Numerical Simulation of Polar Lows and Comma Clouds Using Simple Dry Models

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 (?x, ?y = 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.