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An iterative wavefront sensing algorithm for high-contrast imaging systems

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Research in Astronomy and Astrophysics 2011年第2期11卷 198-204页

作者： Jiang-Pei Dou 1,2,De-Qing Ren 1,2,3 and Yong-Tian Zhu 1,2 1 national Astronomical Observatories/Nanjing Institute of Astronomical Optics & Technology,Chinese Academy of sciences,Nanjing 210042,China 2 Key Laboratory of Astronomical Optics & Technology,Nanjing Institute of Astronomical Optics & Technology,Chinese Academy of sciences,Nanjing 210042,China 3 Physics & Astronomy Department,California State University Northridge,18111 Nordhoff Street,Northridge,California 91330-8268,USA national Astronomical Observatories / Nanjing Institute of Astronomical Optics & Technology Chinese Academy of Sciences Nanjing 210042 China Key Laboratory of Astronomical Optics & Technology Nanjing Institute of Astronomical Optics & Technology Chinese Academy of Sciences Nanjing 210042 China Physics & Astronomy Department California State University Northridge 18111 Nordhoff Street Northridge California 91330-8268 USA

Wavefront sensing from multiple focal plane images is a promising technique for high-contrast imaging ***,the wavefront error of an optics system can be properly reconstructed only when it is very *** paper presents an iterative optimization algorithm for the direct measurement of large static wavefront errors from only one focal plane *** first measure the intensity of the pupil image to get the pupil function of the system and acquire the aberrated image on the focal plane with a phase error that will be *** we induce a dynamic phase on the tested pupil function and calculate the associated intensity of the reconstructed image on the focal *** algorithm will then try to minimize the intensity difference between the reconstructed image and the aberrated test image in the focal plane,where the induced phase is a variable of the optimization *** simulation shows that the wavefront of an optical system can theoretically be reconstructed with high precision,which indicates that such an iterative algorithm may be an effective way to perform wavefront sensing for high-contrast imaging systems.

关键词： techniques: image processing -- methods: numerical -- planetarysystems

Ionospheric longitudinal variations at midlatitudes: Incoherent scatter radar observation at Millstone Hill

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science China(Technological sciences) 2012年第5期55卷 1153-1160页

作者： COSTER Anthea HOLT John FOSTER John ERICKSON Phil MIT Haystack Observatory Off Route 40 Westford MA 01866 USA

Incoherent scatter radar （ISR） extra-wide coverage experiments during the period of 1978-2011 at Millstone Hill are used to investigate longitudinal differences in electron density. This work is motivated by a recent finding of the US east-west coast difference in TEC suggesting a combined effect of changing geomagnetic declination and zonal winds. The current study pro- vides strong supporting evidence of the longitudinal change and the plausible mechanism by examining the climatology of electron density Ne on both east and west sides of the radar with a longitude separation of up to 40% for different heights within 300-450 kin. Main findings include： 1） The east-west difference can be up to 60% and varies over the course of the day, being positive （East side Ne 〉 West side Ne） in the late evening, and negative （West side Ne 〉 East side Ne） in the pre-noon. 2） The east-west difference exists throughout the year. The positive （relative） difference is most pronounced in winter; the negative （relative） difference is most pronounced in early spring and later summer. 3） The east-west difference tends to enhance toward decreasing solar activity, however, with some seasonal dependence; the enhancements in the positive and negative differences do not take place simultaneously. 4） Both times of largest positive and largest negative east-west differences in Ne are earlier in summer and later in winter. The two times differ by 12-13 h, which remains constant throughout the year. 5） Variations at different heights from 300-450 km are similar. Zonal wind climatology above Millstone Hill is found to be perfectly consistent with what is expected based on the electron density difference between the east and west sides of the site. The magnetic declination-zonal wind mechanism is true for other longitude sectors as well, and may be used to understand longitudinal variations elsewhere. It may also be used to derive thermospheric zonal winds.

关键词： midlatitude ionosphere geomagnetic declination thermospheric zonal winds longitudinal variation incoherent scatterradar

An active coronagraph using a liquid crystal array for exoplanet imaging:principle and testing

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Research in Astronomy and Astrophysics 2012年第5期12卷 591-600页

作者： Xi Zhang De-Qing Ren Yong-Tian Zhu Jiang-Pei Dou national Astronomical Observatories/Nanjing Institute of Astronomical Optics&Technology Chinese Academy of SciencesNanjing 210042China Key Laboratory of Astronomical Optics&Technology National Astronomical Observatories/Nanjing Institute of Astronomical Optics&TechnologyChinese Academy of SciencesNanjing 210042China Physics&Astronomy Department California State University Northridge18111 Nordhoff StreetNorthridgeCalifornia 91330-8268USA Graduate University of Chinese Academy of sciences Beijing 100049China

High-contrast imaging coronagraphs, used for the detection of exoplanets, have always adopted passive coronagraph optical components. It is therefore impossible to actively optimize the coronagraphs to achieve their best performance. To solve this problem, we propose a novel high-contrast imaging coronagraph which combines a liquid crystal array （LCA） for active pupil apodization and a deformable mirror （DM） for phase correction. The LCA we use is an amplitude-only spatial light mod- ulator. The LCA is well calibrated and compensates for its amplitude non-uniformity and nonlinear intensity responsivity. We measured the imaging contrasts of the coron- agraph system with the LCA only and without the DM deployed. Imaging contrasts of 10-4 and 10-5 can be reached at an inner working angular distance of 2.5 and 5A/D, respectively. A simulation shows that the phase errors on the coronagraph pupil limit the contrast performance. The contrast could be further improved if a DM is deployed to correct the phase errors induced by the LCA and coronagraph optics.

