binesthat informationwith a short-termforecastto getanoptimalestimateof the landsurfaceinitial conditions.

Cycling thecreationof initial conditionsis tantamountto anextendedmodelintegrationnudgedby observations

andextendsthememoryof thesurfacevariablesbeyondtheforecastduration.Therealismof therepresentationof

landsurfaceprocessesis crucialfor handlingcorrectlythememoryproperties,representedby soil moisturein sub-

tropics and mid-latitudes spring and summer and by snow mass in high latitudes and mountainous areas.

Although the importanceof the landsurfacein theclimatesystemhasbeenestablishedwith, amongothers,the

pioneeringwork of Namias(1958),Budyko(1963;1974andreferencestherein),Manabe(1969),Charney(1975)

–seereview by Garratt(1993)- theimpactof landsurfaceonweatherwasonly recentlyrecognised,ascanbeseen

by thescarceliteratureavailable.In Section2 somegeneralremarkson thesurfaceenergy andwaterbudgetwill

bepresented,while Section3 will elaborateon therole of thelandsurface,with emphasison thelong time scales

characteristicof soil water. In Section4 somesurface-atmosphereinteractionmechanismsthatarerelevantfor the

atmosphericsimulationattheregionaltocontinentalscalesarereviewed,basedonnumericalorobservationalstud-

iesthatarerelevantfor NWP. Section5will illustratewhenandwherethelandsurfacecanaffecttheweather, based

on examplesfrom recentexperienceat theEuropeanCentrefor Medium-RangeWeatherForecasts(ECMWF).A

short summary is presented inSection 6.



ECMWF, 2002 3

2. SURFACE ENERGY AND WATER BUDGET

Fig. 1 representsin a schematicform theenergy andwaterbalanceat thelandsurface.Thesurfacealbedo,con-

trolling thefractionof theincidentshortwaveradiationabsorbedby thesurface,dependsonthesoil andvegetation

typeandstateandon theamountof snow present.Thenetlongwave radiation,LW, dependsalsoon propertiesof

thelandsurface,namelythesurfaceemissivity andthesurfaceskin temperature.Sincethenetradiationflux (the

sumof solarandlongwave radiation)is downward,andbecausethe landsurfacehasa small thermalinertia,the

sumof latentandsensibleheatfluxesmustbeanupwardsflux. Note that thesurfacelatentheatflux, LE, in the

energy budget(left panel)is equalto the latentheat,L, timestheevaporationflux, E, in the waterbudget(right

panel),indicatingthatthewateris availableat thesurfacein acondensedphaseandis passedto theatmospherein

the vapour phase. In that process, the surface undergoes evaporative cooling.

As mentionedin theintroduction,thepartitioningof theenergy availableatthesurfaceinto latentandsensibleheat

dependscrucially on thesoil moisture.Vegetatedcoveredsurfaceshave theability to draw waterfrom a depthof

order1m (therootlayer),while for baregroundonly thewaterin thetopfew cmof soil contributesfor evaporation.

Thelatentandsensibleheatfluxes(LE andH, respectively) playadifferentrole for theatmosphere.Sensibleheat

at thebottommeansenergy immediatelyavailableto theatmosphere,andcontributesto theheatingand/ordeep-

eningof theplanetaryboundarylayer(BL), thatshallow portionof theatmospheredirectlyaffectedby thesurface.

Thesurfaceevaporationflux doesnotdirectlyheattheatmosphere,but providesmoistureto theBL or, in thecase

of deepconvection,to thewholetroposphere.In favourableconditions,thatcontributesto precipitationgeneration

mechanisms,with theassociatedreleaseof latentheatinto thewholetroposphere.It is clearthat,whencompared

to thesensibleheatflux, evaporationcanindirectlyleadto averyefficienttransferof energy affectingamuchdeep-

er atmosphericlayer. For anentireatmosphericcolumn,thenet radiative cooling is balancedby energy involved

in phasechangesinsidethecolumn(condensationof watervapourandevaporationof rain) andsensibleheatflux

at the surface. Land surface processes affect directly or indirectly this balance.

Figure 1. Schematicsof surfaceenergy andwaterfluxes.H, LE, LWandSWstandfor surfacesensibleandlatent

heat flux, surface longwave and shortwave radiation flux, respectively; E, P, andY stand for evaporation,

precipitation and runoff. Numbers at the bottom represent averaged values over all land points for the ERA15

reanalysis (1979–1993).

TABLE 1. MEAN SURFACE ENERGY FLUXES IN THE ERA15ATMOSPHERICREANALYSIS

SW LW H LE G0 Bo=H/LE

Land 138 -63 -23 -41 0 0.6

Sea 163 -51 -10 -104 -2 0.1

Surface energy budget

H

LE LW

SW

23 41 63 138 Wm-2

Surface water budget

E

P

Y

0.8 mmd-12.21.4



ECMWF, 2002

Table1summarisesthesurfaceannualmeanfluxesfor the1979-1993periodcoveredby the15yearEuropeanCen-

trefor Medium-RangeWeatherForecasts(ECMWF)reanalysis(ERA15,Gibsonetal. 1997).Valuespresentedare

globalaveragesover landandseaseparately, in W m-2, anddownwardfluxesarepositive.Thenetheatflux, G0, is

thesumof all thesurfacefluxes.Thecontrastsbetweenlandandseaareclear. Evenfor sucha largetime period,

thenetflux is non-zeroover sea,emphasisingthelarger thermalinertiaof theoceans.Thecontinentshave a fast

responsive surfaceandadjusttheir surfacetemperaturein orderto maintaina zero-heatflux at thesurface,while

theoceanshave a muchlargerthermalinertia,with relatively smallvariationsin surfacetemperatureandflux im-

balancesallowed in muchlongertime scales.The last column,the Bowen ratio, Bo, is the ratio of sensibleand

latentsurfaceheatfluxes.Thelargervaluesoverlandareindicativeof therelativelydifficulty of accessingthewater

at thesurface.Overvegetatedsurfaces,thiscorrespondsto thephysiologicalmechanismscontrollingtranspiration

while over bare ground the water directly accessible for evaporation is limited to the top few soil cm.

Surfaceenergy fluxesfor anentireseason(notshown in thetableabove)still balanceout,indicatingthattheenergy

associatedto theseasonalchangesin soil temperaturearenegligible whencomparedto theindividual surfaceen-

ergy fluxes.However, for thesurfacewaterbalanceon a seasonaltime scale,thestorageterm,or changein total

soil watercontent,canbeof similarmagnitudeto theprecipitationor evaporation,which is self-evidentin any ex-

tendeddroughtperiod.In thenext section,wewill discussthedifferenttimescalesregulatingthesurfaceandreg-

ulated by the surface.

3. TIME SCALES AND THE ROLE OF SOIL MOISTURE

3.1 Global time scales

Figure 2. The global water cycle (fromChahine1992). Units of water in reservoirs: 1015 kg; units of water in

fluxes: 1015 kg yr-1.

