Spatial and Temporal Variability of Antarctic Precipitation from Atmospheric Methods

34 Pages·1998·4.7 MB·English

334 JOURNAL OF CLIMATE VOLUME11
Spatial and Temporal Variability of Antarctic Precipitation from Atmospheric
Methods*
(cid:49)
RICHARD I. CULLATHER AND DAVID H. BROMWICH
PolarMeteorologyGroup,ByrdPolarResearchCenter,OhioStateUniversity,Columbus,Ohio
MICHAEL L. VAN WOERT
OfficeofResearchandApplications,NOAA/NESDIS,CampSprings,Maryland
(Manuscriptreceived9January1997,infinalform23June1997)
ABSTRACT
The spatial and temporal variability of net precipitation (precipitation minus evaporation/sublimation) for
AntarcticaderivedfromtheEuropeanCentreforMedium-RangeWeatherForecastsoperationalanalysesviathe
atmosphericmoisturebudgetisassessedincomparisontoavarietyofglaciologicalandmeteorologicalobser-
vationsanddatasets.Forthe11-yrperiod1985–95,theaveragecontinentalvalueis151mmyr(cid:50)1waterequivalent.
Largeregionaldifferenceswithotherdatasetsareidentified,andthesourcesoferrorareconsidered.Interannual
variabilityintheSouthernOceanstormtracksisfoundtobeanimportantmechanismforenhancedprecipitation
minusevaporation(P(cid:50)E)inbotheastandwestAntarctica.Inrelationtothepresentfindings,anevaluation
oftherawinsondemethodforestimatingnetprecipitationineastAntarcticaisconducted.Estimatesof P(cid:50)E
usingsyntheticrawinsondesderivedfromtheanalysesarefoundtocomparefavorablytoglaciologicalestimates.
Asignificantupwardtrendof2.4mmyr(cid:50)1isfoundfortheAntarcticcontinentthatisconsistentwithfindings
fromtheNationalCentersforEnvironmentalPrediction,formerlytheNationalMeteorologicalCenter,andthe
NationalCenterforAtmosphericResearchReanalysisprecipitationdataset.Despitelargeregionaldiscrepancies,
thegeneralagreementonthemainfeaturesofAntarcticprecipitationbetweenstudiessuggeststhatathreshold
hasbeenreached,wheretheassessmentofthesmallertermsincludingevaporation/sublimationanddriftsnow
lossisrequiredtoexplainthedifferences.
1. Introduction much longer timescales (Fortuin and Oerlemans1990).
Additionally, precipitation in polar regions has been
Precipitation over Antarctica is recognized asanim-
forecast to increase with potential increases in global
portant climatic variable (Bromwich 1990).Therateof
temperature (Kattenberg et al. 1995); hence, the mon-
accumulation of snow and ice is necessaryinformation
itoring of the cryosphere is an important componentof
for the assessment of the stability and motion of the
detecting global change.
Antarctic ice sheets, which in turn play an important
Difficulties in obtaining accurate estimates of Ant-
role in the global sea level budget. The annual precip-
arctic precipitation have been documented (Bromwich
itation over the ice sheets may be thought of in terms
1988). Direct in situ gauge measurements are compli-
of equivalent sea level decrease, which has been esti-
mated to be large ((cid:59)8 mm yr(cid:50)1) in relation to current cated by wind biases and the presence of an unlimited
estimations of global sea level rise ((cid:59)1 to 3 mm yr(cid:50)1) snow field. The introduction of blowing snow creates
(Warrick et al. 1995). The eustatic impact of the ice the problem of distinguishing snow that has been pre-
sheets arises because precipitation may vary rapidly cipitated from that which has been picked up by the
over time, while the ice-sheet response occurs over wind and transported. Additionally, over the interior
Antarctic plateau, snowfall amounts are less than the
minimum gauge resolution. Attempts to correct gauge
values for wind bias have been made on a global basis
*ByrdPolarResearchCenterContributionNumber1057. (e.g., Legates and Willmott 1990). The quality of the
(cid:49)
Currentaffiliation: Atmospheric SciencesProgram,OhioState
corrected precipitation depictions for the Antarctic has
University,Columbus,Ohio.
not been assessed, however.
In contrast togaugemeasurements,accumulationes-
Corresponding author address: David H. Bromwich, Polar Me- timates derived from glaciological methods are gener-
teorologyGroup,ByrdPolarResearchCenter,OhioStateUniversity,
allystraightforwardandconsideredreliable,dueinpart
Columbus,OH43210-1002.
