Abstract

This study explores dynamical and microphysical development of clouds over the Bay of Bengal including days associated with cyclonic storms (Cylone Pahilin, Oct 2013). The vertical transport of sea spray to cloud bases is first quantified‐from Landsat (L8) measurements launched a few months prior to Phailin which whipped up strong winds producing spray droplets, mostly in the coarse mode, at the crest of ocean waves. On calmer days one observed an additional fine mode. These sea salt aerosol distributions were then incorporated into a chemical parcel model yielding realistic cloud droplet spectra which were grown to form much larger rain drops through a process of stochastic coalescence.

The second part of the study focuses on the effect of millimeter scale vortices obtained during both stormy as well non‐stormy days on the quantification of rain rates. In essence, rescaled settling of the small inertial particles (mainly the film mode drops) is obtained as they are ‘centrifuged’ out of vortices and eddies in turbulence. This process enhances the average sedimentation rate of particles lying below a critical radius (~ 20 microns) by about 80% strongly impacting rain rates. Whist earlier studies have shown that multi‐component mixtures perturb the Aerosol‐Cloud Droplet relationships over certain regimes; this first study shows that when multiple aerosol modes are present within convective clouds, ano ther added perturbing effect‐ the enhanced settling rates from the centrifuging action of the micro‐scale eddies increases precipitation rates over the Bay of Bengal.

Wider implications are discussed including recent research on cloud seeding (enhancement of precipitation) and the geo‐engineering modulation of cyclonic storms through sea water injection mechanisms (dissipation of rainfall over oceans prior to landfall).