Regional Bowen Ratio Controls on Afternoon Moist Convection: A Large Eddy Simulation Study

This numerical study examines the effect of the regional Bowen ratio β, the ratio of the domain-averaged surface sensible to latent heat fluxes, on afternoon moist convection. With a temporally evolving but spatially uniform surface available energy over a mesoscale domain under a weak capping inversion, we run large eddy simulation of the convective boundary layer (CBL) from 1100 to 1800 local time (LT). We first prescribe a low β of 0.56 (a grassland surface), and then the reversed high β of 1.80 (an urban surface) by switching the latent and sensible heat fluxes. The spatial variations of the surface fluxes are imposed with two-dimensional perturbation fields created with the Fourier spectra of κ^-3 (strong mesoscale heterogeneity) and κ⁰ (random homogeneity) in the wavelength range from 28 km to 0.1 km. Although the sum of surface latent and sensible heat fluxes is the same for all the cases, the high β cases have stronger vertical fluxes of virtual potential temperature, which produce more vigorous updrafts, than the low β cases. Thus over the heterogeneous surface, with the removal of convective inhibition over a mesoscale subdomain of higher sensible heat fluxes, deep convection develops at 1400 LT. In the low β case, convective clouds develop but do not progress into precipitating convection. Over the homogeneous surfaces randomly distributed shallow clouds develop, with somewhat more and thicker clouds in the high β case. This study also discusses the spectra and cospectra of CBL variables critical for the afternoon moist convection.