Diurnal momentum budget analysis of thermally induced slope winds

Citation
M. Kossmann et F. Fiedler, Diurnal momentum budget analysis of thermally induced slope winds, METEOR ATM, 75(3-4), 2000, pp. 195-215
Citations number
50
Categorie Soggetti
Earth Sciences
Journal title
METEOROLOGY AND ATMOSPHERIC PHYSICS
ISSN journal
01777971 → ACNP
Volume
75
Issue
3-4
Year of publication
2000
Pages
195 - 215
Database
ISI
SICI code
0177-7971(2000)75:3-4<195:DMBAOT>2.0.ZU;2-J
Abstract
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.