Comparison of TOVS-derived Land Surface Variables with Ground Observations

Transcription

1 University f Suth Carlina Schlar Cmmns Faculty Publicatins Earth and Ocean Sciences, Department f Cmparisn f TOVS-derived Land Surface Variables with Grund Observatins Venkataraman Lakshmi University f Suth Carlina - Clumbia, vlakshmi@gel.sc.edu Jel Susskind Fllw this and additinal wrks at: Part f the Earth Sciences Cmmns Publicatin Inf Published in Jurnal f Gephysical Research, Vlume 15, Issue D2,, pages Lakshmi, V. & Susskind, J. (). Cmparisn f TOVS-derived land surface variables with grund bservatins. Jurnal f Gephysical Research, 15 (D2), Jurnal f Gephysical Research, American Gephysical Unin This Article is brught t yu fr free and pen access by the Earth and Ocean Sciences, Department f at Schlar Cmmns. It has been accepted fr inclusin in Faculty Publicatins by an authrized administratr f Schlar Cmmns. Fr mre infrmatin, please cntact SCHOLARC@mailbx.sc.edu.

2 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 15, NO. D2, PAGES , JANUARY 27, Cmparisn f TOVS-derived land surface variables with grund bservatins Venkataraman Lakshmi Department f Gelgical Sciences, University f Suth Carlina, Clumbia Jel Susskind Labratry fr Atmspheres, NASA Gddard Space Flight Center, Greenbelt, Maryland Abstract. The Tirs Operatinal Vertical Sunder (TOVS) Pathfinder Path A retrieved surface skin temperature, surface air temperatures, and surface specific humidity are cmpared with data btained frm three large-scale field campaigns: the First ISLSCP (Internatinal Satellite Land Surface Climatlgy Prject) Field Experiment (FIFE), the Hydrlgic Atmspheric Pilt Experiment (HAPEX) in the Sahel, and the Breal Ecsystem-Atmsphere Study (BOREAS). The lng-term estimates f surface skin temperatures, surface air temperatures, and surface vapr pressure were unbiased, and the standard deviatins f the errrs were abut 4øC, 3.5øC, and 3.5 mbar, respectively. The mnthly mean variables btained frm the TOVS data at fur times f the day (crrespnding t the AM and PM verpass fr each f tw satellites) exhibited realistic diurnal and seasnal cycles when cmpared with crrespndin grund bservatins. 1. Intrductin Cntinental scale hydrlgical mdeling has been limited by the lack f grund bservatins that are needed as input. Grund bservatins suffer frm tw drawbacks. Firstly, grund bservatin netwrks are expensive t maintain fr extended perids f time. Secndly, grund bservatins are pint bservatins and thereby d nt capture the spatial variability ver large regins. Satellite data vercme these tw drawbacks. Mst f the satellite sensrs are in rbit fr mul- tiple years and have 2-4 times f day repeat bservatins f the land surface. This paper carries ut direct cmparisns f satellite and grund-based bservatins f land surface variables f surface temperature, air temperature, and vapr pressure. This study fllws alng the lines f previus studies [Sugita and Brutsaert, 1993; Kalluri and Dubayah, 1995] which cmpared a few days f satellite-estimated skin temperatures with field bservatins. In ur study, we fcus n a lnger time perid f cmparisns (than thse mentined abve) t achieve a cnfidence in ur statistical characterizatin f the Africa (HAPEX-Sahel), and the breal frest in Canada (BOREAS). 2. Data and Methds 2.1. Satellite Data The Tirs Operatinal Vertical Sunder (TOVS) cnsists f the HIRS2 (High-Reslutin Infrared Sunder), the MSU (micrwave sunding unit), and the SSU (Stratspheric Sunding Unit). The HIRS2 has channels in the infrared, visible, and near-infrared regin ( / m), and the MSU cnsists f fur channels in the 5 GHz range (5.3, 53.74, 54.96, and GHz). TOVS is flwn n NOAA satellites [Kidwell, 1995], and ne can btain frm each satellite (sme years have tw different satellites, Table 1), twice a day (ascending and descending rbits), glbal fields f temperature and misture prfiles f the atmsphere, surface skin and air temperatures, as well as estimates f zne amunt in the atmsphere and clud and precipitatin variables [Susskind et al., 1997]. TOVS has been flwn since Nvember 1978 and the prcesse data will sn exist fr the entire perid The Pathfinder Path A prducts als include the lcal time f bservatin. differences between TOVS-retrieved data and grund bservatins. Surface skin temperature cmparisns is emphasized in this paper. In recent years, there have been several field campaigns that have prvided us with imprved data sets fr these cmparisns. The satellite data used in this study is frm The TOVS Pathfinder Path A physically based retrieval systhe TOVS Pathfinder Path A data set [Susskind et al., 1997]. tem uses the Gddard Earth Observing System (GEOS) (4 ø x This data set (TOVS Pathfinder Path A) has been prduced 5 ø) general circulatin mdel 6 hur frecast as the first guess using a cnsistent algrithm and validated extensively [Suss- fr temperature and misture prfiles. The retrieval algrithm kind et al., 1997; Lakshmi and Susskind, 1998; Anyaruba and then mdifies these prfiles and surface skin temperatures s Susskind, 1998]. Our cmparisns were carried ut ver large that the radiances cmputed frm the slutin best match the areas (1 ø x 1 ø) and lng time perids (a year r mre). The clud-crrected radiances fr partially cludy and clud-free cmparisns were made in three cntrasting areas: the tall fields f view. A 2 x 2 array f HIRS2 spts (6 km x 6 km grass prairie f Kansas, United States (FIFE), the bundary f at nadir) and the clsest MSU spt are used t recnstructhe the desert, bush, and savanna in the Sahel, Niger in West clear-sky (upwelling) radiances, i.e., the radiance in the cludfree prtins f the scene [Susskind et al., 1984; Susskind et al., This paper is nt subject t U.S. cpyright. Published in by the 1997]. The radimetric surface temperatures are btained di- American Gephysical Unin. rectly using the infrared channels by crrecting fr atm- Paper number 1999JD9921. spheric attenuatin [Susskind et al., 1984]. The emissivities 2179