关键词： instrumentation: coronagraph liquid crystal array - techniques high-contrast imaging exoplanet detection

INTERRELATION BETWEEN EAST-ASIAN WINTER MONSOON AND INDIAN/PACIFIC SST WITH THE INTERDECADAL VARIATION

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Acta meteorologica Sinica 1998年第3期12卷 275-287页

作者：徐建军朱乾根孙照渤 Lab.of atmospheric sciences and Geophysical Fluid Dynamics(LASG) Institute of Atmospheric PhysicsChinese Academy of SciencesBeijing 100080 Nanjing Institute of Meteorology Nanjing 210044

Investigated statistically is the interrelation between East Asian winter monsoon(EAWM) and SST over sensitive areas of the Indian and Pacific *** focus on the relation of EAWM to strong ENSO signal area.i.e.,the equatorial eastern Pacific(EEP)*** suggests that the EAWM variation is intimately associated not only with the EEP SST but with the equatorial western Pacific“warm pool”and equatorial Indian/northwestern Pacific Kuroshio SST as well:the EAWM and ENSO interact strongly with each other on the interannual time scales, exhibiting pronounced interdecadal variation mainly under the joint effect of the monsoon QBO and the monsoon/SST background field features on an interdecadal basis—when both fields are in the same phase(anti-phase).strong EAWM contributes to EEP SST rise(drop)in the following winter,corresponding to a warm(cold)ENSO cycle;the EAWM QBO causes ENSO cycle to be strong phase-locked with seasonal variation,making the EEP SST rise lasting from April—May to May—June of the next year,which plays an important role in maintaining a warm ENSO phase.

关键词： East-Asian winter monsoon ENSO cycle interdecadal variation QBO(quasibiennial oscillation) equatorial eastern Pacific(EEP)

Numerical Simulation of the 12 May 1997 CME Event

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Chinese Physics Letters 2008年第2期25卷 790-793页

作者：周玉芬冯学尚吴式灿 SIGMA Weather Group State Key Laboratory for Space Weather Center for Space Science and Applied Research Chinese Academy of Sciences Beijing 100080 School of Earth science Graduate University of Chinese Academy of Sciences Beijing 100049 Center for Space Plasma and Aeronomic Research The University of Alabama in Huntsville Huntsville AL 35899 USA

Our newly developed CESE MHD model is used to simulate sun-earth connection event with the well-studied 12 May 1997 CME event as an example. The main features and approximations of our numerical model are as follows：（1） The modifed conservation element and solution element （CESE） numerical scheme in spherical geometry is implemented in our code. （2） The background solar wind is derived from a 3D time-dependent numerical MHD model by input measured photospheric magnetic fields. （3） Transient disturbances are derived from solar surface by introducing a mass flow of hot plasma. The numerical simulation has enabled us to predict the arrival of the interplanetary shock and provided us with a relatively satisfactory comparison with the WIND spacecraft observations.

关键词： CORONAL MASS EJECTION SOLAR-WIND MHD SIMULATION PROPAGATION

Improving the Treatment of the Vertical Snow Burial Fraction over Short Vegetation in the NCAR CLM3

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Advances in atmospheric sciences 2009年第5期26卷 877-886页

作者：王爱慧 Xubin ZENG Nansen-Zhu International Research Center Institute of Atmospheric Physics Chinese Academy of Sciences Beijing 100029 Department of atmospheric sciences The University of Arizona Tucson AZ 85721

One deficiency of the NCAR Community Land Model （CLM3） is the disappearance of the simulated snow even in the middle of winter over a boreal grassland site due to unrealistically modeled high downward turbulent fluxes. This is caused by the inappropriate treatment of the vertical snow burial fraction for short vegetation. A new snow burial fraction formulation for short vegetation is then proposed and validated using in situ observations. This modification in the CLM3 largely removes the unrealistic surface turbulent fluxes, leading to a more reasonable snowmelt process, and improves the snow water equivalent （SWE） simulation. Moreover, global offline simulations show that the proposed formulation decreases sensible and latent heat fluxes as well as the ground temperature during the snowmelt season over short vegetation dominant regions. Correspondingly, the SWE is enhanced, leading to the increase in snowmelt-induced runoff during the same period. Furthermore, sensitivity tests indicate that these improvements are insensitive to the exact functional form or parameter values in the proposed formulation.

