Wide Slow-Moving Rainbands and Narrow Fast-Moving Rainbands Observed in Typhoon 8913

Journal of the Meteorological Society of Japan, Vol. 75, No. 1, pp. 67-80, 1997 67

Wide Slow-Moving Rainbands and Narrow Fast-Moving Rainbands

Observed in Typhoon 8913

By Yoshio Shimazu

MeteorologicalResearch Institute, Tsukuba, Ibaraki 305, Japan

(Manuscript received 16 April 1996, in revised form 19 December 1996)

Abstract

The size, motion, and structure of the rainbands in Typhoon 8913 are presented from observations using a long-range conventional radar and two Doppler radars. These rainbands were classified into wide slow- moving and narrow fast-moving. Wide (50-150km) rainbands have a small angular velocity (<10h-1) and a long lifetime (6-10 h or more). Narrow (25-50km) rainbands are generated in a region more upwind and inward than wide rainbands every 3-4h. They rotate cyclonically around the typhoon center at a large angular velocity (30h-1) during their short lifetime (1-4h), and catch up and merge with wide rainbands. One wide slow-movingrainband is accompanied with a disturbance (convergence,updraft, and high vorticity) inclining outward with a large slope (1/0) on its inside. One narrow fast-moving rainband is associated with a nearly upright disturbance. The structure of the disturbance differs, however, from the inertia-buoyancy waves proposed in some previous studies as the mechanism behind fast-moving rainbands. Both types of rainbands are characterized by stratiform precipitation in which low-levelshallow convective cells are embedded, and make no cold pools in the boundary layer.

1. Introduction stricted to the principal band. Tabata et al. (1992) classified typhoon rainbands into two types-outer The detailed structure of rainbands in tropical cy- and inner rainbands-based on their locations rel- clones has been studied based on aircraft or Doppler ative to the typhoon center, and it was possible to radar observations since the 1980s. It has not been regard them as principal and secondary bands, but certain, however,whether the results of the observa- their three-dimensional wind field analysis appeared tions revealed the general features of tropical cyclone to show a convective scale transient structure rather rainbands or some properties unique to the specific than a mesoscale quasi-steady one. sample because most analyses were confinedto only Tropical cyclone rainbands with strong convective one among multiple rainbands in a tropical cyclone activity have been observed and analyzed minutely (Barnes et al., 1983; Ishihara et al., 1986; Barnes in previous studies (Barnes et al., 1983; Powell, and Stossmeister, 1986;Bluestein and Hazen, 1989; 1990a,; Ryan et al., 1992). They were character- Powell, 1990a,;Barnes et al., 1991;Barnes and Pow- ized by deep convection and made cold pools in the ell, 1995). The detailed structure of all rainbands boundary layer. Rainbands with precipitation dom- in a tropical cyclone is difficult to clarify, so all inated by a stratiform character were also observed rainbands must be categorized by size and motion, in some tropical cyclones (Marks, 1985; Houze et both of which can be estimated easily, and the de- al., 1992), but their structure and mechanism have tailed structure observed in a representative sam- not been clarified sufficiently. Houze et al. (1992) ple for each type of rainband be assumed applica- stated that such a stratiform rainband was an area in ble to other samples. Willoughby et al. (1984) used which ice particles fell, having been seeded by deep such tactics to distinguish the stationary band com- convection in an eyewall. It should have been con- plex (SBC) from moving bands and divide SBC el- sidered, however, whether weak mesoscale updrafts ements into principal and secondary bands by size.1 less than 1 ms-1 and generating cells contributed Their observations were limited to flight level alone, to its formation. Marks and Houze (1987) showed however, and their discussion on wind structure re- that the stratiform precipitation in a tropical cy- 1 Willoughby et al. (1984) stated that another type of con- clone was formed from seeding by deep convection in necting band joined the principal band to the eyewall. an eyewall and feeding by an upper-level mesoscale (c) 1997, Meteorological Society of Japan 68 Journal of the Meteorological Society of Japan Vol. 75, No. 1

