walmsley_wind  (09/12/07)

walmsley_wind (Walmsley et al., 1989)

This module is defined in Classwalmsley_wind and is parametric version of windflow model of Mason and Sykes (1979), referred as MS. The MS windflow model is a computational routine for estimating windflow over three-dimensional topography and is based on Fourier transform techniques and has a division of the inner and outer flow regions. The MS windflow model is linearized and thus only applies to low hills; the model assumes neutral thermal stratification and uniform surface roughness within the simulation region. The MS model is potentially computational costly and Walmsley et al. (1989) derived a simple parametric version of the MS model for estimating wind speed variation induced by small-scale topographic features. These topographical features include 2D hills (ridges), 3D hills, 2D escarpments, 2D rolling terrain, 3D rolling terrain, and flat terrain. Users can define their simulation topographic features from coefficients ‘A’ and ‘B’.

Observations

• none.

Variables

• hru_Uadjust (m/s) - adjusted wind speed due to topography.
• hru_Uchange (m/s) - amount of change in wind speed due to topography.
• WR () - wind ratio between adjusted wind speed and reference wind speed.

Parameters

• Zwind (m) - wind instrument height.
• A () - coefficient for wind speed change due to topography. Values: 0.0 – flat terrain, 2.5 – 2D escarpments, 3.0 – 2D hills, 3.5 – 2D rolling terrain, 4.0 – 3D hills, 4.4 – 3D rolling terrain.
• B () - coefficient for wind speed change due to topography. Values: 0.0 – flat terrain, 0.8 – 2D escarpments, 1.1 – 3D rolling terrain, 1.55 – 2D rolling terrain, 1.6 – 3D hills, 2.0 – 2D hills.
• L (m) - upwind half-width at half height. It is the length from the centre of a hill with height, h to the slope with half height of the hill, h/2. It is also the length from the centre of a depression with depth, h to the slope with half depth of the depression, h/2.
• obs_elev (m) - measurement altitude.
• hru_elev (m) - altitude.

Variable Inputs

• hru_u (m/s) (obs)

Notes

Parameter L is upwind half-width at half height. It is the length from the centre of a hill with height, h to the slope with half height of the hill, h/2. It is also the length from the centre of a depression with depth, h to the slope with half depth of the depression, h/2.

Mason, P.J. and Sykes, R.I. 1979. Flow over an isolated hill of moderate slope.

Walmsley, J.L., Taylor, P.A. and Salmon, J.R. 1989. Simple guidelines for estimating windspeed variations due to small-scale topographic features – an update. Climatological Bulletin 23: 3-14.