7 research outputs found
Effects of stratospheric sulfate aerosol geo-engineering on cirrus clouds
Cooling the Earth through the injection of sulphate into the stratosphere is one of the most discussed geo-engineering (GE) schemes. Stratospheric aerosols can sediment into the troposphere, modify the aerosol composition and thus might impact cirrus clouds. We use a global climate model with a physically based parametrization for cirrus clouds in order to investigate possible microphysical and dynamical effects. We find that enhanced stratospheric aerosol loadings as proposed by several GE approaches will likely lead to a reduced ice crystal nucleation rate and thus optically thinner cirrus clouds. These optically thinner cirrus clouds exert a strong negative cloud forcing in the long-wave which contributes by 60% to the overall net GE forcing. This shows that indirect effects of stratospheric aerosols on cirrus clouds may be important and need to be considered in order to estimate the maximum cooling derived from stratospheric GE
Dust ice nuclei effects on cirrus clouds
In order to study aerosol–cloud interactions in cirrus clouds, we apply a new
multiple-mode ice microphysical scheme to
the general circulation model ECHAM5-HAM.
The multiple-mode ice microphysical scheme allows for analysis of the competition
between homogeneous freezing of solution droplets, deposition nucleation of
pure dust particles, and immersion freezing of coated dust particles and
pre-existing ice. We base the freezing efficiencies of coated and pure dust
particles on the most recent laboratory data. The effect of pre-existing ice,
which has been neglected in previous ice nucleation parameterizations, is to
deplete water vapour by depositional growth and thus prevent homogeneous and
heterogeneous freezing from occurring. As a first step, we extensively tested
the model and validated the results against in situ measurements from various
aircraft campaigns. The results compare well with observations; properties
such as ice crystal size and number concentration as well as supersaturation are
predicted within the observational spread.
We find that heterogeneous
nucleation on mineral dust particles and the consideration of pre-existing
ice in the nucleation process may lead to significant effects: globally, ice
crystal number and mass are reduced by 10 and 5%, whereas the ice
crystals' size is increased by 3%. The reductions in ice crystal number are
most pronounced in the tropics and mid-latitudes in the Northern Hemisphere.
While changes in the microphysical and radiative properties of cirrus clouds
in the tropics are mostly driven by considering pre-existing ice, changes
in the northern hemispheric mid-latitudes mainly result from heterogeneous
nucleation. The so-called negative Twomey effect in cirrus clouds is
represented in ECHAM5-HAM. The net change in the radiation budget
is −0.94 W m−2, implying that both heterogeneous nucleation on dust
and pre-existing ice have the potential to modulate cirrus properties in
climate simulations and thus should be considered in future studies
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A large ozone-circulation feedback and its implications for global warming assessments.
State-of-the-art climate models now include more climate processes which are simulated at higher spatial resolution than ever1. Nevertheless, some processes, such as atmospheric chemical feedbacks, are still computationally expensive and are often ignored in climate simulations1,2. Here we present evidence that how stratospheric ozone is represented in climate models can have a first order impact on estimates of effective climate sensitivity. Using a comprehensive atmosphere-ocean chemistry-climate model, we find an increase in global mean surface warming of around 1°C (~20%) after 75 years when ozone is prescribed at pre-industrial levels compared with when it is allowed to evolve self-consistently in response to an abrupt 4×CO2 forcing. The difference is primarily attributed to changes in longwave radiative feedbacks associated with circulation-driven decreases in tropical lower stratospheric ozone and related stratospheric water vapour and cirrus cloud changes. This has important implications for global model intercomparison studies1,2 in which participating models often use simplified treatments of atmospheric composition changes that are neither consistent with the specified greenhouse gas forcing scenario nor with the associated atmospheric circulation feedbacks3-5.We thank the European Research Council for funding through the ACCI project,
project number 267760. The model development was part of the QESM-ESM project
supported by the UK Natural Environment Research Council (NERC) under contract
numbers RH/H10/19 and R8/H12/124. We acknowledge use of the MONSooN
system, a collaborative facility supplied under the Joint Weather and Climate
Research Programme, which is a strategic partnership between the UK Met Office
and NERC. A.C.M. acknowledges support from an AXA Postdoctoral Research
Fellowship.This is the accepted manuscript. The final version is available from Nature Publishing at http://www.nature.com/nclimate/journal/v5/n1/full/nclimate2451.html