Thermodynamic constraints on the sensitivity of boundary layer clouds to land surface flux partitioning

We combine multiple modeling techniques to examine the atmospheric leg of land-atmosphere coupling. Our premise is that the daytime boundary layer over land should reflect, in its development, the surface heat and moisture fluxes. Details of the vertical profiles of temperature and humidity precondition the atmosphere to respond more favorably to sensible or latent (or a certain ratio of) heat fluxes in terms of cloud formation.

Large eddy simulation (LES) cases have been performed for 92 warm-season days over a periodic 100x100km domain with both realistic prescribed heterogeneous and average homogeneous lower boundary conditions, including specified diurnal variations of latent and sensible heat fluxes driven by analyses over the Southern Great Plains (SGP) region of northern Oklahoma and southern Kansas. Over the same location, seven years of single column model (SCM) simulations with the NCAR Community Atmospheric Model cover the period May-September. Using the SCM, the impact of surface heterogeneity on the daytime evolution of the vertical atmospheric profile is simulated via an ensemble of simulations with the same diurnal time series of specified total surface heat flux to the atmosphere, but a range of partitioning of the total heat into sensible and latent fluxes quantified by evaporative fraction (EF).

Additionally, the heated condensation framework (HCF) is applied hourly to vertical profiles from both the suite of SCM simulations and the daily highest and lowest EF locations from the LES. Given an atmospheric profile, the HCF’s “better than parcel theory” approach to simulate the impact of surface heat and moisture fluxes on a well-mixed boundary layer quantifies the energy input needed for convective triggering, as well as the optimum EF to trigger cloud with the minimum total energy investment.

This tripartite investigation using LES, SCM and HCF model results sheds light on the roles of surface heterogeneity, thermally-induced secondary circulations and surface fluxes on boundary layer development and cloud formation. HCF is also tested as a diagnostic tool to predict time of convection, and where (over what surface conditions) convection is most likely to form, but in the absence of any secondary circulations (i.e., as a 1-D vertical thermodynamic problem). We examine the veracity of HCF in these situations as a diagnostic tool, to examine if it is useful to solve the inverse problem: to infer difficult-to-measure surface heat fluxes over land from the development of the daytime boundary layer, without using computationally expensive dynamical models of the atmosphere.