http://www.geosci-model-dev-discuss.net/ Geoscientific Model Development Discussions Latest Articlesenhttp://www.geosci-model-dev-discuss.net/3/1359/2010/<b>The global middle-atmosphere aerosol model MAECHAM5-SAM2: comparison with satellite and in-situ observations</b><br /><br />Geoscientific Model Development Discussions, 3, 1359-1421, 2010<br /><br />Author(s): R. Hommel, C. Timmreck, and H. F. Graf<br /><br />In this paper we investigate results from a middle-atmosphere aerosol-climate model which has been developed to study the evolution of stratospheric aerosols. Here we focus on the stratospheric background period and evaluate several key quantities of the global dispersion of stratospheric aerosols and their precursors with observations and other model studies. It is shown that the model fairly well reproduces in situ observations of the aerosol size and number concentrations in the upper troposphere and lower stratosphere (UT/LS). Compared to measurements from the limb-sounding SAGE II satellite instrument, modelled integrated aerosol quantities are more biased the lower the moment of the aerosol population. Both findings are consistent with earlier work analysing the quality of SAGE II retrieved e.g. aerosol surface area densities from the volcanically unperturbed stratosphere (SPARC/ASAP, 2006; Thomason et al., 2008; Wurl et al., 2010). <br><br> The model suggests that new particles are formed over large areas of the LS, albeit nucleation rates in the upper troposphere are at least one order of magnitude larger than those in the stratosphere. Hence, we suggest that both tropospheric sulphate aerosols and particles formed in situ in the LS are maintaining the stability of the stratospheric aerosol layer also in the absence of direct stratospheric emissions from volcanoes. Particle size distributions are clearly bimodal, except in the upper branches of the stratospheric aerosol layer where aerosols evaporate. Modelled concentrations of condensation nuclei (CN) are lesser than measured in regions of the aerosol layer where aerosol mixing ratios are largest, due to an overpredicted particle growth by coagulation. <br><br> Transport regimes of tropical stratospheric aerosol have been identified from modelled aerosol mixing ratios and correspond to those deduced from satellite extinction measurements. We found that convective updraft in the Asian Monsoon region significantly contributes to both stratospheric aerosol load and size. The timing of formation and descend of layers of fine mode particles in the winter and spring polar stratosphere (CN layer) are reproduced by the model. Far above the tropopause where nucleation is inhibited due to with height increasing stratospheric temperatures, planetary wave mixing transports significant amounts of fine mode particles from the polar stratosphere to mid-latitudes. In those regions enhanced condensation rates of sulphuric acid vapour counteracts the evaporation of aerosols, hence prolonging the aerosol lifetime in the upper branches of the stratospheric aerosol layer. <br><br> Measurements of the aerosol precursors SO₂ and sulphuric acid vapour are fairly well reproduced by the model throughout the stratosphere.Wed, 01 Sep 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/1317/2010/<b>Development and validation of a size-resolved particle dry deposition scheme for applications in aerosol transport models</b><br /><br />Geoscientific Model Development Discussions, 3, 1317-1357, 2010<br /><br />Author(s): A. Petroff and L. Zhang<br /><br />A size-resolved particle dry deposition scheme is developed, which has been designed for inclusion in large-scale air quality and climate models, where the size distribution and fate of the atmospheric aerosol is of concern. The "resistance" structure is similar to what is proposed by Zhang et al. (2001, 2003), while a new "surface" deposition velocity (or surface resistance) is derived by simplification of a one-dimensional aerosol transport model (Petroff et al., 2008b, 2009). Collection efficiencies are given for the 26 Land Use Categories that decribe the earth surface. Validation of this model with existing measurements is performed on desert, grass, coniferous forest and liquid water surfaces. A comparison of this model with measurements on snow and ice is also given. Even though a qualitative agreement is reached, further size-segegated measurements are needed in order to confirm the model accuracy on this surface. The present analytical model provides more accurate predictions of the aerosol deposition on these surfaces than previous models.Thu, 19 Aug 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/1271/2010/<b>Adaptive method of lines for multi-component aerosol condensational growth and cloud