THE CLIMATE ADJUSTMENT PROCESS - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 154

weather patterns such as the El Nino Southern Oscillation (ENSO) and local factors
such as urbanization and changing vegetation cover, must be considered.

12.1.1 Historical Evidence
Historical evidence concerning the long-term stability of the wind climate is mixed. A
key problem is that there are few wind monitoring stations where the wind has been
measured continuously at the same height and at the same location, with consistent
measurement protocols, and using the same instrument or instrument type, for more
than 10 - 15 years. The dearth of truly homogeneous long-term data sets, coupled with
the effects of normal short-term climatic fluctuations, makes it difficult to reliably
detect trends caused by long-term climate change.
Nonetheless, various researchers have attempted to detect trends in the wind over
time. In one of the most comprehensive studies, a group of European researchers
reviewed observations from hundreds of surface weather stations in the northern hemi-
sphere and from two types of modeled global weather information known as reanalysis
data (1). Almost universally, the surface weather stations show a significant down-
ward speed trend amounting to 5 - 15% over 30 years. The two reanalysis data sets,
however, show much smaller trends that vary from positive to negative depending on
the region.
Which picture is correct? The authors ascribe the trends in surface observations
in part to changes in surface roughness caused by tree growth. While this is plausi-
ble, it is also possible that many of the measurements have been affected by uneven
maintenance practices and changes in instrumentation, height, and location, as well
as the effects of encroaching urbanization, not all of which could be removed by
the QC procedures implemented by the researchers. Most such issues have no bear-
ing on wind energy resources. On the other hand, the global atmospheric models
that generate reanalysis data are driven mainly by observations from weather bal-
loons, satellites, aircraft, and other platforms. Although they have their own issues
(as discussed below), the reanalysis data sets are at least partly insulated from the
inhomogeneities afflicting surface wind observations and thus may provide a truer
picture of long-term trends.
Looking more closely at specific countries or regions can be instructive. One study,
for example, identified a significant decline in mean wind speeds recorded by surface
weather stations in the United States since 1973 (2). As the authors state, it seems likely
that the advent of the Automated Surface Observing System (ASOS) in the mid-1990s,
as well as other changes such as urbanization and tree growth, is responsible for much
of this decrease. They note, “We did not attempt to correct for these inhomogeneities,
but their presence strongly argues for use of the other data sets ” By and large,

the other data sets they mention, mainly reanalysis data, do not support the observed
decline in surface wind speeds nor do long-term observations for the same region
from weather balloons (3).
On balance, there is little reliable evidence to date that wind resources have either
increased or decreased significantly in the past several decades in most areas of the

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