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NASA Technical Reports Server (NTRS) 19980023498: Climate Sensitivity Studies of the Greenland Ice Sheet Using Satellite AVHRR, SMMR, SSM/I and in Situ Data PDF

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Preview NASA Technical Reports Server (NTRS) 19980023498: Climate Sensitivity Studies of the Greenland Ice Sheet Using Satellite AVHRR, SMMR, SSM/I and in Situ Data

NASA-CR-Z05169 Meteorol. Atmos. Phys. 51,239 258{19931 Meteorology and Atmos pheric Physics t Springer-Verlag 1993 Printed in Austria (_ : " /_,P' UC r: 551.583:551:579.3(988) Cooperative Institute for Research in Environmental Sciences, University ofColorado, Boulder, Colorado, U.S.A. Climate Sensitivity Studies of the Greenland Ice Sheet Using Satellite AVHRR, SMMR, SSM/I and In Situ Data K. Steffen, W. Abdalati, and J. Stroeve With 16 Figures Received January 25, 1993 Revised February 8, 1993 Summary the hemispheric circulation through its topography and strong temperature gradient towards the The feasibility of usingsatellite data forclimate research over northwestern Atlantic. The present ice sheet has the Greenland ice sheet is discussed. In particular, we a surface area of 1.75-106 km 2 and a volume of demonstrate the usefulness ofAdvanced Very High Resolution Radiometer (AVHRR) Local Area Coverage [LAC) and 2.65" 106 km 3. These correspond to 11 and 8_o of Global Area Coverage (GAC) data for narrow-band albedo the global glacier surface area and the ice volume, retrieval. Our study supports the use of lower resolution respectively (Ohmura et al., 1991). The ice volume AVHRR (GAC) data for process studies over most of the is also equivalent to 6.7 m of water relative to the Greenland icesheet. Based on LAC data time series analysis, present global sea level. The ice sheet has a typical wecan resolve relative albedo changes on the order of 2 5%. In addition, we examine Scanning Multichannel Microwave shield shape climbing relatively steeply from sea- Radiometer (SMMR)and Special Sensor Microwave Imager level to 1000m and then gently to the highest (SSM/I) passive microwave data for snow typing and other point of 3210m a.s.l, at 72.3'_N and 38.0°W, signals ofclimatological significance. Based on relationships where the thickness of ice is estimated at about between insitu measurements and horizontally polarized 19 3000 m. The average thickness of the ice sheet is and 37GHz observations, wet snow regions are identified. 1515m. The wet snow regions increase in aerial percentage from 9°0 of the total ice surface in June to a maximum of 26% in The Greenland ice sheet is a remnant of the August 1990. Furthermore, the relationship between bright- Pleistocene ice sheet. During the last ice age the ness temperatures and accumulation rates inthe northeastern volume was estimated at 2.92 to 5.59"106km 3 part of Greenland is described. We found a consistent (Ohmura et al., 1991), suggesting that the surface increase inaccumulation rate for the northeastern part ofthe ice sheet from 1981 to 1986. altitude was higher in the past. Presently the equilibrium line of the ice sheet lies at 1800 m a.s.l. 1. Introduction at the southern end, and it descends to 750 m a.s.1. in the northern Greenland (Ambach and Kuhn, 1.1 Greenland Ice Sheet - Overview 1985). The accumulation and ablation area occupy The Greenland ice sheet is one of the major ice 82 and 18%, respectively. Within the accumulation sheets in the world. It is the largest one in the area the dry snow line is found at 3100m in the Northern Hemisphere and is generally thought to south descending to 1650 m in the north, with the have survived throughout the Quaternary. The surface area of the dry snow zone being approxi- ice sheet presently plays an important role in mately 50,q_ of the entire ice sheet. 