Gary G. Gibson

δ-Fit: A fast and accurate treatment of particle scattering phase functions with weighted singular-value decomposition least-squares fitting

Abstract With a limited number of polynomial terms (so-called “streams”), there are significant differences between a phase function and its Legendre polynomial expansion at large scattering angles, which are important to satellite observations. This study finds that while it takes hundreds of Legendre polynomial expansion terms to simulate the backscattering portion of cloud phase functions accurately, the backscattered radiance pattern can be accurately estimated with only 30 Legendre polynomial expansion terms by replacing the regular Legendre polynomial expansion coefficients by coefficients obtained by a weighted singular-value decomposition least-squares fitting procedure. Thus the computing time can be significantly reduced. For satellite remote-sensing purposes, the weighted least-squares Legendre polynomial fitting is an optimal estimation of the cloud phase function.

First Estimates of the Diurnal Variation of Longwave Radiation from the Multiple-Satellite Earth Radiation Budget Experiment (ERBE)

First results for diurnal cycles derived from the Earth Radiation Budget Experiment (ERBE) are presented for the combined Earth Radiation Budget Satellite (ERBS) and NOAA-9 spacecraft for April 1985. Regional scale longwave (LW) radiation data are analyzed to determine diurnal variations for the total scene (including clouds) and for clear-sky conditions. The LW diurnal range was found to be greatest for clear desert regions (up to about 70 W · m−2) and smallest for clear oceans (less than 5 W · m−2). Local time of maximum longwave radiation occurs at a wide range of times throughout the day and night over oceans, but generally occurs from noon to early afternoon over land and desert regions.

Discriminating between spherical and non-spherical scatterers with lidar using circular polarization: a theoretical study

For ground based observations, depolarization of lidar backscatter indicates that the scattering particles are non-spherical. This property provides a useful means to discriminate between ice particles (non-spherical) and water droplets (spherical) in clouds. However, for space based lidar measurements, backscatter from spherical water cloud particles is also depolarized due to multiple scattering. For the spaceborne lidar application, the discrimination between water and ice is not straightforward. An alternative method for water/ice discrimination that is less sensitive to multiple scattering is proposed in this study. The new approach is based on the differences in P44 (an element of the scattering phase matrix) at 180° scattering angle between spherical and non-spherical particles. By transmitting a circularly polarized beam from the lidar and resolving the rotational sense of the polarization in the receiver, discrimination between spherical and non-spherical scatterers can be accomplished even when multiple scattering occurs. When the incident beam is left-hand-circularly polarized, the circular component of backscatter by a non-spherical particle is weak and possibly left-handed, whereas backscatter by a spherical particle is significantly right-hand-circularly polarized. Monte Carlo simulations with full Stokes vector parameterizations indicate that multiple scattering does not affect the rotational sense of the backscatter polarization, making robust discrimination between spheres and non-spheres possible with this new circular polarization approach.

Elevation information in tail (EIT) technique for lidar altimetry.

A technique we refer to as Elevation Information in Tail (EIT) has been developed to provide improved lidar altimetry from CALIPSO lidar data. The EIT technique is demonstrated using CALIPSO data and is applicable to other similar lidar systems with low-pass filters. The technique relies on an observed relation between the shape of the surface return signals (peak shape) and the detector photo-multiplier tube transient response (transient response tail). Application of the EIT to CALIPSO data resulted in an order of magnitude or better improvement in the CALIPSO land surface 30-meter elevation measurements. The results of EIT compared very well with the National Elevation Database (NED) high resolution elevation maps, and with the elevation measurements from the Shuttle Radar Topography Mission (SRTM).

