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An AVHRR investigation of surface emissivity near Lake Eyre, Australia
REMOTE SENSING OF ENVIRONMENT 20:153-163 (1986)
153
An AVHRR Investigation of Surface Emissivity Near Lake Eyre, Australia
I. J. BARTON CSIRO Division of Atmospheric Research, Private Bag No. 1, Mordialloc, 3195, Australia
T. TAKASHIMA Meteorological Research Institute, Ibaraki, 1apan
An attempt is made to gain information on land surface emissivities using only data from the AVHRR instrument on the NOAA-7 satellite. Measurements were taken of the water surface of Lake Eyre to determine the effect on the satellite radiances of the absorption by atmospheric water vapor. The results show that during the night the 11 tzm emissivity is less than that at 12/tm for both sand and salt surfaces, but during the day the reverse is true for the salt surface while the emissivities are equal for the sand. Some radiometric measurements of sand emissivity are also presented. A comparison is made between the lake water surface temperatures derived from standard sea surface temperature algorithms and those obtained from a model of radiative transmission through the atmosphere.
Introduction The various radiometers on the NOAA polar orbiting operational meteorological satellites currently supply measurements of the earth's atmosphere and surface several times daffy. For cloudless skies accurate measurements of sea surface temperature are available from the infrared channels of the AVHRR. Accuracies better than 1 K are obtained in regions where the atmospheres are not heavily laden with water vapor (i.e., not tropical areas). Accurate measurements are possible because the sea surface is homogeneous and horizontal temperature gradients are small. Also the infrared emissivity of sea water is accurately known and corrections for atmospheric absorption are possible using differential absorption techniques (Barton, 1985). The presence of clouds in the radiometer field of view and excess water vapor ©Elsevier Science Publishing Co., Inc., 1986 52 Vanderbilt Ave., New York, IVY 10017
absorption can cause inaccuracies in surface temperature measurements from space. It is, however, possible to get some estimates of sea surface temperature in partly cloudy areas using different analysis techniques; e.g., the N-star technique which compares infrared radiances in contiguous pixels (Smith, 1968). In totally cloudy areas microwave radiometers can be used to give sea surface temperature with an accuracy of 2 - 4 K, but at these accuracies it may be better to use climatological values. During the next decade there is to be a concerted effort to provide more accurate sea surface temperatures from space. The TOGA experiment of the World Climate Research Program has specified the need for global sea surface temperatures to accuracies of 0.3 K. Much work is in progress to refine the multichannel algorithms used to provide measurements from the AVHRR instrument. Also, the 00344257/86/$3.50
154
European Space Agency has included the Along Track Scanning Radiometer on its first remote sensing satellite, ERS-1, due for launch in 1989. This satellite instrument has been specifically designed to give sea surface temperature measurements with an accuracy approaching that required by TOGA. In contrast to these activities over the oceans, there has been little effort to produce reasonably accurate surface temperature measurements over land. The AVHRR instruments give an abundance of data related to the surface temperature but accurate interpretation is most difficult for the following reasons: 1. Land surfaces are usually inhomogeneous, and a large range of surface temperatures can exist over "small distances. 2. The surface emissivity is unknown and is likely to vary with moisture content, vegetation cover, and surface material. 3. The atmospheric absorption is more difficult to assess as large vertical t e m p e r a t u r e gradients and inversions can exist near the surface. 4. The surface temperature has a large diurnal cycle. 5. In situ measurements of surface (radiative) temperature can be quite difficult (Fuchs and Tanner, 1968). Air temperatures just above the surface are usually quite different from the actual surface temperatures. In many applications of satellitederived temperature, the accuracies required are quite coarse and a simple assumption of unity emissivity and a rough estimate of atmospheric absorption based on standard atmospheric profiles of water vapor and temperature are sufficient to
I. ]. BARTON AND T. TAKASHIMA
give a useful measurement. However, there are other applications relating to soil moisture estimation, energy balances at the surface, geologic exploration, and land use management where a more accurate measurement is required. Several reports exist on the laboratory measurement of the emissivity of various soils and rock types using broad band (8-13 /zm) radiometers (Lorentz, 1973). There have also been spectral measurements of the emissivity of specific compounds (e.g., Taylor, 1979; Hovis and Callahan, 1966; Conel, 1969). Vincent and Thomson (1972) presented a dual band technique for remotely estimating the silicate content of various surfaces. All these measurements form a guide to the assessment of the emissivity of natural land surfaces but accurate details of emissivities at the AVHRR wavelengths are not yet available. The Thermal Infrared Multispectral Scanner (TIMS) data discussed by Kahle and Goetz (1983) highlight the difficulty of separating surface emissivity from surface temperature effects. Given the problems associated with the interpretation of satellite measurements the best method of obtaining information on surface emissivity may be from the satellite measurements themselves. In this paper we assess the potential of gaining information about infrared surface emissivity from AVHRR data alone; i.e., no ground truth data are used. The discussion herein will be most useful to those involved in the interpretation of AVHRR data over land surfaces.
Radiometer Measurements
An accurate infrared radiometer was used to measure the emissivity of beach
LANDSURFACEEMISSIVITY
155 z~ J J J J
.4/
J J
RADIOMETER
\ ~ \ \ \
/
SUREAEE THERMOMETER
FIGURE1. The groundbasedradiometermeasurementof sandemissivity. sand. Measurements were taken on an area out of doors on cloudless days. The single channel radiometer has a spectral response that is similar to that of Channel 4 on the AVHRR instrulnent on NOAA-7. The radiometer was used to measure the radiation from the sand for zenith angles between 70 ° and 30 ° . The radiation consists of two components, that emitted by the sand and that of the reflected sky emission. Therefore, measurements of the sky radiation at the same zenith angle were also taken. Figure 1 shows the experimental arrangement. Measurements closer to the vertical than 30 ° were not possible as the reflected component would include a significant proportion from the radiometer and its operator. The surface temperature of the sand was taken using a mercury in glass thermometer placed just below the surface. The radiance measured by the radiometer I n is given by
+[1--Es(O)]BsKY(O), (1) where e~ is the surface emissivity, B(T~)
is the black body radiance at the surface temperature T~, Bsr Y is the measured sky radiance, and /9 is the zenith angle. In Eq. (1) we have assumed that the diffuse reflection of downward sky-emitted radiation can be approximated by the specular reflection of the sky radiation at the appropriate angle of incidence. Although this is the case for rough water surfaces where the emissivity is also near unity (Sidran, 1981), the reflection for land surfaces is expected to be neither Lambertian nor isotropic. In the above equation the emissivity is almost entirely determined by the measured surface temperature and the radiance of the surface so that any errors introduced by this approximation are qttite small. The results for six days of measurements in October 1984 are shown in Fig. 2. The spread of the results at each zenith angle indicates that there are considerable errors involved in this crude technique for emissivity measurement. The general shape of emissivity vs. angle curves for natural surfaces is reasonably well reproduced, and we can estimate that, for our sand, the zenith emissivity is between 0.97 and 0.98.
