Precipitation is the single most important source of freshwater generation on Earth, but our process-level understanding of its formation and evolution at the microphysical level is crude and remains highly uncertain. In particular, the relative roles of ice microphysical processes, from ice initiation and growth to ice splintering are poorly constrained. The goal of this study is to apply a new method that measures the freezing behavior of individual rain and drizzle drops, with the objective to parameterize the susceptibility of rain rate to ice nuclei number concentrations.
INTELLECTUAL MERIT Precipitation forms when drops collide and coalesce, rapidly shunting cloud water into drizzle and rain. To initiate this process a small number of larger hydrometeors must be present, usually assumed to be ice crystals in clouds that are cold enough for heterogeneous ice nucleation to occur. Each precipitation particle contains this seed, provided secondary processes such as ice multiplication or drop breakup did not occur. To analyze the relative importance of primary and secondary processes, individual rain drops will be collected in liquid nitrogen and stored in paraffin. Each raindrop will be placed on a cold stage and the temperature at which it freezes will be measured. The distribution of raindrop freezing temperatures for precipitation from convective storms, synoptic scale winter storms, frontal passages, and remnants of tropical cyclones will be analyzed as a function of the evolution of the precipitating system. All data will be analyzed in the larger meteorological and microphysical context, i.e. rain rate, cloud-top temperature, cloud droplet number concentration, and used to constrain the susceptibility of precipitation to ice nucleation for inclusion in numerical weather and climate prediction models.
BROADER IMPACTS The investigation, although fundamental in nature, touches on questions with important societal implications. It is hypothesized that aerosols from anthropogenic activity affect ice processes and in turn modify precipitation. It is surprising that despite its fundamental importance to society, and more than 200 years after Benjamin Franklins? conjecture implicating ice processes in precipitation formation, we still do not understand the relative importance of the microphysical processes involved. The experiments here will help to constrain this question. In addition to the typical dissemination channels (conferences and peer reviewed contributions to the literature), the investigators will host public lectures to engage the non-science community in the Raleigh/Durham area with this topic. A number of undergraduate and graduate students at N.C. State University will work with the principal investigator on this project.