updraft, but their analysis was restricted to a small 20-km-wide area just outside the eyewall, and this may not be applicable to outer areas. This study classifies Typhoon 8913 rainbands into two types-wide slow-movingand narrow fast- moving-based on observations using a long-range conventional radar. The wind structure of the representative sample for each type is further presented from Doppler radar data. The character of precipitation is also studied using vertical cross sections of radar reflectivity, surface observation, and soundings, and is found to be dominated by stratiform features. The mechanisms behind them are then discussed. 2. Observation and analysis To estimate rainband size and motion, conventional Mt. Fuji 10-cm radar with a 500-km range was used. Data from it represents the CAPPI at about 2km high, and has a 5-km horizontal grid spacing and time intervals of 8-14 minutes. To observe rainband wind structure, two Meteorolog- ical Research Institute Doppler radars, Narita 3-cm radar and Tsukuba 5-cm radar, were used. They are detailed in Sakakibara et al. (1985). The vol- Fig. 1. Typhoon 8913 track and central pressure. ume scan consisted of PPI data at 8 (Narita) or 10 (Tsukuba) elevations collected in 402 seconds. Narita radar RHI data were also obtained. Doppler radar data was analyzed by VAD and rainbands are classified into wide slow-movingand dual-Doppler method. The VAD analysis followed narrow fast-moving as follows: the procedure by Browning and Wexler (1968) and Wide (50-150km) rainbands W1, W2, and W3 only the horizontal wind averaged over a scanned are almost stationary or rotate cyclonically around circle with a diameter of 10km or more was used to the typhoon center with a small angular velocity remove convective-scaledisturbances. Dual-Doppler (<-10h-1),2 and are called wide slow-movingrain- analysis followedIshihara et al. (1986), and its re- bands hereafter. They have long lifetimes of 6-10h sults included a large vertical interpolation error due or more. Once an old wide rainband diminishes, a to the coarse vertical resolution of observation. The new wide one is generated and develops at another error makes some artificial mesoscale convergence azimuth. Rainband W1, for example, weakens and zones similar to real ones in their width (-10km) W2 strengthens during 0000-0300 UTC 6 August, and magnitude (N10-4s-1) and requires careful and W2 becomes weak and W3 is generated and de- analysis. It is useful to distinguish real from artifi- velops during 0300-0900 UTC. cial disturbances by checkingwhether disturbances Narrow (25-50km) rainbands N5, N6, and N7 move with rainbands, because artificial ones are al- are characterized by their fast motion. They are most always stationary relative to the ground unless generated in a region more upwind and inward than the vertical wind profile changes greatly. wide rainband W2 every 3-4h. They rotate fast 3. Results cyclonicallyaround the typhoon center during their short lifetime (1-4h), and catch up and merge with 3.1 Rainband size and motion W2. They are called narrow fast-moving rainbands Typhoon 8913 moved northwest or north- hereafter. Narrow rainband N5 is a spiral with a northwest, made a landfall on the coast of Japan crossing angle of 30 and a large angular velocity at 0600 UTC 6 August, and passed through the dis- of 30h-1 (Fig. 3). Its crossing angle and angular tance of 31km from Narita at 0630 UTC (Fig. 1). velocity are constant temporally and homogeneous It was not affected by the mid-latitude westerly or horizontally. Although its angular velocity at a cer- frontal zone located north of latitude 50N. tain radius from the typhoon center is large, its up- Mt. Fuji radar echoes relative to typhoon center wind and downwind ends move more slowly due to from 0000 UTC through 0900 UTC 6 August (Fig. 2) 2 Rainband W2 has a fairly large angular velocity (15- show no eyewalls,and some rainbands are biased to 20h-1) during development (0000-0300 UTC). the forward (northwest) side of the typhoon. These February 1997 Y. Shimazu 69

Fig. 2. Reflectivity of Mt. Fuji radar echoes at (a) 0000 UTC, (b) 0100 UTC, (c) 0200 UTC, (d) 0300 UTC, (e) 0600 UTC, and (f) 0900 UTC 6 August relative to typhoon center T. The upper side is directed north. Wide slow-movingrainbands are W1, W2, and W3. Narrow fast-moving rainbands are N5, N6, and N7. Hatching denotes 25-35 dBz, and solid >35 dBz. Broken lines represent the maximum radar range (500km). 70 Journal of the Meteorological Society of Japan Vol. 75, No. 1

