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131.
A. A. Deshpande F. D’Alessandro P. M. McCulloch 《Journal of Astrophysics and Astronomy》1996,17(1-2):7-16
Spectral analysis of the residual pulsearrival times of pulsars is a useful tool in understanding the nature of the underlying
processes that may be responsible for the timing noise observed from pulsars. Power spectra of pulsar timing residuals may
be described by one or a combination of powerlaws. As these spectra are expected to be very steep, it is important to ensure
a high dynamic range in the estimation of the spectrum. This is difficult in practice since one is, in general, dealing with
timing measurements made at unevenly placed epochs. In this paper, we present a technique based on, ‘CLEAN’ to obtain high
dynamic range spectra from unevenly sampled data. We compare the performance of this technique with other techniques including
some that were used earlier for estimation of power spectra of pulsar timing residuals. 相似文献
132.
133.
The study reports the age evolution of groundwater as it flows from the recharge area through a regional alluvial aquifer system in North Gujarat-Cambay region in western India. Radiocarbon (14C), 4He and 4He / 222Rn dating methods have been employed. Sediments from a drill core in the Cambay Basin were also analysed for uranium (U) and thorium (Th) concentrations and the measured values have been used to estimate the 4He and 222Rn production rate for groundwater age calculations. Additionally, factors controlling the distribution of 222Rn, 4He and temperature anomalies in groundwater, vis-à-vis their relation to the tectonic framework and lithology of the study area, have also been examined.The multi-isotope study indicated a reasonable correspondence in groundwater age estimates by the three methods employed. The groundwater 14C ages increased, progressively, in the groundwater flow direction: from the foothills of Aravalli Mountains in the east, and reached a value of ∼35 ka towards the region of lowest elevation, linking Little Rann of Kachchh (LRK)-Nalsarovar (NS)-Gulf of Khambhat (GK) in the western part of the study area. In this region, groundwater ages obtained for free flowing thermal wells and springs employing 4He and 4He / 222Rn systematics are in the order of million years. Such anomalous ages are possibly due to enhanced mobilisation and migration of ‘excess helium’ from hydrothermal circulation vents along deep-seated faults. Excluding such anomalous cases and considering all uncertainties, presently estimated 4He and 4He / 222Rn groundwater ages are in reasonable agreement with 14C age estimates in the Cambay Basin for helium release factor (ΛHe) value of 0.4 ± 0.3. The 4He method also indicated west-southwards progression of groundwater ages up to ∼100 ka beyond the Cambay Basin.Large ‘excess helium’ concentrations are also seen to be generally associated with anomalous groundwater temperatures (> 35 °C) and found to overlie some of the basement faults in the study area, particularly along the east and the west flanks of the Cambay Basin. Groundwater 222Rn activities in most of the study area are 800 ± 400 dpm/l. But, a thermal spring at Tuwa on the east flank of the Cambay Basin, having granitic basement at shallow depth, recorded the highest 222Rn activity (∼63,000 dpm/l). 相似文献
134.
The evolution of sea surface temperature (SST) and thermocline (represented by 20 °C isotherm depth, D20) in the east equatorial Indian Ocean (EIO) associated with the Indian Ocean Dipole (IOD) years is studied for the period of 50 years from 1958 to 2007. A new IOD index based on combined anomalies of surface winds, D20 and SST over the equatorial Indian Ocean is defined to identify strong and weak IOD events. It is found that the evolution of strong IOD events is driven by ocean dynamics in the form of thermocline–SST coupling and is strongly interactive with the atmosphere, whereas the weak IOD events are mere response to surface winds without such dynamical coupling. The easterly wind anomalies extend up to the western equatorial Indian Ocean (WIO) during strong IOD years and support enhanced EIO air–sea interactions. On the other hand, the evolution of zonal wind anomalies is weak during the weak IOD years. Thermocline–SST coupling is robust in both EIO and WIO during strong IOD years, which is primarily responsible for the enhanced SST gradient, strong enough to establish anomalous Walker circulation within the Indian Ocean. The strong convection over the WIO associated with the Indian Ocean Walker cell triggers a secondary cell with subsidence over the African landmass. This double cell structure over the equatorial Indian Ocean is not reported before. Such double cell structure is not evident in weak IOD years and instead the convection over WIO extends up to African landmass. These are well supported by the spatial pattern of anomalous precipitable water during strong and weak IOD years. Strengthening of monsoon flow and local Hadley cell associated with strong IOD events enhances precipitation over the Indian subcontinent, whereas weak IOD years have less impact on the Indian summer monsoon circulation and rainfall. Analysis reveals that the EIO thermocline index and combined index could be potential predictors for the central Indian rainfall during summer. 相似文献
135.
