Release Notice for Global Earth Coverage (GEC) Total Ozone Mapping Spectrometer (TOMS) Data rev. May 24, 2001
DATA STREAMS: gectomsX1.a1 derived from gectomsaerosolindexX1.00 gectomsozoneX1.00 gectomsreflX1.00 OVERVIEW These files contain daily values remapped into a regular 1 x 1.25 degree grid, of ozone, aerosol index and reflectivity from the TOMS instrument on the Earth Probe Satellite. Originally, the data obtained from Earth Probe (EP-) TOMS were intended to complement data obtained from ADEOS TOMS, which gave complete equatorial coverage due to its higher orbit. EP-TOMS was launched into a 500 kilometer orbit rather than the originally planned 950 kilometer orbit. The lower orbit decreased the size of the "footprint" of each measurement, which increased the resolution and also increased the ability to make measurements over cloudless scenes. This orbit was chosen to improve the ability of the TOMS instrument to make measurements of UV-absorbing aerosols in the troposphere. However, ADEOS failed in June 1997. The orbit of EP was then boosted to 740 km and circularized to provide coverage that is as complete as possible. As with the pre-boosted measurements, the EP-TOMS data still have missing elements at the equator. There are no EP TOMS data between 5-Dec-1997 and 12-Dec-97. More information about these data can be found at the NASA TOMS WEB site Data are available beginning July 25, 1996. If these EarthProbe/TOMS data are downloaded and used in publication, please give proper credit to the NASA/GSFC TOMS Ozone Processing Team (OPT). DESCRIPTION OF THE FILES: Daily values of ozone, aerosol index and reflectivity at each grid cell are taken from three ascii files and combined into one monthly netCDF file. Data for each day are gridded into 1 degree latitude zones by 1.25 degree longitude zones. Latitudes go from -90 degrees (the south pole) to 0 degrees (the equator) to +90 degrees (the north pole) in 1 degree steps, so there are 180 latitude zones. The first zone extends from -90 to -89, so the grid cell is considered to be centered on -89.5. Similarly, longitudes go from -180 (west longitude) to 0 (Greenwich, England) to +180 (east longitude) in 1.25 degree steps, so there are 288 longitude zones (360/1.25). The first longitude zone extends from -180 to -178.75, so the grid cell is considered to be centered on -179.375. Measurements: OZONE Ozone measurements are given in Dobson units and are integers with 3 significant figures. A Dobson Unit represents the physical thickness of the ozone layer if it were brought to the Earth's surface. A value of 300 Dobson units equals three millimeters or 1/10th of an inch. Missing values denote flagged data, i.e. data that could not be collected due to lack of sunlight or other problems. AEROSOL INDEX The data are an aerosol index formed directly from the measured radiances in two TOMS channels. Positive values generally represent absorbing aerosols (dust and smoke) while negative values represent nonabsorbing aerosols. The identification is not perfect because of geophysical reasons (e.g., aerosol too low to the ground). November 2000 Special Note: After four years in orbit, the Earth Probe TOMS instrument is experiencing some wavelength-dependent calibration drift which is affecting the aerosol index calculation (whereas the effect on total ozone calculation is very small). The problem is being looked into which could lead to a re-computation of recent aerosol index values and re-generation of recent aerosol index images for Earth Probe TOMS. REFLECTIVITY The solar irradiance reflected up to a spacecraft from the surface of the Earth can be used to calculate reflectivity. Reflectivity calculated for specific bandwidths is needed for the calculation of total column ozone from TOMS instrument. Reflected radiation can come from two surfaces, the ground, and the tops of clouds. Reflectivity is determined from the measurements at 380 nm, or 360 nm in the case of Earth Probe. Clouds are clearly defined and recognizable in images produced using TOMS reflectivity data. Data are in percent. For further information, please contact: Laurie Gregory or Richard Wagener 631-344-2266 631-344-5886 gregory@bnl.gov wagener@bnl.gov