The Roles of Aerosols and Entrainment and Mixing in the Warm Rain Process

New Mexico Institute Of Mining And Technology, Socorro NM

Investigators

Abstract

A substantial amount of the rain in the tropics and midlatitudes falls from clouds that are not cold enough to contain ice crystals. This so-called "warm rain" is produced by the collision and coalescence of cloud droplets, which are formed by condensation. The time required for precipitation to develop is determined by the concentration of cloud droplets and their tendency to come together. A typical droplet in a non-precipitating cloud has a diameter of about 10 mm. Drops this small do not collide with each other frequently enough to account for the formation of rain. The creation of drizzle drops (100 mm) in the times observed (about 20 min after the cloud appears) requires the presence of some drops of about 20 mm diameter. These fall fast enough relative to the smaller droplets to collide with them and form still larger drops. In general, the cloud droplet populations that promote collisions and coalescence and can thereby initiate precipitation are those containing a spread of droplet sizes: the greater the spread the greater the collision rate and the faster the conversion of cloud to drizzle and rain. A classical problem in the theory of warm rain is that condensation acting on what are thought to be realistic populations of condensation nuclei does not produce the broad droplet spectra that are conducive to collisions and coalescence. This problem is approached by a combination of numerical modeling and the analysis of observations. The guiding hypothesis is that the broadening of droplet spectra may be explained by the mixing of cloudy air parcels with different histories. The research plan includes computing an ensemble of trajectories of droplets that start from different positions within a developing cloud but reach the same place at the same time. Because of turbulence and different growth histories, the source region can be large. The analysis combines the idea of turbulently-perturbed growth trajectories with the airflow and parameterized precipitation generated by large eddy simulation. A novel aspect of the work is that the calculations are performed backwards in time: the history of a drop of a given size is traced back from a prescribed starting point to an original position at cloud base. Not all sizes will turn out as originating at cloud base. An iterative procedure is used to determine which sizes can be ascribed to droplets that form at cloud base. These, then, are the sizes that are consistent with the model. The model is evaluated by comparing the predicted drop sizes with those measured by aircraft in the Small Cumulus Microphysics Study conducted in Florida in 1995.