Fig. 2. (Continued) its elongation upwind in the developing stage and its shrinking at the downwind end in the weakening stage. Its radial motion at a certain azimuth of the typhoon is outward as the natural character of a spiral moving cyclonically. N6 and N7 shape and motion are similar to N5. To confirm the different motions of the two types of rainbands from data with shorter time intervals and a longer period, the azimuth (relative to the typhoon center)-time cross section of Mt. Fuji radar echo at 175km radius from the typhoon center (Fig. 4) and the radius-time cross section in the northwest direction (Fig. 5) are shown. In Fig. 4, the slow motion of W1, W2, and W3 and the alternation of old and new wide rainbands are seen. The fast motion of rainbands N1-N7 and their fairly cyclic gen- eration in a region more upwind than W1 and W2 are also recognized. In Fig. 5, W2, nearly stationary in the radial direction, and N1-N7 moving outward are seen. The radial velocity of these narrow rain- Fig. 3. Hourly positions and shapes of nar- bands is proportional to the radius from the typhoon row fast-moving rainband N5 relative to center, for example 11 ms-1 at 125km and 15 ms-1 typhoon center T between 2300 UTC 5 at 175km. This confirms that their crossing angle August and 0300 UTC 6 August. The and angular velocity are constant temporally and upper side is directed north. Broken homogeneous horizontally. An eyewall is observed lines represent the hourly positions of a at 120km before 2030 UTC 5 August (Fig. 5). spiral with a crossing angle of 30 and an angular velocity of 30h-1. Num- 3.2 Wind structure bers represent times (UTC). To investigate the wind structure of Typhoon 8913 rainbands, Narita Doppler radar data on rainbands W2, N5, and N6 almost in the northwest rela- determined through time-space transformation as- tive to the typhoon center (Fig. 5) were analyzed by suming the typhoon's quasi-steady state. time composite VAD. The results of 45 VAD anal- In the cross section of radar reflectivity (Fig. 6a), yses were composed, and the radius (from the ty- three rainbands W2, N5, and N6 are seen. Tangen- phoon center)-height cross sections (Fig. 6) were February 1997 Y. Shimazu 71

Fig. 4. Azimuth (relative to the typhoon center-time cross section of Mt. Fuji radar echo at 175km from the typhoon center between 1700 UTC 5 August and 1100 UTC 6 August. Wide slow-moving rainbands are W1, W2, and W3. Narrow fast-moving rainbands are labeled N1-N7. Hatching denotes 25-35 dBz, and solid>35 dBz. Dots represent the region outside the maximum radar range.

Fig. 5. Radius (from the typhoon center-time cross section of the Mt. Fuji radar echo on the northwest side of the typhoon between 1700 UTC 5 August and 0600 UTC 6 August. Wide slow-moving rainband W2 is denoted by polygon W2 and the eyewall by rectangle EW. Line NARITA connects observation points (radii and times of Narita Doppler radar. Letters a, b, c, and d represent points of soundings at Tsukuba. Other notation is the same as for Fig. 4. 72 Journal of the Meteorological Society of Japan Vol. 75, No. 1

Fig. 6. Radius (from the typhoon center)-height cross sections from time composite VAD of Narita radar data. (a) Reflectivity (dBz) contoured at 2.5 dBz intervals. Strong reflectivity (>25 dBz) is indicated by hatching. (b) Tangential wind (ms-1) relative to the typhoon contoured at 2 ms-1 intervals. Strong wind (>30 ms-1) is indicated by hatching. (c) Radial wind (ms-1) relative to the typhoon contoured at 2 ms-1 intervals. Positive values indicate outflow. Inflow is indicated by hatching. (d) Radial divergence (x10-4 s-1) contoured at 2x10-4 s-1 intervals. Convergence is indicated by hatching. (e) Vertical velocity contoured at 0.2 ms-1 intervals. Broken lines represent the contours of negative values. Strong updrafts (>0.2 ms-1) are indicated by hatching. (f) Vorticity of tangential wind (x10-4 s-1) contoured at 2x10-4 s-1 intervals. Broken lines represent the contours of negative values. High vorticity (2x10-4 s-1) is indicated by hatching. In all figures, dots represent the region not analyzed. In (b)-(f), strong reflectivity (>25 dBz) is indicated by shading. On the top of (a) and (d), observation times (UTC) are indicated, rainband locations are W2, N5, and N6, and NO DATA denotes the region where no data was obtained and contours are interpolated. tial wind relative to the typhoon (Fig. 6b) has its though the width of the convergencezone at a cer- maximum area (>36 ms-1) broad in radius (75km tain height is 20km, as a whole, it occupies a wider wide) and shallow in height (0.4km thick). Ra- area of 65km due to its large inclination. Its hori- dial wind relative to the typhoon (Fig. 6c) has a zontal structure and motion are indicated in results boundary layer dominated by strong inflow. On the of dual-Doppler analysis (Figs. 7 and 8). A con- inner side of 180km radius, outflow occures at a vergence zone with a width and magnitude similar 1-2 or 3km height, nearly zero radial wind above, to that obtained from time composite VAD is seen and outflow again at 5-6km. On the outer side of on the inside of W2 (Fig. 7). In radius (from the 180km radius, inflow occurs at 1-4km and outflow typhoon center)-height cross sections made from above. Fig. 6d presents radial divergence in cylin- the results of dual-Doppler analysis (Fig. 8), it is drical coordinates 3Vr/ar+Vr/r, where Vr is radial clear that the convergence zone inclines outward wind and r is the radius from the typhoon center. and is stationary relative to the typhoon center be- Vertical velocity (Fig. 6e) is estimated from the ra- tween 0038 UTC and 0118 UTC 6 August. The dial divergence. Fig. 6f shows vorticity of tangential convergencezone associated with W2 forms an upwind 5V9/Dr+VB/r, where Ve is tangential wind. draft of 0.4 ms-1 (Fig. 6e). A high vorticity area Wide slow-movingrainband W2 is accompanied (>2x10-4 s-1) is also recognized on the inside of with a convergence (2x10-4 s-1) zone inclining W2 (Fig. 6f). They also incline outward with a large outward with a large slope of 110 on its inside. Al- slope. February 1997 Y. Shimazu 73

