Journal of the Meteorological Society of , Vol. 77, No. 2,, pp. 459-482, 1999 459

Structure and Evolution of within Yancy (T9313) in

the Early Developing Stage Observed by the Keifu Maru Radar

By Kazumasa Mori

MeteorologicalResearch Institute, Tsukuba, Japan

Syuji Ishigaki

Nagasaki Marine Observatory, Nagasaki, Japan

Takao Maehira, Masakatsu Ohyal

Climate and Marine Department, Japan MeteorologicalAgency, Chiyoda-ku, Tokyo, Japan

and

Hitoshi Takeuchi

Tokyo District MeteorologicalObservatory, Chiyoda-ku, Tokyo, Japan

(Manuscript received80 March 1998, in revised form 8 January 1999)

Abstract

Typhoon Yancy (T9313), which was in the early gradual developing stage and moved westward over the northwestern Pacific near (19N, 129E), was observed by the Japan Meteorological Agency research vessel Keifu Maru, during 30 August to 1 September 1993. During that period, the circulation center of Yancy approached as close as 80 km to the north of the Keifu Maru. Convection in the major part of Yancy was analyzed using the radar, maritime and upper air observation data obtained on the ship and recently available satellite data. Cell echo tracking winds (CETwinds) were estimated and utilized to supplement low level wind data around Yancy. During the early developing stage, an in-concentric structure of Yancy in which a cloud system existed in a southwest quadrant of a lower-levelcyclonic circulation (LLCC) of 1500 km scale was transformed to a concentric one through a formation of a central dense overcast ('CDO') in the cloud system. After the establishment of the concentric structure, Yancy began rapid development. Various mesoscale (100-500 km) precipitation features (MPFs) were organized and evolved successively within Yancy. The configurations of the MPFs were changed as the early developing process progressed through four sub-stages. In the initial sub-stage, a large (400 km) echo system (LES) was organized in the southwest quadrant of the LLCC, over which a round cloud system appeared. In the second sub-stage, a long lasting mesoscale intense convective area (MICA) was formed around the northwestern edge of the LES, which was a mesoscale precipitation entity of the 'CDO' in the round cloud system. LLCC appeared to be intensified on a 500 km scale after the formation of MICA. In the third sub-stage, LES and the cloud system evolved into a comma-shaped spiral band with length over 500 km in the intense cyclonic circulation. In the final sub-stage, curvature of the spiral band was increased and an inner near-circular spiral band emerged in the further intensified LLCC. The northern head of the comma-shaped cloud system was encircling the LLCC center. Line systems transversal and longitudinal to lower-levelcirculation were formed around MICA in the first sub-stage, and in the second sub-stage, respectively.

Corresponding author: Kazumasa Mon. Present affilia- 1 Present affiliation: Fukuoka District Meteorological Ob- tion: Climate and Marine Department, Japan Meteoro- servatory, Oohori, Chuo-ku, Fukuoka, Japan. logical Agency, Chiyoda-ku, Tokyo, Japan. 1999, Meteorological Society of Japan 460 Journal of the Meteorological Society of Japan Vol. 77, No. 2

LES and MICA constructed a kernel structure of Yancy on the early developing process. The MICA possessed a three dimensionally well organized structure for long lasting intense convection whose echo top attained 16 km in height. The MICA and 500 km scale LLCC appeared to mutually reinforce each other. Several aspects of the MPFs were summarized, which appear to correspond well to those numerically simulated mesoscale convection within developing tropical in Yamasaki (1983, 1986).

1. Introduction able common cloud features in the early developing Understanding a formation and early develop- stage of TCs. ing process and mechanism of a Concerning the central mesoscale features of for- mation and early developing process of TCs, virtu- (TC) has been one of the fundamental problems in tropical meteorology. So far, several case stud- ally no radar observation has been done. Therefore, ies have been made on these issues mainly using it is not clear how the convection is organized un- special observational experiment opportunities con- der the well known cloud feature of CDO. Mesoscale ducted over the tropical ocean. Yanai (1961) an- events analyzed by Zipser and Gautier (1978) first alyzed a typhoon synoptic scale formation process appear to be radar observed central mesoscalefea- transformed from an easterly wave disturbance with tures of TC in the early developingstage. However, cold core structure using special dense upper-air ob- because of the shortage of period and time inter- servation data over the tropical Pacific Ocean. Us- vals of the aircraft radar observation, the evolution ing the Australian Monsoon Experiment (AMEX) process and three dimensional structure were not data, Davidson et al. (1990) analyzed synoptic to described in detail. This is contrastive to mesoscale cyclonic scale changes during the formation of TCs, features within mature TCs, such as eyewall clouds and indicated low level spinup. and spiral bands, which have been investigated ex- As for mesoscale aspects of forming and early de- tensively using airborne Doppler radar (e.g., Barns veloping TCs, rare observations have been done. et al., 1983; Marks and Houze, 1987) and ground Zipserand Gautier (1978)analyzed mesoscaleevents based dual Doppler radar (e.g., Ishiraha et al., 1986; within a tropical depressionwhich intensified around Tabata et al., 1992; Shimazu, 1997). the Global Atmospheric Research Program's At- Since a concept of conditional instability of the lantic Tropical Experiment (GATE) area, and noted second kind (CISK), which is an instability of the that a deep convection mesoscale organization pre- atmosphere in which a large-scale disturbance de- ceded mesoscale . Mesoscale structure velops through interaction with cumulus convec- of a wide in a developing typhoon was tion and large-scale circulation, was presented by investigated using aircraft data in the Equatorial Ooyama (1964) and Charney and Eliassen (1964), MesoscaleExperiment (EMEX) (Ryan et al., 1992). a number of numerical simulations of TC develop- Recently, the Tropical Cyclone Motion field exper- ment have been done using a parameterization of iment (TCM-93) was conducted over the western convection based on the theoretical concept (e.g., tropical Pacific, and a mesoscale convective system Yamasaki, 1968; Ooyama, 1969). Yamasaki (1983) investigated the interaction between cumulus con- (MCS)-embedded in an intensifying typhoon - was investigated using an aircraft-equipped Omega vection and large scale motion during the devel- dropsonde system (Harr and Elsberry, 1996). opment of TC, using a two dimensional numerical In addition to these case studies - which were model in which convection was treated explicitly, fruits of epoch making field experiments-contin- and categorized the simulated mesoscaleconvection uous efforts to incorporate newly obtained meteoro- in the developing TC three types depending on the logical satellite data into the analysis of TC genesis role of surface friction. Further, Yamasaki (1986) and early development have been made since the developeda three-dimensional TC model with a new 1960's. For example, Hawkins and Rubsam (1968) parameterization of convection based on the results described a formation of hurricane Hilda in detail of Yamasaki (1983) and performed a numerical ex- combining satellite photographic data with aircraft periment, in which the simulated mesoscalefeatures and conventional data. Recently, Liu et al. (1994) were discussed using the category of convection, investigated the structure and atmospheric water and compared them with several observational facts. balance of typhoon Nina through its life cycle in- Yamasaki (1989) studied tropical cycloneformation cluding formation and developing stages using data in the intertropical convergence zone (ITCZ). Re- from the Special Sensor Microwave/Imager (SSM/I) cently, Nasuno and Yamasaki (1997) discussed long- on the Defense Meteorological Satellite Program lived mesoscaleconvection simulated in the basically the same Yamasaki model (1983) with extended ar- (DMSP) satellite. Further, a general model of TC development cloud patterns was presented on the eas. basis of the morphology of numerous TCs cloud fea- Continuing the above described progress of tures (Dvorak, 1975), in which the central dense TC numerical experiment development, Yamasaki overcast (CDO) was noted as one of the remark- (1988) remarked that more interaction between ob- April 1999 K. Mori, S. Ishigaki, T. Maehira and et al. 461

Fig. 1. Track and central pressure of Yancy. Thick line is a best track of Yancy, small marks show locations of Yancy at 00 UTC, and numerals near the marks are date and central pressures. On 30 and 31 August, locations of Yancy are marked every six hours. Track of the center of lower-level cyclonic circulation derived from CETwinds during 18 UTC 30 August to 12 UTC 31 August is shown by dotted line in which locations of the center every three hours are marked by larger dots. K' indicates Keifu Maru's location and shaded square shows radar observation area. 'TD', 'TS', ' etc. show intensity of Yancy.

