Abstract
Detailed measurements of the ice particle properties of winter continental cumulus clouds, performed by instrumented aircraft, are described and analyzed. The data presented, based upon measurements in 69 such clouds in the cloud summit temperature range of −5 to −25°C, suggests that the temperature dependence of ice crystal concentrations, at 300 m below cloud tops, corresponds to the mean ice nucleus temperature spectrum within a range of variation of about one order of magnitude. This rather limited discrepancy is proposed to be attributed partly to very limited processes of secondary production of ice crystals and partly to our inability to account, instrumentally, for the activation of nuclei through such mechanisms as contact nucleation.
These findings also suggest that cold continental cumuli do not provide conditions for noticeable effects due to ice crystal multiplication mechanisms. In particular, it is shown that the characteristics of the cloud droplet spectra, in such clouds, are the key factor in determining the ice budget of the clouds as they affect the timing and the nature of the riming processes that were proposed to lead to ice crystal multiplication. These findings are in good agreement with recent laboratory studies suggesting a strong link between temperature and droplet characteristics during the riming process, and ice crystal multiplication.
Consequently and in view of the shown dependence of graupel concentrations on temperature in cold continental cumuli, some conclusions are drawn regarding the formation of either natural or artificially induced precipitation in such colloidally stable clouds. These conclusions seem to be in agreement with observations elsewhere of precipitation likelihood as a function of cloud top temperature.