E-mail:[email protected] to the variety of methods available which may then be
(cid:113)1998AmericanMeteorologicalSociety
MARCH1998 CULLATHER ET AL. 335
FIG.1.Antarcticcontinent showingrawinsondestationsineastAntarctica(filledcircles),Roi
BaudoinStation(notoperationalsincethe1960s,opencircle)andcontoursoftheDrewry(1983)
elevationdataset.
intercompared(Schwerdtfeger1984).Thedisadvantage Inthispaper,weexpandonresultspresentedinBrom-
here is the lack of an adequate and uniform temporal wich et al. (1995) by examining the spatial represen-
resolution for large areas of the continent. As a result, tation of net precipitation (precipitation minus evapo-
only thelong-termsynthesizeddepictionsofthespatial ration/sublimation)derivedfromtheatmosphericmois-
variability in accumulation are presently available ture budget of the European Centre forMedium-Range
(Giovinetto and Bentley 1985; Giovinetto and Bull Weather Forecasts (ECMWF) analyses. A useful exer-
1987), with theinterannualvariabilityandtrendsavail- cise is to intercompare fields derived fromatmospheric
able at some point locations (e.g., Thompson et al. numerical methods with synthesized observational da-
1995). tasetsfromglaciologicalanddirectmeasurementmeth-
Because of this limitation, additional methods using ods. An intercomparison offers a means of validation
atmospheric techniques have been examined, such as aswellasaqualitativemeasureofhowwellthevarious
the derived moisture budget from rawinsonde data fieldsareknown.Severalobservationalandderivedda-
(Bromwich 1979, 1988; Bromwich and Robasky 1993; tasets are introduced for comparison. An appraisal of
Connolley and King 1993). In recent years, the en- the spatial depiction and regionalvariabilityofferedby
hancementofmeteorologicaldataassimilationmethods, available atmospheric methods is relevant to other ef-
includingsatellitedata,hasledtotheuseofatmospheric forts in examining Antarctic ice sheet mass balance,
numericalanalysesandmodelsforthestudyofAntarctic includingtheuseofsatellitealtimetrydataandpotential
precipitation and its variability (e.g., Howarth 1986; glaciological field studiessuchasITASE(International
Masuda 1990; Yamazaki 1992, 1994; Arpe and Cattle Trans-AntarcticScientificExpedition,Mayewski1996).
1993;Bromwichetal.1995;Buddetal.1995;Genthon This evaluation is also of interest to modelers in as-
and Braun 1995; Reid and Budd 1995; Connolley and sessing the quality and sources of available validation
King 1996; Walsh and McGregor 1996; Ohmura et al. data. Some of the key issues to be addressed by this
1996). study are
336 JOURNAL OF CLIMATE VOLUME11
FIG.2.GiovinettoandBentley(1985)long-termaccumulationdistribution:(a)asdrawninw.e.(waterequivalent)unitsof100mmyr(cid:50)1;
(b)digitizedinmmyr(cid:50)1w.e.
(cid:82) What are the qualitative and quantitative differences sublimation). In estimating ice sheet mass balance, the
in the spatial distributions of the datasets? arealaccumulationrateisbalancedagainsticebergcalv-
(cid:82) WhatistheannualandinterannualvariabilityofAnt- ing and basal melting at the bottom of ice shelves. A
arctic precipitation on regional scales? review of these terms is given by Jacobs et al. (1992).
(cid:82) Are the causes for the observed variations in the at- Thedominanttermin(1)isprecipitation,and,toafirst
mospheric moisture budget apparent? order, the spatial distributions of B, P, and P (cid:50) E have
been thought to be comparable (Bromwich 1988), al-
though the magnitudes are known to be different (e.g.,
2. Previous study
Stearns and Weidner 1993).
