3.3 Complex Terrain Locations
the purposes of this guidance, the term “complex terrain” is intended to
mean any site where terrain effects on
meteorological measurements may be significant. Terrain effects include:
- aerodynamic wakes,
- density-driven slope flows,
- flow accelerations over the crest of terrain features,
primarily affect wind speed and wind direction measurements, however,
temperature and humidity measurements may also be affected. The definition
of significance depends on the application; for regulatory dispersion
modeling applications, significance is determined by comparing stack-top
height and/or an estimated plume height to terrain height - terrain which is
below stack-top is classified as simple terrain (see Section 3.2), terrain
between stack-top height and plume height is classified as intermediate
terrain, and terrain which is above plume height is classified as complex
gradients and/or discontinuities in the vertical profiles of meteorological
variables are often significant in complex terrain. Consequently,
measurements of the meteorological variables affecting transport and
dispersion of a plume (wind direction, wind speed, and ) should be made at multiple levels in order to ensure that data
used for modeling are representative of conditions at plume level.
arrangement in complex terrain involves siting a tall tower between the
source and the terrain feature of concern. The tower should be tall enough
to provide measurements at plume level. Other terrain in the area should not
significantly affect plume transport in a different manner than that
measured by the tower. Since there are not many situations where this ideal
can be achieved, a siting decision in complex terrain will almost always be
Monitoring options in complex terrain range from a single tall
tower to multiple tall towers supplemented by data from one or more remote
sensing platforms. Other components of the siting decision include
determining tower locations, deciding whether or not a tower should be sited
on a nearby terrain feature, and determining levels (heights) for
monitoring. Careful planning is essential in any siting decision. Since each
complex terrain situation has unique features to consider, no specific
recommendations can be given to cover all cases. However, the siting process
should be essentially the same in all complex terrain situations.
Recommended steps in the siting process are as follows:
the variables that are needed for a particular application.
as much information as possible to define what terrain influences are
likely to be important. This should include examination of topographic
maps of the area with terrain above physical stack height outlined.
Preliminary estimates of plume rise should be made to determine a range
of expected plume heights. If any site specific meteorological data are
available, they should be analyzed to see what can be learned about the
specific terrain effects on air flow patterns. An evaluation
by a meteorologist based on a site visit would also be desirable.
alternative measurement locations and techniques for required variables.
Advantages and disadvantages of each technique/location should be
considered, utilizingas a starting point the discussions presented above
and elsewhere in this document.
network design by balancing advantages and disadvantages.
is particularly important in complex terrain to consider the end use of each
variable separately. Guidance and concerns specific to the measurement of
wind speed, wind direction, and temperature difference in complex terrain
are discussed in the following sections.
3. SITING AND EXPOSURE
3.1.1 Objectives for Siting
3.1.2 Factors to Consider
3.2 Simple Terrain Locations
3.2.1 Speed and Wind Direction
3.2.2 Temperature, Temperature
Difference, and Humidity
3.3 Complex Terrain Locations
3.3.1 Wind Speed
3.3.2 Wind Direction
3.3.3 Temperature Difference
3.4 Coastal Locations
3.5 Urban Locations