Modeling Multiscale Interactions in Orographic Moist Processes – Convective/Non-convective and Isotropic/Anisotropic Turbulence Regime Transitions

Explicit simulation of convection requires grid spacing on the order of 100m. At these scales, turbulence is partially resolved breaking down the assumptions of planetary boundary layer (PBL) parameterizations. At coarser resolutions, increasing the model resolution or improving the cumulus scheme without considering the turbulence scheme does not necessarily result in better simulations as the interactions among resolved and parameterized physics depend on flow and stability regimes and numerical implementation. In complex terrain, orographic forcing further contributes to scale interactions by triggering gravity waves, inducing blocking and regions of stagnation, and overall modification of impinging flows.  Nogueira and Barros (2015) showed using high-resolution simulations in the Andes that the horizontal scale invariant behavior of atmospheric wind and water fields in the model is a process-dependent transient property that varies with the underlying dynamics.  They found a sharp transition in the scaling parameters between non-convective and convective conditions, which explains different scaling regimes reported in the literature for atmospheric wind, temperature, and moisture observations. Spectral slopes around 2–2.3 arise under non-convective or very weak convective conditions, whereas in convective situations the transient scaling exponents remain under 5/3 in agreement with the Kolmogorov turbulent regime accounting for the intermittency correction. Based on these results, Nogueira and Barros (2015) proposed a new sub-grid scale parameterization of clouds obtained from coarse resolution states alone. 

Here, we focus on the transition in the vertical scaling behavior between isotropic and anisotropic large-scale turbulent processes over the Southern Appalachians as representative of Middle Mountains and in contrast with the High-Mountain Andes.  The vertical structure of zonal wind kinetic energy (KE) spectra produced by WRF is studied for multiple simulations in the Southern Appalachian Mountains under weak (9-12, July 2012) and strong (12-16, May 2014) synoptic forcing for grid spacing in the range of 15 km to 0.25 km with two different microphysics schemes and four planetary boundary layer parameterizations. The results show that for the sub-kilometer grid spacing the lower layers exhibit Kolmogorov and Corrsin-Obukhov -5/3 scaling, while the higher levels approach Bolgiano-Obhukov scaling. However, at 1.25 km grid spacing, the scaling behavior at low levels exhibits a slope flatter than -5/3 that is unphysical, indicative of aliasing of small scales. Higher levels show a scaling behavior similar to the convective/non-convective transition identified by Nogueira and Barros (2015) approaching Bolgiano-Obhukov scaling. Further, it is shown that the scaling behavior of variables such as heat and moisture flux, temperature and mixing ratio statistics, and energy spectra are affected differently by grid resolution, which should have important implications for the effective implementation of “scale-aware” parameterizations. Changes in PBL parameterization result in distinctive complex vertical scaling behavior that exhibits a diurnal cycle at lower levels (< 2,000 m) in the troposphere at large scales (meso-α and larger) and up to mid-levels (< 5,000m) at small scales (meso-β and meso-γ). Topography, surface roughness, diurnal heating and cooling, and thermal stratification, and the direction of weather propagation influence the vertical scaling behavior of the horizontal velocity.

Therefore, a comprehensive analysis and scaling of flow behavior conditional on stability regime for both KE and moist processes (total water, cloud water, rainfall) is necessary to elucidate scale-interactions among different processes in the model for different grid spacing, grid geometry, and mesoscale forcing toward identifying the effective model resolution, that is the scale at which generalized scaling behavior is reproduced.

Nogueira, M., and A. P. Barros (2014), The nonconvective/convective structural transition in stochastic scaling of atmospheric fields, J. Geophys. Res. Atmos., 119, 13,771–13,794, doi:10.1002/2014JD022548.