Asymmetry in Wind Field of Typhoon 0115 analyzed by Triple Doppler Radar Observation

Transcription

1 Asymmetry in Wind Field of Typhoon 115 analyzed by Triple Doppler Radar Observation Hiroshi YAMAUCHI*, Osamu SUZUKI (Meteorological Research Institute Kenji AKAEDA (Japan Meteorological Agency 1. Introduction Doppler radar is one of the most powerful tools for revealing detailed kinetic structure of tropical cyclones (TCs owing to its high spatial and temporal resolution over a hundred kilometers range. However, TC s wind information observed by a single Doppler radar, which is in itself only radial velocity with respect to a radar (Doppler velocity, need to be retrieved to wind fields by some methods such as GBVTD (LEE, 1999 for deeper understanding of TC structures or operational use. Those retrieval methods are based on the assumption of TC s axi-symmetry. Therefore, asymmetric components of wind field in a real TC, especially after its landfall may badly affect accuracy or stability of the methods. In order to improve accuracy and stability of the methods, characteristics of asymmetric components should be known. An airborne or dual (or triple Doppler radar observation is the best way for the purpose. However, observations with such methods are limited in number nor analysis area due to rare chance. Detailed observation of asymmetric dynamics of the hurricane inner core region was performed by Reasor (, but analysis area was limited to the region of 3km radius from the hurricane center. This study reports asymmetry in wind field structure of Typhoon 115 after landfall. The typhoon passed the Kanto district where triple Doppler radar data were available. Triple Doppler radar analysis was performed to investigate the wind field asymmetry ranging to a hundred of kilometers from the center. The results may be useful for understanding of TC structures and for improvement of operational use of radars. 2. Typhoon 115 Typhoon 115 attacked the Kanto district on September 11, 1. FIG. 1 and FIG. 2 show the track of the typhoon that moved north-northeast at a speed of approximately 7 m/s (25km/hour and made landfall near Kamakura around 93 JST (Japan Standard Time. At *Corresponding author address: Hiroshi YAMAUCHI, Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki Japan: hyamauch@mri-jma.go.jp the time of the landfall the typhoon s minimum surface pressure and maximum wind speed ware 97 hpa and 3 m/s, respectively. 3. Data and analysis methods 3.1 Radar observations Three C-band Doppler radars located in the Haneda airport, the Narita airport, and MRI (Meteorological Research Institute, Tsukuba observed reflectivity and Doppler velocity fields of the typhoon. The former are operational radars for airport weather (Ishihara, TABLE 1 shows specifications of the radars. Square marks in FIG. 2 give the locations of the sites. Each of the three radars executed PPI (Plan Position Indicator scans with different elevation angles every 6 minutes. Kanto district Hanada radar observation range (1km Tokyo Haneda Kamakura 9 JST 15 JST MRI Narita Sep. 12, 9JST 982hPa Sep. 11, 9JST 97hPa Sep. 1, 9JST 955hPa Sep. 9, 9JST 95hPa FIG. 1. Track of Typhoon 115 Narita radar observation range (1km the Pacific Ocean FIG. 2 Track of Typhoon 115 in the Kanto district from 8 JST to 16 JST. Three radar sites are also plotted (square marks.

2 (f (g (c (h (d (i (e (j FIG. 3 Horizontal wind (left column and reflectivity (right column fields of Typhoon 115 on Sep Each observation time is indicated at the top left of figure. Locations of the typhoon center are indicated by an intersection of two broken lines in each figure.

