Study of a Wind Front over the Northern South China Sea Generated by the Freshening of the North-East Monsoon (original) (raw)
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Atmospheric front over the East China Sea studied by multisensor satellite and in situ data
Journal of Geophysical Research, 2004
A frontal feature visible on a synthetic aperture radar (SAR) image acquired by the Radarsat satellite over the East China Sea on 19 November 2000 is analyzed in conjunction with data acquired by Quikscat, TOPEX/Poseidon, Tropical Rain Measurement Mission (TRMM), Defense Meteorological Satellite Program (DMSP), and National Oceanic and Atmospheric Administration (NOAA) satellites, and with data obtained from ship measurements. Although this frontal feature is located close to the Kuroshio front, it is demonstrated that it is not a sea surface manifestation of an oceanic front, but rather of an atmospheric front extending over 800 km from an area of the Pacific Ocean northeast of Taiwan to the southern coast of Korea. It is a cold front moving in the southeast direction with a speed of approximately 45-50 km/hour and associated with a 40-km-wide rainband trailing the front. The Radarsat image, which has a resolution of 50 m, reveals fine-scale structures of the atmospheric front, in particular small-scale convective rain cells embedded in the front. Conclusion is drawn that accurate interpretation of frontal features in SAR images requires use of additional meteorological and remote sensing data and information.
2006
Spaceborne SAR images acquired over the ocean in conjunction with weather radar images are well suited to study atmospheric fronts in coastal areas. In this paper we confine ourselves to study quasi-stationary atmospheric fronts off the east coast of Taiwan which are located typically 30- 70 km offshore. These quasi- stationary atmospheric fronts were first detected on ERS SAR images (5). In (5) we have compared mainly ERS SAR images with cloud images from a geostationary satellite, while here we compare Envisat ASAR and MODIS images mainly with weather radar images. Our analysis of three events presented in this paper lends further support to our hypothesis that these coastal quasi-stationary atmospheric fronts are generated by the collisions of the two airflows from opposing directions: one is associated with a weak to moderate easterly synoptic-scale wind blowing against the high coastal mountain range at the east coast of Taiwan and the other with a local offshore wind. The offs...
Photogrammetric Engineering & Remote Sensing, 2007
Sea off the coast of Hong Kong are studied by using multi-sensor satellite and ground-based meteorological data. These include synthetic aperture radar (SAR) images acquired by the Advanced Synthetic Aperture Radar (ASAR) onboard the European ENVISAT satellite, weather radar images from the Hong Kong Observatory (HKO), rain rate data acquired by the Special Sensor Microwave Imager (SSM/I) sensor onboard the F15 satellite of the American Defense Meteorological Satellite Program (DMSP) and the rain sensors onboard the Tropical Rainfall Measurement Mission (TRMM) satellite, cloud image of satellite, sea surface wind maps acquired by the scatterometer onboard the QUIKSCAT satellite, and meteorological data from weather stations in Hong Kong. Three rain events, typical of Hong Kong, are studied. The first event consists of a cluster of rain cells associated with the summer monsoon, the second one of rain cells aligned in a rain band generated by an upper-air trough, and the third one consists of small rain cells embedded in a cold front. It is shown that ASAR images, which have a resolution of 30 m in the Image Mode (IM) and 150 m resolution in the Wide Swath Mode (WSM), yield much more detailed information on the spatial structure of rain events over the ocean than data obtained from SSSM/I and the rain sensors onboard the TRMM satellite. The precipitation radar (PR) onboard TRMM, which is the rain measuring instrument flown in space with the next best resolution, has a resolution of only 4 km. However, the disadvantage of SAR is that it is sometimes difficult to identify SAR signatures visible on SAR images of the sea surface unambiguously as caused by rain events. By comparing SAR images with simultaneously acquired weather radar images of the Hong Kong Observatory, a better knowledge of radar signatures on SAR images resulting from rain events over the ocean is obtained. This knowledge then helps greatly in detecting rain events on SAR images which are acquired over ocean areas, which are not in the reach of weather radar stations. SAR images containing radar signature of rain events allow a much more detailed study of fine-scale structures of rain events over the World's ocean, in particular of clusters of rain cells, than any other sensor presently flown in space.
Journal of Ocean University of China, 2008
Ocean surface winds observed by the Quick Scatterometer (QuikSCAT) satellite prior to the geneses of 36 tropical cyclones (TCs) in the South China Sea (SCS) are investigated in this paper. The results show that there are areas with negative mean horizontal divergence around the TC genesis locations three days prior to TC formation. The divergence term [-(f+ζ)(∂u/∂x+∂v/∂y)] in the vorticity equation is calculated based upon the QuikSCAT ocean surface wind data. The calculated mean divergence term is about 10.3 times the mean relative vorticity increase rate around the TC genesis position one day prior to TC genesis, which shows the important contributions of the divergence term to the vorticity increase prior to TC formation. It is suggested that criteria related with the divergence and divergence term be applied in early detections of tropical cyclogenesis using the QuikSCAT satellite data.