Fig. 7. Divergence at 2km height contoured at 2.5x10-4 s-1 intervals from dual-Doppler analysis at 0111 UTC 6 August. Convergence is indicated by hatching. Strong Tsukuba radar echo (>27.5 dBz) is indicated by shading. W2 indicates rainband location. Dots represent the region where wind was not analyzed.

A convergencezone inclining outward with a large analysis including a large vertical interpolation error slope was also found in a wide slow-movingrainband ofN10-4 s-1. The wind structure of N6 could not observed by Ryan et al. (1992), and may be a com- be obtained due to the lack of a data on its outside mon feature of such rainbands. Although Tabata part (Fig. 6). et al. (1992) found no such inclining convergence 3.3 Precipitation character zone in a wide slow-movingrainband, their analy- To characterize Typhoon 8913 rainband precip- sis appeared to be contaminated by convective-scale itation, vertical cross sections of radar reflectivity transient disturbances. are shown in Fig. 9. Both types of rainbands ap- Narrow fast-moving rainband N5 is associated with a nearly upright disturbance (convergence,up- pear to be dominated by stratiform precipitation with a bright band. Many radar echo cells embed- draft, and high vorticity) (Figs. 6d, 6e, and 6f). ded in the stratiform precipitation are also noted. Since N5 and the observationsite (Narita) move out- Most upper-level cells (at 7-8km) must be generat- ward at p=13 ms-1 and inward at q=9 ms-1 rela- ing cells working as seeders for stratiform precipita- tive to the typhoon center respectively, the apparent tion. The streaks falling from generating cells might width of N5 in Fig. 6 using coordinates fixed to the form some of the low-levelcells. Somelow-level shal- typhoon center, reduces to q/(p+q)=0.4 times of low convective cells must exist, however, because an actual value. Apparent radial divergence domi- some short-term strong rain cores (20-75mm h-1) nated by a radial differential term and apparent ver- are recognized in the surface observation at Narita tical velocity estimated from the radial divergence (Fig. 10). The results of soundings (Fig. 11, loca- become about twice the actual values. Vorticity of tions given in Fig. 5.) show that it is easy to gen- tangential wind is not affected so much because it erate free convection by lifting a low-level air par- is not dominated by a radial differential term. The cel everywhere in and around rainbands. They also actual magnitude of the disturbance, therefore, may show that the convective available potential energy be 1x10-4 s-1 in convergence,0.6 ms-1 at 5.5km in an updraft and 2x10-4 s-1 in vorticity. The con- (CAPE) is small (50-140 J kg-1) and the level of convective equilibrium is fairly low (4-6km). Such vergence is too weak to be detected in dual-Doppler 74 Journal of the Meteorological Society of Japan Vol. 75, No. 1

