Mesoscale Variability of Boundary Layer Properties in the Los Angeles Basin

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Abstract
We employ daytime radiosonde data to analyze boundary layer (BL) variability within the Los Angeles Basin for a case study day. The predominantly mesoscale forcing in the region produces large spatial and temporal variations in BL depth and vertically averaged BL virtual potential temperature and wind velocity. Particularly notable are the shallowing of BL depth—beginning in midmorning at the coast and in the early afternoon inland—and the afternoon decrease in BL potential temperature. The relative importance of terms in the BL continuity, thermodynamics, and momentum equations is evaluated; values for area-averaged entrainment velocity, surface heat flux, and mean BL pressure gradient force (PGF) are derived as residuals. Our budget equations are vertically integrated to the variable BL top and are thus more appropriate to investigations of BL variables than are equations integrated to a fixed height. Advection and divergence are shown to affect BL depth, heating, and acceleration significantly. Mean entrainment velocity is 5.4 cm s−1 for this inhomogeneous region, in agreement with predictions of formulas strictly valid only for homogeneous conditions. Vertical profiles show velocity divergence throughout the BL with compensating convergence immediately above it. Predictions from a linear analysis of the momentum equation agree closely with the observed values.

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