Environmental Conditions Controlling the Morphology of Shallow Orographic Convection

Abstract Quasi-stationary rainbands capable of producing heavy localized precipitation have been observed over the Oregon Coastal Range and other mesoscale mountain ridges. These bands thus present an important forecasting problem, which is challenged by the inability of operational NWP models to accurately resolve them. To aid the prediction of these events, this study synthesizes an observational climatology with idealized large-eddy simulations of shallow convection over the Coastal Range. The climatology identified cases with banded and cellular morphologies over the Coastal Range and determined composite upstream soundings for each morphology. While prominent differences in these soundings included stronger low-level winds and dry static stability in the banded events, these differences alone did not fully determine the resulting cloud morphology in the simulations. Another key factor was the turbulence intensity in the impinging atmospheric boundary layer (ABL), which is partially controlled by the sea–air temperature difference over the eastern Pacific Ocean (Δ T SA ). While banded events mostly exhibit Δ T SA < 0 and weak ABL turbulence, cellular events mostly exhibit the opposite. ABL turbulence was thus hypothesized to favor cells by disrupting the lee-wave circulations responsible for organizing the bands. A new parameter R was developed to predict cloud morphology based on the ratio of the TKE of transient turbulent velocity fluctuations to that of stationary lee-wave perturbations. This parameter accurately predicted the cloud morphology in numerous simulations with varying upstream flows and Δ T SA . It also provides a simple explanation for why the observed characteristics of banded events (stronger low-level winds and ABL stabilities, Δ T SA < 0) all favor band development. Significance Statement This study synthesizes observations and numerical simulations to determine the environmental conditions distinguishing quasi-stationary banded convection from transient cellular convection over the Oregon Coastal Range. The former morphology can concentrate heavy precipitation over narrow regions to greatly enhance flash-flooding risks. The results provide quantitative guidance for forecasters to aid their analysis of operational NWP forecasts.