High- resolution observations and modeling of precipitation processes in the Great Smoky Mountains: the importance of getting the physics right

Complex orographic landscapes characterized by high space-time gradients of precipitation, and thus complex hydrologic response, appear to orchestrate optimally the interplay of natural hazards leading from heavy precipitation to flooding, debris flows and landslides. The Southern Appalachians and the Great Smoky Mountains in particular provide a unique setting for investigating orographic precipitation processes and associated hydrological response in mountainous areas. Here, we present data from the Great Smoky Mountains including a high-resolution network of 32 ridge locations, one flux tower, several streamgauges and several other embedded small networks to measure runoff, soil moisture, and air temperature. First, we demonstrate why reliability and accuracy of event-scale precipitation intensity and accumulation at high spatial and temporal resolution are critical in order to capture flashflood response in headwater catchments. For this purpose, we use data and model simulations from tropical Storm Fay during which the river discharge increased and decreased by two-to-three orders of magnitude within a two-day period. Next, we report on detailed studies to elucidate the space-time variability, seasonality and microphysical properties of clouds and precipitation systems which have an impact on regional hydrometeorology, hydrology and ecology. Specifically, the data show that summer precipitation is characterized by large event to event variability including both stratiform and convective characteristics. The diurnal cycle of precipitation in the winter and spring, summer and in the fall are very different. In the inner region, during fall, stratiform precipitation dominates and rainfall is 3 times more frequent at the ridge than in the valley, corresponding to a 70% increase in cumulative rainfall at high elevations. On a seasonal basis, the orographic enhancement is on the order of 300%. For concurrent rain events, the orographic enhancement effect is on the order of 20%. Evidence of a seasonal signature in the rain DSD (Drop Size Distribution) was found with significantly heavier tails (larger raindrops) for summer DSDs at higher elevations, while no significant differences were observed between ridge and valley locations during fall deployment. Data from convective storms suggest there is a high number of very small drops ( < 0.5 mm), which along with the dominance of coalescense processes explains the observed high rainfall intensities. In the case of tropical storm Fay an increase of one order of magnitude in the number of small drops is apparently needed to estimate rainfall rates consistent with observed stream flow response and raingauge observations regardless of the value of the measured radar reflectivity (ground-based and,or TRMM PR). Boundary layer measurements show evidence of strong ridge-valley circulations in the summer time which appear to be related to isolated convective activity in the afternoon and the local diurnal cycle of cloudiness, which in turn affect surface temperature and soil moisture gradients and energy fluxes.