The climatology of near-equatorial typhoons over the western-north Pacific are fully investigated using the JTWC （Joint Typhoon Warning Center） typhoon record from 1951 to 2006. The result shows that there are seaso...The climatology of near-equatorial typhoons over the western-north Pacific are fully investigated using the JTWC （Joint Typhoon Warning Center） typhoon record from 1951 to 2006. The result shows that there are seasonal and decadal variations, as well as a distinctive spatial distribution, of such events. Among them, Typhoon Vamei is an example of a near-equatorial typhoon that occurred near Singapore in December of 2001. Using the WRF （Weather and Research Forecast） model, we attempt to find out how the well known ＂wind surge＂ of this event contributes to the development of Typhoon Vamei. It is found that the strong wind surge not only helps to provide advection of positive vorticity to Vamei between 800 and 500 hPa, but also increases the convective instability of the lower troposphere, and thus helps to induce convective outbreaks and rapid intensification. Furthermore, sensitivity experiments show that terrain and the land-sea distribution have very limited effects on the formation of Typhoon Vamei in the simulation, but an adequate Coriolis parameter （f） is still needed for the development of Vamei.展开更多
In a sharp contrast to tropical cyclone(TC) genesis over the main development region of the western North Pacific(WNP), near-equatorial(0°-5°N) TCs exhibit a distinctive annual cycle, peaking in boreal winte...In a sharp contrast to tropical cyclone(TC) genesis over the main development region of the western North Pacific(WNP), near-equatorial(0°-5°N) TCs exhibit a distinctive annual cycle, peaking in boreal winter and being inactive in boreal summer. The relative roles of dynamic and thermodynamic background states on near-equatorial TCs formation were investigated based on the observational diagnosis of the genesis potential index(GPI) and high-resolution model simulations. It is found that the background vorticity makes a major contribution to the distinctive annual cycle, while mean temperature and specific humidity fields are not critical. Numerical simulations further indicate that seasonal mean cyclonic vorticity in boreal winter has three effects on TC genesis near the equator. First, the environmental cyclonic vorticity interacts with TC vortex to promote a mid-level outflow, which strengthens boundary layer friction induced ascending motion and thus condensational heating. Second, it produces an equivalent Coriolis effect(via enhanced absolute vorticity), which strengthens positive feedback between primary and secondary circulation. Third, it helps to merge small-scale vortical hot towers(VHTs) into a mesoscale core through vorticity segregation process. However, background vorticity in boreal summer has an opposite effect on TC development near the equator.展开更多
In this study,the differences in spatial distribution and controlling parameters for the formation of near-equatorial tropical cyclones(NETCs)between the western North Pacific(WNP)and the North Atlantic(NA)are investi...In this study,the differences in spatial distribution and controlling parameters for the formation of near-equatorial tropical cyclones(NETCs)between the western North Pacific(WNP)and the North Atlantic(NA)are investigated.NETCs exhibit distinctive spatial variabilities in different basins.Over the past few decades,the majority of NETCs took place in WNP while none was observed in NA.The mechanism behind such a distinguishing spatial distribution difference is analyzed by using statistical methods.It is noted that the dynamical variables such as low-level relative vorticity and vertical wind shear(VWS)are likely the primary controlling parameters.Compared with NA,larger low-level vorticity and smaller VWS appear over WNP.The increase of vorticity attributes a lot to the turning of northeast trade wind.NETCs in WNP tend to occur in the areas with VWS less than 9 m s^(-1),while the VWS in NA generally exceeds 10 m s^(-1).On the other hand,the sea surface temperature in the near-equatorial region of both of the two oceans exceeds 26.5℃and the difference of mid-level moisture is not significant;thus,thermal factors have little contribution to the distinction of NETC activities between WNP and NA.Intraseasonal oscillation(ISO)and synoptic-scale disturbances in WNP are also shown to be more favorable for NETC genesis.More NETCs were generated in ISO active phase.Synoptic-scale disturbances in WNP obtain more energy from the mean flows through the barotropic energy conversion process.The overall unfavorable thermal and dynamic conditions lead to the absence of NETCs in NA.展开更多
The scale-invariant feature transform(SIFT)ability to automatic control points(CPs)extraction is very well known on remote sensing images,however,its result inaccurate and sometimes has incorrect matching from generat...The scale-invariant feature transform(SIFT)ability to automatic control points(CPs)extraction is very well known on remote sensing images,however,its result inaccurate and sometimes has incorrect matching from generating a small number of false CPs pairs,their matching has high false alarm.This paper presents a method containing a modification to improve the performance of the SIFT CPs matching by applying sum of absolute difference(SAD)in different manner for the new optical satellite generation called near-equatorial orbit satellite(NEqO)and multi-sensor images.The proposed method leads to improving CPs matching with a significantly higher rate of correct matches.The data in this study were obtained from the RazakSAT satellite covering the Kuala Lumpur-Pekan area.The proposed method consists of three parts:(1)applying the SIFT to extract CPs automatically,(2)refining CPs matching by SAD algorithm with empirical threshold,and(3)evaluating the refined CPs scenario by comparing the result of the original SIFT with that of the proposed method.The result indicates an accurate and precise performance of the model,which showed the effectiveness and robustness of the proposed approach.展开更多
