On the conversion between mean and eddy components of potential and kinetic energy in isentropic and isopycnic coordinates

Equations for the mean and eddy kinetic energy and for the mean and eddy potential energy in isopycnic and isentropic coordinates are derived, and conversion terms between the four energy forms are identified. Contrary to Lorenz' isobaric formulae, a conversion term between mean kinetic and eddy potential energy is found in the isopycnic/isentropic framework; furthermore, no term governing direct conversion between eddy and mean potential energy exists in isopycnic coordinates. The option of “re-routing” some energy conversion processes to eliminate the formal difference between the isopycnic/isentropic and the isobaric viewpoint is discussed. A numerical simulation of baroclinic instability in an isentropic coordinate model is used to illustrate the energy conversion processes.     

An improved Lorentzian technique for evaluating resonance characteristics of the Earth-ionosphere cavity

The effectiveness of various formulations of the Lorentzian procedure for estimating Schumann resonance (SR) characteristics of the Earth-ionosphere waveguide from transient electromagnetic signals is tested in the limits of a simplified, spherically uniform model of the resonator. It is shown that the major improvement, in comparison with the “classic” Lorentzian formulation, is achieved by consideration of the intra-modal phase interference. The effect of the “limited frequency dispersion” inherent in the “classic” Lorentzian approach – that is of substantial importance at the lowest SR modes – can be effectively neutralized by interpolating the values for the propagation parameter between the adjacent modal frequencies. Several practical aspects of applying the Lorentzian procedure to transient signals are also discussed.     

Refraction of Rossby waves on a multiple β-plane

A multiple β-plane is introduced to explore the relation between plane and spherical Rossby waves. The fundamental problem, the refraction of a plane Rossby wave across a discontinuity in β, is solved. It is shown that refraction on the multiple β-plane agrees in the limit with refraction on the full sphere only if a suitable correction is made for the geometric distortion of the β-plane. The full spherical modes of Rossby waves trapped in a band about the equator (Longuet-Higgins, 1964) have their counterpart in a simple model consisting of an “equatorial” β-plane bounded above and below by “polar” β-planes.     

Some economics of ‘dangerous’ climate change: Reflections on the Stern Review

The Stern Review on the Economics of Climate Change concluded that there can be “no doubt” the economic risks of business-as-usual (BAU) climate change are “very severe” [Stern, 2006. The Economics of Climate Change. HM Treasury, London, p. 188]. The total cost of climate change was estimated to be equivalent to a one-off, permanent 5–20% loss in global mean per-capita consumption today. And the marginal damage cost of a tonne of carbon emitted today was estimated to be around $312 [p. 344]. Both of these estimates are higher than most reported in the previous literature. Subsequently, a number of critiques have appeared, arguing that discounting is the principal explanation for this discrepancy. Discounting is important, but in this paper we emphasise that how one approaches the economics of risk and uncertainty, and how one attempts to model the very closely related issue of low-probability/high-damage scenarios (which we connect to the recent discussion of ‘dangerous’ climate change), can matter just as much. We demonstrate these arguments empirically, using the same models applied in the Stern Review. Together, the issues of risk and uncertainty on the one hand, and ‘dangerous’ climate change on the other, raise very strongly questions about the limits of a welfare-economic approach, where the loss of natural capital might be irreversible and impossible to compensate. Thus we also critically reflect on the state-of-the-art in integrated assessment modelling. There will always be an imperative to carry out integrated assessment modelling, bringing together scientific ‘fact’ and value judgement systematically. But we agree with those cautioning against a literal interpretation of current estimates. Ironically, the Stern Review is one of those voices. A fixation with cost-benefit analysis misses the point that arguments for stabilisation should, and are, built on broader foundations.     

