A Numerical Investigation of Squall Lines. Part III: Sensitivity to Precipitation Processes and the Coriolis Force

Kit Kong Szeto Department of Physics, University of Toronto, Toronto, Ontario, Canada

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Han-Ru Cho Department of Physics, University of Toronto, Toronto, Ontario, Canada

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Abstract

The effects of various microphysical processes and the Coriolis force on the dynamics of squall systems were investigated with a two-dimensional, anelastic numerical model. The incorporation of ice-phase microphysics into the model has been found to be important in the successful simulation of realistic storm structure and evolution of squall lines. The significance of the ice-phase microphysics is largely accounted for by the small terminal velocities of ice particles and cooling by melting.

The response of the atmosphere to the cooling by melting is a complicated one and has been shown to play an important role in shaping the kinematic and precipitation characteristics of the observed and modeled squall systems. The interaction between the front-to-rear (FTR) flow and cooling by melting would both intensify (by enhancing the mesoscale updraft and the FTR flow above the melting layer) and limit (by partially driving the rear-to-front flow at the back of the stratiform region) the stratiform precipitation development.

The Coriolis force has also been found to have significant effects on the simulated squall systems. The rotational component of the storm flow field constrains the strength of the divergent wind field, which in turn limits the horizontal scale of the mesoscale circulation and the associated stratiform region. The model squall lines seemed to be most sensitive to the variations of f in the range between f = 0.7 × 10−4 s−1 and f = 1 × 10−4 s−1.

Abstract

The effects of various microphysical processes and the Coriolis force on the dynamics of squall systems were investigated with a two-dimensional, anelastic numerical model. The incorporation of ice-phase microphysics into the model has been found to be important in the successful simulation of realistic storm structure and evolution of squall lines. The significance of the ice-phase microphysics is largely accounted for by the small terminal velocities of ice particles and cooling by melting.

The response of the atmosphere to the cooling by melting is a complicated one and has been shown to play an important role in shaping the kinematic and precipitation characteristics of the observed and modeled squall systems. The interaction between the front-to-rear (FTR) flow and cooling by melting would both intensify (by enhancing the mesoscale updraft and the FTR flow above the melting layer) and limit (by partially driving the rear-to-front flow at the back of the stratiform region) the stratiform precipitation development.

The Coriolis force has also been found to have significant effects on the simulated squall systems. The rotational component of the storm flow field constrains the strength of the divergent wind field, which in turn limits the horizontal scale of the mesoscale circulation and the associated stratiform region. The model squall lines seemed to be most sensitive to the variations of f in the range between f = 0.7 × 10−4 s−1 and f = 1 × 10−4 s−1.

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