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The influence of soil moisture on the historic 2021 Pacific Northwest heatwave

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Author(s): Robert Conrick, Clifford Mass

Document Type:
Publisher: Monthly Weather Review
Published Year: 2023
Volume: 151
Number: 5
Pages: 1213 to 1228
DOI Identifier: 10.1175/MWR-D-22-0253.1
ISBN Identifier:
Keywords: Atmosphere-land interaction Extreme events Heat wave Mesoscale models Model evaluation/performance

During late June 2021, a record-breaking heatwave impacted western North America, with all-time high temperatures reported across Washington, Oregon, British Columbia, and Alberta. The heatwave was forced by a highly anomalous upper-level ridge, strong synoptic-scale subsidence, and downslope flow resulting in lower-tropospheric adiabatic warming. This study examines the impact of antecedent soil moisture on this extreme heat event. During the cool season of 2020/21, precipitation over the Pacific Northwest was above or near normal, followed by a dry spring that desiccated soils to 50%–75% of normal moisture content by early June. Low surface soil moisture affects the surface energy balance by altering the partitioning between sensible and latent heat fluxes, resulting in warmer temperatures. Using numerical model simulations of the heatwave, this study demonstrates that surface air temperatures were warmed by an average of 0.48°C as a result of dry soil moisture conditions, compared to a high-temperature anomaly of 10°–20°C during the event. Air temperatures over eastern Washington and southern British Columbia were most sensitive to soil moisture anomalies, with 0000 UTC temperature anomalies ranging from 1.2° to 2.2°C. Trajectory analysis indicated that rapid subsidence of elevated parcels prevented air parcels from being affected by surface heat fluxes over a prolonged period of time, resulting in a relatively small temperature sensitivity to soil moisture. Changes to soil moisture also altered regional pressure, low-level wind, and geopotential heights, as well as modified the marine air intrusion along the Pacific coast of Washington and Oregon.

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Suggested Citations:
In Text Citation:
Conrick and Mass (2023) or (Conrick and Mass, 2023)

References Citation:
Conrick, R. and C. Mass, 2023, The influence of soil moisture on the historic 2021 Pacific Northwest heatwave: Monthly Weather Review, Vol. 151, No. 5, pp. 1213-1228, doi: 10.1175/MWR-D-22-0253.1.