Illuminating the structure of convective cold pools with a dense station network

Abstract. From June to August 2020 an observational network of 103 meteorological ground-based stations covered the greater area (50 km × 35 km) of Hamburg (Germany) as part of the Field Experiment on Sub-mesoscale Spatio-Temporal variability in Hamburg (FESST@HH). The purpose of the experiment was to shed light on the sub-mesocale (O(100) m – O(10) km) structure of convective cold pools that typically remains under-resolved in operational networks. During the experiment, 82 custom-built, low-cost APOLLO (Autonomous cold POoL LOgger) stations sampled air temperature and pressure with fastresponse sensors at 1 s resolution to adequately capture the strong and rapid perturbations associated with propagating cold pool fronts. A secondary network of 21 weather stations with commercial sensors provided additional information on relative humidity, wind speed and precipitation at 10 s resolution. The realization of the experiment during the COVID-19 pandemic was facilitated by a large number of volunteers who provided measurement sites on their premises and supported the station maintenance. This article introduces the novel type of autonomously operating instruments, their measurement characteristics and the FESST@HH data set (https://doi.org/10.25592/UHHFDM.8973; Kirsch et al., 2021b). A case study demonstrates that the network is capable of mapping the horizontal structure of the temperature signal inside a cold pool as well as quantifying its size and propagation velocity throughout its life cycle. Beyond its primary purpose, the data set offers new insights into the spatial and temporal characteristics of the nocturnal urban heat island and variations of turbulent temperature fluctuations as an expression of different urban and natural environments.