Microphysical properties and radar polarimetric features within a warm front

On 21 January 2009, the warm front of an extensive low pressure system affected U.K. weather. In this work, macroscopic and microphysical characteristics of this warm front are investigated using in situ (optical array probes, temperatures sensors, and radiosondes) and S-band polarimetric radar data from the Aerosol Properties, Processes and Influences on the Earth’s Climate–Clouds project. The warm front was associated with a warm conveyor belt, a zone of wind speeds of up to 26 ms⁻¹, which played a key role in the formation of extensive mixed-phase cloud mass by ascending significant liquid water (LWC; ~0.22 g m⁻³) at a level ~3 km and creating an ideal environment at temperatures ~ −5°C for ice multiplication. Then, “generating cells,” which formed in the unstable and sheared layer above the warm conveyor belt, influenced the structure of the stratiform cloud layer, dividing it into two types of elongated and slanted ice fall streaks: one depicted by large Z_DR values and the other by large Z_H values. The different polarimetric characteristics of these ice fall streaks reveal their different microphysical properties, such as the ice habit, concentration, and size. We investigate their evolution, which was affected by the warm conveyor belt, and their impact on the surface precipitation.