Fig. 2 representsthesizeof themoisturereservoirs of theterrestrialatmosphereandthemarineatmosphere(rec-

tanglesin thefigure),theexchangesof moisturebetweenthem,andbetweentheatmosphereandthesurfacebelow



ECMWF, 2002 5

(arrows in thepicture).Theseasurfaceevaporatesat thepotentialrate,while over landthereareadditionalmech-

anismsthat reducetheevapotranspirationrate:drynessof thesoil or, over vegetatedareas,physiologicalmecha-

nismsthatcanreduceor shuttranspirationfrom theplant leavesandtrunksmakingthewaterfrom theroot zone

effectively unavailablefor theatmosphereabove.Precipitationover landis abouta quarterof thatover sea.Note

thatprecipitationexceedsevaporationover land,while overseathereverseis true.In orderto haveaclosedbudget

for theterrestrialatmosphere,advectionof moistureacrossaverticalwall projectingover thecontinentboundaries

hasto matchthedifferenceprecipitationminusevaporation.Advectionis roughlyhalf of thewaterevaporatedover

land(seePeixotoandOort 1992for estimatesbasedon radiosondeobservations),suggestinganannualrecircula-

tion ratio (ratioof therainfall comingfrom localevaporationover total rainfall rate)of 67%(71/107).To closethe

hydrologicalcycle, theadvectionhasto bematchedby theriver runoff: theglobalaveragedinflux of freshwater

into theoceanis estimatedin this way as36 x 1015 kg yr-1. For continentalareas,annualrunoff, evaporationand

precipitation are approximately in the ratio 1:2:3.

Rigorousformulationsof theatmosphericbranchof thehydrologicalcyclecanbefoundin Peixoto1973andPeix-

otoandOort1983,1992.So-called“aerologicalestimatesof runoff”, basedonmeasurementsof atmosphericwater

vapourtransportanda closureat thesurface,have beenappliedsuccessfullyto basinswith areaslarger than106

km2 (Rasmusson 1967, 1968, 1971;Peixoto 1973).

Therainfall rateandthesizeof thereservoir canbecombinedto giveatimescale4.5/107= 0.042years= 15days:

theterrestrialatmospherewouldbeemptiedby rainfall in 15days.In asimilarway thereservoir wouldbereplen-

ishedby surfaceevapotranspirationin 23 days(4.5/71years)1. The time scalesassociatedto marinerainfall are

only 7.5days,andthecorrespondingvaluefor evaporationis 6.8days.This suggests(a) a morevigoroushydro-

logical cycle over theocean;(b) a landsurfacecontrolover largetime scales(weeksto months),throughtheeva-

potranspirationflux of waterat the surface.The implication of the above on the extendedpredictabilityof the

atmospheredueto exchangesof waterwith thelandsurfacehasbeendiscussedby many authors(see,e.g.,Namias

1958,Mintz 1984,Dümenil and Bengtsson 1993,Dirmeyer and Shukla 1993).

Theaccuracy of thenumbersshown in Fig. 2 varieswidely: seeChahine1992,for thesourcesusedto produce

theseparticularestimates.Any literaturereview showsavery largedispersionin thosenumbers(see,e.g.,Viterbo

1996):globalestimatesof thetotalcolumnwatervapourcanvaryby asmuchas34%,while runoff estimatesdiffer

by 45%.Independentestimatesof precipitationhavesmallerrangesof uncertainty(notwithstandingtheextensive

areasof theplanetwhereobservationsarevery scarce),but director indirectestimatesof evaporationaresubject

to very large uncertainty.

3.2 The role of soil moisture

In orderto illustratetheroleof soil moisturein shapingtheinteractionsurface-atmosphere,wewill usemodeldata

over theRed-ArkansasRiverbasin,asub-basinof theMississippiRiverbasin.Datais basedon theECMWFrea-

nalysisERA15(Gibsonetal. 1997)usedalsoin Section2.Bettsetal. (1998,1999)havestudiedtheECMWFmod-

el energy andwaterbudgetover theMississippiRiver basin,asdescribedby short-termforecasts.Basin-average

datawasusedfor nineyears,1985–93andanalysedatdifferenttemporalscales;wewill concentratein thissectio

on summertime 5-day average ECMWF data.

1. Theglobalview ontheatmosphericwaterbudgetpresentedin thetext canberegionalised,allowing, undercertainrestrictiveassumptions,an estimate of how much moisture that precipitates comes out from local evaporation versus horizontal transport. For a discussion on the“intensityof thehydrologicalcycle”, i.e.,thetimescalesassociatedto theemptyingandrepleneshingof theatmosphericwaterreservoir atdif-ferent locations on the globe, seeTrenberth (1999).



ECMWF, 2002

Figure 3. (a) Scatterplot of 5-day average evaporative fraction over warm soils (0-7 cm layer soil temperature >

296K) against0-7cmsoil water(SW1). (b) As Fig 3(a),but for pressureheightto thelifting condensationlevel

(PLCL) (from Bettset al. 1998).

We will illustrateherethat theECMWF datashows a similar couplingin themidsummeron the5-daytimescale

betweensoil water, evaporationandlow-level thermodynamicsastheFirstInternationalSatelliteLandSurfaceCli-

matologyProjectField Experiment(FIFE) data2 (BettsandBall 1998)show on thediurnaltime scale.We define

a 5-day mean evaporative fraction as

.

whereH andLE arethe5-dayaveragesof thesurfacesensibleandlatentheatfluxes.Fig. 3 (a) shows thestrong

couplingbetweenEF andthe top layermodelsoil water, SW1.Thedataplottedareall the5-dayvalues(1985-

1993) for which the 0-7 cm mean soil temperatures are > 296 K, representative of the warm months.

Fig.3 (b) showsasimilarcouplingof PLCL (thepressureheightof thelifting condensationlevel,determiningcloud

base)to SW1for thesamedata.Themodelresistanceto evaporationbetweenthesaturatedinteriorof a “leaf” and

thesurroundingair is dependenton soil water, andthis vegetationresistanceis thereforeonekey factorin deter-

miningtheequilibriumsaturationlevel difference,PLCL, in thesaturationpressurebudgetof theBL (BettsandBall

1998).

Wehaveshown 5-dayaverages,but thepatternsandslopesin Figs.3 (a)and(b) aresimilar (but shiftedslightly to

higherPLCL andlowerEF) if the12-hdaytimeaverageareusedinstead.Notethatthelower limit in Fig. 3 (b) (cor-

respondingto very wet soils) is neartheoceanicequilibriumof hPa (e.g.,BettsandRidgway 1989).

The oceanicsurfaceboundaryis saturated,andhasno additionalresistanceof evaporationcorrespondingto the

vegetativeresistanceover land.Figs.3 (a)and(b) areparticularlysignificantbecauseneitherof theserelationships

of EF andPLCL on soil water depend strongly on soil temperature at this warm temperatures.

2. FIFEwasanexperimentmeasuringthedifferentcomponentsof thesurfaceenergy andwaterbudgetof a15x 15km2 of naturalgrasslandlocated in Kansas, US, and heavily instrumented over the period 1987-1989.

