Observed Characteristics of the Afternoon–Evening Boundary Layer Transition Based on Sodar and Surface Data

Abstract

he early-evening boundary layer transition has been defined in the past using a variety of criteria, the most popular of which is the onset of a negative surface heat flux. According to this definition, the transition is an almost-instantaneous event that occurs when the positive daytime heat flux switches to the negative nighttime heat flux. This definition is simplistic, however, because the stable boundary layer does not form instantaneously over a deep layer. Other factors are involved, and many changes occur aloft during the transition period that this definition does not account for?for example, a more gradual reduction in turbulence and an increase in wind speed. The combined use of sodar data, as well as 915-MHz wind profiler, surface temperature, dewpoint, and wind data, provides a more-comprehensive definition of the early-evening boundary layer transition. Sodar backscatter is sensitive to temperature fluctuations, and therefore as the heat flux decreases, the sodar return power exhibits changes from a time-varying convective structure to a more-stratified and steady structure. A relative minimum in intensity and height of the sodar backscatter is one indication that the transition is occurring. As the boundary layer evolves from the unstable convective afternoon conditions to the more stable nocturnal conditions, the finescale temporal variations in many parameters, including temperature, the 10?2-m temperature difference, dewpoint, and wind speed, decrease. There is often a distinct steplike shape in the temperature/wind decrease or dewpoint increase within 30 min of the sodar minimum. In this paper, an analysis of sodar and surface data is presented for low-wind cases to demonstrate the efficacy of this combined sensor technique, and to illustrate the average physical characteristics of the transition period for 21 cases during the summer months (June?August) and 9 cases during the autumn months (November?December) in Huntsville, Alabama.