NO.4 LI Ying, CHEN Lianshou, QIAN Chuanhai, et al. 413

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

1† ¢¡ 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 . The study is based on satellite, surface and sounding data, and 20 km×20 km regional spectral model data released by the 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-β 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- affects 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 confirmed the existence of mesoscale spi- A typhoon usually decays after its landfall. How- ral cloud bands in 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 effects 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- fined 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. †Corresponding 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 verification 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 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 effect 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 affected 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 (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 find 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 difference. insufficient, 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 verification 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 specific 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 specific 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 verified 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. The RSM temperature distribution also conforms well Typhoon Rananim originated at 1200 UTC 8 Au- to the observation. Both of RSM and the observation gust 2004 over the Pacific Ocean in the east of the display that the 0◦C isoline lies on 500 hPa and the Luzon Island of . It moved northwestward value of temperature is about 24◦C at 925 hPa over heading for China’s southeast coastal areas, and in- the typhoon center. tensified into a typhoon in the early morning of 11 In general, we may state that despite the weaker August. It made landfall at 1200 UTC 12 August in intensity in the RSM results, RSM can reproduce de- Wenling City of , with a minimum sea level cently the track and main structural features of Ty- pressure of 950 hPa, and the maximum wind speed

Fig. 2. (a) Accumulated rainfall from 0000 UTC 12 to 0000 UTC 15 August 2004 based on surface observations, (c) topographic height, (c) 500-hPa winds (vectors; m s−1) and geopotential height (gpm), and (d) 850-hPa winds (vectors; m s−1) and temperature (◦C; areas with temperature 6 18◦C are shaded) at 0600 UTC 12 August 2004. Results in (c)–(d) are plotted using the NCEP analysis data. NO.4 LI Ying, CHEN Lianshou, QIAN Chuanhai, et al. 417 around 45 m s−1. Then it moved westward, entered over the Poyanghu Plain in the north of Jiangxi, with Jiangxi as a tropical storm in the morning of 13 Au- its intensity slowly decaying in the next 18 h, which is gust and stagnated there for about 22 h before mov- favorable for sustained typhoon-induced rainfall over ing into the northeastern part of in the morn- there (Xu et al., 2005). At lower levels, the midlati- ing of 14 August. It eventually dissipated in south- tude cold air intruded into the cyclonic circulation of eastern Hubei at 1800 UTC 14 August (see Figs. 1a typhoon via the region between the two highs. Fig- and 1b). ure 2d shows that Rananim’s circulation met a cold Figures 2a and 2b show the accumulated rainfall air center of 12◦C in its northwest at 850 hPa at 0600 distribution from 0000 UTC 12 to 0000 UTC 15 Au- UTC 12 August. Meanwhile, warm wet airflows over gust 2004 associated with Typhoon Rananim and the coastal waters of China were forced into the inland topographic height in eastern China. There are two areas by easterly airflows along the northern flank of noticeable heavy rainfall centers of more than 200 mm the typhoon. Ascending motion and water vapor con- along the typhoon track: one in the coastal area of vergence would be strengthened in the region where Zhejiang and the other over the middle and northern warm wet airflows mingled with cold air. This favored plains of Jiangxi. It is obvious that Rananim induced a rainfall increase, similar to the case studied by Niu a heavy rainfall when passing through the inland ar- et al. (2005). eas, despite its decaying. Rananim had a large-scale circulation pattern fea- 4. MCS activities in typhoon circulation tured with a strong western North Pacific subtropi- 4.1 Activities of mesoscale convective cloud cal high and a continental high, which connected with clusters each other with the 5860 gpm isoline in a strip running from the east to west. As evident in Fig. 2c, Rananim Figure 3 displays satellite infrared images on 13 sat on the southern side of the two highs and just in August 2004. At 0100 UTC 13 August (Fig. 3a), between at 0600 UTC 13 August. Then it stagnated Rananim was traveling across the hilly area in the west

