b. Meteorological Data Fields. Input files can be in these different forms: (1) time variable assumes uniform meteorological conditions over relatively flat terrain; (2) height-time variable meteorology also assumes uniform flow over relatively flat terrain, but allows wind vectors and air temperatures to change as a function of height; (3) latitude and longitude-time variable meteorology also applies to relatively flat terrain but allows meteorological conditions to vary spatially and temporally, such as measured by a field of meteorological towers; and (4) xyz-time variable allows full, three-dimensional variations of meteorological conditions. For the Khamisiyah analysis, VLSTRACK used the last data type.

c. Terrain Data. VLSTRACK does not explicitly account for terrain effects because it does not generate a mass-consistent wind field internally. However, since terrain information is required when specifying meteorological data at several locations (either randomly-spaced stations or regularly-spaced grid points), the terrain is the same as the full resolution of the mesoscale weather model output. Meteorological conditions at the puff centroid are obtained by interpolating the xyz-time variable data set.

d. Mean Field Interpolation and Advection. Meteorological conditions at the puff tracking point are interpolated using the three meteorological grid locations closest to the interpolation point, followed by vertical interpolation. The model obtains the downwind and crosswind puff centroid transport by simple forward differencing:

If the wind speed at the representative puff transport height is less than 0.5 m/s, VLSTRACK uses a wind meander approach, with the wind speed set to 0.49 m/s and the wind bearing set to the random wind direction , i.e.,

e. Dispersion Calculations. VLSTRACK uses the Monin-Obukhov similarity theory to determine the PBL’s governing parameters and describes stability in terms of the Pasquill-Gifford (PG) stability classes. In addition to the temporally and spatially variable wind fields, VLSTRACK uses meteorological model-produced friction velocity and roughness lengths to update stability parameters and perform dispersion calculations. In addition, VLSTRACK uses the heat flux calculated by COAMPS to determine the Monin-Obukhov length and the Golder (1972) nomogram to determine the PG class based on the Monin-Obukhov length and the roughness length.

If the meteorological model does not provide friction velocity, u_*, it can be determined from the wind speed, u_r, at the lowest measurement (or model prediction) height, z_r, the Monin-Obukhov length, L, and the roughness length, z₀, based on this similarity theory.

VLSTRACK assigns the height of the mixing layer, z_r, from Table A-6 (Saucier, 1987):

The model assumes the standard deviations of turbulent velocity fluctuations, denoted by _{u, v}, and _w, are functions of the friction velocity, u_*.

These formulas were adapted from Hanna et al. (1982):

Puff diffusion parameters (²) are based on Taylor’s statistical theory. VLSTRACK currently uses these expressions for vapor puffs ² (Bauer, 1998):

where _u, _v, and _w are turbulent fluctuations of the u, v, and w wind components; T_u, T_v, and T_w are the Lagrangian time scales; t is the current time; and t is the integration time step. The model includes additional factors in these equations for large droplets.

f. Vapor and Liquid Fractions. VLSTRACK normally determines the initial fractions of vapor and liquid based on an energy balance for exploding munitions. For the Khamisiyah analysis, the Dugway tests defined the initial vapor and liquid fractions of the release. Because the Dugway tests also estimated the time distribution of evaporation of agent that initially spilled on wood and soil, all dispersion models treat the release as a pure vapor release.

g. Vapor Decay and Deposition. As described before, GB and GF vapor reacts with pollutants in the air during dispersal. The rate of reaction generally follows an exponential decay form. An exponential decay rate adjustment has been added to the VLSTRACK 3.0.1 (used in the 2000 modeling) vapor transport subroutine.

The version of VLSTRACK used in the 1997 modeling handled deposition by gravitational settling only. For liquid droplet clouds, VLSTRACK allows puffs consisting of droplets or particles of 10 microns or greater in diameter to impact the ground once the puff centroid comes very close to the surface. However, the model does not allow airborne GB and GF vapor to deposit. Recent studies have suggested that the ground surface will absorb the G-agent vapor at a deposition velocity in the range of 0.1 to 2 cm/s. VLSTRACK 3.0.1 used in the 2000 modeling contains an incremental vapor deposition subroutine, where deposition is accounted for through a reflection coefficient. VLSTRACK 3.0.1 assumes a reflection fraction of 0.985, which corresponds to a deposition velocity of 0.1 cm/s.

