The WAMME regional model intercomparison study

Results from five regional climate models (RCMs) participating in the West African Monsoon Modeling and Evaluation (WAMME) initiative are analyzed. The RCMs were driven by boundary conditions from National Center for Environmental Prediction reanalysis II data sets and observed sea-surface temperatures (SST) over four May–October seasons, (2000 and 2003–2005). In addition, the simulations were repeated with two of the RCMs, except that lateral boundary conditions were derived from a continuous global climate model (GCM) simulation forced with observed SST data. RCM and GCM simulations of precipitation, surface air temperature and circulation are compared to each other and to observational evidence. Results demonstrate a range of RCM skill in representing the mean summer climate and the timing of monsoon onset. Four of the five models generate positive precipitation biases and all simulate negative surface air temperature biases over broad areas. RCM spatial patterns of June–September mean precipitation over the Sahel achieve spatial correlations with observational analyses of about 0.90, but within two areas south of 10°N the correlations average only about 0.44. The mean spatial correlation coefficient between RCM and observed surface air temperature over West Africa is 0.88. RCMs show a range of skill in simulating seasonal mean zonal wind and meridional moisture advection and two RCMs overestimate moisture convergence over West Africa. The 0.5° computing grid enables three RCMs to detect local minima related to high topography in seasonal mean meridional moisture advection. Sensitivity to lateral boundary conditions differs between the two RCMs for which this was assessed. The benefits of dynamic downscaling the GCM seasonal climate prediction are analyzed and discussed.     

Book reviews

R. Donaghe, R. Chaney, and M. Silver, eds. 1988. Advanced Triaxial Testing of Soil and Rock, STP 977. Philadelphia: American Society for Testing and Materials. Hard Cover, 904 pp., 625 illus. $120.00.

Adrian F. Richards, ed. 1988. Vane Shear Strength Testing in Soils: Field and Laboratory Studies, STP 1014. Philadelphia: American Society for Testing and Materials. Hard Cover, 312 pp., 253 illus. $79.00.     

Electrical-hydraulic conductivity model for a weatheredfractured aquifer system of Olbanita,Lower Baringo Basin,Kenya Rift

Groundwater yield in the Kenya Rift is highly unsustainable owing to geological variability.In this study,field hydraulic characterization was performed by using geoelectric approaches.The relations between electrical-hydraulic(eh)conductivities were modeled hypothetically and calibrated empirically.Correlations were based on the stochastic models and field-scale hydraulic parameters were contingent on pore-level parameters.By considering variation in pore-size distributions over eh conduction interval,the relations were scaled-up for use at aquifer-level.Material-level electrical conductivities were determined by using Vertical Electrical Survey and hydraulic conductivities by analyzing aquifer tests of eight boreholes in the Olbanita aquifer located in Kenya rift.VES datasets were inverted by using the computer code IP2Win.The main result is that ln T=0.537(ln Fa)+3.695;the positive gradient indicating eh conduction through pore-surface networks and a proxy of weathered and clayey materials.An inverse(1/F-K)correlation is observed.Hydraulic parameters determined using such approaches may possibly contribute significantly towards sustainable yield management and planning of groundwater resources.     

Five hundred million years of punctuated addition of juvenile crust during extension in the Goochland Terrane,central Appalachian Piedmont Province