240 K. Steffenet al. 1.2 Greenland Ice Sheet - Climatology permanent cyclones, the Baffin Bay low to the west and the larger Icelandic low to the southeast. Snow and ice are key variables in the global The ice sheet is located under the weak saddle climate system. They can influence the global heat between the two depressions. The summer circu- budget through their regional feedback mechanism lation is dominated by the pressure ridge extend- in the varying exchange of heat, moisture and ing from the northeast towards the center of momentum between the surface and the atmosphere the ice sheet. Changes in the surface radiation (Dickinson et al., 1987). Therefore, accurate infor- balance and/or in the cloud amount over the ice mation on cryospheric variations (e.g. large scale sheet due to global climate change may alter the surface albedo increases or decreases due to change overall pressure distribution and consequently in dry snow area extent) are essential for the the precipitation. It is beyond the scope of this prediction of future climate change. Information paper to derive precipitation amounts, but the on the response of the ice sheet to climate warming aerial extent of dry snow area and its seasonal and is of crucial importance for reliable projection of annual variations as derived from satellite data future sea level. As the Greenland ice sheet is will help to understand the present situation. located in a relatively warm climate, it is particu- larly vulnerable to climate change. Present com- 1.3 Greenland Ice Sheet - Research Camp puter model simulations of the greenhouse scenario predict a regional cooling for the Greenland ice Scientists from the Swiss Federal Institute of sheet, contrary to the general warming trend Technology (ETH) in Ziirich established a perma- over the Arctic sea ice areas (Manabe et al., 1992). nent research camp at the equilibrium line of the Trends for the annual temperature computed Greenland ice sheet near Jakobshavn (69° 34' N, from land surface stations for the period 1961 to 49° 17' W)in spring of 1990 (Fig. 1).Climatological 1990 show a slight cooling for the southern half measurements were carried out to study the energy of Greenland (Chapman and Walsh, 1993). How- exchange and the mass balance above, at, and ever, our present knowledge of the mass and below the ice surface during April-September energy exchange, and its sensitivity to environ- of 1990 and 1991. Furthermore, the structure of mental conditions, is insufficient to make reliable the atmospheric boundary layer within the kata- quantitative estimates of future changes associ- batic wind regime on the ice sheet was recorded ated with the greenhouse warming. An observed with daily radiosonde profile measurements. Pre- 0.23 m/year thickening of the Greenland ice sheet liminary results from the two field seasons are south of 72°N was reported by Zwally (1989) summarized in progress reports (Ohmura et al., based on satellite radar altimeter measurements. 1991, 1992). The research station has been trans- The northern part of the ice sheet was not covered ferred in summer 1992 to the University of Colo- by satellite altimeter measurements and at this rado at Boulder to continue the field experiments point we don't know if the Greenland ice sheet for three additional seasons. north of 72°N is growing or shrinking. Micro-climatological measurements at a single Annual total precipitation and annual accumu- point are necessary and essential for the study of lation on the Greenland ice sheet were evaluated the magnitude of the different energy fluxes such by Ohmura and Reeh (1991). The analysis isbased as the turbulent fluxes and radiation balance at on accumulation measurements of 251 pits and the ice sheet surface. However, a single point cores obtained from the upper accumulation zone measurement isnot sufficient to discuss the clima- and precipitation measurements made at 35 tology of the entire Greenland ice sheet. Therefore, meteorological stations in the coastal region. The we propose the use of multispectral satellite data mean annual precipitation for all of Greenland to derive certain surface parameters which are was found to be 340 mm water equivalent, with an relevant to the surface climatology. The purpose increasing gradient from north to south. The of this paper is to demonstrate the feasibility of amount of precipitation is regulated primarily by satellite remote sensing as a technique for climate atmospheric conditions, such as stability, water- studies over large homogeneous snow and ice vapor content and circulation. The winter covered areas. The ultimate goal of the project is circulation is strongly dominated by two semi- the statistical analysis of the regional and seasonal Climate Sensitivity Studies of the Greenland Ice Sheet 241 ,- y, 85ON 85oN /; Nord ETH/CU Station Fig. 1. Overview of the Greenland ice sheet. The outline of the ice sheet is shown with a dotted line, and the land is depicted with a solid line. The location of the joint research camp of the Swiss Federal Institute of Tech- nology