(a) (b)

�� +--------------------=

�LCL 60≈



ECMWF, 2002 7

3.3 The interplay between the diurnal and the seasonal time scales

Figure 4. (a) Daytime diurnal cycle of potential temperature (q) and mixing ratio (q) at 2 m from 1145 to 2345

UTC for monthlydry-daycomposites(FIFEaveragesfor 1987);(b) (q,q) plot of surfacedatafor selected28days

from July and August 1987, composited by soil moisture, showing the dependence of mean diurnal cycle on

surface evaporation (fromBettset al. 1996).

Fig. 4 (a) (from Bettset al. 1996)shows thedaytimediurnalcycle of theFIFE 2-m thermodynamicdatafor the

predominantlysunny anddry daysfrom May to October1987.Theaxesarepotentialtemperature(θ) andmixing

ratio (q). This (θ, q) plot canberegardedastheheatandmoisturebudgeton orthogonalaxes(Betts1992).There

are19, 21, 25, 22, 23, and22 datesin eachaveragefrom May to October. Theselectioncriteriawerenear-noon

surfacenetradiationaboveathreshold(whichwas450W m-2 in midsummer, falling to 300W m-2 in October)and

nosignificantdaytimerainfall. Herewecanseethediurnalandseasonalcycletogether. Thepointsareplottedhour-

ly, startingat 1145UTC, shortlyaftersunrisein midsummer. Theseasonalriseandfall of meantemperatureand

mixing ratio canbeseen:July is thewarmestmonth.Octoberis noticeablydrier, afterthevegetationhasdiedand

evaporationis low. Saturationpressurelinesof 970and800hPa areshown dashed.Thesurfacepressureis near

970hPa.It canbeseenthatat themorningminimumtemperature,the2 m air is about30hPa from saturation,ex-

ceptin October, whenit is moreunsaturated.Thediurnalrangeof mixing ratio q is relatively small in all months.

Thereis generallya riseof q in themorning,whentheBL is shallow andcappedby relatively moistair from the

BL of theprecedingday, anda fall in theafternoon,asthegrowing BL entrainsdrier air from higherlevels.May

shows no afternoonfall of q, probablybecauseof thehighersoil moistureandevaporation.May andJunedo not

reachas low afternoonsaturationpressuresas the later monthsof July, August,September, andOctober. This

meansa lower meanlifting condensationlevel (seealsoprevioussection)or cloudbasein spring.Probablythis

reflectsaseasonaldryingof thesurface,althoughchangesin upperair thermodynamicstructuremaybeinvolved.

It is clearthattheafternoonmaximumof equivalentpotentialtemperatureθe is controlledmostlyby theseasonal

shift. Theisoplethsof θe =310,330,350K areshown dotted.Theriseof θe from morningminimumto afternoon

maximum is around 14 K in all months.

Thesumof surfacesensibleandlatentheatfluxesis asurfacesourcefor increasingθe (e.g.,BettsandBall 1998).

This surfaceθe is proportionalto thesumof H+LE, andit is not affectedby theBowenratio. It is entrainmentof

low θeair from above theBL, togetherwith thedeepeningof theBL, thatreducetheBL θe rise,andsofeedback

on boththeshallow andevenmoreimportantlyon precipitatingconvection.Thusoneof theimportantaspectsof

theBL evolutionover landis how largeentrainmentatBL top is. ThedaytimeBL over landis primarily thermally

generated(in strongwinds,shearplaysa role),andthuslinkedto thesurfacevirtual heatflux (which over landis

usuallydominatedby thesensibleheatflux). Henceif thesurfaceBowenratio is large,althoughthesurfaceθeflux

(a)(b)



ECMWF, 2002

maybeunchanged,thelargeH flux drivesmoreentrainment,producesa deeperBL, andthediurnal riseof θe is

reduced.Fig. 4 (b) shows how this diurnal cycle over land dependson soil moistureand,asa consequence,the

surface evaporation.

A total of 28 daysfrom July andAugust1987duringFIFE,which wereaffectedlittle by precipitationor cold air

advection,werecompositedby soil moisture(SM: measuredgravimetricallyin thetop10cm).Thepointsarehour-

ly valuesfrom 1115UTC (nearsunrise)to 2315UTC. Thedry soil composite(SM= 13%,for whichthemeasured

meansurfaceBowenratioatnoonwas0.8)reachesahigherafternoonmaximum K (theθe isoplethsare

shown dotted).In contrast,thewetsoil composite(SM = 23.4%),for whichBowenratioatnoonwas0.4),reaches

a muchcoolerafternoonθ maximum,but a muchwetterq value,sothat theafternoon K. Someof this

shift of θe is associatedwith theshift of theentirediurnalpathof higherq with highersoil moisture,but abouthalf

is theresultof reducedentrainmentof dry low θe into theBL. Overwetsoils,H is muchreducedandtheBL deep-

enslessrapidly. For all the threecomposites,thesurfaceavailablefluxes(net radiationminusgroundheatflux)

werenearlyidentical,sothatthesurfaceθe fluxesweresimilar. This local feedbackbetweensoil moisture,evap-

orationandafternoonθe equilibriumprobablyproduceson largespatialscalesa positive feedbackbetweensoil

moistureandprecipitation,which hasbeenthesubjectof muchresearch(see,for example,Brubakeret al. 1993,

Trenberth1999).Theanalogyover thetropicaloceansis thelink betweenBL andseasurfacetemperature(SST),

whichinfluencestheprevalenceof deepconvectionoverwarmwater. Overlandvariationsin soil moisturecanlead

to aslarge variationsin BL θe aslarger asthoseproducedby several degreesof changein SST. On continental

scales,highersoil moistureandhigherevaporationoverlandwouldleadto ahigherafternoonθemaximumrelative

to thesurroundingoceanandshiftmoreof theglobalprecipitationoverthecontinents(Bettsetal. 1996).Thisfeed-

backhasbeenseenin globalmodels(Mintz 1984).Ontheregionalscale,theMississippifloodcasestudy, present-

edin Section5.1below, suggestedthatthemultipleBLs overtheMidwesternUnitedStatescontrolledthelocation

of precipitation, rather than this mechanism.

3.4 A schematic view of the role of land surface

To concludethis section,a schematicdescriptionof theinteractionsbetweenthesurfaceandtheatmospherewill

bepresented.Inspiredby anearly, muchmorecomplex diagramby Horton(1931),Dooge(1992)(seealsoKuhnel

et al. (1991))summarisedthe interactionbetweenthe landsurfaceandtheatmospherein thepicturereproduced

(with adaptations)in Fig.5 . Thediagramillustratesthebehaviour of thesoil andtheatmospherewithin acomplete

cycle composedof a wet periodfollowedby a dry period.Let usstartjust aftera long episodeof rainfall, point A

in thefigure.Thesoil wateris availablein abundancein theroot layer3 andits evolution is goingto bedetermined

by evaporation.While thesoil hasplentyof water, therateof evaporationis controlledby thenear-surfaceatmos-

phericmoisture:the regime is controlledby theatmosphereandtheevaporationis at thepotentialrate.Below a

certainlevel of soil moisture(pointB in thepicture),physiologicalmechanismswill limit thesupplyof waterfrom

theroot layerinto theatmosphere,andtheevaporationwill dropbelow its maximumvalue(potentialevaporation,

Epot). The regime is undera vegetation(soil) control.Whenprecipitationstarts(pointsC) it will meeta soil dry

enoughduringtheinitial stages,sothatinfiltration (I f, thatpartof waterthatfallsasprecipitationandis effectively

collectedby thesoil for futureuse)will equalprecipitation.Theevolutionof waterin thesoil is oncemoreatmos-

phericcontrolled,via therateof precipitation.Beyonda certainvalue(point D), thesoil doesnot have theability

to soakall precipitation,someof it goesinto runoff. This lastphaseis again soil controlled;thestateof thesoil

determines the rate of infiltration.