I. J. BARTON AND T. TAKASHIMA
1.56 1"00
t
O.9B
>~mA
0-96 oq
o
%. "%
t.u
o
• u
% N •
X
\
0.%
o
\=
,% \ o % 0"92 0
• 20
i t~0
I 60
I 80
ZENITH ANGLE (°) F I G U R E 2. The sand surface emissivity as a function of zenith angle from measurements taken on six days during October 1984. The dashed line is through the mean value of each angle.
The Lake Eyre Region Data Lake Eyre is the largest of the dry salt lakes that abound in the center of the dry Australian continent. There are, however, times (only a few each century) when excess rains in the catchment area in the northeast of the continent cause Lake Eyre to be filled with water. Such was the case in 1984 when the lake was filled in June and then slowly drained and evaporated until it was completely dry again by June 1985. During October and November of 1984 satellite data were collected for four sites in the Lake Eyre region to assess the possibility of obtaining useful information about surface emissivity at the AVHRR wavelengths. Three sites were chosen after careful examination of satellite data in both the visible and infrared channels showed them to be of a uniform nature over a large
area (20 × 20 km). The fourth site was the water in Lake Eyre itself. This site was included to allow an estimation of the atmospheric effect (water vapor absorption and emission) on the satellite measurements over the other three sites. The Lake Eyre region was chosen for this work for several reasons. First, there are large expanses of apparently homogeneous surfaces which relax the requirements for accurate ground location of the satellite data. Second, the measurements of the water in Lake Eyre can be used to give a correction for atmospheric absorption, and, third, the larger dry continental expanses ensure that similar air masses will exist over each of the sites at the same time. The location of Lake Eyre and the other three sites are shown in Fig. 3. Also shown is Woomera from where daily radiosonde data are available. Sites A and C are assumed to be similar and to con-
LAND SURFACE EMISSIVITY
157
sist of large sandy expanses with only very sparse vegetation cover while site B is located in the north of a salt lake, Lake Torrens, which remained dry throughout the period of interest. The satellite data collected were the raw counts in the NOAA-7 AVHRR Channels 4 and 5 at each location together with those of the four adjacent pixels. The five values for each channel at each site were averaged and then converted to brightness temperatures. Measurements of the salinity of the water in Lake Eyre were kindly supplied by the University of Adelaide and daily (0900 local time) radiosonde data for Woomera were obtained from the Bureau of Meteorology.
Land
the dependence has only a minimal effect on the following analysis. Usually A is no more than 5% of /sat" For the AVHRR channels in the 10-13 #m band the following relation applies (Price, 1983): (3)
I = a T 45,
where T is the black body temperature and a is a constant. The satellite brightness temperature can then be expressed as Tsat ~-~-( -4T' 5surf -tf
A/a)
1/4"5"
(4)
In the 10-13 #m window A is small compared to the surface emitted radiance so that the satellite brightness temperature can be approximated by
Measurements
Tsat ~
At 15 times during October and November 1984 satellite data from NOAA-7 were collected for the four sites shown in Fig. 3. Without any ground-based measurements of surface temperature or emissivity, there is only limited information that can be gleaned from the satellite data. In any one channel the radiance measured by the satellite can be expressed as the difference between the ground emitted radiance and a component due to the combined effects of atmospheric absorption and emission, viz., /sat = elsurt -- A ,
(2)
where I is radiance, e is emissivity, and A is the atmospheric component. Although A is weakly dependent on the surface emissivity through the reflectance of downward sky emittance [see Eq. (1)]
~l/4"5Tsurf- A',
(5)
where A' represents the atmospheric effect. The difference between Channel 4 and 5 brightness temperatures is then given by A = Z4sat - Z5sat = Zsurf((e4) 1/4'5 - (£5) 1/4'5) - (A'4 - A'5).
(6)
The emissivities of water for different angles and salinities can be calculated from the data of Friedman (1969) and are shown in Fig. 4. It can be seen that the emissivities for Channels 4 and 5 are approximately equal and are near unity except for very large zenith angles. Therefore, the factor (e4) 1/45 - (%)1/45 is near zero and, from Eq. (6), the dif-
158
I.J. BARTON AND T. TAKASHIMA
~7
FIGURE 3. The location of Lake Eyre (D), the other three sites (A,B,C), and the radiosonde station at Woomera (W). The image is from Channel 2 data on the NOAA-7 AVHRR.
ference in the brightness temperatures in Channels 4 and 5 over water is then proportional to the difference in the atmospheric effect in the two channels. With the assumption that the air mass above the entire study area is uniform, it is possible to derive the effect of the atmosphere from the water surface measurements. Thus by adding superscripts for water ( W ) and land (L), the difference between A for the two sudaces is
given by
~ _ ~ __ ~u~r,[(~4~)~4~ _ t 4~ t ~'~].
(7) Equation (7) shows that if the difference in the brightness temperatures over water is subtracted from the difference over land, the result is an indication of the
159
LAND SURFACE EMISSIVITY
100
i
i
i
" -.'.-.-.- ~ . _ _ _ . ~ I T H
~ ~
0 •9 6
o
.
