Effects of atmospheric emissivity on clear sky temperatures

Pergamon

AtmosphericEnvironmentVol 29, No. 16, pp 2201-2204.1995

Copyright© 1995ElsevierScienceLtd Prmted m Great Bntmn All nightsreserved 1352-2310/95$9 50 + 000

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EFFECTS OF ATMOSPHERIC EMISSIVITY O N CLEAR SKY TEMPERATURES D H I R E N D R A K. P A N D E Y , R O B E R T B. LEE III* and J A C K P A D E N Science Applications International Corporation (SAIC), One Enterprise Parkway, State 250, Hampton, VA 23666-5845, U.S.A.; *Atmospheric Sciences Division, NASA Langley Research Center, Mail Stop 420, Hampton, VA 23681-0001, U.S.A.

(Ftrst received 4 February 1993 and in final forrn 15 May 1994) Abetraet--'Hae accurate determination of atmospheric temperatures from outgoing longwave radiation

depends upon the effective atmospheric emissivity used in Stefan-Boltzmann's law. We reviewed the literature dealing with the atmospheric emissivityequations for clear sky and studied their effects on the clear sky atmospheric temperatures. The clear sky outgoing Iongwave radiation data were taken by the National Aeronautics and Space Administration's (NASA) Earth Radiation Budget Satellite (ERBS) scanning radiometers. The five years (1985-1989) of global annual-mean of clear sky atmospheric temperatures with different emissivity values are presented and discussed in this paper. The effect of emissivity on the retrieval of clear sky atmospheric temperatures are found to vary by 7-8 K (Kelvin). Key word index: Atmospheric emissivity, clear sky temperature, ERBE, ERBS, outgoing longwave radiation, scanning radiometers.

1. INTRODUCTION

The determination of climatic changes has recently become a very important research area of the atmospheric sciences. Global warming/cooling may adversely affect human settlements by modifying sea levels as well as regional climatic patterns. Global warming/cooling trends are derived using surface air temperature data (Hansen and Lebedeff, 1987) measured by thermometers sparsely distributed over the Earth's surface. The surface air temperature data are considered to be contaminated by urbanization, instrument handling, observation timing, and instrument design and calibration procedures (Diaz, 1986; Karl et aL, 1988). To overcome these problems, the use of satellite data for monitoring of global atmospheric temperature variations with a high level of precision is demonstrated by Spencer and Christy (1990). Thus the resulting atmospheric temperatures derived from the satellite data would play a very important role in monitoring the local, regional, and global warming/ cooling trends. The present research work demonstrates the importance of effective emissivity which directly affects the quality of global change phenomena. The retrieval of clear sky atmospheric temperature from the outgoing longwave radiation depends upon the accuracy of the effective atmospheric emissivity as required by the Stefan-Boltzmann law (Kornfield and Susskind, 1977; Becker, 1987; Van de Griend et al., 1991). We reviewed the literature dealing with the

atmospheric emissivity for clear sky and determined the effect of emissivity on the clear sky atmospheric temperatures using the Earth Radiation Budget Satellite (ERBS) outgoing longwave radiation data which were measured by scanning sensors employed on the ERBS spacecraft. The ERBS was launched on 5 October 1984 in a non-sun-synchronous trajectory (altitude = 610 km), whose orbital plane is inclined 57° from the geocentric equatorial plane. The change in global annual-mean of atmospheric temperatures resulting from the different atmospheric emissivity for five years (1985-1989) are presented and discussed in this paper.

2. ERRS CLEAR SKY OUTGOING LONGWAVE RADIATION DATA

The details about the Earth Radiation Budget Experiment (ERBE) project are given by Barkstrom (1984) and Barkstrom and Smith (1986). A schematic diagram of the ERBE scanner instrument is illustrated in Fig. 1. Three scanning radiometers, total (0.2-50/tm), longwave (5.0-50/zm), and shortwave (0.2-5.0/zm), measured the Earth-atmosphere's emitted and reflected radiation fluxes. The complete design and method of operation of the scanners are described by Kopia (1986). The ERBE scanners measured the Earth's radiation fluxes from limb to limb in 4 s. Scanners measured reference fluxes from space as well as from in-flight calibration sources to determine

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Fig. 1. ERBS scanning sensors.

any drifts in the instruments. The detailed descriptions of the scanner count conversion procedures and final count conversion coefficients can be found elsewhere (Halyo et al., 1989; Lee et al., 1989; Paden et al., 1991). The ERBS outgoing longwave radiation fluxes for clear sky at the top of the atmosphere (TOA) were determined for the Earth's atmosphere from 50° north to 50° south latitudes. The TOA altitude is defined as 30 kin. The monthly means of outgoing longwave fluxes for five years (1985-1989) are shown in Fig. 2. The corresponding global annual-means of outgoing longwave radiative fluxes are displayed in Table 1. The standard deviations from the global means are also given in Table 1. These data sets were obtained from National Space Sciences Data Center (NSSDC) at NASA, Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.

