Orographic Land-Atmosphere Interactions and the Diurnal Cycle of Low Level Clouds and Fog

The Southern Appalachian region in the Southeast United States is an area of rich biodiversity that is vulnerable to land use/land cover changes due to its proximity to the rapidly growing population in the area. Persistent near surface moisture and associated microclimates, as well as their effect on highly localized ecosystems, have been consistently observed in this region. The topography in this area, in particular in the inner region between the southeast-northwest oriented ridges, is characterized by nested valleys and ridges. This geometry of the terrain causes distinct diurnal and seasonal local flow patterns that result in complex interactions of this persistent low level moisture with meso- and synoptic-scale cyclones passing through the region. Previous work documented diurnal cycles of precipitation in the Southern Appalachians organized along altitudinal gradients that can be related to the spatio-temporal variability of precipitation microphysics. Landform controls on moisture convergence in the Southern Appalachians have been shown to promote heterogeneity in the vertical structure of low level clouds (LLC) and seeder-feeder interactions (SFI) that significantly impact the space-time patterns of warm season precipitation.

In this study, 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) Intense Observing Period, May-June 2014, are examined using the Advanced Research Weather Research and Forecasting (ARW-WRF) model version 3.5.1, with resolution down to 1 kilometer in the innermost domain.  First, the sensitivity of model simulations to the choice of planetary boundary layer parameterization was investigated in the light of IPHEx observations. The MYNN (Mellor-Yamada-Nakanishi-Niino) scheme led to the formation of low cloud 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: 1) a stationary nocturnal mode at the meso-α scales associated with an extended flow separation zone that supports low level pooling and trapping of cold, moist, stable air (N_m² > 0) in the inner mountain basins on the lee side of the western topographic divide; and 2) a dynamic daytime mode that results from the co-organization of ridge valley-circulations at the meso-γ scale and Rayleigh-Bénard convection at the meso-β scale characterized by widespread low level instability (N_m² < 0) below the envelope orography with limited stagnation above the inner mountain basin.