ATMOSPHERIC TRANSPORT MODULE

NOTE: Local meteorological data are required by the MEPAS air dispersion model to compute patterns of air and soil concentrations. Those data should be from the nearest meteorological observation station that will have weather conditions that are most representative of the area to be modeled. In United States, the Environmental Data Service, NOAA, National Climatic Center (NCC) in Asheville, North Carolina is the major supplier of such data. Alternatively other agencies and organizations supply summaries of meteorological data.

CLIMATOLOGY TAB

AC-LCDREF--Reference weather station
Units: none

An annual summary of the Local Climatological Data (LCD) for the nearest representative national weather service station should be obtained. They are published by the Environmental Data Service, NOAA, National Climatic Center (NCC) in Asheville, North Carolina. Example LCD summaries are provided as Figures 2.32, 2.33, and 2.34. The specific form of the LCD summary may differ from these examples, but all of the information should be found under equivalent headings.

In the United States, NOAA has assigned a five digit “Surface Station Number” to each location where meteorological surface observations are available. Enter the “Surface Station Number” or other appropriate identifier.

Note: If “Joint Frequency Data (JFD)” is loaded later from a file, the identifier will be set to the JFD station name and may be need to be reset if the LCD data are not from the same station as the JFD come from a different station.

AC-MIXAM--Morning mixing height
Units: meters (m)
Range: 50 to 900

Enter the morning mixing height from site documents, if available. Otherwise, use Figure 3.1 to estimate this value for sites in the contiguous United States. Multiply the average morning mixing height for the facility from Figure 3.1 by 100 and enter the value on the worksheet. A common range of values is from 300 to 900 m.

AC-MIXPM--Afternoon mixing height
Units: meters (m)
Range: 50 to 2600

Enter the afternoon mixing height from site documents, if available. Otherwise, use Figure 3.2 to estimate this value for sites in the contiguous United States. Multiply the average morning mixing height for the facility from Figure 3.2 by 100 and enter the value on the worksheet. A common range of values is from 1,000 to 2,600 m.

AC-RAIN--Annual precipitation
Units: inches (in.)
Range: 0.1 to 130.0

Source of these data is the LCD summary (see Section AC-LCDREF). Use the section entitled "Normals, Means, and Extremes" to obtain the average annual total, water equivalent precipitation.

AC-PRENUM--Precipitation days per year
Units: number per year (#/yr)
Range: 0 to 365

Source of these data is the LCD summary (see Section AC-LCDREF). A precipitation day is defined as a day having at least 0.01 inch of precipitation. Obtain the annual number of precipitation days from the LCD summary table (see Figure 2.33). This value is found under the heading of "Mean number of days: Precipitation 0.01 inches or more."

AC-NUMTS--Thunderstorms per year
Units: number per year (#/yr)
Range: 1 to 150

Source of these data is the LCD summary (see Section AC-LCDREF). Enter the number of days per year having thunderstorms as listed in the LCD. This parameter is defined as the number of days that thunder was heard at the station in 1 year.

JOINT FREQUENCY DATA TAB

Wind joint frequency data consist of tables of values that give the relative frequencies of various wind conditions. A joint frequency table for various wind speed groups, wind direction sectors, and atmospheric stability classes (based on Pasquill stability categories) is needed to represent the region surrounding the facility. These data are used to characterize the annual patterns of atmospheric transport and dispersion processes for the facility.

A sample wind joint frequency summary is provided in Table 3.1 (for wind stability classes A through E). This sample summary illustrates one format for a joint frequency distribution. Other formats can be equally useful although the parameters may be listed in a different order. Also, the frequency values in these tables may be expressed in a number of ways; e.g., as fractions, percentages, or number of occurrences. Table 3.1 illustrates frequency values as number of occurrences. The following provides guidance on the selection of data and subsequent entry into the template table.

Figure 3.5 illustrates the number of occurrences of winds for 16 wind directions. The diagram is based on information obtained at the National Weather Service station in Providence, R.I. The plot represents the total number of occurrences (for all wind-speed classes and wind-direction sectors) for stability class E. These data correspond to the fifth page of Table 3.1 under the column "Total."

There are normally two sources of these data. The first source is joint frequency summaries from special meteorological stations located at or in the immediate area of the facility. Facility environmental statements, safety analysis documents, RCRA Part B Permit Applications, and similar publications often contain such data when available. The second source is STAR PROGRAM summaries from the National Climatic Data Center (NCDC) in Asheville, North Carolina, which is operated by the National Oceanic and Atmospheric Administration (NOAA's) Environmental Data Service (Figure 3.6). The facility documents may also contain summaries from this source.

If the summaries are not found in the facility documents, contact the NCDC concerning the summaries that are available or that could be generated for this facility. The NCDC can provide a listing of the locations for which summaries have been generated, and summaries can be obtained for the cost of copying. In addition, the NCDC can generate new summaries for first-order weather stations for a modest fee.

