Convective cold pools in long-term boundary layer mast observations

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  • 1 Meteorological Institute, University of Hamburg, Hamburg, Germany
  • 2 Max Planck Institute for Meteorology, Hamburg, Germany
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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 mid afternoon, 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 mid-tropospheric low-Θe air masses under convective downdrafts.

Corresponding author address: Meteorological Institute, University of Hamburg, Bundesstrasse 55, 20146 Hamburg, Germany. E-mail: bastian.kirsch@uni-hamburg.de

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 mid afternoon, 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 mid-tropospheric low-Θe air masses under convective downdrafts.

Corresponding author address: Meteorological Institute, University of Hamburg, Bundesstrasse 55, 20146 Hamburg, Germany. E-mail: bastian.kirsch@uni-hamburg.de
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