著者
Kensuke NAKAJIMA Eizi TOYODA Masaki ISHIWATARI Shin-ichi TAKEHIRO Yoshi-Yuki HAYASHI
出版者
(公社)日本気象学会
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
vol.82, no.6, pp.1483-1504, 2004 (Released:2005-03-02)
参考文献数
23
被引用文献数
6 7 7

For the purpose of examining the initial development of the atmospheric response to a warm SST anomaly placed at the equator, an ensemble switch-on experiment is conducted with an aqua-planet GCM. An ensemble average of the size of 128 significantly reduces the transient noises caused by both small scale convective activity and large scale intraseasonal variability.In the first three days after the switch-on of the SST anomaly, a convection center develops above the warm SST area. As a barotropic response to the heating of convection center, a global increase of surface pressure occurs outside the low pressure region around the warm SST area. The response after the emergence of the high pressure anomaly is consistent with Gill (1980); a warm Kelvin wave-like anomaly is emitted to the east of the convection center, while a warm Rossby wave-like anomaly is emitted to the west.The Kelvin wave-like signal propagates at a speed slower than that of free Kelvin wave expected from its vertical wavelength, suggesting that the signal is a “moist” Kelvin wave. Transient decrease of precipitation occurs at the moist Kelvin wave front; a decrease of convection associated with the downward motion at the wave front is consistent with its slow propagation. After several days, precipitation recovers and is even intensified because of the surface frictional convergence associated with the Kelvin wave-like equatorial low pressure anomaly. To the west of the warm SST area, on the other hand, precipitation decreases monotonically. The continuous reduction of precipitation is caused by the equatorial surface frictional divergence associated with the relatively high pressure anomaly at the equator of the Rossby wave structure.Finally, there appears a slow zonally symmetric response within the Hadley cell characterized with surface pressure rise in the tropics and westerly wind anomaly in the troposphere. The change of eddy zonal momentum transport, together with the transport toward the lower level by the Hadley circulation and the geostrophic adjustment to the resulting low level westerly acceleration, seems to be responsible for the response.
著者
Hiroki TSUJI Hisanori ITOH Kensuke NAKAJIMA
出版者
(公社)日本気象学会
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
vol.94, no.3, pp.219-236, 2016 (Released:2016-07-02)
参考文献数
29
被引用文献数
4

To understand the basic mechanism governing the size evolution of tropical cyclones (TCs), we systematically perform numerical experiments using the primitive equation system on an f-plane. A simplified, TC-like vortex is initially given and an external forcing mimicking cumulus heating is applied to an annular region at a prescribed distance from the vortex center. Moist process and surface friction are excluded for simplification. We focus on the sensitivity of size evolution to the location of the forcing. The vortex size is defined as the radius of 15 m s-1 lowest-level wind speed (R15). The evolution of R15 depends on the forcing location, and its dependence can be understood by considering radial transport of the absolute angular momentum (AAM) at R15 due to the heat-induced secondary circulation (SC), whose structure is governed by the distribution of inertial stability. When the forcing is applied to the outer part of a vortex but still inside R15, where inertial stability is weak, the SC extends to the outside of R15 and carries AAM inward. Thus, R15 increases. Conversely, when the forcing is applied near the center of the vortex, where inertial stability is strong, the SC closes inside R15 and R15 hardly increases. These results indicate that extension of the heat-induced SC to the outside of R15 is important for the evolution of the vortex size. Moreover, the further beyond R15 the SC extends, the more the vortex size increases. This relationship is consistent with the result of the parcel trajectory analysis; the larger the extent of SC, the longer distances the parcels cover, conserving larger AAM. Finally, when the forcing is applied to the outside of R15, smaller AAM is carried outward by the SC on the inward side of the heating location, resulting in the decrease of R15.