Observations and Modeling of Orographic Enhancement of Precipitation in the Southern Appalachians via Low-Level Bergeron Processes

The need for accurate observations of precipitation accumulations and intensity in regions of complex terrain is well known, due to a lack of in situ observations and obstacles to radar and satellite observations such as beam blockage and ground clutter. Since 2007, a high-elevation dense rain gauge network has been recoding precipitation observations along ridgelines in the Pigeon River Basin in the Southern Appalachians. Observations of the vertical structure of rainfall, surface rain rates and drop size distributions (DSDs) have also been made during this period with Micro Rain Radars (MRRs) and PARSIVEL disdrometers. Data are presented with a focus on diurnal cycles and variability. The data reveal warm season events characterized by heavy surface rainfall in valleys and along ridgelines that are inconsistent with radar observations of the vertical structure of precipitation. An illustrative event investigated by a stochastic column model of advection-coalescence-breakup of warm rain microphysics is presented. The integrated analysis of observations and model simulations suggests that seeder-feeder interactions (i.e. Bergeron processes) between incoming rainfall systems and local fog and/or low level clouds with very high number concentrations of small drops (<0.2 mm) govern surface rainfall intensity through driving significant increases in coalescence rates and efficiency. Specifically, the model shows how accelerated growth of small and moderate sized raindrops (<2 mm) via Bergeron processes can enhance surface rainfall rates by one order of magnitude for durations up to one hour as in the observations. An examination of the fingerprints of seeder-feeder processes on DSD statistics conducted by tracking the temporal evolution of mass spectrum parameters points to the critical need for improved characterization of hydrometeor microstructure evolution, from mist formation to fog and drizzle development to rainfall. To begin to address this observational gap, experimental fog collectors to measure bulk parameters, and a Passive Cavity Aerosol Spectrometer Probe (PCASP) to measure very fine particles in the atmosphere (0.1 – 10 µm) were deployed in the summer of 2013. Preliminary findings from their observations will be presented. In addition, the role of secondary organic aerosols in the formation of haze and fog is explored using high resolution WRF simulations.