3 3.2 Horizontal wind field Horizontal wind fields were derived from triple or dual Doppler radar analysis. The PPI data of the each site were interpolated to make CAPPI (Constant Altitude Plan Position Indicator data. We used the PPI data with elevation angles of less than 7 degrees for the CAPPI processing to avoid contamination of vertical motion of rain droplets. The CAPPI data have a spatial resolution of.5km, and temporal resolution of 6 minutes over a region of 256km x 256km. We used only CAPPI data of 2km altitude. Horizontal wind fields for the constant altitude of 2km were calculated from the CAPPI data with a leastsquares method. Dual Doppler analysis was made in regions in which data of only two radars were available. 3.3 Location and movement of the typhoon center Location of the typhoon center was determined as the center of coaxial structure of high vertical vorticity region (example FIG. 8 calculated from the horizontal wind field. Movement of the typhoon center was calculated from a displacement of the center in two hours. TABLE 2 shows location and movement of the typhoon center. 4. Results and discussion 4.1 Angular wave-number 1 asym metry FIG. 3 shows horizontal wind fields derived from triple Doppler analysis and reflectivity fields observed by MRI radar for to. There is significant angular wave-number 1 asymmetry in each figure of wind fields: wind speeds are large in the southeast region and are small in the northeast region. FIG. 4a and 4b show tangential velocity (Vt and radial velocity (Vr with respect to the typhoon center at. Vt is large in the region of right-hand side towards the system movement direction. Vr is large in the system movement direction. FIG. 5 shows Vt and Vr along circles of radius of km from the typhoon center at as a function of azimuth angle θ. Curve fitting results for FIG. 5a and 5b are as follows: Vt = sin( θ sin(2θ 68 Vr = cos( θ cos(2θ 76 Angular wave-number 1 component (the second term of the right side of each values is consistent with apparent tangential or radial velocity component caused by the movement of the typhoon system (6.9m/s, 25 degrees, at which are approximately expressed as follows: Vt = 6.9 sin( θ 29 Vr = 6.9 cos( θ 29 This consistency means the wave-number 1 components Site Hanada Narita MRI Antenna diameter (m Range resolution (m Azimuth resolution (deg Power (kw 25 Observation range (km TABLE 2 Location and movement of Typhoon 115 on Sep. 11, 1 Location Movement Time Latitude Longitude Speed Direction (JST (degree (degree (m/s (degree N E N E N E N 14.4E N 14.26E (h (i (m/s (m/s FIG. 4 and radial velocity fields with respect to the typhoon center at TABLE 1 Specifications of the three radars Velocity (m/s Velocity (m/s (j Azimuth angle (degree Azimuth angle (degree FIG. 5 and radial velocity as a function of azimuth along circles of radius of km from the typhoon center at 12JST.

4 (a (m/s (f (m/s km (g (h (m/s (m/s (m/s (m/s (c (d (m/s (i (m/s FIG. 6 Tangential (left column and radial (right column velocity fields of Typhoon 115 relative to the moving typhoon center on Sep Each observation time is indicated at the top left of figure. The location of the typhoon center is indicated by an intersection of two broken lines. (a shows tangential velocity field near the typhoon center of on large scale. (e (m/s (j (m/s

5 were mainly caused by the movement of the system. 4.2 Angular wave-number 2 asym metry FIG. 6 shows Vt and Vr fields on the moving coordinate system with the typhoon center for to (hereafter, Vt and Vr indicate those on the moving coordinate system. These figures indicate significant angular wavenumber 2 asymmetry. Vt is large in east and west directions, and small in north and south directions. Vr is in north-east and south-west directions, and in south-east and north-west directions. It is noted that the radius of maximum Vt (broken circle in each figure, FIG. 6(a on large scale seems not to depend on directions. Phase angles of these asymmetry did not change with time. It is also recognized that Vt over the sea was greater than that over the land. FIG. 7 shows Vt and Vr along circles of radius of km from the typhoon center at against azimuth angle θ. Curve fitting results for FIG. 8 are as follows: Vt = sin( θ sin(2θ 67 Vr = cos( θ cos(2θ 79 The wave-number 2 component dominated the asymmetry. Amplitude of the wave-number 1 component on the moving coordinate system was almost half of that of wave-number 2 component. Amplitude values of higher wave-number components were also found to be smaller than the wave-number 2 component. Mechanism that caused the wave-number 2 asymmetry is not clear yet. On the contrary to the significant asymmetry of Vt and Vr, vertical vorticity field shown in FIG. 8 is almost axisymmetry. These wind fields may be explained with deformation (FIG. 9 which makes the wave-number 2 asymmetry without vertical vorticity asymmetry. If a typhoon wind field is approximated with the cyclostrophic wind field applied Velocity (m/s Velocity (m/s Azimuth angle (degree Azimuth angle (degree FIG. 7 Tangential and radial velocity along circle of range of km from the typhoon center against azimuth at after cancellation of the effect of system movement. Vorticity (m/s/km FIG. 8 Vertical Vorticity field of Typhoon 115 at (The corresponding wind field is FIG. 3(d. FIG. 9 Illustration of a deformation flow (m/s (m/s FIG. 1 and radial velocity calculated by using simple cyclostrophic wind model superimposed by a deformation.

