Climatology of Diffusion Potential Classes for Minneapolis

Introduction

The nature of the temperature profile of the lower atmosphere varies considerably during the course of the diurnal period and throughout the year. Commonly, it varies from increasing stability through the night to increasing instability during the daylight hours. The combined effect of atmospheric stability, wind speed, and wind direction fluctuation characterizes the diffusion or mixing potential of the atmosphere. The degree of this potential is dependent upon two mechanisms: thermal and mechanical turbulence (Stull 1995). Atmospheric diffusion is the exchange of fluid parcels, including their contents and properties, between neighboring portions of the atmosphere (Wanta and Lowry 1976). The vertical and horizontal structure, therefore, affects the intensity and nature of the atmospheric turbulence that diffuses or mixes air pollutants.

Several studies have described atmospheric diffusion characteristics in terms of calculated or estimated stability classes that have been related to readily and routinely measured surface parameters. Pasquill (1961, 1974) proposed classes that were based upon synoptic weather observations and modified by Turner (1969). Luna and Church (1972) compared the diffusion estimates based upon the Pasquill classes with measured vertical temperatures and wind speed and directions.

Atmospheric diffusion investigations based upon wind gustiness measurements as described by Singer and Smith (1953) and wind fluctuations defined by the Brookhaven National Laboratory (McCormick and Holzworth 1976), and later refined by Leahey et al. (1995) and Ibarra (1995), added knowledge into diffusion potential for specific stability classes.

Radiosonde soundings provided the database for earlier investigations into inversion frequencies by Hosler (1961) and mixing depth frequencies by Holzworth (1964, 1972). The results of these studies provided a basis for quantifying the meteorological parameters associated with diffusion in the lower atmosphere. Connell and Miller (1995) list possible sources of error associated with the use of radiosondes in measuring diffusion parameters.

Remote sensors such as the lidar (Hashmonay et al. 1991), the acoustic sounder, and the radar profiler (Quintarelli 1993; Angevine et al. 1993) have increased the information about the structure and changes that occur in the boundary layer. Only in a limited number of investigations can remotely sensed data be compared directly with in situ instrumentation, such as that from instrumented towers. Thuillier (1995) found significant variability when comparing simultaneously recorded data taken by different types of instrumentation when measuring the vertical temperature structure and winds.

Short-term direct measurement techniques of the vertical structure of the lower urban atmosphere have included tethered balloonborne profilers (Changnon 1981;Vernekar et al. 1993) and instrumented aircraft (Spangler and Dirks 1974).

Relatively few studies of urban atmospheric diffusion based upon direct, continuous, long-term measurements of both wind and atmospheric stability have been possible due to the limited number of instrumented towers. Vertical temperature profiles and winds measured at Louisville, Kentucky (DeMarrais 1961); Fort Wayne, Indiana (Bowne and Ball 1970); Brookhaven Labs, Upton, New York (Singer and Smith 1953); and Montreal and Ontario, Canada (Munn and Stewart 1967), provided opportunities to assess the low-level urban atmosphere on a continuous basis. Takle et al. (1976) describes a 6-yr study of dispersion characteristics during inversion occurrences in a rural area based on data from an instrumented tower near Ames, Iowa. Riordan et al. (1986) also used tower data to compare morning transition climatologies for two rural sites.

The St. Louis, Missouri, area has been the subject of extensive studies (METROMEX) of the urban effect on regional weather and atmospheric dispersion potential. Fixed, instrumented towers provided continuous vertical temperature profiles and wind data for the Regional Air Monitoring System (RAMS) project as reported by Schreffler (1978, 1979, 1982).

Earlier investigations have largely limited their attention to the description and analysis of temperature profile extremes, either inversion or unstable (mixing) conditions, as they affect dispersion of air pollutants. However, weakly stable (subadiabatic) conditions occur in the lower urban atmosphere with a significant frequency as diurnal transition stages between stable and unstable lapse rates. Relatively little is known about the effect of stability on horizontal flow during these transition periods (Myrup et al. 1986). Subadiabatic conditions can also occur during periods of overcast skies, as well as during moderate to strong winds. Even though this transition may show as subadiabatic when comparing low-level vertical temperature differences, Myrup et al. (1986) defined three stability regimes that occur in the evening transition period.

