Observed characteristics of the afternoon-evening boundary layer transition based on sodar and surface data

The early evening boundary layer transition has been defined in the past using a variety of criteria. The most popular transition definition is the onset of negative 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 is a convenient definition because it is easy to determine, especially if actual flux measurements are available. However, this definition is rather simplistic because the stable boundary layer does not form in a single moment. Other factors are involved and many changes occur during the transition period that this definition does not account for, e.g., a more gradual reduction in turbulence and an increase in wind speed.

The combined use of sodar return power and wind data, as well as surface temperature, dewpoint, and wind data, provide another way to define the early evening boundary layer transition. The sodar return power is sensitive to temperature fluctuations, so 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 height of the sodar signal indicates where the transition begins. The surface data is also used to help with the new definition. As the boundary layer transitions from the unstable convective afternoon conditions to the more stable nocturnal conditions, the temporal variations in temperature, the 10 m to 2 m temperature difference, dewpoint, and wind speed decrease. These decreases often have a distinct step-like decrease (temperature and wind) or increase (dewpoint) within 30 minutes of the sodar minimum, but not usually the same exact instant. In this paper, an analysis of sodar and surface data is presented to demonstrate the veracity of this combined sensor technique, and to illustrate the climatology and physical characteristics of the transition period during the summer months (June-August) in Huntsville, Alabama.