Estimating orographic updraft velocity

FrankHanssen — Thu, 10 Jul 2014 14:15:23 GMT

I am not an mathematician, but I would like to calculate orographic updraft (that is horizontal wind pressed upward against exposed terrain). In their article "Estimating udraft velocity components over large spatial scales: contrasting migration strategies of golden eagles and turkey vultures" Bohrer et.al (2011) presented a very good approach that I want to operationalize in ArcGIS Modelbuilder.

The value of orographic updraft velocity (Wo) can be calculated from horizontal wind speed, wind direction, slope and angle as described below in Bohrer et.al 2011.

From this I have calculated terrain aspect in modelbuilder as ATan((Z / X) / (Z / Y)) and terrain Slope as ATan(Power(Power((Z
/ X), 2) + Power((Z / Y), 2), 0.5)).

Finally the orographic updraft velocity was calculated as Wind speed (m/s) * (Sin(Slope) * Cos(Wind direction - Aspect")) as described in the article. However these calculations did not give me the expected Wo output. Neither did the slope or aspects. I have also tried to use the ESRI Slope and Aspect tools as inputs for Wo without success. Remember that aspect and wind is oriented in opposite directions (e.g. eastward aspect heading to 90 degrees and eastern wind heading from 90 degrees.

I guess for a mathematician this is not so complicated. Could anyone please help me!

Frank

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Estimating orographic updraft velocity