Accumulation and precipitation are related using SincetheInternationalGeophysicalYear(IGY,1957–
(Bromwich 1988) 58), the estimation of accumulation has been part of
(cid:94)B(cid:38) (cid:53) (cid:94)P(cid:38) (cid:50) (cid:94)E(cid:38) (cid:50) (cid:94)D(cid:38) (cid:50) (cid:94)M(cid:38), (1) mostglaciologicalexpeditionsintheAntarctic.Several
recentestimatesfromSouthPole,DomeC,PlateauRe-
where angled brackets represent an areal average and mote, and Wilkes Land are summarized by Thompson
theoverbarrepresentsatimeaverage,Bisaccumulation, et al. (1995). Although year-to-year variability is not
P is precipitation, E is the net of sublimation minus presented, the combined results imply an increase in
deposition of hoarfrost, D is the divergence of snow accumulationduringthelast30yr.Enomoto(1991)has
drift, and M is the divergence of meltwaterrunoff.The reviewed accumulation data from ice cores and snow
first two terms on the right-hand side are hereafter re- pits at eight Antarctic stations. Possible relations be-
ferred to as net precipitation (precipitation minus net tween the long-term variability in snow accumulation
MARCH1998 CULLATHER ET AL. 337
FIG.2.(Continued)
and midlatitude sea level pressure were evaluated. A relatively large values for the east Antarcticcoastales-
relation between precipitation gauge measurementsfor carpment and low elevations in west Antarctica, to es-
the Antarctic Peninsula region and the semiannual os- sentially desert-likeconditionsovertheinteriorplateau
cillation of the Antarctic circumpolar trough has been of east Antarctica.Forreference,Fig.1showstheAnt-
describedbyvanLoon(1972;seealsovanLoon1967). arctic continent contoured using the Drewry elevation
Thisrelationwasalsofoundinacomprehensivereview dataset (Drewry 1983). The most recent accumulation
of variability in the observed precipitation frequency compilation (Giovinetto and Bentley 1985; hereafter,
for the Antarctic Peninsula region given by Turner et GB85), shown in Fig. 2a, isamanualsynthesisofover
al. (1997). 1200 data points. It differs from previous work in that
Numerous authors have previously synthesized gla- thelowestaccumulationvaluesoflessthan50mmyr(cid:50)1
ciological data into a single, long-term annual accu- extendoveramuchlargerareathanhasbeenpreviously
mulation depiction for the Antarctic continent. Com- depicted (Bromwich 1988). This figure has been fre-
pilations produced over the 1960–85 period are dis- quently used for model and analyses validation(Tzeng
cussedbyGiovinettoandBull(1987)andhaveevolved etal.1994;Buddetal.1995;ConnolleyandKing1996;
considerably as the number of available observations Ohmura et al. 1996). In Fig. 2b, the GB85 depiction
has increased. These depictions generally indicate a has been digitized by assigning the contour values to
strongdependenceofaccumulationonelevation—from 0.5(cid:56) (cid:51) 0.5(cid:56) boxes and interpolatingtheremaininggrid.
338 JOURNAL OF CLIMATE VOLUME11
FIG.3.Jaegerprecipitationclimatology(1976):(a)annualaverageinmmyr(cid:50)1w.e.;(b)differenceofJulyminusJanuaryinmmyr(cid:50)1w.e.
Part of the distortion between Figs. 2a and 2b arises confirms the low accumulation values for the area,
fromthelackofminimumvaluesforenclosedcontours. Highametal.(1997)describeanareaofablation(an-
Thiscreatesareasofconstantvalues,withwhichacon- nual accumulation (cid:44)0) confined only to the Lambert
touring program has difficulty. Over the high plateau, Glacier surface, while GB85 show an additionallobe
valuesof30mmyr(cid:50)1havebeenaddedtothefeatureless of values less than zero extending to the south of the
central ridge of the ice sheet for a morerealisticdepic- basin rim. Higham et al. (1997)issignificantbecause
tion. The resulting dataset is in reasonable agreement it establishes values in a region where contradictory
with the manual analysis of Giovinetto and Bentley. accumulation distributions (Allison 1979; McIntyre
At least two regional studies have provided results 1985) had previously been reported between which
that are in significant disagreement with the GB85. GB85hadtochoose.RecentvalidationbyZwallyand
Frolich (1992) determined values for the Antarctic Giovinetto (1995) using passive microwave data has
Peninsula region to be nearly twice that depicted in alsofoundGB85valuestobelow,particularlyinwest
thesynthesis.Therevisedvaluesforthisregionresult Antarctica. While these studies suggest a need for a
ina7%increasefortheentirecontinentalarea.Good- revised accumulation distribution for the continent,
win (1995) and Higham et al. (1997) have also ex- GB85 is the best synthesis of glaciological data cur-
amined the Lambert Glacier Basin inland of the rently available. These studies also point to the haz-
Amery Ice Shelf. Although the revised distribution ards of synthesizing a long-term variable for regions
MARCH1998 CULLATHER ET AL. 339
FIG.3.(Continued)
where values are susceptible to interannual variabil- mulation plot of GB85, monthly depictions of precip-
ity.ZwallyandGiovinetto(1995)haveestimatedthat itation are available from the Jaeger dataset. Figure 3b
GB85 is representative of periods with an overall shows the July minus January seasonal variation. All
range between 1940 and 1976. locationsonthecontinentandadjacenticeshelvesshow
Additional sources of precipitation data are several larger values during the austral winter than summer,
global climatologies that have been compiled for pur- although the situation is dramaticallyreversedoffshore
poses of model validation; only a few actually depict with some locations experiencing annual variations of
values for theAntarcticcontinent,however.TheJaeger 2 m yr(cid:50)1 or more for some locations south of 60(cid:56)S.
