A Near-Global Climatology of Single-Layer and Overlapped Clouds and Their Optical Properties Retrieved from Terra/MODIS Data Using a New Algorithm

Abstract

Cloud overlapping has been a major issue in climate studies owing to a lack of reliable information available over both oceans and land. This study presents the first near-global retrieval and analysis of single-layer and overlapped cloud vertical structures and their optical properties retrieved by applying a new method to the Moderate Resolution Imaging Spectroradiometer (MODIS) data. Taking full advantage of the MODIS multiple channels, the method can differentiate cirrus overlapping lower water clouds from single-layer clouds. Based on newly retrieved cloud products using daytime Terra/MODIS 5-km overcast measurements sampled in January, April, July, and October 2001, global statistics of the frequency of occurrence, cloud-top pressure/temperature (Pc/Tc), visible optical depth (τVIS), and infrared emissivity (ε) are presented and discussed. Of all overcast scenes identified over land (ocean), the MODIS data show 61% (52%) high clouds (Pc < 500 hPa), 39% (48%) lower clouds (Pc > 500 hPa), and an extremely low occurrence (<4%) of Pc between 500 and 600 hPa. A distinct bimodal distribution of Pc is found and peaks at ?275 and ?725 hPa for high and low clouds, thus leaving a minimum in cloud in the middle troposphere. Out of the 61% (52%) of high clouds identified by MODIS, retrievals reveal that 41% (35%) are thin cirrus clouds (ε < 0.85 and Pc < 500 hPa) and the remaining 20% (17%) are thick high clouds (ε ≥ 0.85). Out of the 41% (35%) of thin cirrus, 29% (27%) are found to overlap with lower water clouds and 12% (8%) are single-layer cirrus. Total low-cloud amount (single-layer plus overlapped) out of all overcast scenes is thus 68% (39% + 29%) over land and 75% (48% + 27%) over ocean, which is greater than the cloud amounts reported by the MODIS and the International Satellite Cloud Climatology Project (ISCCP). Both retrieved overlapping and nonoverlapping cirrus clouds show similar mean τVIS of ?1.5 and mean ε of ?0.5. The optical properties of single-layer cirrus and single-layer water clouds agree well with the MODIS standard retrievals. Because the MODIS retrievals do not differentiate between cirrus and lower water clouds in overlap situations, large discrepancies are found for emissivity, cloud-top height, and optical depth for high cirrus overlapping lower water clouds.