HAPpy Hour Seminar: Fine-scale prediction of orographic precipitation in multi-ridge orography

Abstract:

The Wasatch Range of northern Utah is a narrow, meridionally oriented mountain range at the eastern periphery of the Great Basin Provence in the interior western United States, which rises 2000 m from the adjacent Salt Lake Valley (SLV) to 3500 m. Directly east of the SLV, the Wasatch broadens to a nearly 1:1 horizontal aspect ratio, making up what is known locally as the central Wasatch, which features several zonally oriented ridges and canyons. One such canyon, Little Cottonwood Canyon (LCC), abuts some of the highest terrain in the Wasatch, and is one of the snowiest locations in the interior western United States, with mean annual snowfall >1250 cm. A major highway, SR-210, runs the length of the canyon, connecting the SLV to the town of Alta in the upper canyon, ascending >1000 m in 11 km and traversing 50 avalanche paths. Due to steep terrain, frequent heavy wintertime precipitation, and high vehicle density, SR-210 has the highest uncontrolled avalanche hazard index of any major road in the world. Compounding the natural hazards present in LCC, operational forecast models in the United States which have horizontal resolutions ≥3 km underpredict seasonal liquid precipitation equivalent (LPE) at Alta by ~30%, posing challenges towards highway management and operations in the canyon. The goal of this research is to improve the prediction of fine-scale orographic precipitation in mountainous terrain throughout the world. The prediction of orographic precipitation at fine-scales has been minimally studied and has implications for flood and avalanche hazard mitigation as well as water resource management. Here, we use upper LCC as a testbed, given its substantial cool-season precipitation and steep, complex terrain.

In December 2022, a shallow post-cold-frontal northwesterly flow snowstorm produced 84 cm of snow from 22 mm of LPE in 48 hours at Alta, despite a High-Resolution Rapid Refresh (HRRR) model forecast of only 15 cm of snow during the period. In this research, we conduct high resolution (≤1 km) Weather Research and Forecasting (WRF) simulations of that event to analyze factors which contribute to the operational forecasting models’ underprediction of fine-scale orographic precipitation in upper LCC. We will be evaluating a number of hypotheses for why operational models underpredict orographic precipitation including model grid spacing, input terrain resolution, microphysics parameterizations, and turbulence. In this talk, we present preliminary results from simulations including early modeling challenges.

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