Down-valley Winds Research Articles

The effects of ambient winds on valley drainage flows

The interaction of katabatic winds with ambient winds has been investigated for an idealized valley using Clark's nonhydrostatic model. Ambient ridgetop wind speeds ranged from 0.5 to 6 m/s, and made angles with the valley axis ranging from 0 ° to 90 °: cooling of the valley was based on measured values of sensible heat fluxes taken from observations in Colorado's Brush Creek Valley. The depth and strength of the down-valley winds decreased with increasing ambient wind speeds but showed relatively little sensitivity to wind directions in the range of 10 ° to 60 ° from the valley axis. An observed inverse linear decrease of drainage depth with wind speed in a 100 m thick layer above the ridgetops was also found in the simulations for parts of the valley but not near the valley mouth. Vertical motions over the valley showed marked patchiness, and implications of this structure on valley flow dynamics are discussed.

Drainage Flow Characteristics and Inversion Breakup in Two Alberta Mountain Valleys

Abstract Wind and temperature profiles and corresponding acoustic sounder data collected in September 1982 are presented for nocturnal drainage flow and inversion breakup in two Alberta mountain valleys. The results show that the diurnal amplitude of average valley air temperatures for the Canyon Creek Valley is greater than the Elbow River Valley for four case days. The two-dimensional topographic amplification factors for both valleys show that this difference can be explained on the basis of differing valley geometry. The bottom half of the Canyon Creek Valley drainage flow had a weak inversion (1–2 K/100 m) with the top half of much greater strength (7 K/100 m). The Elbow River Valley drainage flow had an inversion strength of 2.9 K/100 m which was invariant with height. In the Canyon Creek Valley when the ridgetop winds had an up-valley component, the maximum down-valley wind speed occurs at 30%–40% of the drainage depth as opposed to 50%–60% when there is no up-valley component. There is also a line...

The development and structure of nocturnal slope winds in a simple valley

The results of an observational and modeling study of the nocturnal slope winds in a simple valley are presented. The valley was approximately 225 m deep in the region of the measurements, and featured a uniform slope angle of approximately 23 ° on one of its sidewalls. The wind and temperature structure of the katabatic flows on the valley sidewalls were measured with tower-mounted instruments, and a Doppler sodar and instruments on a tethered balloon and a 61-m tower were used to determine the atmospheric conditions near the center of the valley. The temperature structure of the slope flows was summarized by characteristic scale parameters h and δT for the inversion depth and strength, respectively. On the sidewalls 50 m above the valley floor, the inversion depths were generally smaller and the inversion strengths were weaker than they were on the sidewalls 100 m higher. These results differ significantly from those obtained over a simple slope of an isolated mountain or ridge. The down-valley winds are shown to be important in limiting the strength of the sidewall inversions. The formation of an inversion in the valley also has a pronounced effect on the structure of the slope flows. Numerical simulations suggest that the presence of adiabatic layers in the valley atmosphere is associated with decreases in the slope-flow inversion depth with increasing downslope distance. The simulations also indicate that the length scales that characterize the momentum and inversion depths behave similarly in flows down simple slopes but not in flows down the sidewalls of a valley.

The vertical structure of summertime local winds in the Wright Valley, Antarctica

Intensive observations of summertime up- and down-valley winds in a “dry” valley utilising airsondes, pilot balloons and a monostatic acoustic sounder are described. Both circulations show a distinctive layered vertical wind and temperature structure. Westerly down-valley flow is typically neutral and is characterised by strong surface winds overlain by light variable winds extending to an inversion between 2000 and 4000m in depth. Above this inversion, gradient winds prevail. This structure is similar to that of downslope winds observed elsewhere. The thermally-induced up-valley easterly flow is shown to be extremely well-developed in terms of its strength, depth and persistence. The strong surface easterly may reach 800 m in depth and usually undercuts the warmer westerly. The boundary between the two regimes is marked by an inversion. During easterly flow a surface-based, super-adiabatic layer of 100–200 m in depth is evident and is associated with weak convective activity. An intriguing aspect of the wind regime is the interaction between the easterly and westerly circulations in the valley. These are separated at the surface by a frontal zone which migrates up and down the valley. Further observational and modelling studies are recommended.

The Role of Valley Geometry and Energy Budget in the Formation of Nocturnal Valley Winds

Diurnally varying up and down-valley winds are a commonly observed feature of mountain meteorology. These winds are produced through the heating and cooling of the land surface but direct connections from the topography to the winds have been difficult to establish. A concept has been proposed which theoretically relates the energy budget and valley geometry to the rate of atmospheric cooling in the valley. The gradient of the along-valley cooling rate will then lead to an along-valley pressure gradient which provides a topographic control of the wind. The ratio of valley width to cross-section area is shown to be the critical topographic parameter which is proportional to the valley cooling rate. Net radiation and the ground heat flux are also critical to the valley cooling rate. An example is given which illustrates that this new concept can produce pressure gradients about 60% larger than the mountain-plain mechanism. Observations of wind and temperature in three valleys in Colorado which include draining and pooling valleys are consistent with the concept.

Experimental Verification Study for System for Prediction of Environmental Emergency Dose Information: SPEEDI, (III)

The performance of a three-dimensional atmospheric transport and diffusion model to simulate an accidental release of radioactivity was evaluated by data sets from field tracer experiments over a mountain and valley area. The model consists of a mass-consistent model for wind fields and a particle random-walk model for atmospheric dispersion. Model simulations of tracer concentrations were compared with observed values from eleven experiments; four were conducted in down-valley winds; two were in up-valley winds and five were in cross-valley winds. The calculated results almost agreed reasonably with observed concentration patterns. However, it became apparent that the model is subject to err from real dispersions in down-valley winds induced by nocturnal drainage flows.

