An Observational and Numerical Study of a Sheared, Convective Boundary Layer. Part I: Phoenix II Observations, Statistical Description, and Visualization

Abstract

Four-dimensional velocity fields derived from dual Doppler radar observations are the basis of a description and statistical analysis of a convective, sheared planetary boundary layer during an afternoon over the High Plains of eastern Colorado. Mean velocities and momentum fluxes are calculated directly from the radar data and are verified with aircraft and tower data. Perturbation pressure and buoyancy fields are recovered for turbulent kinetic energy budgets, and for estimates of horizontal heat advection across the analysis area. The surface layer and lowest third of the observed boundary layer were similar to minimally sheared convective boundary layers, but there were significant differences in the upper two-thirds of the boundary layer. An overrunning residual mountain boundary layer merged with the locally generated convective boundary layer, producing a deep, continuously sheared layer of turbulent activity. Computer visualization reveals a complicated flow characterized by clusters of vortical structures extending well into the slightly stable overrunning region, frequently dominated by clusters of large, long-lived vortices. First- and second-order statistics vary with time of day and averaging volume, suggesting that appropriate parameterizations of similar boundary layers should be functions of the required spatial and temporal scales and mesoscale environment. A number of common simplifying assumptions, scalings, and parameterizations employed for purely convective boundary layers would be inappropriate for this flow.