Satellite retrieval of cloud base height and geometric thickness of low-level cloud based on CALIPSO

Abstract

Satellite-based cloud base and top height (CBH and CTH) and cloud geometrical thickness (CGT) are validated against ground-based lidar measurements and provide new scientific insights. The satellite measurements are done by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO). The retrieval methodology is built on the 333gm resolution low-level water cloud data obtained from the Vertical Feature Mask product of CALIPSO. The methodology is based on the definition that CBH of boundary layer clouds is the lowest cloud base over an area of several tens of kilometers. This allows taking the CBH of the neighboring penetrable shallower cloud as having CBH representative for the entire cloud field. The methodology over the ocean was validated based on observations from two surface-based ceilometer measurements in the islands of Barbados and the Azores, with an error standard deviation of 115gm. Validation over land was based on 4 years data of 138 terrestrial ceilometer sites with an error standard deviation of 220gm. The unprecedented accurate CBH allows us to obtain CGT, which is an essential parameter in the understanding of the aerosol-cloud interaction. Based on this newly developed methodology, we retrieved the annual, seasonal, and diurnal distributions of global CBH, CTH, and CGT for two years and analyzed the variations of CBH and CTH over the ocean and land. Climatology of the annual mean cloud geometrical properties shows the following. (1) The lowest CBH occurs over the eastern margins of the subtropical oceans and increases westward from 300-400 to 800-900gm. The CGT increases from 300 to 1200gm, respectively. In the western part of the tropical oceans, CBH is 500-600gm and CGT is g1/4g1500gm. (2) A narrow band of lower CBH and CGT occurs over the Equator, especially over the eastern parts of the oceans. (3) CBH and CGT over the tropical rain forests (Amazon and Congo) are 1200 and 1500gm, respectively. CBH over the drier tropical land is 1500-2000gm, with CGT of 800-1000gm. (4) CBH decreases towards Antarctica in the Southern Ocean, while CGT deepens. (5) Seasonally, the mid-latitude global oceans have the lowest CBH (mostly below 500gm) and CGT in summer seasons and the highest values in winter. (6) There is just an obvious difference between day and night for the maximum CTH and CGT over the tropics. Over the ocean, there is no discernible difference in CBH, but during night CTH is higher by g1/4g300gm.