Environmental conditions controlling the morphology of shallow orographic convection

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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 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 (ΔTSA). While banded events mostly exhibit ΔTSA < 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 ΔTSA. It also provides a simple explanation for why the observed characteristics of banded events (stronger low-level winds and ABL stabilities, ΔTSA < 0) all favor band development.