Sumatra Island

Figure 6. Japanese Geostationary Meteorological Satellite image at 0232 UTC 27 December 2001. All national weather services in the region reported it as a tropical storm.

(e.g. DeMaria and Pickle, 1988), the spiral cloud bands emanating out from around the center clearly indicate that the storm circulation was on both sides of the equator. Figure 6 shows the Japanese Geostationary Meteorological Satellite image at 0232 UTC of the same day. Feeder bands from both sides of the equator spiral into the center of Vamei, where a small eye is visible. An eye was also observed in TRMM and SSM/I images within the preceding two hours (not shown). The diameter of the eye estimated from different sensors ranges from 28 km to 50 km. Vamei's small size as it formed at the southern end of the South China Sea made it difficult to observe its highest wind speed from ground-based observations or to estimate its intensity from satellite images. As a result, all national weather services in the region reported it as a tropical storm. Without the chance passage by the USS Carl Vinson carrier group through its eyewall, the JTWC may not be able to operationally upgrade the intensity of the storm to that of a typhoon either.

Figure 7 shows Doppler weather radar images from Singapore's Changi Airport during the 12 hours prior to the arrival of Vamei. The rapid development of the eye of Vamei can be readily seen in 3-hour intervals. The eye was just starting to form with an irregular boundary when the storm moved into radar range at 1930 UTC 26 December 2001 (right panel). It became quite well organized 3 hours later, at 2230 UTC (middle panel), with a geometric center of the eye near 1.4°N. By 0130 UTC 27 December (left panel), the eye had become a nearly symmetric round feature.

Evidence of the strength of Vamei and its relationship with the cold surge can be revealed from QuickSCAT satellite scatterometer wind data. Figure 8 shows that the QuickSCAT wind

Figure 7. Changi Airport (Singapore) Doppler weather radar images (the color legend indicates estimated rain rates, in mm/h) in three-hour intervals, with the time sequence of the images organized from right to left: 1930 UTC 26 December (right panel), 2230 UTC (middle panel) and 0130 UTC 27 December (left panel).

Figure 7. Changi Airport (Singapore) Doppler weather radar images (the color legend indicates estimated rain rates, in mm/h) in three-hour intervals, with the time sequence of the images organized from right to left: 1930 UTC 26 December (right panel), 2230 UTC (middle panel) and 0130 UTC 27 December (left panel).

Quickscat Satellite

Figure 8. QuickSCAT satellite scatterometer wind direction and speed (color shading and arrow length) at 2232 UTC 26 December 2001, showing the typhoon strength of Vamei and the remnant of the continuing surge wind upstream in the northern South China Sea. See text for details. (Diagram courtesy of Jet Propulsion Laboratory/NASA.)

Figure 8. QuickSCAT satellite scatterometer wind direction and speed (color shading and arrow length) at 2232 UTC 26 December 2001, showing the typhoon strength of Vamei and the remnant of the continuing surge wind upstream in the northern South China Sea. See text for details. (Diagram courtesy of Jet Propulsion Laboratory/NASA.)

direction and speed at 2232 UTC 26 December 2001 captured both the signal of Vamei as it developed to typhoon strength, and the remnant of the continuing surge wind upstream in the northern South China Sea. At the southern perimeter, the wind speed at a 10 m height has already reached above 27 ms-1 over an area of about 1° latitude x 1° longitude. The northern spiral band extends to about 6°N and is detached from the cold surge wind belt further north.

3. Roles of the Winter Monsoon and Possible Mechanisms

Based on the synoptic sequence of the low-level circulations, Chang et al. (2003) suggested that Vamei formed as a result of an interaction between two prominent features of the Asian winter monsoon: a weak Borneo vortex that drifted into, and remained at, the southern tip of the South China Sea; and a strong and persistent cold surge that created the large background cyclonic vorticity at the equator.

A similar equatorial generation process was proposed two decades ago in the cold surge theory of Lim and Chang (1981), who used the framework of the equatorial beta-plane equatorial wave theory. In their barotropic theory, geostrophic adjustment and potential vorticity conservation following a cross-equatorial surge spin up counterclockwise rotation to the east of the surge axis, where in the real world the Borneo vortex is located. A comparison of Lim and Chang's cold surge theory and the observed low-level flow during the development

EQUATOR! AIL WAVE THEORY

EQUATOR! AIL WAVE THEORY

NCSAP« BR O HPAWiHD ANALYSES
Serveteliu Nerimas Schemos
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Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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