The importance of surface reflectance anisotropy for cloud and NO<sub>2</sub> retrievals from GOME-2 and OMI
<p>The angular distribution of the light reflected by the Earth's surface influences top-of-atmosphere (TOA) reflectance values. This surface reflectance anisotropy has implications for UV/Vis satellite retrievals of albedo, clouds, and trace gases such as nitrogen dioxide (NO<sub&g...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-07-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | https://www.atmos-meas-tech.net/11/4509/2018/amt-11-4509-2018.pdf |
| Summary: | <p>The angular distribution of the light reflected by the Earth's surface
influences top-of-atmosphere (TOA) reflectance values. This surface
reflectance anisotropy has implications for UV/Vis satellite retrievals of
albedo, clouds, and trace gases such as nitrogen dioxide (NO<sub>2</sub>). These
retrievals routinely assume the surface to reflect light isotropically. Here
we show that cloud fractions retrieved from GOME-2A and OMI with the FRESCO
and OMCLDO2 algorithms have an east–west bias of 10 % to 50 %, which
are highest over vegetation and forested areas, and that this bias originates
from the assumption of isotropic surface reflection. To interpret the
across-track bias with the DAK radiative transfer model, we implement the
bidirectional reflectance distribution function (BRDF) from the Ross–Li
semi-empirical model. Testing our implementation against state-of-the-art
RTMs LIDORT and SCIATRAN, we find that simulated TOA reflectance generally
agrees to within 1 %. We replace the assumption of isotropic surface
reflection in the equations used to retrieve cloud fractions over forested
scenes with scattering kernels and corresponding BRDF parameters from a daily
high-resolution database derived from 16 years' worth of MODIS measurements.
By doing this, the east–west bias in the simulated cloud fractions largely
vanishes. We conclude that across-track biases in cloud fractions can be
explained by cloud algorithms that do not adequately account for the effects
of surface reflectance anisotropy. The implications for NO<sub>2</sub> air mass
factor (AMF) calculations are substantial. Under moderately polluted
NO<sub>2</sub> and backward-scattering conditions, clear-sky AMFs are up to 20 %
higher and cloud radiance fractions up to 40 % lower if surface
anisotropic reflection is accounted for. The combined effect of these changes
is that NO<sub>2</sub> total AMFs increase by up to 30 % for backward-scattering
geometries (and decrease by up to 35 % for forward-scattering
geometries), which is stronger than the effect of either contribution alone.
In an unpolluted troposphere, surface BRDF effects on cloud fraction
counteract (and largely cancel) the effect on the clear-sky AMF. Our results
emphasise that surface reflectance anisotropy needs to be taken into account
in a coherent manner for more realistic and accurate retrievals of clouds and
NO<sub>2</sub> from UV/Vis satellite sensors. These improvements will be
beneficial for current sensors, in particular for the recently launched
TROPOMI instrument with a high spatial resolution.</p> |
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| ISSN: | 1867-1381 1867-8548 |