The response of cumulus convection to large-scale temperature perturbations: some insights through Lagrangian particle tracking

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The response of cumulus convection to large-scale temperature perturbations: some insights through Lagrangian particle tracking Tian Yang and Zhiming Kuang Previous studies have documented that deep convection responds more strongly to above-the-cloud-base temperature perturbations in the lower troposphere than to those in the upper troposphere, a behavior that is important to the dynamics of large-scale moist flows such as the convectively coupled waves. A number of factors may contribute to this differing sensitivity, including differences in buoyancy, vertical velocity, and/or liquid water content between cloud updrafts in the lower and upper troposphere. Quantifying the contributions from these factors can help to guide the development of convective parameterization schemes. We tackle this issue by tracking Lagrangian particles embedded in cloud-resolving simulations under a linear response framework. The results show that both the differences in updraft buoyancy and vertical velocity play a significant role, with the vertical velocity being the more important, and the effect of liquid water content is only secondary compared to the other two factors. These results indicate that cloud updraft vertical velocities need to be correctly modeled in convective parameterization schemes in order to properly account for the differing convective sensitivities to temperature perturbations at different heights of the free troposphere. We further examine how updraft vertical velocity and different forces that regulate the vertical momentum budget respond to a small large-scale temperature perturbation, with the goal of informing parameterization of updraft vertical velocity. Using Lagrangian tracking, we illustrate, in a shallow cumulus case, that the effective buoyancy and dynamic perturbation pressure can be approximated to a good extent by a simple updraft model given the cloud radius. This has implications for parameterizing vertical velocity in large-scale models and developing a unified scheme for moist convections.