Solar Backscattering Ultraviolet-2 (SBUV/2) instrument on NOAA-9 orbit on June 1987, solar zenith angles of the observations; plot of weekly average differences between SBUV (Nimbus-7 and SBUV/2 (NOAA-9); radiance comparisons for March 1986; time dependence of relative change between SBUV and SBUV/2; and explicit wavelength dependence are presented in viewgraph format. Each is briefly discussed

Mcpeters, Richard D.

English

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A Technique for Directly Comparing Radiances
From Two Satellites N9 i -- i 9 52 0
Richard D McPeters
Laboratory for Atmospheres
NASA/Goddard Space Flight Center
(in cooperation with NOAA/NESDIS) _ _I i_
The SBUV instrument on Nimbus 7 and the SBUV-2 instrument on
NOAA-9 are almost identical, each measuring the ultraviolet
atmospheric albedo at wavelengths from 2500-3400 A to infer total
ozone and ozone profiles. While derived ozone has been compared
for the two instruments, understanding the calibration
differences between the two instruments requires that the
measured radiances be compared. The problem is that SBUV is in a
noon sun synchronous orbit while SBUV-2 is in an afternoon orbit
(Figure i). Radiances cannot be compared directly because the
solar zenith angles of the observations are different (Figure 2).
Comparison of ozone values avoids this problem because solar
zenith angle effects are implicitly taken into account in the
retrieval algorithm. Figure 3 is a plot of the weekly average
differences between SBUV and SBUV-2 of total ozone and of Umkehr
layer ozone amounts. Initial biases between the two instruments,
5% in total ozone in March of 1985, are probably due to degrada-
tion of the SBUV diffuser plate, which had been in use for six
years when NOAA-9 was launched. The relative change between 1985
and 1987 was significant for total ozone but was especially
striking for ozone in the upper stratosphere, layers 7-10. This
change was likely due to SBUV-2 orbit drift since SBUV degrada-
tion was fairly well known and much less rapid. In order to
understand this change in ozone, it would be very useful to know
the changes in the radiances.
A technique to compare radiances has been developed in which the
measured albedo at each wavelength is put onto a common solar
zenith angle scale. A forward calculation for each wavelength is
done using the derived ozone profile, then repeated for the
common solar zenith angle to predict what each instrument would
have measured had it been at the standard solar zenith angle.
This technique is accurate provided neither the zenith angle nor
the zenith angle correction is large. Direct comparisons of the
instrument measurements (Q values) are then possible. The Q
value is the ratio of the backscattered radiance to the
extraterrestrial solar irradiance scaled by the Rayleigh
scattering phase function.
Such radiance comparisons for March of 1986 are shown in Figures
4 and 5. Differences traceable to calibration should be
independent of latitude; ie., the difference curves should be
flat. Significant deviations occur at large solar zenith angles
(high latitudes) and at wavelengths for which multiple scattering
is important (2975-3058 A), indicating the limits for the
6-1
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technique.
The time dependence of the relative change between the two
instruments is shown in Figure 6, normalized to day i. Between
March 1985 and August 1987 there was almost a 14% relative change
at 2975 A, the longest wavelength shown, and a 20% relative
change at the shortest wavelength, 2735 A. During this period
the SBUV-2 orbit drifted from 2 PM local time to approximately 5
PM local time. The increasing incidence angle on the diffuser
plate during solar flux measurements is being examined as the
possible cause of the apparent SBUV-2 calibration change. To
first order, such goniometric errors would be expected to be
wavelength independent.
The explicit wavelength dependence is shown in Figure 7. The
most striking conclusion is that the differences between SBUV and
SBUV-2 are almost wavelength independent for wavelengths 2735 A
through 2922 A. The results for wavelengths 2975-3058 A are less
reliable because multiple scattering is important for these
wavelengths but was not included in the forward calculation. But
the fact that total ozone changes relative to the Dobson network
are observed indicates that there is some wavelength dependent
error for wavelengths greater than 3000 A. The wavelength
dependence that is seen at the shorter wavelengths is consistent
with the long term degradation of the SBUV diffuser plate, which
is know to greater at shorter wavelengths.
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Attachment 7
SBUV/2 Comparisons
A.J. Miller and R. Nagatani
NOAA/NWS-CAC



A technique for directly comparing radiances from two satellites

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