3 218 LAKSHMI AND SUSSKIND: COMPARISON OF TOVS DATA WITH OBSERVATIONS Table 1. List f Satellites and Nminal Overpass Times fr TOVS Nminal Overpass Satellite Time Perid Tirs N NOAA 6 NOAA 7 NOAA 9 NOAA 1 NOAA 11 3 am/pm 73 am/pm 23 am/pm 23 am/pm 73 am/pm 13 am/pm 73 am/pm 73 am/pm 23 am/pm 73 am/pm December 1978 t December 1979 July 1979 t May 1982' July 1981 t May 1982' uary 1985 t December 1986 December 1986 t July 1991 Nvember 1988 t December 1994 August 1997 t present July 1991 t December 1998 uary 1995 t present June 1998 t present surface and air temperature and humidity measurements, detailed bservatins f the bundary layer using radisndes, and daily sil misture measurements were carried ut at 3 sites. The autmated meterlgical statins cllected data cntinuusly every half hur between May 3, 1987 and Nvember 1, This prvides nearly a 2.5 year data set that was used in this study. The site-averaged data [Betts and Ball, 1998] have been used in this paper. The IRTs were lcated in fenced enclsures with n grazing. Therefre the IRT re- crded surface temperature is cler than bare sil (due t transpiratin by grass) and has a diurnal and seasnal range that is mdulated by the vegetatin. NOAA 12 NOAA HAPEX-Sahel. The Hydrlgic Atmspheric Pilt NOAA 15 Experiment in Sahel (HAPEX-Sahel) was cnducted in western Niger, West Africa, ver a 3-4 year perid with an 8 week Asterisk, presently prcessed. NOAA 6 ends in March 1983 and NOAA 7 extends till December Read 3 am/pm as 3/15 LT. intensive bservatin perid (IOP) frm August 25 t Octber 1, The area was a 1 ø x 1 ø regin (-11 km x 11 km) extending frm 2 ø t 3øE and 13 ø t 14øN. The measurements assumed ver land fr the HIRS2 channel 8 (9 cm - ) is.95 were carried ut with a tempral frequency f 1 hur fr and.85 fr channels 18 and 19 (25 cm- ). The temperature surface air temperature and relative humidity and a 1 min and humidity prfiles are determined using an iterative prcess frequency fr surface skin temperature during the 8 week IOP. that varies the initial prfiles smthly in height in a manner The landscape cnsisted f varius landcver types: tiger bush, that is cnsistent with the channel bservatins. This is millet, and fallw areas. The main purpse f the campaign was t btain a better understanding f the changes in the atmspheric circulatin due t changes in the land surface cnditins in the Sahel frm year t year [Prince et al., 1995; Gutrbe et al., 1994]. The mst imprtant feature f this achieved by calculating radiative transfer thrugh the atmsphere t determine the radiances at the satellite. Surface air temperature and the specific humidity initial guesses are btained by extraplating the GCM first-guess prfiles f air temperature and humidity t the surface pressure level. The single value f retrieved land surface and atmspheric variables are interpreted as averages ver the clud-free prtins f the scene. In general, retrievals can be perfrmed in up t campaign was its larger area (11 km x 11 km) cmpared t FIFE (15 km x 15 km). This allws fr the field data t be interpreted in the cntext f utput frm general circulatin mdels (GCMs) as well as prviding a better match with data 8% fractinal cludiness [Susskind et al., 1997]. retrieved frm satellite sensrs. The TOVS Pathfinder Path A data are available as a gridded 1 ø x 1 ø (latitude by lngitude) prduct fr bth the mrning and the afternn verpasses fr each satellite. The data are als available in pentad (five day averages) and mnthly aver- The data cllectin was rganized spatially int three supersites: the eastern central, western central, and suthern. The values f hurly surface air temperature and relative humidity fr 1992 were used in this study measured by using dry bulb aged frms. The afternn rbit near 133 drifts cnsiderably and wet bulb thermmeters, hygrmeters, respectively, at the in time. The TOVS data used in this paper are frm the NOAA 1, 11, 12, and 14 satellites (nminal lcal nadir bservatin time at the equatr f 73/193, 13/133, 73/ 193, and 23/143 LT, respectively). climate statins. The surface skin temperature data fr the intensive bservatin perid cmes frm many surces: the Institute f Hydrlgy infrared thermmeter data fr the fallw, millet, and tiger bush regins (central west and suthern 2.2. Field Observatins supersite); the data ver millet sil in the central east supersite; data ver the millet and grassland regin; data ver the her- This sectin describes three measurement campaigns that have cllected high spatial and tempral reslutin data which are cmpared with crrespnding satellite-retrieved variables. bacius grass-sparse layer, and the data frm the climate statin (which als cllected air temperature and relative humidity data) FIFE. The First ISLSCP (Internatinal Satellite BOREAS. The Breal Ecsystem-Atmsphere Land Surface Climatlgy Prject) Field Experiment (FIFE) was cnducted near Manhattan, Kansas, between 1987 and One f the bjectives f this campaign was t understand the relatinship between the satellite-measured values f land surface variables and the grund bservatins [Sellers and Hall, 1992]. The FIFE study area was 15 km x 15 km, predminantly cvered by tall grass prairie and surrunded by grassland and Study (BOREAS) was carried ut ver a 1 km x 1 km study regin in Canada. The tw majr study areas in this campaign were placed at the nrthern and suthern ectnes in rder t determine the effect f the different cntrlling factrs that were respnsible fr the varius land-atmsphere interactins in these regins. The nrthern and the suthern study areas (NSA and SSA) were 5 km apart. The NSA was lcated 98ø3'W, 56ø'N and the SSA was lcated 14ø45'W agriculture. The surface skin temperature was measured by and 53ø45'N. There were a ttal f 1 twers that measure nadir-lking infrared thermmeters (IRT), while the air temperature and the surface relative humidity were measured by dry and wet bulb thermmeters. These instruments were lcated in autmated meterlgical statins (AMS). The experiment itself was cnducted as a series f intensive field campaigns fr shrter perids f time in the summer and fall f 1987 and In these perids, half hurly heat flux, fluxes f water, energy, and CO2 and ther atmspheric and land surface variables such as surface skin temperature, air temperature, and relative humidity near the surface in the tw areas. The twers were placed in areas f different vegetatin type such as black spruce, jack pine, and aspen, which reflect the surface type f the area surrunding the twer. The bservatins at these twers began in December 1993 and ended in

4 LAKSHMI AND SUSSKIND: COMPARISON OF TOVS DATA WITH OBSERVATIONS 2181,,, 4:::),