dence of cold pools associated with Typhoon 8913 rainbands with weak convective activity. Data on temperature in the lowerboundary layer up to 200m obtained by an observation tower at Tsukuba (data not shown) confirms the absence of cold pools. The echo top of the inside of W2 inclines outward with a large slope and is collocated with the updraft described in Section 3.2. This suggests that the updraft feeder effect is important in the growth of stratiform precipitation in W2. Such an effect appears large at low levels, because the mean surface rainfall intensity (Fig. 10) is fairly strong (15mm h-1) inside W2 with a low-levelupdraft, but is weaker (5mm h-1) outside with a middle- level updraft. Tropical cyclone rainbands dominated by stratiform precipitation have also been observed in previous studies (Marks, 1985; Houze et al., 1992). Since these studies observed developing or mature tropical cyclones over the tropical or subtropical ocean, rainbands dominated by stratiform character are not peculiar to the mid-latitude weakening tropical cyclones on land like Typhoon 8913. The author thinks more attention should be paid to the structure and mechanismof such rainbands in future research. Fig. 8. Radius (from the typhoon cen- 4. Discussion ter)-height cross sections of divergence averaged over an azimuthal width of 5 A radar echo consisting of wide and narrow rain- (15-20km) contoured at 2x10-4 s-1 bands, located more upwind and inward than wide intervals from dual-Doppler analysis at ones, like Typhoon 8913, was observed in previous (a) 0038 UTC and (b) 0118 UTC 6 Au- studies of tropical cyclones3 (Senn and user, 1959; gust. Convergenceis indicated by hatch- Willoughby et al., 1984;Bluestein and Hazen, 1989; ing. Thick slant lines in each figure indi- Tabata et al., 1992). Although many recent stud- cate the position of a stationary conver- ies used the terms, principal and secondary bands gence zone axis. Strong Tsukuba radar proposed by Willoughby et al. (1984) for these two echo (>27.5 dBz) is indicated by shad- types, Senn and Hiser (1959) called them rain shield ing. Rainband location is W2 above (a). and spiral bands and Tabata et al. (1992) called Dots represent the region where wind was not analyzed. them outer and inner rainbands. All of these wide rainbands were almost stationary or moved slowly. A spiral band observed by Senn and Hiser (1959)- the only case where the motion of a narrow rain- weak convective instability is consistent with the band was estimated in previous studies4-rotated fact that no deep convective cells are found in the cyclonically around the tropical cyclone center with rainbands. As discussed above, Typhoon 8913 rain- a large angular velocity of 40h-1, but its upwind bands have stratiform precipitation with embedded and downwindends did not rotate due to its elonga- low-level shallow convective cells, unlike rainbands tion or shortening. Its radial motion at a certain az- with stronger convective features observed in some imuth of the tropical cyclonewas outward. In other previous studies of tropical cyclones (Barnes et al., words, the motion of the two types of rainbands ob- 1983;Powell, 1990a,; Ryan et al., 1992). served in previous studies of tropical cyclones were The difference in rainband convective activity similar to that in Typhoon 8913. Willoughby et al. causes their thermodynamic effect on the environ- (1984) characterized Hurricane David as including ment to differ. Rainbands with strong convective some narrow fast-moving rainbands separate from activity make cold pools in the boundary layer from the downdrafts of deep convection (Barnes et al., 3 The alternation of old and new wide rainbands is unique 1983;Powell, 1990a, 1990b;Ryan et al., 1992). Sur- to Typhoon 8913. 4 Although Willoughby et al. (1984) regarded secondary face observation of temperature and dew-point at bands as elements of a band complex stationary as a Narita (Fig. 12), however, does not show any evi- whole, they did not estimate the motion of each band. February 1997 Y. Shimazu 75

Fig. 9. RHI reflectivity (dBz) patterns from Narita radar contoured at 2.5 dBz intervals; (a) across rainband W2 at 0005 UTC 6 August, (b) across rainbands W2 and N5 at 0107 UTC 6 August, (c) across rainband N6 at 0306 UTC 6 August, (d) along rainband W2 at 2243 UTC 5 August, and (e) along rainband N6 at 0217 UTC 6 August. Hatching indicates strong reflectivity (>22.5 dBz). Radar is at 0km, and radar beam azimuths are shown with arrows. 76 Journal of the Meteorological Society of Japan Vol. 75, No. 1

Fig. 10. Surface rainfall intensity (mm h-1) at Narita between 2100 UTC 5 August and 0500 UTC 6 August. W2, N5, and N6 are periods of rainband passage.

Fig. 11. Profiles of equivalent potential temperature (8e) and saturation equivalent potential temperature (ee*)from soundings at Tsukuba at (a) 2330 UTC 5 August, (b) 0056 UTC 6 August, (c) 0236 UTC, and (d) 0413 UTC. Sounding (a) is a routine one by the AerologicalObservatory, and other soundings are special ones using the Omega zondes. CAPE (J kg-1) for an air parcel with the highest 8e in the levels between the surface and 700 hPa is also shown. February 1997 Y. Shimazu 77

Fig. 12. Temperature (T) and dew point (Td) of surface air at Narita between 2100 UTC 5 August and 0500 UTC 6 August. W2, N5, and N6 represent periods of rainband passage.