关键词： snow burial fraction model short vegetation global validation

On Multi-Timescale Variability of Temperature in China in Modulated Annual Cycle Reference Frame

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Advances in atmospheric sciences 2010年第5期27卷 1169-1182页

作者：钱诚 Zhaohua WU 符淙斌周天军 Key Laboratory of Regional Climate-Environment Research for Temperate East Asia Institute of Atmospheric Physics Chinese Academy of Sciences Beijing 100029 Department of Meteorology ＆ Center for Ocean-atmospheric Prediction Studies Florida State University Tallahassee Florida USA Institute for Climate and Global Change Research Nanjing University Nanjing 210093 State Key Laboratory of Numerical Modeling for atmospheric sciences and Geophysical Fluid Dynamics Institute of Atmospheric Physics Chinese Academy of Sciences Beijing 100029

The traditional anomaly （TA） reference frame and its corresponding anomaly for a given data span changes with the extension of data length. In this study, the modulated annual cycle （MAC）, instead of the widely used climatological mean annual cycle, is used as an alternative reference frame for computing climate anomalies to study the multi-timescale variability of surface air temperature （SAT） in China based on homogenized daily data from 1952 to 2004. The Ensemble Empirical Mode Decomposition （EEMD） method is used to separate daily SAT into a high frequency component, a MAC component, an interannual component, and a decadal-to-trend component. The results show that the EEMD method can reflect historical events reasonably well, indicating its adaptive and temporally local characteristics. It is shown that MAC is a temporally local reference frame and will not be altered over a particular time span by an exten-sion of data length, thereby making it easier for physical interpretation. In the MAC reference frame, the low frequency component is found more suitable for studying the interannual to longer timescale variability （ILV） than a 13-month window running mean, which does not exclude the annual cycle. It is also better than other traditional versions （annual or summer or winter mean） of ILV, which contains a portion of the annual cycle. The analysis reveals that the variability of the annual cycle could be as large as the magnitude of interannual variability. The possible physical causes of different timescale variability of SAT in China are further discussed.

关键词： modulated annual cycle the Ensemble Empirical Mode Decomposition climate anomaly climate normal variability of surface air temperature in China

Effects of Vertical Wind Shear on Intensity and Rainfall Asymmetries of Strong Tropical Storm Bilis (2006)

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Advances in atmospheric sciences 2010年第3期27卷 552-561页

作者：余锦华谈哲敏 Key Laboratory of Mesoscale Severe Weather/Ministry of Education and School of Atmospheric SciencesNanjing University School of atmospheric sciences Nanjing University of Information Science and Technology

The effects of environmental vertical wind shear （VWS） on the intensity and rainfall asymmetries in Tropical Storm （TS） Bilis （2006） have been analyzed based on TRMM/TMI-estimated surface rainfall data, QuikSCAT wind fields, 850- and 200-hPa winds of the NCEP-NCAR reanalysis, precipitation data at 5-min intervals from automatic weather stations over China's Mainland, and the best track data of TS Bilis （2006）. The results show that the simultaneous and 6-hour-lagged correlation coefficients between VWS and storm intensity （the minimum central sea level pressure） are 0.59145 and 0.57438 （P 〈0.01）, respectively. The averaged VWS was found to be about 11 m s-1 and thus suppressed the intensification of Bilis （2006）. Distribution of precipitation in Bilis （2006） was highly asymmetric. The azimuthally-averaged rainfall rate in the partial eyewall, however, was smaller than that in a major outer rainband. As the storm intensified, the major rainband showed an unusual outward propagation. The VWS had a great impact on the asymmetric distribution of precipitation. Consistent with previous modeling studies, heavy rainfall generally occurred downshear to downshear-left of the VWS vector both near and outside the eyewall, showing a strong wavenumber-one asymmetry, which was amplified as the VWS increased.

关键词： vertical wind shear tropical storm rainfall distribution

Coupling Ensemble Kalman Filter with Four-dimensional Variational Data Assimilation

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Advances in atmospheric sciences 2009年第1期26卷 1-8页

作者： Fuqing ZHANG Meng ZHANG James A. HANSEN Department of Meteorology Pennsylvania State University University Park Pennsylvania USA Navy Research Laboratory Monterrey California USA

This study examines the performance of coupling the deterministic four-dimensional variational assimilation system （4DVAR） with an ensemble Kalman filter （EnKF） to produce a superior hybrid approach for data assimilation. The coupled assimilation scheme （E4DVAR） benefits from using the state-dependent uncertainty provided by EnKF while taking advantage of 4DVAR in preventing filter divergence： the 4DVAR analysis produces posterior maximum likelihood solutions through minimization of a cost function about which the ensemble perturbations are transformed, and the resulting ensemble analysis can be propagated forward both for the next assimilation cycle and as a basis for ensemble forecasting. The feasibility and effectiveness of this coupled approach are demonstrated in an idealized model with simulated observations. It is found that the E4DVAR is capable of outperforming both 4DVAR and the EnKF under both perfect- and imperfect-model scenarios. The performance of the coupled scheme is also less sensitive to either the ensemble size or the assimilation window length than those for standard EnKF or 4DVAR implementations.

关键词： data assimilation four-dimensional variational data assimilation ensemble Kalman filter Lorenz model hybrid method