5 2182 LAKSHMI AND SUSSKIND: COMPARISON OF TOVS DATA WITH OBSERVATIONS Nvember/December 1996 and were carried ut at 15 min AM: NOAA 1 intervals t btain a 3 year bservatin perid fr the regin. The bservatins are made frm twers at a height f 2 m (fr the air temperature and the relative humidity) and a nadirlking infrared thermmeter fr the surface skin temperature. The bjectives f these bservatins and the BOREAS experiment campaign are t bridge varius spatial scales amng physical prcesses cnnected with leaf bichemistry 1 which can be applied t atmspheric general circulatin mdels (AGCMs) and ther carbn cycle mdels [Sellers et al., 1997] Methdlgy The surface skin temperature (Ts), surface air temperature (T,), and the surface vapr pressure (e,) frm the field campaigns were cmpared with the TOVS data fr the 1 x 1 grid bx which includes the field site. The field bservatins at the clsestime f the satellite verpass were cmpared with the satellite data. The difference in time between the satellite bservatins and the grund bservatin was at mst 15 min in the case f FIFE (half hurly bservatins), 3 min in the case f HAPEX-Sahel surface air temperature, and relative humidity (hurly bservatins) and 5 min fr surface skin temperatures (1 min bservatins) and 7.5 min at BOREAS (15 min bservatins). The satellite bservatin lcal time varies cnsiderably ver the scan line. Taking the exact satellite lcal bservatin time int accunt was significant, because surface skin temperature changes quite rapidly, especially during the sunrise and early mrning hurs. In the case f HAPEX-Sahel the campaign area cvers a 1 x 1 grid, and therefre cmparisns with the 1 x 1 gridded TOVS data are justified. The FIFE campaign area was 15 km x 15 km. In the case f BOREAS the field bservatins were made at 1 flux twers in the study area. The bservatins frm each flux twer were cmpared with the crrespnding 1 x 1 grid bx in which it was lcated. The flux twers were placed in areas that represent the surrunding lcatin (usually represent -<1 km2), s the measurement is characteristic f that surrunding regin. The surface vapr pressure was cmputed frm the TOVS data by using the surface specific humidity (which is a standard NOAA lo 5.. d.. ' ' -2,s ø Figure 1. Time series f surface skin temperatures (C): FIFE and NOAA 1. i,'... I... I... I... I... I... 5[crre atin i.9; std :' 4.9 C/// 5 crrelatin:.931 std: 3.34 C/// 4Ibias: -.64C slpe: 4 bias:.677 G FIFE Surface Skin Temperature (C) PM: NOAA 1 'crrelatin' 8 3 = 4 bias: Aslp t, g -lo H Surface Skin Temperature (C) AM: NOAA FIFE Surface Skin Temperature (C) PM: NOAA std: 7.654C ' bias: -.476C slpe: 1.28, HFE Surface Skin Temperature (C) Figure 2. Scatterplt f surface skin temperatures (C): FIFE versus TOVS. The mean difference ver all bservatins (bias = IRT - TOVS) is indicated n the figures as well as the slpe f the regressin line fit, the crrelatin, and standard deviatin f the difference between FIFE and TOVS data. A psitive value f bias means that n the average the FIFEbserved surface temperatures were warmer than the TOVSretrieved data. TOVS data prduct) and the GEOS-GCM grid pint surface pressure. The surface vapr pressure fr the field bservatins at FIFE, HAPEX-Sahel, and BOREAS was cmputed using the bserved relative humidity, surface air temperature and/r wet bulb temperatures. Cmparisns f the field bservatins and the TOVS retrievals were perfrmed fr fur individual times a day in perids when NOAA 11 data existed t supplement either NOAA 1 r NOAA 12. Mean mnthly diurnal curves (i.e. a diurnal cycle based n half-hurly bservatins fr a mnth) were cnstructed using the field bservatins, and the TOVS data were superimpsed n these curves fr each f the 4 times f the day (averaged ver the mnth) fr which bservatins were available. 3. Results Examples f glbal fields f surface skin temperature, surface air temperature, and surface specific humidity fr a single day (June 1, 1987) are shwn in Plate 1. The plate is frm the NOAA 1 satellite ( 73/193 LT). Actual lcal times will vary by mre than _ 1 hur acrss a scan line, and nadir times vary frm 73 with distance frm the equatr. Areas in which n data exists are shwn in gray. Part f this is due t rbit gaps and calibratin cycles, which ccur fr three cnsecutive scan lines in a blck f 4 lines. Other gaps ccur in places where n retrievals culd be prduced, mst ften because f excessive cludiness r precipitatin. The 193 LT surface skin temper-

6 LAKSHMI AND SUSSKIND: COMPARISON OF TOVS DATA WITH OBSERVATIONS 2183 Table 2. Daily Cmparisns in Surface Fields fr Surface Skin Temperature Overpass Variable Experiment Satellite Time n s.d. Crr Slpe Bias Ts FIFE NOAA am 3.3øC øC 13 pm øC NOAA 1 73 am øC øC 73 pm øC øC HAPEX NOAA am øC øC 13 pm øC øC NOAA am 14 3.øC øC 73 pm øC øC BOREAS NOAA 11/14 23 am øC øC 23 pm øC øC NOAA am øC øC 73 pm øC øC atures were warmer than the mrning surface skin temperatures in the equatrial and subtrpical regins alng mst lines f the lngitude. The same was true fr surface air temperatures, which were similar but nt identical t surface skin temperatures. This was reflected in the glbal means (cmputed using a latitudinal area weighting) fr bth the surface skin and the surface air temperatures. Over arid areas, 193 surface air temperatures were cnsiderably warmer than 193 surface skin temperatures, because the maximum air temperature ccurred later in the day than that f skin temperature. The surface specific humidity did nt shw as strng a diurnal difference as the temperatures (bth skin and air). The surface specific humidity in the 3øN-3øS regin varied between 12 and 17 g/kg Daily Cmparisns FIFE. The cmparisns f daily mrning and afternn surface skin temperatures ver the 2.5 year perid determined frm TOVS at the FIFE site, crrespnding t the NOAA 1 (equatrial verpass time 73/193) are shwn in Figure 1. There were mre TOVS bservatins than IRT (infrared thermmeter) data because FIFE first began in June 1987, and many days were missing in the subsequent perids. There was a clear annual cycle t the mrning and afternn PM surface skin temperature and the TOVS retrievals captured this seasnal cycle very well (Figure 1). The vertical spread f surface skin temperatures is due t the day-t-day variability f the surface temperature and the time f bservatin f the TOVS nt being exactly the same n subsequent days (this is a minr effect). Figure 2 gives scatter diagrams f the TOVS and FIFE data fr days in cmmn fr the mrning and afternn verpasses f NOAA 1 and NOAA 11. On the average, there was a slight underestimatin f the mean IRT surface skin temperature by the TOVS surface skin temperature fr the NOAA 1 LT verpass (+.6øC). This underestimatin was greater in the case f IRT surface temperatures greater than 1øC. There AM' NOAA i... i... i... i...,... ' 5 crrelatin:.929 std: 3.816C/ 4 E 3 bias:.818c slpe:.94/ AM: NOAA 11 F crrelatin:.93 std: 3.233C/' 4 Ibias:-.99C slpe: 3 AM' NOAA 1 s 4 bias' -.713C slp 3 AM: NOAA 11 4 Ibias:.326C slpe:.87/ FIFE Surface Air Temperature (C) lo -lo lo Surface Air Temperature (C) FIFE Surface Vapr Pressure (mb) -1[ Surface Vapr Pressure (rb) <, ;> PM' NOAA 1,bias: C sl;; : -lo a a....,...,......, F S ace Air Temperature (C) PM: NOAA 11, bias: 1. 4C slpe: J Surface Air Temperathe (C) Figure 3. Surface air temperatures (C): FIFE versus TOVS. PM NOAA 'bias' -.599C slpe FIFE Surface Vapr Pressure (mb) PM: NOAA 11 'bias: -.862C slpe 1 -lo lo S ace Vapr ess e (mb) Figure 4. Surface vapr pressure (mb)' FIFE versus TOVS.