servational and modelingstudies is desirable for bet- proached within 80 km north of the ship during the ter understanding of the formation and early de- radar observation. Convection within Yancy was veloping process of TC. For substantiation of the organized into various mesoscale precipitation fea- formation and early development TC process in tures (MPFs), whose structure and evolution are the context of interaction between convection and described analytically, relating them with Yancy's larger-scale motions (Yamasaki, 1983, 1986, 1988), early gradual developing process. Existence of a it is important to investigate how the convection is long lasting intense MPF, with three-dimensionally organized within a TC in the early developing stage well organized structures beneath a 'CDO' is em- based on continuous radar observation. phasized. Over the northwestern Pacific Ocean south of In Section 2, observation, the data and the data Japan, the Japan MeteorologicalAgency (JMA) re- analysis method will be described. Overview of search vessel (R/V) Keifu Maru, has made obser- Yancy will be given in Section 3. MPFs within vation cruises including radar, upper air, and mar- Yancy will be identified and described, and the early itime weather observations to observe ev- developing stage of Yancy will be divided into four ery summer. This has been planned and executed sub-stages based on characteristics of the MPFs in by the Climate and Marine Department, JMA. In Section 4. In Section 5, analysis will be focused on the 1993 observation cruise, from 30 August to 1 the long lasting intense MPF. In Section 6, char- September 1993, the Keifu Maru radar observed a acteristics of the MPFs on the early development of major part of Typhoon Yancy (T9313) continuously Yancy will be summarized and compared with those over 30 hours around (19N, 129E) in the early de- of numerically simulated mesoscale features in TC veloping stage. developments (Yamasaki, 1983, 1986) and several In this study, structure and evolution of convec- aspects of the long lasting intense MPF will be dis- tion within Yancy in the early developing stage are cussed. Summary and concluding remarks will be investigated, mainly using the Keifu Maru radar, given in Section 7. upper air and sea surface data. To cover a shortage 2. Observation and data of wind data around Yancy,cell echo tracking winds (CETwinds) are evaluated. Recently available satel- 2.1 Observation lite data are also utilized. Analysis of a lower- The R/V Keifu Maru was deployed around (19N, level cyclonic circulation (LLCC) around Yancy by 129E) from 23 August to 1 September 1993 to ob- the CETwinds indicates that the LLCC center ap- 462 Journal of the Meteorological Society of Japan Vol. 77, No. 2 serve typhoons (Fig. 1). During the observation pe- CAPPI data every 7.5 minutes and hourly echo riod, C-band radar observation, twelve-hourlyupper top height data in 50 km mesh were stored on air observation, automatic recording marine weather floppy disks during 06 UTC 30 August to 03 UTC observation for usual meteorological elements, and 1 September. In addition, hourly three dimensional three- or special one-hourly marine weather observa- data and echo top height data in 5 km mesh were tions were made on board the R/V Keifu Maru. The stored in a magnetic tape during 06 UTC 31 August Keifu Maru radar system can obtain low level (2 km) to 03 UTC 1 September. constant altitude plan position indicator (CAPPI) data, covering 500 km x 500 km square with 2.5 km 2 Data used for larger-scale analysis mesh every 7.5 minutes, and three dimensional data For larger-scale analysis, three-hourly equivalent black body temperature (TBB) data of the Geo- every one hour. More detailed specifications of the radar is available in Mori (1995). stationary Meteorological Satellite (GMS) edited by the Meteorological Satellite Center (MSC), Cloud clusters moving westward along 20N de- veloped into a tropical depression (TD) located at JMA and processed by the Typhoon Research De- partment of the Meteorological Research Institute (19.5N, 138E), at 00 UTC 29 August 1993. The TD became a tropical (TS) named Yancy (MRI) and twelve-hourly objective analysis data with central pressure of 998 hPa and maximum sus- routinely made by the Numerical Prediction Divi- tained wind speed of 18 m/s at (20.2N, 134.2N) to sion, JMA were utilized. Conventionalupper air and the east of the ship approximately 400 km at 00 surface data obtained through the Global Telecom- UTC 30 August. Yancy continued to move west- munication System (GTS) and cloud track winds de- ward approaching the ship with gradual develop- rived by the MSC, JMA were also examined. Best ment on 30 and 31 August (Fig. 1). In the morning track data issued by the Forecast Division, JMA based on the Dvorak (1975)'s method were referred (around 00 UTC) of 30 August, altocumulus covered the sky over the ship beneath the dense cirrus over- for Yancy's locations and intensities. cast and echoes increased from the eastern sector of 3 Recently available satellite data the radar observation area corresponding to the ap- Sea surface winds estimated from scatterometer proach of the gradually developing Yancy (quoted data on the European Remote Sensing satellite- from the observation report of the Keifu Maru ob- 1 (ERS-1) were used. The estimation method is servation team). Considering the routinely analyzed based on the fact that the backscatter signals are location and forecasted track of Yancy by JMA, a function of wind speed, incidence angle, polar- both of which were actually deviated slightly north- ization, frequency and azimuth angle (Freilich and ward from the re-analyzed best track, the observa- Dunbar, 1993). The ERS-1 scatterometer data tion team decided to stay at the observation point cover a 500 km swath, with an effective resolu- (19N, 129E) and intended to observe the southern tion of 50 km (Attema, 1991) approximately twice part of Yancy as much as possible. Continuous radar a day. Precipitable water content (PWC) derived observation was started at 06 UTC 30 August. from SSM/I data on the DMSP satellite using a The major part of Yancy passed westward over interactive method (Shibata, 1994) were also used. the radar observation area and continuous radar The PWC data cover a 1400 km swath with a reso- data were obtained. During the westward passage, lution of 25 km approximately twice a day. a center of lower-levelcyclonic circulation (LLCC) derived from CETwinds was nearest to the north 2.4 Cell echo tracking winds of the ship approximately 80 km around 03 UTC Cell echo tracking winds (CETwinds) were esti- 31 August (dotted line in Fig. 1) as later shown. mated to supplement wind data around Yancy. The Upper air observation was tried successively three tracking method is based on that used with Tatehira times around 00 UTC 31 August under very severe (1961) in which a map of echo cell velocities was weather conditions. These upper air observations, presented for a mature typhoon near Tokyo, Japan. and surface observation on the ship near the LLCC The CETwinds were derived using low level (2 km) center, give us invaluable information about the or- CAPPI data every 7.5 minutes. An echo which sat- isfied the followingthree conditions was defined as a ganization of convection in the central part of the early developing TC. Yancy attained severe tropi- traceable cell echo; 1) life time longer than 15 min- cal storm (STS) intensity at (20.7N, 127.5E) about utes, 2) horizontal scale smaller than 30 km, and 3) 200 km northwest of the ship at 12 UTC 31 August aspect ratio larger than 1/3. As shown later, the and Typhoon (TY) intensity at (22.4N, 125.8E) at whole structure of Yancy changed drastically dur- 03 UTC 1 September, moving away northwestward ing the radar analysis period (from 06 UTC 30 Au- from the ship. At 03 UTC 1 September, echoes gust to 12 UTC 31 August 1993). Therefore, the around the central part of Yancy moved away north- CETwind field was mapped every hour to extract westward from the radar observation area, so the an expected large change of the wind field during continuous radar observation ended. the period. April 1999 K. Mori, S. Ishigaki, T. Maehira and et al. 463

Fig. 2. Intensity change of Yancy. Time change of central pressure and speed of Yancy are shown by solid and broken line, respectively. Shaded column indicates the period when major part of Yancy was observed by the Keifu Maru radar. 'TD', 'TS', etc. show intensity of Yancy.

A map of CETwinds at a given map time was examined. The mean wind speed and direction of made using nine successive CAPPI patterns ob- the CETwinds corresponded well to those of 700 hPa tained for one hour centered at the map time (except winds at each map time. The mean wind speed of for 06 UTC 30 August, when the map was made us- the CETwinds was slightly smaller than that of the ing five CAPPI patterns from 0600 UTC to 0630 700 hPa wind, but the differencebetween them was UTC). On the map, CETwind vectors were drawn smaller than 6 m/s and the direction of the mean by tracking traceable cell echoes. At first, traceable CETwind at each time was within 18 degrees from cell echoes were identified and their traceable posi- that of the 700 hPa wind. Considering these results, tions were plotted on the map. Next, focusing on CETwinds are approximately interpreted as 700 hPa an identified traceable cell echo, a CETwind vector winds in this study, which agrees with the previous centered at the map time was drawn connecting its investigations on the relationship between the cell two positions 7.5 minutes before, and after the map echo tracking winds, and upper winds (Ligda and time. Then, for the same cell echo, CETwind vec- Mayhew, 1954; Kessler and Atlas, 1956). tors not centered at the map time but within the 3. Overview of Yancy one hour period were drawn connecting two posi- tions which indicated movements of the cell echo for The track and intensity change of Yancy are 15 minutes. For one traceable cell echo, tracing pe- shown in Fig. 1 and Fig. 2, respectively. Depend- riods did not overlap each other. Three CETwinds ing on the time change of the central pressure, the could be obtained at a maximum for one traceable lifetime of Yancy was divided into the followingfour cell echo if it lasted over one hour. The same proce- stages; formation stage (from 00 UTC 29 to 00 dures were done for other traceable cell echoes one UTC 30 August), developing stage (from 00 UTC by one. In the case that there were few traceable cell 30 August to 09 UTC 2 September), mature stage echoes, CETwinds using cell echoes with a lifetime (from 09 UTC 2 September to 03 UTC 3 Septem- of only 7.5 minutes, or convective echoes embedded ber), and decaying stage (from 03 UTC 3 Septem- in larger moderate echoes, were also plotted on the ber to 09 UTC 4 September). Further, the devel- map. oping stage could be divided into two stages de- Characteristics of the traceable cell echoes were pending on the developing speed and Yancy's whole examined briefly using the CAPPI data centered at structure; an early developing stage with gradual 12 UTC 30 August, 00 UTC and 12 UTC 31 Au- deepening of central pressure (N-8 hPa/day) un- gust. Number, mean horizontal scale, and mean der an in-concentric structure (from 00 UTC 30 to lifetime were examined using 17 CAPPI patterns 12 UTC 31 August) and a latter developing stage centered on each time. Mean number, horizontal with (- -40 hPa/day) under a scale, and lifetime of the traceable cell echoes for concentric one (from 12 UTC 31 August to 09 UTC the three map times were 76,8 km, and 25 minutes, 2 September) (Fig. 2). The R/V Keifu Maru radar respectively. The relationship between the mean observed a major part of Yancy in the early gradual CETwinds within a circle with a radius of 100 km developing stage on 30 and 31 August 1993. Cen- from the ship, and upper winds obtained on the ship tral pressure of the mature Yancy attained 925 hPa at the above mentioned three map times were also with a maximum sustained wind speed of 50 m/s 464 Journal of the Meteorological Society of Japan Vol. 77, No. 2

