The Dynamics of Thermals and Their Contribution to Mixed Layer Processes
The dynamics of thermal updrafts and compensating environmental downdrafts in the Convective Boundary Layer are examined using observations from the Phoenix 78 field experiment. A new conditional sampling technique based upon the universal mixed layer spectra of vertical velocity is developed and used to distinguish thermals from their environment. The spectra show that the buoyant production of vertical velocity variance occurs primarily at horizontal scales of from 0.1 to 10.0 times the depth of the boundary layer. Therefore, thermals are defined as those regions in which the vertical velocity is upwards at these horizontal scales. Thus, the contribution of mesoscale and inertial subrange eddies to the vertical velocity are eliminated from the determination of the thermal-environmental boundaries. This technique is not limited to conditions of large upwards heat or moisture fluxes as were previous methods. A new diagnostic model is developed which permits the determination, from observations, of the lateral mass exchange between thermal updrafts and environmental downdrafts and the pressure forces acting on the vertical velocity in these two regions. This model consists of the budget equations for horizontal averages of virtual potential temperature, convective mass flux and vertical velocity. These averages are computed separately for the updraft and downdraft regions so that the processes affecting the two legs of the convective circulation can be examined separately. Therefore, the lateral exchanges between the two regions enter into the budgets. These exchanges are commonly referred to as lateral entrainment and detrainment. The time tendency terms in the budgets are small while the effects of lateral mixing and pressure are large. The importance of lateral mixing and pressure effects is shown to result from the large imbalance between the observed gradient production of buoyancy and vertical buoyancy flux divergence. The lateral mixing into thermals is small relative to that out of thermals in the upper boundary layer. Therefore, the proportion in downdrafts of a nonbuoyant contaminant released in the upper boundary layer should increase with time. This result supports the observed initial descent of plume centers in large eddy simulation models.
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- Physics: Atmospheric Science