Fig. 3. Satellite infrared images from 0100 UTC 13 to 0000 UTC 14 August 2004 (number indicating year.month.day.hour, the same below). 418 ACTA METEOROLOGICA SINICA VOL.24 of Zhejiang, without any noticeable rainfall in Jiangxi remnant center of typhoon. At this time, the mid- in the past 24 h. At the noon (0400 UTC) of 13 Au- latitude northerly cold air remained distant from the gust (Fig. 3b), convective cloud clusters (indicated by typhoon remnant, with its front still lingering at 35◦N, arrows) began to form in a patch two latitudinal de- northwest to the remnant. At 0000 UTC 13 August grees northwest of the typhoon center, from Anhui to (Fig. 4b), the northerly airflows approached 29◦N the northern part of Jiangxi. Two hours later (Fig. along 115◦E, and formed a convergence region (de- 3c), the convective cloud clusters boomed up rapidly, noted by the symbol C) over Jiangxi where the basin forming a convective band stretching from north to terrain in “V” shape (see Fig. 2b) allowed the cold air south, containing 6 to 7 convective cells of about 100 to be trapped inside. Easterly warm and wet airflows km each in size, at a meso-β scale level. After that, from coastal waters of China on the northern side of convective cloud clusters went on with merger and de- the typhoon remnant moved westward along with the velopment, and turned themselves into two large con- vortex circulation, with its front entering the Jiangxi vective cloud clusters at 1200 UTC 13 August, ap- basin, where it joined the cold air and formed a weak proaching the remnant center of Rananim in the south MCL indicated by the double dashed line. Rainfall (Figs. 3d, e). Convective cloud clusters went deeper in the next 6 h occurred mainly along the MCL and into the remnant center in the following 12 h. Mean- beneath the remnant clouds. At 0600 UTC 13 Au- while, the remnant center was moving northwestward gust (Fig. 4c), the easterly warm wet airflows and heading for the patch where convective cloud bands the northerly cold airflows intensified and produced a were developing, which enhanced the remnant clouds. remarkable MCL in the northwest of the typhoon rem- The cloud image at 0000 UTC 14 August shows that nant. The convergence was enhanced with a value of the entire Jiangxi was dominated by dense depression –8×10−5 s−1 near the MCL. The convergence occurred clouds (Fig. 3f). From 0000 UTC 13 to 0000 UTC 14 two longitudinal degrees away from the typhoon rem- August, a heavy rainfall center of 200 mm appeared nant center, in a south-to-north strip straddling over in the northern part of Jiangxi. three to four latitudinal degrees. Basically, this area is Satellite images show that meso-β scale convec- where strong convective cloud clusters formed and de- tive cloud clusters were produced and they devel- veloped, as shown in the satellite images. Rainfall in oped further in the northwestern quadrant of Typhoon the next 6 h took place in the warm air mass near the Rananim after it made landfall. Apparently, the in- MCL, with two heavy rainfall centers in the south and land heavy rainfall is mainly caused by the rapidly- north, respectively. Heavy rainfall clusters reached a booming convective cloud clusters and their merging scale of 200 km, with a maximum rainfall of 100 mm in with the remnant vortex clouds. 6 h. At 1200 UTC 13 August (Fig. 4d), the MCL in- tensity decreased, with a smaller wind speed, resulting 4.2 Activities of the MCS on surface in greatly-reduced rainfall near the MCL in the next 6 In this section, we analyze MCS activities in ty- h. The maximum rainfall center shifted to the south- phoon remnant circulation using surface observational eastern flank of the remnant center. This is because data. the cold airflows veered from northerly to westerly on Figure 4 gives distributions of surface wind vec- the southern side of the remnant center, leading to the tors, divergence fields, and next 6-h accumulated rain- formation of another MCL between the westerly and fall, from 1800 UTC 12 to 0000 UTC 14 August 2004. easterly airflows. At 1800 UTC 13 August (Fig. 4e), It is evident that at 1200 UTC 12 August, i.e., 6 h wind vectors at both flanks of the previous MCL be- after Rananim’s landfall, a depression appeared over came much weaker in magnitude and more uniform in the hilly areas of Zhejiang, with a relatively intact and direction, indicating the disappearance of the MCL. isolated vortex circulation (Fig. 4a). Rainfall in the However, rainfall in the next 6 h sustained near next 6 h mainly occurred in the coastal areas near the the remnant center where a new MCL formed by NO.4 LI Ying, CHEN Lianshou, QIAN Chuanhai, et al. 419 convergence of westerly cold air and easterly warm wet gered the MCL formation and the MCL approached air. Rananim entered the Jiangxi basin at 0000 UTC the remnant center in a spiral manner, with its earli- 14 August. Its remnant was basically a cyclonic cir- est appearance in the northwest of the remnant, then culation with the involvement of cold air. The MCL the south, and then the northeast. Heavy rainfall cen- between cold and warm airflows was in the northeast- ters were located in line with the area between cold ern quadrant of Rananim (Fig. 4f). Heavy rainfall and warm airflows, indicating a significant relation be- kept occurring near the remnant center because of the tween mesoscale convective activity and cold air inva- topography induced convergence effect. sion. Surface observations show that Rananim brought 4.3 The MCL structure warm and wet air against the cold air from mid and high latitudes when they invaded deeply into the in- The aforementioned analysis shows that land areas. Cold airflows merged with warm airflows Rananim-induced heavy rainfall over the inland China within the typhoon remnant circulation, which trig- is closely associated with the MCL activities. The

