Secondary instability as a possible mechanism for clear-air turbulence: a case study     

T. N. Venkatesh  Joseph Mathew  Ravi S. Nanjundiah 《Meteorology and Atmospheric Physics》2014,126(3-4):139-160

Many theories and mechanisms have been proposed to explain the phenomenon of clear-air turbulence (CAT), and some of them have been successful in predicting light, moderate and, in some cases, severe turbulence. It is only recently that skill in the forecasting of the severe form of CAT, which could lead to injuries to passengers and damage to aircraft, has improved. Recent observations and simulations suggest that some severe to extreme turbulence could be caused by horizontal vortex tubes resulting from secondary instabilities of regions of high shear in the atmosphere. We have conducted direct numerical simulations to understand the scale relationship between primary structures (larger-scale structures related to one of the causes mentioned above) and secondary structures (smaller-sized, shear structures of the size of aircraft). From shear layer simulations, we find that the ratio of sizes of primary and secondary vortices is of the right order to generate aircraft-scale vortex tubes from typical atmospheric shear layers. We have also conducted simulations with a mesoscale atmospheric model, to understand possible causes of turbulence experienced by a flight off the west coast of India. Our simulations show the occurrence of primary flow structures related to synoptic conditions around the time of the incident. The evidence presented for this mechanism also has implications for possible methods of detection and avoidance of severe CAT.  相似文献   

Tidal variations in the Sundarbans Estuarine System, India   总被引：1，自引：0，他引：1  

MEENAKSHI CHATTERJEE  D SHANKAR  G K SEN  P SANYAL  D SUNDAR  G S MICHAEL  ABHISEK CHATTERJEE  P AMOL  DEBABRATA MUKHERJEE  K SUPRIT  A MUKHERJEE  V VIJITH  SIDDHARTHA CHATTERJEE  ANWESHA BASU  MADHUMITA DAS  SARANYA CHAKRABORTI  ARAVIND KALLA  SURJA KANTA MISRA  SOUMYA MUKHOPADHYAY  GOPAL MANDAL  KANKAN SARKAR 《Journal of Earth System Science》2013,122(4):899-933

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Effect of surface wave inversion non-uniqueness on 1D seismic ground response analysis     

Narayan Roy  Ravi SankarJakka  H. R. Wason 《Natural Hazards》2013,68(2):1141-1153

Surface wave methods are increasingly being used for geotechnical site characterization. The methodology is based on the dispersive characteristic of Rayleigh waves in vertically heterogeneous medium. Experimental dispersion curve is inverted to obtain one-dimensional shear-wave velocity profile by inverse problem solution. Uncertainty associated with this surface wave inversion has drawn much attention. Inverse problem solution can provide different equivalent shear-wave velocity profiles, which may lead to different seismic site response analysis. In this study, a neighborhood algorithm has been used for inversion of dispersion curve to get a set of equivalent shear-wave velocity profiles. These equivalent velocity profiles are then used for 1D ground response analysis for different input motion record of the same earthquake at different epicentral distances. Results show significant variation in amplification spectrum in terms of maximum amplification as well as peak frequency. The extent of this uncertainty largely depends on the characteristics of the ground motion records at different epicentral distances. A linear variation is observed between mean coefficients of variation of amplification spectrum and epicentral distance of ground motion records. A gradual increase in mean value of peak frequency and peak amplification with the epicentral distance is also observed.  相似文献   

Did the Indo‐Asian summer monsoon decrease during the Holocene following insolation?     

Manish Tiwari  Rengaswamy Ramesh  Ravi Bhushan  Madavalm S. Sheshshayee  Bammidipati L. K. Somayajulu  A. J. Timothy Jull  George S. Burr 《第四纪科学杂志》2010,25(7):1179-1188

A few studies from the western Arabian Sea indicate that the Indian summer (or southwest) monsoon (ISM), after attaining its maximum intensity at ca. 9 ka, declined during the Holocene, as did insolation. In contrast, earlier and later observations from both the eastern and the western Arabian Sea do not support this inference. Analysis of multiple proxies of productivity in a new sediment core from the western Arabian Sea fails to confirm the earlier, single‐proxy (e.g. abundance of Globigerina bulloides) based, inference of the Holocene weakening of ISM, following insolation. The reason for the observed decreasing trend in foraminiferal abundance – the basis for the earlier inference – could be the favouring of silicate rather than carbonate productivity by the increased ISM wind strength. Although ISM exhibits several multi‐millennial scale fluctuations, there is no evidence from several multi‐proxy data to conclude that it declined during the Holocene; this is consistent with the phase lag analysis of longer time series of monsoon proxies. Thus, on sub‐Milankovitch timescales, ISM did not follow insolation, highlighting the importance of internal feedbacks. A comparison with East Asian summer monsoon (EASM) records suggests that both ISM and EASM varied in unison, implying common forcing factors on such longer timescales. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Unbiased estimates of galaxy scaling relations from photometric redshift surveys     

