The radiative forcing from aerosols (particularly through their interaction with clouds) remains one of the most uncertain components of the human forcing of the climate. Observation-based studies ...have typically found a smaller aerosol effective radiative forcing than in model simulations and were given preferential weighting in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). With their own sources of uncertainty, it is not clear that observation-based estimates are more reliable. Understanding the source of the model and observational differences is thus vital to reduce uncertainty in the impact of aerosols on the climate. These reported discrepancies arise from the different methods of separating the components of aerosol forcing used in model and observational studies. Applying the observational decomposition to global climate model (GCM) output, the two different lines of evidence are surprisingly similar, with a much better agreement on the magnitude of aerosol impacts on cloud properties. Cloud adjustments remain a significant source of uncertainty, particularly for ice clouds. However, they are consistent with the uncertainty from observation-based methods, with the liquid water path adjustment usually enhancing the Twomey effect by less than 50%. Depending on different sets of assumptions, this work suggests that model and observation-based estimates could be more equally weighted in future synthesis studies.
The HadGEM2 earth system climate model was used to assess the impact of biomass burning on surface ozone concentrations over the Amazon forest and its impact on vegetation, under present-day climate ...conditions. Here we consider biomass burning emissions from wildfires, deforestation fires, agricultural forest burning, and residential and commercial combustion. Simulated surface ozone concentration is evaluated against observations taken at two sites in the Brazilian Amazon forest for years 2010 to 2012. The model is able to reproduce the observed diurnal cycle of surface ozone mixing ratio at the two sites, but overestimates the magnitude of the monthly averaged hourly measurements by 5-15 ppb for each available month at one of the sites. We vary biomass burning emissions over South America by plus or minus 20, 40, 60, 80 and 100% to quantify the modelled impact of biomass burning on surface ozone concentrations and ozone damage on vegetation productivity over the Amazon forest. We used the ozone damage scheme in the "high" sensitivity mode to give an upper limit for this effect. Decreasing South American biomass burning emissions by 100% (i.e. to zero) reduces surface ozone concentrations (by about 15 ppb during the biomass burning season) and suggests a 15% increase in monthly mean net primary productivity averaged over the Amazon forest, with local increases up to 60%. The simulated impact of ozone damage from present-day biomass burning on vegetation productivity is about 230 TgC yr-1. Taking into account that uncertainty in these estimates is substantial, this ozone damage impact over the Amazon forest is of the same order of magnitude as the release of carbon dioxide due to fire in South America; in effect it potentially doubles the impact of biomass burning on the carbon cycle.
On 15–16 October 2017, ex-hurricane Ophelia passed to the
west of the British Isles, bringing dust from the Sahara and smoke from
Portuguese forest fires that was observable to the naked eye and ...reported in
the UK's national press. We report here detailed observations of this event
using the UK operational lidar and sun-photometer network, established for the
early detection of aviation hazards, including volcanic ash. We also use
ECMWF ERA5 wind field data and MODIS imagery to examine the aerosol
transport. The observations, taken continuously over a period of 30 h,
show a complex picture, dominated by several different aerosol layers at
different times and clearly correlated with the passage of different
air masses associated with the intense cyclonic system. A similar evolution
was observed at several sites, with a time delay between them explained by
their different location with respect to the storm and associated
meteorological features. The event commenced with a shallow dust layer at
1–2 km in altitude and culminated in a deep and complex structure that lasted
∼12 h at each site over the UK, correlated with the storm's warm sector.
For most of the time, the aerosol detected was dominated by mineral dust
mixtures, as highlighted by depolarisation measurements, but an intense
biomass burning aerosol (BBA) layer was observed towards the end of the
event, lasting around 3 h at each site. The aerosol optical depth at
355 nm (AOD355) during the whole event ranged from 0.2 to 2.9, with the
larger AOD correlated to the intense BBA layer. Such a large AOD is
unprecedented in the UK according to AERONET records for the last
20 years. The Raman lidars permitted the measurement of the aerosol
extinction coefficient at 355 nm, the particle linear depolarisation ratio
(PLDR), and the lidar ratio (LR) and made the separation of the dust
(depolarising) aerosol from other aerosol types possible. A specific extinction has
also been computed to provide an estimate of the atmospheric concentration of
both aerosol types separately, which peaked at 420±200 µg m−3 for
the dust and 558±232 µg m−3 for the biomass burning aerosols.
Back trajectories computed using the Numerical Atmospheric-dispersion
Modelling Environment (NAME) were used to identify the sources and strengthen
the conclusions drawn from the observations. The UK network represents a
significant expansion of the observing capability in northern Europe, with
instruments evenly distributed across Great Britain, from Camborne in
Cornwall to Lerwick in the Shetland Islands, and this study represents the
first attempt to demonstrate its capability and validate the methods in use.
Its ultimate purpose will be the detection and quantification of volcanic
plumes, but the present study clearly demonstrates the advanced capabilities
of the network.