and the University of Colorado at Boulder is shown at the west side of the ice sheet near the coastal village of Jakobshavn. The labels A through K give the locations of the SMMR brightness temperature time series as given in Fig. 14 and 15 variations of surface parameters. This statistical homogeneous surface, is an ideal test area for the analysis over a period of ten years is needed to development of remote sensing techniques in improve the present knowledge and understanding climate research. Unlike sea ice and land areas, of the Greenland ice sheet surface climatology. the three major surface types of the ice sheet (glacier ice, wet snow, and dry snow) extend over 2. Method several tens of kilometers at a minimum, which makes the application of low resolution satellite 2.1 Multispectral Satellite Data data feasible for surface type classification. Past and ongoing research have shown that several The types of satellite imagery used for this study polar surface properties can be mapped through cover a range of spatial resolutions and wave- a combination of multispectral satellite and aircraft lengths. For the visible and thermal infrared data (Steffen et al., 1993; Haefliger et al., 1993; wavelengths we have used Advanced Very High Williams et al., 1991). These properties include: Resolution Radiometer (AVHRR) Local Area spectrally integrated surface albedo, surface tem- Coverage (LAC: 1.1km at swath nadir) and Global perature, shortwave and longwave radiation Area Coverage (GAC: 4km at swath nadir) data. balance, and snow properties. All of these param- For the passive microwave region, Nimbus-7 eters can be linked to the surface energy balance. Scanning multifrequency microwave radiometer The Greenland ice sheet, due to its large size and (SMMR) data (10/25/80-8/20/87), and DMSP 242 K. Steffen et al. 1 Satellite Input Data: _ p.u,,o ._,o,,.,,o m ( Algorithm SMMR &SSM/I m | -Radiance2sChannels I_ Development - Resolution 50 km (19GHz) m "_ I -o,_elolM_, I I -WoSt .o. I I -o,_G=ac__ce I I -FloodeGd_ae_r ?.........I........ .iiii!iiiiiii(o_p.t)_iiiiiiiiiiiiiiiiiiiiii!iiiiiiiiiiiii_ili iiii_ilE_,r0mii_:!i!iii_ii!ii!_iiiiiii_i!i i!iii!!i i!!iiiii! i!ili iiiiiiiiiiiiii!iiiiii iiiiii!'iiiiiii iiiiiii i!iiii ! Meteorological Data: RNeemhCamp - Radiosonde Profde T,H,V (25 kin) -Air & Sudace Temperature - Incoming Shortwave Radialk:_ - Inco_ng I_ongwave Rad_atK)n - Reflected Shortwave Radiation - Longwave Outgoing Radiation -Emissivity at37 GHz -Dislrib_dion of ,Snow Deplh [.,sfL,,J=A,_o [I ii_iii_iii__iliiii_iiii!:iiiii_iiiii!iiiiiiiiiiiiiiiiii_i!iil Satellite Input Data: VII & TIR NOAA-AVHRR Radiances 5 Channels DMSP OLS Observables --2RReaRsdeoiaslnoucltuextsi)onn6s200C1hkmainnn,e4lskin A-SlgurofarciethmTemp/erature 1 -Snow Types -. - Radiarces 5 Channels -Onset ofMelt -2 P,eso_utio_ 80m (VIS), -AI3edo LANDSAT M1S2S0rn ('FIR) --CRiaod¢¢ia1tsive Fluxes Fig. 2. Processing of multi-spectral satellite data in combination with in situ measurements for the retrieval of the surface parameters relevant to climate process studies special sensor microwave imager (SSM/I) data into discrete data sets (level lb data) at the (7/9/87-9/30/90) were used. They are presently Naval Oceanic Atmospheric Research Laboratory available for both polar regions, including the (NOARL). AVHRR LAC and GAC channel 1 Greenland ice sheet, on CD-ROM from the (0.58 /am-0.68gLm) and 2 (0.725 /_m-l.10 #m) National Snow and Ice Data Center (NSIDC), images have been calibrated over the entire Green- Boulder, Colorado. land ice sheet using pre-launch calibration coeffi- In order to retrieve surface parameters relevant cients to obtain percent albedo values. The cali- for climate process studies from satellite imagery, bration technique follows the description given in a number of sensitivity analyses and intercom- the NOAA users guide (Kidwell, 1991) and in- parisons with ground measurements are needed. cludes the non-linear corrections. In order to These are described below (Fig. 2). correct for the geometric distortions caused by the curvature of the earth and the scanning geometry the images have been geo-registered and mapped 2.2 Processino of AVHRR Data to a polar stereographic projection using a navi- The AVHRR data were received as raw data gation code developed by Baldwin et al. (1993). that have been quality controlled and assembled The accuracy of the navigation routine is at most ClimatSeensitivSitytudieosftheGreenlanIcdeSheet 