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51.
P. R. Fagundes Y. Sahai I. S. Batista M. A. Abdu J. A. Bittencourt H. Takahashi 《Annales Geophysicae》1999,17(8):1053-1063
In December 1995, a campaign was carried out to study the day-to-day variability in precursor signatures to large-scale ionospheric F-region plasma irregularities, using optical diagnostic techniques, near the magnetic equator in the Brazilian sector. Three instruments were operated simultaneously: (a) an all-sky (180° field of view) imaging system for observing the OI 630 nm nightglow emission at Alcântara (2.5°S, 44.4°W); (b) a digisonde (256-Lowell) at São Luis (2.6°S, 44.2°W); and (c) a multi-channel tilting filter-type zenith photometer for observing the OI 630 nm and mesospheric nightglow emissions at Fortaleza (3.9°S, 38.4°W). During the period December 14–18, 1995 (summer in the southern hemisphere), a good sequence of the OI 630 nm imaging observations on five consecutive nights were obtained, which are presented and discussed in this study. The observing period was geomagnetically quiet to moderate (Kp = 0+ to 5+ Dst = 18 nT to −37 nT). On four nights, out of the five observation nights, the OI 630 nm imaging pictures showed formations of transequatorial north-south aligned intensity depletions, which are the optical signatures of large-scale ionospheric F-region plasma bubbles. However, considerable day-to-day variability in the onset and development of the plasma depleted bands was observed. On one of the nights it appears that the rapid uplifting of the F-layer in the post-sunset period, in conjunction with gravity wave activity at mesospheric heights, resulted in generation of very strong plasma bubble irregularities. One of the nights showed an unusual formation of north-south depleted band in the western sector of the imaging system field of view, but the structure did not show any eastward movement, which is a normal characteristic of plasma bubbles. This type of irregularity structure, which probably can be observed only by wide-angle imaging system, needs more investigations for a better understanding of its behaviour. 相似文献
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S. Abhilash A. K. Sahai K. Mohankumar John P. George Someshwar Das 《Pure and Applied Geophysics》2012,169(11):2047-2070
In this paper the impact of Doppler weather radar (DWR) reflectivity and radial velocity observations for the short range forecasting of a tropical storm and associated rainfall event have been examined. Doppler radar observations of a tropical storm case that occurred during 29–30 October 2006 from SHARDWR (13.6° N, 80.2° E) are assimilated in the WRF 3DVAR system. The observation operator for radar reflectivity and radial velocity is included within latest version of WRF 3DVAR system. Keeping all model physics the same, three experiments were conducted at a horizontal resolution of 30?km. In the control experiment (CTRL), NCEP Final Analysis (FNL) interpolated to the model grid was used as the initial condition for 48-h free forecast. In the second experiment (NODWR), 6-h assimilation cycles have been carried out using all conventional (radiosonde and surface data) and non-conventional (satellite) observations from the Global Telecommunication System (GTS). The third experiment (DWR) is the same as the second, except Doppler radar radial velocity and reflectivity observations are also used in the assimilation cycle. Continuous 6-h assimilation cycle employed in the WRF-3DVAR system shows positive impact on the rainfall forecast. Assimilation of DWR data creates several small scale features near the storm centre. Additional sensitivity experiments were conducted to study the individual impact of reflectivity and radial velocity in the assimilation cycle. Radar data assimilation with reflectivity alone produced large analysis response on both thermodynamical and dynamical fields. However, radial velocity assimilation impacted only on dynamical fields. Analysis increments with radar reflectivity and radial velocity produce adjustments in both dynamical and thermodynamical fields. Verification of QPF skill shows that radar data assimilation has a considerable impact on the short range precipitation forecast. Improvement of the QPF skill with radar data assimilation is more clearly seen in the heavy rainfall (for thresholds >7?mm) event than light rainfall (for thresholds of 1 and 3?mm). The spatial pattern of rainfall is well simulated by the DWR experiment and is comparable to TRMM observations. 相似文献
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R. Sahai K. Young N. Patel C. Sánchez Contreras M. Morris 《Astrophysics and Space Science》2008,313(1-3):241-244
Using the Submillimeter Array (SMA), we have obtained high angular-resolution (∼1″) interferometric maps of the submillimeter
(0.88 mm) continuum and CO J=3–2 line from IRAS 22036+5306 (I 22036), a bipolar pre-planetary nebula (PPN) with knotty jets discovered in our HST SNAPshot
survey of young PPNe. In addition, we have obtained supporting lower-resolution (∼10″) 2.6 mm continuum and CO, 13CO J=1–0 observations with the Owens Valley Radio Observatory (OVRO) interferometer. We find an unresolved source of submillimeter
(and millimeter-wave) continuum emission in I 22036, implying a very substantial mass (0.02–0.04M
⊙) of large (i.e., radius ≳1 mm), cold (≲50 K) dust grains associated with I 22036’s toroidal waist. The CO J=3–2 observations show the presence of a very fast (∼220 km s−1), highly collimated, massive (0.03M
⊙) bipolar outflow with a very large scalar momentum (about 1039 g cm s−1), and the characteristic spatio-kinematic structure of bow-shocks at the tips of this outflow. The fast outflow in I 22036,
as in most PPNe, cannot be driven by radiation pressure. The large mass of the torus suggests that it has most likely resulted
from common-envelope evolution in a binary, however it remains to be seen whether or not the time-scales required for the
growth of grains to millimeter sizes in the torus are commensurate with such a formation scenario. The presence of the torus
should facilitate the formation of the accretion disk needed to launch the jet. We also find that the 13C/12C ratio in I 22036 is very high (0.16), close to the maximum value achieved in equilibrium CNO-nucleosynthesis (0.33). The
combination of the high circumstellar mass (i.e., in the torus and an extended dust shell inferred from ISO far-infrared spectra)
and the high 13C/12C ratio in I 22036 provides strong support for this object having evolved from a massive (≳4M
⊙) progenitor in which hot-bottom-burning has occurred. 相似文献
58.
Algorithms for GPS operation indoors and downtown 总被引:6,自引:0,他引:6
Nainesh Agarwal Julien Basch Paul Beckmann Piyush Bharti Scott Bloebaum Stefano Casadei Andrew Chou Per Enge Wungkum Fong Neesha Hathi Wallace Mann Anant Sahai Jesse Stone John Tsitsiklis Benjamin Van Roy 《GPS Solutions》2002,6(3):149-160
The proliferation of mobile devices and the emergence of wireless location-based services has generated consumer demand for
availability of GPS in urban and indoor environments. This demand calls for enhanced GPS algorithms that accommodate high
degrees of signal attenuation and multipath effects unique to the "urban channel." This paper overviews the market for location-based
services and discusses algorithmic innovations that address challenges posed by urban environments.
Electronic Publication 相似文献
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Total ozone data series for 1957–82 at ten locations were subjected to Maximum Entropy Spectral Analysis. Besides the annual, semi-annual, and quasi-biennial oscillations, peaks were noticed at 3.5–4, 6–7, and 10–11 years. For Arosa, Switzerland, for a longer period (1932–71), an additional peak was indicated at about 16 years. 相似文献