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Study on Formation and Development of a Mesoscale Convergence Line in Typhoon Rananim∗

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Study on Formation and Development of a Mesoscale Convergence Line in Typhoon Rananim∗ NO:4 LI Ying, CHEN Lianshou, QIAN Chuanhai, et al. 413 Study on Formation and Development of a Mesoscale Convergence Line in Typhoon Rananim¤ 1y ¢¡ 1 2 3 ¨ © § LI Ying ( ), CHEN Lianshou ( £¥¤§¦ ), QIAN Chuanhai ( ), and YANG Jiakang ( ) 1 State Key Laboratory of Severe Weather (LaSW ), Chinese Academy of Meteorological Sciences, Beijing 100081 2 National Meteorological Center, Beijing 100081 3 Yunnan Institute of Meteorology, Kunming 650034 (Received April 29, 2010) ABSTRACT This study investigated the formation and development of a mesoscale convergence line (MCL) within the circulation of Typhoon Rananim (0414), which eventually led to torrential rainfall over inland China. The study is based on satellite, surface and sounding data, and 20 km£20 km regional spectral model data released by the Japan Meteorological Agency. It is found that midlatitude cold air intruded into the typhoon circulation, which resulted in the formation of the MCL in the northwestern quadrant of the typhoon. The MCL occurred in the lower troposphere below 700 hPa, with an ascending airflow inclined to cold air, and a secondary vertical circulation across the MCL. Meso-¯ scale convective cloud clusters emerged and developed near the MCL before their merging into the typhoon remnant clouds. Convective instability and conditional symmetric instability appeared simultaneously near the MCL, favorable for the development of convection. Diagnosis of the interaction between the MCL and the typhoon remnant implies that the MCL obtained kinetic energy and positive vorticity for its further development from the typhoon remnant in the lower troposphere. In turn, the development of the MCL provided kinetic energy and positive vorticity at upper levels for the typhoon remnant, which may have slowed down the decaying of the typhoon. Key words: typhoon circulation, mesoscale convergence line, convective and symmetric instability, inter- action Citation: Li Ying, Chen Lianshou, Qian Chuanhai, et al., 2010: Study on formation and development of a mesoscale convergence line in Typhoon Rananim. Acta Meteor. Sinica, 24(4), 413{425. 1. Introduction el al., 1997; Montgomery et al., 2002; Black et al., 2004). An MCS not only produces rainstorm, but also The typhoon is a weather phenomenon at the syn- a®ects the typhoon structure/intensity (Chen et al., optic scale. However, it brings up mesoscale convective 2002) through causing extremely asymmetric spatial systems (MCSs) within its circulation. Radar echoes and temporal distributions in typhoon circulation. have already con¯rmed the existence of mesoscale spi- A typhoon usually decays after its landfall. How- ral cloud bands in typhoons long ago (Maynard, 1945; ever, it may produce an active MCS such as a Parrish et al., 1982). In recent years, satellite remote- mesoscale vortex or a mesoscale convergence line sensing data, air-borne radar observations, and drop- within its circulation due to the e®ects of terrain and sonde measurements have been increasingly employed midlatitude weather systems. The active MCS encour- to detect MCS activities within typhoon circulation. ages a sudden increase of typhoon rainfall, and results Meanwhile, mesoscale models have demonstrated re- in a severe disaster pounding the inland areas. Ty- ¯ned mesoscale typhoon structures, and unveiled the phoon Nina (7503) is a typical example in this context. features of MCS activities in typhoon circulation (Liu It bred a mesoscale convergence line (MCL) as a ¤Supported by the National \973" Program of China under Grant No. 2009CB421504, the National Natural Science Foundation of China under Grant Nos. 40730948, 40675033, and 40975032, and the Key Project of the Chinese Academy of Meteorological Sciences under Grant No. 2008LASWZI01. yCorresponding author: [email protected]. (Chinese version to be published) 414 ACTA METEOROLOGICA SINICA VOL.24 result of the interaction between the typhoon remnant ture of the MCS. Section 5 delineates the interaction and terrain in Henan Province. A continuous flow between the MCS and typhoon circulation. Conclu- of small convective vortexes was produced near the sions are given in Section 6. MCL, which brought up excessive rainfall and trig- gered the notorious \75¢8" flash flood in Henan. Al- 2. Data and veri¯cation though Typhoon Talim (0513) degraded into a tropical depression after landfall, it still produced heavy down- In this study, 6-h NCEP analysis data with a res- pour with rainfall rate of 529 mm day¡1 in Lushan of olution of 1±£1± were used to analyze the background Jiangxi Province (He et al., 2006). Another case is Ty- circulation. Surface observations, sounding data, and phoon Rananim (0414). It brought torrential rain over FY-II infrared satellite images from China Meteoro- inland China, inducing severe mountain floods, land- logical