Structural Features of a Microburst-Producing Storm in Colorado Revealed by JAWS Dual-Doppler Radars

Abstract

Some dynamic and thermodynamic structures of a microburst-producing storm, which occurred on 14 July 1982 in Colorado, were studied in detail during the storm's quasi-steady mature stage. Dual-Doppler data from 1646 to 1648 MDT, collected during the project of Joint Airport Weather Studies (JAWS) at Denver's Stapleton International Airport, were objectively analyzed to produce a three-dimensional wind field. The domain of interest had a horizontal dimension of 10 km by 10 km centered on the microburst. There were 19 analysis levels in the vertical, ranging from 0.25 to 8.5 km AGL. The horizontal grid spacing was 0.5 km, while thevertical grid spacing varied from 0.25 km near the surface to 0.5 km at levels above I kin. Vertical velocities were computed by integrating the anelastic continuity equation downward from the storm's top with variational adjustment. Subsequently, fields of deviation-perturbation pressure and virtual temperature were recovered from a detailed wind field using the three momentum equations. These fields were then subjected to internal consistency checks to determine the level of confidence before interpretation. Findings demonsUate that the thermodynamic retrieval method is feasible for investigating the structure and internal dynamics of the storm. Variational adjustment substantially reduces errors in vertical velocity fields. Results show that the microburst being investigated is embedded within the high-refiectivity region with heavy precipitation. A strong downfiow impinges upon the surface, producing a stagnation mesohigh inside the microburst. This high is accompanied by low pressure in the strongest outflow regions, forming a pronounced horizontal perturbation pressure gradient outward from the high-pressure center. Such pressure patterns are in good agreement with the surface observations in similar cases for two different storms. The outflow regions extend from the surface to approximately I km height with maximum divergence in excess of lO-: s-L The outflow air is negatively buoyant due to evaporation in the outsldn of the microburst. In the middle troposphere, hish pressure forms on the upshear side of the main ulxlraft with low pressure on the downshear side due to dynamical interactions between the updraft and the sheared environmental wind. The retrieved buoyancy field agrees well with the updraft-downdraft structure with warming in the updraft and cooling in the downdraft. The combined effects of perturbation-pressure gradients, buoyancy and precipitation loading are responsible for maintaining vigorous convection of the downdrafis which produced the strong diverging outflow at low levels.