Shashi K. Gupta

Seasonal variation of surface radiation budget derived from International Satellite Cloud Climatology Project C1 data

Surface radiation budget data are presented for the midseasonal months of July and October of 1983 and January and April of 1984. These data allow examination for the first time of geographical and seasonal variations of the entire surface radiation budget from pole to pole. The latest flux estimation techniques have been used along with data from the International Satellite Cloud Climatology Project (ISCCP) and the Earth Radiation Budget Experiment (ERBE). Regional, zonal, and hemispheric distributions of the downward and net components of both shortwave and longwave fluxes and of the net total surface fluxes are determined. Seasonal flux variation per region, expressed as flux range, is illustrated for these components also. The estimated fluxes appear to be accurate to about 16 W m−2 on a global average, based on sensitivity analyses and comparisons with ground data. An analysis of flux errors showed that most of the error was attributable to errors in input data.

A Parameterization for Longwave Surface Radiation from Satellite Data: Recent Improvements

Abstract Several improvements have been made recently to the parameterization for surface longwave radiation described by Gupta. Model constants have been modified in order to use meteorological data from the International Satellite Cloud Climatology Project instead of from the TIROS Operational Vertical Sounder data, primarily to take advantage of the vastly superior cloud information available from the former. Additional modifications were made to improve the estimation of cloud effect in the presence of low-level clouds. The latter modifications reduced the systematic error of the overcast-sky fluxes from 10.0 to 1.7 W m−2 and the random error from ±18.9 to ±6.3 W m−2 when compared to the fluxes computed with a detailed radiative transfer model.

Estimation of surface insolation using sun-synchronous satellite data

Abstract A technique is presented for estimating insulation at the Earths surface using only sun-synchronous satellite data. The technique was tested by comparing the insolation results from year-long satellite datasets with simultaneous ground-measured insolation taken at five continental United States sites Monthly average insolation values derived from the satellite data showed a standard error of 4.2 W m−2, or 2.7% of the average ground insulation value.

A parameterization for longwave surface radiation from sun-synchronous satellite data

Abstract A parameterization has been developed for computing downward, upward, and net longwave radiation at the earths surface using meteorological data from NOAAs operational sun-synchronous satellites. The parameterization was developed using a narrowband radiative transfer model and a large meteorological database consisting of satellite and in situ soundings. Clear-sky downward flux was represented as a function of surface and lower tropospheric temperatures and water vapor burden of the atmosphere. Cloud contribution to the downward flux was represented in terms of cloud base temperature and water vapor burden of the atmosphere below the cloud. Upward flux was computed directly from the surface temperature. Results obtained with the parameterization were verified against detailed radiative transfer computations for an independent set of satellite and in situ soundings. The parameterization was applied to satellite soundings for the month of April 1982 from a large region in the tropical Pacific Oc...

A biophysical process-based estimate of global land surface evaporation using satellite and ancillary data II. Regional and global patterns of seasonal and annual variations

A process-based biophysical model of evaporation described previously was run using spatially representative data to calculate global fields of monthly total transpiration, soil evaporation and interception for a period of 24 months (January 1987 to December 1988). Solution of the energy balance equation provided net radiation and sensible heat flux, complementing the evaporative flux. The zonally averaged (area weighted 5° latitude bands) values of annual total evaporation and evaporative fraction (ratio of evaporation and net radiation) are in broad agreement with previous estimates. Transpiration was found to be the dominant component of annual total evaporation in 20 out of the 28 latitude bands. Partitioning of annual total evaporation over the global land area is calculated to be, 52% transpiration, 28% soil evaporation and 20% interception. Seasonal variations of total evaporation and its components are presented for some specific types of vegetation (e.g., tundra, taiga, rainforest, crop land) and compared with field observations.

Downward Longwave Radiation at the Surface from Satellite Measurements

Abstract A new technique is presented for generating downward longwave flux at the Earths surface from satellite meteorological data and a radiative transfer model The technique was tested by using TIROS-N data from 41 passes over a ground site covering a period of one month. Satellite-derived fluxes were compared with those measured by a ground-based pyrgeometer during each overpass. The standard error of the satellite-derived fluxes relative to the mean ground-measured values was found to be 6.5%.

An improved algorithm for retrieving surface downwelling longwave radiation from satellite measurements

Retrieving surface longwave radiation from space has been a difficult task since the surface downwelling longwave radiation (SDLW) are integrations from radiation emitted by the entire atmosphere, while those emitted from the upper atmosphere are absorbed before reaching the surface. It is particularly problematic when thick clouds are present since thick clouds will virtually block all the longwave radiation from above, while satellites observe atmosphere emissions mostly from above the clouds. Zhou and Cess developed an algorithm for retrieving SDLW based upon detailed studies using radiative transfer model calculations and surface radiometric measurements. Their algorithm linked clear sky SDLW with surface upwelling longwave flux and column precipitable water vapor. For cloudy sky cases, they used cloud liquid water path as an additional parameter to account for the effects of clouds. Despite the simplicity of their algorithm, it performed very well for most geographical regions except for those regions where the atmospheric conditions near the surface tend to be extremely cold and dry. Systematic errors were also found for areas that were covered with ice clouds. An improved version of the algorithm was developed that prevents the large errors in the SDLW at low water vapor amounts. The new algorithm also utilizes cloud fraction and cloud liquid and ice water paths measured from the Cloud and the Earths Radiant Energy System (CERES) satellites to separately compute the clear and cloudy portions of the fluxes. The new algorithm has been validated against surface measurements at 29 stations around the globe for the Terra and Aqua satellites. The results show significant improvement over the original version. The revised Zhou-Cess algorithm is also slightly better or comparable to more sophisticated algorithms currently implemented in the CERES processing. It will be incorporated in the CERES project as one of the empirical surface radiation algorithms.

Downward longwave surface radiation from sun-synchronous satellite data - Validation of methodology

Abstract An extensive study has been carried out to validate a satellite technique for estimating downward longwave radiation at the surface. The technique, mostly developed earlier, uses operational sun-synchronous satellite data and a radiative transfer model to provide the surface flux estimates. The satellite-derived fluxes were compared directly with corresponding ground-measured fluxes at four different sites in the United States for a common one-year data period. This provided a study of seasonal variations as well as a diversity of meteorological conditions. Dome heating errors in the ground-measured fluxes were also investigated and were corrected prior to the comparisons. Comparison of the monthly averaged fluxes from the satellite and ground sources for all four sites for the entire year showed a correlation coefficient of 0.98 and a standard error of estimate of 10 W m−2. A brief description of the technique is provided, and the results validating the technique are presented.

New data set on surface radiation budget available on‐line

A new long-term global data set of monthly average surface shortwave (SW) and longwave (LW) fluxes extending from July 1983 to June 1991 is now available. It is the product of computationally fast radiative transfer algorithms that compute global surface radiative fluxes using satellite data. The data set can be used in the development of general circulation models and in climate studies, including those that examine surface processes and interannual climate anomalies such as El Nino/Southern Oscillation and regional floods and droughts.