Cloud optical parameter data from the Moderate Resolution Spectroradiometer (MODIS) aboard the Aqua satellite and collocated cloud radar data from CloudSat are used to study the relationship among macrophysical variables (cloud thickness, liquid water path) and microphysical variables (effective radius, droplet concentration) in warm oceanic clouds across the tropics and subtropics. Remote Pacific warm clouds tend to have low droplet number concentrations, large effective radii, and high drizzle frequency. In areas influenced by continental aerosols and pollution, such as off the Asian Coast and over the Gulf of Mexico, droplet size is smaller by several microns, and droplet concentrations are larger.
Warm cloud top height, liquid water path, effective radius, and droplet concentration are examined as a function of drizzle rate, and cloud top height and liquid water path substantially increase as drizzle increases. Particle size also increases with dBZ, but tends to level off as dBZ>0, except off the Asian Coast and over the Gulf of Mexico, where a monotonic increase of particle size with drizzle rate occurs. Droplet radius is considerably different by several microns for a given dBZ in different regions, which suggests that the radius is not a useful indicator of precipitation. Droplet concentration tends to decrease only slightly in remote marine areas with drizzle rate, but decreases dramatically off the Asian Coast and over the Gulf of Mexico. Drizzle itself may remove a large concentration of smaller drops in high number concentration regimes, or alternatively lower droplet concentrations and large cloud droplets may be more effective for warm rain collision and coalescence.
The probability of drizzle is also examined as a function of cloud macrophysics and microphysics. An analysis of eight separate regions reveals that drizzle frequency increases dramatically and nearly uniformly from one region to another when cloud tops grow from one to two km. When looking at both liquid water path and number concentration effects together, drizzle frequency decreases for a fixed amount of cloud liquid water as the number concentration increases. Even in regions where drizzle occurs very infrequently on the whole, it becomes likely when clouds acquire sufficient liquid water and have low droplet concentrations.
Finally, we compare observations of drizzle intensity and frequency as a function of various macrophysical and microphysical regimes with calculations from a simple warm rain continuous collection model.