6 by Schloemer s expression for TC s pressure fields (Schloemer, 1954, Vt and Vr are expressed as follows, { ( f r + 4V m( Vm + f Rm ( Rm r exp( Rm r f r} 1 2 Vt = 1 2 Vr = where f is the Coriolis parameter, tangential velocity, R m is the maximum is the radius of maximum tangential velocity, r is radius from the typhoon center. If a deformation field superimpose to the cyclostrophic wind field, following terms are added to the above Vt and Vr, respectively, Vt = A r cos( 2θ Vr = A r sin( 2θ where A is a constant, θ is azimuth angle with respect to the typhoon center. FIG. 1 shows Vt and Vr fields calculated from this model for V = 3m/s, R = km, A =.5m/s/km, with the typhoon center at. FIG. 1a and 1b are similar to real fields (FIG. 6c and 6h. These wave-number 2 asymmetry can be described as that air flowed along an oval stream line illustrated in FIG. 11. Wind speed reached its maximum in the direction of the minor axis of the oval and its minimum in the direction of major axis. 5. Conclusions Asymmetric structures in the horizontal wind field of Typhoon 115 at an altitude of 2km after landfall in the Kanto district was investigated by triple Doppler radar observation. There were two types of significant asymmetry in the wind field relative to the ground: angular wave-number 1 component and angular wave-number 2 component. The angular wave-number 1 component was mainly caused by movement of the typhoon system. Amplitude of the component, approximately 7 m/s, was consistent with that calculated from the system movement. The angular wave-number 2 component dominated the asymmetry of the wind field respect to the moving typhoon system. Phase angles of the asymmetry component showed little change with time. Tangential velocity reached its maximum in east and west directions and its minimum in north and south directions. Radial velocity was in north-east and south-west directions, and in south-east and north-west directions. The radius of maximum tangential velocity did not depend on directions. Amplitude of the asymmetry was approximately 3-4 m/s at range of 5 km from the typhoon center. Possibility that asymmetry wind field around Typhoon m V m m Typhoon center : minimum : : : maximum : : FIG. 11 Image of oval stream line presumed from the wave-number 2 asymmetry of Typhoon can be expressed relatively simple model over several tens of kilometers area from the center was suggested. The cause of the wave-number 2 asymmetry is unknown. What determine the amplitude and direction dependence of the asymmetry should be clear in the feature. REFERENCES Ishihara, M., and K. Hata, 1995: Operational Doppler weather radar for airport in Japan. Preprints 27 th Conf. Radar Meteor., Amer. Meteor. Soc., Lee, W.-C., B. J.-D. Jou, P.-L. Chang, and S.-M. Deng, 1999: Tropical cyclone kinematic structure retrieved from single Doppler radar observations. Part I: Interpretation of Doppler velocity patterns and the GBVTD technique. Mon. Wea. Rev., 127, Reasor, P. D., M. T. Montgomery, F. D. Marks and J. F. Gamache, : Low-Wavenumber Structure and Evolution of the Hurricane Inner Core Observed by Airborne Dual-Doppler Radar. Mon. Wea. Rev., 128, Schloemer, R. W., 1954: Analysis and synthsis of hurricane wind patterns over Lake Okeechobee, Florida, Hydrometeorological Report, 31, 49