climatology(1976),obtainedfromtheNationalCenters As noted previously, direct precipitation gauge esti-
forEnvironmentalPrediction(NCEP),formerlytheNa- mates for the Antarctic continent are inflicted with nu-
tional Meteorological Center–National Center for At- merous errors associated with wind biases. A recent
mospheric Research (NCAR) Reanalysis CD-ROM effort to correct gauge errors in a global climatology
(Kalnay et al. 1996), is shown in Fig. 3a. Annually has been made by Legates and Willmott (1990; data
averaged, Fig. 3a is qualitatively similar to the accu- obtained from NCAR, also available from NCEP–
mulationsynthesisofCameron(1964),indicatinglarger NCAR CD-ROM). In the Antarctic, the most severe
values in Wilkes Land and a general dependence on obstacle to a corrected climatology is the sparsenessin
elevation and distance to the coast. Unlike the accu- observingstations.Manypolarstationsforegostandard
340 JOURNAL OF CLIMATE VOLUME11
FIG.4.LegatesandWillmott(1990)annualaverageprecipitationclimatologyinmmyr(cid:50)1w.e.
meteorologicalprecipitationobservationsbecauseofthe ancies are to be expected. A synthesis of gauge mea-
uncertaintyinvolvedwiththemeasurements(Schwerdt- surements from former Soviet stations has also been
feger1984),andtheresultingnumberofavailableAnt- produced by Bryazgin (1982), with a discussion of er-
arctic stations is meager, with a large concentration in rorsassociatedwithgaugemeasurements.TheBryazgin
the vicinity of the Antarctic Peninsula (Legates and composition was supplemented by available glaciolog-
Willmott 1990, see their Fig. 1). The record for these ical data, however.
stations is also very short, some as short as 5 yr (D. R. Use of the atmospheric moisture budget for net pre-
Legates1997,personalcommunication).Thecontoured cipitation estimations in Antarctica has a relatively re-
LegatesandWillmottclimatology,showninFig.4,con- centhistory.Severalstudieshaveexaminedthemoisture
tains large discrepancies with the climatologies previ- budget from a global or hemispheric perspective (e.g.,
ouslyshown,whichappeartoresultfromthesparseness Starr et al. 1969; Peixoto and Oort 1983). Bromwich
of interior data points. In particular, the sharp gradient (1979,1988,1990)andBromwichetal.(1995)utilized
associated with the coastal escarpment is not asclearly theexisting east Antarcticcoastrawinsondenetworkto
defined as in GB85 or Jaeger (1976), and the precipi- estimate P (cid:50) E for asectordefinedfrom0(cid:56)–110(cid:56)Eand
tation quantities are extremely large in comparison to 68.4(cid:56)–78.2(cid:56)S.Theresultsindicategoodagreementwith
theGB85accumulationvalues.Giventhesparsenessof glaciological data when the entire rawinsonde network
gauge measurements on the continent, these discrep- is utilized. The area has been evaluated separately by
MARCH1998 CULLATHER ET AL. 341
FIG.5.AnnualaverageofprecipitationfieldfromtheNCEP–NCARReanalysisfor1982–94inmmyr(cid:50)1w.e.
ConnelleyandKing(1993,1996).Incomparisontothe 12 h) forecasts. These predictions are more model
glaciologicalestimatefortheregionandselectedresults dependent than the analyzed fields (Kalnay et al.
from the United Kingdom Meteorological Office Uni- 1996; Genthon and Braun 1995; Arpe and Cattle
fied Climate Model, Connelley and King (1996) found 1993). Genthon and Braun (1995) have recently an-
the rawinsonde method overestimates sector accumu- alyzed the ECMWF ensemble forecast precipitation
lation; however, the treatment of the rawinsonde trans- for 1985–91. Genthon and Braun indicate that the
ports differs with that employed by Bromwich (1988, ECMWF depiction ‘‘does a fairly good job’’ withthe
1990). Bromwich utilized the SANAE station (70.3(cid:56)S, atmosphere–surface water exchange over ice sheets.
