The Nocturnal Boundary Layer: Model Calculations Compared with Observations

Abstract

The structure and evolution of the nocturnal boundary layer (NBL) is simulated using a model which includes the transfer of energy by radiation and turbulence. The radiation scheme is an accurate narrow band model which simulates the absorption and emission of infrared radiation by water vapor and carbon dioxide. For the transfer of energy by turbulence a model was used in which a prognostic equation for the turbulent kinetic energy is solved together with a diagnostic length scale equation for the turbulence. We adapted the ?force restore method? to account for the transfer of energy through the soil. For the vegetation layer a simple scheme was used to describe the energy flux. The performance of this combined model is compared with detailed observations of the mean thermodynamic and turbulence structure throughout the NBL (up to 200 m) for two cloudless nights in 1978. These observations were collected at the meteorological observational site near the village of Cabauw in the Netherlands. During the first night the wind speed above the NBL was approximately 10 m s?1, while during the second night it was 6 m s?1. From the comparison we conclude that the agreement between the calculated and observed profiles of turbulence and mean thermodynamic variables and between the calculated and observed evolution of the surface fluxes is satisfactory. However, the detailed structure of these profiles depends upon the local terrain inhomogeneities. Because the model simulates a horizontally homogeneous flow, these detailed structures cannot be reproduced accurately. Moreover, we find that the inclusion of radiative cooling within the atmosphere increases the calculated boundary layer height by about 25%.