A comparison of National Water Model retrospective analysis snow outputs at snow telemetry sites across the Western United States. Issue 1 (27th January 2022)

• Record Type: Journal Article
• Authors: Garousi‐Nejad, Irene; Tarboton, David G.
• Is Part Of: Hydrological processes. Volume 36:Issue 1(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-01-27
• Subjects: MODIS -- National Water Model -- Noah‐MP -- remote sensing -- SNOTEL -- snow parameterization -- snow water equivalent -- snow‐covered area fraction; Hydrology -- Periodicals; Hydrology -- Research -- Periodicals; Hydrologic models -- Periodicals; Hydrological forecasting -- Periodicals; 631.432
• DOI: 10.1002/hyp.14469
• Languages: English
• ISSNs: 0885-6087

Abstract

Abstract: This study compares the US National Water Model (NWM) reanalysis snow outputs to observed snow water equivalent (SWE) and snow‐covered area fraction (SCAF) at snow telemetry (SNOTEL) sites across the Western United States. SWE was obtained from SNOTEL sites, while SCAF was obtained from moderate resolution imaging spectroradiometer (MODIS) observations at a nominal 500 m grid scale. Retrospective NWM results were at a 1000 m grid scale. We compared results for SNOTEL sites to gridded NWM and MODIS outputs for the grid cells encompassing each SNOTEL site. Differences between modelled and observed SWE were attributed to both model errors, as well as errors in inputs, notably precipitation and temperature. The NWM generally under‐predicted SWE, partly due to precipitation input differences. There was also a slight general bias for model input temperature to be cooler than observed, counter to the direction expected to lead to under‐modelling of SWE. There was also under‐modelling of SWE for a subset of sites where precipitation inputs were good. Furthermore, the NWM generally tends to melt snow early. There was considerable variability between modelled and observed SCAF as well as the binary comparison of snow cover presence that hampered useful interpretation of SCAF comparisons. This is in part due to the shortcomings associated with both model SCAF parameterization and MODIS observations, particularly in vegetated regions. However, when SCAF was aggregated across all sites and years, modelled SCAF tended to be more than observed using MODIS. These differences are regional with generally better SWE and SCAF results in the Central Basin and Range and differences tending to become larger the further away regions are from this region. These findings identify areas where predictions from the NWM involving snow may be better or worse, and suggest opportunities for research directed towards model improvements. Abstract : We compared the National Water Model (NWM) v2.0 reanalysis snow water equivalent (SWE) and snow‐covered area fraction (SCAF) against observations from snow telemetry (SNOTEL) and moderate resolution imaging spectroradiometer (MODIS), respectively, and found that NWM generally under‐estimates SWE, over‐estimates SCAF, and melts snow early across the SNOTEL sites and that these discrepancies vary regionally. Under‐modelling of SWE is in part due to downward bias in precipitation but also due to model errors as the downward bias still exists even when model precipitation is relatively accurate.