How to Use the Oklahoma Dispersion Model

To understand the model better, please read the Model Description section. The focus of the Model is the evaluation of dispersion conditions with respect to downwind concentrations at distances of 1/4 mile and greater. In addition to dispersion conditions, it is also important to know the transport direction of such material (e.g., drift). Interpretation of the Dispersion Model output is discussed below, with dispersion conditions and transport direction described separately.

With respect to dispersion conditions, the Oklahoma Dispersion Model is used in conjunction with weather conditions reported by the Oklahoma Mesonet to produce a map of current dispersion conditions across Oklahoma; another part of the Model is used in conjunction with the latest National Weather Service 60-hour NGM MOS forecasts to produce maps of forecast dispersion conditions at 3-hour intervals into the future. With respect to transport direction, the Mesonet is used to create a map of current wind conditions (as well as temperature and relative humidity), while the MOS forecasts are used to create similar maps at 3-hour intervals into the future. In addition, table output based on the NGM forecasts showing future dispersion and transport conditions is available for specific sites.

Dispersion Conditions

Based on the current or forecasted weather conditions, the Oklahoma Dispersion Model assigns a "Dispersion Condition", which is reflective of downwind concentrations at distances of 1/4 mile or greater. There are six categories as seen below:

	Dispersion Conditions	Code	Color of Maps
	Excellent	EX	Dark Green
	Good	G	Green
	Moderately Good	MG	Light Green
	Moderately Poor	MP	Beige
	Poor	P	Orange
	Very Poor	VP	Red

This color scheme is utilized both in the map for current dispersion conditions as well as in the MOS-based future dispersion condition maps. "Excellent" would indicate the lowest possible downwind concentrations, while "Very Poor" would indicate the highest downwind concentrations. The green colors thus represent the recommended times for the near-surface release of gases and particulates. Of course, EX is better than G, and G is better than MG. The beige color signifies possibly acceptable conditions if the wind direction is suitable to minimize complaints, while the orange and red colors represent times to avoid.

The MOS-based table output lists the category code for the dispersion rating (EX, G, MG, MP, P, or VP). In addition, the MOS table includes something called the "Downwind Pollution Index" (DPI). This is a number which indicates the relative severity of downwind concentrations. Ranging from 1 to 120+, the higher the value of the DPI, the greater the concentrations downwind. It also can be used to compare dispersion conditions for different times through the forecast period. For example, if the MOS forecast for 3 p.m. indicated a DPI of 2, while that for 3 a.m. indicated a DPI of 100, that would mean the downwind concentrations at 3 a.m. would be about 50 times (100/2) worse than those at 3 p.m.

The Model is such that during the daytime excellent (EX) through poor (P) categories can occur, while during the nighttime only good (G) through very poor (VP) conditions can occur. During the daytime, excellent and good conditions occur during times of strong to moderate solar radiation or with strong winds. During the nighttime, very poor and poor conditions are associated with light winds and mostly clear skies, which lead to surface temperature inversions which greatly reduce dispersion.

The Model-calculated dispersion categories are best applicable for flat uniform terrain and no precipitation. Under light wind conditions, especially on clear nights, gases and particulates tend to "drain" gravitationally downslope and dispersion conditions may be worse than what the Model suggests, especially in valleys. With variable terrain and vegetation (e.g., forested hilly terrain), results may also be different. During periods of precipitation, dispersion may be enhanced, resulting in better dispersion conditions than the model suggests. In general, however, the Oklahoma Dispersion Model provides useful information for the wide range of situations to be encountered throughout Oklahoma; it is not designed as a site-specific model taking local topography and vegetation into account.

Transport Direction

A map showing current wind speed and direction, as well as temperature and relative humidity, is available using the most recently observed conditions from the Oklahoma Mesonet. In addition, maps going back 6 hours in 15-minute intervals are available through the link at the bottom of the web page (these maps are useful to see if any wind shifts are moving toward your area).

An example of the station model plot utilized in this map is shown at left. Winds blow from the barbed end of the line segment toward the solid circle at the Mesonet station's location. The wind speed is obtained by summing the values represented by the barbs: a half-barb means 5 mph; a full barb, 10 mph; and a flag, 50 mph. Any wind gusts over 20 mph are denoted by a "G" followed by the gust value at the top of the station plot. Calm conditions are denoted by a larger circle surrounding the solid circle. Finally, temperature in Fahrenheit appears at the upper left of the station plot, and relative humidity in percentage at the bottom left.