7 2184 LAKSHMI AND SUSSKIND: COMPARISON OF TOVS DATA WITH OBSERVATIONS Table 3. Daily Cmparisns in Surface Fields fr Surface Air Temperature Overpass Variable Experiment Satellite Time n s.d. Crr Slpe Bias r S FIFE NOAA am øC øC 13 pm øC øC NOAA 1 73 am øC øC 73 pm øC øC HAPEX NOAA am øC øC 13 pm øC øC NOAA am øC øC 73 pm øC øC BOREAS NOAA 11/14 23 am øC øC 23 pm øC øC NOAA am øC øC 73 pm øC øC were als a few cases f verestimatin f the FIFE surface skin temperatures between øc and 3øC by the TOVS surface skin temperatures fr the mrning verpass. There were at least fur cases f lw surface temperatures as reprted by the IRT at FIFE during the summer mnths, as can be seen frm Figure 1. This underestimate is 15øC in sme f the cases. The IRTs at FIFE were mstly placed in grassland sites. During the warm afternns and evenings when the bare sil temperature was much higher, the IRT reprted the canpy temperature that tends t be cler because f evaptranspiratin. This culd be the reasn why we bserve many IRT data pints n the bttm edge (f the band f pints) fr the NOAA 2 LT verpass. The average bias (as seen in Table 2) shws that the IRT was 2.øC cler than the TOVS surface temperature, which is cnsistent with ur interpretatin. The slpe f the least squares fit line t the scatterplt f TOVS versus IRT data was.99 fr the NOAA 1 mrning verpass and.992 fr the NOAA 1 afternn verpass, which were clse t cmplete agreement (1.). The crrelatin between the TOVS and the FIFE data was.91 fr the NOAA 1 mrning and.87 fr the NOAA 1 afternn verpasses, and the standard deviatin f the difference was 4.9øC and 5.2øC fr the NOAA 1 mrning and afternn verpasses, respectively (Table 2). In the case f NOAA 11, there was an underestimatin (bias =.68øC) fr the mrning verpass and an verestimatin (bias = -.48øC) fr the afternn verpass. The slpes f the regressin lines are clse t unity, and the crrelatins are high (.93 and.85, respectively). The standard deviatins fr the NOAA 11 mrning and afternn surface skin temper- atures were 3.3øC and 7.1øC, respectively. The larger standard deviatin fr the NOAA 11 afternn case was due t the greater variability f the TOVS surface skin temperatures during the early afternn hurs near 133 LT. The results fr the cmparisn f the surface air temperature frm TOVS and FIFE are shwn as a scatterplt in Figure 3. The standard deviatin f the differences were 3.8øC and 4.8øC fr the mrning and afternn satellite verpasses f NOAA 1 and 3.2øC and 4.3øC fr the NOAA 11 mrning and afternn verpasses. The respective biases were.8øc and -4.øC, and -1.øC and 1.5øC, the crrelatins were.93 and.89, and.93 and.93, while the slpes were.94 and.91 and.93 and.92 fr the mrning and afternn verpasses f NOAA 1 and mrning and afternn verpasses f NOAA 11, respectively. The striking difference between cmparisns f the surface skin temperature and the surface air temperature (Figures 2 and 3) was the tighter gruping f the pints arund the regressin line fr the surface air temperature, which was reflected in the higher crrelatin and the lwer standard deviatins. This might be a result f air temperature varying mre slwly in space and time than the skin temperatures and sampling differences in space and time being less significant. The crrespnding results fr the vapr pressure are shwn in Figure 4, with standard deviatin differences f 4.3 and 4.3 mb, and 2.9 and 3.9, biases f -.7 and -.6 mb, and.3 and -.9 mb, crrelatins f.87 and.84, and.9 and.9, and slpes f 1. and.9, and.9 and 1. fr the mrning and afternn verpasses f NOAA 1 and mrning and afternn verpasses f NOAA 11, respectively. The range f TOVS Table 4. Daily Cmparisns in Surface Fields fr Surface Vapr Pressure Overpass Variable Experiment Satellite Time s.s. Crr Slpe Bias FIFE NOAA am 8 13 pm 182 NOAA 1 73 am pm 562 HAPEX NOAA am pm 138 NOAA am 1 73 pm 157 BOREAS NOAA 11/14 23 am pm 576 NOAA am pm mbar 3.8mbar 4.3mbar 4.3mbar 3.9mbar 2.9mbar 4.mbar 3.6mbar 2.3mbar 2.7mbar 2.6mbar 2.1mbar mbar -.9mbar -.7mbar -.6mbar 1.mbar -2.mbar -.2mbar - 1.5mbar -.2mbar -.6mbar -.7mbar.mbar

8 ... LAKSHMI AND SUSSKIND: COMPARISON OF TOVS DATA WITH OBSERVATIONS 2185 vapr pressures was smaller than that f surface measurements at FIFE. This is the case fr the afternn verpass f NOAA 1 and mrning verpass f NOAA 11. The maximum ver- estimatin and underestimatin in the case f the NOAA 11 verpasses is less than 1 mb. There is a 25-3 mbar range in the vapr pressure that is captured by the TOVS data. Tables 2-4 summarize the cmparisn statistics fr the vari- 8 = ables shwn in Figures 1-4. In general, the standard deviatin =- f the differences between TOVS and FIFE was lwer fr the -4 NOAA 11 mrning verpass cmpared t the afternn ver- The standard deviatin f the difference between TOVS and HAPEX-Sahel surface skin temperature was a maximum at the fl. afternn verpass f NOAA 11 (equatrial verpass time f i 133 LT), as was fund with cmparisn f TOVS and FIFE data. The lcal meterlgical cnditins were such that the! - maximum gradient in surface skin temperatures are at 133 LT > -4 in bth cases. The magnitudes f the standardeviatin f the differences are arund 3.5øC. The crrelatin between the TOVS and the HAPEX-Sahel data was lw fr the NOAA ( > 1 crrelatin.631 std 3.53C bias 3 185C slpe Grund NOAA 11 13am -, i crrelatin:.247 std' 2.797C bias: 1.56C slpe: Grund crrelatin.818 std 1.767C bias 543C slpe 1.95,.,i i... i... i... -1;'"" Grund NOAA 11 13pm are included in Tables 2-4. The biases in the surface air temperature fr HAPEX-Sahel were abut 1.ø-1.5øC higher than 6[... ' 4 e.ti'"' 1...""" 1...'"" I crrelatin:.622 std: 4.42C ' -lol.....,...:,6.!.,.c...,.?!,..p.,,i...9:?,!., Grund Figure 5. Surface skin temperatures: HAPEX versus TOVS. Dts dente the pints btained frm averaging surface skin temperature measurements at eight sites during the intensive bservatin perid (IOP) between August 25 and Octber 1, NSA (Black Spruce) ' 'cdr/elhtlh: ':943' s'td:' 5.549C E 4 bias:.823c 4 } ' , ;1.,, i,,. i.,, i.., i, pass HAPEX-Sahel. The cmparisns fr the surface SSA (Old Aspen) skin temperatures in HAPEX-Sahel are shwn in Figure 5 as a ' 'c6ffelhtlh! '.'929' gtd:' 5.797,/7 line f best fit fr the data pints NSA (Old Jack Pine) 'c6ffelhfih! ' 941' s'tct :' 5.527C., i, bias: SSA (Old Jack Pine) 'c6r/elhfih! 