243 6 to 8km dueto errorsin thetime tags.The relations which are described below, require that imageshavebeenfittedtoamapgridto reduce the data be left in image format. For such applica- thiserror to lessthan1 pixel.TheCPUtimes tions an ice mask was developed from a vector file necessartyocalibrateandnavigatetheLACand of the ice sheet boundaries which was digitized GACdatafortheentireGreenlandicesheetare from the Geological Survey of Greenland (GGU) 5minforaGACimage(325×625pixels)and84 geological map (scale= 1:250,000). This mask is minfora LAC image(1500×2500pixels)ona applied to the images and the result was a set of IBM6000workstation.OnaSUNII workstation images of the Greenland ice sheet with no land- 13minforGACand188minforLACareneeded contaminated pixels. to processthesatelliteimagery. Other applications such as time series investi- Ourpresenctomputingfacilitydoesnotpermit gations require that data points from the same theprocessingofalargenumberofLACscenes location(s) be extracted from all of the images. dueto extensivecomputingtime necessaryto The first such application is the retrieval of time calibrateandnavigatetheAVHRRLACimages series of brightness temperatures along a horizontal for theentireGreenlandicesheet.Thus,it is transect across Greenland. The transect chosen in desirableto usetheGACimagesfor longtime the first stage of the data analysis is the one that seriesanalysis.In section3 wepresenta pilot passes through the ETH/CU research station programto testthefeasibilityofusingAVHRR (Fig. 1). In addition to providing information GACimagesfor longtermclimatestudiesover about the brightness temperatures along a west/ theGreenlandicesheet. east profile across the ice sheet, this time series provides a means of identifying days of bad data. 2.3 Processing of SMMR and SSM/I Data The brightness temperatures between 318-325 ° E The SMMR isa ten-channel, five frequency linearly longitude are relatively stable with little fluctua- polarized passive microwave radiometer system. tion. Consequently, days that show very high The instrument measures surface brightness deviation from the normal (greater than 4 sigma) temperatures at 6.6, 10.7, 18.0, 21.0 and 37.0 GHz. in this longitude range are flagged as "bad" days, Vertical and horizontal polarization are provided and are linearly interpolated from the nearest for each frequency. The SSM/I operates at four "good" days. frequencies, namely 19.35, 22.24, 37.0, and 85.0 GHz. From the transect, the pixel that corresponds to Both polarizations are provided for each frequency the station location is extracted, and a time series except 22.24GHz. For this study we used the 19 of data specifically for the station is produced. In and 37GHz frequencies which have effective field addition, the horizontally polarized gradient ratio ofview dimensions of 69 × 43 km and 37 × 28km, is calculated using the following formula: respectively (along track × cross track). The GR =(19H- 37H)/(19H + 37H) (1) SMMR and SSM/I data provided by NSIDC are gridded in polar stereographic projection in 25 × where 19 and 37 correspond to the channel 25 km grid cells. frequencies in GHz, and H indicates horizontal The passive microwave data are read from the polarization. Similar time series can be produced CD-ROM, and all points within the rectangle for any location on the ice sheet, but the climate whose edges contain the outermost limits of the station is chosen initially so that comparisons to Greenland boundary are extracted. Then the in situ data can be made. ocean mask (which is provided on each CD- One final item in the processing of the passive ROM) is applied. This procedure is followed for microwave data pertains to the relationships all of the available dates, and it yields a set of between SMMR and SSM/I data. An extended images 60 x 109 pixels in size for each channel time series is important for the assessment of and each day of SSM/I and SMMR coverage. A climatological trends, but in order to effectively total of 1611 images for each SMMR channel and combine the SMMR and SSM/I data sets, a 1186 images for each SSM/I channel have been comprehensive understanding of how the two are extracted. related must be obtained. Figure 3 shows the Some applications, such as snow mapping relationship between the 4 common channels on (described in section 3.2) or SMMR/SSM/I cor- the two instruments for the overlapping days of 244 K. Steffen et al. 260 250 ....... SSM/I 19v ....... SSM/I 37v v __ SMMR 18V __ SMMR 57V 250 240 £ £ & 240 E 230 , ,. -'--'--'----,.... ..... ,- ,.._, ..... c c _c:,,250 _,220 220 210 10 20 50 40 10 20 30 40 Days Past July 9, 1987 Days Past July 9, 1987 220 220 ....... SSM/I 19H ....... SSM/I 57H _C __ SMMR 18H __ SMMR 57H 210 2t0 2o o & 200 zoo " L/- --*" _ 190 m 19o c_ & 180 180 o.......................................... 0 10 20 30 40 10 20 50 40 D(_ys Post July 9, 1987 Days Past July 9, 1987 Fig. 3. SMMR and SSM/I brightness temperatures for the overlapping days in 1987 (July 9-August 20) at 72°N and 37°W longitude 28O I I I I I __ Tb(SMMR) = 0.80 xTb(SSM/I) + .!,7.0, R = 0.94 Calculated from Greenland Data 260 24O rv- 220 09 2O0 180 Fig. 4. Relationship between S MMR 160 'J, , , I , . I , , . I . , , I , , , I and SSM/I data over the entire ice 160 180 200 220 240 260 280 sheet for the full period of overlapping SSMI coverage operation in the highest area of the ice sheet (72°N These relationships were addressed for Antarctica and 37° W). Alarge discrepancy (nearly 10degrees by Jezek et al. (1991), but they have not yet been consistently) is apparent between the 19GHz defined for the Greenland ice sheet. For all of the SSM/I channels and the 18GHz SMMR channels. overlapping days, July 9, 1987 through August 20, Similar discrepancies, though not as pronounced, 1987, SSM/I 19GHz vertically polarized bright- exist for the 37GHz channels. ness temperature measurements are compared to Climate Sensitivity Studies ofthe Greenland Ice Sheet 245 the corresponding SMMR observations (18 GHz 8O LAC vertically polarized), and the relationships are 0 assessed. The results, which correspond well to "0 Jezek et al. (1991) results are shown in Fig. 4. The _ 60 0 small discrepancy can most likely be credited to the fact that Antarctica has much colder regions C0 _ 40 than Greenland. Consequently, the data points t A E; span a much broader range of temperatures, so o the behavior of the correlation curve is better L o 20 i i t I defined at the low end. Furthermore there is Z 49.8 49.3 48.8 48.3 47.8 47.3 simply more data from Antarctica because of its Longitude (degrees west) size. The similarity between the two regression lines, however is encouraging• 8O __ LAC Based on this comparison, the SMMR time O series can be extended to include the SSM/I data, "ID 70 _0 _D for a data set that currently spans over 13 years. O Similar relationships need to be determined for -O 60 E the other channels• O C © I 5O 8 3. Results L ZD 40 I I I I 3.I Narrow-Band Albedo of Snow 47.5 44.5 41.5 38.5 35.5 32.5 Longitude (degrees west) The variation of solar radiation absorbed and reflected by the earth is a key factor in the Fig. 6. West-east transect plots of narrow-band planetary understanding of climate change. If the solar albedo (0.58-0.68pm) for AVHRR Local Area Coverage (LAC: dotted curve) and Global Area Coverage (GAC: line radiation reflected by the earth were to increase, curve) data at 69.6_'N of the Greenland ice sheet. Four a trend toward the extension of the winter season regions for June 06, 1990 are represented. Region A shows would result, as can be shown in a simple energy the bare ice, region Bthe wet snow, region C the dry snow, feedback mechanism (Budyko, 1974). The snow and region D the clouds 1.0- Very New Snow NewSnow 0s- OnsetofMelt Z NewSnow 0,6_ WetSnow 2 i h _ 0.4- 0.2- Fig. 5. Hemispheric spectral albedo for very new snow (a few hours old), new snow (1 2 days old), O.O- onset ofmelt, and melting snow surface inthe spectral 3OO 1200 1500 1800 2100 2400 270O range 300-2500nm measured at the ETH/CU re- Wavelengths (run) search camp in 1991 246 K. Steffenetal. cover is one of the most reflective naturally Region C: Dry Snow occurring materials, and therefore, an under- Over dry snow where the albedo remains relatively standing of its visible solar reflectance is of great constant, the agreement between the LAC and the importance. The spectral albedo of different snow GAC images are very high, with mean differences types at the ETH/CU research camp was measured practically zero. In this region the albedo is very for the purpose of calibrating and interpreting homogeneous and thus very little difference between satellite derived values (Fig. 5). All measurements the two resolutions is expected (Table 1). were carried out at solar zenith angles between 50° and 68°.The snow grain diameter varied for Region D: Clouds the different