Atmospheric water species budget in mesoscale simulations of lee cyclones over the Mackenzie River Basin
A moisture budget over the Mackenzie River Basin (MRB) was computed using a high‐resolution mesoscale model with explicit microphysics for 3 lee cyclogenesis events. A unique feature of the calculation is that all the budget terms are calculated from the model and no residual terms are required. It was found that during the initial formative period of the lee cyclones, a large influx of moisture occurs at the western boundary. However, as the cyclone moves further east, a significant amount of moisture is withdrawn through the eastern and southern boundaries of the basin. Surface evaporation was found to be relatively large during the local day time and plays a vital ròle in initiating convection in the presence of frontal lifting south of 60°N within the basin. In 2 of the 3 cases, the total water in the basin increases over the history of the simulation as a result of substantial lateral flux convergence of total water content even though the total precipitation in these two events was nearly 1.4× the surface evaporation. For the 3rd cyclone, the total water in the basin decreases substantially because of precipitation and large outward moisture flux at the boundary. The dominant microphysical processes governing the transformation of various water species were condensation, deposition, autoconversion and accretion of cloud water by rain, accretion of cloud water by ice, melting of ice to rain water and evaporation of cloud and rain water. In the net horizontal flux convergence of water species, the largest was water vapor, followed by ice and cloud water. The net flux convergence of rainwater into the basin was small and the effect of the graupel processes is negligible.
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Document Type: Research Article
Publication date: March 1, 2000