Limited-are a modelling of stratocumulus over South-Eastern Pacific

Abstract

Abstract. This paper presents application of the Weather Research and Forecasting (WRF) model to limited-area modeling of atmospheric processes over the subtropical southeastern Pacific, with the emphasis on the stratocumulustopped 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.

Marine Boundary Layer Clouds Associated with Coastally Trapped Disturbances: Observations and Model Simulations

“…When we compare output from the various WRF configurations to the observations, our results are similar to previous studies (e.g., Rahn and Garreaud 2010;Andrejczuk et al 2012;Chen et al 2015). Most notably, the MBL top is too low in WRF for all of the BOAS soundings.…”

“…Furthermore, the regional models display the most variability in MBL depth. Model initialization and lateral boundary conditions (LBCs) are the suggested culprits behind the pervasive underestimation in MBL height (e.g., Andrejczuk et al 2012).…”

A Neural Network‐Based Scale‐Adaptive Cloud‐Fraction Scheme for GCMs

“…These are attributed to biases in the ECMWF‐AUX data. For example, the ECMWF model tends to underestimate the inversion layer height in the eastern subtropical ocean (e.g., the southeast Pacific; Andrejczuk et al., 2012; G. Chen et al., 2015; G. Chen & Wang, 2016b), causing underestimated relative humidity in the upper cloud layer. As there is not a well‐defined quantitative relationship between the sub‐grid CF and the grid‐mean relative humidity, it is difficult to find a physical‐based method to detect these anomalous data.…”

“…These are attributed to biases in the ECMWF-AUX data. For example, the ECMWF model tends to underestimate the inversion layer height in the eastern subtropical ocean (e.g., the southeast Pacific; Andrejczuk et al, 2012;G. Chen et al, 2015;G.…”