The aerosol shortwave, direct radiative effects of smoke plumes from Chilean wildfires in 2017 and 2023 were derived from satellite observations in both cloud‐free and cloud scenes. At the top of the ...atmosphere, the aerosol DRE changes sign when aerosol overly clouds or open ocean, confirmed by both measurements and a simulation study. The cloud‐free daily‐mean DRE, computed using an offline radiative transfer model (RTM), was 66 W m−2 in 2023 and 42 W m−2 in 2017, due to absorption by smoke. However, the total radiative effects were larger in 2017 due to a larger plume size compared to 2023. The method presented here provides a new conceptual model to quickly assess the radiative effects of wildfire smoke plumes using satellite measurements and pre‐computed RTM results. The presented estimates are strongly affected by the uncertainty of aerosol optical thickness retrievals from satellite, which can be large in the presence of clouds.
Plain Language Summary
From 30 January to the end of February 2023, central Chile experienced over 400 individual wildfires, consuming over 430,000 ha of native sclerophyllous forests, with 25 fatalities as of 14 February. Wildfires are common in central Chile during dry periods, but seem intensified due to the mega‐drought since 2010. Next to health effects, the smoke from these wildfires has important climatic impacts through the change of solar insolation in the atmosphere: smoke strongly absorbs solar radiation and heats the atmosphere, changing the vertical stability. In this paper, the horizontal and vertical smoke distribution in the atmosphere is presented using satellite observations, and the radiative effects in the atmosphere and at the surface are quantified for the recent fires and as well as for the record‐breaking wildfires in 2017, which were the most devastating in the modern history of central‐Chile. Our results show that the radiative effects of the smoke from the recent wildfires were stronger in magnitude during the first few days, but confined to a smaller area, reducing their overall effect.
Key Points
The radiative effects of the recent 2023 Chilean wildfire smoke plumes are assessed and compared to the 2017 Chilean wildfires smoke plumes
Pre‐computed radiative transfer model results can provide readily available aerosol direct radiative effects in clear‐sky for smoke
Satellite measurements can further improve the aerosol direct effect and direct forcing estimates for both clear sky and cloud scenes
The direct radiative effect (DRE) of aerosols above
clouds has been found to be significant over the south-east Atlantic
Ocean during the African biomass burning season due to elevated smoke
layers ...absorbing radiation above the cloud deck. So far, global
climate models have been unsuccessful in reproducing the high DRE
values measured by various satellite instruments. Meanwhile, the
radiative effects by aerosols have been identified as the largest
source of uncertainty in global climate models. In this paper, three
independent satellite datasets of DRE during the biomass burning
season in 2006 are compared to constrain the south-east Atlantic
radiation budget. The DRE of aerosols above clouds is derived from the
spectrometer SCanning Imaging Absorption spectroMeter for
Atmospheric CHartographY (SCIAMACHY), the polarimeter Polarization and
Directionality of the Earth's Reflectances (POLDER), and
collocated measurements by the spectrometer Ozone Monitoring
Instrument (OMI) and the imager Moderate Resolution Imaging
Spectroradiometer (MODIS). All three datasets confirm the high DRE
values during the biomass season, underlining the relevance of local
aerosol effects. Differences between the instruments can be attributed
mainly to sampling issues. When these are accounted for, the
remaining differences can be explained by a higher cloud optical
thickness (COT) derived from POLDER compared to the other instruments
and a neglect of aerosol optical thickness (AOT) at shortwave infrared
(SWIR) wavelengths in the method used for SCIAMACHY and OMI–MODIS. The
higher COT from POLDER by itself can explain the difference found in
DRE between POLDER and the other instruments. The AOT underestimation
is mainly evident at high values of the aerosol DRE and accounts for
about a third of the difference between POLDER and OMI–MODIS DRE. The
datasets from POLDER and OMI–MODIS effectively provide lower and upper
bounds for the aerosol DRE over clouds over the south-east Atlantic,
which can be used to challenge global circulation models (GCMs). Comparisons
of model and satellite datasets should also account for sampling
issues. The complementary DRE retrievals from OMI–MODIS and POLDER may
benefit from upcoming satellite missions that combine spectrometer
and polarimeter measurements.
