Trond Iversen

Intercomparison of models representing direct shortwave radiative forcing by sulfate aerosols

The importance of aerosols as agents of climate change has recently been highlighted. However, the magnitude of aerosol forcing by scattering of shortwave radiation (direct forcing) is still very uncertain even for the relatively well characterized sulfate aerosol. A potential source of uncertainty is in the model representation of aerosol optical properties and aerosol influences on radiative transfer in the atmosphere. Although radiative transfer methods and codes have been compared in the past, these comparisons have not focused on aerosol forcing (change in net radiative flux at the top of the atmosphere). Here we report results of a project involving 12 groups using 15 models to examine radiative forcing by sulfate aerosol for a wide range of values of particle radius, aerosol optical depth, surface albedo, and solar zenith angle. Among the models that were employed were high and low spectral resolution models incorporating a variety of radiative transfer approximations as well as a line-by-line model. The normalized forcings (forcing per sulfate column burden) obtained with the several radiative transfer models were examined, and the discrepancies were characterized. All models simulate forcings of comparable amplitude and exhibit a similar dependence on input parameters. As expected for a non-light-absorbing aerosol, forcings were negative (cooling influence) except at high surface albedo combined with small solar zenith angle. The relative standard deviation of the zenith-angle-averaged normalized broadband forcing for 15 models was 8% for particle radius near the maximum in this forcing (∼0.2 μm) and at low surface albedo. Somewhat greater model-to-model discrepancies were exhibited at specific solar zenith angles. Still greater discrepancies were exhibited at small particle radii, and much greater discrepancies were exhibited at high surface albedos, at which the forcing changes sign; in these situations, however, the normalized forcing is quite small. Discrepancies among the models arise from inaccuracies in Mie calculations, differing treatment of the angular scattering phase function, differing wavelength and angular resolution, and differing treatment of multiple scattering. These results imply the need for standardized radiative transfer methods tailored to the direct aerosol forcing problem. However, the relatively small spread in these results suggests that the uncertainty in forcing arising from the treatment of radiative forcing of a well-characterized aerosol at well-specified surface albedo is smaller than some of the other sources of uncertainty in estimates of direct forcing by anthropogenic sulfate aerosols and anthropogenic aerosols generally.

Application of the CALIOP layer product to evaluate the vertical distribution of aerosols estimated by global models: AeroCom phase i results

[1] The CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) layer product is used for a multimodel evaluation of the vertical distribution of aerosols. Annual and seasonal aerosol extinction profiles are analyzed over 13 sub-continental regions representative of industrial, dust, and biomass burning pollution, from CALIOP 2007–2009 observations and from AeroCom (Aerosol Comparisons between Observations and Models) 2000 simulations. An extinction mean height diagnostic (Za) is defined to quantitatively assess the models’ performance. It is calculated over the 0–6 km and 0–10 km altitude ranges by weighting the altitude of each 100 m altitude layer by its aerosol extinction coefficient. The mean extinction profiles derived from CALIOP layer products provide consistent regional and seasonal specificities and a low inter-annual variability. While the outputs from most models are significantly correlated with the observed Za climatologies, some do better than others, and 2 of the 12 models perform particularly well in all seasons. Over industrial and maritime regions, most models show higher Za than observed by CALIOP, whereas over the African and Chinese dust source regions, Za is underestimated during Northern Hemisphere Spring and Summer. The positive model bias in Za is mainly due to an overestimate of the extinction above 6 km. Potential CALIOP and model limitations, and methodological factors that might contribute to the differences are discussed.

A synthesis of regional climate change simulations—A Scandinavian perspective

Four downscaling experiments of regional climate change for the Nordic countries have been conducted with three different regional climate models (RCMs). A short synthesis of the outcome of the suite of experiments is presented as an ensemble, reflecting the different driving atmosphere-ocean general circulation model (AOGCM) conditions, RCM model resolution and domain size, and choice of emission scenarios. This allows the sources of uncertainties in the projections to be assessed. At the same time analysis of the climate change signal for temperature and precipitation over the period 1990–2050 reveals strong similarities. In particular, all experiments in the suite simulate changes in the precipitation distribution towards a higher frequency of heavy precipitation.

Aerosol-climate interactions in the CAM-Oslo atmospheric GCM and investigation of associated basic shortcomings

The paper discusses some challenges in aerosol-climate modelling. CAM-Oslo, extended from NCAR-CAM3, employs an aerosol module for sea-salt, dust, sulphate, black carbon (BC) and particulate organic matter (OM). Primary aerosol size-distributions are modified by condensation, coagulation and wet-phase processes. Aerosol optics and cloud droplet numbers use look-up tables constructed from first principles. Ground level sulphate and sea-salt are generally well modelled, BC and OM are slightly underestimated (uncertain), and dust is considerably (factor ∼2) underestimated. Since non-desert dust, nitrate, anthropogenic secondary organics, and biological particles are omitted, aerosol optical depths (0.12) are underestimated by 10–25%. The underestimates are large in areas with biomass burning and soil dust. The direct and indirect forcing of aerosol increments since pre-industrial time are estimated at +0.031 Wm−2 and −1.78 Wm−2, respectively. Although the total absorption AOD probably is slightly underestimated, the BC contributes to DRF with double strength compared to the AeroCom average. Main reasons for this include: internal BC-mixing (+0.2 Wm−2), accumulation mode BC-agglomerates (+0.05 Wm−2), assumed aitken-mode OM-BC mixture (+0.1 Wm−2), large BC fraction (36%) above 500 hPa, and high low-level cloudiness. Using a prognostic CDNC and process parametrized CCN activation instead of assuming CDNC are equal to CCN, the indirect forcing is 36% smaller.

Modelled and measured transboundary acidifying pollution in Europe— verification and trends

Abstract The routine acid deposition model at MSC-W EMEP has been run for 5 years. Routine measurements taken by countries participating in EMEP are so far available from CCC EMEP for hour of these years. This paper aims at demonstrating the usefulness of having access to simultaneous sets of data, such as measurements and model-calculated quantities for which one can vary the determining parameters. By comparing the data sets with emissions used in the calculations, one can identify both important model shortcomings as well as probable errors in measurements and emissions. Furthermore, trends in manual deposition and concentration levels can be investigated by controlling the meteorological input in order to filter out the effect of meteoroligical variability. In general, errors in modelled concentrations can only be identified with confidence if a statistically sufficient number of stations agree. This is the case for NO 2 in air for which the model seems to underestimate systematically the measured concentrations, and for sulphate and nitrate in precipitation for which the model gives persistently lower concentrations than measured over peripheral areas in Europe. A majority of the “outliers” in the scatter plots comparing measurements and calculations are seen to be connected with clear misfits between emission data and measured values. Furthermore, at a few sites measurements show much larger variations from year to year than anticipated. The trend experiments emphasize the importance of meteorological variability as compared to emission changes. Even if the impacts of emission trends can be larger than those of meteorological variations at some sites, the complex spatial pattern caused by the latter completely masks the former. Measurements, being influenced by changes in both conditions, are thus of limited value for a year-to-year monitoring of emission reduction effects. Keeping the meteorology the same from year to year in the model calculations, reveals that calculated air concentrations and depositions correspond to the reported emissions. Trends in measurements and calculations generally agree well, except that for sulphur in air 1989 measured concentrations are smaller than calculated. This is believed to be a consequence of too high emission numbers for SO 2 in 1989. The conclusions that can be drawn from this study are direct consequences of the distribution of measurement sites in Europe as part of EMEP. This distribution favours central European areas north of the Alps. The statistical confidence and representativity of the results would be much improved by a more evenly distributed measurement network.

Advancing Polar Prediction Capabilities on Daily to Seasonal Time Scales

AbstractThe polar regions have been attracting more and more attention in recent years, fueled by the perceptible impacts of anthropogenic climate change. Polar climate change provides new opportunities, such as shorter shipping routes between Europe and East Asia, but also new risks such as the potential for industrial accidents or emergencies in ice-covered seas. Here, it is argued that environmental prediction systems for the polar regions are less developed than elsewhere. There are many reasons for this situation, including the polar regions being (historically) lower priority, with fewer in situ observations, and with numerous local physical processes that are less well represented by models. By contrasting the relative importance of different physical processes in polar and lower latitudes, the need for a dedicated polar prediction effort is illustrated. Research priorities are identified that will help to advance environmental polar prediction capabilities. Examples include an improvement of the p...

Modeling of the Wegener?Bergeron?Findeisen process?implications for aerosol indirect effects

A new parameterization of the Wegener–Bergeron–Findeisen (WBF) process has been developed, and implemented in the general circulation model CAM-Oslo. The new parameterization scheme has important implications for the process of phase transition in mixed-phase clouds. The new treatment of the WBF process replaces a previous formulation, in which the onset of the WBF effect depended on a threshold value of the mixing ratio of cloud ice. As no observational guidance for such a threshold value exists, the previous treatment added uncertainty to estimates of aerosol effects on mixed-phase clouds. The new scheme takes subgrid variability into account when simulating the WBF process, allowing for smoother phase transitions in mixed-phase clouds compared to the previous approach. The new parameterization yields a model state which gives reasonable agreement with observed quantities, allowing for calculations of aerosol effects on mixed-phase clouds involving a reduced number of tunable parameters. Furthermore, we find a significant sensitivity to perturbations in ice nuclei concentrations with the new parameterization, which leads to a reversal of the traditional cloud lifetime effect.

The Norwegian IPY–THORPEX: Polar Lows and Arctic Fronts during the 2008 Andøya Campaign

From a weather forecasting perspective, the Arctic poses particular challenges for mainly two reasons: 1) The observational data are sparse and 2) the weather phenomena responsible for severe weather, such as polar lows, Arctic fronts, and orographic influences on airflow, are poorly resolved and described by the operational numerical weather prediction (NWP) models. The Norwegian International Polar Year (IPY)– The Observing System Research and Predictability Experiment (THORPEX) project (2007–10) sought to significantly improve weather forecasts of these phenomena through a combined modeling and observational effort. The crux of the observational effort was a 3-week international field campaign out of northern Norway in early 2008, combining airborne and surface-based observations. The main platform of the field campaign was the Deutsches Zentrum fur Luft- und Raumfahrt (DLR) research aircraft Falcon, equipped with lidar systems for profiling of aerosols, humidity, and wind, in addition to in situ measu...

High‐resolution limited‐area ensemble predictions based on low‐resolution targeted singular vectors

The operational limited-area model, HIRLAM, at the Norwegian Meteorological Institute is used at 0.25° latitude/longitude resolution for ensemble weather prediction over Northern Europe and adjacent parts of the North Atlantic Ocean; this system is called LAMEPS. Initial and lateral boundary perturbations are taken from coarse-resolution European Centre for Medium-Range Weather Forecasts global ensemble members based on targeted singular vectors (TEPS). Five winter and five summer cases in 1997 consisting of 20 ensemble members plus one control forecast are integrated. Two sets of ensembles are generated, one for which both initial and lateral boundary conditions are perturbed, and another with only the initial fields perturbed. The LAMEPS results are compared to those of TEPS using the following measures: r.m.s. ensemble spread of 500 hPa geopotential height; r.m.s. ensemble spread of mean-sea-level pressure; Brier Skill Scores (BSS); Relative Operating Characteristic (ROC) curves; and cost/loss analyses. For forecasts longer than 12 hours, all measures show that perturbing the boundary fields is crucial for the performance of LAMEPS. For the winter cases TEPS has slightly larger ensemble spread than LAMEPS, but this is reversed for the summer cases. Results from BSS, ROC and cost/loss analyses show that LAMEPS performed considerably better than TEPS for precipitation, a result that is promising for forecasting extreme precipitation amounts. We believe this result to be linked to the high predictability of mesoscale flows controlled by complex topography. For two-metre temperature, however, TEPS frequently performed better than LAMEPS. Copyright

Targeted ensemble prediction for northern Europe and parts of the north Atlantic Ocean

The targeting procedure developed at ECMWF is used to make ensembles specially designedfor northern Europe and parts of the north Atlantic Ocean. A total of 35 ensembles are integrated, consisting of 20 winter cases and 15 summer cases in 1997, each consisting of 20 membersplus one control forecast. The ensembles are run up to day 10, and the ensemble spread insidethe target area continues to increase all through the 10 days. Two distinct regimes of increasecan be found, the first increase is consistent with the perturbations moving in and through thetarget area, it is hypothesised that the latter increase in ensemble spread around forecast day 5–7is connected with increasing non-linearity. The performance of the experimental ensembles iscompared to the operational ensemble prediction system (EPS) at ECMWF, both with all 50members and with only 20 members. The spread increases when the number of members in theensemble prediction system is increased, and the spread increases inside the target area whentargeting is applied.We find that the increase in spread when going from EPS with 50 membersto the targeted ensembles is larger than when going from 20 to 50 ensemble members of theoperational sets. Clearly targeting must be an option when predicting for a sub-domain of thehemisphere. Looking at other measures, such as the Brier skill score (BSS), relative operatingcharacteristic (ROC) curves and cost/loss analyses, the impact of the targeting is modest forthe winter cases, but the impact for the summer cases is evident. For the winter cases a largepart of the operational perturbations were located in the same area as the targeted perturbations, and the differences for the two sets are small. For the summer cases the operational perturbationswere mostly split between two locations and hence the targeting will give results differing morefrom the operational.