Normally, the preferred data summary is from the station closest to the facility. Special considerations such as data quality or local influences on atmospheric transport and dispersion, may dictate the selection of a summary from some station other than the closest. In such a case, note the source of information and the reason.

When otherwise equivalent summaries are available for multiple measurement heights, use the data from the height closest to the releast height of constituents.

If summaries are available over different time periods, give preference to the summary based on the longer time period (i.e., a 5-year summary is preferable to a 1-year summary). The minimum time period for a summary is 1 year, and all observations each day at a station should be taken into account.

There are two situations in which it may be appropriate to attach more than one summary for the facility:

1. The release sites are geographically separated, and joint frequency summaries are available for the different locations.
2. The release sites are in complex terrain, and the local winds are expected to be significantly different from the regional winds.

AJ-STATNM--Data station
Units: none

Enter a name that describes the location at which the joint frequency data were taken. Use a descriptive name to designate the location where onsite data were taken. For the NCDC data, enter the name of the climatic station (normally the name of a nearby city). Also enter the name of the state in which the station is located. Document the source of the joint frequency summary data.

AJ-ANEMHT--Anemometer height
Units: meters (m)
Range: 0.0 to 999.9
Typical environmental range: 3 to 15

The height at which the winds were measured for the joint frequency distribution can normally be obtained either from the source of the joint frequency data (i.e., facility documentation for onsite data) or from the last page of the LCD summary for NCDC data. The typical value for joint frequency anemometer height is 3 m. This parameter is critical only for data sets obtained from towers; 3 m will be adequate for most near-surface data.

AJ-RLEN--Average roughness length
Units: centimeters (cm)
Range: 0.0 to 999.9
Typical environmental range: 0 to 150

Estimate an average roughness length for the area surrounding the weather station used for the wind joint frequency summary. The typical value for roughness length is 10 cm.

AJ-WINDS(6)--Wind speed midpoints
Units: meters per second (m/s)
Range: 0.00 to 99.99
Typical environmental range: 1.0 to 10.0

Midpoint wind speed values of the six wind-speed categories are needed. If ranges of wind speeds are given in the summaries, enter the ranges in the template and compute the midpoint by averaging the two range values. If the midpoints are given in the summary, enter these in the "Reported Midpoint" column (and ignore the "Range" columns). If the category has no defined range as in the last category shown in Table 3.2 (>21 KTS), use the defining value as the midpoint.

Combination of the values in the data tables is required for more than six wind-speed groups. If combination is required, combine the wind-speed categories so that the coverage of the range of wind-speed conditions is as even as possible. The frequency of occurrence of conditions within wind-speed groups will provide some guidance as to which adjacent groups to combine--groups already having high frequencies are generally left separate, and groups with relatively low frequencies are combined.

If fewer than six wind-speed groups are encountered, enter the information for all available wind-speed groups. Enter a value of 0 for the undefined wind-speed group midpoints.

AJ-CALMS--Wind joint frequency calms
Units: number, percent, or fraction

Enter total frequencies of calm conditions and the units. If no value is given for calms, enter a value of zero. The units selected also determine the units for all the joint frequency data in the stability class tabs (Class A through Class G).

Import Joint Frequency Data
Units: none

If you wish to use joint frequency data in an externally created joint-frequency data file, click on the "Import Joint Frequency Data" button and select the file containing the data. An externally prepared data file should be given an extension of ".JFD" and have the following format. The first line of the file is used to define two variables, ORD and FRM, which are numeric and character variables, respectively. FORTRAN formats are used to specify data input formats. This file has the following structure:

Line Number	Contents	Format of Data
1	ORD, FRM	FORMAT(I1,A30)
2 to (N+1)	N Lines of Joint Frequency Date	FRM from Line 1
N+2 to N+3	2 Lines of Calm Frequency Data	FRM from Line 1
N+4	Wind Speed Mid-Points (m/s)	FRM from Line 1

where ORD is always equal to 6. FRM is the FORTRAN expression enclosed in parenthesis for reading subsequent data. A typical format to enter fractions of occurrences has the form of (F10.6). ORD equal to "6." indicates a structure of six columns (wind-speed categories) per line and 16 lines (wind direction clockwise starting with North) per stability class block. Alternative options for the value of ORD are no longer supported. The joint frequency data are listed as blocks of data for each stability class starting with the most unstable case (A stability) and ending with the most stable case (G stability).

The next two lines of the file contain frequency of calm conditions for each stability class. The last line contains the mid-points of the wind speed groups used in generating the joint frequency table. The following is an example JFD file:

6(6F10.6) ,24157,"SPOKANE               ","WA", -8,"N",47.63,"W",117.53, 721, 6.4, 10., 1.0000,111,111,111,111,120.
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   .771666  2.057776  3.858330  6.331618  9.343415 12.709790

Joint Frequency Data
Units: number, percent or fraction of observations

Input the data in the joint frequency table using the same units as was specified for AJ-CALMS. The following guidelines should be used:

1. Entries are limited to five significant digits including the decimal point. The use and placement of the decimal point depends on the form of the data:

Fractions of total time (table sum will equal 1.0) are entered as thousandths; e.g., 0.0013, 0.0232, 0.0000.

