illc incan wind s p e d . thc 'above coinbinstioii of wind s p e d and dominant
Eulerian period of oscillation implies a mean eddy diameter of 1200 m. or
slightly more than three times the height above ground. Since other investigators have found a similar ratio (Pasquill. 1962), the '*rural" Eulcrian
spectral-peak frequency appears fairly representative. and this is confirmed
by the location of the spectral peak at a nondimensional or reduced
frequency (nZiV. where V is wind s p e d at height Z ) of 0.15. rather close to
the mean value of 0.30 obtained by Busch and Panofsky (1968).
Figure 4 shows the composite Eulerian and Lagrangian spectra based o n
the 26 hourly intervals when the tower was downwind of the city and the
associated 14 tetroon flights passed directly over the city from the launch site
indicated in Fig. 2. The mean tetroon height of 390 m (YO m above equilibrium level) is the result of the updrafts induced by the city. but these
updrafts, relfecting a low-frequency contribution to the total variance.
should not significantly intluence either the Lagrangian or Eulerian spectralpeak frequencies. The Eulerian spectral peak is now found at a period of
0.8 min (in comparison with 5.4 min in the rural case) and the Lagrangian
spectral peak ;it a period of 10.6 min (in comparison with 14.0 min in the
rural case). A shift toward higher frequency of the Eulerian spectral peak
would be anticipated because of the twofold greater wind speed in the urban
case (13 versus 7 m sec I ) , but most of the observed sevenfold shift in dominant period of oscillation must be due to the presence of relatively smallscale eddies formed by the urban roughness elements located 5- 10 k m
upwind of the tower (Pasquill. 1972). Inasmuch as the tetroon is within such
an urban bouiidary layer for only ii short time, the derived mean fl of 1.3 is
believed to be unrepresenlative.
I n this connection we note that. with the assumption of frozen turbulence.
when the towcr is downwind of the city the mean eddy diameter at a height
of 355 m is about 300 ni. or approximately the height above ground.
However. as pointed out earlier, other investigators have found the eddy size
to be about three times the height above ground, and this strongly suggests
that the "urban '* Eulerian spectral-peak frequency is atypical. as docs the
value of0.57 for the reduced frequency of this spectral peak. It is interesting.
though, that the magnitude of the spectral peak appears fairly representative. the mean stress of4 dynes c m - 2 resulting in a value of0.7 for the ratio
of (spectral peak) vertical velocity variance and square of the friction velocily. similar to the mean value of 0.5 obtained by Busch and Panofsky.
One should itlso consider the representativeness of the Lagrangian data
obtained iit Oklnhoma City. l h c dominant period of 14 min in rural areas at
a iiieiin height of 310 ni is in good agreement with the value of 18 min
obtained iII ;I mean height of 460 m near Las Vegas. Nevada. (Angel1 et uI.,
1971) ;itid thc vulue of 16 min obtained at a mean height of 800 m at
Eulerian period of oscillation implies a mean eddy diameter of 1200 m. or
slightly more than three times the height above ground. Since other investigators have found a similar ratio (Pasquill. 1962), the '*rural" Eulcrian
spectral-peak frequency appears fairly representative. and this is confirmed
by the location of the spectral peak at a nondimensional or reduced
frequency (nZiV. where V is wind s p e d at height Z ) of 0.15. rather close to
the mean value of 0.30 obtained by Busch and Panofsky (1968).
Figure 4 shows the composite Eulerian and Lagrangian spectra based o n
the 26 hourly intervals when the tower was downwind of the city and the
associated 14 tetroon flights passed directly over the city from the launch site
indicated in Fig. 2. The mean tetroon height of 390 m (YO m above equilibrium level) is the result of the updrafts induced by the city. but these
updrafts, relfecting a low-frequency contribution to the total variance.
should not significantly intluence either the Lagrangian or Eulerian spectralpeak frequencies. The Eulerian spectral peak is now found at a period of
0.8 min (in comparison with 5.4 min in the rural case) and the Lagrangian
spectral peak ;it a period of 10.6 min (in comparison with 14.0 min in the
rural case). A shift toward higher frequency of the Eulerian spectral peak
would be anticipated because of the twofold greater wind speed in the urban
case (13 versus 7 m sec I ) , but most of the observed sevenfold shift in dominant period of oscillation must be due to the presence of relatively smallscale eddies formed by the urban roughness elements located 5- 10 k m
upwind of the tower (Pasquill. 1972). Inasmuch as the tetroon is within such
an urban bouiidary layer for only ii short time, the derived mean fl of 1.3 is
believed to be unrepresenlative.
I n this connection we note that. with the assumption of frozen turbulence.
when the towcr is downwind of the city the mean eddy diameter at a height
of 355 m is about 300 ni. or approximately the height above ground.
However. as pointed out earlier, other investigators have found the eddy size
to be about three times the height above ground, and this strongly suggests
that the "urban '* Eulerian spectral-peak frequency is atypical. as docs the
value of0.57 for the reduced frequency of this spectral peak. It is interesting.
though, that the magnitude of the spectral peak appears fairly representative. the mean stress of4 dynes c m - 2 resulting in a value of0.7 for the ratio
of (spectral peak) vertical velocity variance and square of the friction velocily. similar to the mean value of 0.5 obtained by Busch and Panofsky.
One should itlso consider the representativeness of the Lagrangian data
obtained iit Oklnhoma City. l h c dominant period of 14 min in rural areas at
a iiieiin height of 310 ni is in good agreement with the value of 18 min
obtained iII ;I mean height of 460 m near Las Vegas. Nevada. (Angel1 et uI.,
1971) ;itid thc vulue of 16 min obtained at a mean height of 800 m at