Observations and Numerical Simulation of Atmospheric Cellular Convection over Mesoscale Topography

Abstract

Radar and satellite images provide observations of convective rolls and other structures in the convective boundary layer (CBL), but the data are intermittent, and neither radar nor satellite gives a complete picture of roll circulation in the observed cases. As a consequence, numerical modeling is a useful complement to the observations, to investigate the temporal and spatial details of convective rolls. In this paper, observations of convective rolls over the south of England are described. Numerical simulations have been performed to investigate these rolls using the Boundary Layer Above Stationary Inhomogeneous Uneven Surfaces (BLASIUS) model, a relatively simple boundary layer code for flow over topography. The numerical results indicate that most of the features of the convective structures can be successfully reproduced, notably the roll orientation and spacing and the basic features of the cloud field. These features are in good agreement for two case studies, one with distinct rolls and the other with more dispersed convective structures and a time-dependent basic state. The model tends to predict the initial occurrence of rolls later than observed, and this time of occurrence is found to be influenced by model resolution. The presence of low topography (with maximum height on the order of 30% of the CBL depth) may have a small influence on the average orientation and spacing, and the time of initial occurrence of modeled rolls. Local flow anomalies related to the hills are much more pronounced. These anomalies appear to be related to coherent patterns in the model cloud fields, with a tendency for more cloud cover upstream and over hills, and cloud clearing in the lee as a result of descent suppressing convective eddies. When the satellite imagery is combined with topography data, this kind of orographic control of the shallow convection by the topography is evident. The CBL height varies considerably in the early stages of CBL evolution over hilly topography, but when the convection is fully developed the CBL height is almost constant over the domain.