Abstract
Cold pools are mesoscale features that are key for understanding the organization of convection, but are insufficiently captured in conventional observations. This study conducts a statistical characterization of cold-pool passages observed at a 280-m-high boundary layer mast in Hamburg (Germany) and discusses factors controlling their signal strength. During 14 summer seasons 489 cold-pool events are identified from rapid temperature drops below −2 K associated with rainfall. The cold-pool activity exhibits distinct annual and diurnal cycles peaking in July and midafternoon, respectively. The median temperature perturbation is −3.3 K at 2-m height and weakens above. Also the increase in hydrostatic air pressure and specific humidity is largest near the surface. Extrapolation of the vertically weakening pressure signal suggests a characteristic cold-pool depth of about 750 m. Disturbances in the horizontal and vertical wind speed components document a lifting-induced circulation of air masses prior to the approaching cold-pool front. According to a correlation analysis, the near-surface temperature perturbation is more strongly controlled by the pre-event saturation deficit (r = −0.71) than by the event-accumulated rainfall amount (r = −0.35). Simulating the observed temperature drops as idealized wet-bulb processes suggests that evaporative cooling alone explains 64% of the variability in cold-pool strength. This number increases to 92% for cases that are not affected by advection of midtropospheric low-Θe air masses under convective downdrafts.
Significance Statement
Cold pools are areas of cool and dense air underneath precipitating clouds that often trigger new convection as they spread outward. Although cold pools are key for correctly representing convection in numerical simulations, their observational characterization is insufficient, focusing on few cases and surface measurements. We analyze meteorological observations of nearly 500 cold-pool passages sampled during 14 years at a 280-m-high mast in Hamburg (Germany). The robust data basis shows that the typical temperature perturbation associated with cold pools is only −3.3 K and weakens with height. The surface temperature signal is mainly driven by evaporative cooling of below-cloud air by rainfall, whereby the saturation deficit is a much better predictor than the often used precipitation amount.
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