Graziano Rossi  Ravi K. Sheth 《Monthly notices of the Royal Astronomical Society》2008,387(2):735-740

Many physical properties of galaxies correlate with one another, and these correlations are often used to constrain galaxy formation models. Such correlations include the colour–magnitude relation, the luminosity–size relation, the fundamental plane, etc. However, the transformation from observable (e.g. angular size, apparent brightness) to physical quantity (physical size, luminosity) is often distance dependent. Noise in the distance estimate will lead to biased estimates of these correlations, thus compromising the ability of photometric redshift surveys to constrain galaxy formation models. We describe two methods which can remove this bias. One is a generalization of the V _max method, and the other is a maximum-likelihood approach. We illustrate their effectiveness by studying the size–luminosity relation in a mock catalogue, although both methods can be applied to other scaling relations as well. We show that if one simply uses photometric redshifts one obtains a biased relation; our methods correct for this bias and recover the true relation.  相似文献   

Ichnology and Depositional Environment of the Cambrian Nagaur Sandstone (Nagaur Group) Along the Dulmera Section,Bikaner-Nagaur Basin,Rajasthan     

Birendra P. SINGH  O. N. BHARGAVA  Ravi S. CHAUBEY  Naval KISHORE 《《地质学报》英文版》2014,88(6):1665-1680

Abstract: The Lower Cambrian Nagaur Sandstone (Marwar Supergroup) has yielded trace fossils Treptichnus, Cruziana, serially repeated Rusophycus, Diplichnites, Monomorphichnus, Bergaueria, arthropod swimming traces and leap frogging marks of Cruziana ichnofacies. This ichnofossils assemblage is dominantly preserved in fine to medium grained red sandstone beds of the Mohra Member (Nagaur Sandstone). The presence of graded rip-up clasts, current ripples, dune cross-stratification with mud drapes and tidal bundles indicates a subtidal paleoenvironment for the deposition of Nagaur Sandstone Formation, which is corroborated by the Cruziana ichnofacies trace fossil assemblage.  相似文献   

Nonlinear finite element modeling of beams on two-parameter foundations     

Ravi Mullapudi  Ashraf Ayoub 《Computers and Geotechnics》2010

This paper presents an inelastic element for the analysis of beams resting on two-parameter foundations. The element is derived from a two field mixed formulation with independent approximation of forces and displacements. The values for the two parameters of the foundation are derived through an iterative technique that is based on an assumption of plane strain for the soil medium. This iterative behavior is repeated at each time step of the nonlinear solution algorithm. The nonlinear response of structures resting on this improved two-parameter foundation model is analyzed following both a Vlasov and a Pasternak approach. Numerical examples that clarify the advantage of the newly developed model are conducted. These studies confirmed the importance of accounting for the foundation second parameter, and the efficiency and accuracy of the proposed model.  相似文献   

Halo abundances in the f_nl model     

Tsz Yan Lam  Ravi K. Sheth 《Monthly notices of the Royal Astronomical Society》2009,398(4):2143-2151

We show how the excursion set moving barrier model for halo abundances may be generalized to the local non-Gaussian f _nl model. Our estimate assumes that the distribution of step sizes depends on f _nl, but that they are otherwise uncorrelated. Our analysis is consistent with previous results for the case of a constant barrier, and highlights some implicit assumptions. It also clarifies the basis of an approximate analytic solution to the moving barrier problem in the Gaussian case, and shows how it might be improved.  相似文献   

Photometric study of W UMa type binaries in the old open cluster Berkeley 39     

Kandulapati Sriram  Yellapragada Ravi Kiron  Pasagada Vivekananda Rao 《中国天文和天体物理学报》2009,9(10)

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Mathematical analysis of a chaotic model in relevance to monsoon ISO     

Sandipan Mukherjee  Ravi Prakash Shukla  Ashok Kumar Mittal  Avinash Chandra Pandey 《Meteorology and Atmospheric Physics》2011,114(3-4):83-93

Mathematical analysis of a predominantly bimodal chaotic attractor, Lu system, was carried out to investigate a possible application of the model as a prototype of monsoon intra-seasonal oscillation (ISO). Bifurcation structures of the system are explored as the system parameter c and the forcing parameter F are varied. Stability criteria of equilibrium points of the forced Lu system are also explored in detail. A sensitivity study is carried out, by changing forcing parameter F, to explore relationships between some of the derived variables of the model and, based on such relationships, an empirical rule is used for extended range prediction. Analogous variables are also derived from the ISO data and prediction results are compared. Application of the prediction rule of regime transition to the observed ISO and chaotic model data is purely based on the bimodal characteristics of ISO and neglects some of the intricate mechanisms therein. We have found that a forced Lu system can be a good prototype in the prediction of peak anomaly of the monsoon ISO when growth rates around a threshold value are taken as predictors.  相似文献