Untangling microphysical and dynamic effects in precipitation formation

One of the objectives of the recent COPE experiment, which took place in southwestern England during the summer of 2013, is to investigate the relative importance of different microphysical pathways on the production of precipitation in mixed-phase convective clouds. Clouds that formed on successive days under similar thermodynamic conditions, yet which differed markedly in how precipitation was initially produced, are examined.

On August 2, 2013, observations show surprisingly strong reflectivity due to warm rain production within central regions of clouds at temperatures as cold as -7 °C. Ground-based polarimetric radar shows elevated ZDR columns in these regions. In situ measurements from an instrumented aircraft were analyzed. Raindrops with diameters greater than 3 mm were observed routinely, including a super-large raindrop with diameter in excess of 7 mm within the updraft core at the -6 °C level.

In contrast, on August 3, 2013, drop growth through warm-processes appeared to be less important to overall precipitation formation even though maximum cloud droplet concentration was lower. Values of ZDR were less and ZDR columns were confined to warmer temperatures. In situ observations within similar regions of clouds as on the previous day show raindrops of much smaller size, generally less than 1.5 mm. Also, formation of ice was much more prevalent resulting in higher concentrations of frozen precipitation.

Here we contrast the observations from these two cases and use a one-dimensional parcel model with detailed microphysics to help unravel some of the differences. The model ascends at a rate proportional to the buoyancy of the sounding for each case, and thus is used to illustrate the importance of any differences in the cloud updrafts from these two days, and how these differences impact cloud microphysical processes.