Pan-African high pressure granulites from SE-Kenya: Petrological and geothermobarometric evidence for a polycyclic evolution in the Mozambique belt

Two different Pan-African tectono-metamorphic events are recognised in the Taita Hill Tsavo East National Park/Galana river area, SE-Kenya (Mozambique belt) based on petrographic and geothermobarometric evidence. Structurally, this area can be subdivided into four units: (1) the easternmost part of the basement along the Galana river is characterized by subhorizontal slightly to the west and east dipping foliation planes. Migmatic paragneisses with intercalated marbles, calcsilicates and metapelites and bands of amphibolites are the dominant rock type. (2) The western part of the Galana river within the Tsavo East National Park is a ca. 25 km wide shear zone with subvertical foliation planes. The eastern part shows similar rocks as observed in unit 1, while towards west, metasedimentary units become rare and the main rock types are tonalitic gneisses with intercalated amphibolites. (3) A 10 km wide zone (Sagala Hills zone) between the strike slip zone (unit 2) and the Taita Hills (unit 4) is developed. This zone is characterized by elongated and folded felsic migmatic amphibole and garnet bearing orthogneiss bodies with intercalated bands of mafic rocks. (4) The Taita Hills are a slightly to the N dipping nappe stack. The main rock type in the Taita Hills are amphibole–biotite–plagioclase–quartz ± garnet ± clinopyroxene ± scapolite bearing migmatic gneisses with mafic bands. In the southern part, metapelites, marbles and some amphibolites are common.Although the geological structures are different in units 1 and 2, the calculated PT conditions are similar with peak PT of 760–820 °C and 7.5–9.5 kbar. Temperatures in unit 3 (Sagalla Hills zone) and unit 4 (Taita Hills) are slightly higher ca. 760–840 °C, but pressure is significantly higher, ranging from 10 to 12 kbar. Sillimanite growth around kyanite, garnet zonation pattern, mineral reaction textures, and PT calculations constrain a “clock-wise” PT-path with near isobaric cooling following the peak of metamorphism. The different PT conditions, tectonic setting, and a different age of metamorphism are evidence that units 1 and 2 (Galana river) belong to a different tectono-metamorphic event than unit 3 (Sagala Hills zone) and 4 (Taita Hills). The major shear zone (unit 2) marks a tectonic suture dividing the two different tectono-metamorphic domains. It is also likely that it played an important role during exhumation of the granulite facies rocks from units 3 and 4.     

Studying an interaction between tropical cyclones and jet streams using the numerical simulation data

Discussed are the results of studying an evolution of tropical cyclones (TCs) in the Pacific Ocean using the data of computation of ETA and WRF NMM mesoscale numerical atmospheric models. Computed are the trajectories of TCs and the fields of meteorological variables in the typhoons, of the wind speed and kinetic energy in the subtropical jet stream during the development of Parma, Melor, and Lupit typhoons. Carried out are the analysis and comparison of computed fields of pressure, wind, kinetic energy, and trajectories of TCs obtained using these models and their comparison with the actual fields. It is demonstrated that both models computed rather well the complex trajectories and the fields of wind and kinetic energy varying in the course of the interaction. Proposed is an explanation of processes taking place during the interaction between the vortices and the subtropical jet stream and the polar front.     

North Atlantic atmospheric circulation and surface wind in the Northeast of the Iberian Peninsula: uncertainty and long term downscaled variability

The variability and predictability of the surface wind field at the regional scale is explored over a complex terrain region in the northeastern Iberian Peninsula by means of a downscaling technique based on Canonical Correlation Analysis. More than a decade of observations (1992–2005) allows for calibrating and validating a statistical method that elicits the main associations between the large scale atmospheric circulation over the North Atlantic and Mediterranean areas and the regional wind field. In an initial step the downscaling model is designed by selecting parameter values from practise. To a large extent, the variability of the wind at monthly timescales is found to be governed by the large scale circulation modulated by the particular orographic features of the area. The sensitivity of the downscaling methodology to the selection of the model parameter values is explored, in a second step, by performing a systematic sampling of the parameters space, avoiding a heuristic selection. This provides a metric for the uncertainty associated with the various possible model configurations. The uncertainties associated with the model configuration are considerably dependent on the spatial variability of the wind. While the sampling of the parameters space in the model set up moderately impact estimations during the calibration period, the regional wind variability is very sensitive to the parameters selection at longer timescales. This fact illustrates that downscaling exercises based on a single configuration of parameters should be interpreted with extreme caution. The downscaling model is used to extend the estimations several centuries to the past using long datasets of sea level pressure, thereby illustrating the large temporal variability of the regional wind field from interannual to multicentennial timescales. The analysis does not evidence long term trends throughout the twentieth century, however anomalous episodes of high/low wind speeds are identified.     

