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Radiative cooling in the nocturnal boundary layer

S.A. Tjemkes

In this thesis the transfer of infrared radiation (electromagnetic waves with a wavelength between 3.6 and 100 μm) through a cloudfree nocturnal boundary layer is studied. To simulate the transfer of infrared radiation an accurate narrow band model which simulates the absorption and emission of longwave radiation by water vapor and carbon dioxide is described. The model is tested against 44 cases of surface downward fluxes as well as against independent calculations. From these two comparison studies it is shown that the narrow band model simulates the surface downward flux with 3 W/m2 and the cooling rate within 0.2 K/day.
To analyse the interaction between turbulence and infrared radiation in a horizontally homogeneous cloudless nocturnal boundary layer, the radiation scheme is combined with a turbulence model, in which a prognostic equation for the turbulent kinetic energy is solved together with a diagnostic length scale equation for the turbulence. The performance of this combined radiation turbulence model is compared with detailed observations of the mean thermodynamic and turbulence structure throughout the nocturnal boundary layer.
From this comparison study it is shown 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.
Moreover it is shown that the inclusion of radiative cooling within the boundary layer increases the boundary layer height by about 25%.
The combined radiation-turbulence model is also used to study the influence of the geostrophic wind velocity on the surface minimum temperature. From these model calculations a parametrization scheme is derived which is able to reproduce the surface minimum temperature over an almost saturated vegetated clay soil within 2.5 K.
The accuracy of the narrow band model used in the above described studies, is at the expense of a considerable amount of computer time. Thus the narrow band model is not recommended for use on a routine base in a boundary layer forecast model.
Therefore an alternative radiation code for use in boundary layer models which is presented, which is accurate as well as efficient.

Bibliografische gegevens

S.A. Tjemkes. Radiative cooling in the nocturnal boundary layer
KNMI number: WR-88-05, Year: 1988, Pages: 107

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