2010 Primary Sulfur Dioxide (SO2)

National Ambient Air Quality Standard (NAAQS)

Technical Analyses and Information Supporting the

Designation Recommendation for Oklahoma

Effects of Terrain on Wind Flow in Muskogee, Oklahoma

Prepared by the Technical Resource and Projects Section of the Data and Planning Unit

Air Quality Division

Oklahoma Department of Environmental Quality

September 10, 2015
1

Meteorological data used in the modeling analysis

Modeling guidance calls for the use of surface and upper-air data, representative of the modeled 

facility location, as input. Modeling of the Muskogee area, for the demonstration of compliance 

with the 2010 1-hour SO2 NAAQS, used the integrated surface hourly dataset (ISHD) from the 

automated surface observing system (ASOS) at Davis Field Airport (KMKO) in Muskogee, 

Oklahoma from the National Center for Environmental Information (NCEI) and upper-air data 

originating from weather balloons launched twice daily from the National Weather Service 

(NWS) office at Max Westheimer Airport (OUN) in Norman, Oklahoma. Prevailing winds from 

the southeast occurred on days with sulfur dioxide (SO2) measurements greater than 50 parts per 

billion by volume (ppb) at the monitored location. Comparative analyses were conducted using 

data from the ambient air monitoring station.

Figure 1: Map of study area

ISH Site - KMKO-Muskogee Airport;

Monitor - SO2 Ambient Air Monitoring Site;

Mesonet Site - Porter, Oklahoma Mesonet Site;

WEBR – Webbers Falls, Oklahoma Mesonet Site;

OG&E - OG&E Muskogee Generating Station;

Gap - Area where GAP effects could be influencing local meteorology; and 

Elevated Terrain – Specific area of terrain which influences local meteorology.
2

Local terrain

Figure 1 shows the area around the OG&E Muskogee Generating Station. The base elevation of 

the area is defined by the Arkansas River which flows through the area. The United States Army 

Corps of Engineers maintains a normal pool elevation of the Webbers Falls Reservoir along the 

McClellan-Kerr Arkansas River Navigation System at 490 feet above mean sea level. An area of 

hilly terrain 200 feet to 350 feet above the river (700 to 850 feet above mean sea level) exists 

about 2 to 5 miles from the south to the east of the OG&E Muskogee Generating Station. The 

OG&E Muskogee Generating Station emits significant levels of SO2 from three smokestacks: 

two at 350 feet and one at 500 feet above ground level. The ground elevation at the OG&E 

Muskogee Generating Station is approximately 525 feet above mean sea level.

Atmospheric stability

In assessing atmospheric stability(3), we can determine whether a parcel of air will rise or fall

based on the parcel temperature (Tp) in relation to the environmental temperature (Te). This can 

be further represented in terms of the change in temperature of a parcel of air with respect to the 

dry adiabatic lapse rate (dalr) of 9.8 K km-1and the environmental lapse rate (elr). If Tp > Te 

then the parcel of air will keep rising such as in an unstable environment. In a stable 

environment, were dalr > elr, a parcel of air will sink because the air around it is warmer.

The term “adiabatic” relates to a thermodynamic process occurring without gain or loss of heat. 

In terms of atmospheric stability, it is used to represent changes in an air parcel’s temperature 

due to expansion or compression, i.e. no heat is added or taken away from the parcel of air.

“Radiative cooling inversions” form stable layers in the atmosphere. Other inversions occur 

when cold air advances at low levels or when warm air moves over cold ground. The radiative 

inversion often occurs at night. In another method for formation of an inversion, a high-pressure 

system moves into an area. Adiabatic compression increases the temperature of the sinking air 

that typically accompanies this type of system. Because the upper-level vorticity leads to 

convergence and sinking air aloft, which creates high pressure at the surface, this sinking takes 

place. This subsidence warms the top layer of the boundary layer more than the bottom.

Cooling of the air aloft and warming of the surface air destabilizes the atmosphere. Cold air 

moving aloft from extratropical cyclones or clouds emitting infrared radiation to space cools air 

aloft. Surface heating warming the surface air, low-level warm-air advection, and cold air 

moving over a warm surface also destabilize the atmosphere. This destabilization can happen 

ahead of a cold front or in situations, such as lake-effect snow. In stable atmospheric boundary 

layer1, airflow is critical to pollution levels. At the surface, friction and heat fluxes influence the 

height of the atmospheric boundary layer. Turbulence primarily characterizes the atmospheric 

boundary layer; temperature gradients can either generate or suppress this turbulence.
3

The structure of the atmospheric boundary layer changes cyclically throughout the day. The 

atmospheric conditions around Muskogee during the morning hours typically begin with light 

winds near the ground level. The first gust of wind comes as the sun rises and heats the ground. 

