A "Frozen Drop" precipitation mechanism over an open ocean and its effect on rain, cloud pattern, and heating

Videosonde data collected over the East Asian monsoon area showed very low concentrations of ice crystals in clouds over the open ocean in contrast to the many observed over the maritime continent. This result corresponds with the weak lightning activity reported over the ocean. Here, “Frozen” rain process works while the “Mixed” rain processes do over the continent.

A three-dimensional microphysical cloud model was used to examine the effects of these different precipitation mechanisms on cloud organization, rain production, and the vertical latent heat profile. Initially, multiple cloud cells were generated randomly at low levels with low-level wind shear and a circulating wind. A rainband was simulated by warming low-level air in a narrow, central strip. “Mixed”, “Frozen Drop”, and “Warm Rain” processes were formed by modifying cloud and ice nuclei concentrations and drop freezing. The environmental conditions were typical of tropical marine air.

With the “Mixed” process, with time the cells coagulated and the clouds were well organized, forming 60 km rain rings with linear wind shear or spiral rainbands with a circulating wind. The highest rainfall production occurred with the “Mixed” process. The “Frozen” process produced slower cloud organization, however, intense rainfall persisted at the rear of the rainband. Here, the conversion rate from water vapor to raindrops is higher, later in time. The vertical latent heat profile showed two peaks: by drop condensation at the lower level and at higher levels by ice crystal depositional growth. With low-level wind shear and circulating winds, however, drop evaporation dominated in small clouds, leading negative latent heat in the lower levels,

The reason for such a difference in ice crystal concentration between an open ocean and a maritime continent is unclear. Videosonde data suggest, however, that over the ocean, the quick formation of large frozen drops may influence ice crystal production.