Characteristics of the earth's radiation budget derived from the first year of data from the Earth Radiation Budget Experiment

The first year of broadband Earth Radiation Budget Experiment (ERBE) data is analyzed for top-of-theatmosphere regional variations of outgoing longwave (LW) flux and planetary albedo for total scene as well as clear-sky conditions. The annual variation of radiative parameters is examined for February 1985 through January 1986 for selected regions, latitude zones, and the entire globe. Results show significant seasonal variations for both LW fluxes and albedo. A broad longwave flux maximum (with a relative minimum corresponding to the Intertropical Convergence Zone in the middle) covers the Tropics and the sub-tropics with its center moving about 200 latitude between seasonal extremes. Minimum albedo (about 20%) occurs within 15° of the Equator. In the Tropics and midlatitudes, there is a tendency toward higher albedos during the summer. Larger albedos at the higher latitudes are caused by solar zenith angle effects and by increased snow and ice cover. Net warming occurs between 35°N and 35°S latitude near the equinoxes and in a 90°-wide latitude band at the solstices centered around 35° latitude in the summer hemisphere. This energy surplus at lower latitudes coupled with an energy deficit in the poleward regions is the primary driver of atmospheric circulations. For the year, the global net radiation is nearly in balance. Clouds were found to have a net cooling effect on Eartlfs climate for all seasons. The annual mean net cloud radiative cooling for the globe from ERBE is 18 Wm-2.

On the Determination of the Optimal Scan Mode Sequence for the TRMM CERES Instrument

Abstract Clouds and the Earth’s Radiant Energy System (CERES) is a NASA spaceborne measurement program for monitoring the radiation environment of the earth–atmosphere system. The first CERES instrument is scheduled to be launched on board the Tropical Rainfall Measuring Mission (TRMM) satellite in late 1997. In addition to gathering traditional cross-track fixed azimuth measurements for calculating monthly mean radiation fields, this single CERES scanner instrument will also be required to collect angular radiance data using a rotating azimuth configuration for developing new angular dependence models (ADMs). Since the TRMM single CERES instrument can only be run in either one of these two configurations at any one time, it will need to be operated in a cyclical pattern between these two scan modes to achieve the intended measurement goals. To minimize the errors in the derived monthly mean radiation field due to missing cross-track scanner measurements during this satellite mission, determination of the...

Time dependence of the earth's radiation fields determined from ERBS and NOAA-9 satellites

Satellite measurements from the Earth Radiation Budget Experiment (ERBE) are providing important quantitative data on the diurnal variability of broadband shortwave and longwave radiation. The results derived from the combination of the Earth Radiation Budget Satellite (ERBS) and NOAA-9 indicate that the largest diurnal variations in longwave radiation occur typically over deserts and over land areas which experience intense convective activity. Maximum values of the albedo diurnal amplitude factor are over oceans. Seasonal and cloud cover variations have important effects on the diurnal cycles of Earths radiation budget. ERBE results derived for individual regions are in substantial agreement with the diurnal results derived from the Geostationary Operational Environmental Satellite (GOES) measurements.

CERES: The Next Generation of Earth Radiation Budget Measurements

NASAs Earth Observing System (EOS) is part of an international program for studying the Earth from space using a multiple-instrument, multiple-satellite approach. The Clouds and the Earths Radiant Energy System (CERES) experiment is designed to monitor changes in the Earths radiant energy system and cloud systems and to provide these data with sufficient simultaneity and accuracy to examine critical cloud/climate feedback mechanisms which may play a major role in determining future changes in the climate system. The first EOS satellite (Terra), scheduled for launch this year, and the EOS-PM satellite, to be launched in late 2000, will each carry two CERES instruments. The first CERES instrument was launched in 1997 on the Tropical Rainfall Measuring Mission (TRMM) satellite. The CERES TRMM data show excellent instrument stability and a factor of 2 to 3 less error than previous Earth radiation budget missions. The first CERES data products have been validated and archived. The data consist of instantaneous longwave and shortwave broadband radiances, top- of-atmosphere fluxes, scene types, and time and space averaged fluxes and albedo. A later data product will combine CERES radiances and high-resolution image data to produce cloud properties and fluxes throughout the atmosphere and at the surface.

Seasonal cloud-radiative forcing over land and ocean derived from ERBE satellites

Earth Radiation Budget Experiment (ERBE) data are analyzed to determine seasonal variations of radiative parameters. Particular attention is given to annual variations of the top-of-the atmosphere zonally averaged outgoing long-wave (LW) and absorbed short-wave (SW) flux, as well as the short- and long-wave components of cloud-radiative forcing. The ERBE results reveal significant seasonal variations in both outgoing LW and absorbed SW flux, and a pronounced difference between oceanic and continental surfaces.