A unified cloud-convection prognostic model for the diurnal cycle of deep convection

Predicting the diurnal cycle of deep convection is valuable for applications ranging from day-to-day weather forecasting and aviation safety to climate modelling and resource management. However, current weather and climate models often struggle to accurately capture the timing of deep convective events, frequently predicting the peak of convective precipitation and the onset of storm formation too early. This study suggests that these timing inaccuracies stem from the absence of cloud-convection interactions in many models. Such interactions represent rapid feedback mechanisms with timescales similar to the transition from shallow to deep convection within a diurnal cycle (Vraciu et al., 2024). By contrast, the typical convective parameterization schemes used by the weather prediction and climate models only incorporate interactions between convection and a uniform environment, which produces feedback mechanisms too slow to align with the diurnal cycle's timing.

To address this gap, this work introduces a unified cloud-convection model that includes both cloud-convection and convection-environment interactions, applicable to both shallow and deep convection. The proposed model comprises a set of prognostic equations for the fractional areas of different cloud types and the convective updraft velocity at varying levels. In addition, following the framework of Arakawa and Schubert (1974), a prognostic equation is included to account for the cloud feedback on the large-scale environment for each cloud type. The model is tested using idealized large-eddy simulations of the shallow-to-deep transition in a diurnal cycle, yielding promising results. Furthermore, the role of cold pools is discussed in the new proposed model, based on simulations where cold pool effects are suppressed. The prognostic model presented here may form the basis for a new class of cumulus parameterization schemes with unified cloud-convection representation and unified shallow and deep treatment.

 

References:

Arakawa, A., & Schubert, W. H. (1974). Interaction of a cumulus cloud ensemble with the large-scale environment, Part I. Journal of the Atmospheric Sciences, 31(3), 674-701.

Vraciu, C. V., Savre, J., & Colin, M. (2024). The rapid transition from shallow to precipitating convection as a predator-prey process. ESS Open Archive, DOI: 10.22541/au.170964875.54219458/v2.