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 (NO2). 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 NO2 air mass
factor (AMF) calculations are substantial. Under moderately polluted
NO2 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 NO2 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
NO2 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.
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). 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 air mass factor (AMF) calculations are substantial. Under moderately polluted NO.sub.2 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 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 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.
This overview paper highlights the successes of the Ozone Monitoring Instrument (OMI) on board the Aura satellite spanning a period of nearly 14 years. Data from OMI has been used in a wide range of ...applications and research resulting in many new findings. Due to its unprecedented spatial resolution, in combination with daily global coverage, OMI plays a unique role in measuring trace gases important for the ozone layer, air quality, and climate change. With the operational very fast delivery (VFD; direct readout) and near real-time (NRT) availability of the data, OMI also plays an important role in the development of operational services in the atmospheric chemistry domain.
The vulnerability of the European airspace to volcanic eruptions was brought to the attention of the public and the scientific community by the 2010 eruptions of the Icelandic volcano ...Eyjafjallajökull. As a consequence of this event, ash concentration thresholds replaced the “zero tolerance to ash” rule, drastically changing the requirements on satellite ash retrievals. In response to that, the ESA funded several projects aiming at creating an optimal end-to-end system for volcanic ash plume monitoring and prediction. Two of them, namely the SACS-2 and SMASH projects, developed and improved dedicated satellite-derived ash plume and sulfur dioxide level assessments. The validation of volcanic ash levels and height extracted from the GOME-2 and IASI instruments on board the MetOp-A satellite is presented in this work. EARLINET lidar measurements are compared to different satellite retrievals for two eruptive episodes in April and May 2010. Comparisons were also made between satellite retrievals and aircraft lidar data obtained with the UK's BAe-146-301 Atmospheric Research Aircraft (managed by the Facility for Airborne Atmospheric Measurements, FAAM) over the United Kingdom and the surrounding regions. The validation results are promising for most satellite products and are within the estimated uncertainties of each of the comparative data sets, but more collocation scenes would be desirable to perform a comprehensive statistical analysis. The satellite estimates and the validation data sets are better correlated for high ash optical depth values, with correlation coefficients greater than 0.8. The IASI retrievals show a better agreement concerning the ash optical depth and ash layer height when compared with the ground-based and airborne lidar data.
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