The Impact of Different Terrain Configurations on the Formation and Dynamics of Katabatic Flows: Idealised Case Studies

Abstract

Impacts of different terrain configurations on the general behaviour of idealised katabatic flows are investigated in a numerical model study. Various simplified terrain models are applied to unveil modifications of the dynamics of nocturnal cold drainage of air as a result of predefined topographical structures. The generated idealised terrain models encompass all major topographical elements of an area in the tropical eastern Andes of southern Ecuador and northern Peru, and the adjacent Amazon. The idealised simulations corroborate that (i) katabatic flows develop over topographical elements (slopes and valleys), that (ii) confluence of katabatic flows in a lowland basin with a concave terrainline occur, and (iii) a complex drainage flow system regime directed into such a basin can sustain the confluence despite varying slope angles and slope distances.

Wind Regimes above and below a Temperate Deciduous Forest Canopy in Complex Terrain: Interactions between Slope and Valley Winds

“…Unfortunately, most eddy flux towers were not tall enough to observe the jet-maximum of drainage flow above the canopy. Only a few studies (including this study) detected that the height of maximum wind velocity could occur over the canopy [21,28,30]. However, determining the whole profile of the valley-wind in any site needs high tower or other techniques (e.g., tethersonde balloon and sodar) [29].…”

“…Highly complex terrain can be viewed as a series of valleys and slopes, with small slopes converging into small valleys or basins, which in turn merge to form larger and larger scale slopes or valleys [10,21]. In such topography, thermally driven winds may be generated at many different scales, with flow in the thin layer nearest to the ground responding to the local slope and local forcings and thicker flows responding to large-scale slopes and large-scale forcings [22,36].…”

“…For example, the height of wind velocity maximum on the low-angle (1.6°) side-wall of the floor of Utah's Salt Lake Valley occurred at 10 to 15 m height [20], but could reach 40 m in another slope in the same valley [48]. Most studies in forest sites reported that the drainage flows were restricted in the subcanopy layer, which characterized with relative open canopy and insignificant understory vegetation [8,13,21], or at the upper part of a long slope [39,40], or very gentle slope [39]. We did not detect the possible subcanopy wind maximum as many previous studies [13,28,31,41,42] due to only one observation point below canopy.…”

“…The nocturnal flows are typically down-slope and/or down-valley due to a surface temperature inversion, while the daytime airflows are normally up-slope and/or up-valley resulting from radiative heating and temperature lapse conditions [15]. On relatively simple sloping topography, the strength of the drainage flows (down-slope and down-valley winds) depends on slope gradient, distance from the crest, atmospheric stability and canopy structure [9,[16][17][18][19]; on complex topography, the strength may be influenced by interactions between slope winds and valley winds [20][21][22][23]. In typical valley terrains, as the air cools down due to a negative radiation budget on the slope, a temperature inversion layer near the ground is developed before sunset.…”

“…In typical valley terrains, as the air cools down due to a negative radiation budget on the slope, a temperature inversion layer near the ground is developed before sunset. The cool air at the sidewalls moves downward along the slope, and then these flows converge on the valley floor with the flows downward along the valley axis [21]. Because the down-slope and down-valley winds are typically in orthogonal directions in most valleys, there is often a considerable directional shear through the slope wind layer [20].…”