Driving Rain Sum Graph

[catodiaz]

Hi:

How do you calculate the values in the graph of Driving Rain Sum [mm/a] in the "Analyze" section of the "Weather Data"?. Using the same climate data of the WAC file, I am trying to replicate the graph in excel, the trends in prevailing orientations are similar the but the shape of the loop is never the same. For different climates, the loops in WUFI are always round, regular close-to-elliptical shapes. I have combine wind speed and precipitation factors in different ways, but I obtain irregular loops with peaks at certain orientations every time.

Do you use a particular equation different from the classic: Driving Rain=Coefficient*Wind speed*Horizontal Rain?

I will appreciate your help

[Christian Bludau]

Hello catodiaz,
the amount of driving rain is calculated according to our model as follows:

R_wdr = R_h * ( R1 + R2 * V_10 * cos(theta) )

with
R_wdr: Amount of Driving Rain
R_h: normal rain (on horizontal surface)
R1: driving rain coefficient (depends on inclination)
R2: driving rain coefficient (depending on wind velocity) *V_10*cos(theta))
V_10: velocity of wind 10m over ground
theta: angle between wind direction and surface normal

If I remember right, for the R2 0.2 is used in the WUFI diagram.

Christian

[Thomas]

I obtain irregular loops with peaks at certain orientations every time.

Hi catodiaz,

in the WUFI-style driving rain diagrams you can never have single peaks. Please note that these are not "climate diagrams" showing the directional distribution of the driving rain. They are "driving rain load diagrams" (or whatever you wish to call them) showing how much driving rain hits facades with various orientations.

The difference is this:

Suppose at your location any driving rain always comes strictly from the west. A diagram showing the directional distribution of driving rain would have one large peak pointing west, and zero rain in all other directions.

However, this strictly western driving rain creates a non-zero driving rain load on _all_ facades facing the western half of the horizon (S - SW - W - NW - N). The maximum rain load would be on a facade facing due west, with decreasing rain loads for facades facing north of west, or south of west. The decrease for non-west orientations is described by the cos(theta) factor in the formula posted by Christian. Due to the properties of the cosine function, the plot describing this situation in a driving rain load diagram is a circle.

So each single (larger or smaller) peak in a climatic driving rain diagram creates a (larger or smaller) circle in WUFI's driving rain load diagram. The rain load diagrams are thus a superposition of circles of various sizes, each of these corresponding to a peak in the climatic driving rain diagram. The rain load diagrams cannot have peaks.

Regards,
Thomas

[catodiaz]

Thank you very much for both responses. Together they helped me to understand.

Now I can plot these diagrams for individual months, which is why I wanted to know how the values are calculated.