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 mounta
in range (Germany) are used to analyze the thermal structure and the moment
um budget associated with thermally induced slope winds. Acceleration of th
e air close to the ground is found to be directed nearly vertically downwar
d 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 def
lected by the slope surface in the down-slope direction, whereas during day
time stable stratification above the heated slope layer is required to esta
blish 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 d
etail with respect to the driving forces (buoyancy and pressure gradient fo
rce) and friction It is found that a small imbalance between forcing and fr
iction is responsible for the diurnal change in slope flow intensity. The a
long-slope components of the horizontal pressure gradient force and the buo
yancy 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 ne
glected 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 win
ds react very quickly.