On the importance of a viscous surface layer to describe the lower boundary condition for temperature

Abstract

A viscous sublayer was introduced into a PBL model in order to specify the lower boundary condition for temperature. The simulated results have been compared against available observations. However, for such a comparison, some of the variables and parameters that are necessary are not known but can be deduced from observed data. In this way, surface temperature and thermal diffusivity of the soil, representative for the four-day period studied here, have been estimated from measured data. An optimized relation for the thickness of the viscous sublayer δ$\delta$ was found that includes the diurnal variation of the properties of the air flow. Including this approach in the model, simulated temperatures in the ground at different levels as well as temperature in the atmosphere agree very well with the observations. The applicability for a wider range of wind speeds was demonstrated by calculating daily maximum temperatures Tmax$T_{\text{max}}$. An analysis of long-term observations for the summer season at different operational weather stations consistently show a distinct maximum of Tmax$T_{\text{max}}$ for a 10‑m wind between 2–3 m/s, which can be explained by the interaction between the molecular transport of heat within the viscous sublayer of thickness δ$\delta$ and the turbulent heat flux.