Generic Guidance and Optimum Model Settings for the CALPUFF ...

In CALPUFF, by default, a calm period is defined as that when the puff transport speed is less
than the user-supplied threshold speed which has a current default value of 0.5 m/s. The default
calm threshold speed is used to identify periods when the transport distances are minimal, but not
zero.
In CALPUFF, several adjustments are automatically made to the normal algorithms to simulate
calm periods. These adjustments affect the way the slugs are released, the way gradual plume
rise is addressed, the way near-source effects are simulated and the way the puff size changes
during each sampling step. Conceptually, under calm conditions it is expected that a fresh release
will rise virtually straight up from the source and disperse as a function of time due to wind
fluctuations about a mean of zero.
The following adjustments are made to puffs released into a calm period
� Slugs are released as puffs, the length of the slug is zero
� All mass for the period (typically one hour) is placed into one puff
� The distance to final rise is set to zero (therefore no gradual plume rise)
� Building downwash effects are not included
� The growth of σy and σz is based on time, rather than distance traveled during the
sampling step, regardless of the dispersion option chosen by the user in the control file
� Minimum values of turbulence velocities for σv and σw are imposed.
When CALMET has been used, u* and w* may be available even when the puff transport speed is
less than the threshold, so that turbulence can be estimated. However, it is recognized that this
may not be a robust procedure if the wind data used by CALMET includes true calms, since
under these conditions estimates of turbulence velocities σv and σw can be indeterminate.
CALPUFF relies on these velocities to grow the puffs using time dependent dispersion formulas
during periods that are calms which can occur under both stable and convective conditions.
There are two ways to improve CALPUFF’s behaviour in calm conditions, the first is to use subhourly
meteorological data and the second is to use sub hourly meteorological data combined
with true measured turbulence parameters, σv and σw. This is discussed below;
3.6.1 Sub hourly meteorological data and its usage in CALPUFF
Steady state Gaussian regulatory models are traditionally limited to a one hour time step and one
hour meteorological data even though sub-hourly meteorological data is typically recorded and
stored at most Automatic weather stations around the world. Of the currently available regulatory
models CALPUFF is the only regulatory model that is able to use sub hourly meteorological and
emissions data. The consequences of this for realistically modelling calm conditions are
significant.
True calm/stagnation events seldom last longer than several consecutive hours at a time before
some instability, mechanical or convective destroys’ the event. Traditional models with their
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