Regional assessment of microphysical properties of marine boundary layer cloud using the PATMOS‐x dataset

Abstract

Cloud droplet number concentration and geometrical thickness of marine boundary layer clouds are inferred from 25 years of National Oceanic and Atmospheric Administration's AVHRR Pathfinder Atmospheres‐Extended (PATMOS‐x) Level 2b retrievals of optical thickness and cloud droplet effective radius over the period 1982 through 2009. A novel approach to addressing nonphysical values of cloud droplet number concentration N owing to satellite orbital drift is applied by normalizing estimated droplet number concentrations with respect to local observation time. Cloud geometrical thickness H is also normalized to a common reference time by scaling H against diurnal values from a passive microwave liquid water path climatology. The effectiveness of the methods applied to correct N and H are evaluated. Both quantities are spatially and temporally characterized in several subtropical subsidence regions for likely drizzle‐free observations. Estimated liquid water path from PATMOS‐x is further validated against 20 years of liquid water path values from the Special Sensor Microwave/Imager (SSM/I). Good agreement between SSM/I and PATMOS‐x is found in coastal regions. Cloud droplet number concentrations in excess of 300 cm−3 are found along the western boundaries of the American and African continents, with greatly lower values found further out to sea with no observed long‐term trends in cloud properties.