Remote sensing of droplet number concentration of aerosol-induced clouds during the 2019 fire event in Borneo, Indonesia

Abstract

Abstract The droplet number concentration of liquid water clouds N is estimated based on the optical thickness τ and particle effective radius reff retrieved from the measurements of the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard of Terra satellite. Two measurements of MODIS taken from 15 June 2019 (case A) and 15 September 2019 (case B) are analyzed. They represent a condition before and during the 2019 fire event in Borneo, Indonesia. The result shows, that the domains of τ and reff are comparable for the two cases. While there is no sign of systematic bias in case of τ, the analysis of r eff tells differently. Clouds with smaller droplets, less than 12 µ m, are more prominent in case B, indicating the present of aerosol-induced clouds. In case A, the frequency distributions of clouds with larger droplets, more than 12 µ m, are systematically higher. The magnitudes of N in case A are mostly less than 100 cm-3, which exhibit a background condition. Extreme magnitudes of N, from 200 to 800 cm −3, are found in case B. Those excessive numbers are attributed to the region where the aerosol-induced clouds are observed.