The principal objective of this investigation was to develop a diurnal and annual diffusion climatology for Minneapolis and St. Paul (the Twin Cities), Minnesota, by analyzing continuously measured low-level vertical temperature profiles and associated wind speed and direction data from an instrumented tower. Thus, this study provides a generalized view of diffusion potential for upper-midwest urban regions where continental-type climate typically predominates. A second objective of this investigation was to analyze the occurrence of weakly stable lapse rates at the same time as very stable and unstable lapse rates were considered. Finally, a system of diffusion potential classes was established and related to the synoptic conditions that produced them.

Site instrumentation and data

The temperature and wind data used in this investigation were measured at the 21.3-m level and at the top of a 152.4-m tower (KSTP television) that is a self- supporting type with a triangular cross section. In 1960, temperature and wind sensors were installed at 21.3-, 51.0-, and 152.4-m heights on the end of 1.8-m arms extended out to the south side of the south leg (Baker et al. 1969). The tower is located on the heavily urbanized boundary between St. Paul and Minneapolis at 45°N, 93°13′W. Most of the surrounding land surface is relatively flat and lies between 260 and 290 m mean sea level (MSL). A 15-m hill lies 0.4 km west and the Mississippi River valley is located 1.2 km southwest of the tower. The tower is located 4.8 km east-southeast of downtown Minneapolis and 8.8 km west-northwest of downtown St. Paul. At 271 m MSL, it was situated within industrial, commercial, and residential zones dominated by two-story frame buildings with three- and four-story buildings immediately to the southeast. This setting, including wooded areas, has changed little during the past 30 years and is representative of other settings surrounding the downtown areas. It is also representative of other upper-midwest urban regions. Most of the change in the metropolitan area has occurred as urban sprawl and the marked increase in numbers and heights of structures in the downtown areas, especially in Minneapolis.

Temperature

Air temperatures were measured at 21.3 m, and temperature differences (ΔT) obtained between 21.3 and 152.4 m were used in this study. Three aspirated Thermohm temperature sensors and a Leeds and Northrup Speedomax G recorder with very short response time were used to sense and record temperatures instantaneously once every 2 min at each level. The shielded temperature sensors were checked daily and calibrated once per year.

Wind

Wind speed and direction were continuously recorded at each level by an Aerovane wind system. Wind speed data used in this investigation were limited to the 21.3-m level since they most closely approximated ground level speeds. The wind speed grouping of less than 4 m s⁻¹ and greater than or equal to 4 m s⁻¹ was based upon the National Weather Service’s designation of winds of less than 4 m s⁻¹ as light winds (Munn 1970; Holzworth 1967). This speed results in only limited horizontal transport and is of such low magnitude that it results in little mechanical mixing. The anemometer used to measure wind speed consisted of a spinning rotor assembly connected directly to a generator. As the wind propelled the rotor, the generator output signal was received by a dual-channel Aerovane recorder, where it was displayed as a pen line marking on one of the channels containing a 0–60 m s⁻¹ chart scale. Sensors were checked daily and calibrated once a year by comparing them with a standardized Aerovane from a wind tunnel.

Wind directions at 152.4 m were used because they were the least affected by airflow around structures and closely approximated the surface wind directions at the National Weather Service’s airport station (United States Air Force 1971). The wind direction data were collected by means of a wind vane system, which positions a synchronous generator that is electrically connected to a synchronous motor located at the recording station. The matching synchromotor activates the recording mechanism of the dual-channel recorder and a continuous record of wind direction from 0° through 360° is traced. These wind direction data were grouped into 12 sectors of 30° each beginning with the 0°–30° sector.

Data used in the study were available for approximately 87% of the time for July 1961 through December 1964, and May 1965 through July 1968. The original strip chart readings for wind speed, direction, and temperature were reduced to 2-h averages.

