Moisture Transport Diagnosis of a Wintertime Precipitation Event in the Mackenzie River Basin

Abstract

Wintertime precipitation events in the Mackenzie River basin (MRB) play an important role in the hydrology of the region because they contribute substantially to water storage prior to the spring runoff maximum. The Mesoscale Compressible Community (MC2) Model is used to simulate a representative wintertime MRB precipitation event. The MC2 simulation, gridded analyses, and raw observations are used to (i) document meteorological conditions associated with the precipitation event, (ii) assess the ability of the model to reproduce the precipitation event and antecedent large-scale moisture transport, and (iii) identify which planetary- and synoptic-scale features are responsible for the observed moisture transport using piecewise quasigeostrophic potential vorticity (QGPV) inversion. Precipitation in the MRB develops north of an intense frontal boundary as a southwesterly flow of moisture originating over the Pacific Ocean is lifted over cold, dense arctic air near the surface. A lee cyclone forms along the frontal boundary as an upper-tropospheric disturbance approaches from the west. The MC2 model adequately represents the lee cyclone formation, the observed precipitation event, and large-scale moisture transport, as determined through comparison of the model output with analyses and raw observations. A plume of moisture advances northeastward from the subtropical Pacific Ocean toward the MRB during the 24?36-h period prior to the precipitation event. Piecewise QGPV inversion demonstrates that the background climatological flow and a cyclonic QGPV anomaly located over the eastern Pacific Ocean are associated with the initial moisture transport into the Gulf of Alaska. Later, a second cyclonic QGPV anomaly centered over the Gulf of Alaska is associated with moisture transport from over the Gulf of Alaska into the MRB. The moisture flux is generally largest in the lower troposphere owing to the larger concentration of water vapor there. The Rocky Mountains, located west of the MRB, block much of the eastward moisture transport below the 800-hPa level. Moisture transport in the layer between 700 and 800 hPa is therefore crucial for MRB precipitation in situations where the moisture originates over the Pacific. QGPV inversions based on a vertically partitioned QGPV field indicate that QGPV anomalies located below the dynamic tropopause are associated with larger moisture transport at the 700-hPa level than their tropopause-based counterparts.