5D.5 ESTIMATION OF THE TANGENTIAL WINDS AND ASYMMETRIC STRUCTURES IN INNER CORE REGION USING HIMAWARI-8 Taiga Tsukada * and Takeshi Horinouchi Hokkaido University, Hokkaido,

1. INTRODUCTION Himawari-8. We analyzed the daytime observations over 8.5 hours since 22:30 UTC on October 20, 2017. Geostationary meteorological satellites seamlessly By the time, Lan has developed a clear with a observe tropical cyclones (TCs) throughout their life radius of 35 km. cycle without interruption. Their observations are used We corrected parallax from the entire Himawari-8 to derive Atmospheric Motion Vectors (AMVs; see data. These were projected onto the azimuthal Menzel, 2001 and the references therein). AMVs are equidistant projection with respect to the TC center. We assimilated not only in global numerical weather derived the TC centers every 30 minutes by prediction systems, but also in forecasts of TCs (Velden subjectively examining the corrected images, and the et al., 1998). Even the state-of-the-art AMV products do results were interpolated with time by the cubic spline not cover TC’s inner core region (Oyama et al., 2018), interpolation. The projected images were sampled on presumably because the operational methods do not the polar coordinate. treat rotation. A space-time power spectrum is computed every 30 The latest-generation geostationary satellite minutes by using 24 images over 1 hour by using a two- “Himawari-8” has been operating since July 2015. Its dimensional FFT along the azimuth and time for each spatio-temporal resolutions were greatly improved from of the radial grid points at a 1-km resolution, and the its predecessor (Bessho et al., 2016). The satellite results are binned over 5 km. The power spectrum is observes TCs every 2.5 minutes, which is called the further binned as a function of azimuthal phase velocity, target observation. Since the clouds in TCs’ eyes are with a special treatment of the aliasing of mostly confined in the boundary layer where the counterclockwise motion onto clockwise motion. Finally, tangential winds are maximized, wind derivation from a representative tangential wind is derived as a the clouds there should be useful to study and monitor weighted average of the power over the top 80% bins. TCs. In this study, we propose a method to derive the 3. RESULTS tangential winds from high-frequency imaging as done by Himawari-8. This method utilizes space-time 3.1 TANGENTIAL WINDS OF TYPHOON LAN spectral analysis to obtain tangential winds as a function of the radius. It is remarkedly different from the Figure 1 shows the time series of the tangential conventional method using cross-correlations (Leese winds and the rotational angular velocities derived et al., 1971). every 30 minutes for radii of 10 to 30 km (dots). The The results of the method can be used to visualize angular velocities fluctuate relatively greatly at a radius and quantify the asymmetric wind components as the of 10 km, which is presumably due to the frequent deviations from the axi-symmetric components. It has presence of clear air regions and asymmetric velocity been suggested that the asymmetric components such components. The five-point temporal running-mean as mesovortices play the important roles to intensify angular velocities (curves) inside radius of 25 km are TCs (Schubert et al., 1999). Observational studies nearly uniform, suggesting a high degree of horizontal have been documented the presence of mesovortices mixing. The angular velocities are increased gradually in the TCs inner core region (Kossin and Schubert, through the 8.5 hours from ~1.1x10-3 to ~1.2x10-3 rad/s, 2004). Some observational studies further quantified suggesting an intensification. The rotation at a radius the vorticity of mesovortices in the eyewall region of 30 km is faster throughout the analysis period. This (Marks et al., 2008; Wingo and Knupp, 2016), but this region is characterized by striating clouds which study is the first to report a quantification of existed over a half of the inner edge of the eyewall. We mesovortices within the eyes. derived its angular velocity by visual inspection, and the result was ~1.75x10-3 rad/s (stars). 2. METHODOLOGY

We used visible reflectivity at 0.64 μm (Band 03) from the target observations of Typhoon Lan (2017) with