Landsurfaceparametrizationshave to representcorrectlythesurfacefluxesandtheevolution of soil moisturein

3. For the purpose of this discussion, field capacity may be considered as the threshold beyond which there is a minimum canopy resistanceto evaporation.

θe 352≈

θe 361≈



ECMWF, 2002 9

all four phasesof thecycle,andto switchfrom theatmosphericcontrolinto thesoil controlregime.Theevolution

of soil moisturewill determinewhenpoint D will occur, andtheevaporationformulationwill determinepoint B.

Thecrucialareas,from thepointof view of theatmosphere,arethequadrantsBC andCD.Duringspringandsum-

mer(wheretheatmosphericdemandcanbevery large),thesystemremainsmuchlongerin thehalf-circumference

BCD than in the opposite part of the cycle.

Figure 5. The hydrological rosette: Schematic depiction of the interaction between the soil hydrology and the

atmosphere (adapted fromDooge (1992)). Epot and If represent potential evaporation and infiltration, respectively.

4. IMPACT OF LAND SURFACE ON WEATHER: A BRIEF LITERATURE SURVEY

Bettsetal. (1996)review theimpactof landsurfacein thecontext of globalnumericalweatherprediction:Diurnal

andseasonalfeedbackloopsarediscussedaswell asfeedbackloopscontrollingtheBL evolution.We will high-

light heretypical mechanismscontrolling the interactionbetweenlandsurfaceandtheatmosphere,over theUS,

Europe, and the tropics.

Earlymodellingefforts (BenjaminandCarlson1986;Laniccietal. 1987)haveshown thesensitivity of precipita-

tion in theUSGreatPlainsto evaporationupstream,in theMexicanPlateau.Thecharacteristicstormenvironment,

leadingto heavy precipitationovertheMidwest,involvesthebreakdown of acappinginversionformedby anover-

lying pre-existing boundarylayer from theMexicanplateau,which overliesthecool moistBL originatingin the

Gulf of Mexico. This complex patternof differentialadvectionimpactson thestrengthof thecappinginversion,

andthestrengthof the inversionis controlledby evaporationupstreamof theprecipitationarea.Lower valuesof

evaporationleadto a strongercappinginversion,andthelow level flow from theGulf will not breakthroughthe

inversionuntil muchfurthernorth.Thelocationandextentof theheavy precipitationassociatedwith theJuly1993

USfloodswasfoundto behighly sensitiveto thecorrectrepresentationof thesemechanismsin theECMWFmodel

(Beljaarset al. 1996;Viterbo and Betts 1999a;Section 5.1).

Spatialgradientsof soil moisturecanalsoenhancethedifferentialheatingmaintainingandreinforcingthesurface

front in apre-stormenvironmentandintensifyingthethermallydirect(ageostrophic)circulation(ChangandWet-

zel 1991;FastandMcCorcle1991).A similar mechanismis active in a cold front associatedwith a severesquall

line developing explosively (Kochet al. 1997).



ECMWF, 2002

Theseasonalityof leafareaindex impactsonthesystematicerrorsof USMidwestlower tropospherictemperature

in summer(Xue et al. 1996;Yang et al. 1994).At a smallerscale,therearemany observationalandmodelling

studiesdemonstratingthe importanceof mesoscalefluxesandsmaller-scaleheterogeneity, speciallyat low wind

speeds.

OverEurope,RowntreeandBolton(1983)demonstratedtheroleof localandnon-localresponseof medium-range

rainfall forecaststo anomaliesin theinitial soil. Themechanismsrelevantto this soil moisture-precipitationfeed-

backwerescrutinisedin Schäretal. (1999)in astudydemonstratingtheimpactof idealised(andlarge)anomalies

of soil wateron theEuropeansummercirculation.Unlike thedesert-albedofeedbackhypothesis(Charney1975),

theEuropeansoil-precipitationfeedbackis notof alarge-scaledynamicalnature,i.e.,it is notassociatedto changes

in large-scaleflow. Precipitationrecycling hasalsoa small role in Europe.Threemainfeedbackloopshave been

identified.Wet soils,associatedwith low Bowenratios,leadto thebuild-up of a shallow BL, concentratingmoist

entropy atlow levelsandgiving highervaluesof convectiveavailablepotentialenergy (CAPE).Additionally, lower

Bowenratiosleadto higherrelativehumidity, loweringthelevel of freeconvection.Finally, apositive feedbackof

radiativeorigin, with increasedcloudcover, but largernetradiativeflux, leadsto largermoistentropy andconvec-

tive instability.

All theexamplesabove referto extratropicsspringandsummerexamples,in snow freesituations.Section5.2be-

low presentsspringexamplesin thepresenceof snow. Thereis little ornoimpactof landsurfaceontheatmospheric

circulation in winter (see Giorgi (1990) andSection 5.3 below).

Despiteanextensive list of publicationsontheroleof land-surfacein thetropicalclimate,thereis scantyevidence

of its impact for short-andmedium-rangeforecasts.A notableexceptionis the work of Walker andRowntree

(1977)whodemonstratedtheroleof enhancedsoil moisturegradientsontheshort-range(1-2daysahead)forecast

of the generation of easterly waves, using a simplified model over West Africa.

5. EXAMPLES FROM ECMWF RECENT EXPERIENCE

5.1 Soil moisture

July 1993showedanomalouslyhigh precipitationover theCentralUSA, with exceptionalfloodingof theMissis-

sippi(Changnon1996).Duringthismonth,thenew versionof theECMWFmodel(CY48)andthethenoperational

version(CY47)4, wererunningin parallelat full resolution(spectraltruncationT213,grid-pointspacing~ 60km),

includingdataassimilation.Beljaarset al. (1996)comparedtheperformanceof the two schemes,looking at the

averageof all one-,two- andthree-dayforecastsverifying between9 and25July. While thedayoneprecipitation

of thetwosystemswereverysimilar, andsimilarto theobservedprecipitation,theforecastsatday3weremarkedly

different.In thenew system,the locationandintensityof themaximumprecipitationwassimilar to theobserva-

tions (40 N, 95 W), while theold systemhadlessthenhalf theprecipitationamountin theareaof theobserved

maximumandhadaspuriousmaximumof precipitationdisplaced800km NE. In theold system,therewasagrad-

ual reductionin precipitationfrom day1 to day3, while thenew systemwasableto bettermaintaintheintensity.