Establishing diffusion potential classes

Further reduction of data included the grouping of ΔT between 21.3 and 152.4 m and the associated wind speed at 21.3 m into six diffusion classes as shown in Table 1. This altitude range was used instead of a shorter one because an earlier investigation (Baker et al. 1969), using the same tower data for the same time period, revealed that inversions between 21.3 and 51.0 m averaged only 2.5 per 100 operational days compared to 26.3 between 51.0 and 152.4 m. Studies by Duckworth and Sandberg (1954) found similar findings in which urban nocturnal inversions are frequently underlaid by subadiabatic or superadiabatic layers. Godowitch et al. (1985) found that urban–nonurban contrasts in the base height, formation time and vertical temperature structure of inversions, and the subsequent growth of the mixing height could be explained both by differing thermal properties of surface materials and mechanical turbulence.

Little evidence from tower data alone exists that verifies that the 152.4-m level rests within the isothermal–inversion layer instead of in the residual mixing layer above the inversion. Fisher (1977), however, reports that the tops of surface-based nocturnal inversions over the Twin Cities exceed 250 m 30% to 40% of the time during both winter and summer.

Climatology of diffusion potential classes

Category I (isothermal–inversion conditions)

Lapse rates in this category occurred primarily during the nighttime throughout the year. A summer maximum (Fig. 1) extended from the first week of June through the first week of November and was distinct from the winter minimum period that occurred during the balance of the year. The winter to summer change amounted to a 15% increase in frequency of occurrence. The frequency change was less abrupt from summer to winter. This transition in the isothermal–inversion frequency pattern is believed to be associated with the average spring and fall passage of the polar front (Chang 1972). The nocturnal duration during the summer (June–August) was typically 6 to 8 h, while the winter (December–February) duration commonly lasted 10 to 12 h.

Class Ia dominated category I annually and occurred more than 75% of the time that the atmosphere was very stable. Table 2 shows that the conditions satisfying Ia criteria occurred most often in July and August. Class Ib occurrences were relatively most frequent in April and May and again in October and November. These peaks in Ib are believed to also be associated with the spring and fall passages of the polar front.

Very stable conditions tended to be associated with the least frequent wind directions. Isothermal–inversion conditions were most often associated with light winds from either the southwest or the northeast. In fact, class Ia conditions were observed over 70% of the time with southwest to westerly winds and almost 60% of the time when winds were northeasterly (Fig. 2). In contrast, class Ia conditions occurred only 50% of the time when light winds were southeasterly and northwesterly. Figure 2 shows that class Ib conditions occurred most frequently (almost 50% of the time) when winds were south to southwesterly but no more than 25% from all other wind directions.

Category II (subadiabatic conditions)

This category was primarily an evening, nighttime, and morning phenomena, except for the winter season when it was commonly observed throughout the daylight hours. Figure 3 shows that the nocturnal subadiabatic occurrence was highest during early October through February, while the lowest occurrence lasted from June through early October. The highest afternoon occurrence was from mid-November through mid-January.

An interesting feature of the diurnal subadiabatic occurrence was that a morning and evening peak occurred with the evening frequency being the highest. These were considered as transitional peaks resulting from the morning change of very stable to unstable conditions and the change from afternoon unstable to evening stable lapse rates. These peaks were most pronounced during June through December.

Class IIb was more pronounced during the nighttime subadiabatic hours throughout most of the year, as presented in Table 3. Class IIa was dominant only during July and August while class IIb predominated during the times of seasonal change, especially April and October. The midday occurrences for each class were equal during the November through January period, while class IIa dominated during August and September.

Subadiabatic conditions occurred over 60% of the time that the nocturnal winds were southeasterly and northwesterly, while they occurred less than 30% of the time when the winds were northeasterly and southwesterly. Most of the time that the nocturnal winds of greater than or equal to 4 m s⁻¹ were easterly, southeasterly, or northwesterly, subadiabatic conditions existed (Fig. 2). Class IIa tends to occur favorably with northerly and southeasterly flows.

Afternoon occurrences were high for easterly and southeasterly flows, as shown in Fig. 2. The wind directions most often associated with nocturnal subadiabatic conditions were also the directions of greatest overall frequency.

Category III (superadiabatic conditions)

This condition was primarily a daytime feature of the lower urban atmosphere and dominated the early afternoon, occurring at least 70% of the time from mid- February until mid-September, and again from mid-October to early November. Figure 4 shows minimum frequency of 50% to 70% occurred from late November through early January. Nocturnal superadiabatic conditions occurred from 10% to 30% of the time from early November through mid-May but less than 10% of the time during the remainder of the year.