Percentages of total time (table sum will equal 100.) are entered as hundredths of percent; e.g., 00.13, 02.32, and 00.00.

Number of occurrences [table sum is 8760 for one non-leap year of hourly observations (24 x 365)] are expressed as whole numbers; e.g., 11.,2.,0., and so on.

2. If frequencies are reported for an "unknown" or other undefined category, the entries may be ignored. The model will automatically adjust the input table to account for missing values. Data sets from which more than 15% is missing should normally not be used.

3. The table allows seven stability categories to be input. If only the first six stability categories are provided, enter the available data along with zeros for the seventh stability class. If some other combination of six or fewer stability categories are encountered, enter the data in the appropriate sections of the table with zeros entered for missing data. If only wind direction and wind-speed information are available, enter those data in the fourth stability category (D).

4. The table has room for up to six wind-speed groups, as defined in AJ-WINDS. If fewer than six groups are encountered, enter zeros for the frequencies in the undefined groups.

5. Do not record summary, total, and average values in the table.

TOPOGRAPHICAL DATA TAB

AR-TOPTYP--Use regional topographical data
Units: none

AR-TOPTYP indicates whether or not terrain heights will be needed for input. If this option is checked, the influence of the local terrain will be considered. This option should only be used in cases with an elevated plume released from a point source and non-flat terrain occurs for distances within a 25 km radius of the source. For a point source release, checking this box will result in activation of the "Terrain Heights" tab, which requires entry of the regional distribution of terrain heights (AR-TOPHTS)used to evaluate the intersection of elevated plumes with terrain. For other cases (with a ground level release and/or flat terrain), this option should not be used.

AR-TOPBAS--Elevation of release site
Units: meters (m)
Range: -50 to 8840

The elevation of the release site corresponding to inputs of AR-TOPHTS is required if the "Use regional topographical data" box is checked. The source of this data is a topographic map.

AR-REGSUR--Regional surface roughness lengths (z_o's)
Units: centimeters (cm)
Range: 0.0 to 999.9
Typical environmental range: 0 to 150

Roughness lengths (z_o's) are needed for the region surrounding the facility. A roughness length characterizes the sizes of surface elements that can influence atmospheric dispersion processes. Roughness lengths must be defined for eight direction sectors at four predetermined distances from the facility.

Figure 3.7 illustrates how surface roughness affects the wind profile using three different environments. The vertical wind profile at the left in Figure 3.7 is for bare ground. The z_o represents surface roughness. For bare ground the z_o is small (z_o = 1 cm). The middle wind profile is for an area with residential housing. These buildings increase the surface resistance to the wind (z_o = 10 cm) and reduce the wind speed. This reduction of wind speed occurs up into the boundary layer. Over a forest, the z_o is 50 cm, which results in a further reduction in wind speeds in the lower portion of the boundary layer.

An order-of-magnitude definition of average roughness lengths will be sufficient. The roughness values reflect major differences in surface cover. Figure 3.8 provides a summary of typical roughness heights associated with various types of surface. Using the information obtained in Section 1.0 along with facility photographs and other descriptive material, compute an average roughness length. For example, if one segment is about 50% agricultural (10 cm) and 50% low forest (50 cm), assign it a value of 30 cm. If the surface cover in a segment is completely unknown, enter a typical value for grasslands as 10.

If the surface cover in a segment is predominantly water, enter a surface roughness of 0 cm in the table. For a zero input value, the model computes the roughness of the surface as a function of wind speed and stability. The roughness over water should only be used in cases that require a spatial averaging of roughness values, i.e., for a segment containing significant fractions of both land and water surfaces. The procedure for defining roughness lengths requires that the directions and distances be overlaid on a map of the region surrounding the facility. Map(s) are needed to cover the region out to a radius of about 75 km (approximately 50 mi). For the shortest distances, facility photographs, maps, and descriptions will be most useful in defining roughness lengths. Table 3.3 shows the ranges of distances to be considered for each entry. Figure 3.9 illustrates a typical grid layout for determining the average roughness length for each direction and distance.

AR-TOPHTS--Regional distribution of terrain heights
Units: meters (m)
Range: -50 to 8840

The source of these data is a topographic map. A circular grid with a 25-km radius centered on the release site is plotted or overlain on the topographic map(s). This circular grid is defined by dividing the area into 16 direction sectors (N, NNE, NE, etc.). Each of these sectors is divided into annular segments by circles with radii of 3, 10, and 25 km. For the thus-defined annular segments, the maximum height of the topography for each segment is found by inspection and entered as the terrain height value relative to the AR-TOPBAS value. There should be 64 terrain height values entered.