Time filter

Source Type

Li W.,George Mason University | Yanga C.,George Mason University | Yang C.,CAS Institute of Remote Sensing Applications
International Journal of Geographical Information Science | Year: 2010

The increased popularity of standards for geospatial interoperability has led to an increasing number of geospatial Web services (GWSs), such as Web Map Services (WMSs), becoming publicly available on the Internet. However, finding the services in a quick and precise fashion is still a challenge. Traditional methods collect the services through centralized registries, where services can be manually registered. But the metadata of the registered services cannot be updated timely. This paper addresses the above challenges by developing an effective crawler to discover and update the services in (1) proposing an accumulated term frequency (ATF)-based conditional probability model for prioritized crawling, (2) utilizing concurrent multi-threading technique, and (3) adopting an automatic mechanism to update the metadata of identified services. Experiments show that the proposed crawler achieves good performance in both crawling efficiency and results' coverage/liveliness. In addition, an interesting finding regarding the distribution pattern of WMSs is discussed. We expect this research to contribute to automatic GWS discovery over the large-scale and dynamic World Wide Web and the promotion of operational interoperable distributed geospatial services. © 2010 Taylor & Francis. Source

Duan A.,CAS Institute of Atmospheric Physics | Wang M.,University of Chinese Academy of Sciences | Lei Y.,CAS Institute of Remote Sensing Applications | Cui Y.,CAS Institute of Atmospheric Physics
Journal of Climate | Year: 2013

The impacts of the thermal forcing over the Tibetan Plateau (TP) in spring on changes in summer rainfall in China are investigated using historical records from the period between 1980 and 2008. The spring sensible heat (SH) flux and snow depth over the TP both decreased over this time period, although the trend in SH was more significant than that in snow depth. The similarity between patterns of precipitation trends over China and corresponding patterns of regression coefficients on the leading mode of spring SH change over the TP demonstrates the distinct contribution of changes in TP SH during spring. Enhanced precipitation in southern China was accompanied by increases in heavy rainfall, precipitation intensity, and the frequency of precipitation events, while reduced precipitation in northern China and northeastern China was primarily associated with decreases in the frequency of precipitation events. Further analysis using observational data and numerical simulations reveals that the reductions in SH over the TP have weakened the monsoon circulation and postponed the seasonal reversal of the land-sea thermal contrast in East Asia. In addition, the positive spring SH anomaly may generate a stronger summer atmospheric heat source over the TP due to the positive feedback between diabatic heating and local circulation. © 2013 American Meteorological Society. Source

Lei Y.,CAS Institute of Remote Sensing Applications | Duan A.,CAS Institute of Atmospheric Physics
International Journal of Climatology | Year: 2011

Prolonged dry episodes, defined by the 90th percentile of long durations without efficient precipitation (above the 0.1 mm/day threshold) in both the wet and dry seasons, have been investigated from 1958 to 2008 at 404 stations over China. Associated with droughts over northern China in summer, the enhancement of the prolonged dry episode duration is an essential feature, together with a lack of precipitation and the negative Palmer drought severity index (PDSI) in the wet season. In the dry season, durations of prolonged dry episodes have significantly increased over southern China and the Yellow River valley during the last 51 years. The prolonged dry episodes highlight the impact of a decrease in precipitation frequency, and are useful for representing short-term droughts, particularly over semi-arid regions and in the dry season. The occurrence of the maximum prolonged dry episodes over vulnerable regions in the early twenty-first century is suggestive of a greater risk of droughts during both the wet and dry seasons in a warmer climate over China. © 2010 Royal Meteorological Society. Source

Li X.,College Park | Zhang J.A.,National Oceanic and Atmospheric Administration | Yang X.,CAS Institute of Remote Sensing Applications | Pichel W.G.,College Park | And 3 more authors.
Bulletin of the American Meteorological Society | Year: 2013

Sea surface imprints of 83 hurricanes show features such as eye structure, mesovortices, rainbands, and arc clouds, along with rarities such as high winds within an eye. Atlantic tropical cyclones and western Pacific counterpart typhoons have been extensively monitored from operational polar-orbiting and geostationary satellite sensors. The striking tropical cyclone cloud pictures taken by these conventional weather satellites have appeared in many journal/magazine covers, newspapers, and television programs. These images are acquired by passive remote sensing instruments operating in the visible and infrared (IR) bands. These images are viewed to gather information about the cloud-top structure of the tropical cyclones at kilometer spatial resolution. Microwave data are also used extensively for tropical cyclone analysis with advance of spaceborne microwave remote sensing. Source

Yang X.,CAS Institute of Remote Sensing Applications | Li X.,National Oceanic and Atmospheric Administration | Pichel W.G.,The Center for Satellite Applications and Research | Li Z.,CAS Institute of Remote Sensing Applications
IEEE Transactions on Geoscience and Remote Sensing | Year: 2011

In this paper, we perform a comparison of wind speed measurements from the ENVISAT Advanced Synthetic Aperture Radar (ASAR), the MetOp-A Advanced Scatterometer (ASCAT), the U.S. National Data Buoy Center's moored buoys, and the U.S. Navy Operational Global Atmospheric Prediction System (NOGAPS) model. These comparisons were made in near U.S. coast regions over a 17-month period from March 2009 to July 2010. The ASAR wind speed retrieval agreed well with the scatterometer and model estimates, with mean differences ranging from-0.69 to 0.85 m/s and standard deviations between 1.16 and 1.77 m/s, depending upon the ASAR beam mode type. The results indicate that ASAR-derived ocean surface wind speeds are as accurate as the ASCAT and NOGAPS wind products. Comparisons between ASCAT winds and synthetic aperture radar (SAR) winds averaged at different spatial resolutions show very little change. This demonstrates that it is suitable that the scatterometer wind retrieval geophysical model function, i.e., CMOD5, is used for SAR wind retrieval. The impact of C-band VV polarization SAR calibration error on wind retrieval is also discussed. © 2011 IEEE. Source

Discover hidden collaborations