Impacts of absorbing aerosols on South Asian rainfall

Anthropogenic aerosols in the lower troposphere increase the absorption and scattering of solar radiation by air and clouds, causing a warmer atmosphere and a cooler surface. It is suspected that these effects contribute to slow down the hydrological cycle. We conducted a series of numerical experiments using a limited area atmospheric model to understand the impacts of aerosol radiative forcing on the rainfall process. Experiments with different radiative conditions under an idealized setting revealed that increasing atmospheric forcing and decreasing surface forcing of radiation causes reductions in rainfall. There was no relationship of top of the atmosphere forcing to the rainfall yield. The model was then used to simulate a domain covering southern part of Sri Lanka, over for the period from November 2002 to July 2003. For a given radiative forcing, instances with lower rainfall yields showed larger fractional reductions in rainfall. The trends in seasonal rainfall observed over the site in past 30 years in a different study confirms this finding. We conclude that the negative impact of increase of anthropogenic aerosols on rainfall would be more severe on regions and seasons with lower rainfall yields. The consequences of this problem on the industries that critically depend on well-distributed rainfall like non-irrigated agriculture and on the general livelihood of societies in low-rain areas can be serious.     

Climate Change Impacts on Urban Flooding

This paper estimates changes in thepotential damage of flood events caused by increasesof CO₂ concentration in the atmosphere. It ispresented in two parts: 1. the modelling of floodfrequency and magnitude under global warming andassociated rainfall intensities and 2. the use ofgreenhouse flood data to assess changes in thevulnerability of flood prone urban areas, expressingthese in terms of direct losses.Three case studies were selected: theHawkesbury–Nepean corridor, the Queanbeyan and UpperParramatta Rivers. All three catchments are located insoutheastern Australia, near Sydney and Canberra.These were chosen because each had detailed buildingdata bases available and the localities are situatedon rivers that vary in catchment size andcharacteristics. All fall within a region that willexperience similar climate change under the availablegreenhouse scenarios. The GCMs' slab model scenariosof climate change in 2030 and 2070 will cause onlyminor changes to urban flood damage but the doubleCO₂ scenarios estimated using the StochasticWeather Generator technique will lead to significantincreases in building damage.For all the case studies, the hydrological modellingindicates that there will be increases in themagnitude and frequency of flood events under thedouble CO₂ conditions although these vary fromplace to place. However, the overall pattern of changeis that for the Upper Parramatta River the 1 in 100-year flood under currentconditions becomes the 1 in44-year event, the 1 in 35-year flood for theHawkesbury–Nepean and the 1 in 10 for Queanbeyan andCanberra. This indicates the importance of usingrainfall-runoff modelling in order to estimate changesin flood frequencies in catchments with differentphysical characteristics.     

An Operational Forecasting Model for the Variation of Mean Maximum Mixing Heights Across the Coastal Zone

Variation exists in the maximum mixing height (MMH) across the coastal zone because of major differences in heat capacity between land and sea. By assuming that synoptic and microscale effects are all small as compared to the contribution from mesoscale temperature differences, a model is proposed to explain this variation. The model states that the variation of the MMH across the coastal zone is due primarily and is linearly proportional to the difference in maximum temperature between land and sea. Since the MMH is one of the most important parameters in the computation of distribution and concentration of aerosols, water vapor, and pollutants, a simple equation is also provided for the operational forecasting of the MMH or the mixing height over the sea. All inputs into the equation are routinely available. The model has been verified by available data and a relevant field experiment.     

Nonlinear interactions of spherical Rossby modes

Weakly nonlinear triad interactions between spherical Rossby harmonics are studied. Each of the harmonics has the form AP_n^m(sin θ)exp[i(mλ − σt)], where A is an amplitude and P_n^m is the associated Legendre function. Equations for the amplitudes are derived and resonance conditions are analysed. The resonance conditions differ substantially from the usual resonance conditions on a β-plane and include a Diophantine equation and a few inequalities for the integer wavenumbers n and m of the interacting modes. Particular analytical series of solutions to the resonance conditions are constructed, which show that modes with arbitrary large wavenumbers can participate in the interactions. A numerical study of the resonance conditions reveals that no more than 21% of the Rossby harmonics can participate in the triad interactions and that chains of the interacting triads soon break. Thus precise interactions (for which the resonance conditions hold exactly) do not result in any significant redistribution of energy over the spectrum. On the other hand, approximate interactions (for which the resonance conditions hold approximately) generate an intensive energy redistribution among short Rossby modes with typical scales much smaller than the Earth's radius. Thus the energy cascade is concentrated mainly in the short-wave part of the spectrum and is very weak in the large-scale domain. The efficiency of the triad interaction of Rossby modes with scales much smaller than the Earth's radius depends strongly on the existence of the so-called interaction latitude at which the local wave-vectors and frequencies of the interacting modes satisfy resonance conditions for plane Rossby waves on the β-plane approximating the neighbourhood of the latitude. If the interaction latitude exists, the interaction is intensive; in the opposite case, it is weak.     