Thermal radiative fluxes through inhomogeneous cloud fields: a sensitivity study using a new stochastic cloud generator

We analyze the effects of flat and bumpy top, fractional and internally inhomogeneous cloud layers on large area-averaged thermal radiative fluxes. Inhomogeneous clouds are generated by a new stochastic model: the tree-driven mass accumulation process (tdMAP). This model is able to provide stratocumulus and cumulus cloud fields with properties close to those observed in real clouds. A sensitivity study of cloud parameters is done by analyzing differences between 3D fluxes simulated by the spherical harmonic discrete ordinate method and three “standard” models likely to be used in general circulation models: plane-parallel homogeneous cloud model (PPH), PPH with fractional cloud coverage model (FCPPH) and independent pixel approximation model (IPA). We show that thermal fluxes are strong functions of fractional cloud coverage, mean optical depth, mean geometrical thickness and cloud base altitude. Fluctuations of “in-cloud” horizontal variability in optical depth and cloud-top bumps have negligible effects in the whole. We also showed that PPH, FCPPH and IPA models are not suitable to compute thermal fluxes of flat top fractional inhomogeneous cloud layer, except for completely overcast cloud. This implies that horizontal transport of photon at thermal wavelengths is important when cloudy cells are separated by optically thin regions.     

The influence of the coast on the dynamics of upwelling fronts: Part I. Laboratory experiments

We describe laboratory experiments on the instability and later evolution of a front in a two-layer rotating fluid. In particular, we focus on the influence of a nearby boundary on instability growth and eddy formation. The front is produced through the adjustment of a buoyant fluid initially confined within a bottomless cylinder. Typically a front in quasi-cyclostrophic balance establishes after two rotation periods, after which it becomes unstable. Measurements of the velocity and vorticity fields at the surface are made which provide detailed information on the evolution of the front as the instability grows to finite amplitude. We focus on the time evolution of the vorticity and distinguish between the cyclonic and anticyclonic components. The spatial averages of the cyclonic and anticyclonic vorticity first grow exponentially. This growth saturates when eddies form and are advected across the front. The growth rate depends upon two nondimensional parameters: the width W of the upwelling region in units of the internal radius of deformation and the depth ratio δ between the two layers. Measurements of the growth rates for the average of the cyclonic and anticyclonic vorticity are compared to the values inferred from a simplified model for baroclinic instability. A good agreement is obtained when the front develops far from the boundary (i.e. W1). However, the agreement is only qualitative when the front is near the boundary (i.e. W1). We find that, as W decreases, the growth of cyclonic eddies consisting of dense—“coastal”—water is enhanced compared to that of anticyclonic vorticity consisting of buoyant—“off-shore”—water. This crucial effect of the boundary with respect to the instability of the front has significant impact on exchanges across the front.     

Analyses and numerical modeling of a polar low over the Japan Sea on 19 December 2003

A meso-α-scale polar low was observed over the Japan Sea on 19 December 2003. It initialed around 11 UTC over the northwestern part of the Japan Sea within a synoptic-scale parent low under the influence of baroclinic environment and disappeared over the eastern edge of Japan Islands with a lifetime of about 20 h. It is of interest that this polar low had “concentric eye-walls” and “warm core” structure at its mature stage. The evolutionary process and spatial structure of this polar low were investigated by using almost all available observational data, including the Geostationary Operational Environmental Satellite (GOES)-9, the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery, the Final Analyses (FNL) data issued by National Centers for Environmental Prediction (NCEP), the surface observational data and the 9-station sounding data of Japan Islands. In order to study its development mechanism, a 24-h numerical simulation using the version 4.4 of the Regional Atmospheric Modeling System (RAMS) starting from 12 UTC 19 December 2003 with an 8 km × 8 km resolution was performed. It is shown that the RAMS model reproduced the main features of the polar low reasonably well. The vorticity budget analyses indicate that the stretching term is the major contributor for the vorticity increase of the polar low. The baroclinic background seems to play significant role for the initial development of this polar low. However, the effect of the diabatic heating for its later development is also significant.     

The climatology of tornadoes and waterspouts in Italy

In this work 10 years of reports collected by weather amateurs are used to define a preliminary climatology of tornadoes and waterspouts in Italy. The results show behaviors different from those observed in other countries. Generally, tornadoes and waterspouts are more frequent in late summer and autumn than in the other seasons. The seasonality of tornadoes and waterspouts appears different for different Italian zones, in particular in the Po Valley and Friulian plain and coast (south to the Alps) tornadoes and waterspouts are more frequent in spring and early summer while in the Tirrenian and Ionian coasts (western and southern Italy), tornadoes and waterspouts are more frequent in late summer and autumn. As observed in other studies (Brooks, H., E. and Doswell, C. A. III, 2001. Some aspects of the international climatology of tornadoes by damage classification. Atmos. Res., 56, 191–201.) Italian tornadoes and waterspouts are statistically weaker than in other countries but this difference cannot be completely ascribed to the presence of waterspouts. The “CAPE Storm-Relative-Helicity diagrams” and “Shear Magnitude diagrams” obtained for Italian tornadoes and waterspouts show different characteristics than those obtained for US. The cause of these differences is still unknown, it can rely in the sample selection (problems with the concept of proximity sounding) or in a real climatic effect.     

