Lisø PhD Dissertation Manuscript - NTNU

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was unavailable at that time) to create a scalar-value
driving rain index, and present weather observations to
develop driving rain roses showing relative directional
dependence at different stations. Lacy, however,
stopped short of combining the two into a single index
that could be used to quantitativelycompare directional
dependence at different locations.
Multiplying directional rainfall by wind speed yields a
driving rain index that is expressed in units that are not
intuitive (Lacyused m 2 /s, for example). Lacyfound, in a
studyof 75 rainfall events over a 16-year period, that
1m 2 /s corresponded to 0.206 m (or 206 mm) driving rain
on a vertical surface [7]. We have adopted this
conversion factor in the presentation of our driving
rain data below. Directional driving rain totals for the
four stations are shown in Figs. 4a–d. As the directional
dependence of the mean wind speed is relativelysmall,
and the mean wind speeds at the four stations
considered here are roughlycomparable, the distributions
shown in Fig. 4 are qualitativelyquite similar to
those shown in Fig. 2.
Next, we used the data in Figs. 4a–d to calculate a
driving rain wall index (I y) for anydirection y (which
represents the angle between north and a line normal to
mm/yr
(a)
mm/yr
(b)
mm/yr
(c)
mm/yr
(d)
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J.P. Rydock et al. / Building and Environment 40 (2005) 1450–1458
the wall):
I y ¼ 0:206 X
vDrD cosðD yÞ; (1)
D
where I y is expressed in mm/yr, 0.206 is the conversion
factor (in s/m) from Lacy [6], discussed above, nD is the
average annual wind speed (in m/s) from direction D, rD
is the average annual rainfall (in mm) with wind from
direction D, D is the wind direction (angle from north).
The summation is taken over all angles D representing
a wind blowing against the wall (This includes the sector
from y 801 to y+801.) Note: We do not applythe 8/9
exponent to the rD term in Eq. (1), commonlydone
when calculating driving rain intensities from hourly
rain and wind data [3,4], because an exponential
dependence has not been shown to be valid for rain
data expressed as average annual values. The results
from the four stations are shown in Figs. 5a–d.
4. Comparison with earlier work
As Hoppestad’s work [2] is still the onlynationwide
driving rain compilation available in Norway, it is
Oslo
10 40 70 100 130 160 190 220 250 280 310 340
Degrees from north
Bergen
10 40 70 100 130 160 190 220 250 280 310 340
Degrees from north
Trondheim
10 40 70 100 130 160 190 220 250 280 310 340
Degrees from north
Tromsø
10 40 70 100 130 160 190 220 250 280 310 340
Degrees from north
Fig. 5. Driving rain wall index (I y (mm/yr)) vs. wind direction for the period 1974–2003 for (a) Bergen, (b) Trondheim, (c) Oslo and (d) Tromsø.