Air Stagnation Index
Calculation Method and Literature Review
Atmospheric pollution manifests itself in many ways, ranging from reduced visibility to dangerous respiratory problems and discomfort. Atmospheric pollution can be gaseous (e.g. ozone, sulfur dioxide, nitrogen oxides) and/or particulate (e.g. soot, dust). The degree of pollution is dependent on a number of factors: source, transport from source, and temporal build up through stagnation. The stagnation index is intended as an indication of the latter only.
In this context, stagnation is considered to consist of light winds so that horizontal dispersion is at a minimum, a stable lower atmosphere that effectively prevents vertical escape, and no precipitation to wash any pollution away. These conditions are most frequently met when there is a persistent or slow moving high pressure system.
One of the most prevalent air pollutants present in the lower levels of the atmosphere during the late spring and summer seasons is ground level ozone. This ozone is produced when pollutants from cars and factories are "cooked" by a hot summer sun. Consequently, concentrations of ground level ozone are typically highest during periods of high temperature and become a health problem particularly when air is stagnant.
During the summer of 2002, conditions from Missouri east to New Jersey and down through South Carolina were conducive to air stagnation. Compared to the summer of 2000, the percentage of days with air stagnation conditions nearly doubled. The maps below show conditions in July 2000 and July 2002.
Shown below are maps of ground ozone concentrations provided by the Environmental Protection Agency for one day in July. Click here to see maps for all 31 days. The yellow areas indicate a moderate 8-hour average peak concentration whereas the orange and red areas indicate locations where the concentration of ozone was deemed unhealthy.
The Air Stagnation Index maps provided here are based on the work of Wang and Angell (1999), but with some slight modifications. To make assessment of stagnation totally objective, they defined a stagnation day as one with sea level geostrophic wind less than 8m/sec, 500mb wind less than 13m/sec, and no precipitation. If there is a temperature inversion below 850mb the 8m/sec is relaxed by 10% (to 8.8m/sec). We calculated a stagnation day using the same method but performed the calculations over a finer grid (0.25 x 0.25 degree instead of 2.5 x 2.5 degree). The Wang and Angell stagnation index is the number of 4-day stagnation periods. The stagnation index depicted on this page is the percentage of days in the month that meet the requirements for stagnation conditions.
Gridded mean sea level pressure, 500mb wind components, and surface and 850mb temperature data were obtained from the NCEP/NCAR reanalysis/CDAS system (Kalnay et al. 1996). These data are available at ftp.cdc.noaa.gov/Datasets/ncep.reanalysis.dailyavgs/ and are updated on a daily basis.
Gridded daily precipitation data were obtained from CPC. Data for years prior to 2007 are available at ftp://ftp.cpc.ncep.noaa.gov/precip/CPC_UNI_PRCP/GAUGE_CONUS/V1.0/. More current data can be obtained from ftp://ftp.cpc.ncep.noaa.gov/precip/CPC_UNI_PRCP/GAUGE_CONUS/RT/.
All gridded datasets cover the area of the Lower-48 states. The reanalysis data are based on 2.50 x 2.50 degree grids, while precipitation has a much finer, 0.250 x 0.250 degree, resolution. To reconcile this discrepancy, the variables obtained from reanalysis data were interpolated to obtain 0.250 x 0.250 degree grid point values before calculation of the stagnation index.
- Kalnay, V.E., and co-authors, 1996: The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteor. Soc., 77, 437-471.
- Wang, J.X.L., and J.K. Angell, 1999: Air Stagnation Climatology for the United States (1948-1998). NOAA/Air Resources Laboratory ATLAS, No.1