Morphology and growth of convective cold pools observed by a dense station network in Germany

Abstract

This study explores the morphology of convective cold pools, i. e., their size, shape, and structure, as well as factors controlling their growth using surface-based observations of the Field Experiment on Sub-mesoscale Spatio Temporal Variability in Lindenberg (FESSTVaL). FESSTVaL featured a dense network of 99 custom-built, low-cost measurement stations covering a circular area of 30 km in diameter at sub-mesoscale resolution (distances between 0.1 km and 4.8 km) and was held at the Lindenberg observatory near Berlin (Germany) from May to August 2021. The station network sampled 42 cold-pool events during the 103-d measurement period. The morphological properties of cold pools are derived by spatially interpolating the temperature observations to a Cartesian grid and defining cold pools as individual objects at a given time with a temperature perturbation ΔT stronger than -2 K. The sample of 1232 cold-pool objects whose extents are sufficiently captured by the network has a median equivalent diameter of 8.5 km. The objects exhibit aspect ratios between 1.5 and 1.6 independent of their size and strength, meaning they are generally not circularly shaped. On average, ΔT is strongest at the cold-pool center and decreases linearly towards the edge. For the growth phase of four selected events, the cold-pool object area A_CP scales linearly with the radar-observed, area-integrated rainfall accumulation, while object-mean temperature perturbation strengthens most efficiently early in the life cycle. The global, radial expansion velocity decreases as the cold pool gets stronger and larger, in contradiction with density-current theory. Instead, A_CP is a better predictor of the expansion rate. These findings identify the cold-air import by precipitation, both through evaporative cooling and convective downdrafts, as the dominant driver of the observed growth.