On the birth rate of thermals in convective layers

Rising thermals are a fundamental component of atmospheric convection. Representing small parcels of air with relatively low density compared to their environment, thermals are known to carry a significant part of the vertical mixing in convective layers. Coherent structures in various modes of convection have been observed to consist of sets of thermals, in the form of vertical chains but also exhibiting more complex spatial structures. The recent focus on the spatial organization of convection has renewed interest in this topic, with meteorological field experiments providing new insights into thermal behavior. However, key aspects of thermal life cycle and population statistics remain unknown. To fill this data gap, this study tracks thermals in large-eddy simulations of a diurnal cycle of convection over land. Based on measurements on 30 August 2016 during the Hi-SCALE field campaign at the ARM SGP site, the case features a shallow convective boundary layer transitioning into deep convection during the late afternoon. Adopting previously proposed tracking algorithms, hundreds of thousands of thermals are thus identified and analyzed. Apart from investigating thermal life-cycle statistics and their diurnal evolution, a key research objective is to gain insight into what controls the birth rate of such thermals. Good scaling of thermal birth rates with various integrated buoyancy scales is reported, distinguishing between various layers and various thermal classes. The birth rate of dry thermals in the sub-cloud layer scales well with the surface-driven Deardorff convective velocity scale. Thermal presence in the cloud layer is found to be partially driven by local buoyancy scales, but is significantly boosted by thermals rising into the cloud layer during the shallow convective phase. This surface coupling disappears during the transition to deeper convection in the late afternoon, after which thermal birth rates are generally lower but scale well with the cloud layer buoyancy flux. The implications and potential use of these results for the conceptual modeling of convective organization and its representation in larger-scale circulation models are briefly discussed.