An improved tropospheric nitrogen dioxide (NO2) retrieval algorithm from the Global Ozone Monitoring Experiment-2 (GOME-2) instrument based on air mass factor (AMF) calculations performed with more ...realistic model parameters is presented. The viewing angle dependency of surface albedo is taken into account by improving the GOME-2 Lambertian-equivalent reflectivity (LER) climatology with a directionally dependent LER (DLER) dataset over land and an ocean surface albedo parameterisation over water. A priori NO2 profiles with higher spatial and temporal resolutions are obtained from the IFS (CB05BASCOE) chemistry transport model based on recent emission inventories. A more realistic cloud treatment is provided by a clouds-as-layers (CAL) approach, which treats the clouds as uniform layers of water droplets, instead of the current clouds-as-reflecting-boundaries (CRB) model, which assumes that the clouds are Lambertian reflectors. On average, improvements in the AMF calculation affect the tropospheric NO2 columns by ±15 % in winter and ±5 % in summer over largely polluted regions. In addition, the impact of aerosols on our tropospheric NO2 retrieval is investigated by comparing the concurrent retrievals based on ground-based aerosol measurements (explicit aerosol correction) and the aerosol-induced cloud parameters (implicit aerosol correction). Compared with the implicit aerosol correction utilising the CRB cloud parameters, the use of the CAL approach reduces the AMF errors by more than 10 %. Finally, to evaluate the improved GOME-2 tropospheric NO2 columns, a validation is performed using ground-based multi-axis differential optical absorption spectroscopy (MAXDOAS) measurements at different BIRA-IASB stations. At the suburban Xianghe station, the improved tropospheric NO2 dataset shows better agreement with coincident ground-based measurements with a correlation coefficient of 0.94.
The retrieval of geophysical parameters is increasingly
dependent on synergistic use of satellite instruments. More
sophisticated parameters can be retrieved and the accuracy of retrievals
can be ...increased when more information is combined. In this paper,
a synergistic application of Ozone Monitoring Instrument (OMI), on the Aura platform, and Moderate Resolution Imaging Spectroradiometer (MODIS), on the Aqua platform, Level 1B reflectances is described, enabling the retrieval of the aerosol direct
radiative effect (DRE) over clouds using the differential aerosol
absorption (DAA) technique. This technique was first developed for
reflectances from the SCanning Imaging Absorption spectroMeter for
Atmospheric CHartographY (SCIAMACHY) on the Environmental Satellite (Envisat), which had the unique
capability of measuring contiguous radiances from the ultraviolet (UV)
at 240 to 1750 nm in the shortwave-infrared (SWIR), at a moderate
spectral resolution of 0.2 to 1.5 nm. However, the spatial resolution
and global coverage of SCIAMACHY was limited, and Envisat stopped
delivering data in 2012. In order to continue the DRE data retrieval,
reflectances from OMI and MODIS, flying in formation, were combined from
the UV to the SWIR. This resulted in reflectances at a limited but
sufficient spectral resolution, available at the OMI pixel grid, which
have a much higher spatial resolution and coverage than SCIAMACHY. The
combined reflectance spectra allow the retrieval of cloud microphysical
parameters in the SWIR, and the subsequent retrieval of aerosol DRE over
cloud scenes using the DAA technique. For liquid cloud scenes in the
south-east Atlantic region with cloud fraction (CF) >0.3, the area-averaged instantaneous aerosol DRE over clouds in June to August 2006 was 25 Wm−2 with a
standard deviation of 30 Wm−2. The maximum area-averaged
instantaneous DRE from OMI–MODIS in August 2006 was 75.6±13 Wm−2. The new aerosol DRE over-cloud dataset from OMI–MODIS is
compared to the SCIAMACHY dataset for the period 2006 to 2009, showing a
very high correlation. The OMI–MODIS DRE dataset over the Atlantic Ocean
is highly correlated to above-cloud AOT measurements from OMI and MODIS.
It is related to AOT measurements over Ascension Island in 2016, showing
the transport of smoke all the way from its source region in Africa over
the Atlantic to Ascension and beyond.