other tropical cyclones with both wide and narrow phoon 8913, which do not form cold pools (Section rainbands. Hurricane David's case should be consid- 3.3). ered a transient state, however, because David had Regarding the mechanism of fast-moving rain- both types of rainbands except for a short period of bands, Tatehira (1962) and Atlas et al. (1963) pro- about 2h. posed that radar echoes successively generated at The difference in motion between slow-moving the point source propagating outward are carried by and fast-moving rainbands offers powerful clues to advection due to tangential wind and line up into the mechanisms behind them. In slow-movingrain- a spiral shape. Although this theory explains the bands, two theories have been proposed in previous large angular velocity, the elongation upwind, and numerical or theoretical studies. One is that the the shortening at the downwind end of fast-moving wind in a tropical cyclone's environment contributes rainbands as seen in Typhoon 8913, the physical to the formation of slow-movingrainbands or up- interpretation of the point source is difficult. An- drafts (Jones, 1977a, 1980; Tuleya and Kurihara, other theory, that these rainbands are related to 1984;Jones, 1993;Elsberry, 1993). Since most pre- fast-movingwaves with a finite wavelength both az- vious numerical experiments without environmen- imuthally and radially, namely with a spiral shape, tal wind (Kurihara and Tuleya, 1974; Jones, 1977b, seems more appropriate. Rainband elongation up- 1980) did not form slow-movingrainbands, this the- wind when developingcan be explained if we assume ory seems reasonable. The detailed process of rain- that these rainbands appear when the updrafts of band formation remains to be clarified, however. fast-moving spiral waves overlap with a large-scale Jones (1977a, 1986) stated that the interaction of a convergence area like that enclosed in broken lines tropical cyclone vortex with its environmental wind in Fig. 14 (shown schematically in Section 5). created slow-movingrainbands, but the relationship The inertia-buoyancy waves were proposed as a between interaction and rainbands was not clear. mechanism behind fast-moving rainbands in some Although Tuleya and Kurihara (1984) emphasized numerical studies (Kurihara and Tuleya, 1974; the large sea surface flux in an asymmetric strong Jones, 1977b, 1980). It was studied as to whether wind area (caused by environmental wind) along a they apply to the narrow fast-moving rainbands in slow-movingrainband, they should have conducted Typhoon 8913. Kurihara (1976) and Willoughby a sensitive test with homogeneous sea surface flux (1977, 1978a, 1978b) showed that inertia-buoyancy independent of wind speed to verify their statement. waves in tropical cyclones developed primarily Jones (1993)showed that a mesoscaleupdraft is cre- through the transformation of kinematic energy ated on the down-shear side of a tropical cyclone from a basic flow, i. e., a tangential wind with the in sheared environmental wind, and Elsberry (1993) radial shear of angular velocity, to the waves and related this to the differential advection of tropical the positive correlation between tangential (Vs')and cyclone vorticity. It was not confirmed, however, radial (Vr') wind anomalies was essential for energy whether the updraft in the Jones' dry numerical transformation. The distribution of V8'and Vr' in N5 model could form a rainband in a wet model. was obtained from time composite VAD (Fig. 13).5 The other theory for slow-moving rainbands is Since the correlation between V8'and Vr' is zero or that the cold pool made from downdrafts of deep 5 Anomalies from the radial means over a width of 20km convection converges with warm moist air in the (at 150-170km) including the entire disturbance are rainband's environment and successively generates shown in Fig. 13. Although anomalies like this can be affected by radial variation with scale larger than each new convectioe cells (Yamasaki, 1986). This is not rainband, particularlyin estimatingVe', such variation in applicable to the wide slow-movingrainbands in Ty- V8is small on the outer sideof 100km radius (Fig. 6b). 78 Journal of the Meteorological Society of Japan Vol. 75, No. 1

