MOISTURE TRANSPORT DIAGNOSIS OF A WINTERTIME PRECIPITATION EVENT IN THE MACKENZIE RIVER BASIN

Wintertime precipitation events in the Mackenzie River basin (MRB) pla y on important role in the hydrology of the region because they contri bute 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 simulati on, gridded analyses, and raw observations are used to (i) document me teorological conditions associated with the precipitation event, (ii) assess the ability of the model to reproduce the precipitation event a nd antecedent large-scale moisture transport, and (iii) identify which planetary-and synoptic-scale features are responsible for the observe d moisture transport using piecewise quasigeostrophic potential vortic ity (QGPV) inversion. Precipitation in the MRB develops north of an in tense frontal boundary as a southwesterly now 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-tr opospheric disturbance approaches from the west, The MC2 model adequat ely represents the lee cyclone formation, the observed precipitation e vent, and large-scale moisture transport, as determined through compar ison of the model output with analyses and raw observations. A plume o f moisture advances northeastward from the subtropical Pacific Ocean t oward the MRB during the 24-36-h period prior to the precipitation eve nt. Piecewise QGPV inversion demonstrates that the background climatol ogical Bow and a cyclonic QGPV anomaly located over the eastern Pacifi c Ocean,rs associated with the initial moisture transport into the Gul f of Alaska. Later, a second cyclonic QGPV anomaly centered over the G ulf of Alaska is associated with moisture transport from over the Gulf of Alaska into the MRB. The moisture nux is generally largest in the lower troposphere owing to the larger concentration of water vapor the re. The Rocky Mountains, located west of the MRB. block much of the ea stward moisture transport below the 800-hPa level. Moisture transport in the layer between 700 and 800 hPa is therefore crucial for MRB prec ipitation in situations where the moisture originates over the Pacific . QGPV inversions based on a vertically partitioned QGPV field indicat e that QGPV anomalies located below the dynamic tropopause are associa ted with larger moisture transport at the 700-hPa level than their tro popause-based counterparts.