136.
Numerical simulation of a typical tropical thunder storm event at Pune (18.53°N, 73.85°E), India, has been performed using the three nested domain configuration of Weather Research and Forecasting-Advanced Research Weather Model (version 3.2). The model simulations have been compared with observations. Sensitivity to cumulus parameterization schemes, namely Betts–Miller (BM), Grell–Devenyi (GD), and Kain–Fritsch (KF), for simulation of vertical structure and time evolution of weather parameters has been evaluated using observations from automatic weather station and global positioning system radiosonde ascents. Comparison of spatial distribution of 24-h accumulated rain with Tropical Rainfall Measuring Mission data shows that BM scheme could simulate better rain than GD and KF schemes. The BM scheme could well simulate the development of storm and heavy rain as it could generate sufficiently humid and deep layer in the lower and middle atmosphere, along with co-existence of updrafts and downdrafts and frozen hydrometeors at the middle level and rain water near the surface. 相似文献
137.
Measurements of the concentration and size distribution of aerosol particles in the size-ranges of 0.5–20 μm and 16–700 nm
diameters were made during six fog episodes over the south Indian Ocean. Observations show that concentrations of particles
of all sizes start decreasing 1–2 hours before the occurrence of fog. This decrease is more prominent for coarse particles
of >1 μm diameter and continues until 10–20 minutes before the onset of fog when particle concentrations in all size ranges
rapidly increase by one/two orders of magnitude in ∼20 minutes. Thereafter, concentrations of particles of all sizes gradually
decrease until the dissipation of fog. After the fog dissipation, concentrations of coarse mode particles rapidly increase
and restore to their pre-fog levels but concentrations of the Aitken mode particles decrease slowly and reach their pre-fog
levels only after 1–2 hours. The net effect of fog is to change the bimodal size distributions of aerosols with a coarse mode
at 1.0 μm and an accumulation mode at 40–60 nm to a power law size distribution. It is proposed that the preferential growth
and sedimentation of the coarse mode hygroscopic particles in the initial phase cause a large decrease in the aerosol surface
area. As a result, the low vapour pressure gases which were initially being used for the growth of coarse mode particles,
now accelerate the growth rates of the accumulation and Aitken mode particles. 相似文献
138.
Impact of physical parameterization schemes on numerical simulation of super cyclone Gonu 总被引:2,自引:0,他引:2
The objective of this study is to investigate in detail the sensitivity of cumulus, planetary boundary layer and explicit
cloud microphysics parameterization schemes on intensity and track forecast of super cyclone Gonu (2007) using the Pennsylvania
State University-National Center for Atmospheric Research Fifth-Generation Mesoscale Model (MM5). Three sets of sensitivity
experiments (totally 11 experiments) are conducted to examine the impact of each of the aforementioned parameterization schemes
on the storm’s track and intensity forecast. Convective parameterization schemes (CPS) include Grell (Gr), Betts–Miller (BM)
and updated Kain–Fritsch (KF2); planetary boundary layer (PBL) schemes include Burk–Thompson (BT), Eta Mellor–Yamada (MY)
and the Medium-Range Forecast (MRF); and cloud microphysics parameterization schemes (MPS) comprise Warm Rain (WR), Simple
Ice (SI), Mixed Phase (MP), Goddard Graupel (GG), Reisner Graupel (RG) and Schultz (Sc). The model configuration for CPS and
PBL experiments includes two nested domains (90- and 30-km resolution), and for MPS experiments includes three nested domains
(90-, 30- and 10-km grid resolution). It is found that the forecast track and intensity of the cyclone are most sensitive
to CPS compared to other physical parameterization schemes (i.e., PBL and MPS). The simulated cyclone with Gr scheme has the
least forecast track error, and KF2 scheme has highest intensity. From the results, influence of cumulus convection on steering
flow of the cyclone is evident. It appears that combined effect of midlatitude trough interaction, strength of the anticyclone
and intensity of the storm in each of these model forecasts are responsible for the differences in respective track forecast
of the cyclone. The PBL group of experiments has less influence on the track forecast of the cyclone compared to CPS. However,
we do note a considerable variation in intensity forecast due to variations in PBL schemes. The MY scheme produced reasonably
better forecast within the group with a sustained warm core and better surface wind fields. Finally, results from MPS set
of experiments demonstrate that explicit moisture schemes have profound impact on cyclone intensity and moderate impact on
cyclone track forecast. The storm produced from WR scheme is the most intensive in the group and closer to the observed strength.
