Orographic land-atmosphere interactions and the diurnal cycle of low-level clouds and fog

Previous work illuminated landform controls on moisture convergence in the southern Appalachian Mountains (SAM) promoting heterogeneity in the vertical structure of low-level clouds (LLC) and seeder-feeder interactions (SFI) that significantly impact warm season precipitation. Here, the focus is on elucidating orographic land-atmosphere interactions associated with the observed diurnal cycle of LLC and fog in the region. Three distinct hydrometeorological regimes during the Integrated Precipitation and Hydrology Experiment (IPHEx) are examined using the Weather Research and Forecasting Model. Sensitivity to the choice of planetary boundary layer parameterization was investigated in the light of IPHEx observations. Simulations using the Mellor-Yamada-Nakanishi-Niino scheme exhibit LLC and fog patterns most consistent with observations, albeit without capturing SFI. Independently of synoptic regime, the simulations reveal two distinct modes of orographic controls on atmospheric moisture convergence patterns that explain the diurnal cycle of LLC and fog. First, a stationary nocturnal mode at the meso-a scale associated with an extended flow separation zone supports low-level pooling and trapping of cold, moist, stable air in the inner mountain on the lee side of the western topographic divide. Second, a dynamic daytime mode that results from the coorganization of ridge-valley circulations at the meso-β scale and Rayleigh-Bénard convection at the meso-ß scale is associated with widespread low-level instability below the envelope orography. Orographic decoupling results in the formation of a shallow stagnation zone between the western and eastern topographic divides at night that contracts westward during daytime. Predominantly easterly and southeasterly low-level moisture convergence patterns support early afternoon LLC formation in the inner SAM.