18. Modelling Enhancements

User Guide MAIN MENU

Input Parameters for the MET File

The following parameters in the 'casename'.MET file are required if the user decides to use the meteorological processor to calculate the Monin-Obukhov length and the friction velocity, and the equivalent Pasquill/Gifford stability class. The procedure overrides the stability class specified in the main input files for various modules (see Chapter 9 of the HGSYSTEM 3.0 Technical Reference Manual). The file is read with free-format. There must be one entry per line. All data must be given in the specified order. There are no default values assumed by the model for the parameters.

Line No.	Real/ Integer	Remarks
1	R	Site latitude in degrees (positive for north)
2	R	Site longitude in degrees (positive for east)
3	R	Time zone (e.g., 5.0 for EST, 8.0 for PST)
4	I	Year (e.g., 1994)
5	I	Month (e.g., 10 for October)
6	I	Day
7	R	Hour (e.g., 1.5 = 1:30 AM)
8	R	Albedo with sun directly overhead, see below for more instructions
9	R	Moisture availability of the soil, see below for more instructions
10	R	The lower limit in meters for the Monin-Obukhov length during stable conditions, see below for more instructions
11	R	Anthropogenic heat flux, W/m2, (usually 0.0 for rural areas)
12	R	Ratio of ground heat flux to net radiation, see below for more instructions
13	R	Fractional cloud cover (0. = clear, 1. = overcast)

The reader is referred to Hanna and Chang (1991, 1992) for a detailed description of the choice of site characteristics (i.e., albedo, moisture availability, the lower limit on the Monin-Obukhov length, anthropogenic heat flux, and ratio of ground heat flux to net radiation). Some instructions are repeated below.

Suggested input values for albedo (Iqbal, 1983) as a function of land use type and season are given in the following table. Further information regarding albedo for specific ground covers is given by Iqbal (1983).

Land Use Type	Spring1	Summer2	Autumn3	Winter4
Water (fresh water and sea water)	0.12	0.10	0.14	0.20
Deciduous forest	0.12	0.12	0.12	0.50
Coniferous forest	0.12	0.12	0.12	0.35
Swamp	0.12	0.14	0.16	0.30
Cultivated land	0.14	0.20	0.18	0.60
Grassland	0.18	0.18	0.20	0.60
Urban	0.14	0.16	0.18	0.35
Desert shrubland	0.30	0.28	0.28	0.45
1Spring: Periods when vegetation is emerging or partially green. This is a transitional situation that applies to one to two months after the last killing frost in spring.
2Summer: Periods when vegetation is lush and healthy, typical of mid-summer, but also of other seasons where frost is less common.
3Autumn: Periods when freezing conditions are common, deciduous trees are leafless, crops are not yet planted or are already harvested (bare soil exposed), grass surfaces are brown, and no snow is present.
4Winter: Periods when surfaces are covered by snow, and when temperatures are sub-freezing. Winter albedo also depends on whether a snow cover is present continuously, intermittently, or seldom. Albedo ranges from about 0.30 for bare snow cover to about 0.65 for continuous cover.

The moisture availability, a, describes the wetness of the ground on a scale from 0.0 (dry) to 1.4 (saturated). The parameter a is preferred to the commonly-used Bowen ratio, Br, because a is not a function of temperature. However, there have only been a few observations of a while the textbooks are full of tables of observed Br. Note that although a relation between a and Br can be derived, that relation depends greatly on time of day and the magnitude of the sensible heat flux (Hanna and Chang, 1991). Holtslag and van Ulden (1983) found that a ~ 0.4 to 0.6 over dry agricultural fields with vegetation, and a ~ 0.9 over these same fields when the soil was wet. The following ranges of values of a are proposed (Hanna and Chang, 1992), based on information presented by Beljaars and Holtslag (1989 and 1991):

	a = 0.0 - 0.2	dry desert with no rain for months
	a = 0.2 - 0.4	arid rural area
	a = 0.4 - 0.6	crops and fields, mid-summer during periods when rain has not fallen for several days
	a = 0.5 - 1.0	urban environment, some parks
	a = 0.8 - 1.2	crops, fields, or forests with sufficient soil moisture
	a = 1.2 - 1.4	large lake or ocean with land more than 10 km away

The meteorological processor recognises that the stability of the atmosphere at night is limited by the presence of the mechanically well-mixed layer. Since the depth of the mechanically well-mixed layer is about two or three times the representative building height (Uno at al., 1988), and the Monin-Obukhov length, L, can be thought of as the depth of the mechanically-mixed layer, the meteorological processor uses a minimum L during stable conditions. The following subjective scheme for minimum L is based on the EPA/Auer (1978) land use classification system, and can be refined as the user gains experience with the system. Note that, as shown in the table below, the effects of minimum L are more important for urban areas.

Land Use Type	Minimum L
Commercial, > 40 story buildings	150 m
Commercial, 10 - 40 story buildings	100 m
Commercial, < 10 story buildings	50 m
Industrial	50 m
Compact residential	50 m
Residential	50 m
Agricultural	2 m

When using the meteorological processor, the user also needs to specify the ratio of the ground heat flux to the net radiation flux. A value of 0.1 for the ratio is recommended as default. This value is characteristic of an agricultural crop or a field. A value of 0.3 for the ratio is recommended for urban areas because of the associated heat-island effects.

Input Parameters for the DEP File

The following parameters in the DEP file are required if the user decides to calculate the dry and wet deposition fluxes (Chapter 9 of the HGSYSTEM 3.0 Technical Reference Manual). The file is read with free-format. There must be one entry per line. All data must be given in the specified order. There are no default values assumed by the model for the parameters.

