Modification of the loop current warm core eddy by Hurricane Gilbert (1988)     

Xiaodong Hong  Simon W. Chang  Sethu Raman 《Natural Hazards》2007,41(3):501-514

Numerical investigation of Hurricane Gilbert (1988) effect on the Loop Current warm core eddy (WCE) in the Gulf of Mexico is performed using the Modular Ocean Model version 2 (MOM2). Results show that the storm-induced maximum sea surface temperature (SST) decrease in Gilbert’s wake is over 2.5°C, as compared with the 3.5°C cooling in the absence of the WCE. The near-inertial oscillation in the wake reduces significantly in an along-track direction with the presence of the WCE. This effect is also reflected between the mixed layer and the thermocline, where the current directions are reversed with the upper layer. After two inertial periods (IP), the current reversal is much less obvious. In addition, it is demonstrated that Hurricane Gilbert wind stress increases the current speed of the WCE by approximate 133%. With the forcing of Gilbert, the simulated translation direction and speed of the WCE towards the Mexican coast are closer to the observed (42% more accurate in distance and 78% more accurate in direction) compared with the simulation without the Gilbert forcing. The simulated ocean response to Gilbert generally agrees with the recent observations in Hurricane Fabian.  相似文献   

Sea-breeze-initiated rainfall over the east coast of India during the Indian southwest monsoon   总被引：1，自引：0，他引：1  

Matthew Simpson  Hari Warrior  Sethu Raman  P. A. Aswathanarayana  U. C. Mohanty  R. Suresh 《Natural Hazards》2007,42(2):401-413

Sea-breeze-initiated convection and precipitation have been investigated along the east coast of India during the Indian southwest monsoon season. Sea-breeze circulation was observed on approximately 70–80% of days during the summer months (June–August) along the Chennai coast. Average sea-breeze wind speeds are greater at rural locations than in the urban region of Chennai. Sea-breeze circulation was shown to be the dominant mechanism initiating rainfall during the Indian southwest monsoon season. Approximately 80% of the total rainfall observed during the southwest monsoon over Chennai is directly related to convection initiated by sea-breeze circulation.  相似文献   

A Seasonal Statistical Evaluation of COAMPS® over the Arabian Gulf Region     

Rebecca E. Eager  Sethu Raman  Teddy R. Holt  Douglas Westphal  Jeffrey Reid  Jason Nachamkin  Ming Liu  Abdulla Al Mandoos 《Pure and Applied Geophysics》2007,164(8-9):1747-1764

A statistical evaluation of the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS^®) was performed over the Arabian Gulf region for the period, 1 August to 5 October, 2004. Verification skill scores of bias and root-mean-square error were estimated for surface variables and for vertical profiles to investigate any diurnal variations. The model predictions of boundary-layer heights are compared with the observations at Abu Dhabi, United Arab Emirates. The Middle East presents challenges to numerical weather prediction due to complex land-ocean-land mesoscale processes. An independent data set of surface measurements from 50 stations in the UAE was available from the Department of Water Resources Studies, Abu Dhabi for model verification. The results indicate a diurnal variation in the model errors. The errors are small considering the magnitudes of the observed variables. Errors in the coastal region can be attributed to the differences in the timing of the onset of sea and land breeze circulations in the simulations as compared to the observations. Errors are relatively smaller in the offshore locations.  相似文献   

Marine Boundary-Layer Variability Over The Indian Ocean During Indoex (1998)   总被引：1，自引：1，他引：0  

Vijayakumar Manghnani  Sethu Raman  Devdutta S. Niyogi  Vinayaka Parameswara  John M. Morrison  S. V. Ramana  J. V. S. S. Raju 《Boundary-Layer Meteorology》2000,97(3):411-430