Annually, class IIIb occurred twice as often as class IIIa during the time that the afternoon atmosphere was unstable. Table 4 shows that class IIIa was more common only during July, occurring over half of the time.

Afternoon superadiabatic conditions were most often associated with westerly winds, while the least frequent directions were east and southeasterly. Class IIIa winds (Fig. 2) were primarily southwesterly, while class IIIb winds were westerly to northerly. Nocturnal winds associated with superadiabatic conditions were primarily northwesterly through northeasterly resulting from cold air advection.

The synoptic aspects of diffusion potential classes

The relationship between diffusion potential classes and synoptic situations producing them was assessed for winter and summer nocturnal periods. This preliminary study provided greater insight into forecasting the occurrence of the classes and the sequence in which they may occur.

The synoptic conditions were identified from daily weather maps prepared by the National Weather Service, U.S. Department of Commerce. These maps were prepared from observations taken at midnight central standard time. Generalized or composite synoptic maps (Figs. 5a,b) were drawn based upon typical winter and summer patterns observed during the review of daily weather maps used for this study period. Christenson and Bryson (1966) derived similar synoptic patterns based upon observed cases in their study, in which the weather for each day was classified into a weather type for winter and summer synoptic situations for Madison, Wisconsin, and Minneapolis–St. Paul, Minnesota. As in their work, the distances in these synoptic patterns are relative, and the location and orientation of the pressure systems and the degree of maturity of the low centers are generalized. The relative position of the Twin Cities within the synoptic system was plotted on the map for each observation. Each plot was identified by the diffusion potential class that was occurring in the Twin Cities at that time.

Winter conditions

The synoptic setting for nocturnal wintertime lapse rates is shown in Fig. 5a. The composite synoptic map typifying a polar air mass was prepared from a total of 266 observations representing the 1963–64, 1964–65, and 1966–67 winter seasons.

In order for the Twin Cities metropolitan area to experience greater than or equal to 4 m s⁻¹ winds, it must be situated near the low pressure center or near the associated front, as shown in Fig. 5a. The stronger winds produced by the large pressure gradient resulted in moderate to strong winds and frequent nighttime occurrences (more than 90% of the time) of classes IIb and IIIb. These two classes were also influenced both by moderate to heavy sky cover and cold air advection. According to Christenson and Bryson (1966), the region immediately west and northwest of a low center is characterized by extensive sky cover. These classes may also result from weakly unstable or stable (nearly neutral) lapse rates produced by very strong winds. Also, the same region may experience a shift in wind direction from northwest to west, which results in a change from IIIb to IIb conditions due to less cold air advection.

Class IIIb is dominant (more than 90% of the time in the core of the class IIIb sector, as shown in Fig. 5a) behind the cold front, where winds blow mainly from the northwest. During winter nights an unstable lapse rate can be produced by colder air over the relatively warmer city, where considerable heat is added to the lower atmosphere.

Along a westward transect from the cold front to the high pressure center, a transition from less stable to more stable air occurs along with a gradual decrease of wind speed. Subadiabatic conditions prevail during the nighttime period midway along this transition zone. Nearer to class IIIb, class IIb is dominant due to moderate winds stirring the air, while class IIa prevails nearer to the high pressure center where light winds occur due to a weak pressure gradient and subsidence. Warm air advection produces class IIa conditions in the area north to northwest of the high pressure center. Again, the class domination in the core of each class in Fig. 5a exceeds 90%.

In a high pressure center the night air is usually calm. In a continental polar mass, the air is also relatively dry and cloud-free, thereby allowing for considerable radiative heat loss from the earth’s surface. As a result category I conditions formed over the Twin Cities more than 90% of the time when a high pressure center was present at night.

North and west of the high center weakly stable lapse rates occurred. Southerly and southeasterly winds dominate this area resulting in warm air advection. Christenson and Bryson (1966) identified the area south and southwest of a high as one of considerable sky cover. The greater cloudiness and the moderate to strong winds southwest of the high result in winter nighttime subadiabatic lapse rates. The warm sector of the cyclonic wave, which is east of the approaching cold front, is dominated by nocturnal subadiabatic lapse rates due to sky cover and moderate warm air advection. The stronger winds nearer the cold front account for the prevalence of class IIb.