Researchers in the remote sensing field use different types of images from satellite systems and simulator devices, such as goniometers. However, no device can simulate the new generation of optical satellite system c...Researchers in the remote sensing field use different types of images from satellite systems and simulator devices, such as goniometers. However, no device can simulate the new generation of optical satellite system called near-equatorial satellite system to perform different kinds of remote sensing applications in equatorial regions. This study proposed a newly invented laboratory and fieldwork goniometer designed to simulate and capture intensity variation and measure the bidirectional spectral reflectance of earth surface. The proposed goniometer is a multi-purpose and multi-field device. It is able to simulate different satellite systems and measure the intensity variation and spectral reflectance of earth’s surface features with freely azimuth and zenith angles of sensors and illumination source in fieldwork and/or laboratory. However, the system of invention is focusing on specific satellite orbital to work with the parameters and properties of NEqO satellite system in order to obtain NEqO system imagery for performing different applications such as geometric correction, relative radiometric normalization and change detection for future work. The significant of this invention is that most of the invented goniometers of remote sensing are able to work just in field or just in laboratory and use, carry just optical sensor or hyperspectral sensor. Specifically, our invention can do all these functions that are not available in existing goniometers. The proposed device offers several advantages, namely, high measurement speed, flexibility, low cost, efficiency, and possible measurement depending on the free zenith/azimuth angles of sensors and illumination sources. The proposed goniometer includes ten parts, and two different sensors (optical and hyperspectral).展开更多
基金supported by Chinese State Key program 2009CB421500the National Natural Science Foundation of China under Grant Nos.40921160380 and 40975059the Scientific Research Foundation of the Ministry of Education of China
文摘The climatology of near-equatorial typhoons over the western-north Pacific are fully investigated using the JTWC （Joint Typhoon Warning Center） typhoon record from 1951 to 2006. The result shows that there are seasonal and decadal variations, as well as a distinctive spatial distribution, of such events. Among them, Typhoon Vamei is an example of a near-equatorial typhoon that occurred near Singapore in December of 2001. Using the WRF （Weather and Research Forecast） model, we attempt to find out how the well known ＂wind surge＂ of this event contributes to the development of Typhoon Vamei. It is found that the strong wind surge not only helps to provide advection of positive vorticity to Vamei between 800 and 500 hPa, but also increases the convective instability of the lower troposphere, and thus helps to induce convective outbreaks and rapid intensification. Furthermore, sensitivity experiments show that terrain and the land-sea distribution have very limited effects on the formation of Typhoon Vamei in the simulation, but an adequate Coriolis parameter （f） is still needed for the development of Vamei.
基金Supported by the National Natural Science Foundation of China(41630423 and 41875069)NOAA of U.S.(NA18OAR4310298)+2 种基金National Science Foundation of U.S.(AGS-1643297)China Scholarship Council(CSC,N201808320286)Postgraduate Research and Practice Innovation Program of Jiangsu Province(KYCX18_1002)。
文摘In a sharp contrast to tropical cyclone(TC) genesis over the main development region of the western North Pacific(WNP), near-equatorial(0°-5°N) TCs exhibit a distinctive annual cycle, peaking in boreal winter and being inactive in boreal summer. The relative roles of dynamic and thermodynamic background states on near-equatorial TCs formation were investigated based on the observational diagnosis of the genesis potential index(GPI) and high-resolution model simulations. It is found that the background vorticity makes a major contribution to the distinctive annual cycle, while mean temperature and specific humidity fields are not critical. Numerical simulations further indicate that seasonal mean cyclonic vorticity in boreal winter has three effects on TC genesis near the equator. First, the environmental cyclonic vorticity interacts with TC vortex to promote a mid-level outflow, which strengthens boundary layer friction induced ascending motion and thus condensational heating. Second, it produces an equivalent Coriolis effect(via enhanced absolute vorticity), which strengthens positive feedback between primary and secondary circulation. Third, it helps to merge small-scale vortical hot towers(VHTs) into a mesoscale core through vorticity segregation process. However, background vorticity in boreal summer has an opposite effect on TC development near the equator.
