REQUEST FOR A SPECIAL PROJECT

Transcription

1 REQUEST FOR A SPECIAL PROJECT MEMBER STATE: Switzerland Principal Investigator 1 : Affiliation: Prof. Ulrike Lohmann Institute for Atmospheric and Climate Science, ETH Zurich Address: Universitaetstrasse 16 CH-8092 Zurich Switzerland Other researchers: Project Title: Ulrike.Lohmann@env.ethz.ch Dr. Peter Spichtinger, Ana Cirisan, Fabian Fusina, Hanna Joos, Dr. Andreas Mühlbauer, Sara Nottelmann Cloud Aerosol Interactions If this is a continuation of an existing project, please state the computer project account assigned previously. Starting year: (Each project will have a well defined duration, up to a maximum of 3 years, agreed at the beginning of the project. For projects started before 2009, please state 2009 as the start year.) SP CHCLAI 2009 Would you accept support for 1 year only, if necessary? YES NO Computer resources required for : (The maximum project duration is 3 years, therefore a continuation project cannot request resources for 2012.) High Performance Computing Facility (units) Data storage capacity (total archive volume) (gigabytes) An electronic copy of this form must be sent via to: Electronic copy of the form sent on (please specify date): special_projects@ecmwf.int 30 th of April, 2009 Continue overleaf 1 The Principal Investigator will act as contact person for this Special Project and, in particular, will be asked to register the project, provide an annual progress report of the project s activities, etc. April 2009 Page 1 of 1 This form is available at:

2 Principal Investigator: Project Title: Prof. Ulrike Lohmann Cloud Aerosol Interactions Extended abstract Motivation: Aerosol particles affect the atmosphere via several mechanisms: First, they can scatter and absorb solar radiation. Second, they can scatter, absorb and emit thermal radiation. Third, they can act as cloud condensation nuclei (CCN) or ice nuclei (IN). Whereas the first two effects are referred as direct aerosol effects and are not subject of our proposed project, the latter is referred as the indirect aerosol effect (see e.g. Lohmann and Feichter, 2005) and will be subject of our proposed research together with other atmospheric properties influenced by aerosols. Clouds themselves are an important regulator of the Earth s radiation budget. One of the most crucial issues for predicting future climate change is the role of clouds (IPCC, 2007). Unfortunately, our knowledge of the contribution of clouds to radiative forcing is quite limited. Clouds warm and cool the atmosphere depending on their properties, such as water content, droplet sizes, cloud height etc. Because of the insufficient representation of cloud processes in existing climate models it is difficult to predict the role of clouds in a changing climate and a changing hydrological cycle. Collins et al. (1994) indicate that small changes to macrophysical (coverage, structure, altitude) and microphysical properties (droplet size, phase) have significant effects on climate. For instance a 5% increase of the shortwave cloud forcing would compensate the increase in greenhouse gases between the years (Ramaswamy et al., 2001). Consequently the growing interest in the impact of aerosols on climate stimulated the development of better physically based parameterizations in climate models. Nevertheless, the lack of understanding feedbacks of external forcings on clouds remains one of the largest uncertainties in climate modeling and climate change prediction (Cess et al., 1990; Houghton et al., 1996). Short description of indirect aerosol effects, relevant for this proposal: Indirect aerosol effect for clouds with fixed water amounts (clouds albedo/twomey effect): The aerosol particles act as CCN and under the assumption of constant available water vapour more numerous but smaller droplets are formed; the more numerous smaller cloud particles reflect more solar radiation (Twomey, 1974). This effect can influence all types of clouds (water/mixed-phase/ice clouds) Indirect aerosol effect for clouds with varying water amounts (cloud lifetime effect): The aerosol particles act as CCN and more numerous but smaller droplets are formed; the smaller clouds particles decrease the precipitation efficiency and thereby the cloud lifetime is prolonged (Albrecht, 1989). Glaciation indirect effect: In mixed-phase clouds more available ice nuclei can increase the precipitation efficiency (Lohmann, 2002). Riming indirect effect: In mixed-phase clouds the smaller cloud droplets decrease the riming efficiency. Longwave effect: In addition to the cloud albedo effect, an increase in ice clouds would lead to an increased greenhouse effect of clouds. Negative Twomey effect: In cirrus clouds, heterogeneous nucleation can modify homogeneous nucleation events leading to a reduction of the ice crystal number densities (Kärcher et al., 2006) April 2009 Page 2 of 2 This form is available at:

3 Proposed projects in details: Projects including a significant amount of high computing resources: Aerosol-cloud interactions in numerical weather prediction models: Homogenous nucleation of water droplets, i.e., direct formation of liquid drops from the water vapour phase, is not possible under atmospheric conditions. Instead, the heterogeneous nucleation, i.e., the condensation of water vapour on existing particles/aerosols, is the dominant process. Anthropogenic influences, as increasing aerosol concentrations, change the radiative properties of the cloud and, therefore, change the climate. More CCN leads to more but smaller droplets and the cloud albedo increases. As discussed above, the cloud albedo and the cloud life-time effect lead to a cooling at the earth surface. In the case of shallow orographic clouds the aerosol-cloud interaction may cause a displacement of precipitation from the upslope side of a hill towards the downslope side (Givati and Rosenfeld 2004; Borys et al. 2003). Therefore, the aspect of aerosol-cloud interaction in relation with orographic precipitation may also be of major importance in numerical weather prediction (NWP). During a first phase of the project, the possible mechanisms how aerosol may affect orographic precipitation development in warm-phase and mixed-phase clouds were investigated. For this purpose an aerosol scheme was coupled to the mesoscale nonhydrostatic weather prediction model COSMO and first 2D and 3D simulations were carried out (Muhlbauer and Lohmann, 2008, 2009; Zubler et al., submitted). These simulations will be extended including effects of convective processes. Additionally, mixed-phase clouds should receive more emphasis in future, as most of the precipitation in mid-latitudes originates via the ice phase: Mixed-phase clouds are very frequent in mid-latitude and polar regions. At temperatures below 0 C, water droplets do not freeze spontaneously, but can persist as super cooled liquid until approximately -38 C. In this temperature range, freezing of droplets occurs only through heterogeneous nucleation on ice nuclei. Ice nuclei are predominantly insoluble particles, such as mineral dust or pollen. The relative importance of the different nucleation modes (deposition, immersion, condensation and contact freezing) is still unclear. Precipitation formation is very efficient due to the Bergeron-Findeisen process. For anthropogenic ice nuclei, such as soot, this may cause a glaciation indirect effect, on the other hand, if IN are coated with solute material and they can act as immersion instead of contact nuclei glaciation would be retarded (Hoose et al., 2008; Storelvmo et al., 2008). This project will address two cloud processes, which are particularly important for mixedphase clouds: aerosol processing, and heterogeneous freezing on mineral dust and other components. The existing aerosol-cloud module, which was implemented into COSMO is currently extended in a PhD project. It will be based on the developments for freezing properties of mineral dust and aerosol processing, which were carried out for the ECHAM model (Hoose et al., 2008a,b). Using such a new aerosol-processing removal and scavenging processes could be investigated in idealized 2D and 3D studies on mesoscale cloud systems leading to deeper insights on the impact of aerosol processing on cloud formation/development as well as precipitation formation. Within the first year, the existing parameterisation is enhanced including aerosol processing and different heterogeneous freezing parameterisations. After some tests, idealized 2D/3D studies should be carried out. April 2009 Page 3 of 3 This form is available at:

4 Impact of dynamics and aerosols on cirrus clouds (cloud-resolving models): In this project we focus on high-level clouds consisting only of pure ice crystals (cirrus clouds). A positive global forcing of cirrus clouds on the radiation budget (i.e. a warming) is possible (IPCC, 2007), but at this time it is rather difficult to provide global estimates because only little is known about the lifecycle of cirrus clouds and their potential formation regions, the ice supersaturated regions (see e.g. Spichtinger et al., 2005a,b). There are two different formation mechanisms for ice clouds. First, in deep convective clouds large water droplets freeze and form ice crystals in the strong updraft regions of the convective system. Second, in moderate upward motions, which induce adiabatic cooling, ice crystals form from super cooled solution droplets (homogeneous nucleation, see e.g. Koop et al., 2000) or on aerosol particles (heterogeneous nucleation, see e.g. DeMott et al., 2003). In our project we focus only on non-convective clouds. There were some studies with trajectory models concerning the impact of aerosols vs. mesoscale dynamics (Haag et al., 2003; Haag and Kärcher, 2004) and also global studies using the model ECHAM (Kärcher and Lohmann, 2003; Kärcher et al., 2006; Lohmann et al., 2004) but still the impact of heterogeneous nucleation vs. the impact of dynamics on cirrus clouds is unclear. Recently, a new ice microphysics scheme was implemented (Spichtinger and Gierens, 2009a) into the 2D/3D an-elastic model EuLag (Smolarkiewicz and Margolin, 1997). This scheme contains several classes of ice. One class is reserved for ice crystals formed by homogeneous nucleation (due to Koop et al., 2000), while arbitrary many classes can be used for ice crystals formed by heterogeneous nucleation with different types of aerosols. Using this model in a cloud-resolving resolution we are able to investigate the impact of heterogeneous nucleation on homogeneous freezing events within an idealized 2D setup (Spichtinger and Gierens, 2009b). The existing parameterisations for heterogeneous nucleation should be extended. In further studies within an idealized 2D or even 3D framework we want to investigate the impact of aerosols (i.e. heterogeneous ice nuclei) in contrast to the impact of mesoscale dynamics (i.e. gravity waves). Here, we will first start using idealized scenarios (e.g. orographic waves) progressing to more complex cases. In order to control the large-scale environment and its impact on smaller scales (small scale dynamics and microphysics) a new concept was developed and is now available for simulating cirrus clouds (Spichtinger and Dörnbrack, in prep.). Using the anelastic equations in perturbation form, it is possible to prescribe a time/spatially dependent background state in order to control the large-scale dynamics and to study the impact of varying large-scale forcings on smaller scales. This concept was successfully applied to some idealized cases of atmospheric flows (Spichtinger and Dörnbrack, 2008). Within a PhD project ( ), this new method will be used for multiscale simulations of cirrus clouds investigating the impact of dynamics on small-scale features inside cirrus clouds. After implementation and testing this concept in the EULAG, idealized 2D/3D cases of representative large-scale situations triggering cirrus clouds will be carried out. Inhomogeneities in cirrus clouds and their impact on radiation (cloud-resolving models): As it is well known from former investigations, cloud inhomogeneities can crucially influence the radiation budget of layer clouds as cirrus (Carlin et al., 2002). However, this impact was not quantified up to now and also the origin of cirrus cloud inhomogeneities is not well known. From some former and more recent simulations there are some indications that convection inside ice-supersaturated regions or even in already existing cirrus layers could trigger high internal variability in ice crystal number and mass concentration, respectively, leading to 2D/3D inhomogeneities (Marsham and Dobbie, 2005; Spichtinger, to be submitted). The impact of convectively driven cirrus inhomogeneities will be investigated using former 2D model simulations as an input for extensive radiative transfer calculations (LibRadTran). Additional simulations in 2D and maybe also 3D will be carried out in order to study the impact of environmental conditions on the formation of convective cells, their structure and their radiative impact in contrast to April 2009 Page 4 of 4 This form is available at:

5 homogeneous layer clouds. Especially, the 3D simulations require a large amount of computing time for the cloud-resolving model EULAG. Projects mostly dealing with data investigations: For all projects described above, meteorological analyses from the operational data as well as from the ERA-40 project will be very helpful: Use of data as initialisation of representative/real cases Comparison of in situ data (measurements) and model results Use of climatological data in order to estimate trends and changes derived from single case studies with the model Use of data as input for trajectory calculations Benefits for numerical weather prediction: Numerical weather prediction will benefit from this work in the following respects: Implementation of an aerosol-cloud microphysics scheme into the IFS model system Estimates on the impact of aerosols on warm clouds and orographic precipitation Improvement of the representation of warm cloud processes due to aerosols Estimates of the importance of glaciation effects and their impacts on precipitation; this might also result into improvements of the representation of mixed-phase clouds for NWP models Estimates of the impact of aerosols on cirrus clouds; also this might result in an improvement of the representation of cirrus clouds in NWP models including aerosol effects All these improvements that include possible important aerosol effects hopefully might lead to better weather forecasts at some time. References: Albrecht, B., 1989: Aerosols, cloud microphysics, and fractional cloudiness. Science, 245, Borys, R., D. Lowenthal, S. Cohn, and W. Brown, 2003: Mountaintop and radar measurement of anthropogenic aerosol effect on snow growth and snowfall rate. Geophys. Res. Lett., 30, Collins, W. D., Conant, W. C., and Ramanathan, V., 1994: Earth radiation budget, clouds, and climate sensitivity, in: The Chemistry of the Atmosphere: Its Impact on Global Change, edited by: J. G. Calvert, pp , Blackwell Scientific Publishers, Oxford, UK. DeMott, P. D. Cziczo, A. Prenni, D. Murphy, S. Kreidenweis, D. Thomson, R. Borys, D. Rogers, 2003: Measurements of the concentration and composition of nuclei for cirrus formation. Proc. Nat. Acad. Sciences 100: Givati, A., and D. Rosenfeld, 2004: Quantifying Precipitation Suppression Due to Air Pollution. J. Appl. Meteor., 43, Haag, W., B. Kärcher, J. Ström, A. Minikin, U. Lohmann, J. Ovarlez, and A. Stohl, 2003: Freezing thresholds and cirrus cloud formation mechanisms inferred from in situ measurements of relative humidity, Atmos. Chem. Phys., 3, Haag, W. and B. Kärcher, 2004: The impact of aerosols and gravity waves on cirrus clouds at midlatitudes. J. Geophys. Res. 109, D12202, doi: /2004jd Hoose, C., Lohmann, U., Erdin, R. & Tegen, I. (2008a), Global influence of dust mineralogical composition on heterogeneous ice nucleation in mixed-phase clouds, Environ. Res. Lett., accepted. Hoose, C., Lohmann, U., Stier, P., Verheggen, B. & Weingartner, E. (2008b), Aerosol processing in mixed-phase clouds in ECHAM5-HAM: Model description and comparison to observations, J. Geophys. Res. p. doi: /2007jd009251, in press. IPCC, 2007: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge April 2009 Page 5 of 5 This form is available at:

6 University Press, Cambridge, United Kingdom and New York, NY, USA, 996 pp. Kärcher, B. and U. Lohmann, 2002: A Parameterization of cirrus cloud formation: Homogeneous freezing including effects of aerosol size. J. Geophys. Res. 107 (D23), 4698 Kärcher, B., and U. Lohmann, 2003: A Parameterization of cirrus cloud formation: Heterogeneous freezing, J. Geophys. Res., 108, doi: /2002jd Kärcher, B., J. Hendricks, U. Lohmann, 2006: Physically based parameterization of cirrus cloud formation for use in global atmospheric models. J. Geophys. Res. 111, doi: /2005jd Koop, T., B. Luo, A. Tsias, T. Peter, 2000: Water activity as the determinant for homogeneous ice nucleation in aqueous solutions. Nature 406, Lohmann, U., 2002: A glaciation indirect aerosol effect caused by soot aerosols. Geophys. Res. Lett., 29, doi: /2001GL Lohmann, U. and J. Feichter, 2005: Global indirect aerosol effects: a review. Atmos. Chem. Phys., 5, Lohmann, U., B. Kärcher, and J. Hendricks, 2004: Sensitivity studies of cirrus clouds formed by heterogeneous freezing in the ECHAM GCM. J. Geophys. Res. 109, D16204, doi: /2003jd Marsham, J., and S. Dobbie, 2005: The effects of wind shear on cirrus: A large-eddy model and radar case-study. Q. J. R. Meteorol. Soc., 131, Muhlbauer, A. and U. Lohmann, 2008: Sensitivity studies of the role of aerosols in warm-phase orographic precipitation in different dynamical flow regimes. J. Atmos. Sci., 65, Muhlbauer, A. and U. Lohmann, 2009: Aerosol-cloud-precipitation interactions in mixed-phase orographic clouds. J. Atmos. Sci., in press. Ramaswany, V., Boucher, O., Haigh, J., Hauglustaine, D., Haywood, J., Myhre, G., Nakajima, T., Shi, G. Y., and Solomon, S., 2001: Radiative Forcing of Climate Change, in: Climate Change 2001: The Scientific Basis. Contribution of working group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, edited by: J. T. Houghton, Y. Ding, D. J. Griggs, M. Noguer, P. J. van der Linden, X. Dai, K. Maskell, and C. A. Johnson, pp , Cambridge Univ. Press, New York. Smolarkiewicz, P., L. Margolin, 1997: On Forward-in-Time Differencing for Fluids: an Eulerian / Semi-Lagrangian Non-Hydrostatic Model for Stratified Flows. Atmos.-Ocean special 35, Spichtinger, P., K. Gierens, H. Wernli, 2005a: A case study on the formation and evolution of ice supersaturation in the vicinity of a warm conveyor belt s outflow region. Atmos. Chem. Phys., 5, Spichtinger, P., K. Gierens, A. Dörnbrack, 2005b: Formation of ice supersaturation by mesoscale gravity waves. Atmos. Chem. Phys., 5, Spichtinger, P. and K. Gierens, 2009a: Modelling Cirrus Clouds. Part 1a: Model description and validation. Atmos. Chem. Phys., 9, Spichtinger, P. and K. Gierens, 2009b: Modelling Cirrus Clouds. Part 2: Competition of different nucleation mechanisms. Atmos. Chem. Phys., 9, Spichtinger, P. and A. Dörnbrack, 2008: Multiscale modelling of atmospheric flows with EULAG. Geophysical Research Abstracts, 10, EGU2008-A Spichtinger, P. and A. Dörnbrack: Multiscale modelling of atmospheric flows with EULAG. Computers and fluids, in prep. Spichtinger, P: Cirrus cloud dynamics a source for ice supersaturation. J. Atmos. Sci, to be submitted. Storelvmo, T., J. E. Kristjansson and U. Lohmann, 2008: Aerosol influence on mixed-phase clouds in CAM-Oslo. J. Atmos. Sci., Submitted Twomey, S., 1974: Pollution and planetary albedo. Atmos. Environ., 8, Zubler, E., U. Lohmann, D. Lüthi, A. Muhlbauer, C. Schär: A glaciation indirect aerosol effect in 2D sensitivity studies of mixed-phase orographic precipitation. J. Atmos. Sci., submitted. April 2009 Page 6 of 6 This form is available at: