Johannes Hendricks

Physically based parameterization of cirrus cloud formation for use in global atmospheric models

Motivated by the need to study the climatic impact of aerosol-related cirrus cloud changes, a physically based parameterization scheme of ice initiation and initial growth of ice crystals in young cirrus clouds has been developed. The scheme tracks the number density and size of nucleated ice crystals as a function of vertical wind speed, temperature, ice saturation ratio, aerosol number size distributions, and preexisting cloud ice, allowing for competition between heterogeneous ice nuclei and liquid aerosol particles during freezing. Predictions of the parameterization are compared with numerical parcel simulations of ice nucleation and growth from atmospheric aerosols, with a special focus on explaining the indirect effects of ice nuclei on the properties of young cirrus clouds. The uncertainties of the parameterization are discussed and its implementation in a general circulation model is briefly outlined. This new scheme establishes a flexible framework for a comprehensive assessment of indirect aerosol effects on and properties of cirrus clouds in global climate, chemistry transport, and weather forecast models.

Climate impact of biofuels in shipping: global model studies of the aerosol indirect effect.

Aerosol emissions from international shipping are recognized to have a large impact on the Earths radiation budget, directly by scattering and absorbing solar radiation and indirectly by altering cloud properties. New regulations have recently been approved by the International Maritime Organization (IMO) aiming at progressive reductions of the maximum sulfur content allowed in marine fuels from current 4.5% by mass down to 0.5% in 2020, with more restrictive limits already applied in some coastal regions. In this context, we use a global bottom-up algorithm to calculate geographically resolved emission inventories of gaseous (NO(x), CO, SO(2)) and aerosol (black carbon, organic matter, sulfate) species for different kinds of low-sulfur fuels in shipping. We apply these inventories to study the resulting changes in radiative forcing, attributed to particles from shipping, with the global aerosol-climate model EMAC-MADE. The emission factors for the different fuels are based on measurements at a test bed of a large diesel engine. We consider both fossil fuel (marine gas oil) and biofuels (palm and soy bean oil) as a substitute for heavy fuel oil in the current (2006) fleet and compare their climate impact to that resulting from heavy fuel oil use. Our simulations suggest that ship-induced surface level concentrations of sulfate aerosol are strongly reduced, up to about 40-60% in the high-traffic regions. This clearly has positive consequences for pollution reduction in the vicinity of major harbors. Additionally, such reductions in the aerosol loading lead to a decrease of a factor of 3-4 in the indirect global aerosol effect induced by emissions from international shipping.

Model studies on the sensitivity of upper tropospheric chemistry to heterogeneous uptake of HNO3 on cirrus ice particles

[1] A chemistry box model is applied to investigate potential chemical perturbations in the midlatitude upper troposphere caused by the reactive and nonreactive uptake of trace gases on cirrus ice particles. Chemical implications of denoxification caused by heterogeneous reactions with HNO 3 as a product and adsorption of gas phase HNO 3 on ice surfaces are the special focus. The role of denitrification due to gravitational settling of ice particles is investigated. The simulations suggest that cirrus cloud chemistry has the potential to induce strong local reductions in upper tropospheric HNO 3 and NO x . Because of NO reduction the OH/HO 2 ratio and the OH concentration decrease. As a consequence of these effects a significant reduction in the net ozone production rate is modeled. Sensitivity studies were performed varying the HNO 3 adsorption efficiency, the particle sedimentation efficiency, and the ambient NO x concentration. The sensitivity experiments reveal that the modeled cirrus cloud impacts are mainly driven by the nonreactive uptake of HNO 3 on ice particles and the subsequent particle sedimentation. The effects strongly depend on the efficiency of HNO 3 adsorption. If a very efficient uptake of HNO 3 is assumed, decreases in the ozone mixing ratio up to 14% are modeled. The simulations also suggest that the cirrus cloud impact on ozone is sensitive to the ambient NO x concentration. The heterogeneous reactions on ice with HNO 3 as a product appear to be of secondary importance in the upper troposphere. Chlorine activation due to heterogeneous reactions on cirrus ice particles has a minor effect on model ozone chemistry under the conditions regarded. The model calculations further suggest that chemical perturbations caused by the uptake of OH, HO 2 , and, H 2 O 2 on ice are potentially significant only in periods of high cirrus cloud activity. Since observational data on cirrus chemical impacts in the upper troposphere are currently too sparse to perform a detailed model validation, it remains unclear whether the potentially large chemical perturbations caused by HNO 3 adsorption on cirrus ice particles are of relevance for the real atmosphere.