During the late morning hours thermals of warm air climb to greater heights, and with this 

vertical motion horizontal wind speeds increase. The primary static stability conditions for air 

pollution management are unstable, neutral, and stable conditions. This determines the flow of 

the atmosphere. Unstable conditions will promote continued acceleration of a parcel of air until 

it reaches an environment in which it is cooler. These turbulent conditions typically lead to 

lower pollution levels due to increased mixing.

A parcel of air displaced in a neutral environment will feel no buoyant forces with moderate to 

strong winds and little heating or cooling from the surface. These occur during overcast 

conditions, often associated with bad weather. In a stable environment, the parcel will 

experience buoyant forces opposite to the direction of displacement due to the environmental 

increase of temperature with height. Stable layers of air, are associated with light winds and a 

surface that is cooler than the air. In addition to these factors, the height of the boundary layer is 

important for pollution dispersion throughout the region. Flow can become turbulent in statically 

stable air if the wind shear is strong enough. Such dynamic stability is indicated by the 

dimensionless bulk Richardson number. When referenced with the critical Richardson number 

of 0.25, a Ri < Ric will represent flow that is dynamically unstable and turbulent.

It is the combination of the buoyance and turbulent drag on a parcel that determine the vertical 

motion of a parcel of air. When this wind approaches an obstacle such as the terrain features of 

Muskogee, much of the air will flow around this obstacle depending on the stability of the 

atmosphere which is further explaining through downward momentum transport and gap winds 

influence airflow near the OG&E Muskogee Generating Station. While emissions out of the top 

of the stack often have strong internal turbulence, this quickly decays, leaving the majority of the 

dispersing to the ambient flow. The direction that the plume travels is controlled by the synoptic 

weather and general circulation. In a given volume of air, the rate of smoke dispersion depends 

on the velocity variance. Thus, if turbulence is isotropic, then a smoke puff would tend to 

expand equally in all directions such as in a neutral environment. Anisotropic conditions are 

prevalent during the daytime over bare land, rising thermals create stronger vertical motions than 

horizontal. Hence, a smoke puff would disperse more in the vertical. At night, vertical motions 

are very weak, while horizontal motions can be larger. This causes smoke puffs to fan out 

horizontally.

Downward momentum transport

On a microscale, these hills, which do not continue westward toward the Davis Field airport,

contribute to downslope winds at the SO2 monitor during prevailing south-southeasterly flow 
4

conditions. These winds help to create microscale wind conditions not found over or west of the 

City of Muskogee in the direction on the mean wind flow. The OG&E facility sits to the lee side 

of this elevated terrain. After air moves upward over the hills south of the facility, the restoring 

force of buoyancy acts on the air. In a stable atmosphere, an air parcel originating on the 

windward side of the formation cools adiabatically as it rises over the hills. Once this air parcel

reaches the top of the hills, it descends along the surface toward equilibrium on the lee side of 

the hills. Momentum can cause higher wind speeds in this local area than in areas without

orographic influence. To surmount this orographic feature, air at the surface must exhibit an

initial velocity strong enough to overcome gravitational potential energy. This can be 

represented mathematically in the form of KE/PE > 1, which is the ratio of Kinetic Energy to 

Potential Energy. In a statically stable atmosphere, the restoring force is the difference between 

ambient temperature and the temperature after lifting a parcel of air dry-adiabatically to a new 

height such as the top of mountain. This restoring force of KE/PE is also known as the Froude 

number Fr. This can further be represented as the cross barrier wind component/BruntVaisalla(

2

) frequency x height of mountain = U/hmN . Given the orography near Muskogee, it is 

much easier for flow to make it over this obstacle with flow slowing as it ascends and 

accelerating as it descends the elevated terrain.

Gap winds

Terrain around a narrow channel to the southeast of the monitor rises as high as 800 ft above 

mean sea level within one mile on both sides of the gap. This gap provides an opportunity for 

gap winds(

4

) to form. The depth of approaching air increases due to blocking effects of the 

terrain. As wind approaches the gap, the air occupies a decreasing area, causing the wind to 

slow, resulting in convergence. This convergence creates an area of high pressure at the gap 

entrance. At the gap exit, flow spreads and thins with divergence; the air occupies an increasing 

area. This increase results in lower pressure with winds accelerating over the exit region. This 

thinning enhances the pressure gradient thus increasing wind flow. If there is a strong 

temperature inversion preventing the depth of flow from changing through the gap, then air 

volume conservation gives: Vd=Ds/Dd * Vs where D is the width of flow, subscript s represents 

the upstream flow, and subscript d represents flow in the narrowest part of the mountain pass.

