Upslope Enhanced Extreme Rainfall Events over the Canadian Western Plains: A Mesoscale Numerical Simulation

R. L. Raddatz Prairie Weather Centre, Atmospheric Environment Service, Winnipeg, Manitoba, Canada R3C 3V4

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M. L. Khandekar W.M.O. Lecturer and Caribbean Meteorological Institute, Barbadoes, West Indies

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Abstract

A limited-area numerical model, previously applied to the study of cold easterly circulations over the Canadian Western Plains, is used to simulate upslope enhanced extreme rainfall events over the Saskatchewan River and adjacent drainage basins. Following Lavoie (1972) the atmospheric structure was represented by three layers: a constant flux layer in contact with earth's surface, a well-mixed planetary boundary layer capped by an inversion, and a deep stratum of overlying stable air. The governing primitive equations were averaged through the depth of the mixed layer with interactions between the mixed layer and both the underlying and overlying air being suitably parameterized.

A 47.6 km grid mesh of 1369 (37×37) points covering the Canadian Prairie Provinces was used to represent the variables. The governing equations were solved numerically for the mixed layer with terrain influences, surface roughness, temperature variations, moisture fluxes and the release of latent heat allowed to perturb the mixed layer from its initial conditions.

The model successfully simulated mean persistent (3-day) extreme rainfall distributions associated with occluded cyclones over the Saskatchewan River Basin. A case study based on observed initial conditions showed that the model could reproduce mesoscale rainfall distributions associated with an individual storm. Implications of such a simulation for developing short-term streamflow forecasts over the Saskatchewan River and adjacent drainage basins are considered.

Abstract

A limited-area numerical model, previously applied to the study of cold easterly circulations over the Canadian Western Plains, is used to simulate upslope enhanced extreme rainfall events over the Saskatchewan River and adjacent drainage basins. Following Lavoie (1972) the atmospheric structure was represented by three layers: a constant flux layer in contact with earth's surface, a well-mixed planetary boundary layer capped by an inversion, and a deep stratum of overlying stable air. The governing primitive equations were averaged through the depth of the mixed layer with interactions between the mixed layer and both the underlying and overlying air being suitably parameterized.

A 47.6 km grid mesh of 1369 (37×37) points covering the Canadian Prairie Provinces was used to represent the variables. The governing equations were solved numerically for the mixed layer with terrain influences, surface roughness, temperature variations, moisture fluxes and the release of latent heat allowed to perturb the mixed layer from its initial conditions.

The model successfully simulated mean persistent (3-day) extreme rainfall distributions associated with occluded cyclones over the Saskatchewan River Basin. A case study based on observed initial conditions showed that the model could reproduce mesoscale rainfall distributions associated with an individual storm. Implications of such a simulation for developing short-term streamflow forecasts over the Saskatchewan River and adjacent drainage basins are considered.

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