ABSTRACT

The Goochland Terrane is an enigmatic crustal block in the Appalachian Piedmont Province of central Virginia, USA. Sparse exposures of terminal Mesoproterozoic and late Neoproterozoic igneous rocks in the central Goochland Terrane offer the opportunity to investigate both the continental affinity of the terrane during the Proterozoic Eon and the timing and mechanisms of crustal growth. We apply multiple geochemical tools to these rocks: tectonic discrimination using whole-rock major and trace element abundances; whole-rock Sm-Nd isotopes; O, U-Pb, and Lu-Hf isotope analyses of spots in zircon; and measurement of O isotopes in multi-grain quartz separates. Eruption of the Sabot Amphibolite protolith is difficult to date, but we tentatively assign an age of 552 ± 11 Ma. Goochland Terrane continental crust first separated from the mantle prior to ca. 1050–1010 Ma intrusion of the Montpelier Anorthosite and the State Farm Gneiss protolith. The granitic magma that became the State Farm Gneiss protolith could have been derived entirely from partial melting of this initial Goochland Terrane crust. In contrast, the magmas that became the Montpelier Anorthosite, Neoproterozoic granitoid, and the Sabot Amphibolite were mixtures of mantle melt and preexisting Goochland Terrane crust. This production of juvenile continental crust occurred during continental extension and, eventually, rifting. The timing and compositions of terminal Mesoproterozoic magmatism in the Goochland Terrane closely match those in the nearby Blue Ridge Province. Although the compositions of the Neoproterozoic magmas in the two regions are similar, intrusion and possibly eruption occurred about 10 M.y. later in the Goochland Terrane.     

Numerical investigation of an oceanic resonant regime induced by hurricane winds

The oceanic mixed layer (OML) response to an idealized hurricane with different propagation speeds is investigated using a two-layer reduced gravity ocean model. First, the model performances are examined with respect to available observations relative to Hurricane Frances (2004). Then, 11 idealized simulations are performed with a Holland (Mon Weather Rev 108(8):1212–1218, 1980) symmetric wind profile as surface forcing with storm propagation speeds ranging from 2 to 12 m s⁻¹. By varying this parameter, the phasing between atmospheric and oceanic scales is modified. Consequently, it leads to different momentum exchanges between the hurricane and the OML and to various oceanic responses. The present study determines how OML momentum and heat budgets depend on this parameter. The kinetic energy flux due to surface wind stress is found to strongly depend on the propagation speed and on the cross-track distance from the hurricane center. A resonant regime between surface winds and near-inertial currents is clearly identified. This regime maximizes locally the energy flux into the OML. For fast-moving hurricanes (>6 m s⁻¹), the ratio of kinetic energy converted into turbulence depends only on the wind stress energy input. For slow-moving hurricanes (<6 m s⁻¹), the upwelling induced by current divergence enhances this conversion by shallowing the OML depth. Regarding the thermodynamic response, two regimes are identified with respect to the propagation speed. For slow-moving hurricanes, the upwelling combined with a sharp temperature gradient at the OML base formed in the leading part of the storm maximizes the oceanic heat loss. For fast propagation speeds, the resonance mechanism sets up the cold wake on the right side of the hurricane track. These results suggest that the propagation speed is a parameter as important as the surface wind speed to accurately describe the oceanic response to a moving hurricane.     

Using stable water isotopes to identify spatio-temporal controls on groundwater recharge in two contrasting East African aquifer systems

Understanding the spatio-temporal variability in groundwater recharge is a prerequisite to sustainable management of aquifers. Spatial analysis of groundwater stable isotopes uncovered predominant controls on groundwater recharge in the Nairobi aquifer system (NAS) and the South Coast aquifer (SC), two exemplar East African aquifers relied upon by 7 million people. We analysed 368 samples for stable isotopes and basic physico-chemical parameters. The NAS groundwater isotopes are controlled by precipitation orographic effects and enriched recharge from impounded lakes/wetlands; the SC isotopes are correlated with water-table depth influencing evapotranspiration. Global Network of Isotopes in Precipitation (GNIP) data revealed groundwater recharge during months of heavy rains in the NAS, whilst the SC experiences spatio-temporally diffuse recharge. Inferred “isoscapes” show: in the NAS, (1) direct, rapid recharge favoured by faults, well-drained soils and ample rainfall in uplands, (2) delayed recharge from impounded lakes and wetlands in mid-lands, and (3) focused, event-based recharge in floodplains; and in the SC, diffuse recharge complicated by significant water-table evapotranspiration processes.