Fig. 3. Large-scale cloud feature and surface circulation around Yancy from 12 UTC 29 August to 00 UTC 1 September 1993. Surface winds are plotted to the east of 120E. Full wind barb is for 5 m/s. Contour of TBB is for -30C. Areas of TBB lower than -60C and -75C are hatched and shaded, respectively. on 2 September, two days after the observation pe- formation (TBB <-60C) in the early develop- riod, and landed on the southern part of Japan on ing stage, appeared during 18 UTC 30 August to 3 September (Fig. 1). 03 UTC 31 August. The threshold TBB value of Large scale cloud features around Yancy were ex- -75C was used because the lower (< -75C) TBB amined every three hours and composite maps of area approximately corresponded to the echo area the large scale cloud features, and surface circula- as shown later. Between 03 UTC and 12 UTC 31 tion in the formation and developing stages every August, the cloud system changed from round to 12 hours are shown in Fig. 3. In the formation comma shape corresponding to the development of stage at 12 UTC 29 August, several cloud systems Yancy from TS to STS. Around 15 UTC 31 Au- defined by low (<-60C) TBB areas appeared to gust, the cloud area was surrounding a center of the be loosely organized around (19.6N, 135.5E) in the comma-shaped cloud system. A round-shaped cloud southwest quadrant of surface cyclonic circulation. system with a diameter about 600 km was organized These cloud systems had been originated from cloud around (23N, 126E) at 00 UTC 1 September, which clusters formed around (20N, 145E) on 27 August was concentric with the surface cyclonic circulation and identified as a TD at 00 UTC 29 August (not (Fig. 3). shown). The latter rapid developing stage started around In the early gradually developing stage, the TD 12 UTC 31 August and Yancy developed to TY in- intensified to a TS named Yancy located at (20.2N, tensity at 03 UTC 1 September (Fig. 2) when a cloud 134.2E) at 00 UTC 30 August, and a round-shaped hole, or , appeared in the round-shaped cloud cloud system with a diameter approximately 400 km system. At the 200 hPa level, an upper level trough formed around (20.8N, 132.1E) in the same circu- existed to the northeast of Yancy during 00 UTC 28 lation quadrant by 12 UTC 30 August. A central August to 00 UTC 30 August. Divergent flow over dense overcast ('CDO') which is defined as a cen- Yancy was established after 00 UTC 31 August (not tral higher cloud (TBB <-75C) in a round cloud shown). April 1999 K. Mori, S. Ishigaki, T. Maehira and et al. 465

(a) Lower-levelcirculation around Yancy was exam- ined in detail incorporating the ERS-1 sea surface winds into the analysis (Fig. 4). Around 00 UTC 30 August, cyclonic circulation with wind speed of 5-20 m/s and a diameter about 1500 km already existed around an angular cloud system associated with Yancy. The circulation center appeared to be located around the eastern edge of the northern side of the angular cloud system, as the best track data indicated. Over the radar observation area, northerly wind with weak cyclonic shear prevailed with wind speeds of 5-10 m/s in its northern area and 3-5 m/s in the southern area (Fig. 4a). Around 12 UTC 31 August, intensified cyclonic circulation with maximum wind speeds over 20 m/s associated with Yancy's comma-shaped cloud sys- tem was clearly detected by the ERS-1 surface winds. Stronger (> 18 m/s) wind prevailed mainly in the north to east sector of the circulation, about (b) 150 km from the circulation center which was lo- cated around the inner edge of the northern head of the comma-shaped cloud system. Intense cyclonic shear appeared around both sides of the southern trail of the comma-shaped cloud system (Fig. 4b). Time changes of cloud area and minimum TBB within 10 degree longitude x 10 degree latitude re- gion centered at the centroid of the cloud system during 09 UTC 27 August to 21 UTC 2 September are shown in Fig. 5. Upper air observation indicated that temperature of -60C, -75C, -80C corre- sponded to the temperature at 13.5 km, 15 km, and 16 km height, respectively and tropopause height was about 16 km. MinimumTBB lowerthan -85C appeared only in the CDO, which infers that the convection associated with the CDO was most ac- tive during that period. Diurnal variations of convection area (TBB < -75C) with a maximum around morning (00 UTC, local time 09 h) followed by that of cloud area Fig. 4. Lower-level circulation around (TBB < -60C) with maximum around noon (03 Yancy at 00 UTC 30 August and 12 UTC, local time 12 h) was observed on 31 August UTC 31 August. Thick larger wind and 1 September, which is roughly similar to a diur- barbs are low level (850 hPa) winds nal variation of eyewall and spiral band convection by sonde and thick smaller wind barbs in mature typhoons (Muramatsu, 1983). Formation marked with black dots are satellite of CDO from 30 to 31 August appeared to be over- cloud winds, respectively. Thin small lapped on the convection diurnal variation (Fig. 5). wind barbs are sea surface winds ob- In summary, at the beginning of early gradual de- tained through the GTS. Sea surface velopment, larger-scale (N 1500 km) and lower-level winds by ERS-1 data at 02 UTC 30 August and 14 UTC 31 August are su- cyclonic circulation (LLCC) with wind speed of 5- 20 m/s already existed around Yancy, but the cloud perimposed in (a) and (b), respectively. Full wind barb is for 5 m/s. Contour system associated with the LLCC was not concen- of TBB is for -30C and areas of TBB tric with the circulation. At the end of the early lower than -60C are hatched. Radar gradual development (and beginning of latter rapid observation area is enclosed by a square. development), the LLCC intensified (> 20 m/s) and T' is a center of Yancy by the best' track its center was located around the central part of the data. comma-shaped cloud system. Combining these re- sults with the overview of cloud and surface circu- lation around Yancy (Fig. 3), it is confirmed that 466 Journal of the Meteorological Society of Japan Vol. 77, No. 2

Fig. 5. Time changes of cloud area and minimum TBB in cloud system of Yancy. Cloud area of TBB lower than -60C, -75C, and -85C and minimum TBB in a box of 10 degree latitude x 10 degree longitude centered at cloud system of Yancy are indicated by solid lines and broken line, respectively. TD', 'TS', etc. show intensity of Yancy. Large and small marks on the abscissa indicate 00' UTC and 12 UTC, respectively.

the structure was transformed from in-concentricto gust. Cloud area (TBB < -60C) spread over these concentric in the relationship between LLCC and MPFs (Fig. 7a). cloud system during the early developing stage in Second sub-stage (between 18 UTC 30 and 03 which CDO was formed. UTC 31 August); It is stressed that a mesoscale 4. Early developing process of precipitation, (100 km) intense convective area (MICA) was formed around the northwestern edge (19N, 129E) cloud, and lower-level circulation of Yancy of the LES around 18 UTC 30 August and main- Low level (2 km) precipitation pattern and tained for nine hours by 03 UTC 31 August, over CETwinds during 06 UTC 30 to 12 UTC 31 August which CDO spread in the central area of round are shown in Fig. 6. How the convection in Yancy cloud system. The lowest (< -85C) TBB area was organized, how the characteristics of the orga- appeared (Fig. 7b) just over the MICA. It is in- nized convection changed as Yancy developed, and teresting to note that corresponding to the forma- what was a precipitation entity of the CDO are sub- tion of MICA, northwesterly flow to the northwest stantially described by the Keifu Maru radar data of MICA was intensified to 20-25 m/s with increas- and CETwinds. Based on the characteristics of the ing cyclonic shear. The center of the LLCC with identified MPFs, LLCC (Fig. 6), and cloud pattern weaker (< 10 m/s) flow appeared to the north- (Fig. 3), the early developing stage was divided into east of MICA at 18 UTC 30 August, moved west- four sub-stages as summarized in Fig. 7. northwestward faster than MICA, located about Initial sub-stage (between 06 UTC and 18 UTC 80 km to the northeast of MICA at 00 UTC 31 Au- 30 August); A westward moving large (-400km) gust. To the east of the center, cyclonic southerly echo system (LES) with scattered convective echoes flows of 15-20 m/s were observed around 00 UTC was organized from loosely organized echoes in the 31 August, which inferred that an intensified (15- southwest quadrant of LLCC at 09 UTC. A slight 25 m/s) LLCC was established all around the cir- cyclonically sheared northerly flowof about 15 m/s culation center in a 500 km scale. At 03 UTC 31 corresponding to a surface circulation detected by August, Keifu Maru arrived at the position (19.2N, the ERS-1 data at 02 UTC 30 August (Fig. 4a) was 128.8E) within MICA which had approached the gradually intensified to the west of the LES, in which ship slowly from the east. Westerly CETwinds line systems transversal to the lower-levelflow (TL 1, about 30 m/s were analyzed just to the west of TL2) evolved and moved around the LES cycloni- the MICA. In the intensified northwesterly flow to cally between 09 UTC and 15 UTC. The developing the northwest of MICA, longitudinal line systems TL2 passed over the ship southwestward around 12 (LL1-LL4) evolved and successivelyturned cycloni- UTC and dissipated by 18 UTC 30 August. Be- cally around the center. Cell echoes constructing hind the dissipating TL2, line systems longitudi- LL2 moved southeastward and merged with MICA nal to the intensified north-northwest flow formed except for outer echoes around (20N, 127E), which and moved southwestward around 18 UTC 30 Au- moved south-southwestward around 21 UTC 30 Au- April 1999 K. Mori, S. Ishigaki, T. Maehira and et al. 467

