STAR / SMCD / SPB Solar Backscatter Ultraviolet (SBUV/2) Project - Data Products - Version 8 Algorithm Description

Version 8 Algorithm Description

The Solar Backscatter Ultraviolet instruments, SBUV on Nimbus 7 and SBUV/2s on NOAA-9, -11, -14, -16 and -17, are nadir-viewing instruments that infer total column ozone and the ozone vertical profile by measuring sunlight scattered from the atmosphere in the ultraviolet spectrum. Heath et al. (1975) describes the SBUV flown on Nimbus-7. Frederick et al. (1986), and Hilsenrath et al. (1995) describe the follow-on SBUV/2 instruments flown on the NOAA series of spacecraft.

The instruments are all of similar design: nadir-viewing double-grating monochromators of the Ebert-Fastie type. The instruments step through 12 wavelengths in sequence over 24 seconds, while viewing the Earth in the fixed nadir direction with an instantaneous field of view (IFOV) on the ground of approximately 180 km by 180 km. To account for the change in the scene-reflectivity due to the motion of the satellite during the course of a scan, a separate co-aligned filter photometer (centered at 343 nm on SBUV; 380 nm on SBUV/2) makes 12 measurements concurrent with the 12 monochromator measurements. Each sequence of measurements is separated by 8 seconds from the next, producing a complete set every 32 seconds on the daylight portions of an orbit.

The instruments are flown in polar orbits to obtain global coverage. Since the SBUV ozone measurements rely on backscattered solar radiation, data are only taken on the dayside of each orbit. There are about 14 orbits per day with 26 degrees of separation at the equator. Unfortunately, the early NOAA polar orbiting satellites are not sun-synchronous. For example the NOAA-11 equator crossing times drifted from 1:30 pm (measurements at 30 degrees solar zenith angle at the equator) at the beginning of 1989 to 5:00 pm by the end of 1995 (measurements at 70 degrees solar zenith angle). As the orbit drifts, the terminator crossing location moves to lower latitudes and coverage decreases.

Ozone profiles and total column amounts are derived from the ratio of the observed backscattered spectral radiance to the incoming solar spectral irradiance. This ratio is referred to as the backscattered albedo. The only difference in the optical components between the radiance and irradiance observations is the instrument diffuser used to make the solar irradiance measurement; the remaining optical components are identical. Therefore, a change in the diffuser reflectivity will result in an apparent trend in ozone. This is the key calibration component for the SBUV(/2) series. See Hilsenrath et al. (1995) for a longer discussion.

The spectral resolutions for SBUV(/2) monochromators are all approximately 1.1 nm, full-width at half-maximum (FWHM) with triangular bandpasses. The bandwidths of the photometers are approximately 3 nm FWHM. The wavelength channels used for Nimbus 7 SBUV were: 256, 273, 283, 288, 292, 298, 302, 306, 312, 318, 331, and 340 nm. The wavelengths for NOAA-9 and the other NOAA SBUV/2 instruments were very similar except that the shortest channel was moved from 256 nm to 252 nm in order to avoid emission in the nitric oxide gamma band that contaminated the SBUV Channel 1 measurement. Data from the 256 and 252 nm channels are not used in this Version 8 processing for any of the instruments.

Version 8 SBUV(/2) Ozone Profile Retrieval Algorithm (V8A)

The Version 8 SBUV(/2) ozone profile retrieval algorithm combines backscattered ultraviolet measurements and a priori profile information in a maximum likelihood retrieval. See Rodgers (1990) for an analysis of this class of retrievals. It improves on the Version 6 SBUV(/2) algorithm described in Bhartia et al. (1996). Among the improvements are the following:

1. The V8A has a new set of a priori profiles varying by month and latitude, leading to better estimates in the troposphere (where SBUV/2 lacks retrieval information) and allowing simplified comparisons of SBUV/2 results to other measurement systems (in particular, to Umkehr ground-based ozone profile retrievals which use the same a priori data set).
2. The V8A has a true separation of the a priori and first guess. This simplifies averaging kernel analysis. Examples and further information are provided below.
3. The V8A has improved multiple scattering and cloud and reflectivity modeling. These corrections are updated as the algorithm iterates toward a solution.
4. Some errors present in the V6A will be reduced. These include the elimination of errors on the order of 0.5% by improved fidelity in the bandpass modeling.
5. The V8A incorporates several ad hoc Version 6 algorithm improvements directly. These include better modeling of the effects of the gravity gradient, better representation of atmospheric temperature influences on ozone absorption, and better corrections for wavelength grating position errors.
6. The algorithm uses improved terrain height information and gives profiles relative to a climatological surface pressure.
7. The V8A is also designed to allow the use of more accurate external and climatological data and allow simpler adjustments for changes in wavelength selection.
8. Finally, the V8A is designed for expansion to perform retrievals for hyperspectral instruments, such as the Ozone Monitoring Instrument (OMI), the Global Ozone Monitoring Experiment (GOME-2) and the Nadir Profiler in the Ozone Mapping and Profiler Suite (OMPS).

The atmospheric ozone absorption decreases by several orders of magnitude over the 252 to 340 nm wavelength range. The V8A uses a variable number of backscattered ultraviolet measurements depending on the solar zenith angle (SZA) of the observations to maintain its sensitivity to ozone changes in the lower atmosphere. For small SZAs (the sun high in the sky), only six wavelengths are used in the retrievals. They are at 273 nm, 283 nm, 288 nm, 292 nm, 298 nm, and 302 nm. As the SZA increases the 306 nm, then the 313 nm and finally the 318 nm channels are added to the retrieval.

Version 8 Algorithm A Priori Profiles

The a priori profile database is provided in a data file: climat.txt, (TXT, 20 KB). The profile data set gives the climatological averages for 18 10-degree latitude bands and 12 months. These profiles can be used to determine the information used in a specific retrieval by interpolating in latitude and day with a FORTRAN code, (TXT, 18 KB). The lowest layer from the a priori program will differ from that used in the retrieval if the surface pressure is not 1 atmosphere. Another data file: terrain.txt, (TXT, 1.31 MB) and FORTRAN code, (TXT, 2 KB) are provided to generate the surface pressure for a given latitude and longitude.

The a priori covariance is constructed as follows: the diagonal elements correspond to 50% variance and the non-diagonal covariance elements fall off with a correlation length of twelve fine layers (approximately two Umkehr layers). The measurement covariance is diagonal and corresponds to radiance errors of 1% in each channel.