Abstract
Abstract. This paper presents application of the Weather Research and Forecasting (WRF) model to limited-area modeling of atmospheric processes over the subtropical south-eastern Pacific, with the emphasis on the stratocumulus-topped boundary layer. The simulations cover a domain from the VAMOS (Variability of the American Monsoon Systems) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) field project conducted in the subtropical south-eastern Pacific in October and November 2008. We focus on a day where the UK's BAe-146 research aircraft encountered Pockets of Open Cells (POCs) at the very western edge of its flight track, rather than on the entire campaign as investigated in previous limited-area modeling studies. Model results are compared to aircraft observations with the main conclusion that the simulated stratocumulus-topped boundary layer is significantly too shallow. This appears to be a combination of an already too shallow boundary layer in the dataset used to provide initial and lateral boundary conditions, and the inability of the WRF model to increase the boundary-layer height. Several sensitivity simulations, applying different subgrid-scale parameterizations available in the model, a larger computational domain and longer simulations, as well as a different dataset providing initial and lateral boundary conditions were all tried to improve the simulation. These changes appeared to have a rather small effect on the results. The model does simulate the formation of mesoscale cloud-free regions that one might consider similar to Pockets of Open Cells observed in nature. However, formation of these regions does not seem to be related to drizzle-induced transition from open- to closed-cell circulations as simulated by LES models. Instead, the cloud-free regions appear to result from mesoscale variations of the lower-tropspheric vertical velocity. Areas of negative vertical velocity with minima (a few cm s−1) near the boundary layer top seem to induce direct evaporation of the cloud layer. It remains to be seen in LES studies whether the mechanism seen in the model is realistic or if it is simply an artifact of interactions between resolved and parameterized processes.
Highlights
Numerical models are the only tools that can be used to objectively predict evolution of the state of the atmosphere
SM1, red - SM2; gray points - variatiobniallitdyataofpomintes a(gsruayresmymebnoltss).show some variability of Figures 2 to 6 show profiles derived from observed variables temperature and moisture profiles, but not the boundary layer for locations 1 to 5, respec- depth
We presented an application of the off-the-shelf version 3.0 of the Weather Research and Forecasting (WRF) model to a limited-area case-study-type simulations of stratocumulus clouds over the South-Eastern Pacific during the VOCALS-REx field project
Summary
Numerical models are the only tools that can be used to objectively predict evolution of the state of the atmosphere. The model solutions depend on the initial and boundary conditions as well as on spatial and temporal resolutions, and on specific parametrizations applied in the simulations This especially applies to limitedarea modeling because of the disparity between model horizontal gridlength (typically ∼10 km) and gridlengths required to resolve boundary-layer processes, turbulent transports in particular. The effects of variable (in space and time) large-scale conditions are difficult to impose, and the interactions between small-scale (boundary-layer) processes and the largerscale dynamics (e.g., mesoscale free-tropospheric waves) cannot be considered. Such interactions can impose significant forcing on the boundary layer, and on the stratocumulus cloud in particular.
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