3006Z 19.69N 128.56E 3015Z 19.51N 128.49E

3009Z 19.62N 128.52E 3018Z 19.44N 128.54E

3012Z 19.56N 128.48E 3021Z 19.36N 128.62E

Fig. 6. Evolution of low level (2 km) precipitation pattern and CETwinds during 06 UTC 30 August to 12 UTC 31 August. Arrows are CETwind vectors and dotted line is isotach (m/s). Surface winds on board are shown by bold arrows with the same scale as CETwinds. Center of LLCC is shown by C'. Sharp triangular slit is a beam cut area. Observation time and ship location are indicated on' the upper side of each figure. 468 Journal of the Meteorological Society of Japan Vol. 77, No. 2

31002 19.28N 128.69E 3109Z 19.08N 129.OOE

3103Z 19.23N 128.79E 3112Z 19.14N 129.08E

3106Z 19.03N 128.92E (mm/h) ■ <1 ■1～4 ■4～16 ■16～32 ■32～64 ■64≦

Fig. 6. (Continued) gust. The direction of MICA was almost perpendic- like comma-shaped system with length over 500 km ular to the lower-levelnorthwesterly flow (Fig. 7b). (SB1) in cyclonic southerly to southeasterly flow of Third sub-stage (between03 UTC and 06 UTC 31 15-25 m/s. At 06 UTC 31 August, another spiral August); The LES rapidly extended northward and band like convective band with length about 300 km southwestward and transformed into a spiral band (SB2) also developed, of which the northern part April 1999 K. Mori, S. Ishigaki, T. Maehira and et al. 469

(a) initial sub-stage (c) third sub-stage

(b) second sub-stage (d) final sub-stage

Fig. 7. Precipitation and cloud pattern of Yancy in the initial sub-stage (a), second sub-stage (b), third sub-stage (c), and final sub-stage (d) in the early developing stage. Square is radar observation area. Shaded regions, inner contours, and black regions enclose areas of precipitation intensity over 0.5 mm/h, 4 mm/h and 16 mm/h, respectively. Broken lines are for TBB of -60C, -75C, and -85C. 'K' indicates location of Keifu Maru and 'C' marks the LLCC center. Stream line of lower-level circulation is figured using CETwinds in Fig. 6.

was connected with SB1. An intense convective line Final sub-stage (between06 UTC and 1 UTC 31 in the northern part of SB2 was originated from the August); The northern head of SB1 moved away MICA and the southern part of SB2 was formed northwestward from the radar range, its southern from LL3 and LL4 which developedaround 05 UTC trail dissipated and SB2 developed with increasing behind the LL3 (not shown). The LLCC center cyclonic curvature between 06 UTC and 09 UTC 31 was located on the northwest side of SB1 and SB2 August. At 09 UTC 31 August, the LLCC center around (20.5N, 128.OE).Lower-level flow appeared was analyzed at (20.3N, 127.9E). Around the center, to be confluent around SB1 and SB2. Directions of cyclonic circulation prevailed, in which southerly SB1 and SB2 were approximately along the lower- CETwinds were over 25 m/s to the east of the center levelflow, and confluent shear appeared around SB1 about 50-200 km. and SB2. The cloud system also changed from round At 12 UTC 31 August, the LLCC center moved to comma shape corresponding to the formation of to (21.1N, 127.1E) around which an eye-like struc- SB1 and SB2. The CDO dissipated (Fig. 7c). ture of echo appeared. The eye-like structure orig- inated from a echo hollow around the inner edge 470 Journal of the Meteorological Society of Japan Vol. 77, No. 2

(21N, 128E) of SB2 at 10 UTC (not shown). SB1 al- UTC 31 August was investigated using the three di- most disappeared and SB2 matured to a near circu- mensional reflectivity data stored on magnetic tape lar wide, spiral band with width about 100 km. Cy- at that time. The northern part of the convective clonically turning echoes came out and merged with line in SB2 originated from the MICA and the time each other to next form near circular spiral bands (07 UTC 31 August) was not so far from the time (SB3) inside of SB2, and a maximum CETwind when MICA was maintained (by 03 UTC 31 Au- (30 m/s) zone. LLCC with a weak (< 10 m/s) gust). Therefore, it may not be irrational to suppose flowaround the circulation center, and stronger (15- that the three dimensional structure of the northern 30 m/s) winds 100-200 km from the center pre- part of the convectiveline in SB2 at 07 UTC 31 Au- vailed. Intense southerly CETwinds about 30 m/s gust might possess similar characteristics to that of were derived to the east of the center, about 200 km. MICA around 03 UTC 31 August. SB 1 and corresponding cloud trails almost disap- SB1 and SB2 were clearly detected by radar peared. (Fig. 8a). Echo top height was over 14 km in the The northern head of the comma-shaped cloud northern part of, and to the northeast of convec- system encircled the LLCC center. A new sharply tive line in SB2, about 12 km in the other part of curved comma-shaped cloud system was formed by the convectiveline in SB2 and the northern head of the northern head and SB2, which developed in the SB1, and lower than 10 km in the southern trail of cyclonic southerly flow of 15-30 m/s. Around the SB1 (Fig. 8b). end of this stage (12 UTC 31 August), SB3 emerged The west-northwest to east-southeast vertical in a cyclonic westerly flow about 20 m/s inside of cross section of the northern part of the convective SB2 and the maximum CETwind (N 30 m/s) zone. line in SB2 and SB1 indicates that the echo top of The LLCC center located in the southern part (in- the convectiveline extended to 16 km height and the ward side) of the northern head of the cloud system intense echo (reflectivity > 41dBZ (13.3 mm/h)) in associated with an eye-likeecho structure (Fig. 7d). SB2 attained 7 km height. Moderate echo (reflec- Southerly wind, about 30 m/s, prevailed approxi- tivity of 26-36dBZ (1.5-6.5 mm/h)) originated from mately 200 km to the east of the center, which is the lower level intense echo reached the 14 km level consistent with the ERS-1 surface wind field at 14 around the east side (r 20 km) of the lower level in- UTC 31 August (Fig. 4b). tense echo. A relatively uniform and moderate echo After that, a round-shaped cloud system with a with a height of 8-10 km spread in SB 1 (Fig. 8c) diameter of about 600 km, in which almost circular was seen. North-northeast to south-southwest ver- spiral bands concentric with the LLCC were orga- tical cross sections of SB1 and SB2 indicate that the nized and a cloud hole or eye appeared (not shown). echo top height of the convectiveline in SB2 was over Simultaneously,Yancy started rapid development. 14 km for the 150 km scale. Within the convective During 18 UTC 30 to 03 UTC 31 August, weak line, intense (> 41dBZ) echoes reached 8 km and to moderate echoes in LES to the southeast of the moderate (26-36dBZ) echoes spread up to 12 km MICA were partly attenuated by the very heavy height, mainly northward from the intense echoes in MICA. However, major features of echoes (Fig. 8d). In the southern trail of SB1, moderate described above appear to be not as affected by the echo with height about 6 km uniformly extended attenuation. During 21 UTC 30 August to 06 UTC (not shown). 31 August, Keifu Maru crossedjust under the MICA These figures indicate that the most active and from its northwest side to southeast side relatively, deepest (-16 km) convectionexisted in a convective and remarkable mesoscalechanges of severe sea sur- line in SB2 and extended north-northeastward. The face weather and upper weather were observed on uniform and moderate echoes in the southern trail of board (as shown later). SB1 indicated that this part was not as active, which is consistent with the observational fact that SB1 5. Meso Y to convective-scale aspects of was rapidly dissipating around that time (Fig. 6). MICA 5. Maritime weather on board the R/V Keifu Maru In this section, we focus on meso-y to convective- Figure 9 shows time sequences of maritime scale (30-3 km) aspects of the MICA which was weather elements observed on board the R/V Keifu the most active mesoscalefeature among the identi- Maru from 15 UTC 29 August to 15 UTC 1 Septem- fied MPFs and mesoscaleprecipitation entity of the ber. During that period, several notable features CDO. of maritime weather were observed associated with 5.1 Three dimensional structure of SB1 and SB2 MPFs. Corresponding to the southwestward pas- The three dimensional structure of MICA could sage of TL2 around 12 UTC 30 August, surface wind not be directly investigated because three dimen- changed from northerly at about 10 m/s to westerly sional reflectivitydata was not stored before 06 UTC at about 8 m/s and temperature decreased by 5C 31 August. Instead, structure of SB1 and SB2 at 07 associated with precipitation of 9.5 mm. TL2 moved April 1999 K. Mori, S. Ishigaki, T. Maehira and et al. 471

southwestward at a speed of about 10 m/s in a lower- level northerly flow at approximately 15 m/s, and an active convective line formed in the northeast side of TL2 (windward side of the lower-levelflow) at 14 UTC 30 August (not shown). After the passage of TL2, west-northwesterly wind speed gradually increased to 17 m/s around 18 UTC 30 August when the MICA was forming about 100 km (Fig. 6) to the east of the ship. Fur- ther, it grew to 20 m/s associated with the pas- sage of LL2 around 21 UTC 30 August (Fig. 9b). This increase of surface wind speed corresponds to the intensification of west-northwest CETwind near MICA around that time (Fig. 6). Relative weak sur- face wind, about 13 m/s around 00 UTC 31 August, appears to be consistent with the LLCC pattern de- rived by CETwinds which had weak flow (< 10 m/s)

