Enhancement of Extratropical Cyclogenesis by a Mesoscale Convective System

Da-Lin Zhang Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada

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Richard Harvey Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada

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Abstract

Considerable progress has been made in the past decades on understanding the life cycle of rapidly deepening winter cyclones. However, little attention has been paid to the role that mesoscale convective systems (MCSs) play during extratropical cyclogenesis within weak baroclinic environments. In this study, the impact of an MCS on the subsequent surface cyclogenesis is investigated by extending the previously documented 21-h simulation of the 10–11 June 1985 PRE-STORM squall line to 36 hours. The model reproduces the meteorological events from the initiation to the dissipation of the squall system and then to the formation of a surface cyclone and the amplification of midlevel baroclinic waves, as verified against all available observations.

It is found that the squall line is initiated ahead of a weak surface cold front with the aid of baroclinic forcing. Once initiated, however, the prefrontal squall system is primarily driven by the interaction of convectively generated circulations with a conditionally unstable environment. As it rapidly intensifies and accelerates east-ward, the squall system amplifies a midlevel short wave by warming the upper troposphere and cooling the lower troposphere, and then forces it to move with the system. On the other hand, the movement of a low to midlevel thermal wave is primarily determined by adiabatic processes. Thus, the convective system tends to enhance the larger-scale baroclinicity and increase the phase lag between the pressure and thermal waves such that the baroclinic environment becomes more favorable for the subsequent surface cyclogenesis.

The role of moist convection in the surface cyclogenesis is examined by comparing simulations with and without the convective system. It is found that, in the absence of moist convection, the model also produces a surface cyclone, but with much weaker intensity, much smaller horizontal extent, and much slower displacement. The relationships of convectively generated mesovortices and wake lows to the surface cyclogenesis are also examined.

Abstract

Considerable progress has been made in the past decades on understanding the life cycle of rapidly deepening winter cyclones. However, little attention has been paid to the role that mesoscale convective systems (MCSs) play during extratropical cyclogenesis within weak baroclinic environments. In this study, the impact of an MCS on the subsequent surface cyclogenesis is investigated by extending the previously documented 21-h simulation of the 10–11 June 1985 PRE-STORM squall line to 36 hours. The model reproduces the meteorological events from the initiation to the dissipation of the squall system and then to the formation of a surface cyclone and the amplification of midlevel baroclinic waves, as verified against all available observations.

It is found that the squall line is initiated ahead of a weak surface cold front with the aid of baroclinic forcing. Once initiated, however, the prefrontal squall system is primarily driven by the interaction of convectively generated circulations with a conditionally unstable environment. As it rapidly intensifies and accelerates east-ward, the squall system amplifies a midlevel short wave by warming the upper troposphere and cooling the lower troposphere, and then forces it to move with the system. On the other hand, the movement of a low to midlevel thermal wave is primarily determined by adiabatic processes. Thus, the convective system tends to enhance the larger-scale baroclinicity and increase the phase lag between the pressure and thermal waves such that the baroclinic environment becomes more favorable for the subsequent surface cyclogenesis.

The role of moist convection in the surface cyclogenesis is examined by comparing simulations with and without the convective system. It is found that, in the absence of moist convection, the model also produces a surface cyclone, but with much weaker intensity, much smaller horizontal extent, and much slower displacement. The relationships of convectively generated mesovortices and wake lows to the surface cyclogenesis are also examined.

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