A new model of tropical waves incorporating momentum mixing by cumulus convection

A comparison is made between the magnitudes of observed large-scale weather waves over the tropical Pacific and the magnitudes of the corresponding waves, predicted by wave-CISK theories, which are driven by the observed amount of latent heating (i.e., precipitation). The theoretical wave fields of meridional velocity, vorticity, and temperature rate are shown to exceed the observed quantities by an order of magnitude. An attempt is made to simulate the observed balance between the diabatic heating and adiabatic cooling within the context of the inviscid theories. For a broad class of heating profiles, geometries and basic states, it is found that this compensation without temperature change cannot be satisfactorily modelled, regardless of the vertical shape of the heating, when the vertical wavelength of the disturbance exceeds about 6 km.Scale analysis demonstrates that an important dynamical term has been neglected in the inviscid models, viz., the vertical transport of horizontal momentum by cumulus clouds. When this process is included in the wave model, velocities, relative vorticity, and the time rate of temperature change are all comparable to the observed values. The general behavior of a system where both forcing and cumulus “friction” are proportional to each other has not been previously examined. We find the behavior of such a system to have several novel features. For example, we find that for a wide range of precipitation amplitudes (from 1/4 to 4 times the precipitation amplitudes of waves in the western Pacific) we get essentially constant amplitudes for wind. The implications of this and other features for various aspects of tropical wave modelling are discussed.     

A comparison of two methods for the assimilation of altimeter data into a shallow-water model

Two simple methods for assimilating ocean surface pressure data into a three-layer adiabatic primitive equation model are tested and compared using a twin-experiment approach. One of the methods is based on a recently presented direct insertion scheme using a quasi-geostrophic model. The method explicitly avoids making changes to the potential vorticity fields in the lower layers and the vertical current structure after assimilation is determined by this criterion. The modification required for a primitive equation framework are discussed and a comparison is made with the more familiar ‘nudging’ method of assimilation. We use root mean square errors to quantify the response of the model to the assimilation of a single complete surface pressure field and also to repeated or intermittent data available every 20 days for a period of 1 year. The two methods are almost equally effective in the short term, although the direct insertion scheme appears to be more effective in the longer term. It is suggested that this is because insertion maintains a geostrophic balance at all levels and thus avoids the generation of internal gravity waves which are needed when nudging is used to restore geostrophic currents in the deeper layers.     

The Kuroshio Current System as a jet and twin “relative” recirculation gyres embedded in the Sverdrup circulation