The aim of this study is to investigate the potential of the
Global Ozone Monitoring Experiment-2 (GOME-2) instruments, aboard the
Meteorological Operational (MetOp)-A, MetOp-B and MetOp-C satellite ...programme platforms, to
deliver accurate geometrical features of lofted aerosol layers. For this
purpose, we use archived ground-based lidar data from stations available
from the European Aerosol Research Lidar Network (EARLINET) database. The
data are post-processed using the wavelet covariance transform (WCT) method
in order to extract geometrical features such as the planetary boundary
layer (PBL) height and the cloud boundaries. To obtain a significant number
of collocated and coincident GOME-2 – EARLINET cases for the period between
January 2007 and September 2019, 13 lidar stations, distributed over
different European latitudes, contributed to this validation. For the 172
carefully screened collocations, the mean bias was found to be −0.18 ± 1.68 km,
with a near-Gaussian distribution. On a station basis, and with a
couple of exceptions where very few collocations were found, their mean
biases fall in the ± 1 km range with an associated standard deviation
between 0.5 and 1.5 km. Considering the differences, mainly due to the
temporal collocation and the difference, between the satellite pixel size
and the point view of the ground-based observations, these results can be
quite promising and demonstrate that stable and extended aerosol layers as
captured by the satellite sensors are verified by the ground-based data. We
further present an in-depth analysis of a strong and long-lasting Saharan
dust intrusion over the Iberian Peninsula. We show that, for this
well-developed and spatially well-spread aerosol layer, most GOME-2
retrievals fall within 1 km of the exact temporally collocated lidar
observation for the entire range of 0 to 150 km radii. This finding further
testifies for the capabilities of the MetOp-borne instruments to sense the
atmospheric aerosol layer heights.
The FRESCO (Fast Retrieval Scheme for Clouds from the Oxygen A band)
algorithm is a simple, fast and robust algorithm used to retrieve cloud
information in operational satellite data processing. It ...has been applied to
GOME-1 (Global Ozone Monitoring Experiment), SCIAMACHY (Scanning Imaging
Absorption Spectrometer for Atmospheric Chartography), GOME-2 and more
recently to TROPOMI (Tropospheric Monitoring Instrument). FRESCO retrieves
effective cloud fraction and cloud pressure from measurements in the oxygen A
band around 761 nm. In this paper, we propose a new version of the
algorithm, called FRESCO-B, which is based on measurements in the oxygen B
band around 687 nm. Such a method is interesting for vegetated
surfaces where the surface albedo is much lower in the B band than in the A
band, which limits the ground contribution to the top-of-atmosphere
reflectances. In this study we first perform retrieval simulations. These
show that the retrieved cloud pressures from FRESCO-B and FRESCO differ only
between −10 and +10 hPa, except for high, thin clouds over
vegetation where the difference is larger (about +15 to +30 hPa),
with FRESCO-B yielding higher pressure. Next, inter-comparison between
FRESCO-B and FRESCO retrievals over 1 month of GOME-2B data reveals that the
effective cloud fractions retrieved in the O2 A and B bands are
very similar (mean difference of 0.003), while the cloud pressures show a
mean difference of 11.5 hPa, with FRESCO-B retrieving higher
pressures than FRESCO. This agrees with the simulations and is partly due to
deeper photon penetrations of the O2 B band in clouds compared to
the O2 A-band photons and partly due to the surface albedo bias in
FRESCO. Finally, validation with ground-based measurements shows that the
FRESCO-B cloud pressure represents an altitude within the cloud boundaries
for clouds that are not too far from the Lambertian reflector model, which
occurs in about 50 % of the cases.
The ultraviolet (UV) Absorbing Aerosol Index (AAI) is widely used as an indicator for the presence of absorbing aerosols in the atmosphere. Here we consider the TROPOMI AAI based on the 340 nm/380 nm ...wavelength pair. We investigate the effects of clouds on the AAI observed at small and large scales. The large-scale effects are studied using an aggregate of TROPOMI measurements over an area mostly devoid of absorbing aerosols (Pacific Ocean). The study reveals that several structural features can be distinguished in the AAI, such as the cloud bow, viewing zenith angle dependence, sunglint, and a previously unexplained increase in AAI values at extreme viewing and solar geometries. We explain these features in terms of the bidirectional reflectance distribution function (BRDF) of the scene in combination with the different ratios of diffuse and direct illumination of the surface at 340 and 380 nm. To reduce the dependency on the BRDF and homogenize the AAI distribution across the orbit, we present three different AAI retrieval models: the traditional Lambertian scene model (LSM), a Lambertian cloud model (LCM), and a scattering cloud model (SCM). We perform a model study to assess the propagation of errors in auxiliary databases used in the cloud models. The three models are then applied to the same low-aerosol region. Results show that using the LCM and SCM gives on average a higher AAI than the LSM. Additionally, a more homogeneous distribution is retrieved across the orbit. At the small scale, related to the high spatial resolution of TROPOMI, strong local increases and decreases in AAI are observed in the presence of clouds. The BRDF effect presented here is a first step – more research is needed to explain the small-scale cloud effects on the AAI.