9
ANGLE 0°
8
. -
I
I
S°°
~
094
difference between the land surface emissivities for the two channels. For our data the differences in the brightness temperatures were calculated for each of the four sites and then AL - Aw obtained for the three land surfaces. The results are shown in Fig. 5 with the ordinate giving the orbit numbers in the order of Table 2. For emissivities near unity the difference between the emissivities is only weakly dependent on the emissivity itself and Eq. (7) can be reduced to
o i. ~-I ~EAWATER
0 92
J
- -
CHANNEL4
---
CHANNELS I
160
I
200
300
SALINITY (ppf.) FIGURE 4. The water surface emissivity dependence on angle and salinity for AVHRR Channels 4 and 5. The values are calculated from the data given by Friedman (1969).
e
=4.5(AL
-
.I
L
(8)
This final equation has been tested by using an atmospheric transmission model (Barton, 1985) with a variety of atmospheres and a selection of surface emissivities. In all cases the difference in the surface emissivity of the two channels could be reproduced with an accuracy of 15%. For example, with the standard mid-
SITE A (SAND) SITE B (SALT) SITE [ (SAN0) DAY NIGHT DAY NIGHT DAY NIGHT 2 I
° • OOO•00
<~
0 O0 O0
-I
•
o°o
-2
o•
o•
0%0
•
t 17237 FIGURE 5.
17187
ORBIT NO.
17519
17625
The values of ~L _ Aw for each site. The ordinate is the satellite orbit number in the order of Table 2.
160
I. J. BARTON AND T. TAKASHIMA TABLE 1 Emissivity Differences in AVHRR Channels 4 and 5 for the Three Land Surfaces SITE
SURFACE
A
sand
B
salt
C
sand
DAY/NIGHT
day night day night day night
latitude summer model of MeClatchey et al. (1972) and land surface emissivities of 0.95 and 0.97 in Channels 4 and 5, respectively, the transmission model gave brightness temperatures over water and land that gave an emissivity difference of 0.019. By setting T~Lrf to 290 K at night and 310 K for the daytime satellite passes the differences in the emissivities of the two channels can be estimated. The results are given in Table 2. The major errors in this analysis are due to the noise in the satellite-derived brightness temperatures. In both channels the noise-equivalent temperature o T is 0.1 K. In Eq. (8) of the expression AI~_ hw thus has a standard error of
A L -- ATM
E4 -- 8 5
+ 0.1 ( + 0.2) - 1.1 + 0.9 - 0.5 + 0.2 - 1.2
0.001 ( +_0.003) - 0.017 0.013 - 0.008 0.003 - 0.018
(40~) x/2, i.e., 0.2 K. The effect of any error in T~rf is negligible compared to this error, and thus the standard error in (e~4~- e L) due to the satellite instrument noise is 0.003. There is also an error introduced when using Eq. (8) as an approximate form of Eq. 7. This error can be estimated by expanding the relations e 1/45 = (1 - q)1/4.5 with q << 1. The resultant error in e 4 - e ~ for emissivities near 0.9 is only 2%, which again is negligible compared to the above standard error. Figure 5 shows that the results for the sandy site C are less variable than those for the sandy site (A) and the salt lake site (B). For both surfaces the daytime emissivity difference (e L - e L) is larger
TABLE 2 Lake Eyre Surface Temperature T. Calculated by (i) the Transmission Model of Barton (1985) and (ii) Using the Standard Surface Temperature Algorithms of McClain (1984) DArE
ORBIT
T4
T5
T4 - T5
(Barton)
(McClain)
Day
26/10 29/10 1/11 6/11 12/11 13/11 15/11
17237 17279 17322 17392 17477 17491 17519
293.4 206.2 294.2 294.3 294.7 295.1 293.6
292.9 294.9 293.5 293.2 294.2 294.0 292.8
0.5 1.3 0.7 1.1 0.5 1.1 0.8
205.2 300.3 296.8 297.4 297.1 298.3 297.9
294.8 299.7 296.1 297.2 296.1 298.0 295.7
Night
23/10 27/10 28/10 30/10 1/11 14/11 16/11 23/11
17187 17244 17258 17286 17315 17498 17526 17625
290.8 292.0 294.2 291.7 290.6 292.6 292.2 289.3
289.6 291.2 293.2 289.9 289.9 291.6 291.3 288.3
1.2 0.8 1.0 1.8 0.7 1.0 0.9 1.0
295.4 294.2 296.7 297.5 293.0 295.4 296.3 292.5
294.2 294.5 297.3 296.8 292.7 295.6 294.6 291.1
LAND SURFACE EMISSIVITY
than that at night by 0.020. The two sand surfaces show good agreement with both having differences that are 0.010 less than the salt surface. The difference between the day and night values indicates that the nature of the surface (e.g., moisture content) changes from day to night. The variability in the values at each site is most likely due to minor variations at the site combined with the difficulties of accurate navigation of the satellite data (ground location errors are approximately 5 km). At typical surface temperatures a variation in surface emissivity of 0.02 is equivalent to a change in brightness temperature of 1.5 K. For thermal inertia studies where the difference between day and night temperatures is often less than 10 K, the surface emissivity obviously becomes an important factor. The nighttime results of emissivity difference for sand are in general agreement with other measurements that give an emissivity of 11 /~m that is slightly less than that at 12 /~m. The differences between the day and night measurements obviously require further study. Water Measurements
Algorithms for obtaining accurate sea surface temperatures from AVHRR data have been developed by McClain (1984) and Barton (1985). For NOAA-7 the best results have been obtained using Channels 4 and 5 (10.8 and 12.0 /xm) as the shorter wavelength Channel 3 is contaminated by reflected sunlight during the daytime and has excess noise at night. The NOAA-7 satellite has a daytime pass at mid- to late-afternoon local time and a nighttime pass near 3 a.m. For the open sea at midlatitudes accuracies better than 1 K are now possible using simple linear
161
algorithms. The difference between the sea surface temperature and the satellitemeasured brightness temperature is primarily a function of the weighted water vapor content of the atmosphere. There are, however, certain atmospheric conditions involving large anomalous temperature gradients near the surface when these simple algorithms do not apply (P. McClain, private communication). Unforttmately, such is the case for our study area. All our measurements are taken under cloudless conditions and so there is considerable heating of the surface during the day and strong radiative cooling during the night. For the daytime satellite pass the land surface temperature is well in excess of 30°C, the temperature of the water surface is 20-25°C, and the temperature of the top of the surface mixed layer at 300-500 m above the surface is 20-25°C. Advection of the hot surface air over the cooler lake surface gives the large vertical temperature gradients that prevent the use of the algorithms. At night the land cools down to between 10 and 15°C while the temperature of the air above the surface layer is similar to that during the day. Typical temperature profiles are shown in Fig. 6. These have been derived from studies of aircraft and balloon measurements over central Australia by Clarke and Brooke (1979). Because the standard water surface algorithms do not apply here it is necessary to use a transmission model [in this case that of Barton (1985)] to obtain an estimate of the "atmospheric effect" on the satellite measurements. Due to evaporation the salinity of the water in Lake Eyre increased from 110 to 175 ppt during the measurement period. The dependence of the infrared emissivity of the water on
162
I.J. BARTON AND T. TAKASHIMA BOO
800
l
x ~- Z~O0
I
/\ 10
20
/\
30
L,0 10 20 TEHPERATURE (°C)
LAND
FIGURE 6.
