Ice Initiation in Supercooled Cumulus Clouds

1. Ice Initiation in Supercooled Cumulus Clouds Bill Cotton Colorado State University

2. Observations of high ice particle concentrations Rangno and Hobbs (1994, 1991) and Hobbs and Rangno (1985) report on aircraft observations of ice particle initiation in maritime and continental cumulus clouds. They find that ice particle initiation is a two-stage process. During stage 1, initial ice particles seem to originate through the freezing of larger cloud droplets in concentrations com-parable to that estimated with the formula of Meyers et al. (1992) derived from IN measurements. During stage 2, however, ice particle concentrations of tens to 100's per liter form in less than ten minutes. Such high concen-trations appear coincident with or soon after the formation of graupel particles. Supercooled raindrops or drizzle drops need not be present. In some of their cases the time-scales of high ice particle concentration formation is too short for the rime-splinter process to account for the observations. In other cases it might explain the observa-tions. We conclude that there still remain mysteries about ice formation in some clouds.

3. Ice generation during evaporation of ice particles Oraltay and Hallett (1989) and Dong et al. (1994) examined the generation of secondary particles during the evaporation of ice particles. At relative humidities below 70%, as many as 30 pieces per crystal formed. This did not occur with columns or solid plates. Evaporation of simulated graupel particles of a few millimeters produced several hundred ice particles under certain conditions. This process has not been further quantified in the laboratory or been inserted in cloud models to examine its potential importance. Circumstantially this process fits with numerous observations which suggest enhancements of ice crystal concentrations during evaporation of clouds or in heavily mixed regions. What needs to be determined is will these evaporated ``bits" survive long enough during entrainment events to serve as embryos for further ice crystal growth?

4. Proposed Observations in cumuli • First attempt to replicate Hobbs and Rangno observations • A single well-instrumented aircraft like the UWYO King Air would be perfect. • It should have LWC, PMS probes, CPI, CCN, GCCN(ideal), and IN(CFDC), WCR would be nice too but I know that it will not fit with above. • Soundings(T, RH, winds, CCN, GCCN, IN) before and after cloud penetrations • Obtain multiple penetrations at temperatures around -10C and colder to define lifecycle of ice evolution • The study should follow a case study approach to enable modelers to determine if current theories can account for rapid ice formation

5. Locations • Begin in State of Washington east and west of Cascades in locations and time of year similar to Hobbs and Rangno to see if their observations can be replicated • Do the same say in NE Colorado • Do the same in central Texas

6. Summary • The proposed observations should provide a good cross section of how ice in supercooled cumuli evolves in different airmasses and provide case studies that can be used for numerical simulations.