Reactions of alkoxy radicals under atmospheric conditions: The relative importance of decomposition versus reaction with O2

The reactions of alkoxy radicals determine to a large extent the products formed during the atmospheric degradations of emitted organic compounds. Experimental data concerning the decompositions, 1,5-H shift isomerizations and reactions with O₂ of several classes of alkoxy radicals are inconsistent with literature estimations of their absolute or relative rate constants. An alternative, although empirical, method for assessing the relative importance under atmospheric conditions of the reactions of alkoxy radicals with O₂ versus decomposition was derived. This estimation method utilizes the differences in the heats of reaction, (H)=(H_{decomposition}–H_{O 2 reaction}), between these two reactions pathways. For (H)[22–0.5(H_{O 2 reaction})], alkoxy radical decomposition dominates over the reaction with O₂ at room temperature and atmospheric pressure of air, while for (H)[25-0.5(H_{O 2 reaction})], the O₂ reaction dominates over decomposition (where the units of H are in kcal mol^–1). The utility and shortcomings of this approach are discussed. It is concluded that further studies concerning the reactions of alkoxy radicals are needed.     

Influences on surface energy fluxes in simulated present and doubled CO2 climates

The surface energy fluxes simulated by the CSIRO9 Mark 1 GCM for present and doubled CO₂ conditions are analyzed. On the global scale the climatological flux fields are similar to those from four GCMs studied previously. A diagnostic calculation is used to provide estimates of the radiative forcing by the GCM atmosphere. For 1 × CO₂, in the global and annual mean, cloud produces a net cooling at the surface of 31 W m^–2. The clear-sky longwave surface greenhouse effect is 311 W m^–2, while the corresponding shortwave term is –79 W m^–2. As for the other GCM results, the CSIRO9 CO₂ surface warming (global mean 4.8°C) is closely related to the increased downward longwave radiation (LW ). Global mean net cloud forcing changes little. The contrast in warming between land and ocean, largely due to the increase in evaporative cooling (E) over ocean, is highlighted. In order to further the understanding of influences on the fluxes, simple physically based linear models are developed using multiple regression. Applied to both 1 × CO₂ and CO₂ December–February mean tropical fields from CSIRO9, the linear models quite accurately (3–5 W m^–2 for 1 × CO₂ and 2–3 W m^–2 for CO₂) relate LW and net shortwave radiation to temperature, surface albedo, the water vapor column, and cloud. The linear models provide alternative estimates of radiative forcing terms to those from the diagnostic calculation. Tropical mean cloud forcings are compared. Over land, E is well correlated with soil moisture, and sensible heat with air-surface temperature difference. However an attempt to relate the spatial variation of LWt within the tropics to that of the nonflux fields had little success. Regional changes in surface temperature are not linearly related to, for instance, changes in cloud or soil moisture.     

Three-dimensional modeling of synthetic cold fronts interacting with northern Alpine foehn

Summary The effect of the Alpine orography on prototype cold fronts approaching from the west is investigated by three-dimensional numerical model simulations. The numerical experiments cover a range of parameter constellations which govern the prefrontal environment of the front. Especially, the appearance and intensity of prefrontal northern Alpine foehn varies from case to case.The behaviour of a cold front north of the Alps depends much on the prefrontal condition it encounters. It is found that prefrontal foehn can either accelerate or retard the approaching front.An important feature is the pressure depression along the northern Alpine rim that results from the southerly foehn flow. In cases where this depression compensates the eastward directed pressure gradient associated with the largescale flow, the front tends to accelerate and the foehn breaks down as soon as the front passes. In contrast, the foehn prevents the front from a rapid eastward propagation if it is connected with a strong southerly wind component.No-foehn experiments are performed for comparison, where either the mountains are removed, or the static stability is set to neutral. Also shown are effects of different crossfrontal temperature contrasts.List of Symbols c _F propagation speed of a front - x, y horizontal grid spacing (cartesian system) - , horizontal grid spacing (geographic system) - t time step - z vertical grid spacing (cartesian system) - cross-frontal potential temperature difference - _i potential temperature step at an inversion - E turbulent kinetic energy - f Coriolis parameter - FGP frontogenesis parameter (see section 2.2) - g gravity acceleration (g=9.81 m s^–2) - vertical gradient of potential temperature - h terrain elevation (above MSL) - h _i height of an inversion (h _i =1000 m MSL) - H height of model lid (H=9000 m MSL) - K _M exchange coefficient of momentum - K _H exchange coefficient of heat and moisture - longitude - N Brunt-Väisäla-frequency - p pressure - Exner function (=T/) - latitude - q _v specific humidity - R _d gas constant of dry air (R _d =287.06 J kg^–1 K^–1) - density of dry air - t time - T temperature - potential temperature - TFP thermal front parameter (see section 2.2) - u, v, w cartesian wind components - u _g ,v _g geostrophic wind components - horizontal wind vector - x, y, z cartesian coordinates Abbreviations GND (above) ground level - MSL (above) mean sea level - UTC universal time coordinated With 20 Figures     

Averaging Intervals For Spectral Analysis Of Nonstationary Turbulence

We formulate a method for determining the smallest time interval Tover which a turbulence time series can be averaged to decompose it intoinstantaneous mean and random components. From the random part the method defines the optimal interval (or averaging window) AW over which this part should be averaged to obtain the instantaneous spectrum. Both T and AW vary randomly with time and depend on physical properties of the turbulence. T also depends on the accuracy of the measurements and is thus independent of AW. Interesting features of the method are its real-time capability and the non-equality between AW and T.     

Methane,carbon monoxide and light non-methane hydrocarbon emissions from African savanna burnings during the FOS/DECAFE experiment

Atmospheric samples from savanna burnings were collected in the Ivory Coast during two campaigns in January 1989 and January 1991. About 30 nonmethane hydrocarbons from C₂ to C₆, carbon monoxide, carbon dioxide and methane were measured from the background and also at various distances from the burning. Concentrations in the fire plume reached ppmv levels for C₂-C₄ hydrocarbons, and 5300, 500 and 93 ppmv for CO₂, CO and CH₄ respectively. The excess in the mixing ratios of these gases above their background level is used to derive emission factors relative to CO and CO₂. For the samples collected immediately in the fire plume, a differentiation between high and low combustion efficiency conditions is made by considering the CO/CO₂ ratio. Ethene (C₂H₄), acetylene (C₂H₂), ethane (C₂H₆) and propene (C₃H₆) are the major NMHC produced in the flaming stage, whereas a different pattern with an increasing contribution of alkanes is observed in samples typical of post flaming processes. A strong correlation between methane and carbon monoxide suggests that these compounds are produced during the same stage of the combustion. In samples collected at a distance from the fire and integrated over a period of 30 minutes, the composition is very similar to that of flaming. NMHC/CO₂ is of the order of 0.7%, CH₄/CO₂ of the order of 0.4% and CO/CO₂ of the order of 6.3%. From this study, a global production by African savanna fires is derived: 65 Tg of CO-C, 4.2 Tg of CH₄-C and 6.7 Tg of NMHC-C. Whereas acetylene can be used as a conservative tracer of the fire plumes, only ethene, propene and butenes can be considered in terms of their direct photochemical impact.     

Equilibrium and transient climatic warming induced by increased atmospheric CO2

The change in the Earth's equilibrium global mean surface temperature induced by a doubling of the CO₂ concentration has been estimated as 0.2 to 10 K by surface energy balance models, 0.5 to 4.2 K by radiative-convective models, and 1.3 to 4.2 K by general circulation models. These wide ranges are interpreted and quantified here in terms of the direct radiative, forcing of the increased CO₂, the response of the climate system in the absence of feedback processes, and the feedbacks of the climate system. It is the range in the values of these feedbacks that leads to the ranges in the projections of the global mean surface warming. The time required for a CO₂-induced climate change to reach equilibrium has been characterized by an e-folding time _e with values estimated by a variety of climate/ocean models as 10 to 100 years. Analytical and numerical studies show that this wide range is due to the strong dependence of _e on the equilibrium sensitivity of the climate model and on the effective vertical thermal diffusivity of the ocean model. A coupled atmosphere-ocean general circulation model simulation for doubled CO₂ suggestes that, as a result of the transport of the CO₂-induced surface heating into the interior of the ocean, _e 50 to 100 years. Theoretical studies for a realistic CO₂ increase between 1850 and 1980 indicate that this sequestering of heat into the ocean's interior is responsible for the concomittant warming being only about half that which would have occurred in the absence of the ocean. These studies also indicate that the climate sytem will continue to warm towards its as yet unrealized equilibrium temperature change, even if there is no further increase in the CO₂ concentration.     

Stability Analysis of the Climate-Vegetation System in the Northern High Latitudes

The stability of the climate-vegetation system in the northern high latitudesis analysed with three climate system models of different complexity: A comprehensive 3-dimensional model of the climate system, GENESIS-IBIS, and two Earth system models of intermediate complexity (EMICs), CLIMBER-2 andMoBidiC. The biogeophysical feedback in the latitudinal belt 60–70° N, although positive, is not strong enough to support multiple steady states: A unique equilibriumin the climate-vegetation system is simulated by all the models on a zonal scale for present-day climate and doubled CO₂ climate.EMIC simulations with decreased insolation also reveal a unique steady state. However, the climate sensitivity to tree cover, T_F, exhibits non-linear behaviour within the models. For GENESIS-IBIS and CLIMBER-2, T_F islower for doubled CO₂ climate than for present-day climate due to a shorter snow season and increased relative significance ofthe hydrological effect of forest cover. For the EMICs, T_F is higher for low tree fraction than for high treefraction, mainly due to a time shift in spring snow melt in response to changes in tree cover. The climate sensitivity to tree coveris reduced when thermohaline circulation feedbacks are accounted for in the EMIC simulations. Simpler parameterizations of oceanic processes have opposite effects on T_F: T_F is lower in simulations with fixed SSTs and higher in simulations with mixed layer oceans. Experiments with transient CO₂ forcing show climate and vegetation not in equilibrium in the northern high latitudes at the end of the 20thcentury. The delayed response of vegetation and accelerated global warming lead to rather abrupt changes in northern vegetation cover in the first halfof the 21st century, when vegetation cover changes at double the present day rate.     

Effects of heat and water vapor transport on eddy covariance measurement of CO2 fluxes

Flux densities of carbon dioxide were measured over an arid, vegetation-free surface by eddy covariance techniques and by a heat budget-profile method, in which CO₂ concentration gradients were specified in terms of mixing ratios. This method showed negligible fluxes of CO₂, consistent with the bareness of the experimental site, whereas the eddy covariance measurements indicated large downward fluxes of CO₂. These apparently conflicting observations are in quantitative agreement with the results of a recent theory which predicts that whenever there are vertical fluxes of sensible or latent heat, a mean vertical velocity is developed. This velocity causes a mean vertical convective mass flux (= _cw for CO₂, in standard notation). The eddy covariance technique neglects this mean convective flux and measures only the turbulent flux _c w. Thus, when the net flux of CO₂ is zero, the eddy covariance method indicates an apparent flux which is equal and opposite to the mean convective flux, i.e., _c w = – _c w. Corrections for the mean convective flux are particularly significant for CO₂ because _cw and _c w are often of similar magnitude. The correct measurement of the net CO₂ flux by eddy covariance techniques requires that the fluxes of sensible and latent heat be measured as well.     

Carbonyl sulfide emissions from biomass burning in the tropics

Carbonyl sulfide emissions from biomass burning have been studied during field experiments conducted both in an African savanna area (Ivory Coast) and rice fields, central highland pine forest and savanna areas in Viet-Nam. During these experiments CO₂, CO and C₂H₂ or CH₄ have also been also monitored. COS values range from 0.6 ppbv outside the fires to 73 ppbv in the plumes. Significant correlations have been observed between concentrations of COS and CO (R ²=0.92,n=25) and COS and C₂H₂ (R ²=0.79,n=26) indicating a COS production during the smoldering combustion. COS/CO₂ emission factors (COS/CO₂) during field experiments ranged from 1.2 to 61×10^–6 (11.4×10^–6 mean value). COS emission by biomass burning was estimated to be up to 0.05 Tg S/yr in tropics and up to 0.07 Tg S/yr on a global basis, contributing thus about 10% to the global COS flux. Based on the S/C ratio measured in the dry plant biomass and the COS/CO₂ emission factor, COS can account for only about 7% of the sulfur emitted in the atmosphere by biomass burning.     

Relations between Light Hydrocarbon, Carbon Monoxide, and Carbon Dioxide Concentrations in the Plume from the Combustion of Plant Material in a Furnace

To systematically explain relations between light hydrocarbons, CO, and CO₂ concentrations/emissions of biomassburning, we measured concentrations/emissions of carbon gases – CO,CO₂, light hydrocarbons (CH₄, C₂H₆,C₂H₄, C₂H₂, C₃H₈, C₃H₆,n-C₄H₁₀, i-C₄H₁₀, n-C₅H₁₂,i-C₅H₁₂), and THC (total hydrocarbon) – in the burning of dead plant material, mainly Imperata grass, byclosed-chamber experiments and by time-series analyses of gas concentrations in combustion plumes in relatively efficient and inefficient combustion situations. Concentrations of hydrocarbons measured were well correlated to [CO] although [C₂H₂] was exceptionally well correlated to[CO₂]. The phase diagrams (relation between [CO]/ [CO₂] and [hydrocarbon]/ [CO₂]) obtained by the time-seriesexperiments well illustrated the variation in the overall emission rates of the closed-chamber experiments. The higher rates of decrease in hydrocarbon concentration with increasing carbon number in the efficient case compared with the inefficient case probably reflected the rate of oxidation and the amount of radicals. The overall concentrations (or emissions) of C₂H₄ and C₃H₆ were higher thanthose of C₂H₆ and C₃H₈, suggesting a linkage to mechanisms in whichthe predominant path of hydrocarbon oxidation is through the degradation of alkyl radicals, which can be immediately converted into or formed from alkenes. For C₃ and C₄ species, normal-chain species hadhigher emissions than iso-chain species under lower combustion efficiency. This may be attributable to the presence of tertiary C–H bonds in iso-species,which show more reactivity in the abstraction of H than secondary C–H bonds unless the carbon number is large.     

Biomass burning in the tropical savannas of Ivory Coast: An overview of the field experiment Fire of Savannas (FOS/DECAFE 91)

FOS/DECAFE 91 (Fire of Savannas/Dynamique et Chimie Atmosphérique en Forêt Equatoriale) was the first multidisciplinary experiment organized in Africa to determine gas and aerosol emissions by prescribed savanna fires. The humid savanna of Lamto in Ivory Coast was chosen for its ecological characteristics representative of savannas with a high biomass density (900 g m^–2 dry matter). Moreover the vegetation and the climate of Lamto have been studied for more than twenty years. The emission ratios (X/CO₂) of the carbon compounds (CO₂, CO, NMHC, CH₄, PAH, organic acids and aerosols), nitrogen compounds (NO_x, N₂O, NH₃ and soluble aerosols) and sulfur compounds (SO₂, COS and aerosols) were experimentally determined by ground and aircraft measurements. To perform this experiment, 4 small plots (100×100 m) and 2 large areas (10×10 km) were prepared and burnt in January 1991 during the period of maximum occurrence of fires in this type of savanna. The detailed ecological study shows that the carbon content of the vegetation is constant within 1% (42 g C for 100 g of vegetal dry matter), the nitrogen content (0.29 g N for 100 g of dry matter) may vary by 10% and the sulfur content (0.05 g S/100 d.m.) by 20%. These variations of the biomass chemical content do not constitute an important factor in the variation of the gas and particle emission levels. With the emission ratios characteristic of humid savanna and flaming conditions (CO/CO₂ of 6.1% at the ground and 8% for airborne measurements), we propose a set of new emission factors, taking into account the burning efficiency which is about 80%: 74.4% of the carbon content of the savanna biomass is released to the atmosphere in the form of CO₂, 4.6% as CO, 0.2% as CH₄, 0.5% as NMHC and 0.7% as aerosols. 17.2% of the nitrogen content of the biomass is released as NO_x, 3.5% as N₂O, 0.6% as NH₃ and 0.5% as soluble aerosols.     

Numerical integration errors in calculated tropospheric photodissociation rate coefficients

Tropospheric photodissociation rate coefficients (J values) were calculated for NO₂, O₃, HNO₂, CH₂O, and CH₃CHO using high spectral resolution (0.1 mm wavelength increments), and compared to the J values obtained with numerically degraded resolution (=1, 2, 4, 6, 8, and 10 nm, and several commonly used nonuniform grids). Depending on the molecule, substantial errors can be introduced by the larger increments. Thus for =10 nm, errors are less than 1% for NO₂, less than 2% for HNO₂, +6.5% to -16% for CH₂O, -6.9% to +24% for CH₃CHO, and -24% to +110% for O₃. The errors for CH₂O arise from the fine structure of its absorption spectrum, and are prevalently negative (underestimate of J). The errors for O₃, and to a lesser extent for CH₃CHO, arise mainly from under-resolving the overlap of the molecular action spectrum and the tropospheric actinic flux in the wavelength region of stratospheric ozone attenuation. The sign of those errors depends on whether the actinic flux is averaged onto the grid before or after the radiative transfer calculation. In all cases studied, grids with 2 nm produced errors no larger than 5%.     

In-water and remote measurements of ocean color

Spectral measurements of downwelling irradiance, E _d(), above the surface, and of upwelling irradiance just below the surface, E _u(), allow computation of spectral values of the diffuse reflectance R() = E _u()/E _d(); this yields full information about the true color and brightness of the ocean. Typical results are presented and interpreted for waters very different in turbidity and phytoplankton content. Conversely, the possibility of infering the water content from R() data at selected wavelengths is examined in terms of the respective number of equations and unknowns. The necessary use of assumptions and of empirical laws is emphasized.The magnitude of the useful signal emerging from the water, and the magnitude of the additional signals due to specular reflexion at the interface and to atmospheric scattering, are compared on the basis of spectroradiometric measurements performed within and above the sea, from different altitudes. These unwanted signals are dominant, causing a drastic change in the spectral composition of the light received by a remote sensor. The evaluation of the atmospheric effect must be very precise in order to recover the marine signal with a sufficient accuracy for a meaningful application of any kind of algorithm.This work was supported in part by the Centre National d'Exploitation des Océans (under contract CNEXO 77/1695) and in part by the Centre National de la Recherche Scientifique (RCP 247 & ERA 278).     

Climate model simulation of anthropogenic influence on greenhouse-induced climate change (early agriculture to modern): the role of ocean feedbacks

We present further steps in our analysis of the early anthropogenic hypothesis (Ruddiman, Clim Change 61:261–293, 2003) that increased levels of greenhouse gases in the current interglacial, compared to lower levels in previous interglacials, were initiated by early agricultural activities, and that these increases caused a warming of climate long before the industrial era (～1750). These steps include updating observations of greenhouse gas and climate trends from earlier interglacials, reviewing recent estimates of greenhouse gas emissions from early agriculture, and describing a simulation by a climate model with a dynamic ocean forced by the low levels of greenhouse gases typical of previous interglacials in order to gauge the magnitude of the climate change for an inferred (natural) low greenhouse gas level relative to a high present day level. We conduct two time slice (equilibrium) simulations using present day orbital forcing and two levels of greenhouse gas forcing: the estimated low (natural) levels of previous interglacials, and the high levels of the present (control). By comparing the former to the latter, we estimate how much colder the climate would be without the combined greenhouse gas forcing of the early agriculture era (inferred from differences between this interglacial and previous interglacials) and the industrial era (the period since ～1750). With the low greenhouse gas levels, the global average surface temperature is 2.7 K lower than present day—ranging from ～2 K lower in the tropics to 4–8 K lower in polar regions. These changes are large, and larger than those reported in a pre-industrial (～1750) simulation with this model, because the imposed low greenhouse gas levels (CH₄ = 450 ppb, CO₂ = 240 ppm) are lower than both pre-industrial (CH₄ = 760 ppb, CO₂ = 280 ppm) and modern control (CH₄ = 1,714 ppb, CO₂ = 355 ppm) values. The area of year-round snowcover is larger, as found in our previous simulations and some other modeling studies, indicating that a state of incipient glaciation would exist given the current configuration of earth’s orbit (reduced insolation in northern hemisphere summer) and the imposed low levels of greenhouse gases. We include comparisons of these snowcover maps with known locations of earlier glacial inception and with locations of twentieth century glaciers and ice caps. In two earlier studies, we used climate models consisting of atmosphere, land surface, and a shallow mixed-layer ocean (Ruddiman et al., Quat Sci Rev 25:1–10, 2005; Vavrus et al., Quat Sci Rev 27:1410–1425, 2008). Here, we replaced the mixed-layer ocean with a complete dynamic ocean. While the simulated climate of the atmosphere and the surface with this improved model configuration is similar to our earlier results (Vavrus et al., Quat Sci Rev 27:1410–1425, 2008), the added information from the full dynamical ocean is of particular interest. The global and vertically-averaged ocean temperature is 1.25 K lower, the area of sea ice is larger, and there is less upwelling in the Southern Ocean. From these results, we infer that natural ocean feedbacks could have amplified the greenhouse gas changes initiated by early agriculture and possibly account for an additional increment of CO₂ increase beyond that attributed directly to early agricultural, as proposed by Ruddiman (Rev Geophys 45:RG4001, 2007). However, a full test of the early anthropogenic hypothesis will require additional observations and simulations with models that include ocean and land carbon cycles and other refinements elaborated herein.     

Relative roles of climate sensitivity and forcing in defining the ocean circulation response to climate change

The response of the ocean’s meridional overturning circulation (MOC) to increased greenhouse gas forcing is examined using a coupled model of intermediate complexity, including a dynamic 3-D ocean subcomponent. Parameters are the increase in CO₂ forcing (with stabilization after a specified time interval) and the model’s climate sensitivity. In this model, the cessation of deep sinking in the north “Atlantic” (hereinafter, a “collapse”), as indicated by changes in the MOC, behaves like a simple bifurcation. The final surface air temperature (SAT) change, which is closely predicted by the product of the radiative forcing and the climate sensitivity, determines whether a collapse occurs. The initial transient response in SAT is largely a function of the forcing increase, with higher sensitivity runs exhibiting delayed behavior; accordingly, high CO₂-low sensitivity scenarios can be assessed as a recovering or collapsing circulation shortly after stabilization, whereas low CO₂-high sensitivity scenarios require several hundred additional years to make such a determination. We also systemically examine how the rate of forcing, for a given CO₂ stabilization, affects the ocean response. In contrast with previous studies based on results using simpler ocean models, we find that except for a narrow range of marginally stable to marginally unstable scenarios, the forcing rate has little impact on whether the run collapses or recovers. In this narrow range, however, forcing increases on a time scale of slow ocean advective processes results in weaker declines in overturning strength and can permit a run to recover that would otherwise collapse.     

On the Response of the Greenland Ice Sheet to Greenhouse Climate Change

Numerical computations are performed with the three-dimensional polythermal ice-sheet model SICOPOLIS in order to investigate the possible impact of a greenhouse-gas-induced climate change on the Greenland ice sheet. The assumed increase of the mean annual air temperature above the ice covers a range from T = 1°C to 12°C, and several parameterizations for the snowfall and the surface melting are considered. The simulated shrinking of the ice sheet is a smooth function of the temperature rise, indications for the existence of critical thresholds of the climate input are not found. Within 1000 model years, the ice-volume decrease is limited to 10% of the present volume for T 3°C, whereas the most extreme scenario, T = 12°C, leads to an almost entire disintegration, which corresponds to a sea-level equivalent of 7 m. The different snowfall and melting parameterizations yield an uncertainty range of up to 20% of the present ice volume after 1000 model years.     

A statistical-dynamical downscaling procedure for global climate simulations

Summary A statistical-dynamical downscaling procedure for global climate simulations is described. The procedure is based on the assumption that any regional climate is associated with a specific frequency distribution of classified large-scale weather situations. The frequency distributions are derived from multi-year episodes of low resolution global climate simulations. Highly resolved regional distributions of wind and temperature are calculated with a regional model for each class of large-scale weather situation. They are statistically evaluated by weighting them with the according climate-specific frequency. The procedure is exemplarily applied to the Alpine region for a global climate simulation of the present January climate.List of Symbols west-east mesh size in geographic coordinates south-north mesh size in geographic coordinates N number of large-scale weather classes n number of regional-scale event classes p pressure P probability Ø large-scale event regional-scale event q _v specific humidity potential temperature u west-east wind component v south-north wind componentAbbreviations AGL above ground level - LT local time - UTC universal time coordinated With 13 Figures     

Sensitivity of North Atlantic subpolar gyre and overturning to stratification-dependent mixing: response to global warming

We use a reduced complexity climate model with a three-dimensional ocean component and realistic topography to investigate the effect of stratification-dependent mixing on the sensitivity of the North Atlantic subpolar gyre (SPG), and the Atlantic meridional overturning circulation (AMOC), to idealized CO₂ increase and peaking scenarios. The vertical diffusivity of the ocean interior is parameterized as κ ∼ N ^−α, where N is the local buoyancy frequency. For all parameter values 0 ≤ α ≤ 3, we find the SPG, and subsequently the AMOC, to weaken in response to increasing CO₂ concentrations. The weakening is significantly stronger for α ≥ α_cr ≈ 1.5. Depending on the value of α, two separate model states develop. These states remain different after the CO₂ concentration is stabilized, and in some cases even after the CO₂ concentration has been decreased again to the pre-industrial level. This behaviour is explained by a positive feedback between stratification and mixing anomalies in the Nordic Seas, causing a persistent weakening of the SPG.     

Three-dimensional modeling of synthetic cold fronts approaching the Alps

Summary A numerical model was used to study the behaviour of prototype cold fronts as they approach the Alps. Two fronts with different orientations relative to the Alpine range have been considered. One front approaches from west, a second one from northwest. The first front is connected with southwesterly large-scale air-flow producing pre-frontal foehn, whereas the second front is associated with westerly largescale flow leading to weak blocking north of the Alps.Model simulations with fully represented orography and parameterized water phase conversions have been compared with control runs where either the orography was cut off or the phase conversions were omitted. The results show a strong orographic influence in case of pre-frontal foehn which warms the pre-frontal air and increases the cross-frontal temperature contrast leading to an acceleration of the front along the northern Alpine rim. The latent heat effect was found to depend much on the position of precipitation relative to the surface front line. In case of pre-frontal foehn precipitation only falls behind the surface front line into the intruding cold air where it partly evaporates. In contrary, precipitation already appears ahead of the front in the case of blocking. Thus, the cooling effect of evaporating rain increases the cross-frontal temperature difference only in the first case causing an additional acceleration of the front.List of symbols C _pd specific heat capacity of dry air at constant pressure (C _pd =1004.71 J kg^–1 K^–1) - C _pv specific heat capacity of water vapour at constant pressure (C _pv =1845.96 J kg^–1 K^–1) - C _f propagation speed of a front - x, y horizontal grid spacing (cartesian system) - , horizontal grid spacing (geographic system) - t time step - E turbulent kinetic energy - f Coriolis parameter - g gravity acceleration (g=9.81 ms^–1) - h terrain elevation - H height of model lid (H=9000 m) - k Karman constant (k=0.4) - K _Mh horizontal exchange coefficient of momentum - K _Hh horizontal exchange coefficient of heat and moisture - K _Mz vertical exchange coefficient of momentum - K _Hz vertical exchange coefficient of heat and moisture - l mixing length - l _c specific condensation heat (l _c =2500.61 kJ kg^–1) - l _f specific freezing heat (l _f =333.56 kJ kg^–1) - l _s specific sublimation heat (l _s =2834.17 kJ kg^–1) - longitude - m ₁,m ₂,m ₃ metric coefficients - p pressure - Exner function - Pr Prandtl number - latitude - _M profile function - q _v specific humidity - q _c specific content of cloud droplets - q _i specific content of cloud ice particles - q _R specific content of rain drops - q _S specific content of snow - R _d gas constant of dry air (R _d =287.06 J kg^–1 K^–1) - R _v gas constant of water vapour (R _v =461.51 J kg^–1 K^–1) - r _E radius of earth (r _E =6371 km) - Ri _F flux Richardson number - density of dry air - t time - T temperature - _dia period of diastrophy - potential temperature - _v virtual potential temperature - _e equivalent potential temperature - U relative humidity - u, v, w cartesian wind components - u _F ,v _F front-normal and front-parallel wind components - x, y, z cartesian coordinates - w ^* transformed vertical wind component - W _R speed of falling rain - W _S speed of falling snow - z ^* transformed vertical coordinate Abbreviations GND (above) ground level - MSL (above) mean sea level With 12 Figures     

Quantifying the Impact of Global Climate Change on Potential Natural Vegetation

Impacts of climate change on vegetation are often summarized in biome maps, representing the potential natural vegetation class for each cell of a grid under current and changed climate. The amount of change between two biome maps is usually measured by the fraction of cells that change class, or by the kappa statistic. Neither measure takes account of varying structural and floristic dissimilarity among biomes. An attribute-based measure of dissimilarity (V) between vegetation classes is therefore introduced. V is based on (a) the relative importance of different plant life forms (e.g. tree, grass) in each class, and (b) a series of attributes (e.g. evergreen-deciduous, tropical-nontropical) of each life form with a weight for each attribute. V is implemented here for the most used biome model, BIOME 1 (Prentice, I. C. et al., 1992). Multidimensional scaling of pairwise V values verifies that the suggested importance values and attribute weights lead to a reasonable pattern of dissimilarities among biomes. Dissimilarity between two maps (V) is obtained by area-weighted averaging of V over the model grid. Using V, present global biome distribution from climatology is compared with anomaly-based scenarios for a doubling of atmospheric CO2 concentration (2 × CO2), and for extreme glacial and interglacial conditions. All scenarios are obtained from equilibrium simulations with an atmospheric general circulation model coupled to a mixed-layer ocean model. The 2 × CO2 simulations are the widely used OSU and GFDL runs from the 1980's, representing models with low and high climate sensitivity, respectively. The palaeoclimate simulations were made with CCM1, with sensitivity similar to GFDL. V values for the comparisons of 2 × CO2 with present climate are similar to values for the comparisons of the last interglacial and mid-Holocene with present climate. However, the two simulated 2 × CO2 cases are much more like each other than they are to the simulated interglacial cases. The largest V values were between the last glacial maximum and all other cases, including the present. These examples illustrate the potential of V in comparing the impacts of different climate change scenarios, and the possibility of calibrating climate change impacts against a palaeoclimatic benchmark.