00 30
~.0
WATER
Typical profiles of temperature for day and night over land and water surfaces.
salinity can be determined from the data given by Friedman (1969) and is included in the transmission model calculations. The errors in derived temperature introduced by neglecting these emissivity variations are quite small being of the order of 1 K between pure and salt saturated water. To estimate surface temperature from the satellite data, it is necessary to determine first the effect of the atmosphere on the upwelling radiation. Initially the midlatitude summer standard atmosphere of McClatchey et al. (1972) was used to specify the atmosphere above the water surface. The temperature at 400 m above the surface was set at 23°C, and then the water vapor content at different levels was adjusted along with the water surface temperature until the atmospheric transmission model reproduced the observed satellite brightness temperatures in the two channels. When adjusting the water vapor profiles, note was taken of the radiosonde data obtained at Woomera some 200 km south of Lake Eyre. A water surface temperature was thus obtained, and the values are shown in Table 2. It must be remembered that these values are radiative temperatures of the top few microns of the surface which, in stable
conditions, can be quite different to the bulk temperature of the water (Robinson et al., 1984). A good example of this is orbit number 17279 on October 29 when the surface radiative temperature is in excess of 300 K. Table 2 also includes values of surface temperature derived using the standard sea surface temperature of McClain (1984). Agreement between the two surface temperatures in most cases is better than 1 K but on two occasions the difference is near 2 K.
Conclusions In this paper we have investigated the possibility of using accurate AVHRR measurements over land surfaces. We have shown that it is possible to gain some information on the relative values of the surface emissivity at different wavelengths without the need for ground truth data. It is evident, however, that ground truth data must be combined with the satellite measurements before it will be possible to get accurate satellite measurements of land surface temperatures from the AVHRR instruments. The apparent difference between the day and night relative emissivities for the AVHRR Channels 4 and 5 requires specific inves-
LAND SURFACE EMISSIVITY
tigation. Hitherto, there have been many measurements across the entire 8 - 1 3 / ~ m band, but now data are required to match the AVHRR thermal infrared bands. Until this is done the use of these channels in thermal inertia studies relating to soil moisture measurement and geologic exploration will be limited.
The satellite data were collected and analysed by Ms. J. Bathols. The salinity data for Lake Eyre were supplied by Dr. Bill Williams from Adelaide University and the radiosonde data for Woomera were supplied by the Australian Bureau o f Meteorology. References Barton, I. J. (1985), Transmission model and ground truth investigation of satellite derived sea surface tempera~res, I. Clim. Appl. Meteorol. 24:508-516. Clarke, R. H., and Brooke, R. R. (1979), The Koorin Expedition, Australian Department of Science and the Environment, Australian Government Publishing Service, Canberra, Australia, 359 pp. Conel, J. E. (1969), Infrared emissivities of silicates; experimental results and a cloudy atmosphere model of spectral emission from condensed particulate mediums, I. Geophys. Res. 74:1614-1634. Friedman, D. (1969), Infrared characteristics of ocean water (1.5-15 /~m) Appl. Opt. 8:2073-2078. Fuchs, M., and Tanner, C. B. (1968), Surface temperature measurement of bare soils, I. Appl. Meteorol. 7:303-305. Hovis, W. A., and CaUahan, W. R. (1966), Infrared reflectance spectra of igneous rocks, tufts and red sandstone from 0.5 to 22 I~m, 1. Opt. Soc. Am. 56:639-643. Kahle, A. B., and Goetz, A. F. H. (1983), Mineralogic information from a new airborne thermal infrared multispectral scanner, Science 222:24-27.
163
Lorentz, D. (1973), Die radiometrische Messung der Boden-und Wasser-oberflachen temperatur und ihre Anwendung insbesendere atd dem Gebeit der Meteorologic, Z. Geophys. 39:627-701. McClain, E. P. (1984), Multi-channel sea surface temperatures from the AVHRR on NOAA-7. Satellite derived sea surface temperature: Workshop-II, Pasadena, Ca, JPL Publication 84-5, pp. 1-8. McClatchey, R. A., Fenn, R. W., Selby, J. E. A., Volz, F. E., and Goring, J. s. (1972), Optical properties of the atmosphere. Environmental Research Paper No. 411, AFCRL-72-0497, Air Force Cambridge Research Laboratories, Bedford, MA, 90 pp. Price, J. C. (1983), Estimating surface temperatures from satellite infrared data--a simple formulation for the atmospheric effect, Remote Sens. Environ. 13:353-361. Robinson, I. S., Wells, N. C., and Charnock, H. (1984), The sea surface thermal boundary layer and its relevance to the measurement of sea surface temperature by airborne and spaceborne radiometers, Int. J. Remote Sens. 5:19-45. Sidran, M. (1981), Broadband reflectance and emissivity of specular and rough water surfaces, Appl. Opt. 20:3176-3183. Smith, W. L. (1968), An improved method for calculating tropospheric temperature and moisture from satellite radiometer measurements, Month. Weath. Rev. 96:387-396. Taylor, S. E. (1979), Measured emissivity of soils in the southern United States, Remote Sens. Environ. 8:359-364. Vincent, R. K., Rowan, L. C., GiUespie, R. E., and Krapp, C. (1975), Thermal-infrared spectra and chemical analyses of twenty six igneous rock samples, Remote Sens. Environ. 4:199-209. Vincent, R. K., and Thomson, F. (1972), Spectral compositional imaging of silicate rocks, ]. Geophys. Res. 77:2465-2472. Received 27 1anuary 1986; revised 27 May 1986.
