Size spectra of ice particles in frontal clouds: Correlations between spectrum shape and cloud conditions

Ice particle size spectra measured by optical particle size spectrometers during airborne sampling of clouds associated with several frontal precipitation systems in the Pacific Northwest are presented and discussed. In the majority of the spectra measured, the concentration densities of particles <2 mm in diameter deviated significantly below or above the exponential curve fitting larger diameters (we term these sub-exponential and super-exponential spectra, respectively). Sub-exponential spectra were found to be dominant in regions of weak stratiform cloud, where liquid water was scarce. Super-exponential spectra were dominant in convective regions between the -1 and -9°C levels, where supercooled liquid water was present in amounts up to 0.2 gm-3. Exponential spectra were dominant in convective regions at the -11 °C level and above. Microphysical processes as well as shear-induced particle size sorting were found to play a role in the development of spectrum shapes. Depletion of small particles due to aggregation appears to have been an important cause of sub-exponential spectra. Secondary ice crystal production appears to have been responsible for the formation of super-exponential spectra. the role of particle size was largely to broaden the range of spectrum shapes present, rather than to promote the dominance of a particular spectrum type. The impact of spectrum deviations on the relationship between radar reflectivity factor and snowfall rate, though observable, is found to be small with respect to other sources of error often found in radar estimates of snowfall.