Mesoscale Convective Vortices Observed during BAMEX. Part Ⅰ: Kinematic and Thermodynamic Structure

摘要

Five cases of mesoscale convective vortices (MCVs) are described from observations collected during the Bow Echo and MCV Experiment (BAMEX) over the central United States during the period from 20 May to 6 July 2003. In the present paper, the kinematic and thermodynamic structure of each vortex and its environment are emphasized. Data consist of BAMEX dropsondes, the National Oceanic and Atmospheric Administration profiler network, and National Weather Service soundings. In addition, Weather Surveillance Radar-1988 Doppler observations documented the signatures of nascent MCVs within nocturnal convection systems as well as the spatial pattern of convection within MCVs during the following day. The vertical structure of each vortex was highly dependent on the vertical shear and the presence of upper-tropospheric cyclonic vorticity anomalies. In strong shear, a pronounced downshear tilt of the vortex was evident, but with the presence of an upper-tropospheric trough, the tilt was upshear in the upper troposphere. In only one case did the tangential velocity of the vortex greatly exceed the vertical shear across its depth, and thus the vortex could maintain itself against the shear. The vortices were generally deep structures, extending through 5-8 km in all cases and maximizing their tangential winds between 550 and 600 hPa. In one of the five cases, vertical penetration into the boundary layer was unambiguous. Lower-tropospheric virtual potential temperature anomalies were generally 1-2 K, greatest when not directly beneath the midtropospheric MCV center but rather on its upshear and downshear flanks. Upper-tropospheric warm anomalies were found above and downshear from the midtropospheric MCV center, with a cool anomaly upshear, the latter being stronger in cases with an upshear tropopause-based trough. A diagnostic balance calculation was performed and indicated that the temperature anomalies were approximately balanced on the scale of the vortex.