Orografiske effekter og diabatiske prosesser i kraftige lavtrykk over Nord-Atlanteren

Two intense cyclones over the North Atlantic during summer/autumn 2003 are investigated to determine atmospheric factors contributing to their development. One of the cases, in August 2003, led to heavy precipitation and flooding in Central Norway. The other event was a cyclone that caused heavy snowfall and wind exceeding 30m/s over East-Greenland and strong winds and snowfall over North-Iceland on 20-21 September 2003. The cyclones have been simulated with the numerical model MM5. For both cases sensitivity studies have been carried out to evaluate the significance of orography, latent heating and SST. In addition, the role of SST gradients are investigated in the September case and the role of surface albedo in the August case. This study shows that the September cyclone is strongly affected by the orography of Greenland. When orography is removed, a deep baroclinic low develops rapidly and moves eastward at 75N. Conversely, in the control run, the evolution of the primary baroclinic low is greatly suppressed by the orographic retardation of the warm air ahead of and the cold air behind the low. At the same time, a secondary low off Greenland’s east coast at 68N intensifies due to a coupling between an approaching upper level PV-anomaly and a lower level PV-anomaly generated from lee effects. The secondary cyclone is then transformed into a baroclinic low and moves eastward and causes the extreme weather conditions. In the August case the precipitation is strongly enhanced by orography due to the northely onshore winds. Removing the orography of Scandinavia leads to greatly reduced precipitation and a deformation of the cyclone. When the orography is present, a lee trough is formed over SE-Norway. A simulation with increased albedo is conducted to determine the significans of thermal effects for the trough formation. The lee trough weakened, but the impact of surface albedo was less than that of the mountain effect. In both cases, the simulations reveal that the release of latent heat had a major impact on the deepening of the cyclone in lower troposphere. In the August case, latent heating contributed to 40% of the rapid deepening during the first 30 hours of the simulation. The August cyclone occurred during a period of anomalously high sea surface temperatures(SST). Numerical tests show however that the development of the cyclone is fairly insensitive to the SST. Neighter the September cyclone was significantly influenced by the SST. In the North-Atlantic there are gradients in the sea surface temperature due to the warm Norwegian Atlantic Current and the cold East Greenland Current. The simulations show however that SSTgradients contributed relatively little to the development of the September cyclone

IMPACT OF THE SCANDINAVIAN MOUNTAINS ON A HIGH-IMPACT CYCLONE IN AUGUST 2003

In August 2003, Central Norway was hit by extreme precipitation. The cyclone that caused the precipitation has been simulated with a high-resolution model, and several sensitivity studies have been carried out. The simulations reveal that the release of latent heat had a major impact on the development of the cyclone. The cyclone occurred during a period of anomalously high sea surface temperatures (SST). Numerical tests show however that the development of the cyclone and the extreme precipitation are fairly insensitive to the SST. Removing the orography of Scandinavia leads to a deformation of the cyclone; when the orography is present a lee trough is formed over SE-Norway and the pressure gradient to the west of the low as it moves over SE-Norway is stronger than in the run with no mountains. The results will be helpful in analysing similar events in coarse-resolution climate simulations, where the mountains are poorly resolved

DYNAMICAL PROCESSES RELATED TO CYCLONE DEVELOPMENT NEAR GREENLAND

A cyclone that caused heavy snowfall and winds exceeding 30 m/s over E-Greenland and N-Iceland on 20-21 September 2003 is investigated. Numerical simulations are conducted to assess the role of Greenland\u27s orography for the development, as well as to evaluate the significance of other factors such as latent heating, SST and SST gradients. The simulations reveal that the cyclone evolution is strongly affected by the orography of Greenland. When orography is removed, a deep, well organized baroclinic low develops rapidly and moves eastward at 75°N. Conversely, in the control run the evolution of the primary baroclinic low is greatly suppressed by the orographic retardation of the warm air ahead of and the cold air behind the low. At the same time, a secondary low off Greenland’s east coast at 68°N intensifies due to a coupling between an approaching upper level PV-anomaly and a lower level PV-anomaly generated from lee effects. This secondary low then moves eastward and causes extreme weather conditions, as observed. Further sensitivity experiments show that latent heating contributes to deepen the low, while SST gradients and SST in general contribute relatively little

DYNAMICAL PROCESSES RELATED TO CYCLONE DEVELOPMENT NEAR GREENLAND

A cyclone that caused heavy snowfall and winds exceeding 30 m/s over E-Greenland and N-Iceland on 20-21 September 2003 is investigated. Numerical simulations are conducted to assess the role of Greenland\u27s orography for the development, as well as to evaluate the significance of other factors such as latent heating, SST and SST gradients. The simulations reveal that the cyclone evolution is strongly affected by the orography of Greenland. When orography is removed, a deep, well organized baroclinic low develops rapidly and moves eastward at 75°N. Conversely, in the control run the evolution of the primary baroclinic low is greatly suppressed by the orographic retardation of the warm air ahead of and the cold air behind the low. At the same time, a secondary low off Greenland’s east coast at 68°N intensifies due to a coupling between an approaching upper level PV-anomaly and a lower level PV-anomaly generated from lee effects. This secondary low then moves eastward and causes extreme weather conditions, as observed. Further sensitivity experiments show that latent heating contributes to deepen the low, while SST gradients and SST in general contribute relatively little

Atmospheric concentrations of black carbon are substantially higher in spring than summer in the Arctic

A key driving factor behind rapid Arctic climate change is black carbon, the atmospheric aerosol that most efficiently absorbs sunlight. Our knowledge about black carbon in the Arctic is scarce, mainly limited to long-term measurements of a few ground stations and snap-shots by aircraft observations. Here, we combine observations from aircraft campaigns performed over nine years, and present vertically resolved average black carbon properties. A factor of four higher black carbon mass concentration (21.6 ng m$^{–3}$ average, 14.3 ng m$^{–3}$ median) was found in spring, compared to summer (4.7 ng m$^{–3}$ average, 3.9 ng m$^{–3}$ median). In spring, much higher inter-annual and geographic variability prevailed compared to the stable situation in summer. The shape of the black carbon size distributions remained constant between seasons with an average mass mean diameter of 202 nm in spring and 210 nm in summer. Comparison between observations and concentrations simulated by a global model shows notable discrepancies, highlighting the need for further model developments and intensified measurements

Multi-model simulations of aerosol and ozone radiative forcing due to anthropogenic emission changes during the period 1990-2015

Over the past few decades, the geographical distribution of emissions of substances that alter the atmospheric energy balance has changed due to economic growth and air pollution regulations. Here, we show the resulting changes to aerosol and ozone abundances and their radiative forcing, using recently updated emission data for the period 1990-2015, as simulated by seven global atmospheric composition models. The models broadly reproduce large-scale changes in surface aerosol and ozone based on observations (e.g., -1 to -3%/yr in aerosols over 30 the US and Europe). The global mean radiative forcing due to ozone and aerosol changes over the 1990-2015 period increased by +0.17 ±0.08 Wm-2, with approximately 1/3 due to ozone. This increase is more strongly positive than that reported in IPCC AR5. The main reasons for the increased positive radiative forcing of aerosols over this period are the substantial reduction of global mean SO2 emissions, which is stronger in the new emission inventory compared to that used in the IPCC analysis, and higher black carbon emissions

Global and Regional Trends of Atmospheric Sulfur

The profound changes in global SO[subscript 2] emissions over the last decades have affected atmospheric composition on a regional and global scale with large impact on air quality, atmospheric deposition and the radiative forcing of sulfate aerosols. Reproduction of historical atmospheric pollution levels based on global aerosol models and emission changes is crucial to prove that such models are able to predict future scenarios. Here, we analyze consistency of trends in observations of sulfur components in air and precipitation from major regional networks and estimates from six different global aerosol models from 1990 until 2015. There are large interregional differences in the sulfur trends consistently captured by the models and observations, especially for North America and europe. europe had the largest reductions in sulfur emissions in the first part of the period while the highest reduction came later in North America and east Asia. the uncertainties in both the emissions and the representativity of the observations are larger in Asia. However, emissions from East Asia clearly increased from 2000 to 2005 followed by a decrease, while in India a steady increase over the whole period has been observed and modelled. the agreement between a bottom-up approach, which uses emissions and process-based chemical transport models, with independent observations gives an improved confidence in the understanding of the atmospheric sulfur budget

Reduced Complexity Model Intercomparison Project Phase 1: introduction and evaluation of global-mean temperature response

Reduced-complexity climate models (RCMs) are critical in the policy and decision making space, and are directly used within multiple Intergovernmental Panel on Climate Change (IPCC) reports to complement the results of more comprehensive Earth system models. To date, evaluation of RCMs has been limited to a few independent studies. Here we introduce a systematic evaluation of RCMs in the form of the Reduced Complexity Model Intercomparison Project (RCMIP). We expect RCMIP will extend over multiple phases, with Phase 1 being the first. In Phase 1, we focus on the RCMs' global-mean temperature responses, comparing them to observations, exploring the extent to which they emulate more complex models and considering how the relationship between temperature and cumulative emissions of CO2 varies across the RCMs. Our work uses experiments which mirror those found in the Coupled Model Intercomparison Project (CMIP), which focuses on complex Earth system and atmosphere–ocean general circulation models. Using both scenario-based and idealised experiments, we examine RCMs' global-mean temperature response under a range of forcings. We find that the RCMs can all reproduce the approximately 1 ∘C of warming since pre-industrial times, with varying representations of natural variability, volcanic eruptions and aerosols. We also find that RCMs can emulate the global-mean temperature response of CMIP models to within a root-mean-square error of 0.2 ∘C over a range of experiments. Furthermore, we find that, for the Representative Concentration Pathway (RCP) and Shared Socioeconomic Pathway (SSP)-based scenario pairs that share the same IPCC Fifth Assessment Report (AR5)-consistent stratospheric-adjusted radiative forcing, the RCMs indicate higher effective radiative forcings for the SSP-based scenarios and correspondingly higher temperatures when run with the same climate settings. In our idealised setup of RCMs with a climate sensitivity of 3 ∘C, the difference for the ssp585–rcp85 pair by 2100 is around 0.23∘C(±0.12 ∘C) due to a difference in effective radiative forcings between the two scenarios. Phase 1 demonstrates the utility of RCMIP's open-source infrastructure, paving the way for further phases of RCMIP to build on the research presented here and deepen our understanding of RCMs