Observations of the origin and distribution of ice in cold, warm, and occluded frontal systems during the DIAMET campaign

Abstract

Three case studies in frontal clouds from the Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) project are described to understand the microphysical development of the mixed phase regions of these clouds. The cases are a kata-type cold front, a wintertime warm front, and a summertime occluded frontal system. The clouds were observed by radar, satellite, and in situ microphysics measurements fromtheU.K. Facility forAirborneAtmosphericMeasurements (FAAM) research aircraft. The kata cold front cloud was shallow with a cloud-top temperature of approximately -13°C. Cloud-top heterogeneous ice nucleation was found to be consistent with predictions by a primary ice nucleation scheme. The other case studies had high cloud tops (< -40°C) and despite no direct cloud-top measurements in these regions, homogeneous ice nucleation would be expected. The maximum ice crystal concentrations and ice water contents in all clouds were observed at temperatures around -5°C. Graupel was not observed, hence, secondary ice was produced by riming on snow falling through regions of supercooled liquid water. Within these regions substantial concentrations (10-150L-1) of supercooled drizzle were observed. The freezing of these drops increases the riming rate due to the increase in rimer surface area. Increasing rime accretion has been shown to lead to higher ice splinter production rates. Despite differences in the cloud structure, the maximum ice crystal number concentration in all three clouds was ~100L-1. Ice water contents were similar in the warm and occluded frontal cases, where median values in both cases reached ~0.2-0.3 gm-3, but lower in the cold front case.