基金Supported by the National Natural Science Foundation of China(42088101)。
文摘In this study,the differences in spatial distribution and controlling parameters for the formation of near-equatorial tropical cyclones(NETCs)between the western North Pacific(WNP)and the North Atlantic(NA)are investigated.NETCs exhibit distinctive spatial variabilities in different basins.Over the past few decades,the majority of NETCs took place in WNP while none was observed in NA.The mechanism behind such a distinguishing spatial distribution difference is analyzed by using statistical methods.It is noted that the dynamical variables such as low-level relative vorticity and vertical wind shear(VWS)are likely the primary controlling parameters.Compared with NA,larger low-level vorticity and smaller VWS appear over WNP.The increase of vorticity attributes a lot to the turning of northeast trade wind.NETCs in WNP tend to occur in the areas with VWS less than 9 m s^(-1),while the VWS in NA generally exceeds 10 m s^(-1).On the other hand,the sea surface temperature in the near-equatorial region of both of the two oceans exceeds 26.5℃and the difference of mid-level moisture is not significant;thus,thermal factors have little contribution to the distinction of NETC activities between WNP and NA.Intraseasonal oscillation(ISO)and synoptic-scale disturbances in WNP are also shown to be more favorable for NETC genesis.More NETCs were generated in ISO active phase.Synoptic-scale disturbances in WNP obtain more energy from the mean flows through the barotropic energy conversion process.The overall unfavorable thermal and dynamic conditions lead to the absence of NETCs in NA.
文摘The scale-invariant feature transform(SIFT)ability to automatic control points(CPs)extraction is very well known on remote sensing images,however,its result inaccurate and sometimes has incorrect matching from generating a small number of false CPs pairs,their matching has high false alarm.This paper presents a method containing a modification to improve the performance of the SIFT CPs matching by applying sum of absolute difference(SAD)in different manner for the new optical satellite generation called near-equatorial orbit satellite(NEqO)and multi-sensor images.The proposed method leads to improving CPs matching with a significantly higher rate of correct matches.The data in this study were obtained from the RazakSAT satellite covering the Kuala Lumpur-Pekan area.The proposed method consists of three parts:(1)applying the SIFT to extract CPs automatically,(2)refining CPs matching by SAD algorithm with empirical threshold,and(3)evaluating the refined CPs scenario by comparing the result of the original SIFT with that of the proposed method.The result indicates an accurate and precise performance of the model,which showed the effectiveness and robustness of the proposed approach.
文摘Researchers in the remote sensing field use different types of images from satellite systems and simulator devices, such as goniometers. However, no device can simulate the new generation of optical satellite system called near-equatorial satellite system to perform different kinds of remote sensing applications in equatorial regions. This study proposed a newly invented laboratory and fieldwork goniometer designed to simulate and capture intensity variation and measure the bidirectional spectral reflectance of earth surface. The proposed goniometer is a multi-purpose and multi-field device. It is able to simulate different satellite systems and measure the intensity variation and spectral reflectance of earth’s surface features with freely azimuth and zenith angles of sensors and illumination source in fieldwork and/or laboratory. However, the system of invention is focusing on specific satellite orbital to work with the parameters and properties of NEqO satellite system in order to obtain NEqO system imagery for performing different applications such as geometric correction, relative radiometric normalization and change detection for future work. The significant of this invention is that most of the invented goniometers of remote sensing are able to work just in field or just in laboratory and use, carry just optical sensor or hyperspectral sensor. Specifically, our invention can do all these functions that are not available in existing goniometers. The proposed device offers several advantages, namely, high measurement speed, flexibility, low cost, efficiency, and possible measurement depending on the free zenith/azimuth angles of sensors and illumination sources. The proposed goniometer includes ten parts, and two different sensors (optical and hyperspectral).
