Preliminary global cloud comparisons from the AVHRR, MODIS, and GLAS: cloud amount and cloud overlap

Abstract

Near-global total cloud frequencies and multilayered cloud frequencies derived from AVHRR (Advanced Very High Resolution Radiometer), MODIS (Moderate Resolution Imaging Spectroradiometer), and GLAS (Geoscience Laser Altimeter System) were analyzed and compared. The GLAS retrievals can be used to quantify the amount of cloud that may go undetected from satellite imagers such AVHRR and MODIS and to help validate satellite cloud overlap detection algorithms. Model sensitivity studies indicate that clouds with a total column optical depth of 0.5 or less may often go undetected by AVHRR and MODIS. The GLAS data show that such cloudy observations comprised 18.3% (14.5%) of all cloudy GLAS footprints during the most convectively active (least convectively active) portion of the day. Where the most (least) convectively active time period is defined as local solar noon plus (minus) 12 hours. It was also shown that the zonal mean total cloud frequency from GLAS and AVHRR and GLAS and MODIS are well correlated but often differ in magnitude because of thin clouds or small-scale cloud systems that are missed by the AVHRR and MODIS cloud detection algorithms. With the exception of the polar regions, the AVHRR and GLAS and the MODIS (via the Visible/Infrared Imager/Radiometer Suite algorithm) and GLAS multilayered cloud frequencies are in good agreement.