Diurnal Variability of Regional Cloud and Clear-Sky Radiative Parameters Derived from GOES Data. Part III: November 1978 Radiative Parameters

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  • 1 Atmospheric Sciences Division, NASA Langley Research Center, Hampton, VA 23665
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Abstract

The diurnal variability of the radiation emitted and reflected from the earth-atmosphere is investigated at the regional scale using November 1978 GOES-East visible and infrared data and GOES-derived cloud information. Narrowband GOES data are converted to broadband radiances using spectral calibration functions determined empirically from colocated Nimbus-7 ERB and GOES-East measurements over ocean, land and cloud surfaces. Shortwave radiances are used to estimate radiant exitances with bidirectional reflectance models derived from GOES and aircraft data for ocean, land and clouds.

Average albedo over clear ocean and land changed by factors of 4.2 and 2.2, respectively, for a solar zenith angle range of 0 to 80°. Average cloud albedo changed by a factor of 1.8 for the same range of solar zenith angles, but varied considerably from region to region. Mean clear-sky longwave radiant exitance varied diurnally from 2 W m−2 over some ocean areas to 100 W m−2 in one high elevation desert area in the Andes. In some regions having regular deep convective diurnal cycles, the mean cloud longwave flux had a diurnal range as high as 50 W m−2. The mean difference in net radiation between cloudy and clear areas was -55 W m−2, indicating that the clouds had a net cooling effect for the GOES-East viewing area during this time period.

Monthly mean Earth radiation budget (ERB) measurements from Sun-synchronous satellites were simulated to estimate the magnitude of the errors which would result from limited local time sampling of the resultant radiation field. It was found that the monthly mean regional longwave flux could be estimated with a precision better than 3 W m−2. The precision in estimated regional albedo ranged from 1.5 to 4.0% for averaging which uses directional reflectance models and from 2.6 to 11.1% for simple averaging which is dependent on equatorial crossing time. For simple averaging, regional net radiation errors ranged from 10 to 40 W m−2 with “global” bias errors as high as 36 W m−2 depending on the satellite equatorial crossing time. Similarly, monthly mean regional net radiation errors ranged from 6 to 14 W m−2 with “global” bias errors of up to 7 W m−2 when directional reflectance models were used.

Abstract

The diurnal variability of the radiation emitted and reflected from the earth-atmosphere is investigated at the regional scale using November 1978 GOES-East visible and infrared data and GOES-derived cloud information. Narrowband GOES data are converted to broadband radiances using spectral calibration functions determined empirically from colocated Nimbus-7 ERB and GOES-East measurements over ocean, land and cloud surfaces. Shortwave radiances are used to estimate radiant exitances with bidirectional reflectance models derived from GOES and aircraft data for ocean, land and clouds.

Average albedo over clear ocean and land changed by factors of 4.2 and 2.2, respectively, for a solar zenith angle range of 0 to 80°. Average cloud albedo changed by a factor of 1.8 for the same range of solar zenith angles, but varied considerably from region to region. Mean clear-sky longwave radiant exitance varied diurnally from 2 W m−2 over some ocean areas to 100 W m−2 in one high elevation desert area in the Andes. In some regions having regular deep convective diurnal cycles, the mean cloud longwave flux had a diurnal range as high as 50 W m−2. The mean difference in net radiation between cloudy and clear areas was -55 W m−2, indicating that the clouds had a net cooling effect for the GOES-East viewing area during this time period.

Monthly mean Earth radiation budget (ERB) measurements from Sun-synchronous satellites were simulated to estimate the magnitude of the errors which would result from limited local time sampling of the resultant radiation field. It was found that the monthly mean regional longwave flux could be estimated with a precision better than 3 W m−2. The precision in estimated regional albedo ranged from 1.5 to 4.0% for averaging which uses directional reflectance models and from 2.6 to 11.1% for simple averaging which is dependent on equatorial crossing time. For simple averaging, regional net radiation errors ranged from 10 to 40 W m−2 with “global” bias errors as high as 36 W m−2 depending on the satellite equatorial crossing time. Similarly, monthly mean regional net radiation errors ranged from 6 to 14 W m−2 with “global” bias errors of up to 7 W m−2 when directional reflectance models were used.

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