Terrain slope vs. flow inclination

[Sinisa Knezevic]

Dear,

How does WEng 3.1 calculate slope of terrain and flow inclination in preparation of input file for WAT 3? What is the radius of terrain for slope calculation in WEng?

Based on my experience flow inclination is expected to be lower than terrain slope as the terrain influence reduces with height, but this is not the case here.

Can calculated terrain slope (from WEng) be compared to slopes calculated for evaluating terrain complexity?

Could you please give me some explanation on this issue.


Thank you very much and kind regards,
Sinisa Knezevic

[Morten]

Hi Sinisa,


There is no radius involved in these calculations. The terrain elevation grid map is Fourier transformed as input to the flow model which is formulated in Fourier space. A computationally efficient way to calculate terrain gradients is to multiply each Fourier mode by wave number vectors and then find the gradient map by inverse Fourier transformation. Values for specific sites are found by interpolation in this grid map. The flow inclination angle are found as the angle between the vertical and horizontal wind components and it approaches the terrain slope close to the ground.


You are right that in general the flow inclination angle should decay in the vertical direction, but there can be local exceptions. The reason is that flow perturbations caused by small terrain features decay faster in the vertical direction that perturbations caused by large terrain features. There is a slide from our WEng course showing this and you can download it from http://wasptechnical.dk/Services/Redirect.aspx?token=8c8d0b3b-769d-4b4f-89aa-6e09aa4cca2b


Cheers,
Morten