## 2010 Primary Sulfur Dioxide (SO 2 

## )

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 , 20151 

### M eteorological data used in the modeling analysis

M odel ing guidance calls for the use of surface and upper

- air data

, representative of the model ed facility location , as input

. M odel ing of the Muskogee area, for the demonstration of compliance with the 2010 1

- hour SO 2 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 Envir onmental 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 wind s from the southeast occurred on days w ith s ulfur dioxide ( SO 2 ) measurements greater than 50 parts per billion by volume (ppb ) at the monitored location. Comparative analys e s were conducted us ing data from the ambient air monitoring station

. 

### Figure 1: Map of s tudy a rea

ISH Site

- KMKO

- Muskogee Airport ; Monitor

- SO 2 Ambient Air Monitoring S ite ; 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 influenc es 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 sou th to the east of the OG&E Muskogee Generating Station. The OG&E Muskogee Generating Station emits significant levels of SO 2 from three smokestacks: two at 350 feet and one at 500 feet above ground level . The ground elevation at the OG&E Muskogee Generat ing Station is approximately 5 25 feet above mean sea level.



### Atmospheric s tability

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 - 1 and 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 “a diabatic ” relates to a thermodynamic process occurring with out 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

s ystem 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 c louds 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 fr ont or in situations, such as lake

- effect snow. In stable atmospheric boundary layer 1 , airflow is critical to pollution levels. At the surface, friction and heat fluxes influence the height of the atmospheric boundary layer. Turbulence primarily charact erizes the atmospheric boundary layer; temperature gradients can either generate or suppress this turbulence.3

T he structure of the atmospheric boundary layer changes cyclically throughout the day. The atmospheric conditions around Muskogee during the morn ing hours typically begin with light winds near the ground le vel. The first gust of wind come s 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 u nstable, 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 l ead to lower pollution le vels 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, oft en associated with bad weather. I n 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 stati cally 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 th is 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 ci rculation. 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 w ould tend to expand equally in a ll directions such as in a neutral environment. Anisotropic conditio ns 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 m omentum t ransport

On a microscale

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

, contribute to downslope wind s at the SO 2 monitor during prevailing south

- southeasterly flow4

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 el evated terrain. After air moves upward over the hills south of the facility

, the restoring
force of buoyancy act s on the air. In a stable atmosphere, an air parcel originating on the windward side of the formation cool s adiabatically as it rises over the hills . Once this air parcel reaches the top of the hills, it descend s along the surface toward equilibrium on the lee side of the hills

. Momentum can cause high er 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 gr avitational potential energy. This can be represented mathematically in the form of KE/PE > 1

, which is the ratio of Kinetic Energy to Potential Ener gy. 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 als o known as the Froude number Fr. This can further be represented as the cross barrier wind component /Brunt

Vaisalla ( 2 ) frequency x height of mountain = U/h m N . Given the orography near Muskogee

, it is much easier for flow to make it over this obstacle wi th flow slowing as it ascends and accelerating as it descends the elevated terrain

.



### Gap w inds

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 provid es an opportunity for

ga p 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 co nvergence 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 thin ning 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 a nalysis

During the period from 2012 to 2014

, SO 2 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 c alm dr y winds at the surface prominent throughout th e 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 Chart6 

### Figure 3: 850

- mb chart

A weak low

- level ridge ( Fig

. 3 ) situated over east Texas , and d ue to the ridge axis location

. The previous evening, the surface profile ( Fig

. 4 ) indicated neutral conditions allowing pollutants th at were at higher elevations to be mixed down to the lower levels of the boundary layer.



### Figure 4: Upper air evening sounding

S urface winds were also calm over the region at the surface with a n 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. A lso

, negligible wind speed at the surface8

indicat es 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 SO 2 event.

May 23, 2012

The chart in Fig ure 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 Chart9 

### Figure 8: Upper air evening sounding

July 27, 2012

P recipitation on the previous day kept the humidity high ( Fig

. 9 ) with an upper ridge building back across the S outhern P lains ( Fig

. 10 )

. 

### Figure 9: surface chart10 

### Figure 10: Upper air 500 mb chart

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

w as the main mechanism for any developments. The atmospheric sound ing s show strong moisture profiles throughout the day with little directional flow at the mid and surface levels.



### Figure 11: Upper air evening sounding11

The environment became unstable in the evening

, which help ed decrease the SO 2 readings in the afternoon ( Fig

. 1 1 )

. Summary

A common theme amongst the high SO 2 readings are stable at mospheric layers in the morning often associated with a high

- pressure system or radiative cooling at night . The environmental profile become s 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.

B oth orographic lift and pressure channeling o f 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 pollutan ts 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 b y 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 , 2 nd ed., Brooks/Col e , pp. 65

.

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

.

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

.

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

.