Diurnal momentum budget analysis of thermally induced slope winds

Data measured during the TRACT field campaign at various stations along a 7.5 degrees steep and west-northwest facing slope of the Black Forest mountain range (Germany) are used to analyze the thermal structure and the momentum budget associated with thermally induced slope winds. Acceleration of the air close to the ground is found to be directed nearly vertically downward during the night and nearly vertically upward during the day, rather than parallel to the slope. This means that during the night the airflow is deflected by the slope surface in the down-slope direction, whereas during daytime stable stratification above the heated slope layer is required to establish upslope flow parallel to the slope. The diurnal cycle of the momentum budget of the along-slope wind component near the surface is analyzed in detail with respect to the driving forces (buoyancy and pressure gradient force) and friction It is found that a small imbalance between forcing and friction is responsible for the diurnal change in slope flow intensity. The along-slope components of the horizontal pressure gradient force and the buoyancy force are shown to have the same order of magnitude. This means that for small to moderate slope angles the pressure gradient force cannot be neglected as is done in some analytical slope wind models. The reaction time of the slope flow to changes in forcing is estimated to be in the range of 30 to 120 seconds. which confirms the empirically known fact that slope winds react very quickly.