Ice particle concentrations and precipitation development in small continental cumuliform clouds

Maximum ice particle concentrations ( I M ) in modest (≦3.7 km deep) continental cumuliform clouds, with tops with temperatures between −6 and −25 °C, were found to be better correlated 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 droplet spectrum the warmer was the cloud top at which ice first appeared. Stratification into three cloud‐base‐temperature ( T B ) categories, cool (0 °C ≦ T B −8 °C), cold (−8°C < T B < 0°C), and very cold ( T B ≦ −8°C), produced correlations between I M and cloud‐top temperature ( T T ) of 0.71, 0.88, and 0.89, respectively. The best‐fit lines for these relationships shift to higher I M values as T B increases; this also reflects the effect on I M of the broadness of the droplet spectrum, since the size of the largest drops increases as T B increases. When the clouds contained drops with diameters ≧25 μm, ice particle formation was sudden and prolific. High concentrations of ice particles appeared coincident with, or very soon after, the formation of graupel; these high concentrations were observed in clouds at ambient temperatures between ‐11 and −28°C, including some clouds with T B <0°C. For clouds with similar droplet spectra and T T , the width of the cloud also affected I M ; narrow clouds (<2 km wide) formed less ice than wider, multi‐turreted clouds. Continental cumulus clouds have to be about 50% wider and about 5 degC colder at their tops than maritime cumulus to have the same chance of producing a radar echo. However, the difference in width to produce a radar echo disappears for clouds with widths >4 km, although continental clouds still need to be about 5 degC colder at cloud top than maritime clouds to produce a radar echo. Several extant theories for high ice particle concentrations in clouds are examined but none provides a satisfactory explanation for the observations.