Assimilation of Doppler Weather Radar Radial Velocity and Reflectivity Observations in WRF-3DVAR System for Short-Range Forecasting of Convective Storms     

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   

A massive, dusty toroid with large grains in the pre-planetary nebula IRAS22036+5306     

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   

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   

The thin end of the wedge     

Baldev Sahai 《Journal of the Indian Society of Remote Sensing》1995,23(4):155-156

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Maximum entropy spectral analysis of total ozone     

R. P. Kane  Y. Sahai  N. R. Teixeira 《Pure and Applied Geophysics》1984,122(5):747-762

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   

Diabatic heating profiles over the continental convergence zone during the monsoon active spells     

Rajib Chattopadhyay  Sharmila Sur  Susmitha Joseph  A. K. Sahai 《Climate Dynamics》2013,41(1):205-226

The present paper aims to bring out the robust common aspects of spatio-temporal evolution of diabatic heating during the monsoon intraseasonal active phases over the continental tropical convergence zone (CTCZ). The robustness of spatio-temporal features is determined by comparing the two state-of-the art reanalyses: NCEP Climate Forecast System reanalysis and Modern ERA Retrospective Analysis. The inter-comparison is based on a study period of 26 years (1984–2009). The study confirms the development of deep heating over the CTCZ region during the active phase and is consistent between the two datasets. However, the detailed temporal evolution of the vertical structure (e.g., vertical tilts) of heating differs at times. The most important common feature from both the datasets is the significant vertical redistribution of heating with the development of shallow (low level) heating and circulation over the CTCZ region 3–7 days after the peak active phase. The shallow circulation is found to be associated with increased vertical shear and relative vorticity over certain regions in the subcontinent. This increased vertical shear and relative vorticity in the lower levels could be crucial in the sustenance of rainfall after the peak active phase. Model experiments with linear dynamics affirm the role of shallow convection in increasing the lower level circulation as observed.  相似文献   

Analysing two‐dimensional effects in central loop transient electromagnetic sounding data using a semi‐synthetic tipper approach          下载免费PDF全文

P. Yogeshwar  B. Tezkan 《Geophysical Prospecting》2018,66(2):444-456

We present a simple and feasible approach to analyse and identify two‐dimensional effects in central loop transient electromagnetic sounding data and the correspondingly derived quasi two‐dimensional conductivity models. The proposed strategy is particularly useful in minimising interpretation errors. It is based on the calculation of a semi‐synthetic transient electromagnetic tipper at each sounding and for each observational transient time point. The semi‐synthetic transient electromagnetic tipper is derived from the measured vertical component of the induced voltage and the synthetically calculated horizontal component. The approach is computationally inexpensive and involves one two‐dimensional forward calculation of an obtained quasi two‐dimensional conductivity section. Based on a synthetic example, we demonstrate that the transient electromagnetic tipper approach is applicable in identifying which transient data points and which corresponding zones in a derived quasi two‐dimensional subsurface model are affected by two‐dimensional inhomogeneities. The one‐dimensional inversion of such data leads to false models. An application of the semi‐synthetic transient electromagnetic tipper to field data from the Azraq basin in Jordan reveals that, in total, eight of 80 investigated soundings are affected by two‐dimensional structures although the field data can be fitted optimally using one‐dimensional inversion techniques. The largest semi‐synthetic tipper response occurs in a 300 m‐wide region around a strong lateral resistivity contrast. The approach is useful for analysing structural features in derived quasi two‐dimensional sections and for qualitatively investigating how these features affect the transient response. To avoid misinterpretation, these identified zones corresponding to large tipper values are excluded from the interpretation of a quasi two‐dimensional conductivity model. Based on the semi‐synthetic study, we also demonstrate that a quantitative interpretation of the horizontal voltage response (e.g. by inversion) is usually not feasible as it requires the exact sensor position to be known. Although a tipper derived purely from field data is useful as a qualitative tool for identifying two‐dimensional distortion effects, it is only feasible if the sensor setup is sufficiently accurate. Our proposed semi‐synthetic transient electromagnetic tipper approach is particularly feasible as an a posteriori approach if no horizontal components are recorded or if the sensor setup in the field is not sufficiently accurate.  相似文献   

回线源瞬变电磁法有限体积三维任意各向异性正演及分析          下载免费PDF全文

刘亚军  胡祥云  彭荣华  Pritam Yogeshwar 《地球物理学报》2019,62(5):1954-1968

目前,瞬变电磁法(TEM)数据基本都是基于各向同性模型进行反演解释,这对于存在明显电性各向异性的勘探区域会产生较大的反演解释误差.为分析电各向异性对回线源瞬变电磁信号的影响方式与程度,本文通过求解离散化的全张量电导率时间域Helmholtz方程,实现了基于有限体积法的TEM任意各向异性的三维正演算法.该算法采用基于交错网格的拟态有限体积法(MFV)对时域Maxwell方程组进行空间域离散,并利用后退欧拉算法(Backward Euler Method)进行时间域离散.为提高时域电磁场的求解精度与效率,该算法将时间分段等步长算法与方程直接求解法相结合.通过对一维各向异性模型以及三维复杂各向同性模型进行测试,验证了本算法对于回线源瞬变电磁响应计算的正确性及有效性.最后,通过对几类典型电各向异性介质中大回线源瞬变电磁信号响应的分析,总结了不同电各向异性类型对TEM电磁信号的影响模式,结果表明,主轴各向异性情况下TEM信号主要受水平方向电导率的影响,倾斜各向异性对TEM信号的影响程度远大于水平各向异性,而通过水平各向异性信号能较清晰判断出各向异性主轴方向.  相似文献