h. Tracking and Splitting Vapor Puffs. VLSTRACK originally sets the representative transport height to the average of the puff center height and the height of interest. If this average height is more than one vertical cloud sigma from the centroid height, the representative transport height is reset to one vertical cloud sigma toward the dosage output height. During the 1997 Gulf War modeling effort, VLSTRACK based puff transport on this original methodology for ground-based puff. Dispersion model results strongly depend on the near-surface algorithm due to strong wind shear near the surface. It was found that the original VLSTRACK algorithm often produces questionable results for near-surface releases, thus the following alternative formula was used for the near-surface puff transport height, z_T, in the 2000 modeling:

where _z is the vertical puff sigma, z_b is the mixed layer height, and z_c is the puff center height.

VLSTRACK may split puffs if they: (1) reach a certain size, (2) have enough mass to remain above toxic levels when split, and (3) will not create puffs exceeding the array size imposed by the model. The original puffs are removed from the computations once they are split. For the Khamisiyah analysis, the vapor puff arrays were assigned appropriate dimensions to allow enough splitting to occur to correctly represent the downwind hazard at low toxicity levels.

i. Pollutant Concentration Calculation. A Gaussian distribution approximates the concentration associated with each airborne puff. The basic equation without reflection terms is:

j. Concentration Analysis Grid. VLSTRACK uses a Cartesian coordinate system to quantify its concentration fields. The origin of the output grid is the Khamisiyah Pit location, with the x-direction heading east and the y-direction heading north. VLSTRACK uses 101 x 101 horizontal grid points for the Khamisiyah analysis. The horizontal grid increments were fixed at x = 850.36 m and y = 479.99 m. In comparison, the highest-resolution meteorological grid spacing in COAMPS was 5 km. VLSTRACK has the option of using an evolving grid, where the grid is expanded with the evolving hazard.

a. Model Basics. HPAC/SCIPUFF (Sykes et al., 1998; DSWA, now DTRA, 1997) is a Lagrangian dispersion model. As with VLSTRACK, HPAC/SCIPUFF is based on a Gaussian representation. However, in contrast to other models, HPAC/SCIPUFF applies a generalized moment representation where moments up to second-order (i.e., Q,, and _ij) are used to describe the Gaussian distribution:

HPAC/SCIPUFF solves transport equations for individual puff moments based on a linear representation for the local velocity field:

c. Terrain Data and Mass Consistency. The terrain data was used directly from the HPAC distribution CD-ROM. HPAC 3.1 assumed all surface characteristics are spatially uniform (Version 3.2 included a 1 km land-use data base). HPAC invokes either the HPAC Mass Consistent wind model (MC-SCIPUFF) or the more advanced Stationary Wind Fit and Turbulence (SWIFT) model to prepare a gridded wind field consistent with terrain. Even though SWIFT is more advanced, it allowed only a maximum of 25 profiles, whereas MC-SCIPUFF allowed a maximum of 400 profiles. Therefore, MC-SCIPUFF was used for all Khamisiyah analyses.

d. Mean Field Interpolation/Advection. The puff centroid is advected using a second-order Adams-Bashforth scheme,

Mean meteorological fields are updated every large time step, t_L, using simple linear interpolation. If (t) represents a field at time t, then the time interpolation is defined by:

where T_M is the next time step when the fields are given. HPAC/SCIPUFF enhances efficiency in calculating puff transport by implementing adaptive time stepping. Each puff uses a time step appropriate for resolving its local evolution rate, so the model does not have to use a small step for the entire calculation. HPAC/SCIPUFF uses bilinear interpolation to compute the mean wind and temperature at a puff centroid.

As implemented in HPAC 3.1 (1997 modeling), HPAC/SCIPUFF accepted only one roughness length for the entire domain. HPAC 3.2 (2000 modeling) (DTRA, 1999) includes a land-use database providing a spatially variable roughness length with a 1-km resolution.