Atmospheric fronts along the east coast of Taiwan studied by ERS synthetic aperture radar images
Journal of the atmospheric …, 2007
The existence of quasi-stationary alongshore atmospheric fronts typically located 30-70 km off the east coast of Taiwan is demonstrated by analyzing synthetic aperture radar (SAR) images of the sea surface acquired by the European Remote Sensing Satellites ERS-1 and ERS-2. For the data interpretation, cloud images from the Japanese Geostationary Meteorological Satellite GMS-4 and the American Terra satellite, rain-rate maps from ground-based weather radars, sea surface wind data from the scatterometer on board the Quick Scatterometer (QuikSCAT) satellite, and meteorological data from weather maps and radiosonde ascents have also been used. It is shown that these atmospheric fronts are generated by the collisions of the two airflows from opposing directions: one is associated with a weak easterly synoptic-scale wind blowing against the high coastal mountain range at the east coast of Taiwan and the other with a local offshore wind. At the convergence zone where both airflows collide, air is forced to move upward, which often gives rise to the formation of coast-parallel cloud bands. There are two hypotheses about the origin of the offshore wind. The first one is that it is a thermally driven land breeze/katabatic wind, and the second one is that it is wind resulting from recirculated airflow from the synoptic-scale onshore wind. Air blocked by the mountain range at low Froude numbers is recirculated and flows at low levels back offshore. Arguments in favor of and against the two hypotheses are presented. It is argued that both the recirculation of airflow and land breeze/katabatic wind contribute to the formation of the offshore atmospheric front but that land breeze/katabatic wind is probably the main cause.
SOLA, 2014
The ECMWF 2.5° gridded analysis was used to reexamine the evolution of the synoptic flow patterns and frontal structure of an early summer monsoon trough that occurred during 10−15 June 1975. A blocking pattern that began with an omega shape developed into a Rex pattern on 12 June. During 10−14 June, the blocking low pressure and associated trough axis were almost stagnant. As a result, the Mei-Yu front was quasi-stationary and affected the Taiwan area for more than four days. Similar to other frontal systems during the early summer rainy season over southern China, this Mei-Yu front exhibited baroclinic characteristics in the subtropics. In the lower troposphere, appreciable temperature gradients and maximum frontogenesis due to horizontal deformation between the postfrontal northwesterlies and prefrontal southwesterlies were diagnosed. The western section (~115°E) of this frontal system exhibited a marked northward vertical tilt. An upper-level jet near the tropopause was also present. A moist tongue was located south of the surface cold front within the low-level southwesterlies and extended vertically upward. A thermally direct circulation across the front with ascending motion within the prefrontal warm, moist air and descending motion within the postfrontal cold, dry air underneath the upper-level jet was diagnosed.
Journal of Climate, 2014
About 44% of the cold-season heavy rainfall/flood (HRF) events around the South China Sea require six days or longer to develop from the formation time of their parent cold surge vortices (CSVs). Formations for both the parent CSV and HRF event are involved with interactions of the concerned vortices with two different cold surge flows. The occurrence frequency of the East Asian cold surge flow varies from 4.5 to 6 days. The longevous CSVs enable their developments to interact with the second cold surge flows between formations of these CSVs and HRF events. Two requirements for the formation of HRF events are 1) synchronized occurrence of the HRF event and the northwestern Pacific explosive cyclone and 2) simultaneous occurrence of the maximum speeds among westerlies of the northwestern Pacific explosive cyclone and easterlies of the tropical trade winds and the HRF event. These requirements cannot be met by the CSV at its second maximum peak intensity, but the CSV at this stage pla...
Tellus A, 2007
This study finds a relationship between the distribution of the seven-year mean relative vorticity of surface winds (RVSW) derived from 25-kilometer-resolution measurements of ocean winds from the Quick Scatterometer (QuikSCAT) satellite and the genesis locations of tropical cyclones (TCs) in the South China Sea (SCS). The results show the effects of the background mean RVSW on TC formation in the SCS. During the summer monsoon, TCs are formed in the northern SCS, where monsoon trough and coastal mountains contribute to positive mean RVSW, while during the winter monsoon TCs form in the southern SCS with positive mean RVSW. No TCs formed in the southern part of the SCS during the summer monsoon nor in the northern part during the winter monsoon during the period from 1945 to 2005, both of which coincide with negative mean RVSW induced by the coastal mountains. The mean RVSW prior to TC formation in regions around the TC genesis locations are calculated using the QuikSCAT measurements averaged over 3 d before the geneses of 36 TCs formed in the SCS from July 1999 to June 2006. The value of this mean RVSW in the SCS is 6.19 × 10 −5 s −1 , which is about 1.7 times the planetary vorticity at the TC genesis positions and about 5.0 times the seven-year monthly mean RVSW. The results of these quantitative calculations using the direct satellite observations support the hypothesis given by Gray in 1975 that above-normal low-level relative vorticity is necessary for the occurrence of tropical cyclogenesis.
Atmospheric fronts off the East Coast of Taiwan studied by ERS synthetic aperture radar imagery
Geoscience and Remote Sensing …, 2005
Synthetic aperture radar (SAR) images acquired by the European Remote Sensing satellites ERS-1 and ERS-2 over the east coast of Taiwan around 10:30 local time often show frontal features that are located typically 50 km off the coast. We show that they are not sea surface manifestations of a land breeze front, but of a quasi-stationary atmospheric front that persists over the whole day or even over the next day(s). Evidence for this interpretation is obtained mainly from the comparison of the SAR images with sequences of cloud images acquired by the Japanese Geostationary Meteorological Satellite GMS-4.
Influences of Typhoon Chanchu on the 2006 South China Sea summer monsoon onset
Geophysical Research Letters, 2008
This study reveals some unique features associated with the impact of Typhoon Chanchu on the South China Sea (SCS) summer monsoon (SCSSM) onset in 2006. With Typhoon Chanchu entering the SCS in mid-May, southwesterlies were induced over the SCS and its upstream region due to a thermally forced Rossby wave response, leading to a reversal of low-level winds and a positive meridional temperature gradient (MTG) in the upper troposphere over the SCS. Typhoon Chanchu thus acted as an immediate trigger for the SCSSM onset, and the onset date was suggested to occur in the third pentad of May. After the typhoon landed in the coast of southern China, it still contributed to the establishment of the SCSSM due to warming the air column over the northern SCS, resulting in a persistent positive MTG with the ridge surface tilting northward.