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

Abstract. Lidar-based measurements of cloud base and top height (CBH and CTH) and cloud geometrical thickness (CGT) with greatly improved accuracy provide new scientific insights. However, direct observation of the active Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) cannot penetrate optically thick clouds to their base. A work around for this problem is developed and validated in this study. This method is based on the 333-m resolution low-level water cloud data obtained from the Vertical Feature Mask product of CALIPSO. The methodology can effectively minimize the interference of surface signals, boundary layer aerosols and cloud anvils of CBH retrieval. The methodology overcomes the full attenuation of the lidar in optically thick clouds by assuming that CBH of boundary layer clouds is similar over an area of several tens of km. This allows taking the CBH of the neighbouring penetrable shallower cloud as having CBH representative for the entire cloud field. The retrieved CBH was validated against two surface-based ceilometer measurements in the islands of Barbados and the Azores, with an error standard deviation of ±100 m. The accurate CBH allow us to obtain CGT, which is an essential parameter in 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 analysed the variations of CBH and CTH over the ocean and land. Some highlights from these first of a kind high-precision cloud geometry observations show for annual mean values that: (1) The lowest CBH occurs over the eastern margins of the subtropical oceans and increases westward from 300–400 to 700–800 m. The CGT increases from 300 to 1200 m, respectively. In the western part of the tropical oceans CBH is 500–600 m and CGT is ~1500 m. (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 Amazon and Congo rain forests are 1200 and 1500 m, respectively. CBH over the drier tropical lands is 1500–2000 m, with CGT of 800–1000 m. (4) Lowering CBH towards Antarctica in the Southern Oceans, while deepening CGT. (5) Seasonally, the mid-latitude global oceans have the lowest CBH (mostly below 500 m) and CGT in summer seasons, and the highest cloud in winter, while the CBH and CGT distribution reversed over high-latitude Southern Oceans. (6) There is an obvious diurnal cycle of maximum CTH and CGT over the tropics. Over the ocean, there is no discernible diurnal cycle in CBH, but during night CTH is higher by ~300 m.