Fig. 4. Surface wind vectors, wind convergence (shaded; only values 6–2×10−5 s−1 are plotted with an interval of 2), and the next 6-h accumulated rainfall (isolines; only values > 10 mm are plotted), from 1800 UTC 12 to 0000 UTC 14 August 2004 based on the surface station observations. Double dashed lines denote convergence lines, and white stars indicate locations of the typhoon remnant center. 420 ACTA METEOROLOGICA SINICA VOL.24 most remarkable effect of the MCL on rainfall is seen The MCL (denoted by double dashed lines) at 925 in the northwestern part of the typhoon circulation. hPa was located in a patch 160-km northwest of the We analyze the structure of this MCL using the La- typhoon center (Fig. 5a). The one at 850 hPa was grangian coordinates (at the averaged moving speed 200 km away (Fig. 5b), the one at 700 hPa was 280 of the remnant center on 13 August 2004, namely u0 km away or 80 km further west versus the one at 850 −1 −1 = –3.5 m s , v0 = 1.2 m s ) in order to better hPa, or 120 km further west compared with the one understand its formation mechanism and its effect on at 925 hPa (Fig. 5c). The MCL at 500 hPa was rainfall. Here, a Shuman-Shapiro (Ding, 1989) 9-point much weaker than that at 700 hPa (Fig. 5d). This smoothing operator (with a smooth coefficient of 0.5) indicates that the MCL existed strongly in the lower is used to smooth the 20-km physical fields for five troposphere and was inclined westwards with height times in order to extract mesoscale disturbances with below 700 hPa. At 300 hPa (figure omitted), there main wavelengthes less than 200 km. existed divergence above the MCL, favorable for the Figure 5 shows disturbance wind fields at 925, sustainment of convergence and ascending motion in 850, 700, and 500 hPa at 0600 UTC 13 August 2004. the lower layer. Figure 5d also shows some mesoscale

Fig. 5. Disturbance winds (vectors; m s−1) from the RSM data at (a) 925 hPa, (b) 850 hPa, (c) 700 hPa, and (d) 500 hPa at 0600 UTC 13 August 2004. The circles depict the distance in 200 and 400 kilometers away from the typhoon center, D represents the divergence center, the long arrow denotes the 280-degree wind direction line used for making vertical profiles, and the fan-shaped frame indicates the area for average in the following text. NO.4 LI Ying, CHEN Lianshou, QIAN Chuanhai, et al. 421

