Numerical Simulation of Cross-Valley Plume Dispersion during the Morning Transition Period

Abstract

A two-dimensional dynamical model was used to simulate the daytime boundary-layer evolution and resulting plume dispersion in a cross-valley section of a northwest?southeast oriented narrow valley in the first 4 h after sunrise. Two cases were simulated, one using a summertime heating distribution and a second with a wintertime heating distribution. In each case, additional conservation equations were added to simulate the dispersion of two plumes released 150 m and 650 m above the valley floor. In the summer case, the lower plume migrated to the more strongly heated southwest sidewall in the first 90 min after sunrise, and was then advected up the sidewall in the slope flow for the remainder of the simulation. This result is consistent with observations. The upper plume diffused slowly in the remnants of the nocturnal inversion layer until it was entrained by the growing convective boundary layer 3 h after sunrise. The boundary layer's thermodynamic structure remained nearly symmetric about the valley axis throughout the transition period. The asymmetric dispersion characteristics seen in the summer case were not found in the winter simulation. The seasonal change in solar illumination reduced the differences in surface heat flux between the two sidewalls that gave rise to the asymmetry observed in the summer case.