Event analysis

During the period from 2012 to 2014, SO2 readings on 7 days exceeded 50 ppb during at least 

one clock hour. Following is a description of the primary meteorological conditions that 

occurred during each of the 2012 events.
5

January 15, 2012

The surface chart depicts calm dry winds at the surface prominent throughout the region. A

frontal boundary (Fig. 2) was situated to the northwest of the state, and was expected to cross the 

area the following day.

Figure 2: Surface Chart
6

Figure 3: 850-mb chart

A weak low-level ridge (Fig. 3) situated over east Texas, and due to the ridge axis location. The 

previous evening, the surface profile (Fig. 4) indicated neutral conditions allowing pollutants that

were at higher elevations to be mixed down to the lower levels of the boundary layer.

Figure 4: Upper air evening sounding

Surface winds were also calm over the region at the surface with an absolutely stable morning 

sounding (Fig. 5).
7

Figure 5: Upper air morning sounding

Figure 6: Upper air evening sounding

Neutral conditions returned during the evening with an elevated inversion remaining over the 

area (Fig. 6).

These soundings also indicate low moisture in the morning, which the surface chart confirms, 

showing low dew points along the Gulf of Mexico. Also, negligible wind speed at the surface 
8

indicates little mixing is taking place. With a neutral sounding, one would expect stagnant 

conditions and fumigation to be occurring.

January 27, 2012

This day is a similar setup to January 15 except that the cold front is closer to the region. Again,

there is a stable boundary layer in the morning, but as the surface heats, this layer is gradually 

eroded and the layer becomes neutral. The upper wave associated with the frontal passage 

brought some mid-level moisture into the region along with southerly winds. The front did not 

pass through the area until the early evening, which was a few hours after the high SO2 event.

May 23, 2012

The chart in Figure 7 shows calm southerly winds with a slow moving low pressure system 

positioned over Eastern Colorado. This day followed the previous events with a stable morning 

sounding become more neutral (Fig. 8) as the day progressed. 

Figure 7: Surface Chart
9

Figure 8: Upper air evening sounding

July 27, 2012

Precipitation on the previous day kept the humidity high (Fig. 9) with an upper ridge 

building back across the Southern Plains (Fig. 10).

Figure 9: surface chart
10

Figure 10: Upper air 500 mb chart

Given no primary upper air driver for the local synoptic conditions, afternoon heating 

was the main mechanism for any developments. The atmospheric soundings show strong 

moisture profiles throughout the day with little directional flow at the mid and surface levels.

Figure 11: Upper air evening sounding
11

The environment became unstable in the evening, which helped decrease the SO2 readings in the 

afternoon (Fig. 11).

Summary

A common theme amongst the high SO2 readings are stable atmospheric layers in the 

morning often associated with a high-pressure system or radiative cooling at night. The 

environmental profile becomes more neutral throughout the day as the inversion in the boundary 

layer erodes with increased surface heating. Because of the topography there is significant local 

terrain influence on wind speeds and direction. If any instability is present in the region, 

pollutants are quickly mixed into the atmosphere.

Both orographic lift and pressure channeling of the wind can affect the air flow in the 

vicinity of the OG&E Muskogee Generating Station. During the daytime conditions of free 

convection and anisotropic conditions, thermals cause a form of dispersion that often brings high 

concentrations of pollutants close to the ground. At night, turbulence is suppressed in the 

vertical, causing pollutants to remain aloft. However, if the conditions are neutral with strong 

winds, turbulence is more isotropic with smoke plumes dispersing at roughly equal rates in the 

vertical and later directions. Because most pollutants are emitted from near the surface, and most 

receptors are at the surface, the mean transport and turbulent dispersion of pollutants are 

primarily controlled by boundary layer characteristic. The nature of the turbulence depends on 

the radiatively-driven heating, and the dynamic forces and winds. Once emitted, the dispersion 

depends partly on the static stability of the ambient atmosphere.

References

1. R. Stull, Meteorology For Scientists and Engineers, 2nd ed., Brooks/Cole, pp. 65.

2. J. Holton, An Introduction to Dynamic Meteorology, 4th ed., Elsevier Academic Press, pp. 53.

3. J. Holton, An Introduction to Dynamic Meteorology, 4th ed., Elsevier Academic Press, pp. 51.

4. E. Aguado & J. Burt, Understanding Weather & Climate, 2nd ed., Prentice Hall, pp. 272.