(a) around the LLCC center (Fig. 6 at 00 UTC 31 Au- gust, Fig. 7b). Between 16 UTC 30 and 03 UTC 31 August, when the ship was behind TL2 and in the windward side of MICA, the surface air mass possessedhigh equivalent potential temperature (Oe) higher dew point temperature and lower tempera- ture. This might be the result of an inflow of a more humid air mass behind TL2 as inferred later from the PWC pattern, evaporative cooling of weak pre- cipitation and mixing well at sea surface by stronger sea surface wind. The most remarkable weather changes observed were associated with the northwestward passage of MICA over the ship during 21 UTC 30 August to 06 UTC 31 August (Fig. 6). Very severe weather with heavy rain, strong gale, and wave height of 7 m occurred around MICA which was located about 80 km to the south of the LLCC center around 03 UTC 31 August (Fig. 6 at 03 UTC 31 August). (b) Heavy rainfall of 147 mm for 6 hours during 22 UTC 30 August to 04 UTC 31 August was observed. Be- tween 02 UTC and 03 UTC 31 August, hourly rain- Fig. 8. Three dimensional structure of fall was 44 mm (Fig. 9a). Northward passage of Yancy at 07 UTC 31 August. Low intense convective cells associated with thunder and level (2 km) precipitation pattern and was observed within the heaviest rain, and echo top height pattern are shown in visibility decreased to less than 100 m during 0230 (a) and (b), respectively. Vertical cross UTC to 0345 UTC (quoted from the observational section of echoes along west-northwest field note), partly because of the heaviest rainfall. A to east-southeast and north-northeast to south-southwest are shown in (c) and south-southwest strong gale with a maximum sus- tained wind of 25.6 m/s at 0350 UTC and a peak (d), respectively. Outer contour is for precipitation of 0.6 mm/h (16dBZ), in- gust of 36.0 m/s at 0237 UTC. side contours are for 1.5 mm/h (26dBZ), Temperature decreased from 28.0C to 23.6C 6.5 mm/h (36dBZ), and 13.3 mm/h during the heavy rain and returned to 28.5C af- (41dBZ) in (a), (c), and (d). Contour ter the rain stopped. It dropped relatively rapid for 3.2 mm/h (31dBZ) is added in (c) (-1C/1h) under an almost saturated condition and (d). Maximum echo top height (km) within the heaviest rain around 02 UTC 31 Au- is obtained by doubling the numeral in gust (Fig. 9a). Surface wind changed from west- each 5 km mesh in (b). Area with echo northwest at 13 m/s to west-southwest wind at top height over 14 km is enclosed by 15 m/s around 00 UTC 31 August, roughly corre- solid line in (b). sponding to the start of heavy rain around the north- western periphery of the MICA. Further, it changed 472 Journal of the Meteorological Society of Japan Vol. 77, No. 2

(c)

(d)

Fig. 8. (Continued) to 20-25 m/s south-southwest around 03 UTC 31 ilar longitudinal line systems were formed around August, associated with the heaviest rainfall. West- the ship in the southeast quadrant of Yancy. They erly and southerly air flowsaround MICA possessed passed over the ship from south-southwest to north- ee of 353K and 343K, respectively. After the heavy northeast around 16 UTC and 19 UTC 31 August rain, south-southwesterly wind at 18 m/s with ee (not shown), associated with rainfall of 11 mm and about 348K prevailed. Minimum sea surface pres- 4 mm and temperature decreases of 0.5C and 3C, sure about 996 hPa was observed around 03 UTC respectively (Fig. 9a). Surface wind around these 31 August when the center of LLCC was closest to three times varied inferring confluent flow around the ship. the line systems (Fig. 9b), which is consistent with An outer longitudinal line system in the southeast the confluent flow near the ship detected by the quadrant of Yancy in the latter developing stage was ERS-1 surface wind at 14 UTC 31 August (Fig. 4b). over the ship around 12 UTC (see Fig. 6 at 12 UTC Semidiurnal oscillation with peaks around 02 31 August), associated with rainfall of 0 mm. Sim- UTC and 14 UTC was superimposed on the time April 1999 K. Mori, S. Ishigaki, T. Maehira and et al. 473

Fig. 9. Time sequences of maritime weather observed on board the R/V Keifu Maru. Sea surface temperature (SST), temperature (T), dew point temperature (Td) and rainfall amount every ten minutes are shown in (a). West to east (U) and south to north (V) components of sea surface wind and sea surface wind every one hour are shown in (b). Full wind barb is for 5 m/s in (b). Equivalent potential temperature (Be) and sea surface pressure (P) are shown in (c) and (d), respectively. change of surface pressure (Fig. 9d). Sea surface around (19.2N, 128.8E) between 00 UTC and 03 temperature (SST) decreased from 30.4C to 28.6C UTC and moved northward slowly (-3m/s) after- gradually on 30 and 31 August, probably because of ward (Fig. 6). Assuming the steadiness of the MICA a mixing of near sea surface water by the strong gale around (19.2N, 128.8E) centered at 03 UTC 31 Au- as observed around mature Hurricane (Shay et al., gust, a schematic horizontal feature of the MICA is 1992). shown in Fig. 10. Heavy rainfall started around the northwest side 5.3 Sea surface weather and upper air conditions of the MICA. In the heaviest rain in the MICA un- around MICA der a saturated condition, northward moving con- Between 00 UTC and 06 UTC 31 August, vective cells associated with thunder and lightning the Keifu Maru sifted southeastward very slowly were observed. A cold pool with temperatures of (r.i 1 m/s) from (19.28N, 128.69E) to (19.03N, 24-26C spread slightly to the southeast side of the 128.92E) passing under the MICA, which remained 474 Journal of the Meteorological Society of Japan Vol. 77, No. 2

Fig. 10. Schematic horizontal structure of MICA around the northwestern edge of LES centered at 03 UTC 31 August. Thin line is outer boundary of precipitation area. Thick line encloses area of echo over 4mm/h and hatched area is over 16 mm/h. Sea surface winds on the ship from 23 UTC 30 to 07 UTC 31 August are plotted. Full barb is for 5 m/s. Broken lines with numerals indicate a pattern of temperature (C). Black arrows show surface air flow with numerals of equivalent potential temperature (K). marks the area where thunder and lightning were observed and 'H (N 100 %)' indicates that relative humidity was almost 100 % during 00 UTC to 04 UTC 31 August. Results of upper air observations at 00 UTC, 01 UTC and 02 UTC on 31 August are also shown in the upper portion of the figure. In the result at 01 UTC 31 August, vertical profiles of 0 at 12 UTC 30 August (dotted line) and 12 UTC 31 August (dash dotted line) are also shown.

MICA. Around the northwestern periphery of the of 7 km could not be obtained because of the se- MICA, west-northwesterly and west-southwesterly vere weather conditions. At 00 UTC 31 August near flow about 15 m/s with higher 8e about 353K con- the northwestern periphery of echo area, northwest verged. Just as in the MICA, a south-southwesterly to west-northwest wind over 13 m/s blew below a flow of 23 m/s with lower ee about 343K prevailed. height of 6 km, associated with conditionally un- About 30 km, the southeast of the MICA, south- stable stratification below the 3 km level. Around southwesterly flow of 18 m/s with 0e about 348K the 1 km height, west-northwesterly winds were blew. most intense (-20 m/s). At 01 UTC in the heavy Around 00 UTC 31 August, upper air observa- rain, wind direction changed to west-southwest and tion was tried three times successively under very wind speed was reduced to 13-18 m/s under 2 km. severe maritime weather conditions. Results of the West-northwest flowabout 25 m/s prevailed around three upper air observations are also shown in the 6 km. Stratification was almost moist neutral with upper part of Fig. 10, in which data above a height 8e about 350K. Around the 6 km level, potential April 1999 K. Mori, S. Ishigaki, T. Maehira and et al. 475

Fig. ll. Composite map of precipitable water content (PWC) derived from SSM/I data and precipitation pattern observed by the Keifu Maru radar at 09 UTC and 21 UTC on 30 August. Contours are for PWC (mm). Square is radar observation area in which echo pattern is shown by shaded (> 1mm/h) and black (> 16 mm/s) areas. Areas where observation was missed or PWC could not be estimated because of the existence of land or very heavy rain are enclosed by broken lines in SSM/I observation swath.