Administration (CMA) were employed to ex- slides, and mudslides covering wide areas, and left an plore the MCS activities in typhoon Rananim. The 6- economic loss that is as heavy as in coastal areas. Such h Japan Meteorology Agency Regional Spectral Model a typhoon-induced inland rainfall is closely associated (RSM) data were utilized to examine the structure with MCS activities in the typhoon remnant. of the MCS. RSM is one of the current operational Many previous studies have examined the mech- weather models of Japan. It is a primitive equation anism of MCS activities in landfalling typhoons. Re- model with 40 vertical levels up to 10 hPa. In the sults show that the convergence e®ect of underlying horizontal direction, it uses a spectral representation mountain terrain can generate strong convection and and has a grid-equivalent resolution of 20 km at 30±N vortex systems at small- and mesoscale within typhoon (60±N) on a Lambert projection plane. Previous stud- circulation, leading to heavy rainfall or high winds in ies have shown that RSM is successful in simulating the a®ected area (Chen et al., 2002). Heat fluxes above the multiple polar mesocyclones over the Japan Sea a saturated wetland are required for the sustaining or and a convective line in Meiyu near Taiwan (Fu et al., strengthening of the typhoon remnant, and momen- 2004; Wang et al., 2005). tum fluxes are favorable for the development of MCS We verify the RSM data against observations in in typhoon circulation, and for enhancing local precip- the post-landfall period of Typhoon Rananim. Figures itation (Li and Chen, 2007). In addition, interaction 1a and 1b compare the track and the minimum central between landfalling typhoons and midlatitude troughs pressure of Typhoon Rananim between RSM and the can speed up the development of MCS, which in turn CMA best analysis from 1200 UTC 12 to 1200 UTC induces a noticeable change of rainfall intensity and 14 August 2004. The RSM typhoon translates a little distribution (Meng et al., 2002). Some studies an- slower than the observations when it goes deep into the alyzed the features and propagation of mesoscale dis- land. The maximum track deviation is about 50 km in turbances using the wave theory (Luo and Chen, 2003; the last 12 h. We compare the time series of the min- Lu et al., 2002). Unfortunately, our understanding on imum central pressure, and ¯nd that RMS typhoon is the structure of typhoon's inner MCS and the inter- weaker than the best analysis, particularly at the land- action between MCS and typhoon circulation remains fall of 1200 UTC 12 August with a 20-hPa di®erence. insu±cient, as such activities are complicated in na- However, the errors become remarkably smaller after ture. landfall, which is the time period we concern, with the The next section introduces data sources used in maximum error not greater than 5 hPa, and the gen- this study and provides a veri¯cation of model results eral comparison of their trends is favorable. Figures 1c against observations. Section 3 presents an overview of and 1d compare the wind vectors and speci¯c humid- Typhoon Rananim and its synoptic environment. Sec- ity between RSM and the observations on 700 hPa at tion 4 analyzes the formation and development of an 1300 UTC 13 August 2004. It is apparent that RSM MCS in Typhoon Rananim, and examines the struc- reproduces favorably the cyclonic circulation structure NO:4 LI Ying, CHEN Lianshou, QIAN Chuanhai, et al. 415 Fig. 1. Comparison between RSM simulations and observations. (a) Typhoon track with 6-h intervals over mainland China, (b) minimum sea level pressure of typhoon center from 1200 UTC 12 to 1200 UTC 14 August 2004, (c, d) horizontal wind vectors (m s¡1) and speci¯c humidity (g kg¡1) on 700 hPa at 0000 UTC 13 August 2004, and (e, f) height-meridional sections of wind vectors (m s¡1) and temperature (±C) along 29±N at 1200 UTC 13 August 2004. In (a) and (b), the line with squares denote RSM, and the line with solid circles denote observations. Results in (c, e) are from RSM, and those in (d, f) are from observations. including its central position, range, and intensity. drier air zone, caused by a weak intrusion of cold air at The RSM also exhibits the main thermodynamic fea- 112±E in the northwestern quadrant of the typhoon. tures such as the relatively moist typhoon core and the Figures 1e and 1f show the comparison of height- 416 ACTA METEOROLOGICA SINICA VOL.24 meridional sections of wind vectors and temperature phoon Rananim, as veri¯ed against the observations. between RSM and observations along the typhoon cen- Therefore, in the following text, we will use the RSM ter at 1200 UTC 13 August 2004. Both of them indi- data to analyze the mesoscale structure of the typhoon cate that the typhoon centers are located near 116.7±E circulation. with southeasterlies on the eastern side and northeast- 3. Overview of Typhoon Rananim and its erlies on the western side, and the breaking point of synoptic environment wind directions are similar over the typhoon centers.
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