Evaluation of stratocumulus cloud prediction in the Met Office forecast model during VOCALS-REx

Abstract

Abstract.Observations in the subtropical southeast Pacific obtained during the VOCALS-REx field experiment are used to evaluate the representation of stratocumulus cloud in the Met Office forecast model and to identify key areas where model biases exist. Marked variations in the large scale structure of the cloud field were observed during the experiment on both day-to-day and on diurnal timescales. In the remote maritime region the model is shown to have a good representation of synoptically induced variability in both cloud cover and marine boundary layer depth. Satellite observations show a strong diurnal cycle in cloud fraction and liquid water path in the stratocumulus with enhanced clearances of the cloud deck along the Chilean and Peruvian coasts on certain days. The model accurately simulates the phase of the diurnal cycle but is unable to capture the coastal clearing of cloud. Observations along the 20 • S latitude line show a gradual increase in the depth of the boundary layer away from the coast. This trend is well captured by the model (typical low bias of 200 m) although significant errors exist at the coast where the model marine boundary layer is too shallow and moist. Drizzle in the model responds to changes in liquid water path in a manner that is consistent with previous ship-borne observations in the region although the intensity of this drizzle is likely to be too high, particularly in the more polluted coastal region where higher cloud droplet number concentrations are typical. Another mode of variability in the cloud field that the model is unable to capture are regions of pockets of open cellular convection embedded in the overcast stratocumulus deck and an example of such a feature that was sampled during VOCALS-REx is shown.

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

“…This is consistent with several previous investigations, such as Wyant et al (2010), Abel et al (2010), Yang et al (2011. The observed top of the approximately well-mixed boundary layer is between 1 and 1.5 km, but the model predicts the depth of between 0.5 and 1 km.…”

“…Although there was a significant variability of the observed profiles (the largest for the horizontal velocity components within the boundary layer), the variability does not explain the systematic differences in the boundary layer height between the observations and simulations. Overall, significant underprediction of the height is consistent with previous limited-area simulations of cloud-topped marine boundary layer (e.g., Wyant et al, 2010;Abel et al, 2010;Yang et al, 2011;Wang et al, 2011). In our case, the underprediction results from a combination of two factors.…”

“…Model simulations indicate that drizzle can trigger POC formations and subsequently accelerate this process by aerosol depletion, thus pointing to the importance of cloudaerosol interactions. A recent study by Abel et al (2010) shows that a model with a relatively low spatial resolution (horizontal gridlength of 17 km) is able to create cloud-free region within the solid stratocumulus deck, although its relevance to POCs is rather questionable (as noted by Abel et al (2010) in the last paragraph of Sect. 3.6).…”

Assessing regional scale predictions of aerosols, marine stratocumulus, and their interactions during VOCALS-REx using WRF-Chem

“…MetUM simulated a good representation of synoptically induced variability in cloud cover and boundary layer depth during the VOCALS-REx (Abel et al, 2010). However, the exclusion of cloud-aerosol interactions and the model's relatively simple parameterization of cloud-microphysical effects are likely to preclude better agreement with field observations.…”

“…A recent modeling exercise by Abel et al (2010) using the UK Met Office Unified Model (MetUM) is parallel to this model evaluation study. MetUM simulated a good representation of synoptically induced variability in cloud cover and boundary layer depth during the VOCALS-REx (Abel et al, 2010).…”