Diffuse light conditions can increase the efficiency of photosynthesis and
carbon uptake by vegetation canopies. The diffuse fraction of
photosynthetically active radiation (PAR) can be affected by ...either a change
in the atmospheric aerosol burden and/or a change in cloudiness. During the
dry season, a hotspot of biomass burning on the edges of the Amazon
rainforest emits a complex mixture of aerosols and their precursors and
climate-active trace gases (e.g. CO2, CH4, NOx). This
creates potential for significant interactions between chemistry, aerosol,
cloud, radiation and the biosphere across the Amazon region. The combined
effects of biomass burning on the terrestrial carbon cycle for the
present day are potentially large, yet poorly quantified. Here, we quantify
such effects using the Met Office Hadley Centre Earth system model
HadGEM2-ES, which provides a fully coupled framework with interactive aerosol, radiative
transfer, dynamic vegetation, atmospheric chemistry and biogenic volatile
organic compound emission components. Results show that for present day,
defined as year 2000 climate, the overall net impact of biomass burning
aerosols is to increase net primary productivity (NPP) by +80 to +105 TgC yr−1,
or 1.9 % to 2.7 %, over the central Amazon Basin on annual mean. For
the first time we show that this enhancement is the net result of multiple
competing effects: an increase in diffuse light which stimulates
photosynthetic activity in the shaded part of the canopy (+65 to +110 TgC yr−1), a reduction in the total amount of radiation (−52 to −105 TgC yr−1)
which reduces photosynthesis and feedback from climate adjustments in
response to the aerosol forcing which increases the efficiency of biochemical
processes (+67 to +100 TgC yr−1). These results illustrate that despite a
modest direct aerosol effect (the sum of the first two counteracting
mechanisms), the overall net impact of biomass burning aerosols on
vegetation is sizeable when indirect climate feedbacks are considered. We
demonstrate that capturing the net impact of aerosols on vegetation should be
assessed considering the system-wide behaviour.
Cloud physics from space Stephens, Graeme L.; Christensen, Matthew; Andrews, Timothy ...
Quarterly journal of the Royal Meteorological Society,
October 2019 Part A, Letnik:
145, Številka:
724
Journal Article
Recenzirano
Odprti dostop
A review of the progression of cloud physics from a subdiscipline of meteorology into the global science it is today is described. The discussion briefly touches on the important post‐war ...contributions of three key individuals who were instrumental in developing cloud physics into a global science. These contributions came on the heels of the post‐war weather modification efforts that influenced much of the early development of cloud physics. The review is centred on the properties of warm clouds primarily to limit the scope of the article and the connection between the early contributions to cloud physics and the current vexing problem of aerosol effects on cloud albedo is underlined. Progress toward estimating cloud properties from space and insights on warm cloud processes are described. Measurements of selected cloud properties, such as cloud liquid water path are now mature enough that multi‐decadal time series of these properties exist and this climatology is used to compare to analogous low‐cloud properties taken from global climate models. The too‐wet (and thus too bright) and the too‐dreary biases of models are called out underscoring the challenges we still face in representing warm clouds in Earth system models. We also provide strategies for using observations to constrain the indirect radiative forcing of the climate system.
These are different views of ship tracks – a phenomena that expresses the effects of aerosol on low clouds, a topic explored in the article. The lower image, more grainy, is a more historical view of ship tracks from Apollo‐Soyuz on 16 July 1975 at 2221 GMT obtained from Porch et al. (1990). This image emphasizes that more is going on than simply brightening the cloud locally, with regions of suppressed albedo and mesoscale process at play. Adding aerosol to clouds does not always result in an increase in the albedo of cloud fields.
African biomass burning emission inventories for gaseous and particulate species have been constructed at a resolution of 1 km by 1km with daily coverage for the 2000-2007 period. These inventories ...are higher than the GFED2 inventories, which are currently widely in use. Evaluation specifically focusing on combustion aerosol has been carried out with the ORISAM-TM4 global chemistry transport model which includes a detailed aerosol module. This paper compares modeled results with measurements of surface BC concentrations and scattering coefficients from the AMMA Enhanced Observations period, aerosol optical depths and single scattering albedo from AERONET sunphotometers, LIDAR vertical distributions of extinction coefficients as well as satellite data. Aerosol seasonal and interannual evolutions over the 2004-2007 period observed at regional scale and more specifically at the Djougou (Benin) and Banizoumbou (Niger) AMMA/IDAF sites are well reproduced by our global model, indicating that our biomass burning emission inventory appears reasonable.