By analyzing the results of a realistic ocean general circulation model (OGCM) and conducting a series of idealized OGCM experiments, the dynamics of the Kuroshio Current System is examined. In the realistic configuration, the Kuroshio Current System is successfully simulated when the horizontal resolution of OGCMs is increased from 1/2° to 1/10°. The difference between the two experiments shows a jet, the model’s Kuroshio Extension, and a pair of cyclonic and anticyclonic, “relative,” recirculation gyres (RRGs) on the northern and southern flanks of the jet. We call them recirculation gyres because they share some features with ordinary recirculation gyres in previous studies, and we add the adjective “relative” to emphasize that they may not be apparent in the total field. Similar zonal jet and RRGs are obtained also in the idealized model with a rectangular basin and a flat bottom with a horizontal resolution of 1/6°. The northern RRG is generated by the injection of high potential vorticity (PV) created in the viscous sublayer of the western boundary current, indicating the importance of a no-slip boundary condition. Since there is no streamline with such high PV in the Sverdrup interior, the eastward current in the northern RRG region has to lose its PV anomaly by viscosity before connecting to the interior. In the setup stage this injection of high PV is carried out by many eddies generated from the instability of the western boundary current. This high PV generates the northern RRG, which induces the separation of the western boundary current and the formation of the zonal jet. In the equilibrium state, the anomalous high PV values created in the viscous sublayer are carried eastward in the northern flank of the zonal jet. The southern RRG is due to the classical Rhines–Young mechanism, where low PV values are advected northward within the western boundary inertial sublayer, and closed, PV-conserving streamlines form to the south of the Kuroshio Extension, allowing slow homogenization of the low PV anomalies. The westward-flowing southern branch of this southern RRG stabilizes the inertial western boundary current and prevents its separation in the northern half of the Sverdrup subtropical gyre, where the western boundary current is unstable without the stabilizing effect of the southern RRG. Therefore, in the equilibrium state, the southern RRG should be located just to the north of the center of the Sverdrup subtropical gyre, which is defined as the latitude of the Sverdrup streamfunction maximum. The zonal jet (the Kuroshio Extension) and the northern RRG gyre are formed to the north of the southern RRG. This is our central result. This hypothesis is confirmed by a series of sensitivity experiments where the location of the center of the Sverdrup subtropical gyre is changed without changing the boundaries of the subtropical gyre. The locations of the zonal jets in the observed Kuroshio Current System and Gulf Stream are consistent as well. Sensitivities of the model Kuroshio Current System are also discussed with regard to the horizontal viscosity, strength of the wind stress, and coastline.     

Microhydrodynamics of flotation processes in the sea surface layer

The uppermost surface of the ocean forms a peculiarly important ecosystem, the sea surface microlayer (SML). Comprising the top 1–1000 μm of the ocean surface, the SML concentrates many chemical substances, particularly those that are surface active. Important economically as a nursery for fish eggs and larvae, the SML unfortunately is also especially vulnerable to pollution. Contaminants that settle out from the air, have low solubility, or attach to floatable matter tend to accumulate in the SML.Bubbles contribute prominently to the dynamics of air–sea exchanges, playing an important role in geochemical cycling of material in the upper ocean and SML. In addition to the movement of bubbles, the development of a bubble cloud interrelates with the single particle dynamics of all other bubbles and particles. In the early sixties, several in situ oceanographic techniques revealed an “unbelievably immense” number of coastal bubbles of radius 15–300 μm. The spatial and temporal variation of bubble numbers were studied; acoustical oceanographers now use bubbles as tracers to determine ocean processes near the ocean surface. Sea state and rain noises have both been definitively ascribed to the radiation from huge numbers of infant micro bubbles [The Acoustic Bubble. Academic Press, San Diego].Our research programme aims at constructing a hydrodynamic model for particle transport processes occurring at the microscale, in multi-phase flotation suspensions. Current research addresses bubble and floc microhydrodynamics as building blocks for a microscale transport model. This paper reviews sea surface transport processes in the microlayer and the lower atmosphere, and identifies those amenable to microhydrodynamic modelling and simulation. It presents preliminary simulation results including the multi-body hydrodynamic mobility functions for the modelling of “dynamic bubble filters” and floc suspensions. Hydrodynamic interactions versus spatial anisotropy and size of particle clouds are investigated.     

Improvements in the detection efficiency model for the Brazilian lightning detection network (BrasilDAT)

The detection efficiency (DE) is the most important performance gauge of a lightning detection network (LDN). Moreover, the main motivation for evaluating the DE of a LDN is to separate the geographical variations of the CG lightning parameters from the variations regarding the network performance. A review of previous relative DE techniques and simple methods to correct the cloud-to-ground (CG) lightning flash density maps is presented. In addition, recent improvements in the flash DE model for the Brazilian lightning detection network (BrasilDAT) are discussed. The DE estimated values are based on the sensor individual DE probability functions, which are derived from a large amount of CG stroke data provided by the network considering different distances from the sensor and specific peak current ranges. The new approach provides better results when compared with the previous developments, since the calculation of the sensor DE probability functions neglects the lightning data provided by the minimum number of reporting sensors. Hence it is possible to minimize the unrealistic enhancement of the DE closer to the network boundaries (“border effect”) without affecting significantly the performance inside the network. The main result is a more realistic correction of the CG flash density maps, particularly at the outermost network areas, leading to an improvement in the model sensitivity.     