Toward effective emissions of ships in global models

The dispersion and chemical conversion of emissions in the near-field of a single ship are studied with two different modelling approaches to explore the differences between gradual dispersion and instantaneous dilution into a box with a size comparable to the large grid boxes of global scale models or satellite data. While both techniques use the same photochemical box model to solve the chemical equations, the dilution of the exhaust into the background air is different. One approach uses a Gaussian plume model and accounts for the expansion phase of a plume. The other one instantaneously disperses the emissions over a large gridbox, a technique commonly used by large scale models. In a first step we show that differences in the time evolution of ozone between the two model approaches are large for the case studied here, where emissions from a large container ship are released into the marine boundary layer with neutral stability. For emissions at noon, the differences in the ship induced ozone change at the reference time when both boxes are of equal size chosen to be 60 km are largest. The ozone change is then overestimated by the global-model approach by a factor of three. This results from the neglect of sub-grid scale OH loss due to NO2 oxidation in the globalmodel approach, which inhibits hydrocarbon oxidation and thus ozone production. Smallest differences are encountered for emissions released around sunset. One possibility to account for these sub-grid processes in global models is the use of effective emissions, i.e. actual emissions are changed and emissions of additional compounds like ozone are introduced in a way that they take sub-grid processes into account. In a second step we present effective emissions for the particular case discussed above. It is shown for this case that the method is able to account for the neglect of sub-grid processes in global models for different emission times and emission strengths.

Global-Mean Temperature Change from Shipping toward 2050: Improved Representation of the Indirect Aerosol Effect in Simple Climate Models

We utilize a range of emission scenarios for shipping to determine the induced global-mean radiative forcing and temperature change. Ship emission scenarios consistent with the new regulations on nitrogen oxides (NO(x)) and sulfur dioxide (SO(2)) from the International Maritime Organization and two of the Representative Concentration Pathways are used as input to a simple climate model (SCM). Based on a complex aerosol-climate model we develop and test new parametrizations of the indirect aerosol effect (IAE) in the SCM that account for nonlinearities in radiative forcing of ship-induced IAE. We find that shipping causes a net global cooling impact throughout the period 1900-2050 across all parametrizations and scenarios. However, calculated total net global-mean temperature change in 2050 ranges from -0.03[-0.07,-0.002]°C to -0.3[-0.6,-0.2]°C in the A1B scenario. This wide range across parametrizations emphasizes the importance of properly representing the IAE in SCMs and to reflect the uncertainties from complex global models. Furthermore, our calculations show that the future ship-induced temperature response is likely a continued cooling if SO(2) and NO(x) emissions continue to increase due to a strong increase in activity, despite current emission regulations. However, such cooling does not negate the need for continued efforts to reduce CO(2) emissions, since residual warming from CO(2) is long-lived.

A method for comparing properties of cirrus clouds in global climate models with those retrieved from IR sounder satellite observations

A methodology to compare cloud properties simulated by global climate models with those retrieved from observations by satellite-based infrared (IR) sounders has been developed. The relatively high spectral resolution in the CO2 absorption band of these instruments leads to especially reliable cirrus properties, day and night. Additionally, bulk microphysical properties can be retrieved for semi-transparent cirrus, based on the observed spectral emissivity differences between 8 and 11 µm. The particular intention of this study is to compare macro-and microphysical properties of high cloudiness as represented by the model simulations and the satellite data. For this purpose, a method has been developed to process the model output to be comparable to the satellite measurements, as in other observational simulator packages (for example the ISCCP-simulator). This simulator method takes into account i) the differences in horizontal resolution of the model and the observations, ii) the specific observation time windows, iii) the determination of the pressure of a cloud system, identified with the pressure at the middle of the uppermost cloud, and iv) the selection of high clouds with specific cloud optical thickness ranges for the microphysical property retrieval using IR sounder data. Applying this method to simulations by the global climate model ECHAM and TOVS satellite observations has important effects. The frequency of high clouds selected from the model output by using the method is significantly smaller than the total frequency of high cloudiness in the model. Largest differences occur around the equator where the zonal mean frequency of high cloudiness is reduced by about 30 % (relative change). The selection method is essential for the comparison of modelled and observed microphysical properties of high clouds. The selection of high clouds from the ECHAM simulation according to the optical thickness range of the TOVS data results in a reduction of the mean water path of high clouds by factors of more than 3 compared to the case where also high clouds of other optical thicknesses are considered. Furthermore, the selection by optical thickness causes a significant increase in the mean effective cloud particle diameter. These changes significantly reduce the differences between the simulation and the observations. The method can also be applied for comparisons with other IR sounder climatologies such as from AIRS and IASI.

