A Method for Estimating the Hydrologic Input from Fog in Mountainous Terrain

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  • 1 Atmospheric Environment Service, Downsview, Ontario, Canada
  • | 2 Department of Geography, University of Newcastle, Newcastle, New South Wales, Australia
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Abstract

A methodology for obtaining estimates of the spatial distribution of fog water volume collected by a tree canopy in complex terrain is described. The method includes assumptions about the shape and spacing of the trees, their fog water collection efficiency, the fog frequency, and the vertical rate of change of the liquid water content (LWC) within ground-based clouds.

The method was applied to a 655-km2 area surrounding Roundtop Mountain, Quebec, Canada, during a carefully selected sample period from the summer of 1993. Field measurements of fog water volume were used to estimate the cloud-base height and the rate of change of the LWC with height. Topographic data were used both as a forcing function in the wind flow model and as a means of defining the three-dimensional geometry for deposition calculations. The goal is the development of a simple model that can be used over large geographic areas.

Results of the application are presented over various domains ranging from 2 to 164 Km2 in size. Spatial variations in the wind velocity field just above the canopy were found to be related to the main terrain features (summits, ridges, and valleys). The fog water deposition rate was specified as a linear function both of terrain height above cloud base and of wind speed. Near the summit of Roundtop Mountain, variations in terrain height were more pronounced than those of treetop wind speeds. Spatial patterns of fog water deposition, therefore, strongly reflected the pattern of topographic contours, with some modifications being apparent due to spatial variations in wind speed. Calculated deposition values ranged up to 0.69 mm h−1 and were found to be typical of measured values in the literature.

Abstract

A methodology for obtaining estimates of the spatial distribution of fog water volume collected by a tree canopy in complex terrain is described. The method includes assumptions about the shape and spacing of the trees, their fog water collection efficiency, the fog frequency, and the vertical rate of change of the liquid water content (LWC) within ground-based clouds.

The method was applied to a 655-km2 area surrounding Roundtop Mountain, Quebec, Canada, during a carefully selected sample period from the summer of 1993. Field measurements of fog water volume were used to estimate the cloud-base height and the rate of change of the LWC with height. Topographic data were used both as a forcing function in the wind flow model and as a means of defining the three-dimensional geometry for deposition calculations. The goal is the development of a simple model that can be used over large geographic areas.

Results of the application are presented over various domains ranging from 2 to 164 Km2 in size. Spatial variations in the wind velocity field just above the canopy were found to be related to the main terrain features (summits, ridges, and valleys). The fog water deposition rate was specified as a linear function both of terrain height above cloud base and of wind speed. Near the summit of Roundtop Mountain, variations in terrain height were more pronounced than those of treetop wind speeds. Spatial patterns of fog water deposition, therefore, strongly reflected the pattern of topographic contours, with some modifications being apparent due to spatial variations in wind speed. Calculated deposition values ranged up to 0.69 mm h−1 and were found to be typical of measured values in the literature.

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