However, evaporationat theareaof maximumprecipitationwassimilar for theold andnew system,andin both

systemstherewasno evidenceof forecastspindown, stronglysuggestingthat the local evaporationwasnot re-

4. ThemaindifferencesbetweenCY47andCY48rely onthesurfaceandboundarylayerprocessesparametrization.Thesoil modelin CY48has 4 layers, with no heat flux and free drainage bottom boundary condition, with soil properties (heat and water conductivities and diffusivi-ties) dependent in a non-linear way on soil moisture (Clapp and Hornberger 1978). CY47 has 2 layers plus 1 climate layer underneath, withconstant water soil properties. CY48 has a smaller roughness length for heat than for momentum, while in CY47 they are identical. For moredetails seeViterbo and Beljaars (1995).



ECMWF, 2002 11

sponsiblefor thedifferencesin precipitation.It turnsout that themaximumof theevaporationdifferencewaslo-

catedover the Mexican Plateau,1000 km SW of the precipitationmaximum,two to threedaysupstreamas

suggestedby backwardstrajectoriesendingupat750hPa,40N, 95W. Themeanthermodynamicprofiles,similar

for day1 forecasts,werevery differentfor day3 forecasts.Theold modelshoweda too strongcappinginversion

above theBL, with air too warmandtoo dry andmuchlower valuesof CAPE.It is clearthat thedifferentialad-

vectionmechanismcharacteristicof theUS Monsoonwasresponsiblefor thedifferencesin precipitation.When

comparedto thenew model,thesoil ontheMexicanPlateauhadmuchlowervaluesof soil moisturein CY47,giv-

ing amuchreducedevaporation,which in turnproducedawarmanddry air massthatcappedtheBL downstream,

inhibiting convection.In CY47, thesoil modelvalueswerestronglyforcedto anerroneous,too dry, climatology:

In suchadatadensearea,atmosphericprofileswereinitiatedto correctvalues,but duringtheforecastthey slowly

felt the influenceof theerroneoussoil moisturevalues.In CY48 thesoil moisturevalueswereinitialisedto field

capacityatthebeginningof July, consistentwith valuesof Juneprecipitationin theareamuchabovenormal.There

wasnoforcingto climatologyin CY48(ViterboandBeljaars1995)andthemodelwascapableof maintaininghigh

valuesof moisturethroughoutJuly. Monthly integrationsperformedwith CY47 andCY48 suggestedthe impor-

tanceof thememoryassociatedto idealisedsoil moistureanomaliesin theinitial conditions(Beljaarsetal. 1996).

The monthly precipitation fields with CY48 compared much better to observations than those of CY47.

Figure 6. Profiles(so-calledtefigrams)of temperature(solid line) anddewpoint (dashedline) with CY47(a)and

CY48(b) of theaveragesverifying from 9 to 25Julyat theforecastrange78hours.Themodellocationis 40N 95

W. (c) shows theaverageverifying analysis.Theshadingindicatestheareawhereaparcellifted from thesurface

hasalowertemperaturethanthesurroundingair, i.e.,theshadedsurfaceareais ameasurefor thestabilityaparcel

has to overcome before convection can occur (fromBeljaars and Viterbo 1999).

CY48surfacemodelranwith predictedsoil moisturethroughoutthefirst half of 1994.It wasclearthataverylarge

near-surfacewarmanddry biasdevelopedovertheNH continentalareasattheendof springandbeginningof sum-

mer. A schemeto initialisesoil waterbasedontheshort-termforecasterrorsof near-surfaceatmospherichumidity

wasdevelopedto overcomethatproblem(Viterbo1996).In orderto testthenew scheme,threecompletedataas-



ECMWF, 2002

similation-forecastexperimentswereranat T213for themonthof June1994:(a) Control(CY48,no assimilation

of soil moisture);(b) As control,but usingtheinitialisedsoil moisturevalues,and;(c) As in (b), but usingaprog-

nosticcloudschemewith muchmorerealisticcloudcover over land(Tiedtke1993).Thenear-surfacewarmand

dry bias,reducedfrom Controlto theinitialisedsoil waterexperiment,in responseto awettersoil. A lower tropo-

sphericwarmbiasdevelopedin theControlmodelandwasgreatlyreducedwheninitialisationof soil wateris used.

Bothexperimentshadtoolittle cloudcoverover landwith toolargesurfaceshortwaveradiativefluxes,but thewet-

ter soil conditionsof experiment(b) managedto maintainevaporationin thefaceof excessive netradiationat the

surface.Thethird simulationdisplayedevensmallerbiases,associatedwith alarger, morerealisticcloudcoverand

smaller radiative biases.

Thealgorithmto initialise soil wateris successfulin controllingmodeldrifts but dampenstheseasonalcycle and

interannualvariationsof evaporationandsoil moisture.Viterbo andBetts(1999a)revisitedtheJuly 1993simula-

tion, usingtheinitial soil wateralgorithmandtheprognosticcloudscheme.Thenew systemgivespoorerresults

for precipitationthanBeljaarset al. (1996).AlthoughmuchbetterthanCY47, thereis a suggestionof northward

displacementandreductionin theprecipitationmaximumin day2 forecasts.It appearsthattheinitialisationof soil

moistureat field capacityat thebeginningof July in Beljaarset al. wascrucial to obtaina goodsimulationof the

excessive rainfall events.

5.2 Boreal forests

Surfacealbedois theprimeregulatorof thenetenergy availableat thesurface.Thealbedoof snow-freelandsur-

facesrangesfrom valuesof 0.1 in foreststo valuesof 0.35over deserts.For areasseasonallycoveredwith snow,

thatrangecanextendupto 0.85.BettsandBall (1997)analysedtheannualcycleof albedoin theBOREASexper-

iment,performedin Canadaduring1994,focussingon thesnow season,comparingseveralmeasurementsiteslo-

catedovergrass,aspenandconiferous.Representativevaluesfor daily averagedalbedoof snow-coveredgrasssites

are0.75,while correspondingvaluesfor theaspenandconifersitesare0.21and0.13,respectively, with valuesas

highas0.4,oneto two daysaftersnowfall. Theloweralbedoof theborealforestsin thepresenceof snow corrob-

oratesdatafrom otherobservationalstudiesandthefew attemptsof makingahemispheric-satellitebasedestimate

of albedo.

TheECMWF modelversionof 1994,at thetimeof theBOREASexperiment,treatedthealbedoof snow covered

areaswith no regard to the landcover: beyonda critical valuefor snow depth,thealbedoof snow coveredareas

wasrarelyoutsidethe0.7-0.8range.As aresult,whencomparedto experimentalresults,netradiationwastoolow

andnear-surfaceair temperaturesweretoocold.A modificationto theschemewasdesignedsuchasthealbedoof

snow-coveredsurfacestendedto theasymptoticvalueof 0.2 in thepresenceof forestsand0.7otherwise(Viterbo

andBetts1999b).Themodifiedschemehasmuchreducedbiasesin temperatureandradiationin thehighlatitudes.