Synoptic and mesoscale study of a severe convective outbreak with the nonhydrostatic Global Environmental Multiscale (GEM) model

Summary ?A nonhydrostatic 4-km version of the Global Environmental Multiscale (GEM) model, with detailed microphysics included, was used to forecast the initiation, development, and structure of a tornado-producing supercell storm that occurred near Pine Lake (Alberta, Canada) on 15 July 2000. Examination of observations and comparison with conceptual models indicate that this storm is a good example of supercell storms that regularly produce summertime severe weather over Alberta. It was found that the high-resolution model was able to reproduce the early initiation of convective activity along the Rocky Mountains foothills, as well as the rapid northeastward propagation towards the Pine Lake area and the subsequent intensification into a supercell storm. The general structures of the forecasted convective system correspond well with conceptual representations of such events. Overall, this high-resolution forecast of the Pine Lake supercell storm was a significant improvement over the current operational version of the GEM model (24 km), which was not able to intensify the foothills’ convection into a supercell storm. Finally, it was found that the nonhydrostatic version of the model produces better trajectory and propagation speed of the convective system, as compared with the hydrostatic one. Received March 20, 2001; revised August 24, 2001     

A numerical model of the partitioning of trace chemical solutes during drop freezing

Partitioning of volatile chemicals among the gas, liquid, and solid phases during freezing of liquid water in clouds can impact trace chemical distributions in the troposphere and in precipitation. We describe here a numerical model of this partitioning during the freezing of a supercooled liquid drop. Our model includes the time-dependent calculation of the coupled processes of crystallization kinetics, heat transport, and solute mass transport, for a freezing hydrometeor particle. We demonstrate the model for tracer partitioning during the freezing of a 1000 μm radius drop on a 100 μm ice substrate, under a few ambient condition scenarios. The model effectively simulates particle freezing and solute transport, yielding results that are qualitatively and quantitatively consistent with previous experimental and theoretical work. Results suggest that the ice shell formation time is governed by heat loss to air and not by dendrite propagation, and that the location of ice nucleation is not important to freezing times or the effective partitioning of chemical solutes. Even for the case of nucleation at the center of the drop, we found that dendrites propagated rapidly to form surface ice. Freezing then proceeded from the outside in. Results also indicate that the solid-liquid interfacial surface area is not important to freezing times or the effective partitioning of chemical solutes, and that the rate aspects of trapping are more important than equilibrium solid-liquid partitioning to the effective partitioning resulting from freezing.     

Sensitivity of the Earth’s radiation budget to interannual variations in cloud amount

 Using two pairs of coincident long-term satellite derived cloud and earth radiation budget data sets (Nimbus-7 ERB/Nimbus-7 Cloud Climatology and ERBE Scanner/ISCCP-C2), estimates are made of the sensitivity of the top of the atmosphere radiation budget to interannual variations in the total cloud amount. Both sets of analyses indicate that the largest net warming due to interannual cloud cover changes occurs over desert regions, while the largest net cooling occurs in areas of persistent marine stratiform cloud. There is generally a large amount of cancellation between the large shortwave cooling and longwave warming effects in tropical convection regions. However, the Nimbus-7 analysis identifies an area of net warming in the tropical eastern Pacific Ocean which is shown to be associated with the 1982–83 ENSO event. In the zonal mean the Nimbus-7 data sets indicate that interannual cloud cover changes lead to a net warming at low latitudes and net cooling polewards of 25° in both hemispheres. In contrast, the analysis of the ERBE and ISCCP data sets indicates net cooling everywhere except for the Northern Hemisphere equatorwards of 20 °N. For the spatial average between 60 °N and 60 °S the ratio of the shortwave and longwave effects is 0.94 in the Nimbus-7 analysis (i.e. clouds cause a small net warming) and 1.21 in the ERBE-ISCCP analysis (i.e. a net cooling). Given their improved spatial and temporal sampling the analysis using the ERBE and ISCCP data sets should be the more reliable. However, the large differences between the two analyses still raises some issues concerning the confidence with which the sign of the effect of clouds on the radiation budget at these time scales is currently known. Received: 24 October 1995 / Accepted: 8 August 1996