Table of Contents - The Atmospheric Studies Group at TRC

Section 1: Introduction
Slope Flows: Slope flows are computed based on the shooting flow parameterization of Mahrt (1982).
Shooting flows are buoyancy-driven flows, balanced by adjective of weaker momentum, surface drag,
and entrainment at the top of the slope flow layer. The slope flow is parameterized in terms of the terrain
slope, distance to the crest and local sensible heat flux. The thickness of the slope flow layer varies with
the elevation drop from the crest.
Blocking Effects: The thermodynamic blocking effects of terrain on the wind flow are parameterized in
terms of the local Froude number (Allwine and Whiteman, 1985). If the Froude number at a particular
grid point is less than a critical value and the wind has an uphill component, the wind direction is adjusted
to be tangent to the terrain.
Step 2 Wind Field
The wind field resulting from the adjustments described above of the initial-guess wind is the Step 1 wind
field. The second step of the procedure involves the introduction of observational data into the Step 1
wind field through an objective analysis procedure. An inverse-distance squared interpolation scheme is
used which weighs observational data heavily in the vicinity of the observational station, while the Step 1
wind field dominates the interpolated wind field in regions with no observational data.
The resulting wind field is subject to smoothing, an optional adjustment of vertical velocities based on the
O'Brien (1970) method, and divergence minimization to produce a final Step 2 wind field.
Introduction of Prognostic Wind Field Results
The CALMET model contains an option to allow the introduction of gridded wind fields generated by
MM4/MM5, NAM(Eta), WRF, RUC, RAMS and TAPM (or the CSUMM model) as input fields. The
procedure permits the prognostic model to be run with a significantly larger horizontal grid spacing and
different vertical grid resolution than that used in the diagnostic model. This option allows certain
features of the flow field such as the sea breeze circulation with return flow aloft, which may not be
captured in the surface observational data, to be introduced into the diagnostic wind field results. An
evaluation with CAPTEX tracer data indicated that the better spatial and temporal resolution offered by
the hourly MM4 fields can improve the performance of the dispersion modeling on regional scales (EPA,
1995).
If the prognostic wind data are used as the initial guess field, the coarse grid scale data are interpolated to
the CALMET fine-scale grid. The diagnostic module in CALMET will then adjust the initial guess field
for kinematic effects of terrain, slope flows and terrain blocking effects using fine-scale CALMET terrain
data to produce a Step 1 wind field. A second approach is to use prognostic wind data directly as the Step
1 wind field. This field is then adjusted using observational data, but additional terrain adjustments are
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