Evening transition thermal processes and flow dynamics over opposing slopes of an isolated mountain

Abstract The evening transition (ET) in the atmospheric surface layer marks the transition from a daytime convective boundary layer to a nocturnal stable layer around local sunset. Using data from multilevel turbulence towers on the east and west slopes of Granite Mountain during the MATERHORN campaign (September–November 2012), this study investigates atmospheric responses to the decay of solar forcing. To quantify slope ET dynamics and the influence of topographic shading, we compare the sunlit west slope with the earlier‐shaded east slope. Shading on the east side accelerates radiative cooling and advances ET onset relative to the still‐sunlit west side. A sigmoid fit to the 0.5–20‐m potential temperature difference captures a three‐phase evolution: slow cooling, rapid cooling, and a final slow cooling. We define ET onset at the sign reversal of the surface heat flux, marking weakly stable condition, and its ends at the initiation of downslope flows. Stability parameters (Monin–Obukhov parameter, gradient Richardson number, and flux Richardson number) consistently transition from unstable to weakly stable, with topographic shading accelerating and reshaping their evolution. Modeling of the cooling rates at 0.5 and 20 m shows vertical differences. After the heat flux reversal, near‐surface air cools exponentially, whereas potential temperature at 20 m decreases more slowly and nearly linearly. The ET time‐scale is well approximated by the e‐folding time of near‐surface exponential cooling. These results quantify the near‐surface thermal and flow transitions over slopes during ET and highlight the role of topographic shading in microclimate.