Different Behaviors of Moisture and Heat Fluxes across the Terra Incognita

As mesoscale model grid spacing (Δ) becomes smaller than the height of the convective boundary layer (CBL), energy-containing turbulent eddies can be partially resolved. CBL parameterizations assume that energy-containing CBL turbulence scales (l) are much smaller than the model filter scale (Δ_f). At Δ_f ~ l, CBL schemes cannot compensate for partially-resolved CBL eddies. This issue is known as the “terra incognita” (Wyngaard 2004) or “grey zone”.

Recently, new CBL schemes that allow for the contributions of partially-resolved eddies to be reduced as a function of Δ have been proposed (e.g. Shin and Hong 2013, Shin and Hong 2015). These schemes use results from homogeneous quasi-steady state Large Eddy Simulation (LES) results filtered to multiple coarser grid resolutions to determine how normalized heat fluxes should be limited to compensate for partially-resolved eddies. Results of these studies were shown to be promising for dry mesoscale simulations of horizontal convective rolls that used this new parameterization method.

In this work, we look to investigate the applicability of this type of scaling to moisture fluxes which exhibit more passive behavior than heat fluxes. Jonker et al. (1999) and de Roode et al. (2004) both showed that passive scalars can exhibit energy-containing variations on the order of the mesoscale. These variations arise due to bottom-up and top-down fluxes of these passive scalars from the surface and the entrainment zone, respectively. By simulating a homogeneous CBL using the Weather Research and Forecasting (WRF) model configured as LES and including moisture, we investigate how heat and moisture fluxes differ. In addition, by including a diurnal cycle through time-varying surface fluxes, we also investigate how the diurnal cycle plays a role in varying these fluxes.

Results show that normalized heat flux profiles and scaling tend to reach a quasi-steady state throughout the simulation. However, normalized moisture fluxes do not, varying considerably with the diurnal cycle. It will be shown that these differences are attributable to differences in surface to entrainment flux ratios for moisture and heat fluxes, and may provide insight for better parameterizations of CBL moisture in the “terra incognita”.