Fig. 14. Schematic representation of the radar echo distribution for Typhoon 8913. Fig. 13. Radius (from the typhoon center)-height cross sections representing wind anomalies of narrow fast-moving wide slow-movingand narrow fast-moving. Wide rainband N5 obtained from time com- (50-150km) rainbands have a small angular veloc- posite VAD of Narita radar data. ity (<10h-1) and a long lifetime (6-10h or more). Anomalies from radial means over a Once an old wide rainband diminishes, a new wide width of 20km (at 150-170km) are one is generated and develops at another azimuth. shown. Dots represent the region where Narrow (25-50km) rainbands are generated in a re- wind was not analyzed. (a) Tangential gion more upwind and inward than wide rainbands wind anomaly V8'contoured at 0.5 ms-1 every 3-4h. They rotate cyclonicallyaround the ty- intervals. Negative values (hatched) de- phoon center at a large angular velocity (30h-1) note anticyclonic wind. (b) Radial wind during their short lifetime (1-4h), and catch up and anomaly Vr' contoured at 1 ms-1 inter- merge with the wide rainband. Although their an- vals. Negative values (hatched) denote inflow. In each figure, thick lines indi- gular velocity at a certain radius from the typhoon cate the maximum area of V8'. center is large, their upwind and downwind ends move more slowly due to their elongation upwind in the developing stage and their shrinking at the negative, the structure of rainband N5 differs from downwind end in the weakening stage. Wide slow-moving rainbands in Typhoon 8913 inertia-buoyancy waves. This result may not be suf- ficient, however, to deny the relationship between may be of the same type as the principal band fast-moving rainbands and inertia-buoyancy waves (Willoughby et al., 1984), the rain shield (Senn because the disturbance associated with N5 is too and Hiser, 1959), or the outer rainband (Tabata weak to be detected by dual-Doppler analysis (Sec- et al., 1992), but the alternation of old and new tion 3.2). wide rainbands is unique to Typhoon 8913. Narrow fast-moving rainbands may be similar to secondary 5. Conclusions and moving bands (Willoughby et al., 1984), spiral bands (Senn and user, 1959), or inner rainbands The size, motion, and structure of Typhoon 8913 (Tabata et al., 1992), although the motion of sec- rainbands are presented from observations using ondary and inner bands was not determined. a long-range conventional radar and two Doppler The wind structure of a representative sample for radars. Figure 14 is a schematic representation of each rainband type was obtained from Doppler radar the radar echo distribution for Typhoon 8913. There data. One wide slow-movingrainband is accompa- are not any eyewalls, and some rainbands are biased nied with a disturbance (convergence, updraft, and to the forward (northwest) side of the typhoon. high vorticity) inclining outward with a large slope All Typhoon 8913 rainbands are classified into (1/10) on its inside. The magnitude of the distur- February 1997 Y. Shimazu 79 bance is 2x10-4 s-1 in convergence and vortic- References ity and 0.4 ms-1 in an updraft. One narrow fast- Atlas, D., K.R. Hardy, R. Wexler and R.J. Boucher, moving rainbands is associated with a nearly upright 1963: On the origin of hurricane spiral bands. Ge- disturbance with a magnitude of about 1x10-4 s-1 ofisica Iriternacional, 3, 123-132. in convergence, 0.6 ms-1 at 5.5km in an updraft, Barnes, G.M., E. J. Zipser, D. Jorgensen and F. Marks, and 2x10-4 s-1 in vorticity. Jr., 1983: Mesoscale and convective structure of a Both types of rainbands exhibit stratiform pre- hurricane rainband. J. Atmos. Sci., 40, 2125-2137. cipitation in which low-level shallow convective cells Barnes, G. M. and G. J. Stossmeister, 1986: The struc- are embedded. They exist in the environment with ture and decay of a rainband in Hurricane Irene small CAPE and make no cold pools in the bound- (1981). Mon. Wea. Rev., 114, 2590-2601. ary layer. Barnes, G. M., J. F. Gamache, M.A. LeMone and G. J. The mechanisms behind wide slow-moving and Stossmeister, 1991: A convective cell in a hurricane narrow fast-moving Typhoon 8913 rainbands are rainband. Mon. Wea. Rev., 119, 776-794. Barnes, G. M. and M.D. Powell, 1995: Evolution of the speculated to be as follows, judging from their mo- inflow layer of Hurricane Gilbert (1988). Mon. Wea. tion: The wind in a tropical cyclone's