Observations of the Origin and Distribution of Ice in Cold, Warm and Occluded Frontal Systems during the DIAMET campaign

DIAbatic influences on Mesoscale structures in ExTratropical storms (DIAMET) was a UK based campaign that took place during Autumn-Winter of 2011 and the summer of 2012. During the Winter campaign an exceptional spell of unsettled weather produced a series of severe storms across northwest Europe that led to a December North Atlantic Oscillation (NAO) index of 2.52, the highest monthly value in 60 years. This provided an excellent opportunity to investigate the microphysical structure of the different frontal systems associated with Extratropical storms. Here we present a range of in-situ microphysics measurements from the FAAM BAe-146 for 3 case studies involving winter-time cold and warm fronts and a summer-time occluded front. For the cold front case only ground-based remote sensing data provided by The Chilbolton Facility for Atmospheric and Radio Research (CFARR) allowed us to gain a detailed understanding of this particular system.

The use of the Chilbolton Advanced Meteorological Radar (CAMRa) at CFARR allowed us to observe the structure and dynamics of the kata-type cold front case through data on radar reflectivity, differential reflectivity and wind data. These enabled us to understand the microphysical development within the context of the larger scale structure. Radar measurements also supported the detailed insitu measurements of the cloud microphysics from the aircraft, for example regions of high differential reflectivity were found to be associated with the generation of pristine ice crystals in the cloud tops of this kata-type cold front case. We found that ice formation processes between the cases were different due to the different depth of each system. The warm and occluded front presented cloud top temperatures cold enough (< ~ - 37¢ªC) for homogeneous and heterogeneous ice nucleation to occur, while the ice phase in the low altitude, relatively warm cloud tops (~ -13¢ªC) of the kata-cold front, was initiated through heterogeneous ice nucleation only. In all three cases the greatest ice number concentrations and ice water contents were observed in the H-M temperature zone ~ -5¢ªC. The presence of significant liquid water and substantial concentrations of supercooled drops larger than 80 um diameter in all cases is likely to have facilitated the rime-splintering mechanism and contributed directly to generating these high mass and number concentrations.