The morphology of convective cold pools in a dense station network during FESSTVaL

Cold pools are crucial for understanding the organization of atmospheric convection. However, basic properties like their size, shape and temperature structure remain obscured by the insufficient resolution of operational station networks. In summer 2021 the FESSTVaL field experiment aimed to address this observational gap and shed light on the structure and life cycle of cold pools on the sub-mesoscale (100 m to 10 km). The experiment took place in the rural surrounding of the Meteorological Observatory Lindenberg (south-east of Berlin, Germany) and featured a dense network of custom-designed low-cost measurement stations covering an area of 30-km diameter. The instrumental setup included 80 novel APOLLO stations sampling air temperature and pressure at 1-s resolution and 19 supplementary weather stations. A X-band rain radar also provided highly resolved information on rainfall.

The FESSTVaL network recorded 42 cold pool events of different strength and size during the three-month measurement phase. Based on the spatial interpolation of the network observations, morphological properties of more than 1200 identified cold pool objects are derived. The analysis reveals a median cold pool diameter of 8.4 km. According to average aspect ratios between 1.5 and 1.6, the sampled cold pools are not round, independent of their size and strength. Moreover, large cold pools tend to be stronger and more heterogeneous than small ones. As FESSTVaL is especially suited to study young cold pools, we further analyse the growth phase of selected events initiated inside the station network. Remarkably, their area scales very linearly with the accumulated rainfall amount suggesting that rainfall is the main driver for the growth of cold pools, both directly by evaporative cooling and indirectly by downward transport of upper air masses.