Thecold biasin temperaturein thecontrolschemeextendedin thevertical to thewhole troposphere,increasing

with forecastrangeandaffectingmostcontinentalareas.Thebottompanelof Fig.7 showstheday5 forecasterror

of 850hPa temperature,averagedfor MarchandApril in 1996.Thevery high albedoinducescoolingerrorsex-

ceeding–3K in NorthAmericaand–7K in Asia.Thetoppanelshowsthecorrespondingfigurefor 1997.In spring

1997,with thenew snow albedoscheme,thecoldbiasover theborealforesthasbeenalmosteliminated.Thenew

schemealsoimproved thequality of themedium-rangeforecasts,asevidencedby betterscoresof 500hPa geo-

potential fields.



ECMWF, 2002 13

Figure 7. Comparison of the average 5-day forecast temperature errors at 850 hPa, for the ECMWF operational

model during March-April 1996 (bottom) and 1997 (top) (fromViterbo and Betts 1999b).

Theforecastresultsabove corroboratethestudyof ThomasandRowntree(1992)on therole of theborealforests

in conditioningtheclimateat high latitudes.Thespringmonthsof two five-yearexperiments,thefirst with a (re-

alistic) snow albedoandthesecondwith thehigh latitudeforestsremovedarecompared.Thelatterexperimentis

colderthantheformerin thecontinentalareasnorthof 50N. PielkeandVidale(1995)suggestedthattheboundary

betweentundraandborealforestsis a regionof enhancedhorizontaltemperaturegradients,actingasapre-condi-

tioner for baroclinicinstability and“locking” the climatologicalpositionof the polar front. In analysingfurther

refinementsto theECMWFsnow model,vandenHurk etal. (2000)show that(a)simulatingtheborealforestcon-

trol on evaporationin spring(reducedtranspirationfrom frozensoils) and(b) increasingthe runoff over frozen

soils, improves the agreement of model results with observations.

5.3 Soil water freezing: A regulator of cold climates

TheoperationalECMWF forecastsfor thewintersof 1993-94throughto 1995-96showeda tendency to produce

a cold biasover continentalareasin winter. This error wasparticularlysevereduring the winter of 1995-96for

Scandinavia, a yearcharacterisedby reducedsnow amountsand,consequently, larger thermalcouplingbetween

thesoil andtheatmosphereabove. Viterbo et al. (1999)diagnosedtwo mainproblemscontributing to thaterror.

Firstly, theenergy involved in phasechangesin thesoil wasnot taken into account.Whenpositive temperatures

approach0 C, asubstantialamountof theexternalcoolingdemand(i.e., infraredcooling)is usedto freezethesoil



ECMWF, 2002

water, therebydecreasingtherateof soil cooling;a similar effect occursin melting.Soil waterphasetransitions

actasa thermalbarrier, increasingthesoil inertiaat temperaturescloseto 0 C. Secondly, thedownwardsensible

heatflux, prevailing in winter conditions,wastoo small, leadingthe modelinto a positive feedbackloop where

coolingreducedtheheatflux, andmadethesoil evencolder. Modelchangesweredesignedto incorporatethemiss-

ingphysicalmechanisms.Separateseasonalintegrationswith bothmodelschemesrevealedagreatlyalleviatedsoil

andnear-surfaceatmosphericcoolingdrift: Screen-level temperaturesreducedfrom –10C to closeto zero.Despite

theconsiderablewarmingin themodelsoil andnear-surfacewinter climatein continentalareas,therewasnegli-

gible impacton thefreeatmospheretemperatureandtheatmosphericflow. In winter, stablesituations,theatmos-

phereis decoupledfrom thesurfaceandchangesat thesurfacedo not propagateupwards,unlessthey affect the

momentum budget.

6. CONCLUSIONS

Theexamplespresentedabovehaveshown thatlandsurfacecanhaveasignificantimpactontheatmosphereat the

synoptic/continentalscalewhenit affectsthepartitioningof thesurfaceenergy into sensible/latentheat,via thesoil

water. Thiseffect canbelocalor non-local.On theotherhand,landsurfacehasasignificantimpacton theatmos-

phereat thesynoptic/continentalscalewhenit affectsthenetenergy at thesurface,e.g.,changeof thealbedoin

snow-coveredareasin spring.However, in winter, staticallystable,conditionsthelandsurfaceis decoupledfrom

theatmosphere:Largevariationsin surfacetemperatureaffect only thelowesthundredmetresof theatmosphere

and do not have a significant impact on the circulation.

RecentexperienceatECMWF, summarisedabove,showsthatforecastssystemsaresensitiveto misrepresentations

of longertimescalesin theland-surface/atmosphereinteraction.Progresscanonly bemadewith sustainedefforts

on validationof themodelcomponents.Dataassimilation/forecastsystemsareideal toolsto validateparameteri-

sationsbecauseof their constantconfrontationwith observations(in a “perfect synoptics”scenario)anda very

largecommunityof usersrequiringaccuratediurnalcycleof weatherparameters.In thiscontext, GEWEX(Global

Energy andWaterExperiment)ContinentalScaleExperimentsandtheCoordinatedEnhancedObservationPeriod

plannedfor 2002will play a prominentrole. If thesurfaceandupper-air observationsarrive at theNWP centres

they canbeassimilated.Analysisandshorttermforecastsfrom thosecentresarethebesthopeof having acoherent

picture of surface and atmosphere variables and of the surface fluxes.



ECMWF, 2002 15

Figure 8. Historyof monthlybiases(thick lines)andstandarddeviations(thin lines)with respectto observations

of thedaytime(72-hour:redline) andnight-time(60hour:blueline) operationaltwo-metretemperatureforecasts,

averaged for all available surface stations in the European area (30º N to 72º N and 22º W to 42º E).

An importantfeedbackloop in thesurface-atmosphereinteractionis thelink betweentheanomaliesof soil mois-

tureandprecipitation.Its correctrepresentationin themodelrelieson the interplayof severalparameterisations:

evaporationandsoil moisture,boundarylayer processes,cloudsandconvection.Most NWP modelshave their

poorestscoresonprecipitationin thetropicsandspring/summerextratropicsandimprovementsontheunderstand-

ing of thesoil moisture-convectioninteractionwill mostlikely greatlyalleviatethatdeficiency. Validationof model

resultsagainstresultsfrom theLargeScaleBiosphere-AtmosphereExperimentin Amazonia(LBA) (seeforthcom-

ing SpecialIssueof J. Geophys.Res.2001)observationsof thediurnalcycleof precipitationandatmosphericther-

mal and humidity profiles over the Amazon River Basin will probably be a key for progress in that area.

Probablythebestsummaryof theimpactof landsurfaceonweathercanbeshown onFig.8 , displayingthehistory

of ECMWF operationalshort-rangeforecasterrorsof 2 m temperatureover Europeasa time-seriesof monthly

averages.Theseerrorsshow alargeannualcycle,aredifferentfor nightandday(72and60hourforecastsverifying

at12and00UTC, respectively), andhavearich historyof themany modelchangesthatweremadeovertheyears.

We will discuss only the model changes made from 1993 onwards.

In August1993,asurfaceschemewith aclimatologicaldeep-soilboundaryconditionfor temperatureandmoisture

wasreplacedby thefree-runningfour-layerscheme(ViterboandBeljaars1995),but theimpactisnotveryobvious.

