Tuesday, 4 June 2002
Angular Variations of Cloud Properties Derived From VIRS and MODIS Data for CERES
The Clouds and the Earth's Radiant Energy System (CERES) experiment utilizes broadband and narrowband scanners aboard the same spacecraft to obtain coincident measurements of broadband radiances and cloud properties. The high spatial resolution cloud properties allow for the development of improved bidirectional reflectance functions that in turn yield increased accuracy in earth radiation budget (ERB) products. The ERB improvements displayed by the CERES project are highly dependent on the accuracy of the narrowband cloud properties. The CERES instrument is currently flying aboard two satellites, Tropical Rainfall Measuring Mission (TRMM) and Terra, with a third expected in Spring 2002 after the launch of the Aqua satellite. TRMM flies in a precessing orbit with a 46-day repeat cycle while Terra is sun-synchronous with 1045 and 2245 local overpass times. Aqua is designed to achieve 0130 and 1330 overpass times, thereby completing the suite of measurements and allowing for extensive studies of cloud and radiative properties and their diurnal cycles. The narrowband instrument from which cloud properties are determined on TRMM is Visible and InfraRed Scanner (VIRS) while Terra and Aqua utilize MODerate-resolution Imaging Spectroradiometer (MODIS). The 0.65, 1.6, 3.7, 11.0, and 12.0-Ám channels from VIRS and MODIS are used to derive cloud properties using multispectral techniques. A Visible Infrared Solar-infrared Split-window Technique (VISST) and a Solar-infrared Infrared Split-window Technique (SIST) are applied at daytime and nighttime, respectively, to estimate cloud fraction, phase, effective particle size, optical depth, altitude, and ice/liquid water path. The orbits of the CERES satellite provide a unique coverage of nearly all viewing and solar zenith angles as well as many relative azimuth angles. This paper examines the dependencies of the CERES-derived cloud properties as a function of solar zenith, viewing zenith, and relative azimuth angles to examine the effects of non-plane parallel clouds on retrievals that are based on plane-parallel models. The impact of cloud thickness on the percieved cloud fraction as a a function of viewing zenith angle is also examined. These angular variations will be useful for understanding the effects of real clouds on an idealistic retrieval and perhaps provide some means for correcting for the non-plane-parallel effects. Comparisons of cloud properties derived from coincident VIRS and MODIS data, thereby allowing multi-angle looks at the same clouds. The accuracy of CERES bidirectional reflectance functions is directly impacted by these dependencies as is the functions' application in radiance-to-flux conversions.