153
An AVHRR Investigation of Surface Emissivity Near Lake Eyre, Australia
I. J. BARTON CSIRO Division of Atmospheric Research, Private Bag No. 1, Mordialloc, 3195, Australia
T. TAKASHIMA Meteorological Research Institute, Ibaraki, 1apan
An attempt is made to gain information on land surface emissivities using only data from the AVHRR instrument on the NOAA-7 satellite. Measurements were taken of the water surface of Lake Eyre to determine the effect on the satellite radiances of the absorption by atmospheric water vapor. The results show that during the night the 11 tzm emissivity is less than that at 12/tm for both sand and salt surfaces, but during the day the reverse is true for the salt surface while the emissivities are equal for the sand. Some radiometric measurements of sand emissivity are also presented. A comparison is made between the lake water surface temperatures derived from standard sea surface temperature algorithms and those obtained from a model of radiative transmission through the atmosphere.
Introduction The various radiometers on the NOAA polar orbiting operational meteorological satellites currently supply measurements of the earth's atmosphere and surface several times daffy. For cloudless skies accurate measurements of sea surface temperature are available from the infrared channels of the AVHRR. Accuracies better than 1 K are obtained in regions where the atmospheres are not heavily laden with water vapor (i.e., not tropical areas). Accurate measurements are possible because the sea surface is homogeneous and horizontal temperature gradients are small. Also the infrared emissivity of sea water is accurately known and corrections for atmospheric absorption are possible using differential absorption techniques (Barton, 1985). The presence of clouds in the radiometer field of view and excess water vapor ©Elsevier Science Publishing Co., Inc., 1986 52 Vanderbilt Ave., New York, IVY 10017
absorption can cause inaccuracies in surface temperature measurements from space. It is, however, possible to get some estimates of sea surface temperature in partly cloudy areas using different analysis techniques; e.g., the N-star technique which compares infrared radiances in contiguous pixels (Smith, 1968). In totally cloudy areas microwave radiometers can be used to give sea surface temperature with an accuracy of 2 - 4 K, but at these accuracies it may be better to use climatological values. During the next decade there is to be a concerted effort to provide more accurate sea surface temperatures from space. The TOGA experiment of the World Climate Research Program has specified the need for global sea surface temperatures to accuracies of 0.3 K. Much work is in progress to refine the multichannel algorithms used to provide measurements from the AVHRR instrument. Also, the 00344257/86/$3.50
154
European Space Agency has included the Along Track Scanning Radiometer on its first remote sensing satellite, ERS-1, due for launch in 1989. This satellite instrument has been specifically designed to give sea surface temperature measurements with an accuracy approaching that required by TOGA. In contrast to these activities over the oceans, there has been little effort to produce reasonably accurate surface temperature measurements over land. The AVHRR instruments give an abundance of data related to the surface temperature but accurate interpretation is most difficult for the following reasons: 1. Land surfaces are usually inhomogeneous, and a large range of surface temperatures can exist over "small distances. 2. The surface emissivity is unknown and is likely to vary with moisture content, vegetation cover, and surface material. 3. The atmospheric absorption is more difficult to assess as large vertical t e m p e r a t u r e gradients and inversions can exist near the surface. 4. The surface temperature has a large diurnal cycle. 5. In situ measurements of surface (radiative) temperature can be quite difficult (Fuchs and Tanner, 1968). Air temperatures just above the surface are usually quite different from the actual surface temperatures. In many applications of satellitederived temperature, the accuracies required are quite coarse and a simple assumption of unity emissivity and a rough estimate of atmospheric absorption based on standard atmospheric profiles of water vapor and temperature are sufficient to
I. ]. BARTON AND T. TAKASHIMA
give a useful measurement. However, there are other applications relating to soil moisture estimation, energy balances at the surface, geologic exploration, and land use management where a more accurate measurement is required. Several reports exist on the laboratory measurement of the emissivity of various soils and rock types using broad band (8-13 /zm) radiometers (Lorentz, 1973). There have also been spectral measurements of the emissivity of specific compounds (e.g., Taylor, 1979; Hovis and Callahan, 1966; Conel, 1969). Vincent and Thomson (1972) presented a dual band technique for remotely estimating the silicate content of various surfaces. All these measurements form a guide to the assessment of the emissivity of natural land surfaces but accurate details of emissivities at the AVHRR wavelengths are not yet available. The Thermal Infrared Multispectral Scanner (TIMS) data discussed by Kahle and Goetz (1983) highlight the difficulty of separating surface emissivity from surface temperature effects. Given the problems associated with the interpretation of satellite measurements the best method of obtaining information on surface emissivity may be from the satellite measurements themselves. In this paper we assess the potential of gaining information about infrared surface emissivity from AVHRR data alone; i.e., no ground truth data are used. The discussion herein will be most useful to those involved in the interpretation of AVHRR data over land surfaces.
Radiometer Measurements
An accurate infrared radiometer was used to measure the emissivity of beach
LANDSURFACEEMISSIVITY
155 z~ J J J J
.4/
J J
RADIOMETER
\ ~ \ \ \
/
SUREAEE THERMOMETER
FIGURE1. The groundbasedradiometermeasurementof sandemissivity. sand. Measurements were taken on an area out of doors on cloudless days. The single channel radiometer has a spectral response that is similar to that of Channel 4 on the AVHRR instrulnent on NOAA-7. The radiometer was used to measure the radiation from the sand for zenith angles between 70 ° and 30 ° . The radiation consists of two components, that emitted by the sand and that of the reflected sky emission. Therefore, measurements of the sky radiation at the same zenith angle were also taken. Figure 1 shows the experimental arrangement. Measurements closer to the vertical than 30 ° were not possible as the reflected component would include a significant proportion from the radiometer and its operator. The surface temperature of the sand was taken using a mercury in glass thermometer placed just below the surface. The radiance measured by the radiometer I n is given by
+[1--Es(O)]BsKY(O), (1) where e~ is the surface emissivity, B(T~)
is the black body radiance at the surface temperature T~, Bsr Y is the measured sky radiance, and /9 is the zenith angle. In Eq. (1) we have assumed that the diffuse reflection of downward sky-emitted radiation can be approximated by the specular reflection of the sky radiation at the appropriate angle of incidence. Although this is the case for rough water surfaces where the emissivity is also near unity (Sidran, 1981), the reflection for land surfaces is expected to be neither Lambertian nor isotropic. In the above equation the emissivity is almost entirely determined by the measured surface temperature and the radiance of the surface so that any errors introduced by this approximation are qttite small. The results for six days of measurements in October 1984 are shown in Fig. 2. The spread of the results at each zenith angle indicates that there are considerable errors involved in this crude technique for emissivity measurement. The general shape of emissivity vs. angle curves for natural surfaces is reasonably well reproduced, and we can estimate that, for our sand, the zenith emissivity is between 0.97 and 0.98.
