3.3.2 Wind Direction
The
most important consideration in siting a wind direction sensor in complex
terrain is that the measured direction
should not be biased in a particular direction that is not experienced by
the pollutant plume. For example, instruments on a meteorological tower
located at the bottom of a well-defined valley may measure directions that
are influenced by channeling or density-driven up-slope or down-slope flows.
If the pollutant plume will be affected by the same flows, then the tower
site is adequate. Even if the tower is as high as the source's stack,
however, appreciable plume rise may take the plume out of the valley
influence and the tower's measured wind direction may not be appropriate for
the source (i.e., biased away from the source's area of critical impact).
The
determination of potential bias in a proposed wind direction measurement is
not an easy judgement to make. Quite
often the situation is complicated by multiple flow regimes, and the
existence of bias is not evident. This potential must be considered,
however, and a rationale developed for the choice of measurement
location. Research has indicated that a single wind measurement
location/site may not be adequate to define plume transport direction in
some situations. While the guidance in this document is concerned primarily
with means to obtain a single hourly averaged value of each variable, it may
be appropriate to utilize more than one measurement of wind direction to
calculate an "effective" plume transport direction for each hour.
3. SITING AND EXPOSURE
3.1 Representativeness
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
3.2.3 Precipitation
3.2.4 Pressure
3.2.5 Radiation
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
3.6 Recommendations
