Duplex structures in the lewis thrust sheet, crowsnest pass, rocky mountains, Alberta, Canada

The Lewis thrust sheet of the southern Canadian Rocky Mountains contains many spectacular examples of small-scale duplex structures. This paper presents the results of a detailed analysis of such structures found in the Mississippian carbonates of the Banff Formation at Crowsnest Pass, southwestern Alberta.Foreland dipping, hinterland dipping and antiformal stacked duplexes are found in the hangingwall of the Lewis thrust. Out-of-sequence thrusts, back thrusts and folds that push out of the plane of the cross-section, termed lateral lobes, give rise to complex internal geometries. Dominant slip vectors are towards 080–090° but the complex fault geometries have generated significant variations in slip away from this direction. The duplex structures occur as discrete thrust fault-bounded packages with each package having different slip vectors. The panels above and below the duplex structures show consistent slip vectors towards 080–090° whereas the duplexes exhibit a wide scatter of slip vectors from 350–160°. The stacking of duplexes with many horses can be likened to the stacking of many inverted soup bowls, herein termed turtle back structures, and will involve a wide scatter of slip directions, particularly if the horses are of limited lateral extent. Such a stacking mechanism involving out-of-section movement invalidates the assumption of two-dimensional plane strain in the plane of the cross-section that contains the regional tectonic transport direction. Correctly balanced cross-sections cannot be constructed through such stacked duplex structures as described in this paper.     

Electrochemical technique for exploration of base metal sulphides

A method based on the principles of electrochemistry has been applied for the first time with indigenous equipment in the search for base-metal sulphides, following a field exploration technique known as ‘CHIM — 10’, which is in use in the U.S.S.R. for the last few years. The technique utilizes the electro-positive property of the metallic ions. When a large section of the ground is artificially electrolyzed by the introduction of a direct current, the metallic ions within the moist soil profile close to the mineralized zone tend to move towards the energizing cathodes of a powerful widely spaced electrode array. A laboratory model experiment was conducted first which showed encouraging results. Subsequently, a prototype field unit was fabricated and employed to examine its effectiveness in the field on a soil profile over the base-metal sulphide zones of Aladahalli area, Hassan District, Karnataka, where the geology of the deposit is well known.The results obtained during the field trials were quite good and in full conformity with the known geology. However, detailed evaluation of those data after applying necessary corrections for the different variable parameters of the system is yet to be done. Work is in progress to refine the technique with a view to standardizing and developing its use for qualitative and quantitative assessment of a concealed base-metal deposit in a given area, specially where conventional geochemical exploration methods are not suitable.The paper deals with the details of the technique and discusses the results obtained during the trial run in the field.     

North-Eastward propagating 25–50-day mode and its link with Indian summer monsoon activity

Summary Spatio-temporal characteristics of the 25–50-day oscillations are investigated using the empirical orthogonal function (EOF) decomposition and spectral analysis with the Maximum Entropy Method (MEM). Daily pressure values over India during 1978 are used in this study. Power spectra of the temporal coefficients of the two leading EOFs show the existence of a low frequency oscillation with a period range 25–50-day over all India. An analysis using extended empirical orthogonal function (EEOF) indicates North-Eastward propagating 25–50-day mode. The analysis EOF has allowed to establish a relationship between the 25–50-day oscillation and the activity of the summer monsoon. The North-Eastward propagation of this mode is also confirmed by the simple EOF analysis.With 9 Figures     

On the Determination of the Neutral Drag Coefficient in the Convective Boundary Layer

Based on the idea that free convection can be considered as a particular case of forced convection, where the gusts driven by the large-scale eddies are scaled with the Deardorff convective velocity scale, a new formulation for the neutral drag coefficient, C_Dn, in the convective boundary layer (CBL) is derived. It is shown that (i) a concept of C_Dn can still be used under strongly unstable conditions including a pure free-convection regime even when no logarithmic portion in the velocity profile exists; (ii) gustiness corrections must be applied for rational calculations of C_Dn; and (iii) the stratification function used in the derivation of C_Dn should satisfy the theoretical free-convection limit. The new formulation is compared with the traditional relationship for C_Dn, and data collected over the sea (during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) and the San Clemente Ocean Probing Experiment (SCOPE)) and over land (during the BOREX-95 experiment) are used to illustrate the difference between the new and traditional formulations. Compared to the new approach, the traditional formulation strongly overestimates C_Dn and z_o in the CBL for mean wind speed less than about 2 m s^-1. The new approach also clarifies several contradictory results from earlier works. Some aspects related to an alternate definition of the neutral drag coefficient and the wind speed and the stress averaging procedure are considered.     