The variability in boundary-layerstructure over the Indian Ocean during a north-eastmonsoon and the factors influencing it areinvestigated. This study was made possible as acomponent of the Indian Ocean Experiment (INDOEX),conducted from February 19 to March 30, 1998. The dataused are, surface-layer mean and turbulencemeasurements of temperature, humidity and wind, andvertical soundings of temperature and humidity.Significant spatio-temporal variability was observedin the boundary-layer structure throughout the cruise.The ITCZ was characterized as the region withstrongest winds and maximum surface turbulent fluxesof momentum and heat. One of the important findingsfrom this study was a strong influence of continentalair masses on the boundary-layer structure in theNorthern Hemisphere, even at a distance of 600 km offthe Indian coast. This was generally evident in theform of an elevated plume of dry continental airbetween altitudes of 1500 m and 2700 m. Advection ofcontinental aerosols in this layer presents potentialfor significant entrainment into shallow clouds inthis region, which eventually feed deeper clouds atthe ITCZ. This finding provides an explanation foranomalous higher aerosol concentrations found duringprevious studies. The structure of the marineboundary layer was influenced by various factors suchas proximity to land, an anomalous warm pool in theocean and the ITCZ. In the southern hemisphere, theboundary-layer height was primarily governed bysurface-layer sensible heat flux and was found to behighest in the vicinity of the ITCZ. North of theequator it was strongly influenced by land-air-seainteractions. In addition to this synoptic modulation,there was also a significant diurnal variability inthe boundary-layer height.  相似文献   

The Mutual Response of the Tropical Squall Line and the Ocean     

X. Hong  S. Raman  R. M. Hodur  L. Xu 《Pure and Applied Geophysics》1999,155(1):1-32

—The Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) is used to investigate the mutual response of a tropical squall line and the ocean. Simulated squall line compares well with the observations, and consists of counterrotating vortices, and has a bow shape bulge toward the leading edge. In addition to these features, which are also shown in the previous numerical simulations, the unique results from the coupled simulation indicate that the air–sea interaction processes within the squall line are important. They affect both the atmosphere and the ocean locally. Simulated upper ocean displays significant response to the squall line with upwelling and baroclinicity. Depth of the ocean mixed layer in the coupled simulation becomes modified due to feedback processes. Ocean temperature acts as a destabilizing factor, and the salinity as a stabilizing factor. Surface turbulent fluxes from the coupled simulation are about 10% less than that of the uncoupled simulation. The SST in the coupled simulation decreases by about 0.21°C. Predicted squall line in the coupled simulation is weaker as compared to the uncoupled simulation. This is reflected in terms of differences in surface fluxes, cloud water, rain water and vertical velocities between the two simulations.  相似文献   

Discriminating zooplankton assemblages in neritic and oceanic waters: a case for the northeast coast of India, Bay of Bengal   总被引：2，自引：0，他引：2  

Rakhesh M  Raman AV  Sudarsan D 《Marine environmental research》2006,61(1):93-109

Zooplankton species distribution and abundance data at 17 locations in the inshore (10-30 m), shelf (50-200 m) and oceanic (2,500-2,800 m) regions off northeast India (Bay of Bengal) during January 1999-April 2001 revealed 112 taxa represented by 30 divergent groups. Copepods (58 species) dominated (87%) the population numerically. In general zooplankton diversity (Margalef richness d, Shannon-Wiener H', Pielou's evenness J') increased in the direction of the open sea relative to coastal locations with a concomitant decrease both in abundance (ind m(-3)) and biomass (dry mass m(-3)). Based on multivariate analyses, it was possible to distinguish the zooplankton community into different assemblages according to their location (e.g., inshore, shelf, oceanic) and seasonality. While Acrocalanus sp., Oithona sp., Corycaeus danae, Euterpina acutifrons, Paracalanus sp., and Acartia sp. were found characterizing the coastal locations, Oncaea venusta was the discriminating species for shelf waters. In oceanic areas, there was a clear dominance of Labidocera sp., Candacia sp., Euchaeta rimana, Centropages calaninus, Copilia mirabilis and Corycella gibbula. The investigations revealed that changes in zooplankton community structure across water bodies could be associated with differing salinity. During November 1999 (post-monsoon), when salinity in the coastal waters was relatively low (26-28.9 PSU), the zooplankton community consisted of mainly Acrocalanus sp., Salpa, Corycaeus danae, Oikopleura sp., Acartia sp., Evadne tergestina, and Creseis sp. In January 2000 (salinity 32.4-34.1), additionally Corycella gibbula, Labidocera sp., Centropages sp., Microsetella sp., Euterpina acutifrons, Canthocalanus pauper, and Oncaea venusta represented the population discriminating the assemblage from others. In May 2000 (pre-monsoon) when salinity was highest (34.7-35.3), Oithona sp., Paracalanus sp., and Acrocalanus gibber were found important. Chaetognaths formed a distinct group during this period. Results presented during this investigation are considered significant since no previous studies exist for this locale drawing comparisons of the kind made during this study between coastal and oceanic situations.  相似文献   

