ICE PARTICLE CONCENTRATIONS AND PRECIPITATION DEVELOPMENT IN SMALL CONTINENTAL CUMULIFORM CLOUDS

Maximum ice particle concentrations (I(M)) in modest (less-than-or-equ al-to 3.7 km deep) continental cumuliform clouds, with tops with tempe ratures between -6 and -25-degrees-C, were found to be better correlat ed with the broadness of the droplet spectrum near cloud top (r = 0.78 ) than with cloud-top temperature (r = 0.58). Also, the broader the dr oplet spectrum the warmer was the cloud top at which ice first appeare d. Stratification into three cloud-base-temperature (T(B)) categories, cool (0-degrees-C less-than-or-equal-to T(B) less-than-or-equal-to 8- degrees-C), cold (-8-degrees-C < T(B) < 0-degrees-C), and very cold(T( B) less-than-or-equal-to 8-degrees-C), produced correlations between I (M) and cloud-top temperature (T(T)) of 0.71, 0.88, and 0.89, respecti vely. The best-fit lines for these relationships shift to higher I(M) values as T(B) increases; this also reflects the effect on Im of the b roadness of the droplet spectrum, since the size of the largest drops increases as T(B) increases. When the clouds contained drops with diam eters greater-than-or-equal-to 25 mum, ice particle formation was sudd en and prolific. High concentrations of ice particles appeared coincid ent with, or very soon after, the formation of graupel; these high con centrations were observed in clouds at ambient temperatures between -1 1 and -28-degrees-C, including some clouds with T(B) < 0 -degrees-C. F or clouds with similar droplet spectra and T(T), the width of the clou d also affected I(M); narrow clouds (<2 km wide) formed less ice than wider, multi-turreted clouds. Continental cumulus clouds have to be ab out 50% wider and about 5 degC colder at their tops than maritime cumu lus to have the same chance of producing a radar echo. However, the di fference in width to produce a radar echo disappears for clouds with w idths >4 km, although continental clouds still need to be about 5 degC colder at cloud top than maritime clouds to produce a radar echo. Sev eral extant theories for high ice particle concentrations in clouds ar e examined but none provides a satisfactory explanation for the observ ations.