temperature (0) at 00 UTC and 01 UTC 31 August regions. To the northwest of Yancy about 600 km, was higher than that at 12 UTC 30 August and that a dry air belt with PWC lower than 35 mm existed at 12 UTC 31 August by about 8K, which infers re- and its southern end was elongating to Yancy. Up- lease of latent heat by active convection. At 02 UTC per air observation at 12 UTC 30 August at Ishigak- 31 August near the western periphery of the heav- ijima suggests that middle troposphere in the dry air iest rain in MICA, west-southwesterly wind below belt was very dry with relative humidity lower than the 2 km level intensified up to 20 m/s and a west- 25 % (not shown). erly flow about 35 m/s prevailed about 3 km level. By 21 UTC 30 August, the moistest air region Stratification was almost moist neutral in the lower with PWC over 65 mm and spiral-band-like rela- troposphere. tive moist regions, turned around Yancy cycloni- Below the 2 km level, intense convergence cally. The moistest air mass existed on the wind- (10-3 s-1) mainly between west-northwesterly and ward side of the lower-levelnorthwesterly inflow to west-southwesterlyflow is inferred around the north- the MICA (Fig. 6 at 21 UTC 30 August). Consider- western periphery of the echo area, which might be ing the cyclonicallyturning motion of PWC pattern the origin of convectiveupward motion in the MICA. around Yancy, relatively moist bands elongating to Intense west-northwest inflow was concentrated be- west-southwest and northwest from the LES at 21 low 1 km level with higher oe, which would provide UTC 30 August might be the origin of SB1 and 5B2, moisture to the MICA. respectively (see also Fig. 6 at 06 UTC 31 August). The southeastern end of the dry air belt intruded 5.4 Precipitable water content pattern into the southwestern side of the MICA. Precipitable water content (PWC) pattern de- Existence of a moisture rich air mass on the wind- rived by SSM/I data on the DMSP satellite using ward side of lower-levelinflow into the MICA might the interactive method (Shibata, 1994) was exam- be a preferable condition for formation and main- ined and appropriate figures for mesoscale analysis tenance of the MICA. The intrusion of a dry air of Yancy are shown in Fig. 11. At 09 UTC 30 Au- belt to the southwestern side of MICA might rein- gust, a moist air mass with PWC of 60-65 mm sur- force convection in MICA through the intensifica- rounded the northwestern side of LES in which the tion of evaporative cooling by rain drops and the MICA was not yet organized. PWC over 65 mm ex- consequent strengthening of the cold pool, as cloud isted around the northern periphery of the LES. TL1 clusters developed around subtropical Baiu frontal and TL2 were in the spiral-band-likerelatively moist zones (e.g., Ishihara et al., 1995). 476 Journal of the Meteorological Society of Japan Vol. 77, No. 2

(b)

(a)

(intensified l ower•level inflow)

(c)

(heating)

Fig. 12. Schematic figure of the three dimensional structure of the MICA and LES around 03 UTC 31 August. Horizontal (a) and vertical cross sections perpendicular to (b) and along (c) MICA. Thick line is for precipitation intensity of 1 mm/h and barred region is for over 16 mm/h (MICA). Cold pool with low ee is shaded in (a) and (b). Broken lines are for TBB of -75C and -85C in (a) and cloud boundary inferred from the TBB data in (b) and (c). Bold arrows are observed surface winds around MICA. Stream lines show 500 km scale lower-levelcyclonic circulation derived from CETwinds and 'C' is a center of the circulation in (a). Inferred upward motion is shown by white arrow in (b) and (c). Terms surrounded by parentheses indicate speculated important effects for maintenance of the MICA.

5.5 Schematic structure of the MICA converged around the northwestern edge of the large Using synthesizing radar, maritime weather, and echo system (LES). The converged high ee air mass upper air data on board the Keifu Maru, and might blow up over the south-southwesterly flow GMS, ERS-1, and DMSP satellite data, a schematic with low Bewhich might be produced by evaporative structure of the mesoscale intense convective area cooling of heavy precipitation. (MICA) of Yancy around 03 UTC 31 August is pre- For Yancy (500 km) scale, LLCC was intensified sented in Fig. 12. West-northwest to east-southeast with lower level wind speed over 25 m/s, which and north-northeast to south-southwest cross sec- infers that the long lasting intense convection in tions of the MICA are figured, speculating that the MICA might reinforce LLCC through the release three dimensional structure of the convectiveline in of latent heat (see positive deviation of 8 at 8 km SB2 at 07 UTC 31 August shown in Fig. 8 might level around MICA in Fig. 10) and consequent de- possess characteristics of MICA around 03 UTC 31 crease of pressure on mesoscale around MICA (see August to a certain extent. Fig. 9d). Simultaneously, lower-levelinflow of high The MICA was well organized three-dimensio- 9e air mass with large PWC to the MICA might nally for intense and long lasting convection within be also reinforced, which then intensified the con- lower-level cyclonic circulation (LLCC) of which vection in MICA. The 100 km scale convection and the center was located about 80 km north of 500 km scale circulation seem to mutually reinforce MICA. Lower-level west-northwesterly and west- each other through the MICA, which appears to be southwesterly inflow, both of which had high 8e7 well organized early on in the developing process of April 1999 K. Mori, S. Ishigaki, T. Maehira and et al. 477

Yancy. TL2 suggest that TL2 possessed several aspects of Eastward and northward extension of middle-to a classical squall line type MPF. upper-level moderate echoes originated from the in- Longitudinal line systems (LL1-LL2) were formed tense lower-levelechoes in the MICA (Figs. 8c, 8d). in the lower-level intensified cyclonically sheared This suggests similar extension of the convective up- northwesterly flow. Echoes constructing these lon- ward motion (Figs. 12b, 12c). The speculated direc- gitudinal systems moved toward LES and MICA as tion of convectiveupward motion in MICA is consis- if they were sucked into an active convective region tent with the observational fact that lower-levelhigh in LES and MICA (Fig. 6) which might be related 9e air which was the origin of the updraft, entered to the intensified lower level inflow into Yancy con- the MICA from the west-southwest side. Hydrome- centrated lower than 1 km (see vertical wind profile teors in the intense convection might be brought up at 00 UTC 31 August in Fig. 10). to the tropopause (16 km level), and spread north- The comma-shaped spiral band (SB1) with a ward and eastward divergently, and consequently 500 km horizontal scale and life time over 7 hours form the CDO. evolved from LES, and the other spiral band (SB2) in which an active convective line originated from 6. Discussion MICA was involved, formed in the intensified LLCC 6.1 Characteristics of MPFs in Yancy in the early (about 25 m/s) with a diameter about 500 km in the developingstage third sub-stage. On this evolvingprocess of comma- During 06 UTC 30 August to 12 UTC 31 August, shaped spiral bands, CDO associated with MICA in the central part of Yancy in the early develop- disappeared and the cloud feature also changed to ing stage, eleven systems; LES, MICA, TL1-2, LL1- a comma shape. This is because after the MICA 4, and SB1-3 were identified in this study (Fig. 6, transformed to an active convectiveline in SB2. Hy- Fig. 7, Fig. 12). Characteristics of these, such as: drometeors in the upper level cloud shield might configuration, horizontal scale, life time, and max- be mainly produced in the active northern convec- imum echo top height are summarized in order of tive region of the comma-shaped band, including the their appearance time in Table 1. Maximum wind convective line in SB2. SB1 and SB2 increased cy- speed of lower-levelflow (CETwinds) and seasurface clonic curvatures, and SB3 emerged in almost cir- winds around the MPFs are also shown in Table 1. cular lower-levelflow (20 m/s) and inside the max- As described in Section 4, LES and MICA con- imum CETwind (25-30 m/s) zone in the final sub- structed a kernel structure of Yancy in the early stage. After that, other almost circular spiral bands gradual developing stage with larger (-400 km) were formed and merged into each other to form echo area, longer time scale (> 20 h for LES), an active round precipitation system around the ty- and the long lasting (9 h) mesoscale active con- phoon center in the latter rapid developing stage vection (MICA). Transversal and longitudinal line (not shown). systems had approximately the same horizontal Yamasaki (1983) categorized the simulated con- (200-300 km) and time (several to ten hours) vection in formation and intensification stages in scales, though maximum echo-top height of longi- an axially symmetric TC model, depending on the tudinal line systems (LL1-LL4) seems to be lower role of surface friction such that: I) surface fric- than that of the transversal line systems (TL 1, tion does not play any significant role before the TL2). Transversal line systems formed in weaker tangential velocity attains about 10 m/s, II) sur- (-) 15 m/s), slightly cyclonically sheared lower-level face friction becomes important when the tangen- flow in the initial sub-stage. On the other hand, tial velocity attains 10-15 m/s, and III) surface fric- longitudinal line systems appeared in the stronger tion plays an essential role in the formation and (20 m/s) lower-levelflow with intensified cyclonic maintenance of the eye and eyewall when the tan- shear, mainly in the second sub-stage. gential velocity near the vortex center exceeds ap- TL2 extended from south-southeast to north- proximately 20 m/s. Further, Yamasaki (1986) per- northwest and moved to the west-southwest at a formed a numerical experiment of TC development speed about 10 m/s (Fig. 6). The extending di- using a three-dimensional typhoon model based on rection of TL2 was perpendicular to the lower level the results of Yamasaki (1983), and pointed out that vertical vector which was directed to the structure of the simulated rainband depended the east-northeast between the surface and 1.5 km on the developing stage of TC rotation and loca- level and was reversed to the west-southwest be- tion of the rainband in the TC. In case of tween 1.5 km and 4 km level at 12 UTC 30 August in which frictional inflow was not very strong, the (not shown). Temperature was decreased by 5C band orientation had a larger angle with the tangen- around 12 UTC 30 August (Fig. 9c) associated with tial direction, and warm moist air entered the rain- passage of convective echoes of TL2, which mainly bands from the inner side. Transversal rainbands existed in lower level windward (northeast) side of like squall lines were organized in the periphery of matured TL2 (not shown). These characteristics of central parts of TC in pre-TS stage. 478 Journal of the Meteorological Society of Japan Vol. 77, No. 2

Table 1. Characteristics of MPFs within Yancy in the early gradual developing stage. Appearance period of each MPF is indicated by a horizontal line in which mature time is marked by a black dot. Sub stages, CETwind speed around the MPFs, and sea surface wind speed on the ship near the MPFs are referred. Correspondence of sub-stages in this study and Yamasaki's (1983) category of convection is inferred from the characteristics and behaviors of the MPFs and lower-level wind speed.