Updraft velocities strongly control the activation of aerosol particles or that component that act as cloud condensation nuclei (CCN). For kilometer‐scale models, vertical motions are partially ...resolved but the subgrid‐scale (SGS) contribution needs to be parametrized or constrained to properly represent the activation of CCNs. This study presents a method to estimate the missing SGS (or unresolved) contribution to vertical velocity variability in models with horizontal grid sizes up to ∼2 km. A framework based on Large Eddy Simulations (LES) and high‐resolution aircraft observations of stratocumulus and shallow cumulus clouds has been developed and applied to output from the United Kingdom Met Office Unified Model (UM) operating at kilometer‐scale resolutions in numerical weather prediction configuration. For a stratocumulus deck simulation, we show that the UM 1 km model underestimates significantly the variability of updraft velocity with an averaged cloud base standard deviation between 0.04 and 0.05 m s−1 compared to LES and aircraft estimates of 0.38 and 0.54 m s−1, respectively. Once the SGS variability is considered, the UM corrected averages are between 0.34 and 0.44 m s−1. Off‐line calculations of CCN‐activated fraction using an activation scheme have been performed to illustrate the implication of including the SGS vertical velocity. It suggests increased CCN‐activated fraction from 0.52 to 0.89 (respectively, 0.10 to 0.54) for a clean (respectively, polluted) aerosol environment for simulations with a 1 km horizontal grid size. Our results highlight the importance of representing the SGS vertical velocity in kilometer‐scale simulations of aerosol‐cloud interactions.
Key PointsWe seek to improve the aerosol activation behavior in kilometer‐scale modelsA method to constrain the subgrid‐scale updraft velocity is presentedWe highlight the potential implication for aerosol‐cloud interactions modeling
Aerosol-cloud interactions (ACI) are considered to be the most uncertain driver of present-day radiative forcing due to human activities. The non-linearity of cloud-state changes to aerosol ...perturbations make it challenging to attribute causality
in observed relationships of aerosol radiative forcing. Using correlations to infer causality can be challenging when meteorological variability also drives both aerosol and cloud changes independently. Natural and anthropogenic aerosol perturbations from well defined sources provide ‘opportunistic experiments’ (also known as natural experiments) to investigate ACI in cases where causality may be more confidently inferred. These perturbations cover a wide range of locations and spatio-temporal scales, including point sources such as volcanic eruptions or industrial sources, plumes from biomass burning or forest fires, and tracks from individual ships or shipping corridors.We review the different experimental conditions and conduct a synthesis of the available satellite data sets and field campaigns to place these opportunistic experiments on a common footing, facilitating new insights and a clearer understanding of key uncertainties in aerosol radiative forcing. Cloud albedo perturbations are strongly sensitive to background meteorological conditions. Strong liquid water path increases due to aerosol perturbations are largely ruled out by averaging across experiments. Opportunistic experiments have significantly improved process-level understanding of ACI, but it remains unclear how reliably the relationships found can be scaled to the global level, thus demonstrating a need for deeper investigation in order to improve assessments of aerosol radiative forcing and climate change.
For over 6 months, the 2014–2015 effusive eruption at Holuhraun, Iceland, injected considerable amounts of sulfur dioxide (SO2) into the lower troposphere with a daily rate of up to one-third of the ...global emission rate, causing extensive air pollution across Europe. The large injection of SO2, which oxidises to form sulfate aerosol (SO42-), provides a natural experiment offering an ideal opportunity to scrutinise state-of-the-art general circulation models' (GCMs) representation of aerosol–cloud interactions (ACIs). Here we present Part 1 of a two-part model inter-comparison using the Holuhraun eruption as a framework to analyse ACIs. We use SO2 retrievals from the Infrared Atmospheric Sounding Interferometer (IASI) instrument and ground-based measurements of SO2 and SO42- mass concentrations across Europe, in conjunction with a trajectory analysis using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model, to assess the spatial and chemical evolution of the volcanic plume as simulated by five GCMs and a chemical transport model (CTM). IASI retrievals of plume altitude and SO2 column load reveal that the volcanic perturbation is largely contained within the lower troposphere. Compared to the satellite observations, the models capture the spatial evolution and vertical variability of the plume reasonably well, although the models often overestimate the plume altitude. HYSPLIT trajectories are used to attribute to Holuhraun emissions 111 instances of elevated sulfurous surface mass concentrations recorded at European Monitoring and Evaluation Programme (EMEP) stations during September and October 2014. Comparisons with the simulated concentrations show that the modelled ratio of SO2 to SO42- during these pollution episodes is often underestimated and overestimated for the young and mature plume, respectively. Models with finer vertical resolutions near the surface are found to better capture these elevated sulfurous ground-level concentrations. Using an exponential function to describe the decay of observed surface mass concentration ratios of SO2 to SO42- with plume age, the in-plume oxidation rate constant is estimated as 0.032 ± 0.002 h−1 (1.30 ± 0.08 d e-folding time), with a near-vent ratio of 25 ± 5 (µg m−3 of SO2 / µg m−3 of SO42-). The majority of the corresponding derived modelled oxidation rate constants are lower than the observed estimate. This suggests that the representation of the oxidation pathway/s in the simulated plumes is too slow. Overall, despite their coarse spatial resolutions, the six models show reasonable skill in capturing the spatial and chemical evolution of the Holuhraun plume. This capable representation of the underlying aerosol perturbation is essential to enable the investigation of the eruption's impact on ACIs in the second part of this study.
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