Retrieving cloud base height from passive radiometer observations via a systematic effective cloud water content table

Abstract

Cloud base heights (CBHs) play an important role in weather and climate studies, but remain one of the most challenging properties to be inferred using passive satellite spectral instruments. This is because spectral signals detected from space are mainly from cloud tops and contain minimal information on cloud vertical structures owing to limited penetration. This paper presents a new algorithm for retrieving CBH from radiometer measurements, utilizing an improved characterization for mutable cloud water contents (CWCs). This was achieved by introducing a variable named effective CWC (ECWC) to represent the vertically varying CWC; and subsequently investigating the sensitivity of ECWC to both cloud and environmental properties. Considering these characteristics, a systematic ECWC lookup table (LUT) was created based on collocated active and passive satellite observations. We then converted the existing cloud water path (CWP; kg/m2) to cloud geometric thickness (CGT; km) via the developed LUT, and calculated CBH by subtracting the CGT estimates from conventional cloud top height (CTH) retrievals. The algorithm was tested using observations from Moderate Resolution Imaging Spectroradiometer (MODIS) and Advanced Himawari Imager (AHI) as examples, and was evaluated using active measurements from the CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellites and a ground-based Ka-band radar. Overall, our CBH results agreed closely with the active observations, with mean biases of 0.11 ± 1.93 km for MODIS and − 0.12 ± 2.34 km for AHI in single-layer cloud cases. Despite the natural limitations on retrieving CBHs using passive radiometers, the proposed algorithm achieves retrieval accuracies comparable to those of operational CTH products. Moreover, this algorithm works for both single-layer and overlapping clouds, and can be extended to other passive radiometers by reconstructing the LUT. Such accurate CBH information derived from high-resolution passive radiometers would greatly improve our understanding of cloud vertical distributions; and could play an important role in studies of cloud radiative effects and the aerosol-cloud-precipitation interactions.