Katabatic winds: A field case study

Results of a case study of katabatic winds utilizing tethered balloon profiling equipment are presented and discussed. The data are unique in the detail they provide of both vector wind and potential virtual temperature throughout the flow. It is concluded from the data that the one-dimensional models so far proposed in the literature are inappropriate. Direct computation shows that the divergence of radiation responsible for cooling the air is much smaller than the surface radiation flux used in these models. The inadequacy of these models is not due to the underlying assumption of temporal invariance, but rather the implication is that the flow is significantly three dimensional. Use of surface stress as the retarding stress in the dynamical balance is also shown to be erroneous, at least so long as ambient wind does not assist the katabatic flow. The stress between the flow and the ambient air is much more important and this stress is inextricably tied in with the downslope development of the flow which is thus inferred to play a major part in the dynamics of katabatic winds. Katabatic flow is an example of an internal mixing process. The data are shown to suggest a critical gradient Richardson number of about 1/4 for the maintenance of internal mixing above slope flows.