Summer conditions

The summer nocturnal synoptic lapse rate pattern differs markedly from the winter situation. Figure 5b, based on a total of 280 observations during the summer and early fall seasons of 1963–66, shows the regions within the macroscale, composite weather system where each diffusion potential class was dominant. A notable difference between summer and winter synoptic conditions was that a superadiabatic lapse rate was practically nonexistent during summer nights.

A subadiabatic lapse rate dominated most of the region around the low pressure center and along the associated cold front. The combination of sky cover and moderate winds produced class IIb conditions near the low center and associated front, while class IIa occurred on the eastern, southern, and western fringe of the high pressure center. Wind speeds were less around the low and behind the front than during similar conditions in the winter because of the usually weaker pressure gradient during the summer season.

The high pressure center was again dominated by isothermal–inversion lapse rates at night. In fact, the geographic area dominated by class Ia was much larger (approximately 5 times) than the area for winter synoptic conditions. In all summer nocturnal situations, each class shown in Fig. 5b dominated more than 90% of the time in the center of each sector.

Discussion of results

The diffusion potential for the six classes, at the tower location, were assessed by comparing them with the widely used stability classes established by Pasquill (1961, 1974) and discussed by Turner (1969). Pasquill estimated stability classes based on the more readily observed parameters of incoming solar radiation, wind speed, and nighttime cloud cover (Seinfeld 1986). The stability categories in this study, on the other hand, were based on actual measured vertical temperature differences. A comparison of the two schemes reveals strong similarities that point to the possibility that this study corroborates Pasquill’s estimates. The order of classes (ranked from least to greatest diffusion) was Ia, IIa, Ib, IIb, IIIb, IIIa, respectively, which closely resembles Pasquill’s order ranging from most stable through most unstable.

Based on the relative order of the six classes according to their expected diffusion efficiency, the summer period of July and August had both the lowest and the highest diffusion efficiencies during the 24-h period. Light winds and stable conditions (categories I and II) occurred more than 90% of the time during July and August nights. These conditions were the most intensive for any nocturnal period throughout the year but the duration was the shortest, lasting only 8 h. The lowest diffusion efficiency was prevalent (70% to 90% of the time) when winds were westerly, southwesterly, and northeasterly.

On the other hand, the early afternoon period in July and August was marked by excellent potential diffusion conditions. Superadiabatic conditions occurred at least 85% of the time with classes IIIa and IIIb occurring nearly equally often. Class IIIa, which rated relatively higher than class IIIb in overall diffusion potential, occurred most frequently this time of year. The higher ranking given class IIIa is due to the greater extent of vertical transport of pollutants compared to that of class IIIb. If moderate to strong winds occur with convective conditions, the vertical temperature lapse rate is only slightly superadiabatic due to considerable mechanical mixing, resulting in only weak vertical diffusion. Furthermore, stronger winds will spread pollutants more horizontally than vertically. This order conforms to the order of diffusion potential defined by Pasquill (1961).

Acknowledgments

The authors are indebted to the late Prof. Harold J. Paulus of the School of Public Health, University of Minnesota, for providing the data upon which this study was based.

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Climatology of Diffusion Potential Classes for Minneapolis–St. Paul (2)

Climatology of Diffusion Potential Classes for Minneapolis–St. Paul (5)

Climatology of Diffusion Potential Classes for Minneapolis–St. Paul (8)

Climatology of Diffusion Potential Classes for Minneapolis–St. Paul (11)

Climatology of Diffusion Potential Classes for Minneapolis–St. Paul (14)

Table 1.

Six diffusion potential classes as defined by stability characteristics, temperature lapse rate (ΔT 152.4–21.3 m), and wind speed (at 21.3 m). * The division between subadiabatic and superadiabatic is the dry adiabatic lapse rate between 21.3 and 152.4 m, where dT;shdz is approximately −1.3°C;sh131.1 m. This rate also corresponds to a potential temperature dθ;shdz of zero.