I. J. BARTON AND T. TAKASHIMA
1.56 1"00
t
O.9B
>~mA
0-96 oq
o
%. "%
t.u
o
• u
% N •
X
\
0.%
o
\=
,% \ o % 0"92 0
• 20
i t~0
I 60
I 80
ZENITH ANGLE (°) F I G U R E 2. The sand surface emissivity as a function of zenith angle from measurements taken on six days during October 1984. The dashed line is through the mean value of each angle.
The Lake Eyre Region Data Lake Eyre is the largest of the dry salt lakes that abound in the center of the dry Australian continent. There are, however, times (only a few each century) when excess rains in the catchment area in the northeast of the continent cause Lake Eyre to be filled with water. Such was the case in 1984 when the lake was filled in June and then slowly drained and evaporated until it was completely dry again by June 1985. During October and November of 1984 satellite data were collected for four sites in the Lake Eyre region to assess the possibility of obtaining useful information about surface emissivity at the AVHRR wavelengths. Three sites were chosen after careful examination of satellite data in both the visible and infrared channels showed them to be of a uniform nature over a large
area (20 × 20 km). The fourth site was the water in Lake Eyre itself. This site was included to allow an estimation of the atmospheric effect (water vapor absorption and emission) on the satellite measurements over the other three sites. The Lake Eyre region was chosen for this work for several reasons. First, there are large expanses of apparently homogeneous surfaces which relax the requirements for accurate ground location of the satellite data. Second, the measurements of the water in Lake Eyre can be used to give a correction for atmospheric absorption, and, third, the larger dry continental expanses ensure that similar air masses will exist over each of the sites at the same time. The location of Lake Eyre and the other three sites are shown in Fig. 3. Also shown is Woomera from where daily radiosonde data are available. Sites A and C are assumed to be similar and to con-
LAND SURFACE EMISSIVITY
157
sist of large sandy expanses with only very sparse vegetation cover while site B is located in the north of a salt lake, Lake Torrens, which remained dry throughout the period of interest. The satellite data collected were the raw counts in the NOAA-7 AVHRR Channels 4 and 5 at each location together with those of the four adjacent pixels. The five values for each channel at each site were averaged and then converted to brightness temperatures. Measurements of the salinity of the water in Lake Eyre were kindly supplied by the University of Adelaide and daily (0900 local time) radiosonde data for Woomera were obtained from the Bureau of Meteorology.
Land
the dependence has only a minimal effect on the following analysis. Usually A is no more than 5% of /sat" For the AVHRR channels in the 10-13 #m band the following relation applies (Price, 1983): (3)
I = a T 45,
where T is the black body temperature and a is a constant. The satellite brightness temperature can then be expressed as Tsat ~-~-( -4T' 5surf -tf
A/a)
1/4"5"
(4)
In the 10-13 #m window A is small compared to the surface emitted radiance so that the satellite brightness temperature can be approximated by
Measurements
Tsat ~
At 15 times during October and November 1984 satellite data from NOAA-7 were collected for the four sites shown in Fig. 3. Without any ground-based measurements of surface temperature or emissivity, there is only limited information that can be gleaned from the satellite data. In any one channel the radiance measured by the satellite can be expressed as the difference between the ground emitted radiance and a component due to the combined effects of atmospheric absorption and emission, viz., /sat = elsurt -- A ,
(2)
where I is radiance, e is emissivity, and A is the atmospheric component. Although A is weakly dependent on the surface emissivity through the reflectance of downward sky emittance [see Eq. (1)]
~l/4"5Tsurf- A',
(5)
where A' represents the atmospheric effect. The difference between Channel 4 and 5 brightness temperatures is then given by A = Z4sat - Z5sat = Zsurf((e4) 1/4'5 - (£5) 1/4'5) - (A'4 - A'5).
(6)
The emissivities of water for different angles and salinities can be calculated from the data of Friedman (1969) and are shown in Fig. 4. It can be seen that the emissivities for Channels 4 and 5 are approximately equal and are near unity except for very large zenith angles. Therefore, the factor (e4) 1/45 - (%)1/45 is near zero and, from Eq. (6), the dif-
158
I.J. BARTON AND T. TAKASHIMA
~7
FIGURE 3. The location of Lake Eyre (D), the other three sites (A,B,C), and the radiosonde station at Woomera (W). The image is from Channel 2 data on the NOAA-7 AVHRR.
ference in the brightness temperatures in Channels 4 and 5 over water is then proportional to the difference in the atmospheric effect in the two channels. With the assumption that the air mass above the entire study area is uniform, it is possible to derive the effect of the atmosphere from the water surface measurements. Thus by adding superscripts for water ( W ) and land (L), the difference between A for the two sudaces is
given by
~ _ ~ __ ~u~r,[(~4~)~4~ _ t 4~ t ~'~].
(7) Equation (7) shows that if the difference in the brightness temperatures over water is subtracted from the difference over land, the result is an indication of the
159
LAND SURFACE EMISSIVITY
100
i
i
i
" -.'.-.-.- ~ . _ _ _ . ~ I T H
~ ~
0 •9 6
o
.
