Abstract
Details of the structure of a moderate reflectivity microburst were provided by dual-Doppler radar measurements during the Phoenix II convective boundary layer experiment. The dated allowed high resolution of the descending microburst in both time and space. Thermodynamic fields of virtual potential temperature and buoyancy retrieved from the radar measurements indicated that the downdraft was associated with a minimum in virtual potential temperature, rather than coinciding with a maximum in precipitation loading. The physical separation of the downdraft from the reflectivity maximum was especially pronounced during the later stages of the microburst and was partly due to the tilled reflectivity core descending more rapidly than the downdraft. The downdraft corms also descended at a rate slower than the magnitude of the maximum downdraft so that air was continually converging and entraining into the downdraft above the level of its peak value and was detraining and diverging below it. The retrieved pressure fields and simple analytical calculations showed that this slower descent and internal circulation coincided with an upward-directed pressure form. Simple calculations also suggest that this influence of the pressure force on the vertical accelerations depends strongly on the aspect ratio of the negatively buoyant parce1; horizontally narrow and vertically deep negatively buoyant parcels result in stronger downdraft than wider and shallower parcels. Our study suggests the internal circulation and the relatively slow descent of the peak downdraft should be inherent characteristics of microbursts driven by corms of low virtual potential temperature air, while microbursts driven primarily by water loading could be expected to have a different structure. In the case of the microbursts driven by corms of cool air, observation and recognition of the convergence and divergence associated with the internal circulation provides important precursors to microburst activity. In this study, the Doppler measurements showed that the microburst descending into a stable layer may have enhanced the divergence pattern below the peak downdraft.
