Radar analysis of boundaries during the early evening boundary layer transition period

Initiation or intensification of deep convection is frequently observed during the afternoon to evening boundary layer transition. The early evening boundary layer transition period involves decreases in near-surface temperature and wind speed, and increases in dewpoint during clear conditions. Likewise, an increase in wind speed at levels above ~100 m is produced as the turbulent momentum flux decreases. The point of this study is to show that these changes also occur with the presence of boundaries and lead to increased convergence along those boundaries. As higher speed winds behind the boundaries meet the low speed environmental winds during the transition, boundaries are able to be sustained for longer periods of time and lead to more instances of convective initiation as convergence increases.

In order to test this hypothesis and improve understanding of the related physical processes, data is obtained from WSR-88D radar sites that are co-located with radiosonde and ASOS surface stations for 12 locations around the US. The radar data is gridded and wind values along the boundaries are examined to show the development of winds and changes in convergence during the transition. 2-D analyses within the vertical plane perpendicular to the boundary are constructed to examine the kinematic response through the depth of the boundary. VAD analyses are also conducted to examine the changes in convergence within a given range as the boundary approaches the radar.

Temperature and dewpoint changes as obtained from the ASOS stations during this time are also examined to determine how changes associated with a boundary passage differ from typical changes associated with the atmospheric boundary layer transition.

Data are also obtained from the Huntsville, AL STORMNet, including the ARMOR radar, various surface-based profiling instruments, and surface instrumentation. These data provide a very comprehensive view of the evolution of the boundary during this time period. The 2-D and VAD analyses will also be conducted using data from the ARMOR radar.