Understanding and prediction of stable atmospheric boundary layers over land

The main objective of this thesis is to contribute to further understanding of the stable boundary layer (SBL) over land, and its representation in atmospheric models. A SBL develops during night due to radiative surface cooling. Observations in the SBL are difficult since many different physical processes can play a role. These processes are turbulent mixing, radiative transport, a coupling with the vegetation and underlying soil, drainage flows, gravity waves, fog, and aspects of land use heterogeneity. Therefore, the understanding and the representation of the SBL in weather forecast and climate models is relatively poor, especially for calm nights. In this thesis, a detailed column model of the atmosphere-land surface system is used to represent the atmospheric boundary layer over land and ice. As such, LES results and CASES99 field observations are used for comparison and model evaluation. It turns out that the degree of land-surface coupling plays a key role in forecasting the SBL. Also, the sensitivity of the radiation transfer model to vertical resolution is examined. It is found that the SBL can be satisfactorily modelled (except for the low-level jet (LLJ) and intermittency of the turbulence) if the geostrophic wind speed, advection, subsidence, and vegetation and soil properties are known, and if relatively high vertical resolution is used near the surface (both in the soil and in the atmosphere). The column model is used to study the nighttime 2m temperature increase due to additional CO₂ as function of wind speed. Observations show that the temperature increase is similar for windy and calm nights, although this is somewhat counterintuitive. Model results confirm that the temperature increase is indeed similar for windy and calm nights. Next an intercomparison study of limited area weather forecast models (COAMPS, MM5, HIRLAM) is performed for CASES-99. Large errors occur in the forecasted minimum temperature, SBL height and diurnal temperature range. Models that account for a realistic interaction with the land surface are advantageous. A sufficient large domain is required to forecast the LLJ, and the forecasted surface cooling is very sensitive to the choice of the radiation scheme, especially for calm nights. Overall it is possible to upgrade the model performance by using the lessons learnt with the column model. In the thesis, also the role of orographic drag on the SBL is explored. It is shown that orography of 10 m amplitude can produce drag as large as the turbulent drag. Accounting for this in a model gave sufficient cyclone filling and a better forecast of the SBL structure and LLJ. The thesis concludes with studies of the SBL height. Dimensional analysis is used to derive a new and robust formulation for the SBL height for a broad range of latitude land-use conditions.