Continuing current in multiple channel cloud-to-ground lightning

In this study we analyze the effects of continuing current initiated by strokes following a new channel to ground in multiple stroke flashes using high-speed video records, electric field measurements from a fast antenna and lightning detection network data. We observed that the long continuing current initiated by a stroke that follows a new channel also obeys the pattern in the initiation of long continuing current suggested by Rakov and Uman [Rakov, V.A., Uman, M.A., 2003. Lightning: Physics and Effects, 687pp., Cambridge Univ. Press, New York.]. We also verify that the statement of Rakov and Uman [Rakov, V.A. and Uman, M.A., 1990. Some properties of negative cloud-to-ground lightning flashes versus stroke order, Journal of Geophysical Research. 95, 5447–5453.] reporting that: “...strokes initiating long continuing currents tend to have lower initial electric field peak than regular strokes” is valid for strokes that create a new channel to ground and are followed by long continuing current (CC). Apparently the reduction of peak current value (I_p) when the stroke is followed by a long CC is stronger than the I_p increase that is commonly observed when strokes follow a new channel. We also find that the “exclusion zone” proposed by Saba et al. [Saba, M.M.F., Pinto, O. Jr., Ballarotti, M.G., 2006a. Relation between lightning return stroke peak current and following continuing current, Geophysical Research Letters 33, L23807, doi:10.1029/2006GL027455.] is valid for new channels initiating CC, and finally we verify that a number of strokes in the same channel larger than four or the existence of a long CC current do not always consolidate the channel in a multiple stroke flash.     

Formation of spring warm water southwest of the Philippine Islands: Winter monsoon wake effects

From satellite observations and the reanalysis data, the late spring formation of warm water with temperature higher than 30 °C to the southwest of the Philippine Islands (8–18°N, 115–120°E) is investigated. Our analysis suggests that the blockage of the winter monsoon by the Philippine Islands results in this “Luzon warm water” (LWW) to the southwest of the Luzon Island and the “Vietnam cold tongue” (VCT) to the southeast of the Vietnam coast during winter and early spring in the South China Sea (SCS). The VCT is formed by the southward cold advection by the western boundary current and surface heat loss in the SCS. During the winter monsoon, the LWW first forms due to weak winds southwest of the Philippine Islands and the countering effect of warm Ekman advection against cold geostrophic advection. In spring its temperature exceeds 30 °C (LWW30), helped by strong solar radiation and the winter monsoon wake effect lee of the Philippine Islands. With the winter monsoon weakening, LWW30 extends southwestward in late spring but disappears quickly after the summer monsoon onset. Reduced latent heat flux in the winter monsoon wake is the dominant factor for the spring fast warming southwest of the Philippine Islands.Both VCT and LWW persist from winter to early spring as the Philippine Islands block the winter monsoon. Their interannual variations are correlated with the variation of the LWW30 since the blockage of the winter monsoon by the Philippine Islands modifies surface latent heat flux and ocean advection from winter to early spring. These results strongly suggest that the LWW30 is a result of land–sea–winter monsoon interaction.     

Tropical Deforestation and the Kyoto Protocol

The current annual rates of tropical deforestation from Brazil and Indonesia alone would equal four-fifths of the emissions reductions gained by implementing the Kyoto Protocol in its first commitment period, jeopardizing the goal of Protocol to avoid “dangerous anthropogenic interference” with the climate system. We propose the novel concept of “compensated reduction”, whereby countries that elect to reduce national level deforestation to below a previously determined historical level would receive post facto compensation, and commit to stabilize or further reduce deforestation in the future. Such a program could create large-scale incentives to reduce tropical deforestation, as well as for broader developing country participation in the Kyoto Protocol, and leverage support for the continuity of the Protocol beyond the 2008–2012 first commitment period.     

