Sub-kilometer numerical predictions in the nocturnal stable boundary layer

Numerical studies have been conducted using the WRF-ARW mesoscale model at sub-kilometer resolution over the complex ridge and valley topography of central PA. The objective is to learn the modeling requirements for predicting plume transport and dispersion in the nocturnal stable boundary layer (SBL). Under weakly forced, very stable conditions, the SBL is often only tens of meters in depth, leaving the physics and resolutions of most mesoscale models ill-designed to simulate winds, temperatures and turbulence. In the present study we hypothesize that the higher-order numerics and minimal diffusion of the WRF-ARW mesoscale model might allow simulation of the principal characteristics of SBL flow, perhaps including at least the statistics of mesogamma-scale wind variance.

A version of WRF-ARW using a 0.444-km nested grid and very fine vertical resolution is run daily and verified against special local wind observations on a network of 10-m towers in the Nittany Valley of Pennsylvania. Evaluations conducted in autumn 2007 and early summer 2008 show considerably smaller errors on the sub-kilometer grid compared to a coarser 1.333-km grid. Errors are also reduced when near-surface vertical resolution is increased, up to using as many as 10 layers below 50 m above ground level (AGL). Scale decompositions indicate that the mesoscale wind variations are predicted well in the deterministic sense, while statistical characteristics of the higher-frequency sub-mesoscale variability may be predictable for at least some cases. Evidence from WRF indicates the chief sources of near-surface wind variance in the SBL are terrain-induced drainage and transient internal gravity waves in the flow aloft. These lead to meandering-wind patterns in the SBL, which are revealed both in parcel trajectories and in plume predictions calculated using the HPAC-SCIPUFF dispersion model.