2.4(cid:56)W) to computeazonal transportacrossthewestern Precipitation fields have also been produced as the
boundary, while Connolley and King assume cancel- result of the expanded datasets made available from
lation of fluxes across the sector east and west bound- reanalysis projects. In particular, the results of the
aries. The observational network nevertheless offers a NCEP–NCAR Reanalysis have recently been made
means of validating time-varying net precipitation es- available for an extended period of time from 1982–
timations with rawinsonde-derived values. 94(Kalnayetal.1996).TheNCEP–NCARReanalysis
A finalsourceof precipitationdataconsideredhere is of interest due to the project’s eventual goals of
istheoutputprecipitationrateofanumericalweather producinganalysesthatextendfor1957–96.Antarctic
prediction model for an ensemble of short-term (6 or analyses produced over this period may take advan-
342 JOURNAL OF CLIMATE VOLUME11
FIG.6.(a)ECMWFnetprecipitation(P(cid:50)E)averagedfor1985–95,correctedforatmosphericmassbalanceinmmyr(cid:50)1w.e.Thefigure
hasbeensmoothedusinga300-kmradius;(b)ECMWFcorrectednetprecipitationminusuncorrectednetprecipitationaveragedfor1985–
95,inmmyr(cid:50)1.Thefigurehasbeensmoothedusinga300-kmradius.
tage of a substantial atmospheric observational net- ditionally, the presence of other sources of observa-
work in the years of, and for the decades following, tional data will nearly always supersede the low pri-
the IGY. In high southern latitudes, however, there ority given to PAOBs during quality control proce-
are two major concerns regarding the present Rean- dures. Second, perhaps more importantly, errors as-
alysis dataset. First, the incorporation of manually sociatedwithpolarmoisturefieldshavebeenrecently
derived sea level pressure observations, known as investigated (R. Kistler, M. Iredell, and H. Pan 1996,
PAOBs(foradiscussionseeSeamanetal.1993),have personal communication). A simplification in the
been misincorporated during the data assimilation. moisture diffusion parameterization used by the as-
These point estimates are produced in aregularspac- similation model leads to a spurious wave pattern in
ing of 1000–1500 km over the SouthernHemisphere, the polar regions. The impact on the reanalysis pre-
with additional points to locate troughs and ridges. cipitation field is shown to be significantinCullather
Over Antarctica, the extreme elevation of the conti- et al. (1996, see their Fig. 10). Figure 5 shows the
nent diminishes the physicalmeaning ofthesealevel averageprecipitationfieldwiththespectraldistortion
pressure field, implying that the PAOBs errors may clearly visible. In the presence of such large spatial
be at least partially mitigated over the continent.Ad- errors, an examination of the evaporation field is de-
MARCH1998 CULLATHER ET AL. 343
FIG.6.(Continued)
ferred. A reasonable representation of the precipita- mospherethatisnotzerointheanalyses.IntheECMWF
tion field may be retrieved if the data are filtered(M. analyses, velocity data extend to 10 hPa, while atmo-
Serreze 1996, personal communication) or if a suf- spheric moisture is considered negligible above 300
ficientlylargeareaisaveraged(W.N.Ebisuzaki1996, hPa. Equation (2) is written so that the residual P (cid:50) E
personalcommunication).Nevertheless,thesetwoer- is positive for comparison with glaciological accumu-
rors argue for considerable caution in evaluating re- lation. The first term on the right-hand side is the time
sults. derivativeofprecipitablewaterandisreferredtoasthe
storage term. The second term may be rewritten as the
flux of vertically integrated moisture transports across
3. Atmospheric moisture budget from analyses
a specified horizontal boundary using
The atmospheric moisture budget may be written as (cid:82) (cid:69)
P (cid:50) E (cid:53) (cid:50)(cid:93)(cid:93)Wt (cid:50) (cid:61)·1g (cid:69)Psfc qV dp, (2) (cid:94)P (cid:50) E(cid:38) (cid:53) (cid:50)(cid:55)(cid:93)(cid:93)Wt(cid:56) (cid:50) A1 (cid:49) PPtospfc qgV dp(cid:50)·n dl, (3)
Ptop
where W is precipitable water, P is surface pressure, where A is the area of interest, and n is the outward
sfc
q is specific humidity, and V is the horizontal wind pointing normal vector of the area perimeter. Four ad-
vector. The variable P is the highest level of the at- jacentgridpointsareusedtodefineanareaandbound-
top