Transport of Atmospheric Moisture during Three Extreme Rainfall Events over the Mackenzie River Basin

Abstract

Lagrangian trajectories were computed for three extreme summer rainfall events (with rainfall exceeding 100 mm) over the southern Mackenzie River basin to test the hypothesis that the low-level moisture feeding these rainstorms can be traced back to the Gulf of Mexico. The three-dimensional trajectories were computed using the Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT). For all three events, parcel trajectories were identified that originated near the Gulf of Mexico and terminated over the southern Mackenzie River basin. Specifically, the transport of low-level moisture was found to occur along either quasi-continuous or stepwise trajectories. The time required to complete the journey varied between 6 and 10 days. Closer examination of the data suggests that, for the three cases in question, the transport of modified Gulf of Mexico moisture to high latitudes was realized when the northward extension of the Great Plains low-level jet to the Dakotas occurred in synch with rapid cyclogenesis over Alberta, Canada. In this way, modified low-level moisture from the Gulf of Mexico arrived over the northern Great Plains at the same time as a strong southerly flow developed over the Dakotas and Saskatchewan, Canada, in advance of the deepening cutoff low over Alberta. This moist air was then transported northward over Saskatchewan and finally westward over the southern Mackenzie River basin, where strong ascent occurred.

Precipitation, Recycling, and Land Memory: An Integrated Analysis

“…Recycling is estimated by performing back-trajectory calculations from precipitation events occurring over each land grid box (Dirmeyer and Brubaker 1999;Brubaker et al 2001;Sudradjat et al 2003). Accounting is done in terms of water mass, rather than E 2 P, where E is evaporation and P is precipitation (Stohl and James 2005), and unlike pure passive-tracer approaches (e.g., Brimelow and Reuter 2005), surface fluxes are included to pinpoint source regions. Observational precipitation products and the reanalysis of Kanamitsu et al (2002) are used to establish the time and place of each precipitation event.…”

Northern Tales: A Synthesis of MAGS Atmospheric and Hydrometeorological Research

“…Lee cyclones were also found to have great effects on the basins water and energy budgets through storm cloud and precipitation developments and moisture transports (Lackmann et al 1998;Misra et al 2000: Brimelow andReuter 2005). Misra et al (2000) investigated numerically the detailed water budget of the basin during the genesis and passage of three autumn lee cyclones over the basin.…”

“…On the other hand, the MRB could receive substantial moisture from the southern plains or even the Atlantic Ocean through its unobstructed southeastern boundary during the warm season Brimelow and Reuter 2005). Although the large-scale atmospheric forcing weakens, synoptic activities such as lee cyclogenesis are still frequent over the region (Whittaker and Horn 1984).…”

“…6), thereby enhancing the precipitation response of the region to variations in large-scale conditions and increasing the interannual variability of warm-season precipitation in the basin. Recently, Brimelow and Reuter (2005) also found that low pressure systems over the extreme southern basin can act in concert with the Great Plains low-level jet to enhance moisture transport from the southern plains into the region, subsequently contributing to the development of extreme precipitation events in basin.…”

Validation of ET Estimates from the Canadian Prairie Agrometeorological Model for Contrasting Vegetation Types and Growing Seasons

“…Eighty percent of the significant (≥10 mm) summer rain events over the Canadian prairies include thunderstorms (Raddatz and Hanesiak, 2008). The amount of water vapour available to feed warm season rain events is governed by the advection of water vapour into a region (e.g., Brimelow and Reuter, 2005) and surface evapotranspiration (ET) (e.g., Brubaker et al, 1993;Trenberth, 1999). Under quiescent atmospheric conditions the daily contribution of ET to moisture in the convective boundary layer (CBL), primarily through transpiration from the vegetation, can exceed that added through advection (Raddatz, 2005).…”