The heat flux is then calculated from:

f. Decay and Deposition. Activity decay can apply to any material to represent a simple reduction in viable fraction or other linear decay effects. Each puff carries an activity coefficient, f_A which is initialized to unity at release time. During the transport calculation, the coefficient evolves according to the relation

where k_A is the user-specified decay rate. At each time step , the effective mass of each puff is reduced by the factor

HPAC/SCIPUFF models gaseous dry deposition as a continuous process. The user specifies the deposition velocity, V_d, as a constant. At each time step for each puff, a sink term, F_S, is calculated for inclusion in Equation A-19

where the deposition flux at the surface is calculated by integrating the product of V_d and the mean concentration over the surface

The flux Fs drives the zeroth moment equation (A-19), which is coupled with the first and second moment equations (A-20 and A-21, respectively). This modification of the moment equations is the only change to the calculations resulting from a dry deposition effect. The model retains a total reflection boundary condition at the surface, since the flux is assumed to be small.

f. Turbulence Specification. Turbulence profiles, best understood for neutral and free-convection conditions, are analytically described in terms of the primary mechanisms driving the PBL growth: the surface heat and momentum fluxes. In a neutral flow, surface heat flux effects are negligible and mechanical shear (which scales with u_*) is dominant. Free convection occurs under light wind conditions with a positive surface heat flux. In this case, turbulence scales with w_*, the convective velocity scale.

In HPAC/SCIPUFF the turbulence velocity correlation for the small scale turbulence of the PBL is represented using standard analytic shape functions

for the shear-driven component and

g. Puff Dispersion. HPAC/SCIPUFF applies a second-order closure of Reynolds-averaged equations to turbulence modeling and computes the mean and variance concentrations. This method also provides a dynamic relationship between turbulence diffusivities and velocity fluctuation statistics—the fundamental quantities of turbulence (e.g., turbulent flux correlations).

h. Probabilistic Calculations and Output. HPAC/SCIPUFF produces probabilistic output by solving the equations for the mean concentration and concentration fluctuation variance. Once the two moments (mean and variance) of the concentration fields are known, HPAC/SCIPUFF assumes the concentration probability distribution to be clipped normal, p_c, i.e., replacing all negative concentration values in a general Gaussian distribution with zero values. The mean of a clipped normal distribution is related to the mean, µ_G, and variance, _G, of a general Gaussian distribution as

The variance of a clipped normal distribution is related to m _G and s _G as

where erf is the error function. Unless otherwise noted, the HPAC/SCIPUFF results for Khamisiyah show contours for which, based on the uncertainty due to calculated turbulence levels, the probability a person standing on the contour would be exposed to a level exceeding the contour value is 1%. This tends to enlarge the area of the contour (depending on the turbulence intensity) relative to the mean contour.

i. Averaging Time. The concentration fluctuation variance predicted by HPAC/SCIPUFF depends on the velocity fluctuation statistics used to determine the dispersion rates. A meaningful quasi-deterministic prediction can sometimes be obtained by restricting the definition of the turbulent velocity fluctuations. By ignoring the large-scale meandering motions in the turbulence, a conditional average result can be generated where the uncertainty in the plume location is ignored.

Conditional average is obtained by specifying an averaging time parameter, T_av. This time scale is converted into a length scale, L _av, using the local wind speed. The averaging time should be thought of as a sampling time, but also can be considered to be the release duration if this is shorter than the sampling time. The relative dispersion result corresponds to T_av = 0. Commonly used dispersion coefficients for the PG stability class for short-range dispersion from surface releases are based on an averaging time of about 10 minutes. The ensemble average result is obtained for large T_av. The model applies a reduction factor to the velocity variances if the length scale of the velocity fluctuations is larger than the conditional average scale, which is defined as the larger of L _av and the instantaneous cloud scale.

Turbulent motions on scales smaller than the instantaneous cloud are always diffusive and should always be included in the conditional average. If the instantaneous cloud is smaller than the conditional average cloud, the averaging time determines the range of scales.

j. Vapor Puff Splitting and Merging. In HPAC/SCIPUFF, the size of a puff increases through turbulent diffusion and, in the case of inhomogeneous meteorology, through elongation in the wind shear direction. As the puff grows, the local representation of the turbulence and velocity fields at the puff centroid location becomes increasingly inaccurate. HPAC/SCIPUFF uses a grid-based method Sykes and Henn (1992) suggested to split puff into smaller components to more accurately sample the variation in meteorology. HPAC/SCIPUFF also merges puffs if their number becomes excessive, where merging rules also require moment conservation (Sykes et al., 1998).