Fig. 6. Vertical cross-sections of (a) stream line and vertical wind speed (10−3 hPa s−1) and (b) equivalent potential temperature (K) based on the RSM data along the long arrow line in Fig. 5a at 0600 UTC 13 August 2004. Abscissas indicate the distance in kilometer away from the typhoon center, and the double dashed line denotes the MCL location on surface. divergence centers (D) within 100 km of the MCL, The situation at 1200 UTC 13 August (figure omit- which may imply some vertical motion features of ted) showed that both the lower and mid levels were MCSs. dominated by cold air above the MCL, and the lower Figure 6 displays the cross-sections of vertical ve- layer shifted from convective instability to stability, locity and equivalent potential temperature along the leaving no room for further development of vertical 280-degree wind direction line at 0600 UTC 13 Au- convection. gust 2004. It is seen that below 700 hPa, there was a 4.4 Conditional symmetric instability slantwise ascending motion (denoted by negative val- ues) above the MCL, 160 km away from the typhoon Symmetric instability has been widely interpreted remnant center at the lower level and 200 km at the as a cause for the strong ascending motion of mesoscale higher level to the west. The updraft reached the 300- circulation. Conditional symmetric instability (CSI) is hPa level, with the strongest ascending motion seen a type of symmetric instability caused by latent heat near 700 hPa. There was a narrow downdraft patch release in wet air that has a reduced effective static sta- 250–300 km away from the typhoon center. Both the bility. Some studies (Lu et al., 2002) pointed out that updraft and downdraft may form a radial vertical cir- the classic symmetric instability provides a mechanism culation across the MCL (Fig. 6a). The correspond- for the development of atmospheric vortex motion. It ing equivalent potential temperature (θe) cross-section would be interesting, therefore, to explain the convec- shows that the intrusion of cold and dry air allowed the tive motion of the atmospheric vortex using the theory cold and dry air within the remnant vortex circulation of symmetric instability. Under a given condition, the to dominate the western area, 300 km away from the criterion for the existence of CSI in an atmospheric remnant center, with a minimum θe center positioned vortex shall be that the moist potential vorticity (qw) near 700 hPa. Both the low and high levels above the is less than zero, which is MCL were featured with warm and wet air, and the ∂θe low and mid levels with relatively cold and dry air. qw = qw1 + qw2 = −g(ζ + f) ∂p This facilitated convective instability below 900 hPa, ∂u ∂θe ∂v ∂θe −g( − ) < 0, and convective stability above that level (Fig. 6b). ∂p ∂y ∂p ∂x 422 ACTA METEOROLOGICA SINICA VOL.24

∂θe −1 −6 2 −1 −1 Fig. 7. Vertical cross-sections of (a) (isolines; K hPa ) and qw (shadings; 10 m K s kg ; only negative ∂p values are plotted), (b) qw1, and (c) qw2 along the 280-degree wind direction line, averaged between 0000 and 1800 UTC 13 August 2004. The while symbol “∗” denotes the CSI region.

∂υ ∂u where ζ = − , qw1 and qw2 represent the MCL. Between 700 and 900 hPa, there are a shaded ∂x ∂y area (qw < 0 and qw ∼ 0) and a negative-value re- qw barotropic and baroclinic parts of , respectively. ∂θe gion of , indicating the existence of conditional The aforementioned analysis shows that the MCL ∂p was unstable at the lower levels, but tended to be sta- symmetric instability (asterisks). In the distributions ble at the mid and high levels. It was accompanied by of qw1 (Fig. 7b) and qw2 (Fig. 7c), it is found that slantwise ascending flows with relative humidity more qw2 makes a major contribution to the negative value than 90% (figure omitted). We further analyze the in- of CSI, while qw1 is mostly positive. Therefore, the stability based on the moist potential vorticity theory. occurrence of CSI should be associated with the de- The vertical cross-sections of qw, qw1, and qw2 along velopment of moist baroclinicity. In the meantime, we the 280-degree wind direction line averaged between also find that the positive qw1 is near the zero contour ∂θe 0000 and 1800 UTC 13 August 2004 are given in Fig. of , indicating the increase of vertical vorticity. ∂θe ∂p 7. In Fig. 7a, the isoline shows the distribution ∂p The above results indicate that the strong and shadings depict negative qw. It is obvious that moist baroclinicity of MCL leads to CSI genesis ∂θe is positive below 900 hPa, suggesting a shallow in its slantwise updrafts. It is clear that convec- ∂p ∂θe tive instability goes along with symmetric instabil- convective instability, while is negative at higher ∂p ity for the MCL, facilitating the development of the levels, indicating a deep convective stability above the convection. NO.4 LI Ying, CHEN Lianshou, QIAN Chuanhai, et al. 423