In this example, the following information can be interpreted about the conditions at the station: The temperature is 73 degrees Fahrenheit. The relative humidity is 47 percent. Winds are out of the southwest at about 25 miles per hour (mph), with gusts to 38 mph.

The NGM MOS forecast maps employ the same station plot as does the Mesonet map of current conditions. Like the dispersion condition maps based on the MOS forecasts, they are available in 3-hour increments through the duration of the forecast period. Wind directions are denoted by appropriate lettering: a north wind (wind from the north) by N; an east wind by E; a south wind by S; and a west wind by W. A direction denoted by WSW, for example, means a wind from the west-southwest. Wind speeds in mph are also listed, as are other weather variables as well as the Model-calculated Dispersion Conditions and Downwind Pollution Index for that forecast time.

How to Use the Graphical/Text Output

The dispersion condition maps (or MOS-based text output) should be used to assess the atmosphere's ability to disperse near-surface releases of gases and particulates, while the weather maps (or MOS-based text output) should be used to assess the direction the gases and particulates will drift. [Of course, all the forecast maps (whether they be for dispersion or weather conditions) are based on the NGM MOS forecasts, so that the reliability of our maps is only as good as those particular National Weather Service forecasts.]

With respect to current conditions, the user should consult both the current dispersion map and current weather map, both of which are based on the latest Oklahoma Mesonet data. With respect to future conditions three or more hours from the current time, the user should consult the MOS-based maps or text output. For those intermediate time periods between the current time and the first forecast time (that exists 3 or more hours from the current time), the user can use simple interpolation between the two times to estimate both the dispersion and weather conditions.

With respect to the Oklahoma Dispersion Model maps and text output, the following general recommendations regarding dispersion can be made:

Excellent (EX) to Moderately Good (MG) Dispersion Conditions

In general, activities involving the near-surface release of gases and particulates should be planned for those times showing excellent (EX), good (G), or moderately good (MG) dispersion conditions (the GREEN colors). An exception would be if the wind direction is such that it would carry the gases and particulates toward an adjacent sensitive area (e.g., a non-targeted field or very close neighbor).

Moderately Poor (MP) Dispersion Conditions

Times showing moderately (MP) conditions might be acceptable if the wind direction is such that it would carry the gases and particulates away from areas of concern. With winds toward areas of concern, such times would best be avoided.

Poor (P) to Very Poor (VP) Dispersion Conditions

Times showing poor (P) to very poor (VP) conditions should generally be avoided, especially if your topography is such that you have problems with gravitational drainage into sensitive areas during light wind conditions at night. If the wind directions during P or VP conditions are away from areas of concern, it might be possible to conduct a particular activity (but not pesticide application, which requires turbulence for adequate penetration into or onto the target area). However, to be on the safe side, it is best to avoid such times.

Again, these recommendations apply to dispersion alone (and, in particular, the levels of downwind concentrations) and by themselves may not indicate appropriate conditions for a particular activity. Rainfall, for example, is not considered in creating the dispersion maps, so that periods having rain may still show excellent (EX) to good (G) dispersion conditions. However, one would not want to conduct a prescribed burn or apply a pesticide during rainy conditions! Another example has to do with wind speed. The Model may indicate good (G) to excellent (EX) dispersion conditions, but wind speeds could be in the 20-40 mph range. A prescribed burn or pesticide application would normally not be recommended under such windy conditions.

Based on the time of interest, the user would normally look at the current maps and/or each of the MOS-based maps to determine for his/her area the times during when the dispersion conditions and wind directions are acceptable (based on the above guidelines). While this may be time consuming, use of the maps enables the user to pinpoint the particular geographical location of interest within the state. A simpler method, but more restrictive in terms of site locations, is to look at the MOS-based table output for a nearby MOS site. By looking at just one table, the user can see both dispersion and wind conditions at all forecast times.

Finally, one should realize that, while the current maps based on Mesonet information are updated every 15 minutes, the MOS-based maps and text output are updated only every 12 hours: generally around 10 a.m. CST (11 a.m. CDT) and 10 p.m. CST (11 p.m. CDT). If it is close to the time of the next update and a decision is to be made based on the forecast output, it would be best (if feasible) to wait until the new forecasts are available.