An Improved Technique for Retrieval of Cloud Properties at Night and in Low Sun Conditions
Heck, P.W.(a), Rapp, A.D.(a), Minnis, P.(b), Smith, Jr., W.L.(b), and Nguyen, L.(b), Analytical Services & Materials, Inc. (a), NASA Langley Research Center (b)
Twelfth Atmospheric Radiation Measurement (ARM) Science Team Meeting
An Improved Technique for Retrieval of Cloud Properties at Night and in Low Sun Conditions The Atmospheric Radiation Measurement (ARM) Program measurements at the surface allow for continued development and validation of techniques for retrieving cloud radiative and microphysical properties throughout day and night. Until recently, ARM satellite cloud properties have primarily been derived with the Layered Bispectral Threshold Method (LBTM). Multispectral daytime and nighttime techniques, the Visible-Infrared-Solar infrared-Split window Technique (VISST) and Solar infrared-Infrared-Split window Technique (SIST), respectively, are now being applied to GOES imagery over a domain that includes the ARM Southern Great Plains site. The SIST, which is also used for retrievals when the solar zenith angle (SZA) is greater than 80°, inverts the cloud physical parameters that produce the minimum difference between observed and modeled brightness temperature differences for each 4-km satellite pixel. Cloud optical depth, effective particle size, water content and temperature (height) of both water and ice clouds are obtained. Although a substantial improvement over the previous infrared-only night time technique used in the LBTM, SIST is limited by the amount of information contained in the available thermal channels: 3.9, 11 and 12 microns. These wavelengths allow discrimination between optically thin and thick clouds but are able to determine the cloud physical properties for optically thin clouds only. SIST is also very sensitive to the input parameters that bound the derived cloud properties and is difficult to apply when SZA is between 80 and 85°. Thus, improved surface emissivity maps and methods to estimate the surface skin temperature are developed to enhance cloud discrimination at night when there is little infrared temperature contrast with the surface. Improvements will also be described in the usage of cloud brightness temperature parameterizations for retrievals of clouds and fog that are warmer than the surface temperature, conditions that are common during winter and in polar regions. Validation of daytime radiative and microphysical properties has been ongoing, while the accuracy of night time SIST-derived properties is comparatively untested. The improved SIST-derived cloud products from GOES will be compared to daytime results and contemporaneous cloud products derived from MODIS and VIRS to assess consistency and the uncertainties will be determined by comparison with ARM surface radar measurements. Since an accurate 24-hour retrieval of cloud properties is vital for studies of cloud diurnal variability, the hydrological cycle, and cloud-radiation interactions, the improved nighttime cloud algorithm, combined with existing daytime results, should provide a consistent data set valuable for modeling validation and, perhaps, initialization.
Note: This is the poster abstract presented at the meeting; an extended version was not provided by the author(s).