'J936' gtd :' bias:.45c slpe t ff ' LT verpass (.25), in which there was very little variabil- Figure 6. BOREAS surface skin temperature versus TOVS ity f the HAPEX skin temperature measurements. surface skin temperature fr fur different sites: (a) nrthern study area (NSA) black spruce, (b) NSA ld jack pine, (c) suth- Crrelatins in ther cases, with larger tempral variabiliern study area (SSA) ld aspen, and (d) SSA ld jack pine. ties, were.62,.63, and.81 fr NOAA , NOAA 12 73, and NOAA 12 73, respectively. The bias fr this time (NOAA verpass) was als larger than the crrespnding FIFE case (arund 1.6øC cmpared t.68øc at FIFE), and the slpes were significantly different frm 1. fr the case f NOAA and 133 (.57 and.71, respectively). The plts shw that the TOVS data were biased with respect t the grund data, especially at 13 and 73. We Surface Skin Temperatures (C) HAPEX-Sahel (1992) wuld expect the grund cllectin f surface skin tempera- NOAA 1273am NOAA 1273pm tures t be biased tward regins f lesser vegetatin amunt, and the satellite data cnsidered equal cntributins frm all... regins. The vegetatin temperatures will generally be cler than bare sil temperatures due t transpiratin. This is cn- 3 sistent with the sign f the differences. The differences culd als be attributed t the surface hetergeneity that existed at the HAPEX-Sahel site. The number f bservatins f surface skin temperatures were restricted t the IOP and ranged between 14 and 27 bservatins depending n the satellite verpass. The standard deviatin f the difference in the case f surface air temperatures and the surface vapr pressure were cmparable t the cunterparts in the FIFE cmparisns. Statistics fr surface air temperature and specific humidity the crrespnding values in FIFE. The crrelatins range frm.47 t.62, and the slpes were lwer (.5-.71). The standard deviatin f the differences range between 3.3 ø and 4.3øC. These are the same values as in FIFE. The slpes are lesser than the ptimal values f unity, which means that the range f the TOVS retrieved surface air temperatures were lesser than the bserved range at HAPEX-Sahel. The number f bser- vatins were cmparable t FIFE fr the NOAA 11 verpasses; hwever, in the case f NOAA 12 the number f bservatins was lwer (arund 15) cmpared t FIFE NOAA 1 (arund 56). This is because the data used in these cmparisns were fr 1 year (1992) nly fr HAPEX-Sahel cmpared t multiple years ( ) fr FIFE. The differences between the HAPEX-Sahel and FIFE statistics n vapr pres-

9 2186 LAKSHMI AND SUSSKIND: COMPARISON OF TOVS DATA WITH OBSERVATIONS -4 NOAA 12 (AM) ' 'c6rfelhtlh: ' 947' gtd :' 5.367C ø'61.,.,.,..,,.,,,.,,, : -4 NOAA 12 (PM) ' 'c6ffelhtlh! 'J945' tct:' 5.47a. C bias:.,..,..., ; , NOAA 11/14 (AM) 'c6ffelhtlh: ' 9' td :' $.82 C bias: NOAA 11/14 (PM) [' 'cdrfel'atlh! ' 94' gtd:' 5.387C 4 t bias: 2.225C slpe:.942/// -4 A,/, Figure 7. Scatterplt f BOREAS versus TOVS surface skin temperature fr NOAA 12 mrning and afternn and NOAA 11 and 14 fr mrning and afternn fr the BOREAS NSA black spruce vegetatin cver. vatins are less than thse f FIFE and HAPEX-Sahel. The maximum value f the standard deviatin (2.7 mb) is less than the minimum value fr the FIFE and the HAPEX-Sahel cases (2.9 mb). The cmparisns between the BOREAS bservatins and the TOVS-retrieved surface skin temperatures are shwn in Figure 6 fr fur different sites with each panel in the figure cntaining data frm all the satellite verpasses fr that site. The cmparisn statistics in the figure d nt indicate any bias with respect t vegetatin cver; that is, the values f crrelatin, bias, standard deviatin, and slpe fr the different sites are quite similar in numerical value. The nly exceptin is the suthern study area ld aspen vegetatin cver site that has a slight negative bias as ppsed t the psitive biases fr all the ther cases. The surface skin temperatures as retrieved frm the TOVS and the bservatins at the BOREAS nrthern study area with black spruce vegetatin cver as shwn in Figure 7. The TOVS retrievals crrespnd t the 1 ø x 1 ø grid bx, which cntains the flux twer at the BOREAS site. This result is displayed as a functin f the verpass times f the NOAA satellites NOAA 12 and NOAA 11 and 14. The greatest amunt f scatter abut the 1:1 line is seen fr NOAA 11 and NOAA 14 mrning verpass (23 LT) at the equatr and an early mrning verpass (NOAA 12, 73) at the BOREAS site and the least amunt f scatter is fr NOAA 12 mrning verpass. The sure are less than that fr surface temperature described abve. The slpes f the regressin lines between the satelliteretrieved and the HAPEX-Sahel-bserved vapr pressure is clse t unity; the crrelatin cefficients were in the range f BOREAS. The cmparisn f the TOVS-retrieved surface skin temperature, surface air temperature, and surface vapr pressure t the crrespnding bservatins at BOREAS are summarized in Tables 2, 3, and 4, respectively. The number f bservatins ver which the cmparisns are carried ut is larger by an rder f magnitude in the case f BOREAS as cmpared t FIFE and by 2 rders f magnitude as cmpared t HAPEX-Sahel. The bias in the surface skin temperature (IRT bservatins- TOVS retrievals) ranges frm 1.5øC fr the NOAA 11 and NOAA 14 mrning verpass t.6øc fr the NOAA 11 and NOAA 14 afternn and the NOAA 12 mrning verpass with the NOAA 12 evening verpass having a bias f.9øc in between these tw values. These values fall within the same range as thse seen in FIFE and HAPEX-Sahel. The standard deviatins range between 5.2øC and 6.2øC, which are slightly higher than thse seen in FIFE and HAPEX-Sahel with the exceptin f the NOAA 11 afternn verpass at FIFE, which has a value f 7.7øC. The crrelatins between the BOREAS- bserved and the TOVS-retrieved surface skin temperatures are greater than.9 in all the satellite verpasses, and the slpes f the regressin lines are clse t 1, indicating a clse agreement in the range f the satellite and the grund bservatins. The surface vapr pressure cmparisns frm Table 4 shw very gd agreements between the satellite-retrieved and the BOREAS-bserved values. The biases are all negative ranging frm. t.69 mb, the crrelatins are higher than.8, and the slpes range between.88 and.97. The values f the standard deviatin between the satellite and the grund bser uary '2 1 '6 ' 24 March 1989 Surface Skin Temperature (C) '8 1'2 1'6 2' May July September February April June lo,, 4 8 1'2 1'6 J 24 August Octber ,! 4 8 i2 i6 24 Figure 8. Diurnal variatins f surface skin temperatures (C)' FIFE versus TOVS. Slid line indicates FIFE bservatins, and circles are the TOVS retrievals.