Boundary-layer structure near the cold front of a marine cyclone during “ERICA”

The boundary layer in the warm sector of a moderately deepening winter cyclone during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) is studied near the cold front. Data from the National Center for Atmospheric Research Electra research aircraft are used to examine mean and turbulence quantities. The aircraft data and supplemental data from ships, drifting buoys and moored buoys reveal an equivalent-barotropic pressure field. The area is found to be dominated by gradients in temperature and in turbulent fluxes, with changes occurring over 100 km horizontally being comparable to changes over 350 m vertically. The horizontal components of the gradients are found to be a maximum in a direction perpendicular to the front. Cross-sections perpendicular to the front are used to illustrate boundary-layer structure. Profiles of wind speed, stress, wind direction and stress direction are estimated from an Ekman model that is modified to take into account the equivalent-barotropic pressure field. Comparison of profiles from the model to the aircraft-measured data show reasonable agreement far from the front (100 km) when the model uses a constant eddy viscosity of approximately 6 kg m^–1 s^–1. Near the front there is less agreement with the model. Profiles of turbulent fluxes of momentum, heat and latent heat are divergent, with along-wind momentum flux negative and decreasing upward, cross-wind momentum flux positive and increasing upward, and heat flux and latent heat flux small, positive and decreasing upward. Far from the front, the turbulent kinetic energy budget shows that dissipation balances shear production. However, near-front behavior has an imbalance at low altitude, with shear production appearing as a TKE sink.     

Single-Column Model Intercomparison for a Stably Stratified Atmospheric Boundary Layer

The parameterization of the stably stratified atmospheric boundary layer is a difficult issue, having a significant impact on medium-range weather forecasts and climate integrations. To pursue this further, a moderately stratified Arctic case is simulated by nineteen single-column turbulence schemes. Statistics from a large-eddy simulation intercomparison made for the same case by eleven different models are used as a guiding reference. The single-column parameterizations include research and operational schemes from major forecast and climate research centres. Results from first-order schemes, a large number of turbulence kinetic energy closures, and other models were used. There is a large spread in the results; in general, the operational schemes mix over a deeper layer than the research schemes, and the turbulence kinetic energy and other higher-order closures give results closer to the statistics obtained from the large-eddy simulations. The sensitivities of the schemes to the parameters of their turbulence closures are partially explored.     

Further results from a laboratory model of the convective planetary boundary layer

The turbulence in a laboratory convective mixed layer is probed more extensively than in the preliminary study of Willis and Deardorff (1974), and results presented. Turbulence intensities, spectra and probability distributions using mixed-layer scaling compare favorably with similarly scaled field measurements not available or plentiful in 1974. However, the velocity spectra in the convection tank exhibit only a short inertial subrange due to the close proximity of the dissipation subrange to the energy-containing range.The turbulence budget suggests that the convergence of the vertical transport of pressure fluctuations is a rather important term.Results on the entrainment rate are also presented, using both mixed-layer scaling and local interfacial scaling.     

Climate trends of the North American prairie pothole region 1906–2000

The Prairie Pothole Region (PPR) is unique to North America. Its millions of wetlands and abundant ecosystem goods and services are highly sensitive to wide variations of temperature and precipitation in time and space characteristic of a strongly continental climate. Precipitation and temperature gradients across the PPR are orthogonal to each other. Precipitation nearly triples from west to east from approximately 300 mm/year to 900 mm/year, while mean annual temperature ranges from approximately 1°C in the north to nearly 10°C in the south. Twentieth-century weather records for 18 PPR weather stations representing 6 ecoregions revealed several trends. The climate generally has been getting warmer and wetter and the diurnal temperature range has decreased. Minimum daily temperatures warmed by 1.0°C, while maximum daily temperatures cooled by 0.15°C. Minimum temperature warmed more in winter than in summer, while maximum temperature cooled in summer and warmed in winter. Average annual precipitation increased by 49 mm or 9%. Palmer Drought Severity Index (PDSI) trends reflected increasing moisture availability for most weather stations; however, several stations in the western Canadian Prairies recorded effectively drier conditions. The east-west moisture gradient steepened during the twentieth century with stations in the west becoming drier and stations in the east becoming wetter. If the moisture gradient continues to steepen, the area of productive wetland ecosystems will shrink. Consequences for wetlands would be especially severe if the future climate does not provide supplemental moisture to offset higher evaporative demand.     

Changes in temperature and precipitation extremes in the CMIP5 ensemble

Twenty-year temperature and precipitation extremes and their projected future changes are evaluated in an ensemble of climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5), updating a similar study based on the CMIP3 ensemble. The projected changes are documented for three radiative forcing scenarios. The performance of the CMIP5 models in simulating 20-year temperature and precipitation extremes is comparable to that of the CMIP3 ensemble. The models simulate late 20th century warm extremes reasonably well, compared to estimates from reanalyses. The model discrepancies in simulating cold extremes are generally larger than those for warm extremes. Simulated late 20th century precipitation extremes are plausible in the extratropics but uncertainty in extreme precipitation in the tropics and subtropics remains very large, both in the models and the observationally-constrained datasets. Consistent with CMIP3 results, CMIP5 cold extremes generally warm faster than warm extremes, mainly in regions where snow and sea-ice retreat with global warming. There are tropical and subtropical regions where warming rates of warm extremes exceed those of cold extremes. Relative changes in the intensity of precipitation extremes generally exceed relative changes in annual mean precipitation. The corresponding waiting times for late 20th century extreme precipitation events are reduced almost everywhere, except for a few subtropical regions. The CMIP5 planetary sensitivity in extreme precipitation is about 6 %/°C, with generally lower values over extratropical land.