PNNL

Abstract

Variability of atmospheric properties in a convective boundary layer occurs over a wide range of spatial scales. Using large-eddy simulations (LES), we examine how the distribution of variability across scales is affected by inhomogeneity of surface properties and external forcing, as well as LES domain size and grid spacing. Two different model setups are analyzed. A semi-idealized configuration uses a periodic domain, at surface, prescribed homogeneous surface heat fluxes, and horizontally uniform profiles of large-scale advective tendencies. A nested LES setup employs a larger domain and realistic initial and boundary conditions, including an interactive land surface model with representative topography and vegetation and soil types. Subdomains of an identical size (~10 km) are analyzed for all simulations. Characteristic structure sizes are quantified using the variability scales L50 and L95, defined such that features smaller than that contain 50% and 95% of the total variance, respectively. It is shown that the progressive increase in L50 from vertical velocity to temperature and moisture structures is systematically reproduced in all simulation configurations. This dependence of L50 on the considered variable complicates development of scale-aware parameterizations for models with grid spacing in the “terra incognita". In simulations using a larger domain with heterogeneous surface properties, development of internal mesoscale patterns on scale 10's and 100's of kilometers significantly affects variance distributions inside analyzed subdomains. Sizes of boundary layer structures also strongly depend on the LES grid spacing and location of the subdomain inside a larger computational domain.