The variation of filament direction in a flaring region     

R. R. Rausaria  K. Sundara Raman  P. S. M. Aleem  Jagdev Singh 《Solar physics》1993,146(1):137-145

We have analyzed the variations in shear angle over a time interval of 30 s during a flare on June 11, 1991, using Kodaikanal Observatory spectroheliogram and photoheliogram data, and assuming H filaments are a proxy for the neutral lines. The changes in shear angles have been analysed at two points of the filament. The orientation of the H filament underwent a considerable change of 55° from June 10, 1991 to prior to the start of the flare on June 11, 1991. The photoheliogram on June 10, 1991 shows considerable twisting of the umbrae (in one common penumbra) and broke into parts before the onset of the flare on June 11, 1991. The twisting of umbrae on June 10, 1991 shows that sunspot proper motion plays an important role in bringing a non-potential character to the field lines. This in turn develops shear and kink and it is argued that changes in filament orientation over a small interval of a half minute triggers the eruption of the flare.  相似文献   

Effects of magnetic shear,spot, and plage rotation on prominence evolution     

R. R. Rausaria  S. S. Gupta  R. Selvendran  K. Sundara Raman  Jagdev Singh 《Solar physics》1993,146(2):259-276

We have studied the evolution of two dark H filaments as prominences during their disk passage from 12 to 19 February, 1992 and 6 to 17 March, 1992, using Kodaikanal Observatory H and Caii K spectroheliograms. Both the filaments were well outside the spot regions. However, they were connected to sunspots by small threads. Outside the spot regions, the filaments were also anchored between opposite polarity plage regions. Both the filaments were almost straight in the beginning. However, they acquired a curved shape (inverted U-shape) as the spot and plages underwent rotation. It is shown that rotation of the plage and spot plays an important role in the evolution of prominences, one serving as the anchor and the other imparting necessary shear. Once the shear reaches a critical value it starts unwinding the filaments, resulting in the fine structure of the two prominences studied.  相似文献   

Sunspot Motions Associated with Flares     

Sundara Raman  K.  Selvendran  R.  Thiagarajan  R. 《Solar physics》1998,180(1-2):331-341

The evolution of five bipolar sunspot groups during their disk passage leading to flares are analysed and studied using Kodaikanal Observatory photoheliogram and spectroheliogram data. The changes in the orientation angle observed in the spot groups show that sunspot proper motion plays an important role in introducing non-potential character to the field lines. This in turn develops shear and once the shear reaches a critical value, the flare eruption is triggered. The rotational motions in the sunspots are measured from the change in their orientation angle and are given as a measure of shear. The sunspots considered for analyses in the present study are not associated with any filament activity.  相似文献   

Repetitive structure in the stellar wind of HD 93843: a normal O-type star     

Raman K. Prinja    Derck Massa  Ian D. Howarth  & Alex W. Fullerton 《Monthly notices of the Royal Astronomical Society》1998,301(4):926-934

We present the results from a 28-day IUE time-series campaign monitoring the stellar wind of the O5-type giant HD 93843. The principal aim was to study variability in the wind of a star with a normal projected rotation velocity. Systematic changes are identified, amidst continuous line-profile variability, in the absorption troughs of the Si  iv and N  v resonance lines. The patterns observed have characteristic time-scales of several days and are mimicked by fluctuations (of several 100 km s⁻¹) in the blue wings of the saturated C  iv P Cygni profile.   Fourier analysis provides support for the repeatability of wind structures in HD 93843 on a 7.1-d 'period'. Power at this frequency is evident only at intermediate and high velocities (i.e., above ∼0.3 of the terminal velocity). The long modulation time-scale suggests that changes in the star itself probably provide the physical source for triggering the onset of wind structure. Unfortunately the rotational, photometric, pulsational and magnetic properties of HD 93843 are too poorly constrained or known to permit a more detailed interpretation of the 7.1-d wind modulation in terms of potential inhomogeneities at the stellar surface. Nevertheless, our study demonstrates that the incidence of cyclic, possibly regular, stellar-wind variability is not restricted to rapid rotators. Comparisons with other OB stars which have exhibited repetitive wind changes on 'periods' of several days suggest that the time-dependent UV properties of HD 93843 are more akin to those of the O4-type supergiant ζ Puppis.  相似文献