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Downdrafts within High Plains Cumulonimbi. Part II: Dynamics and Thermodynamics

Kevin R. KnuppAtmospheric Science and Remote Sensing Laboratory, Johnson Research Center, University of Alabama in Huntsville, Huntsville, Alabama

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Abstract

The dynamical and thermodynamical properties of precipitation-associated downdrafts are examined using a Lagrangian trajectory analysis approach applied to parcels passing through the low-level downdraft of precipitating convection. Both observations and three-dimensional cloud model results for one particular case presented in Part I are included. For this case, negative buoyancy within the low-level downdraft is very rapidly produced over the lowest 2 km by melting and evaporation of precipitation. Cooling profiles from the melting and evaporation cooling components show a significant overlap in the case considered. Both diagnosed and modeled low perturbation pressure located near the 2 km level appear to be generated by the rapid onset of negative buoyancy.

The diagnosed and modeled behavior along two low-level downdraft branches defined in Part I are examined for this particular case. It is found that the low-level downdraft is forced in varying amounts by condensate loading, negative buoyancy produced by precipitation evaporation and melting, and pressure forces. The relative role of cooling by evaporation and melting varies according to trajectory type. Along midlevel trajectories, evaporation is most significant, whereas along up–down trajectories melting is more important. For the particular case examined, the maximum amplitude in downward acceleration was found in the 1–2 km layer, where effects of loading, melting, evaporation/sublimation and negative vertical pressure gradient contributed to downward acceleration.

Abstract

The dynamical and thermodynamical properties of precipitation-associated downdrafts are examined using a Lagrangian trajectory analysis approach applied to parcels passing through the low-level downdraft of precipitating convection. Both observations and three-dimensional cloud model results for one particular case presented in Part I are included. For this case, negative buoyancy within the low-level downdraft is very rapidly produced over the lowest 2 km by melting and evaporation of precipitation. Cooling profiles from the melting and evaporation cooling components show a significant overlap in the case considered. Both diagnosed and modeled low perturbation pressure located near the 2 km level appear to be generated by the rapid onset of negative buoyancy.

The diagnosed and modeled behavior along two low-level downdraft branches defined in Part I are examined for this particular case. It is found that the low-level downdraft is forced in varying amounts by condensate loading, negative buoyancy produced by precipitation evaporation and melting, and pressure forces. The relative role of cooling by evaporation and melting varies according to trajectory type. Along midlevel trajectories, evaporation is most significant, whereas along up–down trajectories melting is more important. For the particular case examined, the maximum amplitude in downward acceleration was found in the 1–2 km layer, where effects of loading, melting, evaporation/sublimation and negative vertical pressure gradient contributed to downward acceleration.

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