droplet activation</b><br /><br />Geoscientific Model Development Discussions, 3, 1271-1315, 2010<br /><br />Author(s): S. Arabas and H. Pawlowska<br /><br />The process of formation of cloud droplets on an ensemble of aerosol particles is modelled by numerous investigators using the method of lines (MOL). The method involves discretization of the aerosol size spectrum into bins whose position and width evolve with time. One of the drawbacks of the method is its poor representation of the aerosol spectrum shape in the region between the unactivated aerosol mode and the activated cloud-droplet mode. An adaptive spectrum refinement procedure that improves the performance of the method is introduced and tested. A model of drop formation on multi-component aerosol is formulated for the purpose of the study. Model formulation includes explicit treatment of the drop temperature evolution. Several examples of the model set-up are used to demonstrate model capabilities. Model results are compared to those without adaptivity, and are compared to the Twomey's formul\ae. A C++ implementation of the model is available as an electronic supplement of the paper.Thu, 19 Aug 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/1223/2010/<b>A new coupled ice sheet-climate model: description and sensitivity to model physics under Eemian, Last Glacial Maximum, late Holocene and modern climate conditions</b><br /><br />Geoscientific Model Development Discussions, 3, 1223-1269, 2010<br /><br />Author(s): J. G. Fyke, A. J. Weaver, D. Pollard, M. Eby, L. Carter, and A. Mackintosh<br /><br />The need to better understand long-term climate/ice sheet feedback loops is motivating efforts to couple ice sheet models into Earth System models which are capable of long-timescale simulations. In this paper we describe a coupled model, that consists of the University of Victoria Earth System Climate Model (UVic ESCM) and the Pennsylvania State University Ice model (PSUI). The climate model generates a surface mass balance (SMB) field via a sub-gridded surface energy/moisture balance model that resolves narrow ice sheet ablation zones. The ice model returns revised elevation, surface albedo and ice area fields, plus coastal fluxes of heat and moisture. An arbitrary number of ice sheets can be simulated, each on their own high-resolution grid and each capable of synchronous or asynchronous coupling with the overlying climate model. The model is designed to conserve global heat and moisture. In the process of improving model performance we developed a procedure to account for modelled surface air temperature (SAT) biases within the energy/moisture balance surface model and improved the UVic ESCM snow surface scheme through addition of variable albedos and refreezing over the ice sheet. <br><br> A number of simulations for late Holocene, Last Glacial Maximum (LGM), and Eemian climate boundary conditions were carried out to explore the sensitivity of the coupled model and identify model configurations that best represented these climate states. The modelled SAT bias was found to play a significant role in long-term ice sheet evolution, as was the effect of refreezing meltwater and surface albedo. The bias-corrected model was able to reasonably capture important aspects of the Antarctic and Greenland ice sheets, including modern SMB and ice distribution. The simulated northern Greenland ice sheet was found to be prone to ice margin retreat at radiative forcings corresponding closely to those of the Eemian or the present-day.Fri, 13 Aug 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/1185/2010/<b>Meteorological and trace gas factors affecting the number concentration of atmospheric Aitken (<i>D</i>_p=50 nm) particles in the continental boundary layer: parameterization using a multivariate mixed effects model</b><br /><br />Geoscientific Model Development Discussions, 3, 1185-1221, 2010<br /><br />Author(s): S. Mikkonen, H. Korhonen, S. Romakkaniemi, J. N. Smith, J. Joutsensaari, K. E. J. Lehtinen, A. Hamed, T. J. Breider, W. Birmili, G. Spindler, C. Plass-Duelmer, M. C. Facchini, and A. Laaksonen<br /><br />Measurements of aerosol size-distribution and different gas and meteorological parameters, made in three polluted sites in Central- and Southern Europe: Po Valley, Italy, Melpitz and Hohenpeissenberg in Germany, were analysed for this study to examine which of the meteorological and trace gas variables affect the number concentration of Aitken (<i>D</i>_p=50 nm) particles. The aim of our study was to predict the number concentration of 50 nm particles by a combination of in-situ meteorological and gas phase parameters. The statistical model needs to describe, amongst others, the factors affecting the growth of newly formed aerosol particles (below 10 nm) to 50 nm size, but also sources of direct particle emissions in that size range. As the analysis method we used multivariate nonlinear mixed effects model. Hourly averages of gas and meteorological parameters measured at the stations were used as predictor variables; the best predictive model was attained with a combination of relative humidity, new particle formation event probability, temperature, condensation sink and concentrations of SO₂, NO₂ and ozone. The seasonal variation was also taken into account in the mixed model structure. Model simulations with the Global Model of Aerosol Processes (GLOMAP) indicate that the parameterization can be used as a part of a larger atmospheric model to predict the concentration of climatically active particles. As an additional benefit, the introduced model framework is, in theory, applicable for any kind of measured aerosol parameter.Fri, 13 Aug 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/1161/2010/<b>IMOGEN: an intermediate complexity model to evaluate terrestrial impacts of a changing climate</b><br /><br />Geoscientific Model Development Discussions, 3, 1161-1184, 2010<br /><br />Author(s): C. Huntingford, B. B. B. Booth, S. Sitch, N. Gedney, J. A. Lowe, S. K. Liddicoat, L. M. Mercado, M. J. Best, G. P. Weedon, R. A. Fisher, P. Good, P. Zelazowski, A. C. Spessa, and C. D. Jones<br /><br />We present a computationally efficient modelling system, IMOGEN, designed to undertake global and regional assessment of climate change impacts on the physical and biogeochemical behaviour of the land surface. A pattern-scaling approach to climate change drives a gridded land surface and vegetation model MOSES/TRIFFID. The structure allows extrapolation of General Circulation Model (GCM) simulations to different future pathways of greenhouse gases, including rapid first-order assessments of how the land surface and associated biogeochemical cycles might change. Evaluation of how new terrestrial process understanding influences such predictions can also be made with relative ease.Wed, 04 Aug 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/1139/2010/<b>A simplified treatment of surfactant effects on cloud drop activation</b><br /><br />Geoscientific Model Development Discussions, 3, 1139-1159, 2010<br /><br />Author(s): T. Raatikainen and A. Laaksonen<br /><br />Dissolved surface active species, or surfactants, have a tendency to partition to solution surface and thereby decrease solution surface tension. Activating cloud droplets have large surface-to-volume ratios, and the amount of surfactant molecules in them is limited. Therefore, unlike with macroscopic solutions, partitioning to the surface can effectively deplete the droplet interior of surfactant molecules. <br><br> Surfactant partitioning equilibrium for activating cloud droplets can be solved numerically from a group of equations. This can be a problem when surfactant effects are examined by using large-scale cloud models. Namely, computing time increases significantly due to the partitioning calculations done in the lowest levels of nested iterations. <br><br> The purpose of this paper is to present analytical equations for surfactant partitioning equilibrium. Some simplifications are needed in deriving the equations, but the numerical errors caused by the simplifications are shown to be very minor. In addition, computing time is decreased roughly by an order of magnitude.Fri, 30 Jul 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/1105/2010/<b>A nonlinear multi-proxy model based on manifold learning to reconstruct water temperature from high resolution trace element profiles in biogenic carbonates</b><br /><br />Geoscientific Model Development Discussions, 3, 1105-1138, 2010<br /><br />Author(s): M. Bauwens, H. Ohlsson, K. Barbé, V. Beelaerts, F. Dehairs, and J. Schoukens<br /><br />A long standing problem in paleoceanography concerns the reconstruction of water temperature from &delta;¹⁸O carbonate, which for freshwater influenced environments is hindered because the isotopic composition of the ambient water (related to salinity) affects the reconstructed temperature. In this paper we argue for the use of a nonlinear multi-proxy method called Weight Determination by Manifold Regularization to develop a temperature reconstruction model that is less sensitive to salinity variations. The motivation for using this type of model is twofold: Firstly, observed nonlinear relations between specific proxies and water temperature motivate the use of nonlinear models. Secondly, the use of multi-proxy models enables salinity related variations of a given temperature proxy to be explained by salinity-related information carried by a separate proxy. Our findings confirm that Mg/Ca is a powerful paleothermometer and highlight that reconstruction performance based on this proxy is improved significantly by combining its information with the information of other trace elements in multi-proxy models. Using Mg/Ca, Sr/Ca, Ba/Ca and Pb/Ca the WDMR model enabled a temperature reconstruction with a root mean squared error of &plusmn;2.19 °C for a salinity range between 15 and 32.Thu, 22 Jul 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/1089/2010/<b>An analytical solution to calculate bulk mole fractions for any number of components in aerosol droplets after considering partitioning to a surface layer</b><br /><br />Geoscientific Model Development Discussions, 3, 1089-1104, 2010<br /><br />Author(s): D. Topping<br /><br />Calculating the equilibrium composition of atmospheric aerosol particles, using all variations of Köhler theory, has largely assumed that the total solute concentrations define both the water activity and surface tension. Recently however, bulk to surface phase partitioning has been postulated as a process which significantly alters the predicted point of activation. In this paper, an analytical solution to calculate the removal of material from a bulk to a surface layer in aerosol particles has been derived using a well established and validated surface tension framework. The applicability to an unlimited number of components is possible via reliance on data from each binary system. Whilst assumptions regarding behaviour at the surface layer have been made to facilitate derivation, it is proposed that the framework presented can capture the overall impact of bulk-surface partitioning. Predictions made by the model across a range of surface active properties should be tested against measurements. The computational efficiency of using the solution presented in this paper is roughly a factor of 20 less than a similar iterative approach, a comparison with highly coupled approaches not available beyond a 3 component system.Tue, 13 Jul 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/1009/2010/<b>The global chemistry transport model TM5: description and evaluation of the tropospheric chemistry version 3.0</b><br /><br />Geoscientific Model Development Discussions, 3, 1009-1087, 2010<br /><br />Author(s): V. Huijnen, J. E. Williams, M. van Weele, T. P. C. van Noije, M. C. Krol, F. Dentener, A. Segers, S. Houweling, W. Peters, A. T. J. de Laat, K. F. Boersma, P. Bergamaschi, P. F. J. van Velthoven, P. Le Sager, H. J. Eskes, F. Alkemade, M. P. Scheele, P. Nédélec, and H.-W. Pätz<br /><br />We present a comprehensive description and benchmark evaluation of the tropospheric chemistry version of the global chemistry transport model TM5 (Tracer Model 5, version TM5-chem-v3.0). A full description is given concerning the photochemical mechanism, the interaction with aerosol, the treatment of the stratosphere, the wet and dry deposition parameterizations, and the applied emissions. We evaluate the model against a suite of ground-based, satellite, and aircraft measurements of components critical for understanding global photochemistry for the year 2006. <br><br> The model exhibits a realistic oxidative capacity at a global scale. The methane lifetime is ~8.9 years with an associated lifetime of methyl chloroform of 5.86 years, which is similar to that derived using an optimized hydroxyl radical field. <br><br> The seasonal cycle in observed carbon monoxide (CO) is well simulated at different regions across the globe. In the Northern Hemisphere CO concentrations are underestimated by about 20 ppbv in spring and 10 ppbv in summer, which is related to missing chemistry and underestimated emissions from higher hydrocarbons, as well as to uncertainties in the seasonal variation of CO emissions. The model also captures the spatial and seasonal variation in formaldehyde tropospheric columns as observed by SCIAMACHY. Positive model biases over the Amazon and eastern United States point to uncertainties in the isoprene emissions as well as its chemical breakdown. <br><br> Simulated tropospheric nitrogen dioxide columns correspond well to observations from the Ozone Monitoring Instrument in terms of its seasonal and spatial variability (with a global spatial correlation coefficient of 0.89), but TM5 fields are lower by 25–40%. This is consistent with earlier studies pointing to a high bias of 0–30% in the OMI retrievals, but uncertainties in the emission inventories have probably also contributed to the discrepancy. <br><br> TM5 tropospheric nitrogen dioxide profiles are in good agreement (within ~0.1 ppbv) with in situ aircraft observations from the INTEX-B campaign over (the Gulf of) Mexico. <br><br> The model reproduces the spatial and seasonal variation in background surface ozone concentrations and tropospheric ozone profiles from the World Ozone and Ultraviolet Radiation Data Centre to within 10 ppbv, but at several tropical stations the model tends to underestimate ozone in the free troposphere. <br><br> The presented model results benchmark the TM5 tropospheric chemistry version, which is currently in use in several international cooperation activities, and upon which future model improvements will take place.Wed, 07 Jul 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/949/2010/<b>SMOKE for Europe – adaptation, modification and evaluation of a comprehensive emission model for Europe</b><br /><br />Geoscientific Model Development Discussions, 3, 949-1007, 2010<br /><br />Author(s): J. Bieser, A. Aulinger, V. Matthias, M. Quante, and P. Builtjes<br /><br />The US EPA regional emission model SMOKE was adopted and modified to create temporally and spatially distributed emission for Europe and surrounding countries based on official reports and public domain data only. The aim is to develop a flexible model capable of creating consistent high resolution emission data for long-term runs of Chemical Transport Models (CTM). This modified version of SMOKE, called SMOKE for EUROPE (SMOKE-EU) was successfully used to create hourly gridded emissions for the timespan 1970–2010. <br><br> In this paper the SMOKE-EU model and the underlying European datasets are introduced. Emission data created by SMOKE-EU for the year 2000 are evaluated by comparison to data of three different state of the art emission models. Differences of SMOKE-EU to those models were in the same range as the differences amongst them. Further, concentrations of criteria pollutants calculated by the CTM CMAQ using the four different emission datasets were compared against EMEP measurements with hourly and daily resolution. Using SMOKE-EU emissions O₃, NO₂ and SO₄ could be modelled most reliably. The amount of simulated concentrations within a factor of 2 (F2) of the observations for these species are: O₃ (F2=0.79 <i>N</i>=329 197), NO₂ (F2=0.55 <i>N</i>=11 465), and SO₄ (F2=0.62 <i>N</i>=17 536). The lowest values were found for NH₄ (F2=0.34 <i>N</i>=7400) and NO₃ (F2=0.25 <i>N</i>=6184). NH₄ concentrations were generally overestimated, leading to a fractional bias (FB) averaged over 22 measurement stations of (FB=0.83&plusmn;0.41) while better agreements with observations were found for SO₄ (FB=0.06&plusmn;0.38, 51 stations) and NO₃ (FB=0.13&plusmn;0.75, 18 stations). <br><br> CMAQ simulations using the three other emission datasets were similar to those modelled using SMOKE-EU emissions. Highest differences where found for NH₄ while O₃ concentrations were almost identical. The results of this comparison confirm that it is adequate to use emissions created by SMOKE-EU as input for CTMs.Wed, 07 Jul 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/889/2010/<b>Modeling global atmospheric CO₂ with improved emission inventories and CO₂ production from the oxidation of other carbon species</b><br /><br />Geoscientific Model Development Discussions, 3, 889-948, 2010<br /><br />Author(s): R. Nassar, D. B. A. Jones, P. Suntharalingam, J. M. Chen, R. J. Andres, K. J. Wecht, R. M. Yantosca, S. S. Kulawik, K. W. Bowman, J. R. Worden, T. Machida, and H. Matsueda<br /><br />The use of global three-dimensional (3-D) models with satellite observations of CO₂ in inverse modeling studies is an area of growing importance for understanding Earth's carbon cycle. Here we use the GEOS-Chem model (version 8-02-01) CO₂ simulation with multiple modifications in order to assess their impact on CO₂ forward simulations. Modifications include CO</sub>2</sub> surface emissions from shipping (~0.19 Pg C/yr), 3-D spatially-distributed emissions from aviation (~0.16 Pg C/yr), and 3-D chemical production of CO₂ (~1.05 Pg C/yr). Although CO₂ chemical production from the oxidation of CO, CH₄ and other carbon gases is recognized as an important contribution to global CO₂, it is typically accounted for by conversion from its precursors at the surface rather than in the free troposphere. We base our model 3-D spatial distribution of CO₂ chemical production on monthly-averaged loss rates of CO (a key precursor and intermediate in the oxidation of organic carbon) and apply an associated surface correction for inventories that have counted emissions of carbon precursor as CO₂. We also explore the benefit of assimilating satellite observations of CO into GEOS-Chem to obtain an observation-based estimate of the CO₂ chemical source. The CO assimilation corrects for an underestimate of atmospheric CO abundances in the model, resulting in increases of as much as 24% in the chemical source during May–June 2006, and increasing the global annual estimate of CO₂ chemical production from 1.05 to 1.18 Pg C. Comparisons of model CO₂ with measurements are carried out in order to investigate the spatial and temporal distributions that result when these new sources are added. Inclusion of CO₂ emissions from shipping and aviation are shown to increase the global CO₂ latitudinal gradient by just over 0.10 ppm (~3%), while the inclusion of CO₂ chemical production (and the surface correction) is shown to decrease the latitudinal gradient by about 0.40 ppm (~10%) with a complex spatial structure generally resulting in decreased CO</sub>2</sub> over land and increased CO₂ over the oceans. Since these CO₂ emissions are omitted or misrepresented in most inverse modeling work to date, their implementation in forward simulations should lead to improved inverse modeling estimates of terrestrial biospheric fluxes.Fri, 02 Jul 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/855/2010/<b>A pre-processor of trace gases and aerosols emission fields for regional and global atmospheric chemistry models</b><br /><br />Geoscientific Model Development Discussions, 3, 855-888, 2010<br /><br />Author(s): S. R. Freitas, K. M. Longo, M. F. Alonso, M. Pirre, V. Marecal, G. Grell, R. Stockler, R. F. Mello, and M. Sánchez Gácita<br /><br />The pre-processor PREP-CHEM-SRC presented in the paper is a comprehensive tool aiming at preparing emissions fields of trace gases and aerosols for use in regional or global transport models. The emissions considered are urban/industrial, biogenic, biomass burning, volcanic, biofuel use and burning from agricultural waste sources from most recent databases or from satellite fire detections for biomass burning. A plumerise model is used to derive the height of smoke emissions from satellite fire products. The pre-processor provides emission fields interpolated onto the transport model grid. Several map projections can be chosen. The way to include these emissions in transport models is also detailed. The pre-processor is coded using Fortran 90 and C and is driven by a <i>namelist</i> allowing the user to choose the type of emissions and the database.Wed, 23 Jun 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/819/2010/<b>On the attribution of contributions of atmospheric trace gases to emissions in atmospheric model applications</b><br /><br />Geoscientific Model Development Discussions, 3, 819-853, 2010<br /><br />Author(s): V. Grewe, E. Tsati, and P. Hoor<br /><br />We present a revised tagging method, which describes the combined effect of emissions of various species from individual emission categories, e.g. the impact of both, nitrogen oxides and non-methane hydrocarbon emissions on ozone. This method is applied to two simplified chemistry schemes, which represent the main characteristics of atmospheric ozone chemistry. Analytical solutions are presented for this tagging approach. In the past, besides tagging approaches, sensitivity methods were used, which estimate the contributions from individual sources based on differences in two simulations, a base case and a simulation with a perturbation in the respective emission category. We apply both methods to our simplified chemical systems and demonstrate that potentially large errors (factor of 2) occur with the sensitivity method, which depend on the degree of linearity of the chemical system. For some chemical regimes this error can be minimised by employing only small perturbations of the respective emission, e.g. 5%. Since a complete tagging algorithm for global chemistry models is difficult to achieve, we present two error metrics, which can be applied for sensitivity methods in order to estimate the potential error of this approach for a specific application.Thu, 10 Jun 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/769/2010/<b>The Lagrangian chemistry and transport model ATLAS: simulation and validation of stratospheric chemistry and ozone loss in the winter 1999/2000</b><br /><br />Geoscientific Model Development Discussions, 3, 769-817, 2010<br /><br />Author(s): I. Wohltmann, R. Lehmann, and M. Rex<br /><br />ATLAS is a new global Lagrangian Chemistry and Transport Model (CTM), which includes a stratospheric chemistry scheme with 46 active species, 171 reactions, heterogeneous chemistry on polar stratospheric clouds and a Lagrangian denitrification module. Lagrangian (trajectory-based) models have several important advantages over conventional Eulerian models, including the absence of spurious numerical diffusion, efficient code parallelization and no limitation of the largest time step by the Courant-Friedrichs-Lewy criterion. This work describes and validates the stratospheric chemistry scheme of the model. Stratospheric chemistry is simulated with ATLAS for the Arctic winter 1999/2000, with a focus on polar ozone depletion and denitrification. The simulations are used to validate the chemistry module in comparison with measurements of the SOLVE/THESEO 2000 campaign. A Lagrangian denitrification module, which is based on the simulation of the nucleation, sedimentation and growth of a large number of polar stratospheric cloud particles, is used to model the substantial denitrification that occured in this winter.Tue, 01 Jun 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/735/2010/<b>Development of an online radiative module for the computation of aerosol optical properties in 3-D atmospheric models: validation during the EUCAARI campaign</b><br /><br />Geoscientific Model Development Discussions, 3, 735-768, 2010<br /><br />Author(s): B. Aouizerats, O. Thouron, P. Tulet, M. Mallet, L. Gomes, and J. S. Henzing<br /><br />Obtaining a good description of aerosol optical properties for a physically and chemically complex evolving aerosol is computationally very expensive at present. The goal of this work is to propose a new numerical module computing the optical properties for complex aerosol particles at low numerical cost so that it can be implemented in atmospheric models. This method aims to compute the optical properties online as a function of a given complex refractive index deduced from the aerosol chemical composition and the size parameters corresponding to the particles. <br><br> The construction of look-up tables from the imaginary and the real part of the complex refractive index and size parameters will also be explained. This approach is validated for observations acquired during the EUCAARI campaign on the Cabauw tower during May 2008 and its computing cost is also estimated. <br><br> These comparisons show that the module manages to reproduce the scattering and absorbing behaviour of the aerosol during most of the fifteen-day period of observation with a very cheap computationally cost.Tue, 01 Jun 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/651/2010/<b>Description and evaluation of GLOMAP-mode: a modal global aerosol microphysics model for the UKCA composition-climate model</b><br /><br />Geoscientific Model Development Discussions, 3, 651-734, 2010<br /><br />Author(s): G. W. Mann, K. S. Carslaw, D. V. Spracklen, D. A. Ridley, P. T. Manktelow, M. P. Chipperfield, S. J. Pickering, and C. E. Johnson<br /><br />A new version of the Global Model of Aerosol Processes (GLOMAP) is described, which uses a two-moment modal aerosol scheme rather than the original two-moment bin scheme. GLOMAP-mode simulates the multi-component global aerosol, resolving sulphate, sea-salt, dust, black carbon (BC) and particulate organic matter (POM), the latter including primary and biogenic secondary POM. Aerosol processes are simulated in a size-resolved manner including primary emissions, secondary particle formation by binary homogeneous nucleation of sulphuric acid and water, particle growth by coagulation, condensation and cloud-processing and removal by dry deposition, in-cloud and below-cloud scavenging. A series of benchmark observational datasets are assembled against which the skill of the model is assessed in terms of normalised mean bias (<i>b</i>) and correlation coefficient (<i>R</i>). Overall, the model performs well against the datasets in simulating concentrations of aerosol precursor gases, chemically speciated particle mass, condensation nuclei (CN) and cloud condensation nuclei (CCN). Surface sulphate, sea-salt and dust mass concentrations are all captured well, while BC and POM are biased low (but correlate well). Surface CN concentrations compare reasonably well in free troposphere and marine sites, but are underestimated at continental and coastal sites related to underestimation of either primary particle emissions or new particle formation. The model compares well against a compilation of CCN observations covering a range of environments and against vertical profiles of size-resolved particle concentrations over Europe. The simulated global burden, lifetime and wet removal of each of the simulated aerosol components is also examined and each lies close to multi-model medians from the AEROCOM model intercomparison exercise.Fri, 28 May 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/627/2010/<b><i>ESCIMO.spread</i> – a spreadsheet-based point snow surface energy balance model to calculate hourly snow water equivalent and melt rates for historical and changing climate conditions</b><br /><br />Geoscientific Model Development Discussions, 3, 627-649, 2010<br /><br />Author(s): U. Strasser and T. Marke<br /><br />This paper describes the spreadsheet-based point energy balance model <i>ESCIMO.spread</i> which simulates the energy and mass balance as well as melt rates of a snow surface. The model makes use of hourly recordings of temperature, precipitation, wind speed, relative humidity, global and longwave radiation. The effect of potential climate change on the seasonal evolution of the snow cover can be estimated by modifying the time series of observed temperature and precipitation by means of adjustable parameters. Model output is graphically visualized in hourly and daily diagrams. The results compare well with weekly measured snow water equivalent (SWE). The model is easily portable and adjustable, and runs particularly fast: hourly calculation of a one winter season is instantaneous on a standard computer. <i>ESICMO.spread</i> can be obtained from the authors on request (contact: ulrich.strasser@uni-graz.at).Fri, 28 May 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/569/2010/<b>Description and evaluation of GMXe: a new aerosol submodel for global simulations (v1)</b><br /><br />Geoscientific Model Development Discussions, 3, 569-626, 2010<br /><br />Author(s): K. J. Pringle, H. Tost, S. Metzger, B. Steil, D. Giannadaki, A. Nenes, C. Fountoukis, P. Stier, E. Vignati, and J. Lelieveld<br /><br />We present a new aerosol microphysics and gas aerosol partitioning submodel (Global Modal-aerosol eXtension, GMXe) implemented within the ECHAM/MESSy Atmospheric Chemistry model (EMAC, version 1.8). The submodel is computationally efficient and is suitable for medium to long term simulations with global and regional models. The aerosol size distribution is treated using 7 log-normal modes and has the same microphysical core as the M7 submodel (Vignati et al., 2004). <br><br> The main developments in this work are: (i) the extension of the aerosol emission routines and the M7 microphysics, so that an increased (and variable) number of aerosol species can be treated (new species include sodium and chloride, and potentially magnesium, calcium, and potassium), (ii) the coupling of the aerosol microphysics to a choice of treatments of gas/aerosol partitioning to allow the treatment of semi-volatile aerosol, and, (iii) the implementation and evaluation of the developed submodel within the EMAC model of atmospheric chemistry. <br><br> Simulated concentrations of black carbon, particulate organic matter, dust, sea spray, sulfate and ammonium aerosol are shown to be in good agreement with observations (for all species at least 40% of modeled values are within a factor of 2 of the observations). The distribution of nitrate aerosol is compared to observations in both clean and polluted regions. Concentrations in polluted continental regions are simulated quite well, but there is a general tendency to overestimate nitrate, particularly in coastal regions (geometric mean of modelled values/geometric mean of observed data ≈2). In all regions considered more than 40% of nitrate concentrations are within a factor of two of the observations. Marine nitrate concentrations are well captured with 96% of modeled values within a factor of 2 of the observations.Thu, 20 May 2010 00:00:00 +0200http://www.geosci-model-dev-discuss.net/3/541/2010/<b>A dynamic probability density function treatment of cloud mass and number concentrations for low level clouds in GFDL SCM/GCM</b><br /><br />Geoscientific Model Development Discussions, 3, 541-568, 2010<br /><br />Author(s): H. Guo, J.-C. Golaz, L. J. Donner, V. E. Larson, D. P. Schanen, and B. M. Griffin<br /><br />Successful simulation of cloud-aerosol interactions (indirect aerosol effects) in climate models requires relating grid-scale aerosol, dynamic, and thermodynamic fields to small-scale processes like aerosol activation. A turbulence and cloud parameterization, based on multivariate probability density functions (PDFs) of sub-grid vertical velocity, temperature, and moisture, has been extended to treat aerosol activation. This dynamics-PDF approach offers a solution to the problem of the scale gap between the resolution of climate models and the scales relevant for aerosol activation and a means to overcome the limitations of diagnostic estimates of cloud droplet number concentration based only on aerosol concentration. <br><br> Incorporated into a single-column model for GFDL AM3, the dynamics-PDF parameterization successfully simulates cloud fraction and water content for shallow cumulus, stratocumulus, and cumulus-under-stratocumulus regimes. The extension to treat aerosol activation predicts droplet number concentrations in good agreement with large eddy simulation (LES). The dynamics-PDF droplet number concentrations match LES results more closely than state-of-the-science diagnostic relationships between aerosol concentration and droplet number concentration.Tue, 11 May 2010 00:00:00 +0200