The summerdaytimebiasof August1993wassmallerthanthat of the previous yearbut, at that time, the soil

schemehasbeenrunningfreely for only two-months(includingtheJuly paralleltestdescribedearlier).Thenext

summershowedapronouncedwarmbiasrelatedto agradualdryingoutof thesoil whichwasreducedin July1994

by resettingthesoil moistureto field capacityover vegetatedareas.A simplesoil moistureanalysisschemewas

introducedin December1994(Viterbo1996)with aclearbeneficialimpacton thedaytimebiasfor summer1995.

Thenight-timetemperatureshave beenbiasedcold for many years,relatedto anoverly largeamplitudeof thedi-



ECMWF, 2002

urnalcycle.Thewinterof 1995/1996wasparticularlybad,mainlybecausetheEuropeanareawasblockedfor most

of thewinter with easterlywindsandvery cold temperatures,althoughchangesto thecloudschemeor theoro-

graphicdragmighthavehadanegativeimpactonnight-timetemperatures.It is interestingthatthereductionof the

daytimebiasactuallyincreasedthenight-timebiasby displacingtheentirediurnalcycle to coldertemperatures.

Soil freezingandincreasedboundarylayerdiffusionin stablelayers,introducedin September1996,improvedthe

monthlyerrorstatisticsconsiderably. Thewintertimebiaswaslargelyeliminatedandtheamplitudeof thediurnal

cycle wasdown to a reasonablelevel. Thesnow albedoreductiondescribedin SectionBorealforestswasintro-

ducedin December1996,but its impactis notclearoverEurope,dueto therelatively smallareacoveredby snow

andtheoverallmagnitudeof theerrorslinkedto theexcessivesoil cooling,correctedthreemonthsearlier. Finally,

a muchmoreselective way of initialising soil moisture(Douville et al. 2000),introducedin April 1999,might be

responsiblefor aslightreductionof thestandarddeviationof temperatureerrorsin thatyear. A new surfacescheme

wasintroducedin June2000(vandenHurk etal. 2000),but it is tooearlyto assessits operationalperformance.It

is fair to point out that thestatisticspresentedin Figure8 areaveragedover a monthandover a largearea.The

errorsonaday-to-daybasiscanstill belarge,but arelesssystematicandareoftenrelatedto errorsin theforecasted

clouds or the presence of snow.

ACKNOWLDEGMENTS

Thisnotesummariseswork doneatECMWFduringthelasttenyears,in closecollaborationwith many colleagues,

in particularAnton Beljaarsand Alan Betts. Furthermore,the different contributions of Martin Miller, Jean-

FrançoisMahfouf, JoãoTeixeira,andEric Woodaregratefullyacknowledged.Part of this work will alsoappear

as a book chapter to be published by Springer-Verlag in 2002 (Viterbo and Beljaars 2002).

REFERENCES

Beljaars, A.C.M., P. Viterbo,M. Miller, andA.K. Betts,1996:Theanomalousrainfall over theUSA duringJuly

1993: Sensitivity to land surface parametrization and soil moisture anomalies.Mon. Wea. Rev., 124, 362-383.

Beljaars, A.C.M., andP. Viterbo,1999:Soil moisture-precipitationinteraction:Experiencewith two landsurface

schemesin theECMWF model.Global energy andwatercycles.K. Browning andR. Gurney, Eds.,Cambridge

University Press, Cambridge, 223-233.

Benjamin, S.G.,andT.N. Carlson,1986:Someeffectsof surfaceheatingandtopography on the regionalsevere

storm environment. Part I: Three-dimensional simulations.Mon. Wea. Rev., 114, 307-329.

Betts, A.K., 1992: FIFE atmospheric boundary layer methods.J. Geophys. Res., 97D, 18,523-18,531.

Betts, A.K., and J.H. Ball, 1997: Albedo over the boreal forest.J. Geophys. Res., 102D, 28,901-28,909.

Betts, A.K., andJ.H.Ball, 1998:FIFEsurfaceclimateandsite-averagedatasets1987-1989. J. Atnos.Sci., 55, 1091-

1108.

Betts, A.K., J.H.Ball, A.C.M. Beljaars,M.J.Miller andP. Viterbo,1996:Thelandsurface-atmosphereinteraction:

A review based on observational and global modeling perspectives.J. Geophys. Res., 101D, 7209-7225.

Betts, A.K., J.H.Ball andP. Viterbo,1999:Basin-scalewaterandenergy budgetsfor theMississippifrom theEC-

MWF reanalysis.J. Geophys. Res., 104D, 19,293-19,306.

Betts, A.K., andW.L. Ridgway, 1989:Climatic equilibriumof theatmosphericconvective boundarylayerover a

tropical ocean.J. Atmos. Sci., 46, 2621-2641.

Betts, A.K., P. Viterbo,andE.F. Wood,1998:Surfaceenergy andwaterbalancesfor theArkansas-Redriverbasin



ECMWF, 2002 17

from the ECMWF reanalysis.J. Climate, 11, 2881-2897.

Brubaker, K.L., D. Entekhabi,andP.S.Eagleson,1993:Estimationof continentalprecipitationrecycling. J. Cli-

mate, 6, 1077-1089.

Budyko, M.I., 1963:Atlasof theheatbalanceof theEarth (in Russian).GlobnaiaGeofiz.Observ., Moscow, 69pp.

Budyko, M.I., 1974:Climate and Life. Academic Press, 510 pp.

Chahine, M.T., 1992: The hydrological cycle and its influence on climate.Nature, 359, 373-380.

Chang, J.-Y., andP.J.Wetzel,1991:Effectof spatialvariationsof soil moistureandvegetationon theevolutionof

a prestorm environment: A numerical case study. Mon. Wea. Rev., 119, 1368-1390.

Changnon, S.A., (Ed.), 1996:The great flood of 1993. Westview Press, xii+324 pp.

Charney, J.G.,1975:Dynamicsof desertsanddroughtsover theSahel.Quart.J. Roy. Meteor. Soc., 101, 193-202.

Clapp, R.B.,andG.M. Hornberger, 1978:Empiricalequationsfor somesoil hydraulicproperties.WaterResources

Res., 14, 601-604.

Dirmeyer, P. andJ. Shukla,1993:Observationandmodellingstudiesof the influenceof soil moistureanomalies

on atmospheric circulation.Prediction of interannual climate variations. J. Shukla, Ed., Springer, Berlin, 1-23.

Dooge, J.,1992:Sensitivity of runoff to climatechange:A Hortonianapproach. Bull. Amer. Meteor. Soc., 73, 2013-

2024.

Douville, H., P. Viterbo,J.-F. Mahouf,andA.C.M. Beljaars,2000:Evaluationof theoptimuminterpolationandthe

nudging techniques for soil moisture analysis using FIFE data. Mon. Wea. Rev., 128, 1733-1756.

Dümenil, L., andL. Bengtsson,1993:Observationandmodellingstudiesof theinfluenceof landsurfaceanomalies

onatmosphericcirculation(futuredirections).Predictionof interannualclimatevariations. J.Shukla,Ed.,Spring-

er, Berlin, 25-47.