Experimental verification study for system for prediction of environmental emergency dose information: SPEEDI. (III). Simulation of field tracer experiments in mountain and valley area.

The performance of a three-dimensional atmospheric transport and diffusion model to simulate an accidental release of radioactivity was evaluated by data sets from field tracer experiments over a mountain and valley area. The model consists of a mass-consistent model for wind fields and a particle random-walk model for atmospheric dispersion. Model simulations of tracer concentrations were compared with observed values from eleven experiments; four were conducted in down-valley winds; two were in up-valley winds and five were in cross-valley winds. The calculated results almost agreed reasonably with observed concentration patterns. However, it became apparent that the model is subject to err from real dispersions in down-valley winds induced by nocturnal drainage flows.

Valley winds and slope winds ? Observations and elementary thoughts

This paper presents a state-of-the-art account of valley wind research, with a bias towards a typical large Alpine valley and towards weak-gradient synoptic conditions. At the center of our attention is the quasiperiodic thermal forcing mechanism which drives the local wind system, in particular the role of slope winds and of topographic relief. Slope winds are at the small-scale end of a whole spectrum of thermally direct circulations which act to transmit the sensible heat input along the slopes to the valley atmosphere via compensating vertical motions. We surmise that the dynamics of slope winds, which react instantly to changes of the insolation or radiation balance, is characterized by local, instantaneous equilibria, rather than by conventional entrainment and boundary layer concepts. As described by Steinacker, the area-height distribution of a valley segment is a fundamental geometric factor which affords a quantitative measure of the slope area available for heat exchange, and of the air volume which must be heated or cooled. Using this concept, one can easily explain why the daily range of the valley mean temperature is, on average, more than twice as large as that of the atmosphere over the adjacent plain. This horizontal temperature contrast between plain and valley, reversing sign twice daily, builds up a corresponding pressure contrast hydrostatically, thereby causing up- and downvalley winds.

Palsas and Other Permafrost Features in the Lower Rock Creek Valley, West-Central Alberta

Three types of permafrost feature occur in a nearly flat alluvial valley bottom in the Foothills of west-central Alberta. (1) Frozen mineral soil underlies wet sedge meadows and fluctuates in lateral extent from year to year. (2) A number of palsas with cores of mineral soil have developed in sedge/willow fens since 1952, and others seem to be actively forming. (3) Frozen soil exists in mounds under spruce trees on an abandoned and subsiding levee, but it appears to be a consequence rather than a cause of those landforms. A fortuitous combination of environmental features permits permafrost to exist in this site, as an outlier far south of the discontinuous permafrost zone. The valley bottom possesses the prerequisite saturated fine materials for ice segregation and palsa formation, and few floods occur to thaw or bury permafrost. Strong down-valley winds ensure little winter snow

Micrometeorological study of an urban valley

Winds, temperatures, and carbon monoxide concentrations were measured in a cross-section of the North Saskatchewan River Valley in central Edmonton on a clear October evening with cross-valley winds. The evolution of a complex asymmetrical valley inversion with vertical temperature gradients up to 12C (100 m)−1 on the north-facing slope and 6C (100 m)−1 on the south-facing slope is described. The inversion is accompanied by downslope winds of about 0.4 m s−1 and a reversal from upvalley to downvalley winds. Carbon monoxide concentrations on the south-facing slope exhibit a well-defined evening maximum and an immediate response to the reversal from upslope to downslope winds.

Plume Dispersion in a Mountainous River Valley during Spring

The dispersion of hot plumes emitted from a smelter complex located In the Columbia River Valley, British Columbia, was evaluated under stable and neutral conditions during two mornings In spring. Spatial measurements of SO2 and temperature within the plume were obtained by immersion probing using fast response helicopter and automobile mounted Instrumentation. In addition, meteorological measurements of vertical wind and temperature profiles at, and downwind from, the smelter were obtained from minisonde balloon releases. With weak down-valley winds, it was found that the plume axis elevations were generally lower during both stable and neutral conditions than would be predicted by Briggs plume-rise formulae. In contrast, plume dispersion, although confined in the horizontal by the steep valley walls during both stability regimes, was significantly enhanced by exceptionally good lateral mixing, particularly close to the source.

Observations of vertical atmospheric structure in a deep mountain valley

A tethered balloon sounder was used to collect vertical temperature and wind structure data in the Gore River Valley of Western Colorado during December, 1975. Observations taken on a clear morning in which a deep inversion was initially present in the valley showed that the inversion top descended at a steady rate of ∼120 m hr−1. reaching the valley bottom after approximately 4 hours. Weak down-valley winds were present within the inversion layer while stronger up-valley winds prevailed above. A hypothesis is presented to account for these observations. A case study is presented for afternoon and evening cooling in which a ground-based inversion developed to a depth of 175 m in less than 2 hours. Winds within the inversion became decoupled from the synoptic-scale winds and remained very weak during the night. The effect of cloud cover during a morning heating cycle was to make the temperature soundings approach isothermal while sensible heating continued throughout the valley volume.

Slope and Valley Winds in the Columbia River Valley *

The behavior of slope and valley winds near the town of Trail in the Columbia River valley system in southern British Columbia was studied, and the results compared with those obtained from similar studies made in the Alps. The hourly wind velocities for two stations—one in the main valley, the other in a side valley—were resolved into components parallel to and at right angles to the valley. This procedure gives the valley and slope winds respectively. The usual diurnal wind pattern was found to prevail in most instances, with up-slope and up-valley winds during the day, and down-slope and down-valley winds at night. These systems were much better developed during the summer than in winter, and approximated the ideal pattern more closely on sunny days when the overall pressure gradient was weak.