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Stephen E. Mudrick

Abstract

A primitive equation channel model is used to integrate an initial state consisting of a zonally independent unstable jet on which is superimposed a small-amplitude perturbation. The jet possesses both horizontal and vertical shear. The horizontal grid spacing is scaled to 100 km, allowing the amplifying disturbance to form fairly sharp surface frontal zones. Two integrations are made, with and without a scale-dependent filter designed to remove small-scale components. The filter removes the subfrontal-scale “noise” without changing significantly the larger scale motion.

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Stephen E. Mudrick

Abstract

A linear, quasi-geostrophic, channel model is used to study the instability of zonally independent jets on a midlatitude beta plane. The jets possess strong horizontal as well as vertical wind shear. The most unstable normal mode solution for a symmetric jet is compared to corresponding solutions for asymmetric jets, where the maximum winds are skewed toward the cyclonic side. A significant increase in the growth rate is found for the asymmetric jets, the rate increasing with the amount of skew. In addition, skewing the jet results in raising the maximum of the eddy momentum flux to the top of the channel, compared to maxima at lower levels for the symmetric jet solution. Thus, the use of more realistic, cyclonically skewed jets as basic states for instability studies may result in more realistic growth rates as well as in more realistic eddy flux patterns.

Results qualitatively similar to those of Brown (1969) are found in that a weak, secondary maximum in the growth rate curve shows up for short, shallow modes for some of the asymmetric jet solutions.

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Stephen E. Mudrick

Abstract

Linear and nonlinear numerical channel models are used to simulate polar low/comma cloud evolution. The purpose of this study is to see how much realism can be obtained using models that do not include water vapor. The study was inspired by several observations of such features that evolved over North America.

The basic state used is zonally independent, with an east-west oriented polar front jet on a beta plane centered at 60°N latitude. Strong horizontal as well as vertical wind shear is present. The lowest 3 km of the channel has a reduced static stability, simulating the effect of destabilization from below by sensible heat fluxes upon the polar air mass. The models contain 10 vertical levels.

The linear results are consistent with earlier studies. The most unstable normal modes given by the linear, quasi-geostrophic model show growth rates increasing as the zonal wavelength decreases down to 700 km, the shortest wavelength for which the model gives meteorologically meaningful results. The mechanism of growth is baroclinic instability. The enhanced growth rates at short wavelength are due to the presence of the layer of reduced stability. Growth rates for wavelengths similar to the scale of polar low/comma clouds, ∼103 km, are as large as observed cases. These short wavelength normal modes are quite shallow, in contrast to some observed polar low/comma cloud cases.

Three-dimensional, fully nonlinear, dry, quasi-geostrophic and primitive equations models are then used to study the evolution of a 1200 km wavelength normal mode superimposed upon the basic state. Results of a “fine-resolution” primitive equations run (Δx, Δy = 50 km) are emphasized. The disturbance undergoes a rapid life cycle lasting ∼3 days and deepens somewhat at midtropospheric levels, in contrast to the linear and nonlinear quasi-geostrophic cases. At the channel bottom a cyclonic vortex ∼500 km across forms, associated with a broad, weak ridge but no closed contour high. An elongated cold front and a short warm front form by day 1. The. frontal configuration can be interpreted alternatively as a cold front with an occluded region nearest the low center. All these feature are similar to observed polar low/comma cloud cases, both on the disturbance scale and on the frontal scale.

The results appear realistic in many ways and suggest that dry baroclinic instability, operating upon regions of reduced static stability in the lower troposphere, can explain the initial stages of polar low/comma cloud development.

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Stephen E. Mudrick

Abstract

Fine-resolution, dry, inviscid, Boussinesq formulations of a quasi-geostrophic model and a primitive equations model are used in a study of frontogenesis. These three-dimensional models employ horizontal and vertical resolution on the order of 100 km and 1 km, respectively; an integration uses about 40 grid points in each horizontal direction and 20 in the vertical. The initial states consist of two baroclinic basic currents upon which are superimposed quasi-geostrophically balanced, small-amplitude perturbations corresponding to the most unstable mode in each case. In the second case, the wave grows by barotropic as well as by baroclinic processes.

The most rapid surface frontogenesis occurs where the synoptic-scale, quasi-geostrophic convergence contributes significantly to the pure deformational increase of the horizontal temperature gradient. In these integrations, this distribution favors formation of warm fronts. The frontal zones in the quasi-geostrophic and primitive equations models agree in structure with earlier theoretical solutions by Stone and Hoskins, respectively.

The horizontal deformation, as well as the “indirect” vertical circulation, is important in producing upper level frontogenesis. The two models generate similar patterns of vertical motion. A feedback mechanism relating the action of the horizontal deformation and the indirect circulation and leading to upper level frontogenesis is postulated.

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Qi Hu, C. M. Woodruff, and Stephen E. Mudrick

Century-long annual precipitation time series at 168 stations in the central United States are analyzed with special attention given to interdecadal variations. The results show statistically significant precipitation variations of interdecadal timescales in the region. In particular, one variation has a quasi 20-yr period, and another one possesses a quasi 12-yr period.

The negative phases of the 20-yr variation match with the major drought periods in the region's history, that is, the 1910s, 1930s, 1950s, and the late 1970s. The positive phases of this variation correspond well to the wet periods between the dry epochs. The 12-yr variation shows an amplification after the mid-1960s, while the 20-yr variation shows a reduced amplitude following this time. Concurrent with these changes, the annual precipitation in the region has increased since the mid-1960s.

A plausible mechanism connecting the interdecadal variations of annual precipitation in the central plains region and slow timescale variations in the midlatitude and subtropical North Atlantic regions is briefly discussed.

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