Control of deep convection by sub-cloud lifting processes:

1. Control of deep convection by sub-cloud lifting processes: Impact on convection and variability of precipitation in a general circulation model Recently, a new conceptual framework for deep convection scheme triggering and closure has been developed and implemented in the LMDZ5B general circulation model (GCM), based on the idea that deep convection is controlled by sub-cloud lifting processes. Such processes include boundary-layer thermals and evaporatively-driven cold pools (wakes), which provide an available lifting energy (ALE) that is compared to the convective inhibition to trigger deep convection, and an available lifting power (ALP) at cloud base, which is used to compute the convective mass flux assuming the updraft vertical velocity at the level of free convection. In this study, this framework is first evaluated on specific case-studies, by comparing 1D simulations with simulations in which convection and clouds are explicitly resolved. The CRM results support the idea of a closure based on sub-cloud processes. We find a much stronger link between the cloud-base mass flux and the third-order moment of vertical velocity at LCL, namely the lifting power, than with CAPE. This favors the concept of closures based on lifting power from sub-grid sub-cloud processes rather than mean environmental properties. It is also shown that the specification of the updraft vertical velocity at the base of convective towers is a key aspect of the closure formulation, in order to make it valid for various environments. Indeed, it is weaker over tropical ocean than over land and weaker in moist mid-latitudes than semi-arid regions. To take this into account, we propose a formulation making this velocity increase with the level of free convection. Even if the representation of the sub-cloud processes still have shortcomings, the control of deep convection by thermals allows to delay the onset of precipitation to late afternoon over land, while the control of deep convection by cold pools allows to maintain it even after sunset. This way, convection is self-sustained and the diurnal cycle of convective precipitation over land is greatly improved. Over ocean, the dependency of the updraft vertical velocity on the level of free convection allows to make the closure valid in both active and suppressed conditions. Then, the impact of the new formulation on the model climate is investigated in full GCM simulations. The version of the GCM including the new set of parameterizations exhibits more realistic patterns of global precipitation and also leads to a better representation of the timing of continental convection, as illustrated for the Northern Hemisphere summer season. As a result, the intra-seasonal variability of precipitation is also profoundly modified. It now appears to be overestimated, while it was dramatically underestimated in the previous version of the model. It is noteworthy that some fundamental behavior of the model versions on constrained 1D case studies are reproduced in the full 3D version of the model. This is true here for the intensity of convective versus non convective precipitation, or for the local hour of maximum rainfall. The use of 1D case studies to improve parameterizations is thus demonstrated to be relevant for identifying parameterizations shortcomings and improving the 3D atmospheric models. Reference: C. Rio, J.-Y. Grandpeix, F. Hourdin, F. Guichard, F. Couvreux, J.-P. Lafore, A. Fridlind, A. Mrowiec, R. Roehrig, N. Rochetin, M.-P. Lefebvre, A. Idelkadi, “Control of deep convection by subcloud lifting processes: the ALP closure in the LMDZ5B general circulation model”, Climate Dynamics

2. Control of deep convection by sub-cloud lifting processes: Impact on convection and variability of precipitation in a General Circulation Model LMDZ physical components Parameterization of deep convection (Emanuel, JAS, 1991) with modified ALE triggering and ALP closure Parameterization of deep convection: New triggering and closure formulations Sub-cloud lifting processes, boundary-layer thermals and cold pools, provide: - an available lifting energy: ALE (J/kg) and - an available lifting power: ALP (W/m2) that control deep convection Parameterization of cold pools (Grandpeix and Lafore, JAS, 2010) Parameterization of thermals (Rio et Hourdin, JAS, 2008) Standard deviation of JJAS daily rainfall anomalies (mm/day) Impact on precipitation variability: - Shift of maximum rainfall over land to late afternoon - Increase of the variability of daily rainfall anomalies over ocean Convection controlled by mean instability (SP) Convection controlled by thermals and wakes (NP) C. Rio, J.-Y. Grandpeix, F. Hourdin, F. Guichard, F. Couvreux, J.-P. Lafore, A. Fridlind, A. Mrowiec, R. Roehrig, N. Rochetin, M.-P. Lefebvre, A. Idelkadi, “Control of deep convection by sub-cloud lifting processes: the ALP closure in the LMDZ5B general circulation model”, Climate Dynamics