Thermal structure and circulation in the great lakes

Abstract

Large enough to include many oceanic phenomena, the Laurentian Great Lakes are more accurately described as inland seas. With the exception of the shallow Western Basin of Lake Erie, the lakes are thermally stratified in summer, homogeneous in winter, with average temperatures passing through the temperature of maximum density of fresh water (4°C) in both the spring and the fall. The circulation is mainly powered by the wind but is strongly modified by thermal stratification and basin geometry. Effects of the earth's rotation are present in all large‐scale flows. Current speeds are typically 10 cm s^?1; they are too small, with rare exceptions, to present difficulties to navigation but a knowledge of the patterns of water movement is essential for interpreting the behaviour of these valuable lakes as complex ecosystems. This paper will review more than a century of physical study of the Great Lakes.     

Application of remote sensing for vegetation mapping — A case study along the northern coastal districts of andhra pradesh

The study deals with the application of Remote Sensing for delineating various vegetation types along the three coastal districts of Andhra Pradesh. The satellite imagery of the region is divided into three types depending upon tone and texture. Each type is correlated with the ground truth. The species composition has been given and discussed.     

Remote Sensing and Geographical Information System for canopy cover mapping

Availability of remote sensing data from earth observation satellites has made it convenient to map and monitor land use/land cover at regional to local scales. A land cover map is very critical for a various planning activities including watershed planning. The spectral and spatial resolutions are major constraints for mapping the crop resources at microlevel. The cropping pattern zones have been mapped using the false color composite, physiography, irrigation and toposheets. The IRS LISS-III data is classified into various categories depending on spectral reflectance from crop canopy and are overlaid on cropping zones map. The re-classified resultant map provides land use/land cover information including dominant cropping systems. The canopy cover is estimated monthly considering the crop calendar for the area.     

Check dam positioning by prioritization of micro-watersheds using SYI model and morphometric analysis — Remote sensing and GIS perspective

Sediment Yield Index (SYI) model and results of morphometric analysis have been used to prioritize watersheds and to locate sites for checkdam positioning in Tarafeni watershed in Midnapur district. West Bengal. Various thematic maps such as land use/land cover, slope, drainage, soil etc. were prepared from 1RS ID LISS III digital data, SOI toposheets of 1:50,000 scale and other reference maps. Morphometric parameters such as bifurcation ratio (R_b). drainage density (D_d), texture ratio (T), length of overland flow (L_o), stream frequency (F_u), compactness coefficient (C_c), circularity ratio (R_c), elongation ratio (E_r), shape factor (B_s) and form factor (R_f) were computed. Automated demarcation of prioritization of micro-watersheds was done by using GIS overlaying technique by assigning weight factors to all the identified features in each thematic map and ranks were assigned to the morphometric parameters. Five categories of priority viz., very high, high, medium, low and very low, were given to all the watersheds in both the methods. Sixty-two micro-watersheds using SYI method and twenty-three micro-watersheds using morphometric have been prioritized as very high priority. Final priority map was prepared by considering the commonly occurred very high-prioritized micro-watersheds in both SYI model and morphometric analysis. Twenty-four suitable sites were identified for check dam construction in 21 highly prioritized watersheds. It is proved that integrated study of SYI model and morphometric analysis yield good result in prioritization of watersheds.     

Characteristics of VHF radiowave scintillations over a solar cycle (1983–1993) at a low-latitude station: Waltair (17.7°N, 83.3°E)

The characteristics of VHF radiowave scintillations at 244 MHz (FLEETSAT) during a complete solar cycle (1983–93) at a low-latitude station, Waltair (17.7°N, 83.3°E), are presented. The occurrence of night-time scintillations shows equinoctial maxima and summer minima in all the epochs of solar activity, and follows the solar activity. The daytime scintillation occurrence is negatively correlated with the solar activity and shows maximum occurrence during the summer months in a period of low solar activity. The occurrence of night-time scintillations is inhibited during disturbed days of high solar activity and enhanced during low solar activity.     

