Abstract
Abstract. A technique is presented that uses attenuated backscatter profiles from the CALIOP satellite lidar to estimate cloud base heights of lower-troposphere liquid clouds (cloud base height below approximately 3 km). Even when clouds are thick enough to attenuate the lidar beam (optical thickness τ≳5), the technique provides cloud base heights by treating the cloud base height of nearby thinner clouds as representative of the surrounding cloud field. Using ground-based ceilometer data, uncertainty estimates for the cloud base height product at retrieval resolution are derived as a function of various properties of the CALIOP lidar profiles. Evaluation of the predicted cloud base heights and their predicted uncertainty using a second statistically independent ceilometer dataset shows that cloud base heights and uncertainties are biased by less than 10 %. Geographic distributions of cloud base height and its uncertainty are presented. In some regions, the uncertainty is found to be substantially smaller than the 480 m uncertainty assumed in the A-Train surface downwelling longwave estimate, potentially permitting the most uncertain of the radiative fluxes in the climate system to be better constrained. The cloud base dataset is available at https://doi.org/10.1594/WDCC/CBASE.
Highlights
The base height z is an important geometric parameter of a cloud, controlling the cloud’s longwave radiative emission, being required in the calculation of the cloud’s subadiabaticity, and setting the level at which aerosol concentration and updraft speed determine the cloud’s microphysical characteristics
Methods that are applicable to A-Train satellites are based on Moderate-Resolution Imaging Spectroradiometer (MODIS; Platnick et al, 2017) cloud properties retrieved near the cloud top and integrated along moist adiabats to determine the cloud thickness (Meerkoetter and Zinner, 2007; Goren et al, 2018) or on active remote sensing by CloudSat (2B-GEOPROF; Marchand et al, 2008) or a combination of CloudSat and Cloud– Aerosol Lidar with Orthogonal Polarization (CALIOP) (2B-GEOPROFLIDAR; Mace and Zhang, 2014)
The input satellite data to our analysis are from the Cloud– Aerosol Lidar with Orthogonal Polarization (CALIOP; Winker et al, 2007) onboard the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite that is part of the A-Train satellite constellation (Stephens et al, 2002) on a sun-synchronous low-Earth orbit with Equator crossings at approximately 13:30 local time
Summary
The base height z is an important geometric parameter of a cloud, controlling the cloud’s longwave radiative emission, being required in the calculation of the cloud’s subadiabaticity, and setting the level at which aerosol concentration and updraft speed determine the cloud’s microphysical characteristics. Methods that are applicable to A-Train satellites are based on Moderate-Resolution Imaging Spectroradiometer (MODIS; Platnick et al, 2017) cloud properties retrieved near the cloud top and integrated along moist adiabats to determine the cloud thickness (Meerkoetter and Zinner, 2007; Goren et al, 2018) or on active remote sensing by CloudSat (2B-GEOPROF; Marchand et al, 2008) or a combination of CloudSat and CALIOP (2B-GEOPROFLIDAR; Mace and Zhang, 2014).
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