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Stratospheric Ozone and Human Health Project
EP/TOMS-derived UV map 

Near-Real Time Global UV Dose Map Generation
Ion Mateescu, Consortium for International Earth Science Information Network, Socioeconomic Data and Applications Center, Columbia University, Palisades, NY
ABSTRACT:
Daily integrated estimates of clear sky erythema-effective UV dose amounts and peak UV Index estimates are being generated at a 1x1 degree latitude/longitude resolution using near-real time total column ozone abundances measured by NASA's Total Ozone Mapping Spectrometer (TOMS) instruments carried on board the Earth Probe (EP) and Advanced Earth Observation Satellite (ADEOS). These data are provided by NASA through the Goddard Space Flight Center (GSFC) in near-real time via an anonymous ftp server.

TABLE OF CONTENTS

  1. Grid Sampling and Datasets
  2. Solar Zenith Angle and Daily Dose Estimation
  3. Image Generation Overview

Grid Sampling and Datasets

The near-real time global maps depicting clear sky erythema-effective UV dose amounts and UV Index values at peak solar zenith angles are presented at a 1x1 degree latitude/longitude resolution. This resolution was selected to match the resolution of the 1x1 degree global elevation dataset employed in the calculations to consider impacts of elevation change on incident UV radiation. Gridded datasets used in the UV dose estimation process are described briefly below:
Global Elevation Data. Effects of topography on the amount of radiation reaching the surface can be significant; higher altitude regions will receive more direct beam radiation than will lower altitude regions since the number of scattering events decrease with the density of the atmosphere, resulting in less radiation being scattered back to space. RAND's global elevation and depth dataset obtained from the National Center for Atmospheric Research's (NCAR) Data Support Section (DSS) was used to scale surface-level UV dose calculations at each grid cell; a nominal scaling factor of 6% increase per kilometer change in altitude was employed to estimate exposure levels for areas above sea level.
Total Ozone Mapping Spectrometer (TOMS). Near-real time total column ozone abundances are provided from the two newest members of the TOMS family, on board the Earth Probe (EP) and Advanced Earth Observing Satellite (ADEOS) platforms. These data are provided at a 1x1.25 degree latitude/longitude resolution. Due to the nature of the platform's orbits, complete global coverage is not obtained; the orbits were designed to provide complete coverage of the high southern latitudes during the Antarctic ozone hole season, and subsequently the high northern latitudes are not completely covered during this time of year. Areas having no data are depicted in black on the maps. Due to the resolution discrepancy between the gridded elevation and ozone datasets, every fifth 1x1 degree grid cell centroid fell on the boundary of two ozone grid cells. In this case the column ozone amount for that cell was taken as the simple average of the 2 adjoining cells.
 

Solar Zenith Angle and Daily Dose Estimation

Daily integrated dose levels are found for each grid cell based on the length of effective daylight estimated at each 1 degree latitude and longitude spacing, and the ozone amount measured for each grid. This process requires several steps. First, UV dose amounts were calculated at three degree solar zenith angle intervals ranging from 0-84 degrees for Dobson Unit values ranging from 100-599 DU. Boundary layer extinction was fixed at 0.2 km-1 for all calculations due to the lack of global information on this variable. Secondly, solar zenith angles are calculated for each 1x1 grid cell (64,800 total) at half-hour increments over the 24-hour period. UV dose levels are then found for the specific Dobson Unit amount at each grid cell by interpolating between pre-calculated values at the three degree zenith angle spacing for those times during the day whose zenith angle is less than or equal to 84 degrees. It is assumed that UV levels at zenith angles greater than 84 degrees will be negligible. These half-hourly dose values are summed to give the daily estimate of ground level exposure.

Image Generation Overview

The mdel calculates estimates for UV Index and Erythemal Dose at each node of a 180x360 grid.  The projection is changed to an equal area Hammer-Aitoff projection to eliminate distorted views at high latitudes.  General Mapping Tools software is used to reproject the grid and to generate postscript code for the map legend and annotations. A rainbow scheme is used for the color palette. Gray is reserved for area with no coverage. No coverage appearing as stripes on the image is due to the low altitude of the Earth Probe orbit. The projected shoreline is then overlayed  over the reprojected data. Ghostscript interprets the postscript code and generate a raster image in ppm format, which is then comverted into GIF by the pbm utilities.
 

Additional sources of information

For more information on the radiative transfer model and UV dose calculations, see the following documents:
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