Line No.	Real/ Integer	Remarks
1	R	Reference height in meters for computing atmospheric resistance (usually 2.0 m)
2	R	Molecular diffusivity, cm2/s, for the particles
3	R	Particle diameter, mm; = 0. for gas
4	R	particle density, kg/m3 (5053.14 for UF6, 6381.86 for UO2F2)
5	R	Precipitation rate, mm/hr; < 0. indicates that raining but rate unknown, = 0. indicates no precipitation
6	I	Form of precipitation; 0 = liquid, 1 = frozen

Input Parameters for the CNY File

The following parameters in the CNY file are required if the user decides to estimate the effects of canyons on the concentrations (Chapter 9 of the HGSYSTEM 3.0 Technical Reference Manual). The file is read with free-format. There must be one entry per line. All data must be given in the specified order. There are no default values assumed by the model for the parameters.

Line No.	Real/ Integer	Remarks
1	R	Building height, m
2	R	Canyon width, m

Input Parameters for the PTL File

The following parameter in the PTL file is required if the user decides to estimate the effects of centreline concentration fluctuation (Chapter 9 of the HGSYSTEM 3.0 Technical Reference Manual). The file is read with free-format. There is no default value assumed by the model for the parameter.

Line No.	Real/ Integer	Remarks
1	R	Percentile (in fractions) on the cumulative distribution curve at which the centreline concentrations are to be estimated

Output for Additional Modeling Enhancements

The meteorological processor is used to calculate the values of the Monin-Obukhov length, L, and the friction velocity, u_*. It is called only once prior to the dispersion calculations. Therefore, the meteorological processor does not have any special output, except for the values of L and u* printed in the regular model output file.

The effects of plume lift-off are included in the regular APR and HSR report files.

The results for the remaining four enhancements are listed in the additional 'casename'.MMO output file (i.e., a file whose file name is the same as the run name, and whose extension is MMO), as detailed in the following.

The effects of (1) canyon, (2) concentration fluctuation, and (3) variation of concentration with averaging time were parameterised as correction factors to the original concentration field. For example, if the model originally predicts the pollutant centreline concentration at a given downwind distance to be 0.0010 kg/m3, and if the existence of a canyon causes the concentration to increase to 0.0012 kg/m3, then a correction factor of 1.2 is assigned to the canyon effects. As another example, if the model originally predicts the pollutant centreline concentration at a given downwind distance to be 0.0010 kg/m3, and according to the concentration fluctuation module the 99th percentile of that concentration is found to be 0.0025 kg/m3, then a correction factor of 2.5 is assigned to the concentration fluctuation effects. If more than one process would influence the predicted concentrations, it is assumed that these processes are independent, and that their associated correction factors can be multiplied to obtain the final results.

The plume thermodynamic and chemistry algorithms in HGSYSTEM assume that no mass is removed from the plume. Consequently, it is assumed that gas or particle deposition fluxes due to dry and wet deposition are small compared to the total flux of material in the plume, which is assumed to remain unchanged. The local gas or particle deposition flux to the ground is calculated as the product of the ground-level concentration (already given by the model) and the deposition velocity. Therefore, as long as the deposition velocity is known, the deposition flux can be estimated. The dry deposition velocity is a constant for any model run and does not vary with space. The wet deposition velocity, on the other hand, varies with downwind distance since it involves an integration over the depth of the plume as it grows vertically with downwind distance.

The correction factors for the effects of (1) canyons, (2) concentration fluctuations, and (3) variations of concentrations with averaging time, together with the dry and wet deposition velocities, are listed in the MMO file for each reported downwind distance of the model.

References

Auer, A.H., 1978: Correlation of land use and cover with meteorological anomalies. J. Applied Meteor., 17, 636-643.

Beljaars, A.C.M., and A.A.M. Holtslag, 1989: A software library for the calculation of surface fluxes over land and sea. Environ. Software, 5, 60-68.

Beljaars, A.C.M., and A.A.M. Holtslag, 1991: Flux parameterization over land surfaces for Atmospheric Models. J. Applied Meteor., 30, 327-341.

Hanna, S.R., and J.C. Chang, 1991: Modification of the Hybrid Plume Dispersion Model (HPDM) for Urban Conditions and Its Evaluation Using the Indianapolis Data Set, Volume I: User's Guide for the HPDM-4.0 Software Package. Prepared for the Electric Power Research Institute, 3421 Hillview Avenue, Palo Alto, CA 94303, by EARTH TECH/Sigma Research, 196 Baker Avenue, Concord, MA 01742.

Hanna, S.R., and J.C. Chang, 1992: Boundary-layer parameterizations for applied dispersion modeling over urban areas. Boundary-Layer Meteor., 58, 229-259.

Hanna, S.R., J.C. Chang, and J.X. Zhang, 1994: Technical documentation of HGSYSTEM/UF6 model. Prepared for Martin Marietta Energy Systems, Oak Ridge, TN 37831, by EARTH TECH/Sigma Research, 196 Baker Avenue, Concord, MA 01742.

Holtslag, A.A.M., and A.P. van Ulden, 1983: A simple scheme for daytime estimates of the surface fluxes from routine weather data. J. Clim. & Applied Meteor., 22, 517-529.

Iqbal, M., 1983: An Introduction to Solar Radiation. Academic Press, 286 pp.

Post, L., 1994: HGSYSTEM 3.0 User's Manual, TNER.94.058, Shell Research Limited, Thornton Research Centre, Chester, England.

Post, L., 1994: HGSYSTEM 3.0 Technical Reference Manual, TNER.94.059, Shell Research Limited, Thornton Research Centre, Chester, England.

Uno, I., S. Wakamatsu, H. Ueda, and A. Nakamura, 1988: An observational study of the structure of the nocturnal urban boundary layer. Boundary-Layer Meteor., 45, 59-82.

User Guide MAIN MENU