Several aspects of MPFs in our real case, sum- wind in Yancy's case, probably because Yamasaki's marized in Table 1 and Fig. 7, corresponded well to (1983) category is based on the results of numerical the results of numerical experiments by Yamasaki simulation by an axially symmetric model. (1983, 1986). Using Yamasaki's (1983) convection To maintain the MICA for a long time, interaction categorization, it might be interpreted that TL1 and between the cold pool made by the cooling through TL2 in the initial sub-stage belong to category I), evaporation of precipitation, and warm moist inflow LL1 to LL4 in the second sub-stage mainly belong from the undisturbed environment, was important to category II), SB1 and SB2 in the third and final as Yamasaki (1986) emphasized for the simulated sub-stages also belong mainly to category II), but rainbands with a large crossing angle in the early they might belong to category III) after 12 UTC developingstage. However,the degree of interaction 31 August as the LLCC intensified. SB3 in the fi- between convection and larger scale circulation ap- nal sub-stage mainly belongs to category III). For pears to be much stronger in the MICA than in the the convection in SB1 and SB2, interaction between simulated rainbands, because the MICA had larger cold pool and frictional inflow might be important crossing angle (almost orthogonal) with the tangen- probably for the convectiveline in SB2 (Fig. 8). For tial direction (Fig. 6 during 18 UTC 30 August to 03 the convection in SB3, which was formed inside of UTC 31 August, Fig. 7b) than the simulated rain- the maximum lowerlevel cyclonic circulation (about band. The MICA was a prominent mesoscale fea- 30 m/s) radius, frictional convergence and conse- ture among the identified MPFs within Yancy and quent upward motion might be important. formed as a most convectivelyactive part of a kernel Coexistence of TL2 and LL1 around 15 UTC 30 structure. On the other hand, the simulated rain- August (Fig. 6) might be related with the lower-level bands were formed evenlyin a sense around a vortex wind pattern, with stronger (20 m/s) wind around that might be a kernel structure of the simulated TC LL1 and weaker (15 m/s) wind around TL2. Tran- in the pre-TY stage. sition of convection from category II) to III) around Lower-levelwind direction in the windward side 12 UTC 31 August appears to not be so definitive. of MICA changed from almost perpendicular to par- In Table 1, wind speed of Yamasaki's (1983) cate- allel to MICA between 00 UTC and 06 UTC 31 gory is weaker than that of CETwinds and surface August adequately after the MICA was established April 1999 K. Mori, S. Ishigaki, T. Maehira and et al. 479

(Fig. 6). During this period, 500 km scale LLCC clogenesis. These studies-including our study- might be intensified through the mutual reinforce suggest that the MICA might have an important mechanism between MICA and LLCC speculated role on the intensification of LLCC on the 500 km in Section 5.5 (Fig. 12), which might consequently scale on the early gradual developing process. change the direction of low level inflow into MICA 7. Summary and concluding remarks from inner to outer side. This change of wind di- rection of lower level wind in the windward side of A major part of westward moving typhoon Yancy MICA seems to be an aspect which appeared pre- (T9313) in the early developing stage was observed ceding transformation of convection within Yancy continuously over 30 hours by a radar of the Japan from Yamasaki's (1983) category II to III. MeteorologicalAgency (JMA) research vessel (R/V) Line and spiral band systems in this case did not Keifu Maru, over the north western Pacific around propagate outward fast, which is different from outer a point (19N, 129E) from 30 August to 1 September internal gravity wave like rainband in mature TC 1993. Using the radar, maritime weather, and up- simulated in Kurihara and Tuleya (1974). In some per air data obtained on board, conventional data, line systems, decreases of temperature by several de- objectively analyzed data, and satellite data such grees were observed associated with precipitation, as Geostationary Meteorological Satellite (GMS), which suggests that cooperative interaction between European Remote Sensing Satellite-1 (ERS-i), and moist convection and large scale lower-levelcircula- Defense Meteorological Satellite Program (DMSP) tion may be fundamentally important for not only satellite data, structure and evolution of convec- MICA, but also these systems as Yamasaki (1983) tion in Yancy in that stage were investigated. To emphasized. supplement wind data, cell echo tracking winds The LES in the first sub-stage (Fig. 6 between 06 (CETwinds) were estimated, which approximately UTC and 18 UTC 30 August) appears to be more corresponded to 700 hPa winds. rigid than ordinary oceanic cloud clusters which was Lower-level cyclonic circulation (LLCC) with not associated with lower-levelcyclonic circulation 1500 km scale already existed at the beginning of (e.g., Mori, 1992). Formation and maintain pro- the early gradual developing stage, and the cloud cess of LES and differencesof the structure between system of Yancy existed in its southwest quadrant. cloud clusters, with and without lower-levelcyclonic Intensified LLCC, concentric with the cloud system circulation, will be studied in the future. of Yancy, was organized at the end of the stage. Af- ter that time, latter rapid development started. 6.2 Mesoscale intense connectivearea (MICA) During the early development, cloud (TBB < CDO has been well known as one of the common -60C) features of Yancy changed from angular to cloud features in the early developing TCs (Dvorak, round, then to comma shape. In the central part of 1975). Gray (1993) noted that convection breaking the round cloud system, lower(< -75C) TBB area, out was associated with wind surge in the early de- which was defined as CDO in this study, was formed veloping stage of TC. Liu et al. (1994) investigated for a relatively long time (9 h). Formation of CDO the water balance of Typhoon Nina using SSM/I appeared to be overlapped on the diurnal variation data and pointed out that the peak of total latent of convection, with its peak around morning. heat release associated with heavy rain occurs about Low level (2 km) precipitation and the lower- three days earlier than the Typhoon's rapid inten- level circulation pattern in the gradually develop- sification. These studies indicate that very active ing Yancy were examined in detail. Convection convection within TC in the early developing stage was organized in various mesoscale (100-500 km) brings about the rapid development in some cases. The MICA, mesoscale precipitation entity of CDO precipitation features (MPFs) such as the large in this case, is also an example of the vigorous con- (-400 km) echo system (LES), transversal and lon- gitudinal line systems, mesoscale intense convective vection within TC in the early developing stage. area (MICA), and spiral band systems, which ap- The MICA formed in the early gradual develop- peared as Yancy developed gradually. The ana- ing stage when the whole structure of Yancy was lyzed lower-levelcyclonic circulation (LLCC) center being transformed from in-concentric to concentric. of Yancy approached as close as 80 km to the north Careful examination of precipitation and CETwinds of the ship. around Yancy in the second sub-stage (18 UTC 30 Based on the configurations of the identified August to 03 UTC 31 August in Fig. 6) suggests that MPFs, the early developing stage was divided into formation of MICA appeared to precede the estab- four sub-stages. In the initial sub-stage, LES associ- lishment of the intensifiedLLCC on the 500 km scale ated with round-shaped cloud was organized in the by about 6 hours. This is consistent with Gray's south to southwest quadrant of LLCC. Northerly (1993) illustration of typical TC cloud formation. flow about 15 m/s with weak cyclonic shear pre- Zipser et al. (1978) also indicated that mesoscale vailed to the west of LES, in which transversal line organized deep convection preceded mesoscale cy- systems evolved. In the second sub-stage, MICA 480 Journal of the Meteorological Society of Japan Vol. 77, No. 2