measurements: very new snow 0.5 mm; new snow 0.1-0.2mm; onset of melt and wet Detection of clouds tends to be difficult since snow 1-5 mm. cloud reflectance is similar, if not identical, to AVHRR LAC and GAC data were processed as snow reflectance. However, thick overcast regions described in section 2.2 to study the differences in show noticeable fluctuations in reflectance (5Y/o). pixel resolution for albedo retrieval over the ice In these regions the agreement between the LAC sheet. Figure 6 shows transect plots of the planetary and the GAC images decreases slightly (Table 1), albedo values for both the LAC and GAC channel 1 but due to the large size of the cloud features (4 km (0.58-0.68/tm) across the ice sheet at 69.6°N. or larger) the GAC data is able to detect the slight Four different regions can be distinguished: variation in reflectance due to the presence of clouds as well as the larger decrease in reflectance Region A: Bare Ice and Slush from the cloud shadows. The ability of the GAC data to detect the cloud shadows could be a useful In this region the majority of the snow has melted tool in cloud discrimination over the Greenland and bare ice remains. Surface lakes tend to form ice sheet. on top of the bare ice, and some slush also exists. Thus, the same information for wet snow, dry The agreement between the LAC and GAC images snow, and clouds can be obtained by using either is low in comparison to the dry snow regions. high or low resolution data. Using the GAC data Such a result is expected because the melt lakes during summer months, when the melt region is are within the 1km foot-print ofthe LAC data but relatively large, should be adequate. When.the the GAC footprint contains information from the melt season isjust beginning, the lower resolution_ surrounding area as well (Table 1). data will not be able to accurately detect the melting since the pixel will be contaminated by Region B: Wet Snow dry snow. Higher resolution data such as LAC In the wet snow region, the agreement between will be needed at the margin of the ice sheet in the LAC and GAC data is much improved. such cases. In addition, to study the surface lakes Although, small scale variation is not detected that form during the melting season, high resolution with the GAC data, the general gradual increase data with pixel size less than 2km (average lake of albedo is well represented (Table 1). diameter) will also be needed. To obtain snow surface albedo maps it is necessary to correct the AVHRR satellite data for the intervening atmosphere. Such a correction can Table 1. Statistical Results ofthe ComparisonBetween LAC be accomplished using a linear relationship between and GAC Datafrom June 06, 1990.Values are given for a clear sky planetary albedo and surface albedo region 300km2region (Koepke, 1989), or by using a radiative transfer model, such as LOWTRAN or 5S, to correct for Region Mean Stddev of Correlation scattering and absorption. Although presently difference% difference there is no consensus among scientists as to which Bare ice - 2.05 3.06 0.890 atmospheric radiative transfer code is best, most Wet snow -0.16 0.89 0.970 codes tend to disagree only for large optical Dry snow -0.04 0.26 0.998 depths of aerosols and large off-nadir angles (60 ° Clouds 0.06 2.21 0.997 or more) (Royer et al., 1988). The 5S code developed Climate Sensitivity Studies of the Greenland Ice Sheet 247 by Tan re et al. (1990) provides reasonably accurate Table 2. Input Parameters for the 5S Code Run for the Lz_cation atmospheric modeling and surface reflectance t_fthe ETH/CU Camp, May 23, 1991 retrieval (Teillet, 1992). The overall scheme involves correcting the narrow-band planetary albedo as Solar zenith angle 49.0 Sensor zenith angle 29.40 '_ viewed by the sensor to the narrow-band surface Solar azimuth angle 285.57' albedo, taking into account illumination, sensor Sensor azimuth angle 243.69" view angle, and atmospheric effects. Atmospheric profile (i) H20=0.62gcm 2 To run the code, illumination and observation (ii) 03 = subarctic summer geometries for each pixel are provided as input. Aerosol model Continental Aerosol level Visibility = 8km The water vapor input can be calculated from Spectral bands NOAA-I 1AVHRR channel 1 the AVHRR channels 4 (10.30 ll.30/tm) and 5 Surface reflectance Dry snow (11.50-12.50/_m) or it can be provided by radio- sonde data that has been collected at the ETH/ CU camp during the summers of 1990 and 1991 (Haefliger et al., 1993). Radiosonde data are also These quantities, which are necessary to calculate available from the summit of the Greenland the surface reflectance, are obtained by running ice sheet and from the coastal station Gothavn the 5S code. and will serve to derive an "average atmosphere" A three day time series for the AVHRR LAC for the data available in 1990 and 1991. Further channel 1was atmospherically corrected using the parameterization is needed to extrapolate for in situ radiosonde profile data (See Table 2 for 5S other years. The ozone concentration is taken to inputs). Figure 7 illustrates the temporal changes be that of subarctic summer. Aerosol content can of narrow-band surface albedo for the dates May vary significantly in Arctic regions (Lindsay and 23, June 01, and June 08, 1991, along a transect Rothrock, 1993) and can result in large errors in at 69.& N (see also Fig. 1).This time series shows deriving the surface reflectance. Unfortunately, a continual decrease in albedo over time. On May optical depths of aerosols over the Greenland ice 23, 1991, the snow cover was dry and the land was sheet are not known and standard atmosphere also covered by snow with many rocky areas assumptions must be made. Using the 5S code, it showing through. ByJune 01, melting had occurred is possible to retrieve surface reflectance by the and the surface reflectance dropped considerably, following expression: and by June 08 the reflectance has decreased further by 5'_/o.The ability of the sensor to detect pi = 100 (Ai + Bi) (2) melt enables us to determine the aerial extent of (100 + (Ai+ Bi)Si) the transition regions along the perimeter of the (pidos)2 (1- 100pa,, ) ice sheet. Figure 8 shows the AVHRR channel 1 Ai - Bi = (3) imagery for the Jakobshavn area on June 8, 1991 TOT.sTvCOS0s' TsZv (see also Fig. 1 for location). Cloud shadows where the indexj stands for the AVHRR channel running north-south on the left side of the image (i = l, 2). (snow covered ice sheet) can be identified. The snow free areas of the ice sheet along the margin surface reflectance for channel 1 or 2 in (bare ice) stand out against the higher reflectance percent of wet snow areas. pO reflectance at the sensor The narrow-band surface albedo as derived atmospheric reflectance from AVHRR channel 1and corrected with the 5S Patm S= atmospheric spherical albedo radiative transfer model for atmospheric scatter- Tg total gas transmittance ing, and absorption was compared with in situ total scattering transmittance in solar measurements at the ETH/CU station. The date "Cs direction used for this intercomparison is 23 May 1991 L, = total scattering transmittance in sensor under clear sky conditions. The in situ hemispheric direction spectral albedo, integrated over the wavelength ds-- solar distance region 580-680nm, is 91.6j.oo/, and the narrow- 0_= solar zenith angle. band albedo derived from AVHRR channel 1 is 248 K. Steffen et al. 1°t° 8O ! 't J :' I :3 / ; , :1 J,-; 60 ! :,. 4 o ¢ I.,/ Ii t/ c b 40 !_ o I ;P .t (; / 2orj __ 25 May .\ ......... 01 June Fig. 7. Narrow-band surface albedo _ _ 08 June (0.58 0.68/_m) time series showing the melt region at 69.6" N. The time 50 W 47W series is from May 23, June 01 and Longitude (degrees west) June 08, 1991 Fig. 8. AVHRR channel 1 imagery for the Jakobshvan area of the Greenland ice sheet, June 8, 1991. Location and size of this imagery is given in Fig. 1. Bare ice, wet snow, dry snow and cloud shadows can be identified by different gray levels ¸¸ I DRY SNOW [CLOUD SHADOWS (reflectance) 90.2/oo/. A recent intercomparison with the same channel 1. Furthermore, the comparison suggests input values but using the LOWTRAN for the that the standard aerosol and ozone concentrations atmospheric correction gave a narrow-band give reasonable results for this case study. However, albedo value of 93.3_ (Haefliger et al., 1993). comparisons for different seasons and a larger This preliminary analysis shows that with in situ number of cases are needed to derive statistically atmospheric temperature and humidity profile significant values. data as input variables for the radiative transfer It should be noted that altitude dependence was model, narrow-band surface albedo can be derived not considered in these computations, as we were within one and two percent accuracy from AVHRR only interested in relative changes of the albedo.

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