Katabatic Forcing Research Articles

Derivation of Turbulent Flux Profiles and Roughness Lengths from Katabatic Flow Dynamics

The dynamics of katabatic flows have been used to determine the vertical momentum flux profiles and surface roughness lengths under stable conditions on glacier surfaces. By assuming a momentum budget balance between katabatic forcing and vertical flux divergence of horizontal momentum in the region beneath the wind maximum, it is possible to derive the vertical momentum flux profile by integrating the temperature deficit. Using the surface flux determined in this way and appropriate profile fits, the roughness length for momentum can be derived. The roughness length obtained in this way agrees well with estimates made under nonkatabatic conditions using standard log-linear fits. The katabatically determined fluxes were compared with eddy correlation measurements and with bulk methods. The eddy correlation measurements were not always in agreement with the katabatic fluxes; the comparison with bulk-derived fluxes, however, was particularly good.

Mesoscale meteorological conditions in Dronning Maud Land, Antarctica, during summer: the momentum budget of the boundary layer

The momentum budget of the boundary layer flew near the Swedish research station Svea in the mountainous region of western Dronning Maud Land, Antarctica, is evaluated using detailed concurrent observations of surface meteorological variables, high-resolution boundary layer profiles and upper-air profiles taken during the summer of 1997–98. Despite the fact that the irregular topographical situation prohibits easy interpretation of the data, interesting features can still be identified. All terms in the momentum budget (katabatic forcing, synoptic pressure gradient, Coriolis force and friction) can be of the same order of magnitude, which indicates that the influence of each term depends largely on the prevailing conditions. More precisely, the relative importance of the various momentum terms varies strongly with altitude, location, time of day and prevailing synoptic conditions. For example, at the undisturbed snow slopes there is a very regular daily variation of katabatically forced flow during the night and a flow dominated by the synoptic pressure gradient during the day, causing a persistent diurnal cycle in surface wind direction. The interpretation of the flow characteristics in terms of the momentum budget agrees favourably with the conclusions drawn on the basis of a general description of the prevailing meteorological conditions, and can therefore be considered consistent.

The Development of Antarctic Katabatic Winds and Implications for the Coastal Ocean

The influence of katabatic winds on the Antarctic coastal waters is examined by using simple models of the ocean and atmosphere. A katabatic flow model incorporating Coriolis dynamics is solved analytically and another with nonlinear friction is solved numerically to provide wind stress to a two-layer coastal ocean model. The resulting solutions are evidently the first to incorporate Coriolis terms with a thermodynamic equation that includes compressional warming effects. The emphasis in this paper is on delineating the parameters that control the relative adjustment of the katabatic wind into alongshore and offshore components. By including nonlinear friction, it is shown that steeper slopes and weaker stratification tend to direct the wind more toward the ocean. It is further demonstrated that the katabatic forcing supports the strong polar easterlies (winds from the east) along the periphery of the continent and that the offshore extent should be dependent on the atmospheric Rossby deformation radius. The ocean model shows that significant downwelling occurs at the coast, while upwelling is predicted at a distance of the order of the ocean Rossby radius. An alongshore coastal jet from the east is found in the model and is evidently the manifestation of the east wind drift. The upwelling offshore may be a significant aspect of polynya formation and maintenance of the Antarctic divergence zone and contribute to the biological productivity of the region.

Heat, momentum and moisture budgets of the katabatic layer over the melting zone of the west Greenland ice sheet in summer

The budgets of momentum, heat and moisture of the atmospheric boundary layer over- lying the melting zone of the west Greenland ice sheet during an 8-day period in summer are calculated. To do so, the governing budget equations are derived and presented in terms of ver- tically averaged quantities. Moreover, stationarity is assumed in the present study. Measurements collected during the GIMEX-91 experiment are used to calculate the contribution of the different terms in the equations to the budget. During smnmer, a well developed katabatic wind system is present over the melting zone of the Greenland ice sheet. The budgets show that advection in the katabatic layer is small for momentum, heat and humidity, when the horizontal length scale of the integration area is sufficiently large (>50 km). This indicates that in principle one-dimensional atmospheric models can be used to study the boundary layer over the melting zone of the Greenland ice sheet. The background stratification plays a crucial role in the heat and moisture budget. Vertical divergence of longwave radiation provides one-third and the turbulent flux of sensible heat the rest of the cooling of the boundary layer. Moisture is added to the boundary layer by evaporation which is a significant term in the moisture budget. Negative buoyancy (katabatic forcing) dominates the momentum budget in the downslope direction. Coriolis forcing is important, stressing the large spatial scale of the katabatic winds on the Greenland ice sheet.

Regional Drainage Flows in the Pacific Northwest

Abstract An analysis of regional drainage flows in the Pacific Northwest is presented using results from a network of surface observations and a series of simulations carried out with a nested mesoscale model. The flows, which occur regularly in southeastern Washington during the late spring and summer months, are marked by an increase in wind speed and a shift to northwesterly wind directions early in the evening. The phenomenon occurs when a deep mixed layer forms cast of the Cascade Range, drawing cooler air from the west over the mountain crest. Anabatic and katabatic forcing, terrain channeling, and turning by the Coriolis force combine to produce the characteristic flow patterns.