k. Concentration Analysis Grid. HPAC/SCIPUFF adapts its puff tracking calculations to changes in spatial scale. Near the source, small spatial scales require small time scales to accurately resolve the dispersion process. As the plume moves downwind, the time scales increase. Figures A-20 and A-21 show the adaptive HPAC/SCIPUFF concentration grid, which provides enhanced resolution where required, at 2 hours and at 3.13 days after the release. The surface grid consists of cells which have sensed non-zero surface concentrations. Once a cell is identified with a non-zero concentration, HPAC/SCIPUFF includes it in dosage calculations for that and all subsequent times.

l. Model Output. HPAC/SCIPUFF describes the concentration field by summing contributions from each puff using the generalized Gaussian formula, (Equation A-15). Dosage calculations numerically integrate concentration with time and involve a summary updated at each time step.

The DoD modeling team used HPAC (version 3.1; DSWA, 1997) and VLSTRACK (version 3.0; Bauer and Gibbs, 1998) dispersion models to assess the possible hazard to US forces from an inadvertent release of nerve agent during a large-scale demolition at the Khamisiyah Pit on March 10, 1991.

Both VLSTRACK and HPAC/SCIPUFF use the COAMPS wind field; the MM5 and OMEGA fields drive HPAC/SCIPUFF only. Even though the same meteorological fields are used, the ways the dispersion models use them are different. HPAC/SCIPUFF uses a set of artificial profiles by selecting a reduced set (i.e., 400) of horizontal grid locations from the meteorological model grid. HPAC/SCIPUFF then uses the MC-SCIPUFF meteorological model to generate a mass-consistent gridded wind field based on refined surface topography. HPAC/SCIPUFF can use the data directly, and thus bypass the mass-consistency calculations, if these data are on a latitude/longitude or UTM grid. However, none of the mesoscale meteorological models used either of the grid systems. The alternative was to interpolate the profiles using the mass-consistency and achieving higher terrain resolution at the same time. VLSTRACK does not have an integrated meteorological model; its three-point interpolation scheme directly uses mesoscale meteorological fields.

Based on the similarity theory, the PBL’s mean wind and temperature profiles and turbulence are primarily functions of the surface roughness (z₀), boundary layer depth (z_i), Monin-Obukhov length (L), and friction velocity (u_*). Both HPAC/SCIPUFF and VLSTRACK use standard tables and equations to specify u_* and z₀ if they are unavailable from the meteorological model outputs. For the Khamisiyah analysis, VLSTRACK uses values consistent with COAMPS. HPAC/SCIPUFF assumed that surface characteristics are spatially uniform for the 1997 analysis. HPAC/SCIPUFF used the land cover in the HPAC LANDSCAN 30 arc-second database to derive the surface roughness, Bowen ratio, and albedo for the 2000 analysis.

VLSTRACK and HPAC/SCIPUFF calculate L and z_i quite differently. For example, VLSTRACK does not directly calculate or use surface heat flux values (H) in modeling the PBL, but uses the Golder nomogram to establish the primary link between the meteorological conditions (captured by the PG stability class) and the Monin-Obukhov stability characterization.

As described above, HPAC/SCIPUFF specifies the PBL parameters according to the calculation mode (Simple, Observation, or Calculated). For the Observation mode, the model either directly accepts the PBL parameters in the input file or calculates them based on the PG stability class. The latter approach is comparable to the VLSTRACK implementation. The Simple mode consists of very simple diurnally-variable formulae. The Calculated mode consists of detailed energy budget methods for determining the surface heat flux and prognostic equations for determining z_i. The Calculated option was always used for the Khamisiyah analyses. However, the heat flux and the mixed-layer height were provided by the meteorological models for the 1997 and 2000 analyses, thus over-riding the internal calculations of these two PBL parameters.

HPAC/SCIPUFF treats phenomena such as puff deformation and concentration fluctuation on a more rigorous theoretical basis. The equation for the concentration fluctuation provides a robust approach to producing probabilistic output. Note that the stochastic uncertainty the HPAC/SCIPUFF methodology estimates includes only contributions due to turbulent fluctuations in the atmosphere. Other sources of uncertainty such as errors in meteorological inputs and in the source term also contribute to the total uncertainty. HPAC/SCIPUFF optionally allows the specification of the meteorological uncertainty in the observational data file. However, these uncertainties were not available for input to HPAC/SCIPUFF.

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