Climate Impact of Transport

Transport impacts the atmospheric composition and the climate by CO2 and non-CO2 emissions. The atmospheric lifetime of most non-CO2 emissions is much shorter than the CO2 lifetime. Nevertheless, the non-CO2 climate effects are large in comparison to the CO2 effect, in particular for aviation and shipping. This is mainly due to triggering new clouds and modifying existing clouds, and to the impact of nitrogen oxides emissions on the abundances of ozone and methane.

Global aerosol modeling with MADE3 (v3.0) in EMAC (based on v2.53): model description and evaluation

Recently, the aerosol microphysics submodel MADE3 (Modal Aerosol Dynamics model for Europe, adapted for global applications, third generation) was introduced as a successor to MADE and MADE-in. It includes nine aerosol species and nine lognormal modes to represent aerosol particles of three different mixing states throughout the aerosol size spectrum. Here, we describe the implementation of the most recent version of MADE3 into the ECHAM/MESSy Atmospheric Chemistry (EMAC) general circulation model, including a detailed evaluation of a 10year aerosol simulation with MADE3 as part of EMAC. We compare simulation output to station network measurements of near-surface aerosol component mass concentrations, to airborne measurements of aerosol mass mixing ratio and number concentration vertical profiles, to groundbased and airborne measurements of particle size distributions, and to station network and satellite measurements of aerosol optical depth. Furthermore, we describe and apply a new evaluation method, which allows a comparison of model output to size-resolved electron microscopy measurements of particle composition. Although there are indications that fine-mode particle deposition may be underestimated by the model, we obtained satisfactory agreement with the observations. Remaining deviations are of similar size to those identified in other global aerosol model studies. Thus, MADE3 can be considered ready for application within EMAC. Due to its detailed representation of aerosol mixing state, it is especially useful for simulating wet and dry removal of aerosol particles, aerosol-induced formation of cloud droplets and ice crystals as well as aerosol–radiation interactions. Besides studies on these fundamental processes, we also plan to use MADE3 for a reassessment of the climate effects of anthropogenic aerosol perturbations.

Dust ice nuclei effects on cirrus clouds in ECHAM5-HAM

Aerosol-cloud interactions are one of the main uncertainties in climate research. Up to now a lot of research has been conducted on aerosol-cloud interactions in warm clouds. The impact of aerosols on ice or mixed-phase clouds is much less understood. Cirrus clouds in an unpolluted environment are assumed to form mainly via homogeneous freezing. The presence of heterogeneous ice nuclei can lead to earlier ice crystal formation and change the microphysical properties of cirrus clouds. Recent box model studies even suggest that heterogeneous freezing can suppress homogeneous freezing, if several conditions are fulfilled. Most likely this would lead to cirrus clouds containing fewer and larger ice crystals. If homogeneous and heterogeneous freezing compete either freezing mechanism may dominate depending mainly on vertical velocity and number density of ice nuclei. Thus, it is not clear yet how number and size of ice crystals are affected.

Global Atmospheric Aerosol Modeling

Global aerosol models are used to study the distribution and properties of atmospheric aerosol particles as well as their effects on clouds, atmospheric chemistry, radiation, and climate. The present article provides an overview of the basic concepts of global atmospheric aerosol modeling and shows some examples from a global aerosol simulation. Particular emphasis is placed on the simulation of aerosol particles and their effects within global climate models.