MOUNTAIN METEOROLOGY FOR WIND ENERGY
Branko Grisogonoa, Kristian Horvathb, Željko Večenaja, Maja Telišman Prtenjaka
a
Department of Geophysics, Faculty of Science, University of Zagreb,
Horvatovac 95, 10000 Zagreb, Croatia, bgrisog@gfz.hr, telisman@rudjer.irb.hr, zvecenaj@gfz.hr
b
Meteorological and Hydrological Service of Croatia,
Grič 3, 10000 Zagreb, Croatia, horvath@cirus.dhz.hr
1. Introduction
Wind energy resources rely heavily on the
underlying airflow characteristics; in particular,
on the mean wind speed and its spatio-temporal
variability (e.g., Horvath et al. 2011). Since
there is hardly any horizontally homogeneous
natural
land-surface,
over
which
a
homogeneous wind field exists (not to mention
stratification effects), mountain meteorology
(MM) becomes an essential ingredient in wind
engineering. Here we skim over three chapters
of MM and show in passing our contribution to
some of these MM chapters: valley flows,
mountain waves and downslope windstorms.
Among other features, we tackle rotors and
Bora-wind pulsations (e.g., Grisogono and
Belušić, 2009). The related atmospheric
processes greatly affect wind energy resource
assessments in Croatia and elsewhere. For
example, the nature of airflow over a narrow
hill is quite different than that over a broad
plateau, even if the terrain height and other
factors are the same.
for e.g., wind energy purposes. The authors
contribute to all these three essential scientific
components.
Figure 1 shows vertical crossection of the
cross-mountain wind component for a highlynonlinear flow having its vertical Froude
number, Fr = U/(NH) = 0.6, where N and H are
the buoyancy frequency and the maximum
terrain height, respectively. Even for constant
input parameters (U, N, surface roughness,
etc.), the flow develops internal critical level(s)
under which the mountain wave breaks yielding
high wind speeds in the immediate lee of the
mountain where rotors or flow pulsations may
occur.
2. Observations, theory and simulations
Since there is no unifying theory for MM,
for all the spatial and temporal scales involved,
our knowledge and understanding of overall
mesoscale
meteorology
still
remains
fragmented and incomplete. Furthermore, we
live in our ever changing laboratory, called the
Earth’s atmosphere; hence, each measurement
is unique and not repeatable. To restate, longlasting, dedicated field measurements are
simply irreplaceable and very precious. Like in
the whole of geophysics, one needs 1) highquality measurements and field campaign data,
2) numerical simulations and 3) development of
theory. Only in this way, we may firmly
proceed
further
substantially
in
the
understanding and eventual applications of MM
Figure 1. 3D simulation of the wind U-component, from
left to right, over idealized terrain (black), for constant
input parameters; vertical Froude number Fr = 0.6, which
is also the inverse of dimensionless mountain height.
Various nonlinear models may simulate such
and more realistic, e.g., input-changing flows.
For Figure 1, MIUU model from Uppsala
University, Sweden, is deployed (e.g.,
Grisogono and Enger, 2004). From a larger
perspective, weather- and wind-systems in the
broader region of Croatia are more variable and
complicated (e.g., Horvath et al. 2008) than, for
example, in the Scandinavia where wind
engineering is probably the strongest in the
world.
3. Valley flows, mountain waves and
downslope windstorms
Systematic valley flows, or even more
simplified - slope flows, may easily reach
sustainable wind speeds over 6 ms-1; however,
these almost never go over 12 ms-1. Gap flows
and low-level jets, on the contrary, may
approach mean wind speeds over 20-25 ms-1.
While linear or weakly nonlinear mountain
waves can produce lee-side wind speeds up to ~
8 ms-1, their fully nonlinear, large-amplitude
forms may give much higher wind speeds.
Namely, severe downslope windstorms that are
related to mountain-wave breaking, including
strong-to-severe Bora, reach wind gusts that are
over 70 ms-1 (e.g., Grisogono and Belušić,
2009).
Our observations (e.g., Večenaj et al. 2010)
show that Bora-related turbulent kinetic energy
(TKE) follows closely its dissipation rate. This
perhaps means that various turbulence transport
terms have systematic contributions to TKE
that do not vary greatly over a few hours.
Moreover, we have just found out observed and
simulated T ≈ 8 min. periodicity, like
pulsations, in a Bora case upstream from Split;
this is a new result of this study. As implied by
Horvath et al. (2008), large variability in
basically all mesoscale processes occur in
Croatia, thus affecting many coastal mountaininduced circulations, and the related airflow
interplays (e.g., Telišman Prtenjak et al. 2010).
It follows then naturally that setting up a wind
energy farm in the broader region of Croatia
should be primarily governed by the local and
sub-regional airflow dynamics and its climatic
future scenarios.
4. Concluding remarks
Several aspects of mountain meteorology are
discussed in terms of data, theory and
numerical
simulations.
These
aspects,
pertaining to airflow dynamics ranging from
turbulence, slope and valley flows, low-level
jets to mountain waves and their nonlinear
forms leading to severe downslope windstorms,
should be carefully assessed when planning for
wind engineering. Bora pulsations, T ≈ 8 min,
are discovered at the mid-NE Adriatic coast.
Our future work will explore vertical
turbulent fluxes and their divergence during
Bora flows and the related detailed simulations
of the wind pulsations at the middle Adriatic
coast.
Acknowledgements
The authors acknowledge the support of the
BORA and WINDEX projects (the Croatian
Ministry of Science, Education and Sports and
UKF research funds).
5. References
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of
the
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