Tuesday, 11 February 2003
Evaluating Water Vapor and Cloud Retrievals from MODIS Over Antarctica
The Moderate Resolution Infrared Spectroradiometer (MODIS) is a 36 channel infrared imager with horizontal resolutions up to 250 meters. It flies onboard the Earth Observing System (EOS) AM platform in a polar orbit, that subsequently converge over the South Pole to provide excellent spatial coverage. Although not designed specifically for sounding applications, retrieval algorithms are being tested by scientists at the Cooperative Institute for Meteorological Studies (CIMSS) using MODIS radiances. Currently, MODIS radiances are being used to retrieve total precipitable water (TPW) in clear 5 by 5 fields-of-view (FOV) yielding a horizontal resolution of 5 kilometers. Cloud detection is also possible at high horizontal resolution. Cloud-top pressures (CTP) are retrieved using a standard CO2 intercept approach. These retrievals have been evaluated against co-located measurements at the Atmospheric Radiation Measurement Program's Cloud and Radiation Testbed (ARM/CART) site in Oklahoma. Results are encouraging for this location. Can these high-resolution retrievals provide useful information to forecasters and numerical models at high latitudes?
Retrievals of TPW and CTP from MODIS have been evaluated over the Antarctic region for one 24-hour period from December 6, 2000 using in situ radiosondes and surface observations. Retrievals have also been inserted in the polar version of the Cooperative Institute for Meteorological Satellite Studies (CIMSS) Regional Assimilation System (PCRAS), running at 60-km resolution over Antarctica. The National Center for Environmental Prediction's (NCEP) Aviation model provides the initial and boundary conditions for the PCRAS. The retrievals were compared to TPW and CTP calculated from the forecast model during a 24 hour forecast initialization period. Approximately 800,000 observations at 5-km resolution were available from 14 satellite passes over the model domain. Comparison statistics indicate that the MODIS retrievals tend to dry the forecast model in the coastal regions surrounding Antarctica and reduce cloudiness throughout the model domain. Smaller differences were present as far north as 45 S. latitude. The retrievals had little effect over the central continent where the atmosphere is typically very dry. Additional cases are planned for further evaluation.