3. CLEAR SKY ATMOSPHERIC EMISSIVITY

The clear sky emissivity can be represented as a function of the dew point temperature T d p . Table 2 lists the important emissivity equations reported in the literature. The third column displays the emissivity values computed by using the emissivity equations for the atmospheric conditions at the top of the atmosphere. The atmospheric conditions at TOA are taken from the U.S. Standard Atmosphere, 1976. The atmospheric vapor pressure at TOA is found to be 11.97 mb. To determine the effective emissivity, the corresponding dew point temperature, Tdp, is computed as follows (Berger et al., 1984): Tap = [5179.25/{20.519 - In (760 Pv/1013)}'l - 273.15 (1) where Pv is water vapor pressure in mb and Tdp is the dew point temperature in °C (Celsius). Thus the Tap at 11.97 mb is found to be 9.5°C. Further, this Tap value

Table 1. The global annual-mean of clear sky outgoing longwave radiative fluxes (Wm -2) measured at the top of the atmosphere by ERBS scanning sensors Year

Mean

Standard deviation

1985 1986 1987 1988 1989

279.47 278.99 279.79 279.74 279.72

_+ 1.75 + 1.35 + 1.62 + 1.75 + 1.49

computed from equation (1) is found to agree within 0.3°C with the Tdp value determined by the historical equation (Tetens, 1930). The emissivity equation of Berdahl and Fromberg (1982) is produced by taking the mean over nighttime and daytime conditions. Brunt (1932) followed Angstrom's (1918) method and produced similar emissivity expressions depending upon the local conditions. Many authors (Idso and Jackson, 1969; Brutsaert, 1975; Aase and Idso, 1978; Idso, 1981) related the clear sky emissivity with the screen level air temperature. Centeno (1982) developed the emissivity equation as a function of the altitude, the ambient temperature, the relative humidity, and the degree of cloudiness, while Sloan et al. (1956) modeled the emissivity in terms of absolute humidity.

4. CLEAR SKY ATMOSPHERIC TEMPERATURE

The clear sky atmospheric temperatures can be inferred from the Stefan-Boltzmann law as follows: T,=y = (QouJSO-) °'25

(2)

where Tsky is the clear sky temperature (K), o is the Stefan-Boltzmann constant equal to 5.67x 10 -s W m -2 K -a, and ~ is the effective atmospheric emissivity. The corresponding atmospheric temperatures at TOA are obtained using the emissivity given in

Clear sky temperatures

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Table 2. Clear sky atmosphere emissivityequations and corresponding emissivityvalues computed at the top of the atmosphere Reference Eisasser (19,;2) Bliss (1961) Clark and Allen (1978) Berdahl and Fromberg (1982) Berger et al (1984) Martin and Berdahl (1984)

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0.756 0.838 0.814 0.792 0.806 0.771

spheric temperature determined by using a constant emissivity value over the years remains the same (Lee et al., 1992). Based on the emissivity equations g~ven in Table 2, one can find the mean clear sky emissivity value of 0.796 at TOA to be appropriate for future use. Further, the emissivity equations lead to a different constant value in the absence of water vapor. This question can be resolved with the future CERES (clouds and the earth's radiant energy system) measurements currently scheduled in 1997. The development of clear sky emissivity based on the global dew point temperatures would be highly beneficial in retrieving the Earth's surface temperature data from the satellite outgoing radiation data. work was funded under NASA Contract NAS1-19570 with the NASA Langley Research Center, Hampton, Virginia. Acknowledgements--This

Table 2 and the outgoing longwave fluxes given in Table 1. Many aspects of the present results obtained for five years (198:,-1989) are discussed.

REFERENCES

5. R E S U L T S

AND

CONCLUSIONS

We computed the clear sky atmospheric temperature at TOA using equation (2). In this equation, the QOLRfrom Table 1 and the effective emissivity values from Table 2 are used. The result of using effective clear sky emissivity on the retrieval of atmospheric temperatures for five years (1985-1989) is depicted in Fig. 3. The nature of the variation of atmospheric temperatures at TOA are the same over the years because the conditions at TOA are considered the same for every yeaL In a real situation this may not be true. Further, Fig. 3 shows the differences in retrieved atmospheric temperatures at TOA based on the clear sky emissivity equations available in the hterature. For example, Elsasser (1942) produces atmospheric temperatures 7-8 K higher than the atmospheric temperatures produced by Bliss (1961). On the other hand, Clark and Allen (1'978) agree closely with the temperature results obtained by Berger et al. (1984). This points out the sensitivity of retrieval of clear sky atmospheric temperatures which clearly depends upon the accuracy of effective clear sky emissivity. However, the nature of the variation in global atmo-

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