An Investigation of Warm Rainfall Microphysics in the Southern Appalachians: Orographic Enhancement via Low-Level Seeder–Feeder Interactions

Abstract

Observations of the vertical structure of rainfall, surface rain rates, and drop size distributions (DSDs) in the southern Appalachians were analyzed with a focus on the diurnal cycle of rainfall. In the inner mountain region, a 5-yr high-elevation rain gauge dataset shows that light rainfall, described here as rainfall intensity less than 3 mm h−1 over a time scale of 5 min, accounts for 30%–50% of annual accumulations. The data also reveal warm-season events characterized by heavy surface rainfall in valleys and along ridgelines inconsistent with radar observations of the vertical structure of precipitation. Next, a stochastic column model of advection–coalescence–breakup of warm rain DSDs was used to investigate three illustrative events. 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-size raindrops (<2 mm) via Bergeron processes can enhance surface rainfall rates by one order of magnitude for durations up to 1 h 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 from drizzle development to rainfall.

Understanding How Low-Level Clouds and Fog Modify the Diurnal Cycle of Orographic Precipitation Using In Situ and Satellite Observations

“…By contrast, mid-day rainfall in the SAM can reach hourly rates similar to convective precipitation (i.e., >10 mm/h). Microphysical and radar observations indicate significant enhancement of number concentrations across the R-DSD spectrum, and in particular in the number of large drop sizes that reach the surface [15,18]. Light rainfall can potentially interact with LLCF (i.e., SFI) consequently leading to significantly enhanced surface rainfall as documented by Prat and Barros [16] and Wilson and Barros [15,18].…”

“…From left to right, they are the production of droplets resulting from coalescence of smaller drops, the removal of droplets resulting from coalescence with other droplets, the gain of droplets due to breakup of larger drops, and the loss of droplets due to their breakup. One distinct aspect of the model is the explicit incorporation of bounce and distinct modes of breakup (neck/filament, sheet, crown and disk) [15,18,50,51] using a We-p parameterization of regimes of collision outcomes after Testik et al [52] and Prat et al [21], where We is the Weber number and p is the ratio of the small to the large diameter of two colliding raindrops (see Figure S1 in the supplementary material).…”

“…As discussed in Section 1, the SAM experience mid-day light rainfall throughout the year that peaks before convective activity develops in the mid and late-afternoon [14,18]. Elsewhere, mid-day light rainfall does not reach the ground due to evaporation in the warm season (e.g., Himalayas) [53].…”

“…They also documented robust ridge-valley differences in the seasonal climatology of R-DSDs with larger drop sizes at lower elevations. Wilson and Barros [18] found that the microphysics of the mid-day rainfall peak at paired ridge-valley sites in the inner mountain region are characterized by very large number concentrations of small drops and light rainfall intensity in the presence of thick fog at both locations (concurrently or not) with large increases in the number of large drops under moderate and heavy rainfall conditions. Concurrent bottom-heavy radar reflectivity profiles further indicated that these microphysical changes took place within the lower 500 m suggesting seeder-feeder interactions (SFI) between rainfall aloft and low-level fog.…”

“…Concurrent bottom-heavy radar reflectivity profiles further indicated that these microphysical changes took place within the lower 500 m suggesting seeder-feeder interactions (SFI) between rainfall aloft and low-level fog. This hypothesis was tested by Wilson and Barros [18] using a raindrop dynamics column model [19][20][21] modified to include fog. Due to the lack of in situ observations, they synthesized fog spectra from the literature for their model experiments and showed that increased coalescence efficiency between precipitating clouds aloft and thick fog in the inner SAM could explain the observed increase in rainfall rate and DSD modification.…”