9
ANGLE 0°
8
. -
I
I
S°°
~
094
difference between the land surface emissivities for the two channels. For our data the differences in the brightness temperatures were calculated for each of the four sites and then AL - Aw obtained for the three land surfaces. The results are shown in Fig. 5 with the ordinate giving the orbit numbers in the order of Table 2. For emissivities near unity the difference between the emissivities is only weakly dependent on the emissivity itself and Eq. (7) can be reduced to
o i. ~-I ~EAWATER
0 92
J
- -
CHANNEL4
---
CHANNELS I
160
I
200
300
SALINITY (ppf.) FIGURE 4. The water surface emissivity dependence on angle and salinity for AVHRR Channels 4 and 5. The values are calculated from the data given by Friedman (1969).
e
=4.5(AL
-
.I
L
(8)
This final equation has been tested by using an atmospheric transmission model (Barton, 1985) with a variety of atmospheres and a selection of surface emissivities. In all cases the difference in the surface emissivity of the two channels could be reproduced with an accuracy of 15%. For example, with the standard mid-
SITE A (SAND) SITE B (SALT) SITE [ (SAN0) DAY NIGHT DAY NIGHT DAY NIGHT 2 I
° • OOO•00
<~
0 O0 O0
-I
•
o°o
-2
o•
o•
0%0
•
t 17237 FIGURE 5.
17187
ORBIT NO.
17519
17625
The values of ~L _ Aw for each site. The ordinate is the satellite orbit number in the order of Table 2.
160
I. J. BARTON AND T. TAKASHIMA TABLE 1 Emissivity Differences in AVHRR Channels 4 and 5 for the Three Land Surfaces SITE
SURFACE
A
sand
B
salt
C
sand
DAY/NIGHT
day night day night day night
latitude summer model of MeClatchey et al. (1972) and land surface emissivities of 0.95 and 0.97 in Channels 4 and 5, respectively, the transmission model gave brightness temperatures over water and land that gave an emissivity difference of 0.019. By setting T~Lrf to 290 K at night and 310 K for the daytime satellite passes the differences in the emissivities of the two channels can be estimated. The results are given in Table 2. The major errors in this analysis are due to the noise in the satellite-derived brightness temperatures. In both channels the noise-equivalent temperature o T is 0.1 K. In Eq. (8) of the expression AI~_ hw thus has a standard error of
A L -- ATM
E4 -- 8 5
+ 0.1 ( + 0.2) - 1.1 + 0.9 - 0.5 + 0.2 - 1.2
0.001 ( +_0.003) - 0.017 0.013 - 0.008 0.003 - 0.018
(40~) x/2, i.e., 0.2 K. The effect of any error in T~rf is negligible compared to this error, and thus the standard error in (e~4~- e L) due to the satellite instrument noise is 0.003. There is also an error introduced when using Eq. (8) as an approximate form of Eq. 7. This error can be estimated by expanding the relations e 1/45 = (1 - q)1/4.5 with q << 1. The resultant error in e 4 - e ~ for emissivities near 0.9 is only 2%, which again is negligible compared to the above standard error. Figure 5 shows that the results for the sandy site C are less variable than those for the sandy site (A) and the salt lake site (B). For both surfaces the daytime emissivity difference (e L - e L) is larger
TABLE 2 Lake Eyre Surface Temperature T. Calculated by (i) the Transmission Model of Barton (1985) and (ii) Using the Standard Surface Temperature Algorithms of McClain (1984) DArE
ORBIT
T4
T5
T4 - T5
(Barton)
(McClain)
Day
26/10 29/10 1/11 6/11 12/11 13/11 15/11
17237 17279 17322 17392 17477 17491 17519
293.4 206.2 294.2 294.3 294.7 295.1 293.6
292.9 294.9 293.5 293.2 294.2 294.0 292.8
0.5 1.3 0.7 1.1 0.5 1.1 0.8
205.2 300.3 296.8 297.4 297.1 298.3 297.9
294.8 299.7 296.1 297.2 296.1 298.0 295.7
Night
23/10 27/10 28/10 30/10 1/11 14/11 16/11 23/11
17187 17244 17258 17286 17315 17498 17526 17625
290.8 292.0 294.2 291.7 290.6 292.6 292.2 289.3
289.6 291.2 293.2 289.9 289.9 291.6 291.3 288.3
1.2 0.8 1.0 1.8 0.7 1.0 0.9 1.0
295.4 294.2 296.7 297.5 293.0 295.4 296.3 292.5
294.2 294.5 297.3 296.8 292.7 295.6 294.6 291.1
LAND SURFACE EMISSIVITY
than that at night by 0.020. The two sand surfaces show good agreement with both having differences that are 0.010 less than the salt surface. The difference between the day and night values indicates that the nature of the surface (e.g., moisture content) changes from day to night. The variability in the values at each site is most likely due to minor variations at the site combined with the difficulties of accurate navigation of the satellite data (ground location errors are approximately 5 km). At typical surface temperatures a variation in surface emissivity of 0.02 is equivalent to a change in brightness temperature of 1.5 K. For thermal inertia studies where the difference between day and night temperatures is often less than 10 K, the surface emissivity obviously becomes an important factor. The nighttime results of emissivity difference for sand are in general agreement with other measurements that give an emissivity of 11 /~m that is slightly less than that at 12 /~m. The differences between the day and night measurements obviously require further study. Water Measurements
Algorithms for obtaining accurate sea surface temperatures from AVHRR data have been developed by McClain (1984) and Barton (1985). For NOAA-7 the best results have been obtained using Channels 4 and 5 (10.8 and 12.0 /xm) as the shorter wavelength Channel 3 is contaminated by reflected sunlight during the daytime and has excess noise at night. The NOAA-7 satellite has a daytime pass at mid- to late-afternoon local time and a nighttime pass near 3 a.m. For the open sea at midlatitudes accuracies better than 1 K are now possible using simple linear
161
algorithms. The difference between the sea surface temperature and the satellitemeasured brightness temperature is primarily a function of the weighted water vapor content of the atmosphere. There are, however, certain atmospheric conditions involving large anomalous temperature gradients near the surface when these simple algorithms do not apply (P. McClain, private communication). Unforttmately, such is the case for our study area. All our measurements are taken under cloudless conditions and so there is considerable heating of the surface during the day and strong radiative cooling during the night. For the daytime satellite pass the land surface temperature is well in excess of 30°C, the temperature of the water surface is 20-25°C, and the temperature of the top of the surface mixed layer at 300-500 m above the surface is 20-25°C. Advection of the hot surface air over the cooler lake surface gives the large vertical temperature gradients that prevent the use of the algorithms. At night the land cools down to between 10 and 15°C while the temperature of the air above the surface layer is similar to that during the day. Typical temperature profiles are shown in Fig. 6. These have been derived from studies of aircraft and balloon measurements over central Australia by Clarke and Brooke (1979). Because the standard water surface algorithms do not apply here it is necessary to use a transmission model [in this case that of Barton (1985)] to obtain an estimate of the "atmospheric effect" on the satellite measurements. Due to evaporation the salinity of the water in Lake Eyre increased from 110 to 175 ppt during the measurement period. The dependence of the infrared emissivity of the water on
162
I.J. BARTON AND T. TAKASHIMA BOO
800
l
x ~- Z~O0
I
/\ 10
20
/\
30
L,0 10 20 TEHPERATURE (°C)
LAND
FIGURE 6.