5. Interaction between the MCL and typhoon indicates that the interaction between the two fields circulation cause the disturbance field to provide positive vortic- ity to the background field. Oppositely, it gets positive MCL does not exist without the remnant circu- vorticity from the background field. lation of typhoon. Here, we analyze the interaction In the studied case, the MCL was extracted us- between the MCL and the typhoon remnant through ing the filtering method and it was present in the calculation of the kinetic energy and vorticity conver- mesoscale disturbance field with wavelength less than sion between the two systems on different spatial scales 200 km, while the typhoon circulation was in the back- (Chen and Xie, 1981). ground field, in which the waves with wavelength more Assuming that a physical quantity A can be writ- than 600 km were kept. We calculate the kinetic en- ten as: ergy and vorticity conversion averaged over the MCL A = A∗ + A0, area, defined as a region between 250- and 330-degree wind direction lines and 100–400 km away from the ∗ 0 where A is the background field, and A is the distur- typhoon remnant center (i.e., the fan-shaped area in bance field. Fig. 5a). The horizontal and vertical conversion terms of Figure 8 shows the conversion of averaged kinetic kinetic energy are given as follows: energy and vorticity during the development period of

∗ ∗ ∗ ∂ ∗ ∗ ∗ the MCL. In terms of kinetic energy conversion (Fig. (V · I )H = −{u [ ((uu) − u u ) ∂x 8; upper panels), the MCL obtained kinetic energy ∂ ∂ + ((uv)∗ − u∗v∗)] + v∗[ ((uv)∗ − u∗v∗) from the lower troposphere below 900 hPa, and pro- ∂y ∂x vided kinetic energy for typhoon circulation at higher ∂ + ((vv)∗ − v∗v∗)] , levels. Horizontal conversion (broken lines) was a key ∂y ª factor, with vertical motion (dotted lines) offering a ∗ ∗ ∗ ∂ ∗ ∗ ∗ (V · I )V = − u [ ((ωu) − ω u )] © ∂p very limited contribution. It could be concluded that ∂ the MCL may gain convective unstable energy from +v∗[ ((ωv)∗) − ω∗v∗)] , ∂p ª lower layers of the background typhoon remnant for where V ∗ · I∗ indicates the conversion of kinetic en- further development. Meanwhile, the development of ergy between the disturbance and background fields, MCL could convert baroclinic energy at higher levels and the subscripts H and V denote the horizontal and into kinetic energy for the sustainment of the back- vertical terms, respectively. Positive values of V ∗ · I∗ ground typhoon remnant circulation. In the early de- mean that the disturbance field provides kinetic energy velopment of MCL (Fig. 8a; 0000 UTC 13 August), to the background field, while negative values imply the kinetic energy provided by the mesoscale distur- that the disturbance field acquires kinetic energy from bance was most noticeable. In the late development of the background field. MCL (Figs. 8b and 8c), the kinetic energy conversion The horizontal and vertical conversion terms of was reduced, which may be associated with the grad- vorticity are then: ual release of unstable energy. For vorticity conversion, the results are different H ∂ ∂ I = − (ζu)∗ − ζ∗u∗ + (ζu)∗ − ζ∗u∗ from the above (Fig. 8; lower panels). The MCL re- ζ n∂xh i ∂xh i ∂ ∂ ceived vorticity (negative value) from the background + (ζv)∗ − ζ∗v∗ + (ζv)∗ − ζ∗v∗ , ∂xh i ∂xh io field in lower layers below 400 hPa, while providing V ∂ ∗ ∗ ∗ vorticity (positive value) to the background at upper Iζ = − h(ζω) − ζ ω i, ∂p levels. In comparison, horizontal conversion played a where Iζ denotes the conversion of vorticity between major role in vorticity conversion, while vertical con- the disturbance and background fields. Here, Iζ > 0 version only changed the vorticity distribution in the 424 ACTA METEOROLOGICA SINICA VOL.24

