The Effect of Large-Scale Convergence on the Generation and Maintenance of Deep Moist Convection

N. Andrew Crook The National Center for Atmospheric Research,Boulder, Colorado

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Mitchell W. Moncrieff The National Center for Atmospheric Research,Boulder, Colorado

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Abstract

The effect of large-scale convergence on the generation and maintenance of deep moist convection is examined with a numerical cloud model. The term large-scale is defused as a scale an order of magnitude greater than the convective scale. Convergence is included in the model by imposing a momentum forcing at the lowest levels of the domain. The simulations are initialized with the linear response to this forcing.

It is shown that large-scale convergence has an important effect both before and after the generation of convection. Before convection commences, the convergence lifts the atmosphere to saturation, or close to saturation, over a wide region. This means that once convection begins, the air entering the system requires little further lifting and it is demonstrated that this allows the system to maintain itself without the additional lifting that evaporative cooling produces. This is in contrast to a system which is generated in an unsaturated environment by an initial warm bubble perturbation that is often critically dependant on the lifting at low levels produced by evaporative cooling.

Large-scale convergence also has an important cited on the mature convection, with the average rainfall rate decreasing by approximately 40% when the convergence is removed. It is also shown, for systems in which the convective time scale is comparable to or larger than the time that air spends in the convergence zone, that the total vertical displacement of air in the convergence region is the important parameter in determining convective intensity.

Finally, some features of the convective structure are discussed. For the first two hours convective activity, the low-level flow has the structure of a gravity wave in that air passes through the system and remains near the surface. The amplitude and structure of this gravity wave, which is primarily forced by evaporative cooling and low-level waterloading, is compared with the predictions of linear theory. Another feature described is the formation of convective cells some 10–15 km ahead of the main updraft and the low-level cold pool.

Abstract

The effect of large-scale convergence on the generation and maintenance of deep moist convection is examined with a numerical cloud model. The term large-scale is defused as a scale an order of magnitude greater than the convective scale. Convergence is included in the model by imposing a momentum forcing at the lowest levels of the domain. The simulations are initialized with the linear response to this forcing.

It is shown that large-scale convergence has an important effect both before and after the generation of convection. Before convection commences, the convergence lifts the atmosphere to saturation, or close to saturation, over a wide region. This means that once convection begins, the air entering the system requires little further lifting and it is demonstrated that this allows the system to maintain itself without the additional lifting that evaporative cooling produces. This is in contrast to a system which is generated in an unsaturated environment by an initial warm bubble perturbation that is often critically dependant on the lifting at low levels produced by evaporative cooling.

Large-scale convergence also has an important cited on the mature convection, with the average rainfall rate decreasing by approximately 40% when the convergence is removed. It is also shown, for systems in which the convective time scale is comparable to or larger than the time that air spends in the convergence zone, that the total vertical displacement of air in the convergence region is the important parameter in determining convective intensity.

Finally, some features of the convective structure are discussed. For the first two hours convective activity, the low-level flow has the structure of a gravity wave in that air passes through the system and remains near the surface. The amplitude and structure of this gravity wave, which is primarily forced by evaporative cooling and low-level waterloading, is compared with the predictions of linear theory. Another feature described is the formation of convective cells some 10–15 km ahead of the main updraft and the low-level cold pool.

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