* Corresponding author address: Taiga Tsukada, Graduate School of Environmental Science, Hokkaido University, N10W5 Sapporo, Hokkaido, 060-0810, Japan; Email: [email protected] 3.2 ASYMMETRIC STRUCTURES IN THE EYE had vorticity whose magnitude is comparable to the vorticity associated with the rotation of the eye. It was To visualize the asymmetric structures in the eye, we suggested that the asymmetric motions transported rotated the satellite images clockwise to compensate angular momentum inward to intensify the near center for the five-point temporal running-mean rotation at a rotation. Our results demonstrate the usefulness of radius of 15 km. As a result, the presence of seven geostationary satellite observations to diagnose and mesovortices was found at the times in Fig. 1b. Their study TCs. This study has been submitted to vorticities relative to the background rotation was Geophysical Research Letters. estimated from three traceable features around mesovortices by visual cloud tracking. Their vorticities 5. ACKNOWLEDGMENTS were comparable to the mean vorticity in the eye. Our result is consistent with earlier studies, since the The Himawari-8 data we used are downloaded from vorticity inside the eye was increased when the the NICT Science Cloud. This work was supported mesovortices were observed. The asymmetric motions partially by the JSPS Grant-in-aid 19H00705. might have transported the high angular momentum associated with the secondary circulation into the eye. 6. REFERENCES

Bessho, K., and Coauthors, 2016: An introduction to Himawari-8/9 Japan’s new-generation geostationary meteorological satellites. J. Meteor. Soc. Japan, 94, 151–183. Kossin, J. P., and W. H. Schubert, 2004: Mesovortices in Hurricane Isabel. Bull. Amer. Meteor. Soc., 85, 151–153. Leese, J. A., C. S. Novak, and B. B. Clarke, 1971: An automated technique for obtaining cloud motion from geosynchronous satellite data using cross- correlations. J. Appl. Meteorol. 10, 118–132. Marks, F. D., P. G. Black, M. T. Montgomery, and R. W. Burpee, 2008: Structure of the eye and eyewall of Hurricane Hugo (1989). Mon. Wea. Rev., 136, 1237–1259. Menzel, W. P., 2001: Cloud tracking with satellite imagery: From the pioneering work of Ted Fujita to the present. Bull. Amer. Meteor. Soc., 82, 33–48. Oyama, R., M. Sawada, and K. Shimoji, 2018: Diagnosis of intensity and structure using upper tropospheric atmosphere motion vectors. J. Meteor. Soc. Japan, 96B, 3–16. Figure 1. (a) Time variation of tangential winds and (b) Schubert, W. H., M. T. Montgomery, R. K. Taft, T. A. rotational angular velocities at the radii of 10 (red), 15 Guinn, S. R. Fulton, J. P. Kossin, and J. P. Edwards, (orange), 20 (green), 25 (blue), 30 (purple) km, 1999: Polygonal eyewalls, asymmetric eye respectively. Dots indicate the 30-minute raw results, contraction, and potential vorticity mixing in while the solid curves show the running means with hurricanes. J. Atmos. Sci., 56, 1197–1223. time over the five samples. The shading indicates ± the Velden, C. S., T. L. Olander, and S. Wanzong, 1998: standard error derived from the variance among the The impact of multispectral GOES-8 wind five samples. Stars indicate the manually derived information on Atlantic tropical cyclone track (angular) velocities of the cloud striations alongside the forecasts in 1995. Part I: Dataset methodology, eyewall. The black lines at the top of (b) show the description, and case analysis. Mon. Wea. Rev., durations when the seven mesovortices are observed. 126, 1202–1218.

Wingo, S. M., and K. R. Knupp, 2016: Kinematic 4. CONCLUSIONS structure of mesovortices in the eyewall of Hurricane Ike (2008) derived from ground-based We have developed a new method for estimating the dual-Doppler analysis. Mon. Wea. Rev., 144, tangential winds of TCs based on the space-time 4245–4263. Fourier analysis of high-frequency geostationary satellite images. The application of the method to Typhoon Lan (2017) revealed the characteristics of rotation in the eye. The flow in the eye was full of asymmetric motions, among which some mesovortices