10 ...,, LAKSHMI AND SUSSKIND: COMPARISON OF TOVS DATA WITH OBSERVATIONS 2187 cmparisns fr this particular site (and fr the ther nine that are nt shwn) are similar t the cmparisns fr the data frm all the sites pled tgether and displayed in Tables 2, 3, and Diurnal Cycles The mean mnthly diurnal variatin f the FIFE surface skin temperature fr 1 mnths in 1989, derived frm hurly data, is shwn in Figure 8. The TOVS data are dented by circles that crrespnd t the fur times f day bservatins by the NOAA 1 and NOAA 11 satellites. There are sme cases when the tw d nt cmpare favrably. The NOAA LT data fr February is an example in which the TOVS data underestimated the field bservatins by almst 3øC. NOAA 11 als underestimated the surface skin temperature in this mnth at 133 LT. The 73 verpass fr March 1989 shwed a 5øC verestimatin with respect t the FIFE diurnal curve. The NOAA 1 (arund 193 LT) data shwed the best agreement t the FIFE-bserved diurnal cycle fr all the 1 mnths. The cmparisn f the fur times f day TOVS-derived surface air temperature and surface specific humidity with the bservatins frm FIFE is shwn in Figures 9 and 1, respectively. The TOVS mnthly mean surface air temperature diurnal cycle has a lesser agreement with the FIFE bservatins as cmpared t the surface skin temperature. There are large biases in the 133 (uary 1989), 133 (February 1989), uary1989 Surface Air Temperature (C) 3 1 February uary 1989 Surface Vapr Pressure (mb) I ' ø I February 1989 March 1989 April ' May 1989 June ' ' 1 July September 1989 :25 15 i 3! August Octber 1989 '8 1'2 1'6 2' 24 Figure 1. Diurnal variatins f surface vapr pressure (mb): FIFE versus TOVS. Slid line indicates FIFE bservatins, and circles are the TOVS retrievals March 1989 i 1'2 1'6 2' 24 April 1989 '4 1'2 1'6 2' 24 May 1989 June , 1'2 1'6 2' 24 July 1989 August '4 i 1'2 1'6 2' '2 1'6 24 September 1989 Octber 1989, 1'2 1' Figure 9. Diurnal variatins f surface air temperatures (C): FIFE versus TOVS. Slid line indicates FIFE bservatins, and circles are the TOVS retrievals. (March 1989), 73 (September 1989), and 133 LT (Octber 1989). These biases are arund 5ø-8øC. The TOVS-retrieved surface air temperatures shw a gd agreement with the FIFE mnthly mean diurnal cycle fr all the mnths, especially the summer mnths f April t August There is a gradual warming between uary and July 1989 which is captured very well by the TOVS data. It can als be seen that the surface air temperature mnthly mean diurnal cycle has a smaller amplitude (15øC) cmpared t a larger amplitude (25ø-3øC) fr the surface skin temperature. The surface vapr pressure des nt shw a strng diurnal cycle like the surface skin and air temperatures, as shwn in Figures 8 and 9. The mnthly mean diurnal cycle is mstly flat with an amplitude ranging frm 2 t 5 mb. In spite f this lw magnitude the TOVS retrievals suffer frm verestimatins fr all the verpasses in September 1989, three (73, 133, and 193 LT) verpasses in April 1989 and tw (73 and 133 LT) verpasses in June These verestimatins range frm 5 t 1 mb. The surface vapr pressure shws a slight diurnal cycle with increases at 73 and 133 LT vapr pressure cmpared t the 133 and 193 LT values. This is very well prnunced in the April 1989 TOVS retrievals. The agreements between the TOVS surface vapr pressure and the mnthly mean diurnal curve frm the FIFE grund-bserved values is gd fr the ther mnths.

