Influence of the Coriolis Force on Two-Dimensional Model Storms

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  • 1 Department of Atmospheric Sciences, University of Washington, Seattle, Washington
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Abstract

Two-dimensional model simulations were made to gage the effect of the Coriolis force on model squall lines. The case chosen for intensive study had low-to-moderate wind shear confined to low levels. With this wind shear, two Coriolis simulations were made, with and without a geostrophically balanced along-line temperature gradient. Additional simulations were made with other wind shear intensifies to test the sensitivity to low-level shear.

Unlike their nonrotational counterparts, none of the Coriolis model storms were able to attain or maintain a “quasi-equilibrium” state. Quasi-equilibrium storms possess mature phases characterized by essentially statistically steady behavior with respect to storm strength, propagation speed, etc. Instead, the Coriolis storms possessed mature phases marked by gradual but definite decay. These are the first model storms created with the present model sounding and wind profiles that have terminal mature phases due to physically realistic forcings. However, the time scale of the decay, at least in these cases, makes it unlikely that Coriolis forcing is the primary mechanism behind the demise of real long-lived, mature squall line thunderstorms.

In each rotational case, the decay phase was marked by two major temporal trends absent in the mature phase of the nonrotational simulations: the continued contamination of the forward environment with storm-induced subsidence warming and the decline in intensity of the rear inflow current. The subsidence warming was slowly eradicating the convective instability of the air flowing into the storm, and the dissipating inflow current appeared to be at least partially responsible for the progressive collapse of the storm's subcloud cold pool. The accumulation of subsidence warming was clearly injuring the model storm. The role that the declining rear inflow played in the decay phase is less clear and requires additional study.

It was found that the inclusion of the geostrophically balanced along-line temperature gradient had small but measurable consequences in this situation. Warm advection at low levels ahead of the storm worked to negate the effect of warm advection aloft on the convective instability, and cold advection into the cold pool opposed the general decline in pool intensity. The net effect was that the Coriolis-associated mature-phase decaying tendency was stowed somewhat, but not arrested.

Abstract

Two-dimensional model simulations were made to gage the effect of the Coriolis force on model squall lines. The case chosen for intensive study had low-to-moderate wind shear confined to low levels. With this wind shear, two Coriolis simulations were made, with and without a geostrophically balanced along-line temperature gradient. Additional simulations were made with other wind shear intensifies to test the sensitivity to low-level shear.

Unlike their nonrotational counterparts, none of the Coriolis model storms were able to attain or maintain a “quasi-equilibrium” state. Quasi-equilibrium storms possess mature phases characterized by essentially statistically steady behavior with respect to storm strength, propagation speed, etc. Instead, the Coriolis storms possessed mature phases marked by gradual but definite decay. These are the first model storms created with the present model sounding and wind profiles that have terminal mature phases due to physically realistic forcings. However, the time scale of the decay, at least in these cases, makes it unlikely that Coriolis forcing is the primary mechanism behind the demise of real long-lived, mature squall line thunderstorms.

In each rotational case, the decay phase was marked by two major temporal trends absent in the mature phase of the nonrotational simulations: the continued contamination of the forward environment with storm-induced subsidence warming and the decline in intensity of the rear inflow current. The subsidence warming was slowly eradicating the convective instability of the air flowing into the storm, and the dissipating inflow current appeared to be at least partially responsible for the progressive collapse of the storm's subcloud cold pool. The accumulation of subsidence warming was clearly injuring the model storm. The role that the declining rear inflow played in the decay phase is less clear and requires additional study.

It was found that the inclusion of the geostrophically balanced along-line temperature gradient had small but measurable consequences in this situation. Warm advection at low levels ahead of the storm worked to negate the effect of warm advection aloft on the convective instability, and cold advection into the cold pool opposed the general decline in pool intensity. The net effect was that the Coriolis-associated mature-phase decaying tendency was stowed somewhat, but not arrested.

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