Coupling Boundary-Layer Thermals and Fair-Weather Cumuli

While our understanding of the fair-weather boundary layer has increased over the past decades our knowledge is still incomplete. In this study, we will examine the behavior of individual thermals identified at several levels in the convective boundary layer. These thermals, measured using an instrumented aircraft during Boundary Layer Experiment 1996 (BLX96) in Oklahoma, are the roots of boundary-layer cumuli. Statistical properties of these thermals are used in a simple cloud model to predict the life cycle of the cumuli.

Thermals are identified using methods similar to those proposed by other authors. In order for a parcel to be considered part of a coherent updraft or downdraft, its vertical velocity (w) must be greater than or less than some threshold w for some length scale (which was defined as a fraction of the mixed-layer depth). Thus, each thermal is defined to be made of a family of parcels. After all the thermals and downdrafts are identified, properties of individual thermals or groups of thermals can be examined. Some key observations are: 1) there is not a relationship between the fraction of the flight leg covered with thermals and the observed cloud cover, 2) the fraction of thermals which are positively buoyant decreases with height in a well behaved way, 3) the observed w of the thermals is greater than w predicted using CAPE.

The Cumulus Potential (CuP) scheme provides a way to couple boundary-layer thermals with the cumuli. The CuP scheme is similar to buoyancy sorting cloud models; the primary difference being that the CuP scheme uses real distributions of mixed-layer thermals or Joint Frequency Distributions (JFDs) of mixed-layer parcels' humidities and temperatures. In the CuP scheme, positively buoyant thermals or parcels are lifted to their level of neutral buoyancy. If a thermal or parcel reaches its lifting condensation level before reaching its level of neutral buoyancy a cloud forms. The cloud continues to rise to its level of neutral buoyancy, moist adiabatically, as a cloudy thermal. Using the CuP model, with either JFDs or thermal statistics from BLX96, many parcels reach the top of the sounding before reaching their level of neutral buoyancy (the soundings were flown to heights above local cloud top). This suggests that other forces or processes are important in the life cycle of the cumuli and that a more detailed cloud model is needed.