environ- Rev., 123, 2348-2368. ment appears to contribute to the formation of Blustein, H.B. and D.S. Hazen, 1989: Doppler-radar slow-moving rainbands, because such rainband (or analysis of a tropical cyclone over land: Hurricane updraft) was formed in previous numerical experi- Alicia (1983) in Oklahoma. Mon. Wea. Rev., 117, ments with environmental wind (Jones, 1977a, 1980; 2594-2611. Tuleya and Kurihara, 1984; Jones, 1993), but could Browning, K. A. and R. Wexler, 1968: The determina- not be created in most previous experiments with- tion of kinematic properties of a wind field using out it (Kurihara and Tuleya, 1974; Jones, 1977b, Doppler radar. J. Appl. Meteor., 7, 105-113. 1980). The detailed process of the formation of Elsberry, R.L., 1993: Advances in dynamical predictions slow-moving rainbands remains to be clarified, how- and modeling of tropical cyclone motion. Naval post- graduate school, Monterey, CA. ever. Although Yamasaki (1986) proposed another Houze, R.A., Jr., F. D. Marks, Jr. and R. A. Solid, 1992: mechanism for slow-moving rainbands in which cold Dual-aircraft investigation of the inner core of Hur- pools from downdrafts of deep convection converged ricane Norbert. Part II: Mesoscaledistribution of ice with warm moist air in the rainband's environment particles. J. Atmos. Sci., 49, 943-962. and successively generated new convective cells, this Ishihara, M., Z. Yanagisawa, H. Sakakibara, K. is not applicable to the wide slow-moving Typhoon Matsuura and J. Aoyagi, 1986: Structure of a ty- 8913 rainbands which form no cold pools. Kurihara phoon rainband observed by two Doppler radars. J. and Tuleya (1974) and Jones (1977b, 1980) have Meteor. Soc. Japan, 64, 923-939. stated that, based on their numerical experiments, Jones, R. W., 1977a: Vortex motion in a tropical cyclone fast-moving rainbands in tropical cyclones were re- model. J. Atmos. Sci., 34, 1518-1527. lated to inertia-buoyancy waves. The wind structure Jones, R.W., 1977b: A nested grid for a three- dimensional model of a tropical cyclone. J. Atmos. of one narrow fast-moving Typhoon 8913 rainband Sci., 34, 1528-1553. does not have the positive correlation between tan- Jones, R.W., 1980: A three-dimensional tropical cyclone gential and radial wind anomalies essential to the model with release of latent heat by the resolvable development of inertia-buoyancy waves, however. scales. J. Atmos. Sci., 37, 930-938. Jones, R. W., 1986: Mature structure and motion of a Acknowledgments model tropical cyclone with latent heating by the resolvable scales. Mon. Wea. Rev., 114, 973-990. The author wishes to express his thanks to H. Jones, S.C., 1993: The motion of dry baroclinic vor- Sakakibara, M. Ishihara, A. Tabata, K. Akaeda, Y. tices: The role of vertical shear and vortex tilting. Misumi, F. Fujibe, T. Tsuyuki, J. Aoyagi, H. Oono Preprints, 20th Conf. on hurricanes and tropical me- and O. Suzuki for their cooperation in data acqui- teorology, A. M.S., Boston, Mass., J24-J25. sition. He also thanks H. Sakakibara, K. Akaeda, Kurihara, Y., 1976: On the development of spiral bands K. Mori and three unknown referees for their help- in a tropical cyclone. J. Atmos. Sci., 33, 940-958. ful comments. He is grateful to H. Okamura, H. Kurihara, Y. and R.E. Tuleya, 1974: Structure of a trop- Nirasawa and the personnel of the Tokyo District ical cyclone developed in a three-dimensional numerical simulation model. J. Atmos. Sci., 31, 893-919. Meteorological Observatory, the New Tokyo Aero- Marks, F.D., Jr., 1985: Evolution of the structure of nautical Local Meteorological Observatory and the precipitation in Hurricane Allen (1980). Mon. Wea. Aerological Observatory, for providing the observa- Rev., 113, 909-930. tional data. The site of the Narita radar was pro- Marks, F.D., Jr. and R.A. 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台風8913号で観測された幅が広く動きの遅いレインバンドと細くて動きの速いレインバンド

島津好男 (気象研究所)

レンジの長い通常レーダーと2台のドップラーレーダーによる観測から、台風8913号のレインバンドの大きさ・動き・構造を示す。これらのレインバンドは、幅が広く動きの遅いものと、細くて動きの速いものに分類される。幅の広い (50-150km) レインバンドは、角速度が10°/h以下と小さく、6-10時間もしくはそれ以上の長い寿命を持つ。細いレインバンドは、幅の広いレインバンドのやや上流かつ内側で3-4時間毎に発生し、台風中心の周りを大きな角速度 (30°/h) で反時計回りに回転して、1-4時間の短い寿命のうちに幅の広いレインバンドに追いつき合体する。幅が広く動きの遅いレインバンドの内側には、1/10の傾斜で外向きに大きく傾いた擾乱 (収束・上昇流・強い渦度) が解析される。細くて動きの速いレインバンドにはほぼ直立した擾乱が対応しているが、その構造は熱帯低気圧中の動きの速いレインバンドの成因として従来のいくつかの研究で提案された慣性内部重力波とは異なっている。どちらの型のレインバンドも下層に浅い対流セルが埋め込まれた層状性降水によって特徴づけられ、境界層にコールドプールを形成することはない。