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READY logoGaussian Dispersion with MOS Forecast Data

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NAM GFS
 

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  • NAM MOS Description
  • GFS MOS Description

    • Background Information:

    This program was originally published in 1981 by Roland R. Draxler as NOAA Technical Memorandum ERL ARL-100, titled, "Forty-eight hour Atmospheric Dispersion Forecasts at Selected Locations in the United States." The program has been updated to produce quick forecasts of atmospheric dispersion via the web by combining the simple techniques of estimating dispersion from Bruce Turner's Workbook of Atmospheric Dispersion Estimates (1994,1969) with National Weather Service (NWS) forecasts of wind direction, wind speed, cloud cover, and cloud ceiling. The NWS forecasts come from the NAM and GFS Model Output Statistics (MOS), which are statistically derived surface conditions produced for over 1000 locations in the CONUS, Alaska, Puerto Rica, and Hawaii.

    Model Assumptions:

    6-hour, straight-line flow, Gaussian dispersion forecasts from 1 to 100 km downwind are produced independently for each 3 hour forecast time given by the MOS forecast. That is, a continuous 6-hour emission is assumed to occur every 3 hours. The user chooses the dispersion forecast time to display on a graph (all the other forecasts are available in the text file) as well as the release height, receptor height, and the maximum daytime and nighttime mixing heights. The wind speed (not the direction) is adjusted to the pollutant source height by the relation:

    u = u10 (z/z10)P

    where u10 is the wind speed at height z10 (10 m) and u is the wind speed at emission height z. The exponent P is a function of the stability (Draxler, 1980). A daytime mixing depth is calculated with a nomogram (Smith and Hunt, 1978). At night an optional constant maximum mixing depth of 400 m is assumed. The constant nighttime value of 400 m will only affect ground-level concentrations during "D" stability at distances greater than 20 km downwind. Hence, the assumption of a constant nighttime maximum mixing depth is not considered to be a serious limitation.

    Draxler, R.R., 1981: Forty-eight hour atmospheric dispersion forecasts at selected locations in the United States, NOAA Technical Memorandum ERL ARL-100, 11 pp.

    Draxler, R.R., 1980: An improved Gaussian model for long-term average air concentration estimates, Atmos. Environ., 14: 597-601.

    Smith, F.B., and R.D. Hunt, 1978: Meteorological aspects of the transport of pollution over long distances, Atmos. Environ., 12: 461-477.

    Turner, D.B., 1994: Workbook of Atmospheric Dispersion Estimates, Second Edition, CRC Press, Inc., 2000 Corporate Blvd., N.W., Boca Raton, FL 33431.

    Turner, D.B., 1969: Workbook of Atmospheric Dispersion Estimates, Public Health Service Publication No. 999-AP-26, U.S. Government Printing Office, Washington, DC 20402, 84 pp.