was formed and maintained around the northwest- this real case are similar to those of the simulated ern edge of the LES for nine hours beneath a central mesoscale features in Yamasaki (1986). However, part of CDO. Northwesterly flow to the northwest of LES and MICA which were the kernel structure of MICA was intensified up to 20 m/s with increasing Yancy in this case, appears to not be simulated in cyclonic shear, in which longitudinal line systems Yamasaki (1986)-probably because of the differ- evolved. Center of LLCC with weaker (< 10 m/s) ence between the real situation in this case, and the flowat first appeared to the northeast of MICA and initial condition used in the simulation. moved west-northwestward faster than MICA. An The MICA was a mesoscaleprecipitation entity of intensified 500 km scale LLCC with maximum wind the CDO which appeared to proceed the establish- speed about 25 m/s around Yancy appeared to be ment of a 500 km scale intensified LLCC. Some as- established after the MICA was formed. In the third pects of MPFs in this case appear to correspond well sub-stage, the LES evolved into a comma-shaped to those of numerically simulated mesoscaleconvec- spiral band with length over 500 km and another tion during TC development. For substantial under- spiral band with length about 300 km in which ac- standing of the typhoon formation process and early tive convectionoriginated from MICA was formed in development, it seems to be an interesting next step the cyclonic southerly flow 20-25 m/s to the south- to simulate a well known but not so well understood east of the LLCC center. The cloud system changed phenomenon-CDO-related vigorous convection as from round- to comma-shape, corresponding to the MICA in our case. A well organized field experiment formation of these spiral bands and CDO was dissi- focusing on TC genesis over the tropical ocean in- pated. In the final sub-stage, the southern part of cluding continuous ship board Doppler radar obser- the comma-shaped spiral band disappeared and the vation is also desirable. These will further substan- northern head of it and southern part of another tiate the Ooyama's (1982) hypothetical interpreta- spiral band formed a new sharply curved comma- tion that the formation of a tropical cyclone may shaped precipitation and cloud system, which encir- be explained by the transition of convection from cled the LLCC center. An almost circular and inner a probabilistic stage to a definitive stage in which spiral band emerged inside of the sharply curved spi- convection is influenced by large scale motion. ral band. Strong southerly wind about 30 m/s pre- Of course, CDO does not always appear in the vailed about 200 km to the east of the LLCC center. early developing TCs. Actually, Dvorak (1975) After that, a round shaped cloud system with a di- presented several developing models of TCs with- ameter about 600 km containing an eye and almost out CDO. Ritchie and Holland (1997) stressed co- concentric with LLCC was formed, and latter rapid operative interaction between environmental and development started. mesoscale dynamics during a TC formation, in LES and MICA constructed a kernel structure of which CDO-related convection appear to not be so Yancy in the early developing stage. A schematic important. Therefore, the above mentioned future structure of the MICA was presented synthesizing work will never give a universal answer on the prob- all of the available data. In the MICA, echo top lem of TC genesis and the early developing process, height reached 16 km and moderate echoes spread but will produce an answer for CDO type TC devel- to the east and north of the intense echoes. The opment. MICA possessed a three dimensionally well orga- nized structure for long lasting intense convection. Acknowledgments Surface west-northwesterly and west-southwesterly inflowwith high ee converged around the northwest The authors would like to thank Dr. H. periphery of MICA, which might produce upward Sakakibara of MRI for his encouragement and help- motion over the south-southwesterly flow with low ful suggestions on this study. They are also grateful 8e which might be related with evaporative cool- to Dr. M.Tanaka of MRI for his constructive dis- ing of precipitation. Then, the very active convec- cussions and comments on this study and Prof. M. tion occurred and maintained for a long time. To- Yamasaki of Tokyo University for his fruitful dis- tal rainfall on the ship within MICA amounted to cussions. Thanks are extended to two anonymous 147 mm for 6 hours. A mutually reinforced process reviewers for their valuable suggestions in revising between convection in MICA and the 500 km scale the manuscript. They wish to express their spe- LLCC through the inferred release of latent heat by cial thanks to Mr. S. Takano, Mr. T. Suzuki and condensation was speculated, but more quantitative members of the observation team of the R/V Keifu study should be required. Maru and Captain S. Taniguchi and all the crew Transversal and longitudinal line systems and spi- of Keifu Maru for their effort during the observa- ral band systems were formed in order as Yancy tion. Thanks are extended to Mr. K.Takahashi developed, and LLCC intensified. This appears to and Dr. K.Aonashi of MRI and Dr. P.M. Wu of be interpretable by Yamasaki's (1983) categoriza- EORC/NASDA for giving them the edited GMS tion of convection. Some aspects of the MPFs in data, PWC pattern by DMSP satellite data, and April 1999 K. Mori, S. Ishigaki, T. Maehira and et al. 481 the ERS-1 surface wind data, respectively. Objec- SSM/I satellite data. Meteor. Meteor. Atmos. Phys., tive analysis data routinely made by the NPD/JMA 54, 141-156. were utilized. The observational cruise was planned Marks, F.D. Jr. and R.A. Houze, Jr., 1987: Inner core and conducted by the Marine Department (present; structure of Hurricane Alicia from airbone Doppler Climate and Marine Department)/JMA. radar observations. J. Atmos. Sci., 44, 1296-1317. Mori, K., 1992: Internal structure and time evolution of References a cloud cluster in the western tropical Pacific region observed by Keifu Maru. J. Meteor. Soc. Japan, 70, Attema, E.P.W., 1991: The active microwaveinstru- 1111-1123. ment on-board the ERS-1satellite. Proc. IEEE, 79, Mori, K., 1995: Equatorial convection observed by 791-799. the research vessel Keifu Maru during the TOGA Barnes, G.M., E.J. Zipser, D.P. Jorgensen and F.D. COARE TOP, November 1992. J. Meteor. Soc. 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啓 風 丸 レ ー ダ ー に よ り 観 測 さ れ た 発 達 初 期 の 台 風Yancy(T9313)内 の

対 流 の 構 造 と 進 化

森 一 正1 (気 象 研 究所)

石 垣 修 二 (長 崎 海洋気 象 台) 前 平 岳 男 ・大 矢 正 克2 (気 象 庁気 候 ・海洋 気象 部)

竹 内 仁 (東 京 管 区気 象 台)

1993年8月30日 か ら9月1日 まで、(19N, 129E)周 辺 の北西 太平 洋上 を西 進 した初 期 の緩 や か な発 達 期 にあ った台風Yancy (T9313) が、 気 象庁 観測 船啓 風 丸 で観測 され た。 この期 間 中、Yancyの 循環 中心 は 啓風 丸 の北80kmま で接近 した。Yancy中 心部 の対 流 が、 船 上 で得 られ た レーダ ー、 海上気 象、 高 層気 象 観 測 デ ー タ と最 近利 用可 能 に なっ た衛 星 デ ー タを用 いて解析 された。 セルエ コー追跡風(CETwinds)が 見 積 もられYancy周 辺 の下層 風 デ ー一タを補 うため に使用 された。 初 期発 達 期 間 中 に、 雲 が1500kmス ケ ール の下層 低 気圧 性 循環(LLCC)の 南西 象 限 に存在 し中心 を一 に しない構 造 が、 雲 シス テム 中心部 の 円形 の厚 い上 層雲('CDo')の 形 成 を経 て同一 中心 を持 つ構 造へ と遷 移 した。 この 同一 中心 を持 つ構 造 の確 立後、Yancyの 後期 の急 激 な発 達 が始 まった。

Yancy内 に様 々な メ ソス ケ ール(100-500km)降 水 体(MPFs)が 次 々に組織 され 時 間発展 した。 この MPFsの 形 態 は台風 初 期発 達 過程 が4つ のサ ブ ステ ー ジ を経 て進 展 す る に したが って変化 した。 第1サ ブ ス テ ージで は大 きな(400km)エ コー システ ム(LES)がLLCCの 南 西象 限 に組 織 され、 その上 に円形 雲 シ ス テ ムが 出現 した・ 第2サ ブ ス テ ージで は、 長続 きす る メソ ス ケー ルの強 い対 流域(MICA)がLESの 北 西端 に組織 され、 それ が 円形 雲 シス テ ム 中の'CDo'の メソ ス ケー ル降水 実体 であ っ た。LLCCはMICA の形 成 後500kmス ケ ー ル で強化 され た ようで あ った。 第3サ ブ ステ ー ジで は、 強 い低気 圧 性 循環 中 で、 LESと 雲 シス テム は500 km以 上 の長 さを持 つ コ ンマ型 ス パ イラルバ ン ドへ と進化 した。 最 終 サブ ス テ ー一 ジで は、 スパ イ ラルバ ンドの 曲率 は増 し、 よ り内側 の ほぼ 円形 に近 い スパ イ ラルバ ンドが更 に強化 され た LLCC中 に現 わ れた。 コ ンマ型 シス テ ムの北側 頭 部 はLLCC中 心 を巻 き込 み つつ あ った。MICA周 辺 に、 下層 の流 れ に垂 直 な線状 シス テ ム と平行 な線状 システ ムが、 第1サ ブ ス テ ージ と第2サ ブ ス テ ー ジ に各 々 形 成 されて い た。

LESとMICAは 初 期発 達過 程 にあるYancyの 核構 造 を構 成 して いた。MICAは、 長 続 きす る、 エ コー 頂 が 高度16kmに 達 す る強 い対 流 に とって3次 元 的 に都 合 よ く組 織 され た構 造 を持 ってい た。MICAと 500kmス ケ ール のLLCCは 互 い に強 め合 ってい る ようであ った。MPFsの い くつ かの特 徴 が まとめ られ、 そ れ らは 山岬(1983, 1986)に よ り数 値 的 に再現 された、 発 達 中の台風 内の メソ対流 の特 徴 と よ く対 応 して い る ようであ った。

1 現所 属:気 象 庁 気候 ・海洋気 象 部 2 現所 属:福 岡管 区気 象 台