Use the Method of PV Inversion to discus Effect of TC on Large-scale Circulation

• Main Authors: Chia-Chung Hsu, 徐家鍾
• Other Authors: 許晃雄
• Format: Others
• Language: zh-TW
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/38449361835221195709

碩士 === 國立臺灣大學 === 大氣科學研究所 === 99 === This study applies typhoon-filtering technique to remove TC disturbance field from the large-scale field, and then use PV inversion to derive the three-dimensional circulation associated with the TC vortex. The period from June to August 2004 was chosen for study because of the active intraseasonal oscillation and its strong clustering effect on TC. A comparison between the original and TC-removed fields yields an estimation of TC contribution to the large-scale circulation variability and to the energy conversion between eddy and mean flow. Similar approach was also applied during the different phases of intraseasonal oscillation. TC is a vortex with its strong circulation, especially along the TC tracks. In the Western North Pacific (WNP), the TCs contribute about 80% of total vorticity variance and account for 60% of eddy kinetic energy at 850 hPa. However, the contribution of TCs decreases with height and accounts for 20% and 10% of the total vorticity variance and eddy kinetic energy, respectively, in the upper troposphere. The barotropic energy conversion associated with the eddy–mean flow interaction is closely related to the TC activity in the WNP. In synoptic-scale, TCs help convert energy from mean flow to eddy in the lower troposphere in the beginning of its lifetime. u''v''(∂u/∂y) and −u''v''(∂v/∂x) are the dominant terms, both positive to the south of 20∘N. This is because the momentum flux difference between the original and TC-removed field is positive in the east of the Taiwan and Philippines, and the background circulation is characterized by ∂v/∂x<0 and ∂u/∂y<0. In the upper troposphere, TCs help convert energy from eddy to mean flow in the latter part of the lifetime. The −u''v''(∂u/∂y) and −u''v''(∂v/∂x) are small and are in reversed phase. The energy conversion term is thus dominated by −u''^2(∂u/∂x) and −v''^2(∂v/∂y) term. In 2004, large-scale circulation is characterized by intrseasonal oscillation with a period around 60 days period between 10°N-20°N and 120°E-150°E in the original field. Removing TCs significantly reduces the intraseasonal variance. Results of the spectral analysis in the original time series, one spectral peak (exceeding 95% confidence level) are found near 60 days, but drops substantially after the removal of TCs. TCs significantly enlarge the fluctuation of the intraseasonal oscillation. During the ISO active phase, the positive barotropic energy conversion in 120°E-150°E and negative barotropic energy conversion near the South China Sea and subtropical high occurred. During the ISO suppress phase, with alternating positive and negative barotropic energy conversion are limited in the region between south of Japan and east of Taiwan. In both situation, the conversion term is dominated by −u''v''(∂u/∂y) and −u''^2(∂u/∂x) term, especially −u''v''(∂u/∂y) term.