11 .,,, 2188 LAKSHMI AND SUSSKIND: COMPARISON OF TOVS DATA WITH OBSERVATIONS 1992 Standard Deviatins (C) NOAA 12 73am 1 8 HAPEX Sahel Observatins 6 TOVS retreivals 4 + +,x.++,x 2,x t-., ++.x+++,++.**. +x, :t,.....,at,,, z:,, *.. :. A S O N Figure 11. Standard deviatins f surface skin temperatures (C)' HAPEX versus TOVS. N 3.4. Seasnal Cycle In the case f BOREAS, there are three years f bserved data and ne can cnstruct seasnal cycles fr the grund and the satellite data. Figure 12 shws a cmparisn f hw the satellite and the grund bservatins f mnthly maximum, mnthly minimum, and mean surface skin temperatures agree with each ther. The satellite retrievals are lwer than the grund bservatins (using the IRT) fr the mnthly minimum temperature and higher than the grund bservatins fr the mnthly maximum surface skin temperatures. These maximum, minimum, and mean surface skin temperatures represent the dynamic range and characteristic fr the whle BOREAS regin (data frm all the 1 statins pled tgether) and fr all satellite verpasses (all satellites, mrning and afternn verpasses pled tgether). The mnthly mean values fr the grund bservatins have been calculated using the 15 min bservatins the twers (96 daily bservatins) and the 4 times f day verpass f the NOAA satellites fr the TOVS. The satellite retrievals capture the range in surface skin temperatures which is clse t 5øC. The differences between the satellite and the grund bservatins f the surface skin temperatures are less than 5øC in the maximum and minimum mnthly surface skin temperatures and less than 2øC in the mean mnthly surface skin temperatures (Table 5). Fr mst f the mnths the difference between the grund and the satellite mean mnthly skin temperatures is less than iøc. The difference exceeds 2øC nly rarely (Octber 1995). The TOVS surface skin temperatures capture the seasnal cycle f the grund bservatins very well; that is, they have the maxima 3.3. Spatial Hetergeneity An imprtant limitatin with regard t use f satellite data is their inability t capture small-scale spatial hetergeneity. Satellite bservatins reprt an average f temperatures in the field weighted higher t warmer temperatures as a result f the nnlinearity f the Planck blackbdy functin. In areas with large variability in land surface cver and/r relief, such as HAPEX Sahel, the questin f the biases in remte sensing estimates due t nnlinear mixing might arise. The TOVS data are gridded n 1 ø x 1 ø basis glbally as an average f all values frm all the satellite spts (nminally 6 km x 6 km) which fall within this grid. The data als include the spatial standard deviatin f the spts in the grid bx. The daily standard deviatin f the surface skin temperatures bserved in HAPEX-Sahel and reprted by TOVS data are shwn in Figure 11 fr August t Nvember In the case f TOVS, if nly a single retrieval ccurred ver the 1 ø x 1 ø regin fr the verpass, the standard deviatin is zer. The zer standard deviatin line is included in the figure. This gives us an idea f the nnzer values f the HAPEX-bserved and the TOVS standard deviatins. The HAPEX-Sahel spatial standard deviatins are between ø and 3øC, with sme values arund 4øC and ne value (NOAA 12 afternn verpass) f 9øC. In cntrast, the TOVSderived standard deviatins are generally smaller, and mst f the values are zer (nly ne bservatin in a i ø x i ø bx). The _. 1 1 _1._. _1... _1_- _ _... 1_ bttlll ltll l l VliltlOl18 f tlll 8UI-lktl2 bkii1 temperature scrvatins reflect the hetergeneity due t surface prcesses. The NOAA 11 (equatrial verpass 133) shws the largest amunts f bserved standar deviatins (between 2øC and 6øC) : -- : _ - _ : BOREAS MONTHLY MINIMUM! BOREAS MONTHLYMAXIMUMSKIN + mt TOVS SKIN TEMPERATURE (C) + ++, +++,_ O OO + +OOO* TEMPERATURE(C) BOREAS MONTHLY MEAN SKIN TEMPERATURE (C) 3 ' !! [811 Figure 12. BOREAS-measured and TOVS-retrieved mnthly minimum and maximum temperatures.

12 LAKSHMI AND SUSSKIND: COMPARISON OF TOVS DATA WITH OBSERVATIONS 2189 Table 5. Seasnal Mean Mnthly Cmparisns Between BOREAS Grund Observatins and TOVS-Retrieved Data fr Skin Temperature, øc Air Temperature, øc Vapr Pressure, mbar Year Mnth Grund TOVS Grund TOVS Grund TOVS NA NA NA NA NA NA NA and minima at the same time f the year and shw similar trends. An exceptin is when the surface skin temperature frm TOVS is lwer in February 1995 as cmpared t uary 1995 when the grund bservatins shw an ppsite trend (warmer in February relative t uary). In the case f surface air temperatures the differences between the grund and the satellite mean mnthly values are larger. There are differences f the rder f 2ø-4øC fr the winter mnths f December, uary, and February. The surface air temperature shws a cycle similar t the surface skin temperature. A reasn fr this culd be due t canpy cver which results in clse surface skin and air temperatures. The surface air temperatures shw gd cmparisns with a few exceptins. In particular, there is a difference f 4øC in uary The surface vapr pressure shws an agreement f.5 mbar r better fr mst f the mnths. The range f the seasnal cycle is arund 12 mbar fr all the three years. There are a few cases f differences greater than 1 mbar such as June The TOVS retrievals capture the seasnal cycle f surface vapr pressure with an accuracy f.5-1. mb. It can be seen that the biases in the case f the seasnal cycle are less than thse crrespnding t the daily cmparisns as seen in Tables 2, 3, and 4. This is an expected reductin due t tempral averaging. This result hlds significance fr the use f TOVS data fr climatlgical and ec- lgical studies which need lng-term data sets t mnitr vegetatin dynamics in respnse t the surface meterlgy. 4. Discussin and Cnclusins The aim f this paper has nt been t assume that the grund bservatins are perfect and characterize an "errr" in the satellite retrievals. Since satellite sensrs and grund-based sensrs have different measurement basis, they cannt be directly cmpared t characterize "errrs" in the satellite data. Instead, the statistics f cmparisns between the grund-based and the satellite-based values have been examined in detail. The validatin studies f Sugita and Brutsaert [1993], Getz et al. [1995], Kalluri and Dubayah [1995] were carried ut ver limited time perids (a few days) and very limited areal extents. A study by Jin et al. [1997] cmpared the difference between the surface skin and the surface air temperatures simulated by the NCAR CCM2 cupled with the BATS (Bisphere-Atmsphere Transfer Scheme) and thse btained frm FIFE bservatins in an indirect manner. Direct cm- parisns carried ut by Andersen [1997] between surface skin temperatures estimated by the split windw technique using AVHRR (advanced very high reslutin radimeter) data and grund bservatins fr 4 days in HAPEX-Sahel yielded dif-