Fig. 8. The mean conversion of the kinetic energy (10−3 W kg−1; upper panels) and vorticity (10−10 s−2; lower pannels) in the MCL area at 0000 UTC (a, d), 0600 UTC (b, e) and 1200 UTC (c, f) 13 August 2004. Dotted lines represent vertical conversion, broken lines represent horizontal conversion, and solid lines denote the sum of the two. vertical direction. As a whole, the MCL obtained pos- vere economic losses and calamities to China in 2004. itive vorticity from the remnant typhoon circulation, It has not only created huge catastrophes to coastal especially during its development period (Fig. 8e; 0600 provinces, but also produced excessive rainfall in the UTC 13 August). However, the vorticity output was inland areas, resulting in flash floods and geological enhanced at 300 hPa in the later period (Fig. 8f; 1200 disasters in inland China. In diagnosing the forma- UTC 13 August), which might have compensated the tion and development of the MCL within the circula- decay of the typhoon remnant. tion of Typhoon Rananim, which contributed to heavy Our analysis results show that the MCL facili- rainfalls in inland areas, we have reached the following tated its own development by obtaining vorticity and conclusions: kinetic energy from the remnant typhoon circulation 1) The original formation of the MCL in the rem- in lower layers, and provided kinetic energy and posi- nant circulation of Rananim was associated with the tive vorticity to the typhoon remnant at upper levels, cold air. The cold airflow intruded into the typhoon which was advantageous to the maintenance of the ty- remnant and met the easterly warm wet airflow at phoon remnant and the rainfall enhancement. This lower levels, which led to the formation of the MCL. bears some similarities with what we have diagnosed The meso-β scale convective cloud clusters occurred with kinetic energy and vorticity budgets of a typhoon and developed near the MCL before merging into the sustaining over land (Li et al., 2004). remnant clouds. The MCL existed obviously below 700 hPa, accompanied by slantwise updrafts and a ver- 6. Conclusions tical circulation. 2) The increase of atmospheric baroclinicity was Rananim is a typhoon that brought about se- responsible for the occurrence of CSI in slantwise NO.4 LI Ying, CHEN Lianshou, QIAN Chuanhai, et al. 425 updrafts. The CSI together with vertical convective Sinica, 62(3), 257–268. (in Chinese) instability created a favorable condition for the devel- —–, and Chen Lianshou, 2007: Numerical study on im- opment of the MCL convection. pact of the boundary layer fluxes over wetland on 3) The MCL facilitated its own development by sustention and rainfall of landfalling tropical cy- acquiring kinetic energy and positive vorticity from clones. Acta Meteor. Sinica, 21(1), 34–46. Liu Y., D. L. Zhang, and M. K. Yau, 1997: A multiscale the typhoon remnant circulation in lower layers, while numerical study of Hurricane Andrew (1992). Part the development of the MCL provided kinetic energy I: An explicit simulation. Mon. Wea. Rev., 125, and positive vorticity to typhoon circulation at higher 3073–3093. levels, which might have slowed down the decay of the Lu Hancheng, Zhong Ke, and Zhang Dalin, 2002: A typhoon. 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