Fourth Study Conference on BALTEX Scala Cinema Gudhjem

4. Surface wind
A closer inspection of the time series at location of the
Gotland Basin (see figure 4) shows the passage of the jet
associated with very strong surface winds. The peak value
for the 10 m wind of 23 m / s coincides with the arrival of an
IGW. For this event at t = 48 h (18.12.1999-00:00) we find
at 2 km height an IGW with high total energy (Etot = 14 m 2 /
s 2 ) associated with horizontal velocity fluctuations of U’ ∝
3.5 m / s. Zonal wind fluctuations are just phase-shifted by
90 ° to the divergence fluctuations
u’ = i / k δ,
which were indicated in the figures above. Hence, the green
lines indicate the positions of maximum wind fluctuations.
The appearant period was found to be τa = 10.4 h.
Passage
of jet &
IGW-s
Figure 4: Time series at the Gotland Basin of 10 m wind
u10 [ m / s ] It is superimposed the linear trend; the arrows
mark those IGW fluctuations indicated in the figures 2 and
3.
5. Precipitation
Heavy precipitation was also associated with IGW activity.
We found a maximum rain rate of 6 mm / 6 h appearing with
the first two IGW-s.
Figure 5 Time series at the Gotland Basin of the
accumulated total precipitation ratio Rtot [ mm ].
The fluctuations of the meridional wind u’ are just phase
shifted by 180 ° to the vertical velocity
w’ = - k / m u’
- 104 -
This means, the minima of u’ are just corresponding to
maxima of w’, which favor the convective updrafts.
Hence, the maximum precipitation should appear in
between the maxima of the wind fluctuations, which are
indicated with green arrows at figure 5. This demonstrates
how IGW-s may modulate the precipitation fields.
6. Summary
We analysed in a case study the impact of IGW-s on the
local wind and precipitation fields. We showed that wind
fluctuations of a 3.5 m / s and rain rates of 6 mm / 6 h
were related to IGW activities, which were generated by
the polar jet.
jet current
Deep convection events
Inertia-gravity waves
(phase: down; energy: up)
geostrophic
adjustment
Inertia-gravity waves
(phase: up; energy: down)
Figure 6: Schematic presentation of the link between jetinduced
IGW-s and deep convection.
For the setup of the mesoscale model a sufficient
resolution is necessary in order to reproduce IGW features
with appropriate intensity. These waves may contribute to
an improved short-term regional weather prediction. The
study of mesoscale ocean dynamics such as mixing events
or build-up of stratification also requires the adequate
treatment of the atmospheric boundary conditions.
Acknowledgement: The funding of the LEWIZ project
(07ATF31) in the frame of the German BMBF-AFO2000
programme is acknowledged. We are also thankful to the
user support groups at DKRZ Hamburg and ECMWF
Reading.
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