13 219 LAKSHMI AND SUSSKIND: COMPARISON OF TOVS DATA WITH OBSERVATIONS ferences between -2 ø and 3øC. Xiang and Smith [1997] reprt an accuracy f 1% in the surface skin temperature and surface emissivity retrieved frm SSM/I (Special Sensr Micrwave eclgical studies. In the future, imprved satellite sensrs will be able t achieve greater precisin, better spatial reslutin, and greater tempral sampling (with the availability f multi- Imager) ver HAPEX-Sahel. Getz [1997] fund an increased ple satellites). crrelatin between higher spatial reslutin remtely sensed data and in situ surface skin temperature at FIFE. The estimates f standard deviatin f the differences be- References tween the satellite-retrieved quantities and the grund data Andersen, H. S., Land Surface Temperature Estimatin based n NOAA-AVHRR data during the HAPEX-Sahel Experiment, J. Hybtained in the field campaigns fr surface skin temperature, drl., , , surface air temperature, and surface vapr pressure vary Anyamba, E., and J. Susskind, A cmparisn f TOVS cean skin and arund (with different values fr different field campaigns and satellite verpass times, these numbers are ur "best" estiair temperatures with ther data sets, J. Gephys. Res., 13, 1,489-1,511, Betts, A. K., and J. H. Ball, FIFE surface climate and site-average data mates) 4øC, 3.5øC, and 3.5 mb, respectively, with part f these set , J. Atms. Sci., 55(7), , differences being due t sampling errrs. The maximum values Czajkwski, K. P., T. Mulhern, S. Gward, J. Cihlar, R. Dubayah, and f these standard deviatins are 7.7øC, 5.4øC, and 4.3 mb, S. Prince, Bispheric envirnmental mnitring at BOREAS using respectively, fr surface skin, surface air temperatures, and AVHRR bservatins, J. Gephys. Res., 12, 29,651-29,662, surface vapr pressure. These estimates are based n the three Getz, S. J., Multisensr analysis f NDVI, surface temperature, and ther biphysical variables at a mixed grassland site, Int. J. Remte field campaigns: FIFE, HAPEX-Sahel, and BOREAS studied Sens., 18, 71-94, in this paper and summarized in Tables 2, 3, and 4. These Getz, S. J., R. Halthre, F. G. Hall, and B. L. Markham, Surface difference estimates can be used as initial guesses t data assimilatin prcedures which assimilate satellite data in land temperature retrieval in a temperate grassland with multi-reslutin sensrs, J. Gephys. Res., 1, 25,397-25,41, surface mdels. Gutrbe, J.-P., et al., HAPEX-Sahel: A large-scale study f landatmsphere interactins in the semiarid trpics, Ann. Gephys., 12, Lakshmi and Susskind [1998] cmpared TOVS-retrieved 53-64, mnthly vapr pressure and air temperature with statin bservatins frm fur different regins (Abilene, Texas, Na- Jin, M., R. E. Dickinsn, and A.M. Vgelmann, A cmparisn f CCM2-BATS skin temperature and surface-air temperature with tinal Airprt, Virginia, Cita and Minsk in the frmer Sviet satellite and surface bservatins, J. Clim., 1, , Kalluri, S. N., and R. O. Dubayah, Cmparisn f atmspheri cr- Unin) acrss the wrld. They fund a bias f -.8 t.15 K rectin mdels fr thermal bands f the advanced very high reslutin fr the mnthly air temperature and mbar fr these radimeter ver FIFE, J. Gephys. Res., 1, 25,411-25,418, fur regins. The slpe and cefficient f crrelatin were clse t unity. The standard deviatin f the differences be- Kidwell, K. B. (Ed), NOAA Plar Orbiter Data Users Guide (TIROS-N, NOAA-6, NOAA-7, NOAA-8, NOAA-9, NOAA-1, NOAA-11, tween 1.1 and 2.4 K fr air temperature and 1. and 2. mbar NOAA-12, NOAA-13 and NOAA-14), NOAA/NESDIS, Natl. Clim. Data Cent., Washingtn, D.C., fr vapr pressure. The better agreement in this case is due t Lakshmi, V., and J. Susskind, Determinatin f land surface skin the use f mnthly data which remves the day-t-day variability and hence reduces the difference between the satellite temperatures and surface air temperature and humidity frm TOVS HIRS2/MSU data, Adv. Space Res., 22(5), , retrievals and grund bservatins. Prince, S. D., et al., Gegraphical, bilgical and remte sensing aspects f the Hydrlgic Atmspheric Pilt Experiment in the Sahel The results f this study are parallel t the study by Prince et (HAPEX-Sahel), Remte Sens. Envirn., 51, , al. [1998] where the authrs estimated differences f the rder Prince, S. D., S. J. Getz, R. O. Dubayah, K. P. Czajkwski, and M. f 3.5øC fr surface skin temperature, 3.9øC fr air temperature and 1.9 mbar fr vapr pressure deficit. Their study Thawley, Inference f surface and air temperature, atmspheric precipitable water and vapr pressure deficit using AVHRR satellite bservatins: Validatin f algrithms, J. Hydrl., , 23-25,1998. fcused n the same three field campaigns (FIFE, HAPEX- Sellers, P. J., and F. G. Hall, FIFE in 1992: Results, scientific gains, and Sahel, and BOREAS) fr the grund-based bservatins and future research directins, J. Gephys. Res., 97, 19,91-19,19, the AVHRR as the surce f satellite data. Sugita and Brutsaert Sellers, P. J., F. G. Hall, G. Asrar, D. E. Strebel, and R. E. Murphy, An [1993] fund that the crrelatin cefficient was.96 and.92 verview f the First Internatinal Satellite Land Surface Climatland the rt mean square errr was 2.2 and 3.3 K fr the gy Prject (ISLSCP) Field Experiment (FIFE), J. Gephys. Res., 97, 18,345-18,371, NOAA 9 and NOAA 1 satellite-retrieved ptential surface Sellers, P. J., et al., BOREAS in 1997: Experimental verview, scientemperatures cmpared t FIFE data fr The values fr tific results, and future directins, J. Gephys. Res., 12, 28,731- surface air temperature cmparisns fr BOREAS (as seen in 28,769, Table 3) can be cmpared against a similar study f Czajkwski Sugita, M., and W. Brutsaert, Cmparisn f land surface temperaet al. [1997] which cmpares the air temperature derived frm tures derived frm satellite bservatins with grund truth during FIFE, Int. J. Remte Sens., 14, , the AVHRR data fr the perid between April and September Susskind, J., J. Rsenfield, and D. Reuter, Remte sensing f weather 1994 (a ttal f 228 bservatins). Their results shw a stan- and climate parameters frm HIRS2/MSU n TIROS-N, J. Gedard deviatin f 4.2øC and a crrelatin f.8. The AVHRR phys. Res., 89, , Susskind, J., P. Pirain, L. Rkke, L. Iredell, and A. Mehta, Characteristics f the TOVS Pathfinder Path A data set, Bull. Am. Meterl. Sc., 78(7), , Xiang, X., and E. A. Smith, Feasibility f simultaneu surface temperature-emissivity retrieval using SSM/I measurements frm HAPEX-Sahel, J. Hydrl., , 33-36, data are at a much higher spatial reslutin than the TOVS (arund 9 km 2 cmpared t 36 km 2 fr the TOVS) which cntributes t the agreement between the satellite-retrieved and the bservatins f surface temperatures. This paper reprts the cmparisn results f the TOVSretrieved land surface variables fr a wide range f land surface cnditins. We hpe that this will prvide the scientific cmmunity with the prper interpretatin f the data in regard t its agreement with field measurements. The data can be used fr validatin, calibratin, and assimilatin in land surface hydrlgical mdels and lng-term climatlgical and V. Lakshmi and J. Susskind, Department f Gelgical Sciences, University f Suth Carlina, Clumbia, SC 298. (vlakshmi@ gel.sc.edu) (Received March 12, 1999; revised August 12, 1999; accepted August 17, 1999.)