The early evening transition of the atmospheric boundary layer from a convective to a more stable condition has not been the focus of many studies. Convective initiation studies during this time period are especially rare. The goal of this study is to determine what changes occur in the atmospheric boundary layer during the early evening transition period and how these changes affect convection.

The 29 August 2007 convective initiation case over Huntsville, AL provides a unique opportunity to develop a detailed profile of convective initiation during the transition period. Convective initiation occurred almost directly over the Mobile Integrated Profiling System (MIPS) located at the National Space Science and Technology Center (NSSTC) in Huntsville. This set of instruments includes a 915 MHz 5-beam profiling radar, a 0.906 m lidar ceilometer, and a 12 channel microwave profiling radiometer (MPR), in addition to surface measurements of pressure, temperature, dewpoint, and relative humidity. The data from these instruments provide a vertical profile of temperature, water vapor, horizontal winds, vertical velocities, and cloud heights. Data from the polarimetric Advanced Radar for Meteorological and Observational Research (ARMOR) located at Huntsville International Airport (located 14 km WSW of the MIPS) help to provide horizontal and vertical views of the ABL and convection through a series of PPI and RHI scans during this transition period.

Prior to convective initiation on 29 August, strong and sustained upward motion was observed for almost an hour prior to convective initiation. Convective initiation occurred at approximately 2250 UTC along an outflow boundary from convection to the south and east propagating westward over Huntsville. The convection continued over NSSTC until around 0000 UTC on 30 August. Convection persisted for a longer period of time over southern Huntsville, leading to flooding in that region. Convection in both regions dissipated or propagated away from the city by shortly after 0100 UTC. The contributions of the boundary layer transition to this pattern of development and dissipation are the focus of this study.