00 30
~.0
WATER
Typical profiles of temperature for day and night over land and water surfaces.
salinity can be determined from the data given by Friedman (1969) and is included in the transmission model calculations. The errors in derived temperature introduced by neglecting these emissivity variations are quite small being of the order of 1 K between pure and salt saturated water. To estimate surface temperature from the satellite data, it is necessary to determine first the effect of the atmosphere on the upwelling radiation. Initially the midlatitude summer standard atmosphere of McClatchey et al. (1972) was used to specify the atmosphere above the water surface. The temperature at 400 m above the surface was set at 23°C, and then the water vapor content at different levels was adjusted along with the water surface temperature until the atmospheric transmission model reproduced the observed satellite brightness temperatures in the two channels. When adjusting the water vapor profiles, note was taken of the radiosonde data obtained at Woomera some 200 km south of Lake Eyre. A water surface temperature was thus obtained, and the values are shown in Table 2. It must be remembered that these values are radiative temperatures of the top few microns of the surface which, in stable
conditions, can be quite different to the bulk temperature of the water (Robinson et al., 1984). A good example of this is orbit number 17279 on October 29 when the surface radiative temperature is in excess of 300 K. Table 2 also includes values of surface temperature derived using the standard sea surface temperature of McClain (1984). Agreement between the two surface temperatures in most cases is better than 1 K but on two occasions the difference is near 2 K.
Conclusions In this paper we have investigated the possibility of using accurate AVHRR measurements over land surfaces. We have shown that it is possible to gain some information on the relative values of the surface emissivity at different wavelengths without the need for ground truth data. It is evident, however, that ground truth data must be combined with the satellite measurements before it will be possible to get accurate satellite measurements of land surface temperatures from the AVHRR instruments. The apparent difference between the day and night relative emissivities for the AVHRR Channels 4 and 5 requires specific inves-
LAND SURFACE EMISSIVITY
tigation. Hitherto, there have been many measurements across the entire 8 - 1 3 / ~ m band, but now data are required to match the AVHRR thermal infrared bands. Until this is done the use of these channels in thermal inertia studies relating to soil moisture measurement and geologic exploration will be limited.
The satellite data were collected and analysed by Ms. J. Bathols. The salinity data for Lake Eyre were supplied by Dr. Bill Williams from Adelaide University and the radiosonde data for Woomera were supplied by the Australian Bureau o f Meteorology. References Barton, I. J. (1985), Transmission model and ground truth investigation of satellite derived sea surface tempera~res, I. Clim. Appl. Meteorol. 24:508-516. Clarke, R. H., and Brooke, R. R. (1979), The Koorin Expedition, Australian Department of Science and the Environment, Australian Government Publishing Service, Canberra, Australia, 359 pp. Conel, J. E. (1969), Infrared emissivities of silicates; experimental results and a cloudy atmosphere model of spectral emission from condensed particulate mediums, I. Geophys. Res. 74:1614-1634. Friedman, D. (1969), Infrared characteristics of ocean water (1.5-15 /~m) Appl. Opt. 8:2073-2078. Fuchs, M., and Tanner, C. B. (1968), Surface temperature measurement of bare soils, I. Appl. Meteorol. 7:303-305. Hovis, W. A., and CaUahan, W. R. (1966), Infrared reflectance spectra of igneous rocks, tufts and red sandstone from 0.5 to 22 I~m, 1. Opt. Soc. Am. 56:639-643. Kahle, A. B., and Goetz, A. F. H. (1983), Mineralogic information from a new airborne thermal infrared multispectral scanner, Science 222:24-27.
163
Lorentz, D. (1973), Die radiometrische Messung der Boden-und Wasser-oberflachen temperatur und ihre Anwendung insbesendere atd dem Gebeit der Meteorologic, Z. Geophys. 39:627-701. McClain, E. P. (1984), Multi-channel sea surface temperatures from the AVHRR on NOAA-7. Satellite derived sea surface temperature: Workshop-II, Pasadena, Ca, JPL Publication 84-5, pp. 1-8. McClatchey, R. A., Fenn, R. W., Selby, J. E. A., Volz, F. E., and Goring, J. s. (1972), Optical properties of the atmosphere. Environmental Research Paper No. 411, AFCRL-72-0497, Air Force Cambridge Research Laboratories, Bedford, MA, 90 pp. Price, J. C. (1983), Estimating surface temperatures from satellite infrared data--a simple formulation for the atmospheric effect, Remote Sens. Environ. 13:353-361. Robinson, I. S., Wells, N. C., and Charnock, H. (1984), The sea surface thermal boundary layer and its relevance to the measurement of sea surface temperature by airborne and spaceborne radiometers, Int. J. Remote Sens. 5:19-45. Sidran, M. (1981), Broadband reflectance and emissivity of specular and rough water surfaces, Appl. Opt. 20:3176-3183. Smith, W. L. (1968), An improved method for calculating tropospheric temperature and moisture from satellite radiometer measurements, Month. Weath. Rev. 96:387-396. Taylor, S. E. (1979), Measured emissivity of soils in the southern United States, Remote Sens. Environ. 8:359-364. Vincent, R. K., Rowan, L. C., GiUespie, R. E., and Krapp, C. (1975), Thermal-infrared spectra and chemical analyses of twenty six igneous rock samples, Remote Sens. Environ. 4:199-209. Vincent, R. K., and Thomson, F. (1972), Spectral compositional imaging of silicate rocks, ]. Geophys. Res. 77:2465-2472. Received 27 1anuary 1986; revised 27 May 1986.