The possible reason attributed for this intensification is the combined effect of reduction in cooling tendencies within the
storm core due to the absence of melting process and reduction of water loading in the model due to absence of frozen hydrometeors
in the WR scheme. We also note a good correlation between evolution of frozen condensate and storm intensification rate among
these experiments. It appears that the Sc scheme has some systematic bias and because of that we note a substantial reduction
in the rain water formation in the simulated storm when compared to others within the group. In general, it is noted that
all the sensitivity experiments have a tendency to unrealistically intensify the storm at the later part of the integration
phase. 相似文献
139.
K. Pandarinath Sushma Prasad R. D. Deshpande S. K. Gupta 《Journal of Earth System Science》1999,108(2):107-116
A 54-m long core was raised from the bed of the Nal Sarovar, a large shallow lake located in the middle of the low-lying region
linking the Gulfs of Kachchh and Khambhat, in western India. A three-layer sequence comprising: Zone-1 (top 3 m), predominantly
silty-clay/clayey; Zone-2 (3–18 m), sandy; and Zone-3 (18–54 m), dominated by sticky silty-clay/clayey-silt with occasional
thin sand layers and basalt fragments was identified. Smectite and illite are the dominant clay minerals with minor amounts
of kaolinite and chlorite. Very high content of smectite (53–97%) in the clays of the lowermost zone (18–54 m) and the geomorphic
features of the surrounding region suggested that the sediments were derived from the basaltic terrain of Saurashtra and/or
via the Gulf of Khambhat. The clay content in the middle zone (3–18 m), dominantly sandy, is very low. Therefore, provenance
for this zone was derived using heavy minerals in the sand fraction. The heavy mineral species in this zone suggested the
mixed metamorphic and igneous terrain of Aravallis as the major source. The grain-size distribution of this zone closely matched
with the sediments underlying the modern Sabarmati riverbed at Ahmedabad, suggesting fluvial depositional environment. Clays
also dominate sediments of the topmost (0–3 m) zone with illite as the dominant (74–81%) specie followed by smectite suggesting
derivation from the mixed metamorphic and igneous terrain of Aravallis. 相似文献
140.
Spatio-temporal heterogeneity and changes in extreme precipitation over eastern Himalayan catchments India 总被引:1,自引:1,他引:0
Vishal?Singh Manish?Kumar?GoyalEmail author 《Stochastic Environmental Research and Risk Assessment (SERRA)》2017,31(10):2527-2546
This study draws attention on the extreme precipitation changes over the eastern Himalayan region of the Teesta river catchment. To explore the precipitation variability and heterogeneity, observed (1979–2005) and statistically downscaled (2006–2100) Coupled Model Intercomparison Project Phase Five earth system model global circulation model daily precipitation datasets are used. The trend analysis is performed to analyze the long-term changes in precipitation scenarios utilizing non-parametric Mann–Kendall (MK) test, Kendall Tau test, and Sen’s slope estimation. A quantile regression (QR) method has been applied to assess the lower and upper tails changes in precipitation scenarios. Precipitation extreme indices were generated to quantify the extremity of precipitation in observed and projected time domains. To portrait the spatial heterogeneity, the standard deviation and skewness are computed for precipitation extreme indices. The results show that the overall precipitation amount will be increased in the future over the Himalayan region. The monthly time series trend analysis based results reflect an interannual variability in precipitation. The QR analysis results showed significant increments in precipitation amount in the upper and lower quantiles. The extreme precipitation events are increased during October to June months; whereas, it decreases from July to September months. The representative concentration pathway (RCP) 8.5 based experiments showed extreme changes in precipitation compared to RCP2.6 and RCP4.5. The precipitation extreme indices results reveal that the intensity of precipitation events will be enhanced in future time. The spatial standard deviation and skewness based observations showed a significant variability in precipitation over the selected Himalayan catchment. 相似文献