Air Stagnation Advisories:

A Research Joint Venture Review

Aaron M. Pollyea, Joseph J. Charney, Warren E. Heilman, and Jeffrey A. Andresen

Introduction

Some National Weather Service (NWS) offices, U.S. Forest Service Smoke Management Programs, and state air pollution regulatory agencies issue Air Stagnation Advisories (ASA) when meteorological conditions conducive to a buildup of pollutants in the atmospheric boundary layer are expected to persist for more than 24 hours. The criteria for issuing an ASA vary according to local meteorological and physiographic conditions, air quality sensitivities, and administrative requirements and directives of the issuing agencies. Generally speaking, existing ASA criteria do not reference a standardized scientific definition of air stagnation, primarily because the atmospheric science community has not established a rigorous definition for the phenomenon and the meteorological conditions that contribute to its occurrence. This review was an initial attempt to document the criteria for issuing an ASA and to compare those criteria against our understanding of the meteorological conditions that promote air stagnation. Furthermore, this study wished to assess if existing forecasting techniques and tools could effectively diagnose the potential for air stagnation events, to recommend new tools or techniques for diagnosing air stagnation events, and finally, to define a scientific standard against which the existing ASA criteria could be compared.

In order to collect information on how and when ASAs are issued operationally, a list of organizations that have in the past issued air stagnation advisories or have an interest in the subject of air stagnation events was compiled. More than 170 inquiries were made either directly (by phone or email) or indirectly (email through NWS regional headquarters) during the fall of 2009 regarding air stagnation advisory guidelines and protocols.Responses were ultimately obtained from 20 organizations, generally from the western seaboard of the country. The responses varied greatly in content, ranging from specific criteria in an individual peer-reviewed paper to uncertainty about responsibility or detailsto information on other organizations that may or may not issue the advisories.

Current Advisory Issuance

The information that was collected indicated a wide range of conditions under which ASAs are issued. Significant differences were found even within the same agencies. For example, the three National Weather Service Weather Forecast Offices (WFOs) in Oregon that responded to our request all required 72 hours of their specific conditions to occur before an advisory would be issued, but those requirements were quite different at each WFO. Specific mixing height levels were a requirement at each of the three offices, but each office had different specific height thresholds (in this case, 1000, 1500, and 2000 feet). Two of the offices had upper wind threshold speeds (both at 5 knots), and two considered the presences of surface inversions. One office allowed for light precipitation in the form of drizzle, freezing drizzle or flurries to occur. In neighboring Washington State, the NWS criteria are completely different. The Seattle WFO calls for an advisory when conditions persist as little as 36 hours, winds less than 10 knots, mixing layer at or below 1500 feet, and the temperature must be at or above 87°F for at least two consecutive days.

A further query was made to both National Weather Service Offices and state level Departments of Environmental Quality/Protection. Of the 38 responses that were returned, 3 groups would issue these with a combination of a forecast tool as well as measurements of either particulate matter or ground level ozone. 28 responded that they issued Air Quality Alerts only at the request of another agency or from data originating from another agency. 5 responded that they do not issue Air Quality or Air Stagnation alerts at all.

The issuing of Air Quality Alerts was a common response to the question posed. These alerts were entirely based upon the Environmental Protection Agency’s Air Quality Index (AQI). The AQI is generated from real time monitoring of pollutants and information from the National Weather Service on whether conditions in the area will be conductive to the increase of pollutant concentrations.

Current State of the definition of Air Stagnation Events.

In general, air stagnation events are associated meteorologically with the presence of low-level inversions or relatively high atmospheric stability, low wind speeds, and the proximity/influence of synoptic-scale anticyclones. Many of the office or agency definitions of air stagnation events stem from the work of Korshover(1976) and Angell (1982). The same ideas and definitions were then brought forward into the study by Wang and Angell (1999), which is arguably the most comprehensive spatial study of air stagnation events in the scientific literature.

The definitions used in these studies were modified over time from an initial mix of objective and subjective measurements (Korshover, 1976) to a fully objective system (Wang and Angell, 1999). Wang and Angell developed the following specific criteria:

  • Derived surface level geostrophic wind speed is less that 8 m/s for at least 4 days. In general, this would correspond to observed surface wind speeds (at anemometer level) less than about 4 m/s. This criterion was based on the results of studies of the Donora, Pennsylvania smog disaster of 1948 (Fletcher, 1949; Willett, 1949; Hewson, 1951).
  • The maximum surface wind speed thresholdis increased to 4.4 m/s during the 4-dayperiod if an inversion exists in the boundary layer (defined as a positive change in temperature with height below 850 mb).
  • With the above criteria, the event is thrown out if there was any precipitation (including trace amounts) during the period.
  • The event was also thrown out if the wind speed at the 500 mb level exceeded 13 m/s.

Overall, Wang and Angell found that thiscombination of conditions tended to limit the events meteorologically to500 mb level high pressure ridges and warm-core high pressure systems since cold core high pressure systems tend not to persist long enough.

Continuation of Air Stagnation Event Study.

We have identified the following six major air stagnation events for future study:

December 10th-11th 2005 in the Pacific Northwest

November 28th-29th 2006 in New York, New York

July 10th 2008 in central Nevada

January 18th-20th 2009 in the Pacific Northwest

January 1st 2010in the pan handle region of Texas and Oklahoma.

August 22nd and 23rd 2010 in the pan handle region of Texas and Oklahoma.

The events, which occur in different regions, seasons, and meteorological environments, can be used to determine whether a single set of criteria (e.g. Wang and Angell, 1999) are capable of identifying air stagnation conditions for different regions of the United States. The North American Regional Reanalysis (NARR) can provide the meteorological conditions that prevailed during these events, which can then be compared against air stagnation criteria.

Upon completion of the analyses of the individual events, the NARR dataset can also be used todocument the historical frequency and distribution of meteorological conditions consistent with air stagnation.

References

Fletcher, R.D. 1949: The Donora smog disaster. A problem in atmospheric pollution.Weatherwise. June, 1949.

Hewson, E.W., 1951: Atmospheric Pollution. Compendium of Meteorology, T.F. Malone, Ed., American Meteorological Society, 1139-1157.

Korshover, J.,1976: Climatology of stagnating anticyclones east of the Rocky Mountains, 1936-1975. NOAA Tech. Memo. ERL ARL-55

Wang, J.X.L., and Angell, J. 1999: Air Stagnation climatology for the United States (1948-1998). NOAA/Air Resources Laboratory ATLAS No. 1

Willett, H.D. 1949: Some meteorological aspects of air pollution. Trans. Conf. on Industrial Wastes, Ind. Hyg.Found., Amer., Trans. Bull.No. 13, Mellon Institute, Pittsburg, PA.