What Causes the Unobserved Early‐Spring Snowpack Ablation in Convection‐Permitting WRF Modeling Over Utah Mountains?. Issue 22 (18th November 2021)

• Record Type: Journal Article
• Authors: He, Cenlin; Chen, Fei; Abolafia‐Rosenzweig, Ronnie; Ikeda, Kyoko; Liu, Changhai; Rasmussen, Roy
• Is Part Of: Journal of geophysical research. Volume 126:Issue 22(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-11-18
• Subjects: snow ablation -- snow melting -- convection‐permitting modeling -- WRF -- snowpack evolution; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2021JD035284
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: Accurate prediction of snowpack evolution and ablation is critical to supporting weather and hydrological applications. Convection‐permitting modeling has been shown to well capture observed snowpack evolution over many western United States (U.S.) mountain ranges, but some significant ablation biases still remain. In this study, we conduct process‐level snowpack analyses of a widely used convection‐permitting (4‐km) weather research and forecasting (WRF) modeling product (WRF4km) for the contiguous U.S. to understand the mechanisms causing its unobserved early‐spring snow ablation over Utah mountains. Analyses across Utah Snowpack Telemetry (SNOTEL) sites show that the unobserved snowpack ablation during mid‐February to late‐March in WRF4km is driven by multiple strong melting events. The melting results from the enhanced downward sensible heat flux to snowpack and enhanced ground solar radiation absorption, with generally larger contributions from the former before early March and from the latter after early March. The enhanced downward sensible heat flux to snowpack is mainly due to the enhanced surface heat exchange coefficient induced by high surface wind speeds. The enhanced ground solar radiation absorption is driven by both enhanced surface downward solar radiation and strong melting‐induced snow cover reduction that is caused by deficiencies in Noah‐MP snow‐related parameterizations used in WRF4km. The substantial snow cover reduction during melting decreases surface albedo and hence triggers a positive albedo feedback that further accelerates melting. Our analyses reveal possible deficiencies in WRF and Noah‐MP (e.g., canopy processes and snow albedo) and shed light on future directions for model improvements. Plain Language Summary: Snowpack plays an important role in modulating surface energy and water balance and land‐atmosphere interaction in the Earth system. Snow melting is key to many important hydrological applications by affecting runoff, fresh water availability, and drought, particularly over western United States (U.S.) mountainous regions. Convection‐permitting modeling with a high spatial resolution (e.g., ≤4 km) can accurately capture observed snowpack evolution over many western U.S. mountain ranges, but some important melting biases still remain. In this study, we conduct analyses of the widely used 4‐km weather research and forecasting (WRF) modeling product developed at the National Center for Atmospheric Research to understand the reasons causing its strong early‐spring snow melting over Utah mountains, which is not seen in observations. We find that the strong melting during mid‐February to late‐March in the model is due to (a) the enhanced downward sensible heat flux to snowpack driven by strong surface winds, and (b) the enhanced ground absorption of solar radiation caused by strong surface downward solar radiation and strong snow cover reduction induced by melting. Our results reveal possible deficiencies in model physics (e.g., canopy processes and snow albedo) and shed light on future directions for model improvements. Key Points: WRF 4‐km early‐spring SWE underestimates are due to strong melting caused by enhanced sensible heat to snow and solar radiation absorption The enhanced surface heat exchange coefficient and thus sensible heat flux to snowpack are driven by strong surface winds in the WRF model The enhanced solar radiation absorption is due to strong surface downward solar radiation and strong melting‐induced snow cover reduction