ALBERT

Description: An analysis of a pollution episode in an urban atmosphere, using a complex model system is presented. The nested atmosphere-chemistry model system simulates the atmospheric conditions during a one week measurement campaign, called FLUMOB, in July 1994 in Berlin-Brandenburg, Germany. The analysis shows that naturally emitted hydrocarbons played the dominant role in the ozone formation in the investigated area. The composition of non-methane volatile organic compounds was made up to 70--80% by biogenically emitted hydrocarbons. During the analysed case, ozone formation was sensitive to hydrocarbon concentrations so that the ozone production was limited by the availability of hydrocarbons and thus especially by the amount which was biogenically emitted. Furthermore, it is shown that the FLUMOB episode was influenced by elevated concentrations of ozone in the free troposphere. In contrast to previous analyses, the importance of ozone produced outside of Europe is emphasized. In spite of the stagnant high pressure situation which occurred during the FLUMOB episode Germany was significantly influenced by long-range transport of ozone. This transport also influenced near surface ozone concentrations.

Description: The availability of near-real time ozone observations from satellite instruments has recently initiated the development of ozone data assimilation systems. In this paper we present the results of an ozone assimilation and forecasting system, in use since Autumn 2000. The forecasts are produced by an ozone transport and chemistry model, driven by the operational medium range forecasts of ECMWF. The forecasts are initialised with realistic ozone distributions, obtained by the assimilation of near-real time total column observations of the GOME spectrometer on ERS-2. The forecast error diagnostics demonstrate that the system produces meaningful total ozone forecasts for up to 6 days in the extratropics. In the tropics meaningful forecasts of the small anomalies are restricted to shorter periods of about two days with the present model setup. It is demonstrated that important events, such as the breakup of the South Pole ozone hole and mini-hole events above Europe can be successfully predicted 4--5 days in advance.

Description: A `virtual' disjunct eddy covariance (vDEC) device was tested with field measurements of biogenic VOC fluxes at a subalpine forest site in the Rocky Mountains of the USA. A PTR-MS instrument was used as the VOC sensor. Daily peak emission fluxes of 2-methyl-3-buten-2-ol (MBO), methanol, acetone and acetaldehyde were around 1.5, 1, 0.8 and 0.4 mg m-2 h-1, respectively. High pass filtering due to long sampling lines was investigated in laboratory experiments, and suggested that VOC losses in PTFA lines are generally governed by diffusion laws. Memory effects and surface reactions did not seem to play a dominant role. Model estimates of MBO fluxes compared well with measured fluxes. The results also suggest that latent heat and sensible heat fluxes are reasonably well correlated with VOC fluxes and could be used to predict variations in VOC emissions. The release of MBO, methanol, acetone and acetaldehyde resulted in significant change of tropospheric oxidant levels and a 10--40% increase in ozone levels, as inferred from a photochemical box model. We conclude that vDEC with a PTR-MS instrument is a versatile tool for simultaneous field analysis of multiple VOC fluxes.

Description: The impact of multiphase reactions involving nitrogen dioxide (NO2) and aromatic compounds was simulated in this study. A mechanism (CAPRAM 2.4, MODAC Mechanism) was applied for the aqueous phase reactions, whereas RACM was applied for the gas phase chemistry. Liquid droplets were considered as monodispersed with a mean radius of 0.1 µm and a liquid content (LC) of 50 µg m-3. The multiphase mechanism has been further extended to the chemistry of aromatics, i.e. reactions involving benzene, toluene, xylene, phenol and cresol have been added. In addition, reaction of NO2 with dissociated hydroxyl substituted aromatic compounds has also been implemented. These reactions proceed through charge exchange leading to nitrite ions and therefore to nitrous acid formation. The strength of this source was explored under urban polluted conditions. It was shown that it may increase gas phase HONO levels under some conditions and that the extent of this effect is strongly pH dependent. Especially under moderate acidic conditions (i.e. pH above 4) this source may represent more than 75% of the total HONO/NO2 - production rate, but this contribution drops down close to zero in acidic droplets (as those often encountered in urban environments).

Description: The efficiency of gas transport to the free and upper troposphere in convective clouds is investigated in an axisymmetric dynamic cloud model with detailed microphysics. In particular, we examine the sensitivity of gas transport to the treatment of gas uptake by different ice hydrometeors. Two parameters are used to describe this uptake. The gas retention coefficient defines the fraction of dissolved gas that is retained in an ice particle upon freezing, which includes also the riming process. We also define a gas burial efficiency defining the amount of gas entrapped in ice crystals growing by vapour diffusion. Model calculations are performed for continental and maritime clouds using a complete range of gas solubilities, retention coefficients and burial efficiencies. The results show that the magnitude of the gas retention coefficient is much more important for gas transport in maritime clouds than in continental clouds. The cause of this difference lies in the different microphysical processes dominating the formation and evolution of hydrometeors in the two cloud types. For highly soluble gases, the amount of gas transported to the free troposphere in maritime clouds falls approximately linearly by a factor of 12 as the retention coefficient is varied between 0 and 1. Gas transport is relatively insensitive to the magnitude of the gas burial efficiency. However, the burial efficiency strongly controls the concentration of trace gases inside anvil ice crystals, which subsequently form cirrus clouds.

Description: The nucleation of NAD and NAT from HNO3/H2O and HNO3/H2SO4/H2O solution droplets is investigated both theoretically and experimentally with respect to the formation of polar stratospheric clouds (PSCs). Our analysis shows that homogeneous NAD and NAT nucleation from liquid aerosols is insufficient to explain the number densities of large nitric acid containing particles recently observed in the Arctic stratosphere. This conclusion is based on new droplet freezing experiments employing optical microscopy combined with Raman spectroscopy. The homogeneous nucleation rate coefficients of NAD and NAT in liquid aerosols under polar stratospheric conditions derived from the experiments are 〈 2 x 10-5 cm-3 s-1 and 〈 8 x 10-2 cm-3 s-1, respectively. These nucleation rate coefficients are smaller by orders of magnitude than the value of ~103 cm-3 s-1 used in a recent denitrification modelling study that is based on a linear extrapolation of laboratory nucleation data to stratospheric conditions (Tabazadeh et al., Science, 291, 2591--2594, 2001). We show that this linear extrapolation is in disagreement with thermodynamics and with experimental data and, therefore, must not be used in microphysical models of PSCs. Our analysis of the experimental data yields maximum hourly production rates of nitric acid hydrate particles per cm3 of air of about 3 x 10-10 cm-3 (air) h-1 under polar stratospheric conditions. Assuming PSC particle production to proceed at this rate for two months we arrive at particle number densities of 〈 5 x 10-7 cm-3, much smaller than the value of ~10-4 cm-3 reported in recent field observations. In addition, the nitric acid hydrate production rate inferred from our data is much smaller than that required to reproduce the observed denitrification in the modelling study mentioned above. This clearly shows that homogeneous nucleation of NAD and NAT from liquid supercooled ternary solution aerosols cannot explain the observed polar denitrification.

Description: The UV-visible absorption spectrum of gaseous IONO2 has been measured over the wavelength range 245--415 nm using the technique of laser photolysis with time-resolved UV-visible absorption spectroscopy. IONO2 was produced in situ in the gas phase by laser flash photolysis of NO2/CF3I/N2 mixtures. Post flash spectra were deconvolved to remove contributions to the observed absorption from other reactant and product species. The resulting spectrum attributed to IONO2 consists of several overlapping broad absorption bands. Assuming a quantum yield of unity for IONO2 photolysis, model calculations show that during sunlit hours at noon, 53° N, the first order solar photolysis rate coefficient (J value) for IONO2 is 4.0 x 10-2 s-1.

Description: The partitioning of nitrogen oxides between ice and air is of importance to the ozone budget in the upper troposphere. In the present study, adsorption of nitrogen oxides on ice was investigated at atmospheric pressure using a chromatographic technique with radioactively labelled nitrogen oxides at low concentrations. The measured retentions solely depended on molecular adsorption and were not influenced by dimerisation, formation of encapsulated hydrates on the ice surface, dissociation of the acids, nor by migration into a quasi-liquid layer or grain boundaries. Based on the chromatographic retention and the model of thermo-chromatography, the standard adsorption enthalpy of -20 kJ mol-1 for NO, -22kJ mol-1 for NO2, -30kJ mol-1 for peroxyacetyl nitrate, -32kJ mol-1 for HON} and -44 kJ mol-1 for HNO3 was calculated. To perform those calculations within the model of thermo-chromatography, the standard adsorption entropy was calculated based on statistical thermodynamics. In this work, two different choices of standard states were applied, and consequently different values of the standard adsorption entropy, of either between -39 kJ mol-1 and -45kJ mol-1, or -164 kJ mol-1 and -169 kJ mol-1 for each nitrogen oxide were derived. The standard adsorption enthalpy was identical for both standard adsorption entropies and thus shown to be independent of the choice of standard state. A brief outlook on environmental implications of our findings indicates that adsorption on ice might be an important removal process of HNO3. In addition, it might be of some importance for HONO and peroxyacetyl nitrate and irrelevant for NO and NO2.

Description: A tracer model, DREAM (the Danish Rimpuff and Eulerian Accidental release Model), has been developed for modelling transport, dispersion and deposition (wet and dry) of radioactive material from accidental releases, as the Chernobyl accident. The model is a combination of a Lagrangian model, that includes the near source dispersion, and an Eulerian model describing the long-range transport. The performance of the transport model has previously been tested within the European Tracer Experiment, ETEX, which included transport and dispersion of an inert, non-depositing tracer from a controlled release. The focus of this paper is the model performance with respect to the total deposition of  137Cs, 134Cs and 131I from the Chernobyl accident, using different relatively simple and comprehensive parameterizations for dry- and wet deposition. The performance, compared to measurements, of using different combinations of two different wet deposition parameterizations and three different parameterizations of dry deposition has been evaluated, using different statistical tests. The best model performance, compared to measurements, is obtained when parameterizing the total deposition combined of a simple method for dry deposition and a subgrid-scale averaging scheme for wet deposition based on relative humidities. The same major conclusion is obtained for all the three different radioactive isotopes and using two different deposition measurement databases. Large differences are seen in the results obtained by using the two different parameterizations of wet deposition based on precipitation rates and relative humidities, respectively. The parameterization based on subgrid-scale averaging is, in all cases, performing better than the parameterization based on precipitation rates. This indicates that the in-cloud scavenging process is more important than the below cloud scavenging process for the submicron particles and that the precipitation rates are relatively uncertain in the meteorological model compared to the relative humidity. Relatively small differences are, however, seen in the statistical tests between the three different parameterizations of dry deposition.

Description: The relationship between nucleation events and spectral solar irradiance was analysed using two years of data collected at the Station for Measuring Forest Ecosystem-Atmosphere Relations (SMEAR II) in Hyytiälä, Finland. We analysed the data in two different ways. In the first step we calculated ten nanometer average values from the irradiance measurements between 280 and 580 nm and explored if any special wavelengths groups showed higher values on event days compared to a spectral reference curve for all the days for 2 years or to reference curves for every month. The results indicated that short wavelength irradiance between 300 and 340 nm is higher on event days in winter (February and March) compared to the monthly reference graph but quantitative much smaller than in spring or summer. By building the ratio between the average values of different event classes and the yearly reference graph we obtained peaks between 1.17 and 1.6 in the short wavelength range (300--340 nm). In the next step we included number concentrations of particles between 3 and 10 nm and calculated correlation coefficients between the different wavelengths groups and the particles. The results were quite similar to those obtained previously; the highest correlation coefficients were reached for the spectral irradiance groups 3--5 (300--330 nm) with average values for the single event classes around 0.6 and a nearly linear decrease towards higher wavelengths groups by 30%. Both analyses indicate quite clearly that short wavelength irradiance between 300 and 330 or 340 nm is the most important solar spectral radiation for the formation of newly formed aerosols. In the end we introduce a photochemical mechanism as one possible pathway how short wavelength irradiance can influence the formation of SOA by calculating the production rate of excited oxygen. This mechanism shows in which way short wavelength irradiance can influence the formation of new particles even though the absolute values are one to two magnitudes smaller compared to irradiance between 400 and 500 nm.

Description: Turbulent fluxes of carbonyl sulfide (COS) and carbon disulfide (CS2) were measured over a spruce forest in Central Germany using the relaxed eddy accumulation (REA) technique. A REA sampler was developed and validated using simultaneous measurements of CO2 fluxes by REA and by eddy correlation. REA measurements were conducted during six campaigns covering spring, summer, and fall between 1997 and 1999. Both uptake and emission of COS and CS2 by the forest were observed, with deposition occurring mainly during the sunlit period and emission mainly during the dark period. On the average, however, the forest acts as a sink for both gases. The average fluxes for COS and CS2 are  -93 ± 11.7 pmol m-2 s-1 and  -18 ± 7.6 pmol m-2 s-1, respectively. The fluxes of both gases appear to be correlated to photosynthetically active radiation and to the CO2 and H2O fluxes, supporting the idea that the air-vegetation exchange of both gases is controlled by stomata. An uptake ratio COS/CO2 of 10 ± 1.7 pmol μmol-1 has been derived from the regression line for the correlation between the COS and CO2 fluxes. This uptake ratio, if representative for the global terrestrial net primary production, would correspond to a sink of 2.3 ± 0.5 Tg COS yr-1.

Description: Uptake of HOBr on sea salt aerosol, sea salt brine or ice is believed to be a key process providing a source of photolabile bromine (Br2) and sustaining ozone depletion cycles in the Arctic troposphere. In the present study, uptake of HOBr on sodium bromide (NaBr) aerosol particles was investigated at an extremely low HOBr concentration of 300 cm-3 using the short-lived radioactive isotopes 83-86Br. Under these conditions, at maximum one HOBr molecule was taken up per particle. The rate of uptake was clearly limited by the mass accommodation coefficient, which was calculated to be 0.6 ± 0.2. This value is a factor of 10 larger than estimates used in earlier models. The atmospheric implications are discussed using the box model "MOCCA'', showing that the increase of the accommodation coefficient of HOBr by a factor of 10 only slightly affects net ozone loss, but significantly increases chlorine release.

Description: Aerosol variability is examined as function of particle size for data collected over the Northern Indian Ocean in February 1999 as part of the INDOEX experiment. It was found that for particles believed to be of terrestrial or oceanic origin, the variability correlated with the average number concentration. For particles that are thought to be formed and grow in the atmosphere through coagulation and condensation an anticorrelation was observed, the minimum in variability coinciding with the maximum in the number concentration. Three altitude ranges were examined (0--1, 4--8 and 8--13 km) and the minimum in variability was found to occur at lower particle sizes in the free troposphere (0.065 mm) than in the boundary layer (0.165 mm). The observed variability has been compared to that generated by a numerical model in order to determine the relative importance of the physical processes. Modelled variability of 0.02 mm particles caused by nucleation was not observed in the measurements. A previously derived empirical relationship for aerosol residence time was compared with the measured variability as a function of bin size. The aerosol variability / residence time relationship was characterised by a coefficient (b) at all altitudes and for both correlating and anticorrelating regimes. By combining the derived coefficient with the model predicted lifetime for 0.020 mm particles we estimated residence times and ages as a function of particle size and altitude. General agreement was found with previous estimates of aerosol residence time. In the upper atmosphere aerosols of 0.065 mm in size have residence times of approximately 1 month and can be transported on a hemispheric scale. The same size aerosol has a lifetime one order of magnitude less in the boundary layer and therefore will not be transported far from the source regions.

Description: New particle formation during the oxidation of a- and b-pinene (C10H16) by ozone, OH and NO3 was studied by measuring the particle size distributions with a scanning mobility particle sizer (TSI 3936). The results indicate a drastically higher nucleation potential of the ozonolysis than in the reaction with either OH or NO3. On the contrary, the contribution of the individual oxidation reactions to form new aerosol volume was found to depend on the location of the carbon double bond to be oxidised: for the endocyclic a-pinene reactions the ozonolysis contributed mostly to the aerosol volume yield, whereas for the exocyclic  b-pinene reactions the oxidation by O3, OH and NO3 yielded a similar aerosol volume. In a second part of this study the influence of water vapour on the nucleation in all three possible oxidation routes was examined. The observations revealed only an effect of water vapour during the ozonolysis reactions.

Description: Biomass burning has long been recognised as an important source of trace gases and aerosols in the atmosphere. The burning of vegetation has a repeating seasonal pattern, but the intensity of burning and the exact localisation of fires vary considerably from year to year. Recent studies have demonstrated the high interannual variability of the emissions that are associated with biomass burning. In this paper I present a methodology using active fire counts from the Along-Track Scanning Radiometer (ATSR) sensor on board the ERS-2 satellite to estimate the seasonal and interannual variability of global biomass burning emissions in the time period 1996--2000. From the ATSR data, I compute relative scaling factors of burning intensity for each month, which are then applied to a standard inventory for carbon monoxide emissions from biomass burning. The new, time-resolved inventory is evaluated using the few existing multi-year burned area observations on continental scales.

Description: Using historical observations and model simulations, we investigate ozone trends prior to the mid-1970s onset of halogen-induced ozone depletion. Though measurements are quite limited, an analysis based on multiple, independent data sets (direct and indirect) provides better constraints than any individual set of observations. We find that three data sets support an apparent long-term stratospheric ozone trend of -7.2 ± 2.3 DU during 1957-1975, which modeling attributes primarily to water vapor increases. The results suggest that 20th century stratospheric ozone depletion may have been roughly 50% more than is generally supposed. Similarly, three data sets support tropospheric ozone increases over polluted Northern Hemisphere continental regions of 8.2 ± 2.1 DU during this period, which are mutually consistent with the stratospheric trends. As with paleoclimate data, which is also based on indirect proxies and/or limited spatial coverage, these results must be interpreted with caution. However, they provide the most thorough estimates presently available of ozone changes prior to the coincident onset of satellite data and halogen dominated ozone changes. If these apparent trends were real, the radiative forcing by stratospheric ozone since the 1950s would then have been -0.15 ± 0.05 W/m2, and -0.2 W/m2 since the preindustrial. For tropospheric ozone, it would have been 0.38 ± 0.10 W/m2 since the late 1950s. Combined with even a very conservative estimate of tropospheric ozone forcing prior to that time, this would be larger than current estimates since 1850 which are derived from models that are even less well constrained. These calculations demonstrate the importance of gaining a better understanding of historical ozone changes.

Description: A simple inverse model is proposed to deduce hemisphere-integrated COS flux based on published time series of total column COS. The global atmosphere is divided into two boxes representing the Northern and Southern Hemispheres, and the total column COS data from several stations are used to calculate hemispheric COS loadings. The integrated flux within each hemisphere is calculated as a linear combination of a steady-state solution and time-varying perturbation. The nature of the time-varying perturbation is deduced using two different approaches: an analytic solution based on a cosine function that was fitted to the original total column COS measurement time series and a Simplex optimization with no underlying assumption about the functional form of the total column time series. The results suggest that there is a steady-state COS flux from the Northern to the Southern Hemisphere. There is a seasonal variation superimposed on this flux that in the Southern Hemisphere has a maximum rate of COS input into the atmosphere around January and a maximum rate of COS removal from the atmosphere around August--September. In the Northern Hemisphere, the maximum rate of COS input into the atmosphere is around May--June, and the maximum rate of COS removal is either August or January, depending on which station in the Northern Hemisphere is considered. The results of the inverse model are compared with the outcome of a forward approach on the temporal and spatial variation of the dominant global sources and sinks published earlier. In general, the deduced hemisphere-integrated flux estimates showed good agreement with the database estimates, though it remains uncertain whether COS removal from the atmosphere in the Northern Hemisphere is dominated by plant and soil uptake in the boreal summer or by oceanic uptake in boreal winter.

Description: We have performed a three-year series of routine lidar measurements at preselected times. The measurements were performed between 1 December 1997, and 30 November 2000, at Kühlungsborn, Germany (54°07' N, 11°46' E). Using a Rayleigh/Mie/Raman lidar system, we measured the aerosol backscatter coefficients at three wavelengths and the extinction coefficient at one wavelength. The present data analysis focuses on after-sunset Raman measurements obtained on cloud-free days. Aerosol backscatter profiles are available for altitudes above 100 m, while the majority of the extinction measurements has been restricted to heights above the residual layer. The residual layer shows an annual cycle with its maximum height in summer (2000 m) and minimum height in winter (850 m). The backscatter coefficients in the residual layer were found to be about 10 times higher than above. The mean aerosol optical depth above the residual layer and below 5 km is 0.3(±1.0) x10-2 in summer, and 1.5(±1.0) x10-2 in winter, which almost is negligible compared to values measured in during daytime in the planetary boundary layer. A cluster analysis of the backward trajectories yielded two major directions of air mass origin above the residual layer and 4 major directions inside. A marked difference between the aerosol properties dependent on the air mass origin could be found for air masses originating from the west and travelling at high wind speeds. Comparing the measured spectral dependence of the backscatter coefficients with data from the Global Aerosol Data Set, we found a general agreement, but only a few conclusions with respect to the aerosol type could be drawn due to the high variability of the measured backscatter coefficients.

Description: We examine the effect of nanometer-sized aircraft-induced aqueous sulfuric acid (H2SO4/H2O) particles on atmospheric ozone as a function of temperature. Our calculations are based on a previously derived parameterization for the regional-scale perturbations of the sulfate surface area density due to air traffic in the North Atlantic Flight Corridor (NAFC) and a chemical box model. We confirm large scale model results that at temperatures T〉210 K additional ozone loss -- mainly caused by hydrolysis of BrONO2 and N2O5 -- scales in proportion with the aviation-produced increase of the background aerosol surface area. However, at lower temperatures (〈 210 K) we isolate two effects which efficiently reduce the aircraft-induced perturbation: (1) background particles growth due to H2O and HNO3 uptake enhance scavenging losses of aviation-produced liquid particles and (2) the Kelvin effect efficiently limits chlorine activation on the small aircraft-induced droplets by reducing the solubility of chemically reacting species. These two effects lead to a substantial reduction of heterogeneous chemistry on aircraft-induced volatile aerosols under cold conditions. In contrast we find contrail ice particles to be potentially important for heterogeneous chlorine activation and reductions in ozone levels. These features have not been taken into consideration in previous global studies of the atmospheric impact of aviation. Therefore, to parameterize them in global chemistry and transport models, we propose the following parameterisation: scale the hydrolysis reactions by the aircraft-induced surface area increase, and neglect heterogeneous chlorine reactions on liquid plume particles but not on ice contrails and aircraft induced ice clouds.

Description: This study investigates the origin of a commonly observed feature in the O3 profiles: mid tropospheric O3 maxima (300--500 hPa) over the tropical Indian Ocean. A comparison and analysis of model simulations, using a 3-D global climate-chemistry model, and measured O3 profiles from the INDOEX campaign is presented. European Centre for Medium-Range Weather Forecast (ECMWF) meteorological analyses have been assimilated into the 3-D model to represent actual meteorology. The model realistically simulates the observed mid-tropospheric O3 maxima. The analysis of the model simulations shows that the major source of the mid-tropospheric O3 maxima is advection of polluted air masses from continental biomass burning areas over Africa, with generally only a small contribution of stratospheric O3. Previous studies hinted at stratosphere-troposphere exchange (STE) along the subtropical jet (STJ) as the primary source of the mid-tropospheric O3 maxima over the Indian Ocean. Analysis of the model simulations shows that the mechanism causing the mid-tropospheric transport of African biomass burning pollution and stratospheric air masses are frontal zones or waves passing along the subtropical jets, causing advection of tropical air masses in the prefrontal (equatorward) zone. Furthermore, the frontal zones or waves also cause STE at the poleward side of the STJ. The model simulations also indicate that the contribution of STE in general is minor compared to advection and in situ tropospheric production of O3 for the mid-tropospheric O3 budget over the Indian Ocean region.

Description: Episodes of ozone depletion in the lowermost Arctic atmosphere (0--2 km) at polar sunrise have been intensively studied at Alert, Canada, and are thought to result from catalytic reactions involving bromine. Recent observations of high concentrations of tropospheric BrO over large areas of the Arctic and Antarctic suggest that such depletion events should also be seen by ozonesondes at other polar stations. An examination of historical ozonesonde records shows that such events occur frequently at Alert, Eureka and Resolute, but much less frequently at Churchill and at other stations. The differences appear to be related to differences in average springtime surface temperatures. The long record at Resolute shows depletions since 1966, but with an increase in their frequency over the period 1966--2000 of 0.66 ± 0.59% per year (95% confidence limits), explaining the apparent increase of Hg in Arctic biota in recent times.

Description: Number concentrations and mean sizes of ice crystals and derived microphysical and optical properties of subvisible cirrus clouds (SVCs) formed by homogeneous freezing of supercooled aerosols are investigated as a function of temperature and updraft speed of adiabatically ascending air parcels. The properties of such clouds are insensitive to variations of the aerosol number and size distribution. Based on criteria constraining the optical extinction, sedimentation time, and existence time of SVCs, longer-lived (〉10min) clouds, capable of exerting a measurable radiative or chemical impact, are generated within a narrow range of updraft speeds below 1-2cm s-1 at temperatures below about 215K, with concentrations of ice crystals not exceeding 0.1cm-3. The clouds do not reach an equilibrium state because the ice crystals sediment out of the formation layer typically before the supersaturation is removed. Two important conclusions emerge from this work. First, the above characteristics of SVCs may provide an explanation for why SVCs are more common in the cold tropical than in the warmer midlatitude tropopause region. Second, it seems likely that a limited number (

Description: In assessing the iris effect suggested by Lindzen et al. (2001), Fu et al. (2002) found that the response of high-level clouds to the sea surface temperature had an effect of reducing the climate sensitivity to external radiative forcing, but the effect was not as strong as LCH found. The approach of FBH to specifying longwave emission and cloud albedos appears to be inappropriate, and the derived cloud optical properties may not have real physical meaning. The cloud albedo calculated by FBH is too large for cirrus clouds and too small for boundary layer clouds, which underestimates the iris effect.

Description: The uptake of HNO3 to deliquescent airborne sea-salt particles (RH = 55%, P = 760 torr, T = 300 K) at concentrations from 2 to 575 ppbv is measured in an aerosol flow tube using 13N as a tracer. Small particles ( 70 nm diameter) are used in order to minimize the effect of diffusion in the gas phase on the mass transfer. Below 100 ppbv, an uptake coefficient (gupt) of 0.50 ± 0.20 is derived. At higher concentrations, the uptake coefficient decreases along with the consumption of aerosol chloride. Data interpretation is further supported by using the North American Aerosol Inorganics Model (AIM), which predicts the aqueous phase activities of ions and the gas-phase partial pressures of H2O, HNO3, and HCl at equilibrium for the NaCl/HNO3/H2O system. These simulations show that the low concentration data are obtained far from equilibrium, which implies that the uptake coefficient derived is equal to the mass accommodation coefficient under these conditions. The observed uptake coefficient can serve as input to modeling studies of atmospheric sea-salt aerosol chemistry. The main sea-salt aerosol burden in the marine atmosphere is represented by coarse mode particles (〉 1 µm diameter). This implies that diffusion in the gas-phase is the limiting step to HNO3 uptake until the sea-salt has been completely processed.

Description: Measurements of the complete isotopic composition of atmospheric CO (3CO, 14CO, C17O, C18O) have been carried out at the high northern latitude stations Spitsbergen, Norway, and Alert, Canada. The annual changes of the isotope signatures reflect the seasonally varying contributions from the individual CO sources and the OH sink. Short-term variability is small at the remote sampling locations. Nevertheless, the interannual variability is considerable, in particular for the summer minimum. The most prominent event was a strong increase in CO in 1998 that persisted for several months. Using the isotope signatures it is possible to clearly identify extraordinarily strong biomass burning during that season as the cause for this large-scale CO anomaly. In 1997, on the other hand, biomass burning emissions were very low, leading to an unusually low summer minimum and corresponding isotope signatures. The results underscore that monitoring of CO and its isotopic composition at remote high latitude stations is a valuable tool to better understand long-term variations of CO that are representative for the whole high northern latitude region.

Description: Polar stratospheric clouds (PSCs) of type 1a or 1a-enh containing high number densities of nitric acid trihydrate (NAT) particles, can act as mother clouds for extremely large NAT particles, termed NAT-rocks, provided the air below the clouds is supersaturated with respect to NAT. Individual NAT particles at the cloud base fall into undepleted gas phase and rapidly accelerate due to a positive feedback between their growth and sedimentation. The resulting reduction in number density is further enhanced by the strong HNO3 depletion within a thin layer below the mother cloud, which delays subsequent particles. This paper introduces the basic microphysical principles behind this mother cloud/NAT-rock mechanism, which produces 10-4 cm-3 NAT-rocks with radii around 10 mm some kilometers below the mother cloud. The mechanism does not require selective nucleation and works even for a monodisperse particle size distribution in the mother cloud.

Description: The uptake and reaction of HOBr with frozen salt surfaces of variable NaCl / NaBr composition and temperature were investigated with a coated wall flow tube reactor coupled to a mass spectrometer for gas-phase analysis. HOBr is efficiently taken up onto the frozen surfaces at temperatures between 253 and 233 K where it reacts to form the di-halogens BrCl and Br2, which are subsequently released into the gas-phase. The uptake coefficient for HOBr reacting with a frozen, mixed salt surface of similar composition to sea-spray was 10-2. The relative concentration of BrCl and Br2 released to the gas-phase was found to be strongly dependent on the ratio of Cl- to Br - in the solution prior to freezing / drying. For a mixed salt surface of similar composition to sea-spray the major product at low conversion of surface reactants (i.e. Br - and Cl-) was Br2. Variation of the pH of the NaCl / NaBr solution used to prepare the frozen surfaces was found to have no significant influence on the results. The observations are explained in terms of initial formation of BrCl in a surface reaction of HOBr with Cl-, and conversion of BrCl to Br2 via reaction of surface Br -. Experiments on the uptake and reaction of BrCl with frozen NaCl / NaBr solutions served to confirm this hypothesis. The kinetics and products of the interactions of BrCl, Br2 and Cl2 with frozen salt surfaces were also investigated, and lower limits to the uptake coefficients of 〉 0.034, 〉0.025 and 〉0.028 respectively, were obtained. The uptake and reaction of HOBr on dry salt surfaces was also investigated and the results closely resemble those obtained for frozen surfaces. During the course of this study the gas diffusion coefficients of HOBr in He and H2O were also measured as (273 ± 1) Torr cm2 s-1 and (51 ± 1) Torr cm2 s-1, respectively, at 255 K. The implications of these results for modelling the chemistry of the Arctic boundary layer in springtime are discussed.

Description: Tropospheric NO2 plays a variety of significant roles in atmospheric chemistry. In the troposphere it is one of the most significant precursors of photochemical ozone (O3) production and nitric acid (HNO3). In this study tropospheric NO2 columns were calculated by the fully coupled chemistry-climate model ECHAM4.L39(DLR)/CHEM. These have been compared with tropospheric NO2 columns, retrieved using the tropospheric excess method from measurements by the Global Ozone Monitoring Experiment (GOME) of up-welling earthshine radiance and the extraterrestrial irradiance. GOME is part of the core payload of the second European Research Satellite (ERS-2). For this study the first five years of GOME measurements have been used. The period of five years of observational data is sufficiently long to facilitate for the first time a comparison based on climatological averages with global coverage, focussing on the geographical distribution of the tropospheric NO2. A new approach of analysing regional differences (i.e. on continental scales) by calculating individual averages for different environments provides more detailed information about specific NOx sources and of their seasonal variations. The results obtained enable the validity of the model NO2 source distribution and the assumptions used to separate tropospheric and stratospheric parts of the NO2 column amount from the satellite measurements to be investigated.

Description: We revisit a model of feedback processes proposed by Lindzen et al. (2001), in which an assumed 22% reduction in the area of tropical high clouds per degree increase in sea surface temperature produces negative feedbacks associated with upper tropospheric water vapor and cloud radiative effects. We argue that the water vapor feedback is overestimated in Lindzen et al. (2001) by at least 60%, and that the high cloud feedback is small. Although not mentioned by Lindzen et al. (2001), tropical low clouds make a significant contribution to their negative feedback, which is also overestimated. Using more realistic parameters in the model of Lindzen et al. (2001), we obtain a feedback factor in the range of -0.15 to -0.51, compared to their larger negative feedback factor of -0.45 to -1.03. It is noted that our feedback factor could still be overestimated due to the assumption of constant low cloud cover in the simple radiative-convective model.

Description: The concentration of cloud condensation nuclei (CCN) in the marine boundary layer (MBL) was estimated from dimethyl sulfide (DMS) flux, sea salt (SS) emission, and aerosols entrained from the free troposphere (FT). Only under clean air conditions, did the nucleation of DMS derived sulfur (DMS CCN) contribute significantly to the MBL CCN. The accommodation coefficient for sulfuric acid mass transfer was found to be a very important parameter in the modeling the contribution of DMS to MBL CCN. The relationship between seawater DMS and MBL CCN was found to be non-linear mainly due to the transfer processes of sulfuric acid onto aerosols. In addition, sea salt derived CCN (SS CCN) and entrained aerosol from the FT (FT CCN) affected the MBL CCN directly, by supplying CCN, and indirectly, by behaving as an efficient sink for sulfuric acid. The SS CCN explained more than 50% of the total predicted MBL CCN when wind speeds were moderate and high. Sea salt and FT aerosol may often be more efficient sources of MBL CCN than DMS.

Description: An Ultrafine Tandem Differential Mobility Analyser (UF-TDMA) has been used in several field campaigns over the last few years. The investigations were focused on the origin and properties of nucleation event aerosols, which are observed frequently in various environments. This paper gives a summary of the results of 10 nm and 20 nm particle hygroscopic properties from different measurement sites: an urban site, an urban background site and a forest site in Finland and a coastal site in western Ireland. The data can be classified in four hygroscopic growth classes: hydrofobic, less-hygroscopic, more-hygroscopic and sea-salt. Similar classification has been earlier presented for Aitken and accumulation mode particles. In urban air, the summertime 10 nm particles showed varying less-hygroscopic growth behaviour, while winter time 10 nm and 20 nm particles were externally mixed with two different hygroscopic growth modes. The forest measurements revealed diurnal behaviour of hygroscopic growth, with high growth factors at day time and lower during night. The urban background particles had growth behaviour similar to the urban and forest measurement sites depending on the origin of the observed particles. The coastal measurements were strongly affected by air mass history. Both 10 nm and 20 nm particles were hygroscopic in marine background air. The 10 nm particles produced during clean nucleation burst periods were hydrofobic. Diurnal variation and higher growth factors of 10 nm particles were observed in air affected by other source regions. External mixing was occasionally observed at all the sites, but incidents with more than two growth modes were extremely rare.

Description: A 1-D marine stratocumulus cloud model has been supplemented with a comprehensive and up-to-date aqueous phase chemical mechanism for the purpose of assessing the impact that the presence of clouds has on gas phaseHOx, NOx and O3 budgets in the marine boundary layer. The simulations presented here indicate that cloud may act as a heterogeneous source of HONOg. The conversion of HNO4(g) at moderate pH (~ 4.5) is responsible for this, and, to a lesser extent, the photolysis of nitrate (NO3-). The effect of introducing deliquescent aerosol on the simulated increase of HONOg is negligible. The most important consequences of this elevation in HONOg are that, in the presence of cloud, gas phase concentrations of NOx species increase by a factor of 2, which minimises the simulated decrease in O3(g), and results in a regeneration of OHg. This partly compensates for the removal of OHg by direct phase transfer into the cloud and may have important implications regarding the oxidising capacity of the marine boundary layer.

Description: We have compared the 14-year record of satellite derived tropical tropospheric ozone columns (TTOC) from the NIMBUS--7 Total Ozone Mapping Spectrometer (TOMS) to TTOC calculated by achemistry-transport model (CTM). An objective measure of error, based on the zonal distribution of TTOC in the tropics, is applied to perform this comparison systematically. In addition, the sensitivity of the model to several key processes in the tropics is quantified to select directions for future improvements. The comparisons indicate a widespread, systematic (20%) discrepancy over the tropical Atlantic Ocean, which maximizes during austral Spring. Although independent evidence from ozonesondes shows that some of the disagreement is due to satellite overestimate of TTOC, the Atlantic mismatch is largely due to a misrepresentation of seasonally recurring processes in the model. Only minor differences between the model and observations over the Pacific occur, mostly due to interannual variability not captured by the model. Although chemical processes determine the TTOC extent, dynamical processes dominate the TTOC distribution, as the use of actual meteorology pertaining to the year of observations always leads to a better agreement with TTOC observations than using a random year or a climatology. The modeled TTOC is remarkably insensitive to many model parameters due to efficient feedbacks in the ozone budget. Nevertheless, the simulations would profit from an improved biomass burning calendar, as well as from an increase in NOx abundances in free tropospheric biomass burning plumes. The model showed the largest response to lightning NOx emissions, but systematic improvements could not be found. The use of multi-year satellite derived tropospheric data to systematically test and improve a CTM is a promising new addition to existing methods of model validation, and is a first step to integrating tropospheric satellite observations into global ozone modeling studies. Conversely, the CTM may suggest improvements to evolving satellite retrievals for tropospheric ozone.

Description: The relationship between nucleation events and numerous physical and meteorological parameters was analysed using data collected at the Station for Measuring Forest Ecosystem-Atmosphere Relations (SMEAR II) in Hyytiälä, Finland. To do this, measurements of solar radiation (ultraviolet [UV], global, photosynthetically active radiation [PAR], net, reflected global radiation and reflected PAR), gas concentrations, temperature, humidity, wind direction, horizontal and vertical wind speed, horizontal and vertical wind variances and particle concentrations were collected over a 4 year period. For the year 1999 a detailed analysis of data were completed by examining parameters in order to determine the physical and meteorological conditions favourable to the formation of new particles. A comparison of different wavelength bands during the bursts of new particles led to the suggestion, that UV-A solar radiation seems to be the most probable radiation band concerning the photochemical reactions involved in the production of condensable vapours. Furthermore a high correlation between the daily curves of UV-A irradiance and the concentration of 3 - 5 nm particles was found throughout the year and examples will be given for two days. During the whole year the concentration of H2O is very low at times nucleation occurs compared to the average of the corresponding month. Especially in June and July many non-event days with high solar irradiance show high amounts of water molecules. To combine these results a "nucleation parameter" was calculated for the year 1999, by dividing UV-A solar radiation by the concentration of H2O and temperature. Throughout the year nearly all nucleation event days reach a value of the "nucleation parameter" of at least 5.4 x 10-25 W m molecules-1 K-1.  Non-event days with high values (〉 2.7 x 10-25 W m molecules-1 K-1) are mostly accompanied by high concentrations of existing particles.

Description: The cascade of Solina/Myczkowce dam reservoirs situated in the West Carpathian Mts was investigated. The cascade consists of two reservoirs, the upper one built 35 yr ago on the San and Solinka Rivers of 2100 ha area separated from another one, of 190 ha area, with hypolimnion cold water supplied from the upper one. The offshore zone was investigated monthly (6 sites of upper, 3 sites of lower reservoir) using a tube bottom sampler. Besides numbers, biomass and biodiversity indices, the proportion of predators, plant/detritus-feeders and filter-feeders was assessed. In upper reservoir Solina, with a lack of filter-feeders, a clear preponderance of plant-detritus feeders was observed. Predators, consisting mostly of leeches and hydranchnellids were present. In lower reservoir Myczkowce, all three trophic classes were present in the first year of study in almost equal proportions and in 1998 with slight preponderance of plant/detritus feeders was observed. The Oligochaeta/Chironomidae index in both reservoirs was low, indicating low degree of eutrophication. Only one site in Solina reservoir (near a fish culture) and one in Myczkowce (influenced by tourist activity) showed a high value of this index.

Description: The edge effect on weevils and spiders was tested along oak forest – meadow transects using sweep-net samples at the Síkfökút Project in Hungary. For spiders the species richness was significantly higher in the forest edge than either in the meadow or the forest interior. For weevils the species richness of the forest edge was higher than that of the meadow, but the difference was not statistically significant whereas the species richness of the forest interior was significantly lower than that of the forest edge and the meadow. The composition of the spider assemblage of the edge was more similar to the forest, while the composition of weevils in the edge was more similar to the meadow. Our results based on two invertebrate groups operating on different trophic levels suggest that there is a significant edge effect for the studied taxa resulting in higher species richness in the edge.

Description: Biotic and abiotic data on undisturbed or moderately disturbed lotic sites from a number of studies carried out in northern Greece showed that large rivers differ from small rivers, streams or creeks in terms of diversity, dominant groups and the kind of taxa (concerning the sensitivity of the taxa according to Biological Monitoring Working Party (BMWP) biotic scores). This is mainly due to the differences in their physical characteristics. Correlation of the environmental variables using MDA (multiple discriminant analysis) showed that the chief differentiating factors among the above water bodies are substrate, total suspended solids (TSS), conductivity, slope and temperature. Additionally, there is no clear phenological seasonality in the majority of the dominant benthic macroinvertebrate groups when undisturbed or moderately disturbed sites of mountainous creeks and small rivers are examined. By contrast, in downstream sites of long rivers, seasonality characterizes the dominant benthic macroinvertebrate groups, as it does for other Mediterranean animals.

Description: Bracken is a major problem for livestock-based, extensive agriculture in many parts of the world. It also causes problems for conservation, recreation, game management and forestry and is hence subject to management in order to control it. This paper reviews current bracken control strategies in Great Britain to assess whether they can be improved, and reviews recent work on combining bracken control with vegetation restoration to derive guidelines for maximising the cost-effectiveness of these measures to increase biodiversity. Bracken control in Great Britain is currently, mainly undertaken by aerial spraying of herbicide. A large-scale survey showed that only a small proportion (25%) of sites were likely to show long-term control, the developing vegetation was not that desired by the instigator of control, and there was a large geographic variation in success. The major conclusion was that large-scale treatment often exceeded the area that could be adequately treated by follow-up measures. Experimental studies demonstrate that to obtain “desirable” vegetation (usually Calluna vulgaris-dominated heath in Great Britain) a number of steps usually have to be followed. However, the steps that have to be taken may differ between sites. Deep litter sites, where stock numbers are low, need the litter disturbed in some way and seed of suitable species added. On sites with higher stock numbers, litter disturbance has in effect already been carried out, so that management must involve seed addition and the exclusion/reduction of stock. It is not yet known how long or to what level stock must be removed before the vegetation is able to withstand grazing. It should be noted that management to reverse succession could prove less cost-effective than management that accelerates succession to woodland or forestry. A set of points which highlight the considerations necessary at the commencement of an “integrated” bracken control programme are outlined. Targeting sites in western Britain or sites with residual vegetation present would provide the greatest gains for biodiversity in the short term. However, in many situations management for vegetation restoration must be seen as a key part of this strategy, not as something that will proceed unaided after bracken control has taken place.

Description: Canopy gaps, i.e. openings in the forest cover caused by the fall of structural elements, are considered to be important for the maintenance of diversity and for the forest cycle. A gap can be considered as a young forest patch in the forest matrix, composed of interior surrounded by an edge, both enclosed by a perimeter. Much of the attention has been focused on the gap interior. However, at gap edges the spectrum of regeneration opportunities for plants may be larger than in the interior. Although definitions of gap are still discussed, any definition can describe it in an acceptable way, if justified, but defining edges is complicated and appropriate descriptors should be used. A method to determine gap interior and edge, using light as a descriptor, is presented with an example of gaps from a beech forest (Fagus sylvatica) in Belgium. Also, the relevance and implications of gap edges for plant diversity and calculation of forest turnover is discussed.

Description: The large shallow wetlands that dominate much of the South American continent are rich in biodiversity and complexity. Many of these undamaged ecosystems are presently being examined for their potential economic utility, putting pressure on local authorities and the conservation community to find ways of correctly utilising the available natural resources without compromising the ecosystem functioning and overall integrity. Contrary to many northern hemisphere ecosystems, there have been little long term ecological studies of these systems, leading to a lack of quantitative data on which to construct ecological or resource use models. As a result, decision makers, even well meaning ones, have difficulty in determining if particular economic activities can potentially cause significant damage to the ecosystem and how one should go about monitoring the impacts of such activities. While the direct impact of many activities is often known, the secondary indirect impacts are usually less clear and can depend on local ecological conditions. The use of qualitative models is a helpful tool to highlight potential feedback mechanisms and secondary effects of management action on ecosystem integrity. The harvesting of a single, apparently abundant, species can have indirect secondary effects on key trophic and abiotic compartments. In this paper, loop model analysis is used to qualitatively examine secondary effects of potential economic activities in a large wetland area in northeast Argentina, the Esteros del Ibera. Based on interaction with local actors together with observed ecological information, loop models were constructed to reflect relationships between biotic and abiotic compartments. A series of analyses were made to study the effect of different economic scenarios on key ecosystem compartments. Important impacts on key biotic compartments (phytoplankton, zooplankton, ichthyofauna, aquatic macrophytes) and on the abiotic environment (nutrients and sediment resuspension) were observed through model analysis. These models results do not indicate a definite relationship between activity and a possible impact, but a potential impact that can be further studied and modelled. Likewise, the model is not intended to be an end in itself, but as a tool to help focus further ecological study, monitoring and modelling. In the real world of wetland management, it is not always possible to conduct extensive (and expensive) analysis of all the principal ecological compartments. In the same manner, the construction of larger and more complex models for resource management usually needs to be focused to those areas most likely to effect resource quality or ecosystem functioning. In this light, the development of qualitative models was considered as a first step to help researchers and decision makers focus their efforts (and economic resources) in an intensive ecological sampling programme and the construction of predictive models.

Description: Avifauna on Greek wetland sites is used as a model for the implementation of the Set Covering Problem in selecting nature reserves. Three site conservation values, which depend on species presence, are used as selection criteria. Their calculation is based upon species richness, species rarity and species-danger status. The conservation values must be inserted in the linear programming problem’s objective function by the form of weighting factors. Optimal solutions according to the three ecological criteria are produced. These solutions belong to the set of alternative optimal solutions of the basic Set Covering Problem with no other criterion taken into account except that of the whole species-list coverage. The set of alternative optimal solutions is generated by the explicit exclusion method. The relative value of goal programming and weighing up-criteria methods in producing a unique solution based on the three criteria simultaneously is assessed. Both methods coincide with the same alternative solution that is thus regarded as the final optimal one incorporating all the three ecological criteria.

Description: By artificially baiting roads, it is shown that simple counts of wildlife corpses found on roads are severe underestimate of the actual road casualty rate. The death rate can be 12–16 times that observed by simply counting corpses particularly from a moving vehicle. Removal rates depend on a range of factors including species of predator, species of prey, type and topography of road, road traffic, season, time of day and weather conditions. It is suggested that a greater importance should be afforded, than has previously been the case to the “gross” casualty rate rather than accept the “net” number of casualties as an indicator of loss.

Description: Recent work has suggested that in highly correlated systems, such as sandpiles, turbulent fluids, ignited trees in forest fires and magnetization in a ferromagnet close to a critical point, the probability distribution of a global quantity (i.e. total energy dissipation, magnetization and so forth) that has been normalized to the first two moments follows a specific non-Gaussian curve. This curve follows a form suggested by extremum statistics, which is specified by a single parameter a (a = 1 corresponds to the Fisher-Tippett Type I ("Gumbel") distribution). Here we present a framework for testing for extremal statistics in a global observable. In any given system, we wish to obtain a, in order to distinguish between the different Fisher-Tippett asymptotes, and to compare with the above work. The normalizations of the extremal curves are obtained as a function of a. We find that for realistic ranges of data, the various extremal distributions, when normalized to the first two moments, are difficult to distinguish. In addition, the convergence to the limiting extremal distributions for finite data sets is both slow and varies with the asymptote. However, when the third moment is expressed as a function of a, this is found to be a more sensitive method.

Description: Recent observations and analyses evidenced that the magnetotail, as well as the magnetospheric dynamics are characterised by a scale-free behaviour and intermittence. These results, along with numerical simulations on cellular automata, suggest that the observed scale-invariance may be due to forced and/or self-organised criticality (FSOC), meaning that the magnetotail operates near a marginally stable state (Chang, 1999). On the other hand, it was underlined that a complex magnetic field topology in the geotail regions may play a relevant role in the impulsive energy relaxation (Consolini and Chang, 2001.

Description: A basing of the diffusion approximation derivation for the Hasselmann kinetic integral describing nonlinear interactions of gravity waves in deep water is discussed. It is shown that the diffusion approximation containing the second derivatives of a wave spectrum in a frequency and angle (or in wave vector components) is resulting from a step-by-step analytical integration of the sixfold Hasselmann integral without involving the quasi-locality hypothesis for nonlinear interactions among waves. A singularity analysis of the integrand expression gives evidence that the approximation mentioned above is the small scattering angle approximation, in fact, as it was shown for the first time by Hasselmann and Hasselmann (1981). But, in difference to their result, here it is shown that in the course of diffusion approximation derivation one may obtain the final result as a combination of terms with the first, second, and so on derivatives. Thus, the final kind of approximation can be limited by terms with the second derivatives only, as it was proposed in Zakharov and Pushkarev (1999). For this version of diffusion approximation, a numerical testing of the approximation properties was carried out. The testing results give a basis to use this approximation in a wave modelling practice.

Description: Tropical cyclones and storms, hurricanes, powerful thunderclouds, which generate tornadoes, destructive extratropical cyclones, which result in catastrophic floods, are the powerful cloud systems that contain huge amount of water. According to the hypothesis argued in this paper, an electric field coupled with powerful clouds and electric forces play a cardinal role in supporting this huge mass of water at a high altitude in the troposphere and in the instability of powerful clouds sometimes during rather a long time duration. Based on this hypothesis, a highly effective method of volume electric charge neutralization of powerful clouds is proposed. It results in the decrease in an electric field, a sudden increase in precipitation, and subsequent degradation of powerful clouds. This method, based on the natural phenomenon, ensures the prevention of the intensification of tropical and extratropical cyclones and their transition to the storm and hurricane (typhoon) stages, which makes it possible to avoid catastrophic floods. It also ensures the suppression of severe thunderclouds, which, in turn, eliminates the development of dangerous thunderstorms and the possibility of the emergence and intensification of tornadoes.

Description: Magnetic reconnection is an important process providing a fast conversion of magnetic energy into thermal and kinetic plasma energy. In this concern, a key problem is that of the resistive diffusion region where the reconnection process is initiated. In this paper, the diffusion region is associated with a nonuniform conductivity localized to a small region. The nonsteady resistive incompressible MHD equations are solved numerically for the case of symmetric reconnection of antiparallel magnetic fields. A Petschek type steady-state solution is obtained as a result of time relaxation of the reconnection layer structure from an arbitrary initial stage. The structure of the diffusion region is studied for various ratios of maximum and minimum values of the plasma resistivity. The effective length of the diffusion region and the reconnection rate are determined as functions of the length scale and the maximum of the resistivity. For sufficiently small length scale of the resistivity, the reconnection rate is shown to be consistent with Petschek's formula. By increasing the resistivity length scale and decreasing the resistivity maximum, the reconnection layer tends to be wider, and correspondingly, the reconnection rate tends to be more consistent with that of the Parker-Sweet regime.

Description: The stationary self-focusing of whistler waves with frequencies near half of the electron-cyclotron frequency in the ionospheric plasma is considered in the framework of a two-dimensional generalized nonlinear Schrödinger equation including fourth-order dispersion effects and nonlinearity saturation. New types of soliton-like (with zero topological charge) and vortex-like nonlinear waveguides are found, and their stability confirmed both analytically and numerically.

Description: The study of the robust features of the magnetosphere is motivated both by new "whole system" approaches, and by the idea of "space climate" as opposed to "space weather". We enumerate these features for the AE index, and discuss whether self-organised criticality (SOC) is the most natural explanation of the "stylised facts" so far known for AE. We identify and discuss some open questions, answers to which will clarify the extent to which AE's properties provide evidence for SOC. We then suggest an SOC-like reconnection-based scenario drawing on the result of Craig (2001) as an explanation of the very recent demonstration by Uritsky et al. (2001b) of power laws in several properties of spatiotemporal features seen in auroral images.

Description: The thermal conductivity of mantle materials has two components, the lattice component klat from phonons and the radiative component krad due to photons. These two contributions of variable thermal conductivity have a nonlinear dependence in the temperature, thus endowing the temperature equation in mantle convection with a strongly nonlinear character. The temperature derivatives of these two mechanisms have different signs, with ∂klat /∂T negative and dkrad /dT positive. This offers the possibility for the radiative conductivity to control the chaotic boundary layer instabilities developed in the deep mantle. We have parameterized the weight factor between krad and klat with a dimensionless parameter f , where f = 1 corresponds to the reference conductivity model. We have carried out two-dimensional, time-dependent calculations for variable thermal conductivity but constant viscosity in an aspect-ratio 6 box for surface Rayleigh numbers between 106 and 5 × 106. The averaged Péclet 〈 Pe 〉 numbers of these flows lie between 200 and 2000. Along the boundary in f separating the chaotic and steady-state solutions, the 〈 Pe 〉 number decreases and the Nusselt number increases with internal heating, illustrating the feedback between internal heating and radiative thermal conductivity. For purely basal heating situation, the time-dependent chaotic flows become stabilized for values of f of between 1.5 and 2. The bottom thermal boundary layer thickens and the surface heat flow increases with larger amounts of radiative conductivity. For magnitudes of internal heating characteristic of a chondritic mantle, much larger values of f , exceeding 10, are required to quench the bottom boundary layer instabilities. By isolating the individual conductive mechanisms, we have ascertained that the lattice conductivity is partly responsible for inducing boundary layer instabilities, while the radiative conductivity and purely depth-dependent conductivity exert a stabilizing influence and help to control thermal chaos developed in the deep mantle. These results have been verified to exist also in three-dimensional geometry and would argue for the need to consider the potentially important role played by radiative thermal conductivity in controlling chaotic flows in time-dependent mantle convection, the mantle heat transfer, the number of hotspots and the attendant mixing of geochemical anomalies.

Description: Local scaling and singularity properties of solar wind and geomagnetic time series were analysed using Hölder exponents . It was shown that in analysed cases due to the multifractality of fluctuations, α changes from point to point. We argued there exists a peculiar interplay between regularity/irregularity and amplitude characteristics of fluctuations which could be exploited for the improvement of predictions of geomagnetic activity. To this end, a layered back-propagation artificial neural network model with feedback connection was used for the study of the solar wind magnetosphere coupling and prediction of the geomagnetic Dst index. The solar wind input was taken from the principal component analysis of the interplanetary magnetic field, proton density and bulk velocity. Superior network performance was achieved in cases when the information on local Hölder exponents was added to the input layer.

Description: An oscillating grids turbulence generator was constructed for studies of two new effects associated with turbulent transport of particles, turbulent thermal diffusion and clustering instability. These effects result in formation of large-scale and small-scale inhomogeneities in the spatial distribution of particles. The advantage of this experimental set-up is the feasibility to study turbulent transport in mixtures with controllable composition and unlimited observation time. For flow measurements we used Particle Image Velocimetry with the adaptive multi-pass algorithm to determine a turbulent velocity field and its statistical characteristics. Instantaneous velocity vector maps, flow streamlines and probability density function of velocity field demonstrate properties of turbulence generated in the device.

Description: Magnetic fluctuations are recognized in a large variety of space plasmas by increasingly high resolution, in situ observations as mirror wave mode structures. A typical requirement for the excitation of mirror modes is a dominant perpendicular pressure in a high-beta plasma environment. Contrary, we demonstrate from a realistic kinetic analysis how details of the velocity space distributions are of considerable significance for the instability threshold. Introducing the most common characteristics of observed ion and electron distributions by a mixed suprathermal-loss-cone, we derive a universal mirror instability criterion from an energy principle for collisionless plasmas. As a result, the transition from two temperature Maxwellians to realistic non-thermal features provides a strong source for the generation of mirror wave mode activity, reducing drastically the instability threshold. In particular, a number of space-related examples illuminate how the specific structure of the velocity space distribution dominates as a regulating excitation mechanism over the effects related to changes in the plasma parameters.

Description: We investigate the nature of nonlinear waves propagating transverse to the magnetic field in a bi-ion plasma including plasma pressure. By using the conservation laws derived from the multi-ion fluid equations the system may be described by a single order differential equation whose properties control the structure of the flow and the magnetic field. Compressive solitons exist in specific ranges of the characteristic Mach numbers. Various features of solitons differ in different existence "windows". For example, there are solitons that contain a strong proton rarefaction core embedded in the main compressional structure. Compressive solitons are found in a wide range of flow parameters. Finite ion pressure introduces critical Mach numbers. In contrast to a plasma consisting only of protons and electrons these singular points are reached where a specific combination of ion and electron speeds lies on particular locii, in multi-parameter space, which corresponds to the generalized "sonic point" of the compound system.

Description: In this paper, we show that the behavior of weakly nonlinear waves in a 2-layer model of baroclinic instability on a b-plane with varying viscosity is determined by a single, degenerate codimension three bifurcation. In the process, we show how previous studies, using the method of multiple scales to derive evolution equations for the slowly varying amplitude of the growing wave, arise as special limits of the general evolution description.

Description: In the Forschungszentrum Karlsruhe an experiment has been constructed which demonstrates a homogeneous dynamo as is expected to exist in the Earth's interior. This experiment is discussed within the framework of mean-field dynamo theory. The main predictions of this theory are explained and compared with the experimental results. Key words. Dynamo, geodynamo, dynamo experiment, mean-field dynamo theory, a-effect

Description: Neutral interstellar H-atoms penetrate into the inner heliosphere and upon the event of ionization are converted into pick-up ions (PUIs). The magnetized solar wind flow incorporates these ions into the plasma bulk and enforces their co-motion. By nonlinear interactions with wind-entrained Alfvén waves, these ions are then processed in the comoving velocity space. The complete pick-up process is connected with forces acting back to the original solar wind ion flow, thereby decelerating and heating the solar wind plasma. As we show here, the resulting deceleration cannot be treated as a pure loading effect, but requires adequate consideration of the action of the pressure of PUI-scattered waves operating by the PUI pressure gradient. Hereby, it is important to take into proper account the stochastic acceleration which PUIs suffer from at their convection out of the inner heliosphere by quasi-linear interactions with MHD turbulences. Only then can the presently reported VOYAGER observations of solar wind decelerations and heatings in the outer heliosphere be understood in view of the most likely values of interstellar gas parameters, such as an H-atom density of 0.12 cm-3 . Solar wind protons (SWPs) appear to be globally heated in their motion to larger solar distances. Ascribing the needed heat transfer to the action of suprathermal PUIs, which drive MHD waves that are partly absorbed by SWPs, in order to establish the observed SWP polytropy, we can obtain a quantitative expression for the solar wind proton pressure as a function of solar distance. This expression clearly shows the change from an adiabatic to a quasi-polytropic SWP behaviour with a decreasing polytropic index at increasing distances. This also allows one to calculate the average percentage of initial pick-up energy fed into the thermal proton energy. In a first order evaluation of this expression, we can estimate that about 10% of the initial PUI injection energy is eventually transfered to SWPs independent of the PUI injection rate.

Description: Modulation of short surface ripples by long surface or internal waves by a cascade mechanism is considered. At the first stage, the orbital velocity of the long wave (LW) adiabatically modulates an intermediate length nonlinear gravity wave (GW), which generates a bound (parasitic) capillary wave (CW) near its crest in a wide spatial frequency band. Due to strong dependence of the CW amplitude on that of the GW, the resulting ripple modulation by LW can be strong. Adiabatic modulation at the first stage is calculated for an arbitrarily strong LW current. The CWs are calculated based on the Lonquet-Higgins theory, in the framework of a steady periodic solution, which proves to be sufficient for the cases considered. Theoretical results are compared with data from laboratory experiments. A discussion of related sea clutter data is given in the conclusion.

Description: One of the interesting observations from the FAST satellite is the detection of strong spiky waveforms in the parallel electric field in association with ion cyclotron oscillations in the perpendicular electric fields. We report here an analytical model of the coupled nonlinear ion cyclotron and ion-acoustic waves, which could explain the observations. Using the fluid equations for the plasma consisting of warm electrons and cold ions, a nonlinear wave equation is derived in the rest frame of the propagating wave for any direction of propagation oblique to the ambient magnetic field. The equilibrium bulk flow of ions is also included in the model to mimic the field-aligned current. Depending on the wave Mach number M defined by M = V/Cs with V and Cs being the wave phase velocity and ion-acoustic speed, respectively, we find a range of solutions varying from a sinusoidal wave form for small amplitudes and low M to sawtooth and highly spiky waveforms for nearly parallel propagation. The results from the model are compared with the satellite observations.

Description: This paper presents the analysis of symmetric circulations of a rotating baroclinic flow, forced by a steady thermal wind and dissipated by Laplacian friction. The analysis is performed with numerical time-integration. Symmetric flows, vertically bound by horizontal walls and subject to either periodic or vertical wall lateral boundary conditions, are investigated in the region of parameter-space where unstable small amplitude modes evolve into stable stationary nonlinear solutions. The distribution of solutions in parameter-space is analysed up to the threshold of chaotic behaviour and the physical nature of the nonlinear interaction operating on the finite amplitude unstable modes is investigated. In particular, analysis of time-dependent energy-conversions allows understanding of the physical mechanisms operating from the initial phase of linear instability to the finite amplitude stable state. Vertical shear of the basic flow is shown to play a direct role in injecting energy into symmetric flow since the stage of linear growth. Dissipation proves essential not only in limiting the energy of linearly unstable modes, but also in selecting their dominant space-scales in the finite amplitude stage.

Description: We have studied a hybrid model combining the forest-fire model with the site-percolation model in order to better understand the earthquake cycle. We consider a square array of sites. At each time step, a "tree" is dropped on a randomly chosen site and is planted if the site is unoccupied. When a cluster of "trees" spans the site (a percolating cluster), all the trees in the cluster are removed ("burned") in a "fire." The removal of the cluster is analogous to a characteristic earthquake and planting "trees" is analogous to increasing the regional stress. The clusters are analogous to the metastable regions of a fault over which an earthquake rupture can propagate once triggered. We find that the frequency-area statistics of the metastable regions are power-law with a negative exponent of two (as in the forest-fire model). This is analogous to the Gutenberg-Richter distribution of seismicity. This "self-organized critical behavior" can be explained in terms of an inverse cascade of clusters. Small clusters of "trees" coalesce to form larger clusters. Individual trees move from small to larger clusters until they are destroyed. This inverse cascade of clusters is self-similar and the power-law distribution of cluster sizes has been shown to have an exponent of two. We have quantified the forecasting of the spanning fires using error diagrams. The assumption that "fires" (earthquakes) are quasi-periodic has moderate predictability. The density of trees gives an improved degree of predictability, while the size of the largest cluster of trees provides a substantial improvement in forecasting a "fire."

Description: In a recent paper on the theory of the Earth's magnetic field and key features of Sunspot activity (de Paor, 2001), a central role in the calculation of secular variations of the geomagnetic field was played by a newly-introduced parameter called the deflection (abbreviated def ). In this note, the significance of def is elucidated and the method used to calculate it is explained.

Description: Blowing snow and snow drifts are common features in the Arctic. Due to sparse vegetation, low temperatures and high wind speeds, the snow is constantly moving. This causes severe problems for transportation and infrastructure in the affected areas. To minimise the effect of drifting snow already in the designing phase of new structures, adequate models have to be developed and tested. In this study, snow distribution in Arctic topography is surveyed in two study areas during the spring of 1999 and 2000. Snow depth is measured by ground penetrating radar and manual methods. The study areas encompass four by four kilometres and are partly glaciated. The results of the surveys show a clear pattern of erosion, accumulation areas and the evolution of the snow cover over time. This high resolution data set is valuable for the validation of numerical models. A simple numerical snow drift model was used to simulate the measured snow distribution in one of the areas for the winter of 1998/1999. The model is a two-level drift model coupled to the wind field, generated by a mesoscale meteorological model. The simulations are based on five wind fields from the dominating wind directions. The model produces a satisfying snow distribution but fails to reproduce the details of the observed snow cover. The results clearly demonstrate the importance of quality field data to detect and analyse errors in numerical simulations.

Description: Large explosive volcanic eruptions are capable of injecting considerable amounts of particles and sulphur gases (mostly sulphur dioxide) above the tropopause, causing increases in the stratospheric aerosol optical depth that may be even larger than one order of magnitude. The e-folding particle lifetime in the stratosphere is much longer than in the troposphere (one year versus a few days) so that climatic perturbations in a timeframe of a few years are produced after major volcanic eruptions. A climate-chemistry coupled model is used here to study the dynamical effects of the radiative forcing due to stratospheric aerosols formed after the June, 1991 cataclysmic eruption of Mt. Pinatubo in the Philippines. It is shown that the dynamical perturbation is twofold: (a) the stratospheric mean meridional circulation is affected by local aerosol radiative heating (mostly located in the tropical lower stratosphere); (b) the planetary wave propagation in the mid- to high-latitude lower stratosphere is altered as a consequence of decreasing atmospheric stability due to the climatic perturbation. Dynamical results of the climate model are compared with available observations; a discussion is made regarding the similarities with the dynamical regime of the easterly phase of the equatorial quasi-biennial oscillation. Major findings of this study are: (a) radiatively forced changes in the stratospheric circulation during the first two years after the eruption may, to a large extent, explain the observed trend decline of long-lived greenhouse gases (CH4 and N2O, in particular); (b) the dynamical perturbation helps explain why simple photochemical studies of the ozone trends during 1991–1993 generally fail in reproducing the satellite observed feature consisting of a 2% additional global ozone depletion during 1993 with respect to 1992. In both cases we conclude that an increase in the mid- to high-latitude downward flux at the tropopause is the key factor for explaining the behaviour of these atmospheric tracers during 1991/92.

Description: Connections between El Niño – Southern Oscillation (ENSO) phenomena and indices of solar activity and geomagnetic disturbance were investigated. Spectral analysis of the ENSO-data was carried out. Oscillations with periods of about 11–12, 5–6, 2–3 years were found. Correlative and cross-spectral analysis was carried out to estimate connections between ENSO data, and solar and geomagnetic indices. Functions of coherency and phase were calculated.

Description: The experience of calibrating Eglit’s avalanche model using data about the avalanche travel distance lengths and velocity data from the Zailiyskiy Alatau range (Kazakhstan) is discussed. Empirical equations are proposed that describe the dependence of the friction coefficients on the avalanche volume and type.

Description: A numerical model for unsteady shallow water flow over initially dry areas is applied to a case study in a small drainage area at the Spanish Ebro River basin. Several flood mitigation measures (reforestation, construction of a small reservoir and channelization) are simulated in the model in order to compare different extreme rainfall-runoff scenarios.

Description: In order to better understand the interaction between powder snow avalanches and defence structures, we carried out physical experiments on small-scale models. The powder snow avalanche was simulated by a heavy salt solution in a water tank. Quasi two-dimensional and three-dimensional experiments were carried out with different catching dam heights. For the reference avalanche, the velocity just behind the nose in the head was greater than the front velocity. For the 2-D configuration, the ratio Umax/Ufront was as high as 1.6, but it depends on the height. For the 3-D configuration, this ratio differed slightly and was even greater (up to 1.8). The vertical velocity rose to 106% of the front velocity for the 3-D simulation and 74% for the 2-D simulation. The reduction in front velocity due to the presence of dams was an increasing function of the dam height. But this reduction depended on topography: dams were more effective on an open slope avalanche (3-D configuration). The ratio Umax/Ufront was an increasing function of the dam’s height and reached a value of 1.9. The obstacle led to a reduction in vertical velocity downstream of the vortex zone.

Description: The search of improvement of protective techniques against natural phenomena such as snow avalanches continues to use classic methods for calculating flexible structures. This paper deals with a new method to design avalanche protection nets. This method is based on a coupled analysis of both net structure and snow mantle by using a Discrete Element Method. This has led to the development of computational software so that avalanche nets can be easily designed. This tool gives the evolution of the forces acting in several parts of the work as a function of the snow situation.

Description: Identification and mapping of landslide deposits are an intrinsically difficult and subjective operation that requires a great effort to minimise the inherent uncertainty. For the Staffora Basin, which extends for almost 300 km2 in the northern Apennines, three landslide inventory maps were independently produced by three groups of geomorphologists. In comparing each map with the others, large positional discrepancies arise (in the range of 55–65%). When all three maps are overlain, the locational mismatch of landslide deposit polygons increases to over 80%. To assess the impact of these errors on predictive models of landslide hazard, for the study area discriminant models were built up from the same set of geological-geomorphological factors as predictors, and the occurrence of landslide deposits within each terrain-unit, derived from each inventory map, as dependent variable. The comparison of these models demonstrates that statistical modelling greatly minimises the impact of input data errors which remain, however, a major limitation on the reliability of landslide hazard maps.

Description: Results of laboratory experiments are presented in which a finite suspension of sawdust particles was released instantaneously into a rectangular channel immersed in a water tank. Two kinds of gravity currents were studied: currents with or without entrainment of particles from the bed. Experiments were repeated for two slopes: 30° and 45°. We observed that the velocity of the front was significantly in-creased as particle entrainment occurred. In addition, our experiments showed that the front kept a quasi-constant velocity for both runs. This might suggest that the flow regime corresponded to the "slumping regime" or "adjustment phase" described earlier by Huppert and Simpson (1980).

Description: An experimental investigation with dry granular flows passing over an obstacle down a rough inclined channel has been performed. The aim is to improve our understanding of the interaction between dense snow avalanches and defence structures. Specific attention was directed to the study of the zone of influence upstream from the obstacle, linked to the formation of a dead zone. The dead zone length L was systematically measured as a function of the obstacle height H and the channel inclination θ, for several discharges. In a whole range of channel inclinations, all the data are shown to collapse into a single curve when properly scaled. The scaling is based on the introduction of a theoretical deposit length (depending on H, θ and the internal friction angle of the material, φ) and a Froude number of the flow depending on the obstacle height.

Description: The EU programme CADZIE (Catastrophic Avalanches: Defence Structures and Zoning in Europe), was established after the catastrophic 1999 avalanche winter in Europe. The overall objective of the programme is improved snow avalanche risk management by: (1) improved avalanche hazard zoning by computational models; and (2) improved understanding of the interaction between defence structures and avalanches.  One contribution to meet the objectives is a database of well-documented extreme or deflected avalanche events in the six countries of the partners of the programme. The database contains observational, topographical and meteorological snow avalanche data with reliability, as well as references, copyrights, etc., all in a convenient framework based on common formats.  The structure, contents, and potential use of the database are described. Example calculations of extreme and deflected events made by the NGI user interface "SKRED", for practical use of avalanche computational models, present applications of the database. Finally, further development of the database and of the computational models to meet the future needs in avalanche hazard zoning is proposed.

Description: A joint experiment was carried out on 10 February 1999 by the Swiss Federal Institute for Snow and Avalanche Research (SFISAR) and the Austrian Institute for Avalanche and Torrent Research (AIATR, of the Federal Office and Re-search Centre for Forests, BFW) to measure forces and velocities at the full scale experimental site CRÊTA BESSE in VALLÉE DE LA SIONNE, Canton du Valais, Switzerland. A huge avalanche could be released artificially, which permitted extensive investigations (dynamic measurements, im-provement of measurement systems, simulation model verification, design of protective measures, etc.). The results of the velocity measurements from the dual frequency pulsed Doppler avalanche radar of the AIATR and the recalculation with the numerical simulation model SAMOS are explained in this paper.

Description: In order to determine the effect of steep slopes on snowdrift generated by snow fences, we have conducted physical modeling experiments in the CSTB (Centre Scientifique et Technique du Bâtiment) cold wind tunnel as part of the European project "Access to Large Facilities". After an overview of previous studies and an accurate description of the drifting snow process inside the experimental chamber,  we present the main results obtained. (1) On flat areas, even for high wind speed, the acknowledged results for moderate wind are still valid: the porous snow fence (50%) is the most efficacious and the bottom gap increases the efficacy of the dense snow fence. (2) The steeper the slope is, the less effective all tested snow fences are. Their effectiveness decreases considerably: the snow catch is approximately divided by two for a slope of 10°. (3) Contrary to flat areas, on steep slopes, the "efficacy" is greater for a dense snow fence.

Description: This paper presents new results for 1D BGK electron solitary wave (phase-space electron hole) solutions and, based on the new results, extends the solutions to include the 3D electrical interaction (E ~ 1/r 2) of charged particles. Our approach for extending to 3D is to solve the nonlinear 3D Poisson and 1D Vlasov equations based on a key feature of 1D electron hole (EH) solutions; the positive core of an EH is screened by electrons trapped inside the potential energy trough. This feature has not been considered in previous studies. We illustrate this key feature using an analytical model and argue that the feature is independent of any specific model. We then construct azimuthally symmetric EH solutions under conditions where electrons are highly field-aligned and ions form a uniform background along the magnetic field. Our results indicate that, for a single humped electric potential, the parallel cut of the perpendicular component of the electric field (E⊥) is unipolar and that of the parallel component (E||) bipolar, reproducing the multi-dimensional features of the solitary waves observed by the FAST satellite. Our analytical solutions presented in this article capture the 3D electric interaction and the observed features of (E|| ) and E⊥. The solutions predict a dependence of the parallel width-amplitude relation on the perpendicular size of EHs. This dependence can be used in conjunction with experimental data to yield an estimate of the typical perpendicular size of observed EHs; this provides important information on the perpendicular span of the source region as well as on how much electrostatic energy is transported by the solitary waves.

Description: Variations of the plasma pressure in a magnetic flux tube can produce MHD waves evolving into shocks. In the case of a low plasma beta, plasma pressure pulses in the magnetic flux tube generate MHD slow shocks propagating along the tube. For converging magnetic field lines, such as in a dipole magnetic field, the cross section of the magnetic flux tube decreases enormously with increasing magnetic field strength. In such a case, the propagation of MHD waves along magnetic flux tubes is rather different from that in the case of uniform magnetic fields. In this paper, the propagation of MHD slow shocks is studied numerically using the ideal MHD equations in an approximation suitable for a thin magnetic flux tube with a low plasma beta. The results obtained in the numerical study show that the jumps in the plasma parameters at the MHD slow shock increase greatly while the shock is propagating in the narrowing magnetic flux tube. The results are applied to the case of the interaction between Jupiter and its satellite Io, the latter being considered as a source of plasma pressure pulses.

Description: A three-wave model of auroral radio emissions near the electron plasma frequency was proposed by Chian et al. (1994) involving resonant interactions of Langmuir, whistler and Alfvén waves. Chaos can occur in the nonlinear evolution of this three-wave process in the magnetosphere. In particular, two types of intermittency, due to either local or global bifurcations, can be observed. We analyze the type-I Pomeau-Manneville intermittency, arising from a saddle-node bifurcation, and the crisis-induced intermittency, arising from an interior crisis associated with a global bifurcation. Examples of time series, power spectrum, phase-space trajectory for both types of intermittency are presented through computer simulations. The degree of chaoticity of this three-wave process is characterized by calculating the maximum Lyapunov exponent. We suggest that the intermit-tent phenomena discussed in this paper may be observed in the temporal signal of magnetospheric radio emissions.

Description: We study properties of nonlinear magnetic fluctuations in the turbulent boundary layer (TBL) over polar cusps during a typical TBL crossing on 19 June 1998. Interball-1data in the summer TBL are compared with that of Geotail in solar wind (SW) and Polar in the northern TBL. In the TBL two characteristic slopes are seen: ~ - 1 at (0.004- 0.08) Hz and ~ - 2.2 at (0.08-2) Hz. We present evidences that random current sheets with features of coherent solitons can result in: (i) slopes of ~ - 1 in the magnetic power spectra; (ii) demagnetization of the SW plasma in "diamagnetic bubbles"; (iii) nonlinear, presumably, 3-wave phase coupling with cascade features; (iiii) departure from the Gaussian statistics. We discuss the above TBL properties in terms of intermittency and self-organization of nonlinear systems, and compare them with kinetic simulations of reconnected current sheet at the nonlinear state. Virtual satellite data in the model current sheet reproduce valuable cascade-like spectral and bi-spectral properties of the TBL turbulence.

Description: An impulse-based model is developed to represent a coupling between turbulent flow in the atmosphere and turbulent flow in the ocean. In particular, it is argued that the atmosphere flowing horizontally over the ocean surface generates a velocity fluctuation field in the latter's near-surface flow. The mechanism for this can be understood kinematically in terms of an exchange of tangentially-oriented fluid impulse at the air-sea interface. We represent this exchange numerically through the creation of Lagrangian elements of impulse density. An indication of the efficacy of such a model would lie in its ability to predict the observed fractal dimension of lateral trajectories of submerged floats set adrift in the ocean. To this end, we examine the geometry of lateral tracer-paths determined from the present model.

Description: Idealised global circulation model simulations with circumpolar and localised (one and two) storm tracks are re-analysed to determine scaling, intermittency and phase-space structures. In a hundred year experiment with a circumpolar storm track, the spectrum S(f ) of the first principal component of the zonal wind fluctuations shows the following power law regimes: (a) a short-term memory between f- -4 and f  -2 up to 50 days and (b) a long-term memory f -1 from 50 to 400 days and f -0.24 beyond 400 days, similar to observed maritime single station near-surface air temperature data. In the presence of localised storm tracks, the wave number two dominates the dynamics and a long-term memory cannot be detected. The recurrence plot is introduced as a novel tool to comprehensively visualise the evolution of the dynamical system in terms of state separations (distances) in phase space. The patterns allow for a qualitative interpretation of the underlying local phenomena in phase space, such as waves, analogs, extremes, and global regimes. Attractor dimensions are, in general, larger than 10, but they appear to be lower in the wave-dominated regimes of the double storm track experiment.

Description: We investigate the evolution of magnetic helicity under kinetic magnetic reconnection in thin current sheets. We use Harris sheet equilibria and superimpose an external magnetic guide field. Consequently, the classical 2D magnetic neutral line becomes a field line here, causing a B ≠ 0 reconnection. While without a guide field, the Hall effect leads to a quadrupolar structure in the perpendicular magnetic field and the helicity density, this effect vanishes in the B ≠ 0 reconnection. The reason is that electrons are magnetized in the guide field and the Hall current does not occur. While a B = 0 reconnection leads just to a bending of the field lines in the reconnection area, thus conserving the helicity, the initial helicity is reduced for a B ≠ 0 reconnection. The helicity reduction is, however, slower than the magnetic field dissipation. The simulations have been carried out by the numerical integration of the Vlasov-equation.

Description: The method of recurrence plots is extended to the cross recurrence plots (CRP) which, among others, enables the study of synchronization or time differences in two time series. This is emphasized in a distorted main diagonal in the cross recurrence plot, the line of synchronization (LOS). A non-parametrical fit of this LOS can be used to rescale the time axis of the two data series (whereby one of them is compressed or stretched) so that they are synchronized. An application of this method to geophysical sediment core data illustrates its suitability for real data. The rock magnetic data of two different sediment cores from the Makarov Basin can be adjusted to each other by using this method, so that they are comparable.

Description: Based on quasi-linear theory (involving pitch angle scattering), the resonant interactions between ions and waves in an anisotropic multi-component plasma are discussed. In particular, electromagnetic Alfvén and ion-cyclotron waves propagating along or obliquely to the magnetic field are considered. A set of reduced (with respect to the perpendicular velocity component) quasi-linear diffusion equations is derived, involving reduced 1-D velocity distribution functions (VDFs), as they occur in wave dispersion relations. A 2-D model VDF can be constructed when using the Gaussian approximation. Wave-particle heating and acceleration rates are calculated.

Description: In a spirit akin to the sandpile model of self-organized criticality, we present a simple statistical model of the cellular-automaton type which simulates the role of an asperity in the dynamics of a one-dimensional fault. This model produces an earthquake spectrum similar to the characteristic-earthquake behaviour of some seismic faults. This model, that has no parameter, is amenable to an algebraic description as a Markov Chain. This possibility illuminates some important results, obtained by Monte Carlo simulations, such as the earthquake size-frequency relation and the recurrence time of the characteristic earthquake.

Description: We studied a three-wave coupling process occurring in an active experiment of microwave power transmission (MPT) in the ionospheric plasma by performing one-dimensional electromagnetic PIC (Particle-In-Cell) simulations. In order to examine the spatial variation of the coupling process, we continuously emitted intense electromagnetic waves from an antenna located at a simulation boundary. In the three-wave coupling, a low-frequency electrostatic wave is excited as the result of a nonlinear interaction between the forward propagating pump wave and backscattered wave. In the simulations, low-frequency electrostatic bursts are discontinuously observed in space. The discontinuity of the electrostatic bursts is accounted for by the local electron heating due to the bursts and the associated modification of the wave dispersion relation. In a case where the pump wave propagates along the geomagnetic field Bext , several bursts of Langmuir waves are observed. Since the first burst consumes a part of the pump wave energy, the pump wave is weakened and cannot trigger the three-wave coupling beyond the region where the burst occurs. Since the dispersion relation of the Langmuir wave is variable, due to the local electron heating by the burst, the coupling condition eventually becomes unsatisfied and the first interaction becomes weak. Another burst of Langmuir waves is observed at a different region beyond the location of the first burst. In the case of perpendicular propagation, the upper hybrid wave, one of the mode branches of the electron cyclotron harmonic waves, is excited. Since the dispersion relation of the upper hybrid wave is less sensitive to the electron temperature, the coupling condition is not easily violated by the temperature increase. As a result, the three-wave coupling periodically takes place in time and eventually, the transmission ratio of the microwaves becomes approximately 20%, while almost no attenuation of the pump waves is observed after the first electrostatic burst in the parallel case. We also examined the dependency of the temporal growth rate for the electrostatic waves on the amplitude of the pump wave.

Description: The dissipation power and size of auroral blobs are investigated in detail to examine the possible analogy between the dynamic magnetosphere and a forced and/or self-organized critical system. The distributions of these auroral parameters are sorted in terms of different levels of activity, namely substorms, pseudo-breakups, and quiet conditions. A power law (scale-free) component is seen in all these distributions. In addition, a peak distribution is found for substorm intervals and a hump for pseudo-breakup intervals. The peak distribution is present prominently during magnetic storms, i.e. when the magnetosphere is strongly driven by the solar wind. It is interpreted that the scale-free component is associated with the activity of the diffuse aurora, corresponding to disturbances at all permissible scales within the plasma sheet. Ionospheric feedback appears to be essential for the presence of two components in the distribution for auroral dissipation power. These results are consistent with the concept that the magnetosphere is in a forced and/or self-organized critical state, although they do not constitute conclusive evidence for the analogy.

Description: We present a geomorphological method to evaluate landslide hazard and risk. The method is based on the recognition of existing and past landslides, on the scrutiny of the local geological and morphological setting, and on the study of site-specific and historical information on past landslide events. For each study area a multi-temporal landslide inventory map has been prepared through the interpretation of various sets of stereoscopic aerial photographs taken over the period 1941–1999, field mapping carried out in the years 2000 and 2001, and the critical review of site-specific investigations completed to solve local instability problems. The multi-temporal landslide map portrays the distribution of the existing and past landslides and their observed changes over a period of about 60 years. Changes in the distribution and pattern of landslides allow one to infer the possible evolution of slopes, the most probable type of failures, and their expected frequency of occurrence and intensity. This information is used to evaluate landslide hazard, and to estimate the associated risk. The methodology is not straightforward and requires experienced geomorphologists, trained in the recognition and analysis of slope processes. Levels of landslide hazard and risk are expressed using an index that conveys, in a simple and compact format, information on the landslide frequency, the landslide intensity, and the likely damage caused by the expected failure. The methodology was tested in 79 towns, villages, and individual dwellings in the Umbria Region of central Italy.

Description: This paper presents two avalanche forecasting applications NXD2000 and NXD-REG which were developed at the Swiss Federal Institute for Snow and Avalanche Re-search (SLF). Even both are based on the nearest neighbour method they are targeted to different scales. NXD2000 is used to forecast avalanches on a local scale. It is operated by avalanche forecasters responsible for snow safety at snow sport areas, villages or cross country roads. The area covered ranges from 10 km2 up to 100 km2 depending on the climatological homogeneity. It provides the forecaster with ten most similar days to a given situation. The observed avalanches of these days are an indication of the actual avalanche danger. NXD-REG is used operationally by the Swiss avalanche warning service for regional avalanche forecasting. The Nearest Neighbour approach is applied to the data sets of 60 observer stations. The results of each station are then compiled into a map of current and future avalanche hazard. Evaluation of the model by cross-validation has shown that the model can reproduce the official SLF avalanche forecasts in about 52% of the days.

Description: The problem of optimization of analytical and numerical approximations of Hasselmann's nonlinear kinetic integral is discussed in general form. Considering the general expression for the kinetic integral, a principle to obtain the optimal approximation is formulated. From this consideration it follows that the most well-accepted approximations, such as Discrete Interaction Approximation (DIA) (Hasselmann et al., 1985), Reduced Integration Approximation (RIA) (Lin and Perry, 1999), and the Diffusion Approximation proposed recently in Zakharov and Pushkarev (1999) (ZPA), have the same roots. The only difference among them is, essentially, the choice of the 4-wave configuration for the interacting waves. To evaluate a quality of any approximation for the 2-D nonlinear energy transfer, a mathematical measure of relative error is constructed and the meaning of approximation efficiency is postulated. By the use of these features it is shown that DIA has better accuracy and efficiency than ZPA. Following to the general idea of optimal approximation and by using the measures introduced, more efficient and faster versions of DIA are proposed.

Description: Preliminary results are presented which suggest that scaling and singularity characteristics of solar wind and ground-based magnetic fluctuations appear to be a significant component in the solar wind-magnetosphere interaction processes. Of key importance is the intermittence of the "magnetic turbulence" as seen in ground-based and solar wind magnetic data. The methods used in this paper (estimation of flatness and multifractal spectra) are commonly used in the studies of fluid or MHD turbulence. The results show that single observatory characteristics of magnetic fluctuations are different from those of the multi-observatory AE-index. In both data sets, however, the influence of the solar wind fluctuations is recognizable. The correlation between the scaling/singularity features of solar wind magnetic fluctuations and the corresponding geomagnetic response is demonstrated in a number of cases. The results are also discussed in terms of patchy reconnection processes in the magnetopause and forced and/or self-organized criticality (F/SOC) of internal magnetosphere dynamics.

Description: The coupled interaction of long standing hydrodynamic waves with a deformable non-Newtonian seabed is examined using a two-layer model for which the upper layer fluid is inviscid and the lower layer is bi-viscous. The two-dimensional response of the system to forcing by a predominantly longitudinal (cross-shore) standing wave perturbed by a small transverse (along-shore) component is determined. With a constant yield stress in the bi-viscous lower layer, there is little amplification of these transverse per-turbations and the model response typically remains quasi-one-dimensional. However, for a bi-viscous layer with a pressure-dependent yield stress (which represents the effect that the seabed deforms less readily under compression and hence renders the rheology history dependent), the initially small transverse motions are amplified in some parameter regimes and two-dimensional, permanent bedforms are formed in the lower layer. This simple dynamical model is, therefore, able to explain the formation of permanent bedforms with significant cross- and along-shore features by predominantly cross-shore standing wave forcing.

Description: Results from an experimental study of vortices in a rotating shear layer are presented. The data are in the form of maps of the instantaneous horizontal velocity field obtained by a particle tracking technique. Two fundamentally different methods to analyse time series of these velocity fields are presented and compared. One technique is the empirical orthogonal function (EOF) analysis, and the other method describes the flow field in terms of a few individual localised vortices. The flows discussed here are time-dependent two-vortex flows, which could either be described as a global mode 2 or as a collection of four unequal vortices. The results show that, while EOF analysis is a very powerful tool to detect fairly regular travelling modes or stationary features, it cannot detect local dynamics. The vortex identification technique is very good at detecting local structures and events but cannot put them into the context of a global flow structure. The comparison of the techniques shows indications that the time-dependence found in the system for low mode numbers could arise from an interaction of the large scale, global-mode flow with a local mechanism of vortex generation and shedding at a solid boundary.

Description: Long-term evolution of westward-travelling non-local modons on the β-plane, i.e. dipolar vortices imbedded in slowly damping Rossby wave fields, is studied numerically. In the framework of the nondivergent (barotropic) model, two stages of the evolution are observed. At the first stage (for about 30 synoptic periods), the parameters and the form of the vortex practically remain constant, whereas at the second stage, vorticity filaments are emitted. Due to the filamentation, the vortex core contracts, the potential vorticity peaks of the vortex pair get closer, and the modon speeds up. In the divergent (equivalent-barotropic) model, nonlocal modons and the Lamb modon (that has no wave field outside the dipolar core) evolve much more slowly, essentially preserving the initial shape and propagation speed until about 100 synoptic periods.

Description: A model is presented for a new type of fast solitary waves which is observed in downward current regions of the auroral zone. The three-dimensional, coherent structures are electrostatic, have a positive potential, and move along the magnetic field lines with speeds on the order of the electron drift. Their parallel potential profile is flattened and cannot fit to the Gaussian shape used in previous work. We develop a detailed BGK model which includes a flattened potential and an assumed cylindrical symmetry around a centric magnetic field line. The model envisions concentric shells of trapped electrons slowly drifting azimuthally while bouncing back and forth in the parallel direction. The electron dynamics is analysed in terms of three basic motions that occur on different time scales characterized by the cyclotron frequency We , the bounce frequency wb , and the azimuthal drift frequency wg. The ordering We 〉〉 wb 〉〉 wg is required. Self-consistent distribution functions are calculated in terms of approximate constants of motion. Constraints on the parameters characterizing the amplitude and shape of the stretched solitary wave are discussed.