Spatial variation of extreme values for freezing rain

It is still a challenge today to get the statistical approach accepted by some transmission line engineers, namely for freezing rain icing loads. They see heavy icing storms as rare events and they still believe that in this case a sound engineering judgment is better than poor statistics. When they are convinced that statistics are good, such as for the flood problem, they do not hesitate to accept the probabilistic approach. Developing extreme values distributions of freezing-rain icing is however a difficult task. Because of the relatively small dimension of major freezing rain storms, freezing-rain icing is not a continuous “variate” at a particular site.One way of improving the statistics is to take simple icing measurements on a fine grid stations network. After only 17 years of measurements with Passive Ice Meters, good fit of extreme values is possible if we use “mesh”extreme values instead of “station” ones. Before pooling extreme values into a “mesh” or “region”distribution, some attention must be given to the homogeneity of the data within a given area. This has been done for four different regions of the province of Québec with very conclusive results. A mesh of about 50 km seems adequate with a temporal resolution of 12 h. Since transmission lines are spatial constructions, this finding can easily satisfy the need of the design engineers in defining icing loads. And, if some caution is given to identify special topographical features, a better knowledge of the spatial variation of icing makes also possible better line routing.     

Coastal-trapped waves with finite bottom friction

Coastal-trapped waves with finite-amplitude bottom friction are explored. “Finite-amplitude” in this context means that the bottom stresses are large enough to change the wave modal structure. The importance of bottom friction is measured by the nondimensional number r/(ωh), where r is a bottom resistance coefficient, ω the wave frequency and h the water depth. Increasing bottom drag causes free wave modes to adjust by having their amplitude maxima for alongshore current translate offshore to the point that, with relatively large bottom stress, the alongshore current variance is trapped entirely on the slope, even though pressure variations remain substantial right up to the coast. In conjunction with these adjustments, wave frequency, hence propagation speed, varies and the wave damping is usually less than would be expected based on a weak-friction perturbation calculation. Stronger density stratification increases wave damping, all else being the same. A mean alongshore flow can strongly affect modal structure and wave damping, although general trends are difficult to discern. Results suggest that bottom friction may cause an observed tendency for lower frequency alongshore current fluctuations to become relatively more important with distance offshore.     

A comparative study of observed and GCM-simulated turbulent surface fluxes at the positions of Atlantic weatherships

If oceanic models are to be driven with transient atmospheric input, data from standard daily analyses or from an atmospheric GCM simulation can be used. The question arises whether these “FF-data” (FF = field forcing) are appropriate to be used as a realistic oceanic forcing in the form of wind stress and turbulent heat fluxes.A series of different FF-data sets is compared with respective “LF-data” (LF = local forcing) derived from in situ weathership observations. We believe these LF data to be the most accurate and reliable long year maritime time series. The study is restricted to 8 Atlantic weatherships and January conditions and to fields obtained with the Hamburg University GCM or derived from analyses of the German Weather Service (DWD).It turns out that DWD based FF data sets are suitable only if long year mean values are required. In general, the interannual and synoptic scale variability is too small for all FF data sets. With respect to the windstress, the empirical formulae to obtain the surface wind (from the sea level pressure field) together with the usage of a windspread dependent drag coefficient yield the best though still unsatisfying results. The approach using generalized similarity theory gives worse results with respect to the synoptic scale and interannual variability.The GCM simulated data set is systematically biased over wide regions which is partly due to a shift in the model's quasistationary Icelandic Low and an increased temperature at the model's lowest level. The transients are simulated at some positions even poorer than those analysed by the DWD, but at other positions superior though still weaker than the LF data's.     

Dealing with displacement: Can “social protection” facilitate long-term adaptation to climate change?

This article explores the possibilities of using social protection to manage and reduce the risks of forced displacement resulting from climate change. It reviews the relevant literature on migration, disasters and climate change, and constructs a model through which international policies may be used to encourage resettlement options that support the capabilities and entitlements of poor and vulnerable populations. By distinguishing between rapid-onset disasters and long-term environmental change, it explores the ways in which cash transfers, asset transfers and conditional cash transfers may be used to break the cycle of vulnerability, destitution and distress migration that can occur during times of severe environmental stress. An important distinction is made between “economic migration,” which implies that households have at their disposal an opportunity to engage in forward-looking analysis about the ways in which they will invest household resources and “distress migration,” which implies that household decisions about investment and migration are largely ad hoc responses to external environmental processes and events. The article reviews recent discussions about the prospects of revising the international refugee regime, and identifies the opportunities and challenges of using social protection to support household decisions that can facilitate economic migration over the long-term.