Ice clouds in extratropical cyclones

The goal of this work is to provide a first step towards the investigation of the role of ice clouds for the evolution and structure of extratropical cyclones. To this end, cyclones are identified from the ERA40 data set and analysed with respect to the 3-dimensional cloud fields in their environment. As a first step results from a detailed global verification of these fields is shown, using satellite data from the POLDER-1 instrument (Nov 1996 - June 1997) that determines the cloud top phase. The results indicate that ice clouds observed by POLDER-1 are fairly well captured by ERA40. However, the ice parameterization used at the ECMWF produces ice also at fairly high temperatures (-20 to 0°C) leading to an overestimation of the occurrence of ice clouds compared to POLDER-1. In the second step the relationship between ice clouds and individual extratropical cyclones is explored in detail. Some of the central questions are: How frequent are and where do occur ice clouds in the vicinity of cyclones? What kind of cyclones produce large-scale ice clouds and in which phase of their life-cycle? The final step is to find out about the origin of the ice clouds in extratropical cyclones. For that purpose a trajectory analysis is made based on the ERA40 data. The advantage of this method is, that the history of the air parcels in the cloud or in its vicinity can be analysed. The vertical displacement of the backward trajectories provides information whether resolved vertical motions like warm conveyor belts, or subgrid-scale processes like deep convection play the key role for ice formation associated with extratropical cyclones.