Fast, J.D.,andM. D. McCorcle,1991:Theeffectof heterogeneoussoilmoistureonasummerbarocliniccirculation

in the Central United States.Mon. Wea. Rev., 119, 2140-2167.

Garratt, J.,1993:Sensitivity of climatesimulationsto landsurfaceandatmosphericboundary-layertreatments—

A Review. J. Climate, 6, 419-449.

Gibson, J.K.,P. Kallberg, S.Uppala,A. Hernandez,A. Nomura,E. Serrano,1997:ERA description.ECMWFRe-

Analysis Project Report Series, 1, 72 pp.

Giorgi, F., 1990:Sensitivity of wintertimeprecipitationandsoil hydrology simulationover the westernUnited

States to lower boundary specification.Atmosp.-Ocean, 28, 1-23.

Horton, R.E.,1931:Thefield,scopeandstatusof thescienceof hydrology. Trans.Amer. Geophys.Union, 12, 189-

202.

Koch, S.E.,A. Aksakal,J.T. McQueen,1997:Theinfluenceof mesoscalehumidity andevapotranspirationfields

on a model forecast of a cold-frontal squall line.Mon. Wea. Rev., 125, 384-409.

Kuhnel, V., J.C.I.Dooge,J.P.J.O’Kane,andR.J.Romanowicz, 1991:Partialanalysisappliedto scaleproblemsin

surfacemoisturefluxes.Land Surface-Atmosphere interactionsfor climatemodeling. Observation,modelsand

analysis. E.F. Wood, Ed., Kluwer, Dordrecht, 221-247.

Lanicci, J.M., T.N. Carlson,andT.T. Warner, 1987:Sensitivity of the GreatPlainssevere-stormenvironmentto

soil-moisture distribution.Mon. Wea. Rev., 115, 2660-2673.



ECMWF, 2002

Manabe, S., 1969:Climateandthe oceancirculation.1. The atmosphericcirculationandthe hydrology of the

earth’s surface.Mon. Wea. Rev., 97, 739-774.

Mintz, Y., 1984.Thesensitivity of numericallysimulatedclimatesto land-surfaceboundaryconditions.GlobalCli-

mate. J. Houghton, Ed., Cambridge University Press, 79-105.

Namias, J.,1958:Persistenceof mid-troposphericcirculationsbetweenadjacentmonthsandseasons.Theatmos-

phere and sea in motion (Rossby memorial volume). B. Bolin, Ed., Rockefeller Institute Press, 240-248.

Peixoto, J.P., 1973:Atmosphericvapourflux computationsfor hydrologicalpurposes.WMO Publ.357,World Me-

teorological Organization, Geneva, Switzerland, 83 pp.

Peixoto, J.P., andA. Oort,1983:Theatmosphericbranchof thehydrologicalcycle andclimate.Variationsof the

global water budget. Street-Perrott, A., M. Beran, and R. Ratcliffe, Eds., Reidel, London, 5-65.

Peixoto, J.P., and A. Oort, 1992:Physics of climate. American Institute of Physics, New York, 520 pp.

Pielke, R.A., andP.L. Vidale,1995:Theborealforestandthepolarfront. J. Geophys.Res., 100D, 25,755-25,758.

Rasmusson, E.M., 1967:Atmosphericwatervaportransportandthewaterbalanceof NorthAmerica.Part I: Char-

acteristics of the water vapor flux field.Mon. Wea. Rev., 95, 403-426.

Rasmusson, E.M., 1968: Atmosphericwater vapor transportand the water balanceof North America.Part II:

Large-scale water balance investigation.Mon. Wea. Rev., 96, 720-734.

Rasmusson, E.M., 1971:A studyof thehydrologyof easternNorthAmericausingatamosphericwatervapordata.

Mon. Wea. Rev., 99, 119-135.

Rowntree, P.R.,andJ.A.Bolton,1983:Simulationof theatmosphericresponseto soil moistureanomaliesoverEu-

rope.Quart. J. Roy. Meteor. Soc., 109, 501-526.

Schär, C.,D. Lhthi, U. Beyerle,andE.Heise,1999:Thesoil-precipitationfeedback:A processstudywith aregion-

al climate model.J. Climate, 12, 722-741.

Thomas, G., and P.R. Rowntree, 1992: The boreal forests and climate.Quart. J. Roy. Meteor. Soc., 118, 469-497.

Tiedtke, M., 1993: Representation of clouds in large-scale models.Mon. Wea. Rev., 121, 3040-3061.

Trenberth, 1999:Atmosphericmoisturerecycling: Roleof advectionandlocal evaporation.J. Climate, 12, 1368-

1381.

vandenHurk, B.J.J.M.,P. Viterbo,A.C.M. Beljaars,andA.K. Betts,2000:Offline validationof theERA40surface

scheme. ECMWF Tech. Mem. 295, 42 pp.

Viterbo, P., 1996:Therepresentationof surfaceprocessesin generalcirculationmodels.Ph.D.Thesis,University

of Lisbon, 201 pp. Available from the author, ECMWF, Shinfield Park, Reading RG2 9AX, England.

Viterbo, P., andA.C.M. Beljaars,1995:An improvedlandsurfaceparametrizationschemein theECMWF model

and its validation.J. Climate, 8, 2716-2748.

Viterbo, P., andA.C.M. Beljaars,2002:Impactof landsurfaceonweather. Vegetation,water, humansandthecli-

mate:A new perspectiveonan interactivesystem.M. Claussen,P. Dirmeyer, J.H.C.Gash,P. Kabat,M. Meybeck.

R. Pielke, Sr., and C. Vörösmarty, Eds. Springer-Verlag, to appear.

Viterbo, P., A.C.M. Beljaars,J.-F. Mahfouf,andJ.Teixeira,1999:Therepresentationof soil moisturefreezingand

its impact on the stable boundary layer. Quart. J. Roy. Meteor. Soc., 125, 2401-2426.

Viterbo, P., andA.K. Betts,1999a:Theimpactof theECMWF reanalysissoil wateron forecastsof theJuly 1993



ECMWF, 2002 19

Mississippi flood.J. Geophys. Res., 104D, 19,361-19,366.

Viterbo, P., andA.K. Betts,1999b:Theforecastimpactof thealbedoof theborealforestsin thepresenceof snow.

J. Geophys. Res., 104D, 27,803-27,810.

Walker, J., andP.R. Rowntree,1977:The effect of soil moistureon circulationandrainfall in a tropical model.

Quart. J. Roy. Meteor. Soc., 103, 29-46.

Xue, Y., M.J.Fennessy, andP.J.Sellers,1996:Impactof vegetationpropertiesonU.S.Summerweatherprediction.

J. Geophys. Res., 101D, 7419-7430.

Yang, R.,M.J.Fennessy, andJ.Shukla,1994:Theinfluenceof initial soilwetnessonmedium-rangesurfaceweath-

er forecasts.Mon. Wea. Rev., 122, 471-485.

role of land surface - ecmwf

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