Solar-induced fluorescence (SIF) data from satellites are increasingly used as a proxy for photosynthetic activity by vegetation, and as a constraint on gross primary production. Here we report on ...improvements in the algorithm to retrieve mid-morning (09:30 hrs local time) SIF estimates on the global scale from GOME-2 sensor on the Metop-A satellite (GOME-2A) for the period 2007-2019. Our new SIFTER (Sun-Induced Fluorescence of Terrestrial Ecosystems Retrieval) v2 algorithm improves over a previous version by using a narrower spectral window that avoids strong oxygen absorption and is less sensitive to water vapour absorption, by constructing stable reference spectra from a 6-year period (2007-2012) of atmospheric spectra over the Sahara, and by applying a latitude-dependent zero-level adjustment that accounts for biases in the data product. We generated stable, good-quality SIF retrievals between January 2007 and June 2013, when GOME-2A degradation in the near infrared was still limited. After the narrowing of the GOME-2A swath in July 2013, we characterized the throughput degradation of the level-1 data in order to derive reflectance corrections and apply these for the SIF retrievals between July 2013 and December 2018. SIFTER v2 data compares well with the independent NASA v2.8 data product. Especially in the evergreen tropics, SIFTER v2 no longer shows the underestimates against other satellite products that were seen in SIFTER v1. The new data product includes uncertainty estimates for individual observations, and is best used for mostly clear-sky scenes, and when spectral residuals remain below a certain spectral autocorrelation threshold. Our results support the use of SIFTER v2 data to be used as an independent constraint on photosynthetic activity on regional to global scales.
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 pollutedNO2 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 andNO2 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 retrieval of geophysical parameters is increasingly dependent on synergistic use of satellite instruments. More sophisticated parameters can be retrieved and the accuracy of retrievals can be ...increased when more information is combined. In this paper, a synergistic application of Ozone Monitoring Instrument (OMI), on the Aura platform, and Moderate Resolution Imaging Spectroradiometer (MODIS), on the Aqua platform, Level 1B reflectances is described, enabling the retrieval of the aerosol direct radiative effect (DRE) over clouds using the differential aerosol absorption (DAA) technique. This technique was first developed for reflectances from the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) on the Environmental Satellite (Envisat), which had the unique capability of measuring contiguous radiances from the ultraviolet (UV) at 240 to 1750 nm in the shortwave-infrared (SWIR), at a moderate spectral resolution of 0.2 to 1.5 nm. However, the spatial resolution and global coverage of SCIAMACHY was limited, and Envisat stopped delivering data in 2012. In order to continue the DRE data retrieval, reflectances from OMI and MODIS, flying in formation, were combined from the UV to the SWIR. This resulted in reflectances at a limited but sufficient spectral resolution, available at the OMI pixel grid, which have a much higher spatial resolution and coverage than SCIAMACHY. The combined reflectance spectra allow the retrieval of cloud microphysical parameters in the SWIR, and the subsequent retrieval of aerosol DRE over cloud scenes using the DAA technique. For liquid cloud scenes in the south-east Atlantic region with cloud fraction (CF) 0.3, the area-averaged instantaneous aerosol DRE over clouds in June to August 2006 was 25 Wm.sup.-2 with a standard deviation of 30 Wm.sup.-2 . The maximum area-averaged instantaneous DRE from OMI-MODIS in August 2006 was 75.6±13 Wm.sup.-2 . The new aerosol DRE over-cloud dataset from OMI-MODIS is compared to the SCIAMACHY dataset for the period 2006 to 2009, showing a very high correlation. The OMI-MODIS DRE dataset over the Atlantic Ocean is highly correlated to above-cloud AOT measurements from OMI and MODIS. It is related to AOT measurements over Ascension Island in 2016, showing the transport of smoke all the way from its source region in Africa over the Atlantic to Ascension and beyond.
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