The simulation of a fair weather marine boundary layer in GATE using a three‐dimensional model

A detailed three‐dimensional model has been used to simulate a cumulus‐topped tropical oceanic boundary layer. A particularly dense set of observational data from surface‐based and airborne instrumentation collected during one day of the GARP Atlantic Tropical Experiment (GATE) has been used enabling comprehensive initial and boundary conditions to be specified. This also permits detailed comparisons to be drawn between the model and observations. The model is found to reproduce the structure of the observed undisturbed boundary layer quite realistically. The clouds developed in the model are similar to those observed in both amount and physical size and the modelled turbulent fluxes in the subcloud layer agree well with observations. Also, the evolution of the modelled boundary layer is similar to that in which the observations were made. The model confirms that the major effects of the turbulent mixing are to warm the subcloud layer but to moisten and cool the cloud layer. The limited scale of the model (2 km x 2 km x 2 km) does, however, lead to systematic differences between measured and modelled variances. The model's turbulent kinetic energy balance for the coupled cloud‐subcloud layers is presented. Although reliable measurements are only available in the subcloud layer, they confirm that the model's values are qualitatively correct except when very close to the bottom boundary. The model's temperature and humidity profiles are strongly affected by mixing occurring across cloud‐base throughout much of the subcloud layer. This is reflected in the shape of the θ_v profile which displays a characteristic minimum midway through the subcloud layer. When scaled using cloudbase parameters, the resulting profiles are similar to those obtained in more unstable conditions over land.