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Hui-Ya Chuang and Peter J. Sousounis

Abstract

A new idealized initialization technique has been developed for the Mesoscale Model version 5 modeling system. The technique allows the specification of baroclinic disturbances that feature vertical variations of the height, temperature, and wind fields in terms of phase lag, wavelength, and phase speed. The technique involves specifying a sounding profile at some reference point, generating the desired height fields using an analytic formulation, constructing the wind fields to be in geostrophic balance, and generating temperature fields using the hydrostatic relationship.

A distinct advantage of this technique over existing ones is that the boundary conditions are not restricted to being specified as periodic. The flexibility means that 1) users do not have to specify a domain whose size is equal to an integer number of wavelengths of the specified flow; 2) users can specify a flow that consists of different wavelengths at different heights, as is typically observed; and 3) any responses that are generated orographically or thermally in the domain and which leave the eastern boundary will not reenter the western boundary. This last item is particularly advantageous because it allows users to study the effects of a preconditioned environment on subsequent development of a featured disturbance rather than studying the repetitive effects of the same forcing mechanism on the same disturbance.

Examples of simulations using initial conditions that are generated with this technique are shown for 1) zonal flow and 2) continuous sinusoidal waves. Flat terrain was adopted for both examples. In example 1, the boundary layer parameterization scheme and surface fluxes were turned off for a simplified zonal flow situation to demonstrate the stability of this technique. During the simulations, the flow remained zonal, exactly as specified, even after 48 h. In example 2, a situation consisting of continuous sinusoidal waves moving across an array of four warm circular lakes was created to demonstrate the utility of the technique for examining how disturbances may be affected by the Great Lakes. Realistic-looking highs, lows, and fronts, along with individual and lake-aggregate enhancements developed by 48 h. Good stability and lack of distortion throughout the domain in both examples add credibility to the technique.

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Hui-Ya Chuang and Peter J. Sousounis

Abstract

The effects of a group (aggregate) of relatively warm circular meso-β-scale lakes on different flow regimes were investigated by conducting a series of idealized numerical experiments. This investigation was motivated by the observed behavior of synoptic-scale cyclones moving through the Great Lakes region during winter. Three with-lake (WL) and three corresponding no-lake (NL) simulations were initialized with 1) zonal flow, 2) a solitary trough, and 3) continuous sinusoidal waves, respectively. The WL experiments were intercompared to examine the importance of a preexisting disturbance and preconditioning. The NL simulations were compared to the corresponding WL simulations to study the contributions of the lake aggregate. The simulation results suggest that the lake aggregate induced or enhanced warm fronts when there were preexisting disturbances. They also suggest that a perturbation mesoscale aggregate vortex was generated in each of the three different flow scenarios even though the lake aggregate alone could only generate a weak meso-α-scale trough.

To identify the physical processes that were altered by the lake aggregate to enhance cyclone development, surface pressure tendency diagnosis using the extended Zwack–Okossi (ZO) equation was applied to the simulation results. The results of the ZO surface pressure (PSFC) tendency diagnosis indicated that the preconditioning from the preceding ridge contributed to the further development of the lake-aggregate–enhanced cyclones. The results also indicated that the lake aggregate not only reduced the PSFC locally through surface sensible heating but also and, more importantly, contributed to large-scale surface pressure deepening by enhancing the surface warm front.

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