Magnetic interpretation of pairs of sheets and their equivalence to dykes

Magnetic anomaly profiles over two thin sheets separated by a small distance resemble those of dykes andvice versa. Interpretation of anomalies over a pair of sheets based on the magnetic properties of dykes predicts a dyke whose centre lies midway between the positions of the sheets. The dyke, on the other hand, is magnetically equivalent to a pair of sheets, both lying at the same depth and having the same magnetization.The magnetic anomalies due to a pair of sheets can be interpreted by framing linear equations between the anomalies and their distances measured from an arbitrary reference. Application of this method to anomalies of dipping sheets with a finite depth extent is indicated.     

Double-difference tomography of P- and S-wave velocity structure beneath the western part of Java,Indonesia

West Java in the western part of the Sunda Arc has a relatively high seismicity due to subduction activity and faults. In this study, double-difference tomography was used to obtain the 3D velocity tomograms of P and S waves beneath the western part of Java. To infer the geometry of the structure beneath the study area, precise earthquake hypocenter determination was first performed before tomographic imaging. For this, earthquake waveform data were extracted from the regional Meteorological, Climatological, Geophysical Agency (BMKG) network of Indonesia from South Sumatra to Central Java. The P and S arrival times for about 1,000 events in the period April 2009 to July 2016 were selected, the key features being events of magnitude > 3, azimuthal gap < 210° and number of phases > 8. A nonlinear method using the oct-tree sampling algorithm from the NonLinLoc program was employed to determine the earthquake hypocenters. The hypocenter locations were then relocated using double-difference tomography (tomoDD). A significant reduction of travel-time (root mean square basis) and a better clustering of earthquakes were achieved which correlated well with the geological structure in West Java. Double-difference tomography was found to give a clear velocity structure, especially beneath the volcanic arc area, i.e., under Mt Anak Krakatau, Mt Salak and the mountains complex in the southern part of West Java. Low velocity anomalies for the P and S waves as well as the v_P/v_S ratio below the volcanoes indicated possible partial melting of the upper mantle which ascended from the subducted slab beneath the volcanic arc.     

A Nd isotopic study of the Kerguelen Islands: Inferences on enriched oceanic mantle sources

The origin of the highly differentiated igneous rocks of the Kerguelen Islands and the nature of their source regions have been investigated by a Nd isotopic study. The Nd isotopic compositions of syenites and granites are identical to those of gabbros and basalts and indicate a common source. The isotopic data preclude the involvement ofold continental crustal material in the genesis of these granitic and alkalic rocks. The data from the Kerguelen samples greatly extend the Nd-Sr isotopic correlation observed for uncontaminated basalts from the oceanic mantle. The large Nd isotopic variations in the Kerguelen samples could be explained by mixing of deep mantle material brought up by a plume and the upper oceanic mantle or by heterogeneities in the lower mantle. An important finding of this study is that there are enriched mantle sources under the oceanic regions. These enriched sources may be ancient in age and are compatible with the 2-b.y. age inferred from the Pb isotope data of these samples. Earth models in future must incorporate this feature of the oceanic mantle in a consideration of mantle-crust evolutionary relationships.     

Application of Two Turbulence Closure Schemes in the Modelling of Tidal Currents and Salinity in the Hooghly Estuary

This paper has two purposes. The first is to study the circulation and salinity in Hooghly Estuary, along the east coast of India and the second is to compare the performance of two turbulence closure schemes by modelling it. A breadth averaged numerical model using a sigma co-ordinate system in the vertical is briefly described. Vertical diffusion of momentum and salt are parameterized by a simple first-order turbulent closure or by a one equation model for turbulent kinetic energy (TKE) which uses a specified mixing length. The results are compared with the available neap and spring tide observations along the estuary for both low and high discharge periods.The computed elevations and currents are in reasonable agreement with the observations showing no major differences in vertical current profiles by both the turbulent schemes. However, there is a slight under-prediction of bottom currents. The salinity profiles predicted by TKE model show better matching with observations. Statistical tests are also conducted to study the comparative performance of the turbulent closure schemes. The maintenance of two layer structure in residual currents and salt variability are also studied by using the model.