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Diagnosis of Tropical Cyclone Intensity and Structure Using Upper Tropospheric Atmospheric Motion Vectors

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2018Journal of the Meteorological Society of Japan, Vol.R. 96B, OYAMA pp. 3−26, et al. DOI:10.2151/jmsj.2017-024, 2018 3 Special Issue on Meteorology and Climate Change Studies by Using the Geostationary Meteorological Satellite Himawari-8 Diagnosis of Tropical Cyclone Intensity and Structure Using Upper Tropospheric Atmospheric Motion Vectors Ryo OYAMA, Masahiro SAWADA Meteorological Research Institute, Japan Meteorological Agency, Tsukuba, Japan and Kazuki SHIMOJI Numerical Prediction Division, Japan Meteorological Agency, Tokyo, Japan (Manuscript received 14 April 2017, in final form 19 July 2017) Abstract The high temporal and spatial resolutions of geostationary satellite observations achieved by recent techno- logical advancements have facilitated the derivation of atmospheric motion vectors (AMVs), even in a tropical cyclone (TC) wherein the winds abruptly change. This study used TCs in the western North Pacific basin to investigate the ability of upper tropospheric AMVs to estimate the TC intensity and structure. We first examined the relationships between the cloud-top wind fields captured by 6-hourly upper tropospheric AMVs derived from images of the Multi-functional Transport Satellite (MTSAT) and the surface maximum sustained wind (MSW) of the Japan Meteorological Agency’s best-track data for 44 TCs during 2011–2014. The correlation between the maximum tangential winds of the upper tropospheric AMVs (UMaxWinds) and MSWs was high, approximately 0.73, suggesting that the cyclonic circulation near the cloud top was intensified by the upward transport of absolute angular momentum within the TC inner core. The upper tropospheric AMVs also revealed that the mean radii of UMaxWinds and the maximum radial outflows shifted inward as the TC intensification rate became large, imply- ing that the low-level inflow was strong for TCs undergoing rapid intensification. We further examined the pos- sibility of estimating the MSW using 30-min-interval UMaxWinds derived from Himawari-8 target observations, which have been used to track TCs throughout their lifetimes. A case study considering Typhoon Lionrock (1610) showed that the UMaxWinds captured the changes in the cyclonic circulation near the cloud top within the inner core on a timescale shorter than 1 day. It was apparent that the increase in the UMaxWind was associated with the intensification of the TC warm core and the shrinkage of UMaxWind radius. These results suggest that Himawari-8 AMVs include useful information about TC intensification and related structural changes to support the TC intensity analysis and structure monitoring. Keywords tropical cyclone; nowcasting; atmospheric motion vector; satellite observation; Himawari 1. Introduction A tropical cyclone (TC) is generally identified by its low-pressure center and axisymmetric structure, which Corresponding author: Ryo Oyama, Meteorological Resear- comprises eyewalls and spiral rainbands. The strong ch Institute, Japan Meteorological Agency, 1-1, Nagamine, Tsukuba, Ibaraki 305-0052, Japan winds and heavy rain associated with the eyewalls and E-mail: [email protected] spiral rainbands cause disastrous destruction of human J-stage Advance Published Date: 10 August 2017 infrastructures. The area within a radial distance 2 – 3 ©The Author(s) 2018. This is an open access article published by the Meteorological Society of Japan under a Creative Commons Attribution 4.0 International (CC BY 4.0) license (http://creativecommons.org/license/by/4.0). 4 Journal of the Meteorological Society of Japan Vol. 96B times the radius of maximum wind (RMW) from the TC intensity estimates in the vast majority of TC cases TC center is generally called the inner core, within (Velden et al. 2006). However, the Dvorak technique which the tangential wind velocity reaches a maxi- can struggle in certain situations wherein TC struc- mum and strong updrafts exist (Li and Wang 2012). tures may fluctuate under the central dense overcast The deep convections within a TC inner core convey (CDO) or in association with rapid intensity changes. the mass and absolute angular momentum upward, Recent technological advancements have enabled with the release of much latent heat. This process observations with high temporal and spatial resolu- plays an essential role in TC development. tions to be made by geostationary satellites, such as The wind structure of a TC can be described as the Multi-functional Transport Satellites (MTSATs), a combination of cyclonic circulation (primary cir- which was functional between 28 June 2005 and culation) and radial–vertical circulation (secondary 07 July 2015, and Himawari-8, which is the first of circulation). The primary circulation is intensified by new-generation geostationary satellites and has been low-level inflows, which convey large absolute angu- in operation since 07 July 2015 (Bessho et al. 2016). lar momentum toward the TC center. The low-level In particular, high-temporal-resolution satellite images convergence, increased by the inflow in the boundary with time intervals shorter than approximately 15 min layer, enhances the inner core convections (Sawada facilitate improved derivations of atmospheric motion and Iwasaki 2007; Rogers 2010). In contrast, the vectors (AMVs; Velden et al. 2005; Oyama 2015), secondary circulation comprises low-level inflows, even in the case of mesoscale phenomena character- updrafts in the TC inner core, and radial outflows near ized by winds that change abruptly. It should be noted the cloud top. The secondary circulation develops the that Himawari-8 can provide limited-domain imagery TC cyclonic vortex (i.e., the primary circulation) ver- for targeted TCs within view at sampling intervals of tically around the inner core and intensifies the warm 2.5 min. core near the TC center by the latent heat release (Vigh AMVs are wind products that are derived by and Schubert 2009; Houze 2010). tracking clouds and water vapor patterns in successive TC wind structures have been investigated in many satellite images. They have been used not only for numerical studies based on non-hydrostatic models. numerical weather prediction (NWP) (Warrick 2016) Bryan and Rotunno (2009) investigated the trajectory but also for atmospheric wind analysis (Molinari and of an air parcel passing through the position of the Vollaro 1989; Apke et al. 2016). For NWP, centers maximum wind speed in a TC and showed that the air such as the European Centre for Medium-Range parcel moves upward along the isosurfaces of absolute Weather Forecasts (ECMWF), JMA, and the National angular momentum and entropy, which tilt outward. Centers for Environmental Prediction (NCEP) of the Stern and Nolan (2011) studied the vertical profiles National Oceanic and Atmospheric Administration of tangential winds with respect to the RMW and the (NOAA) have improved their NWP products using maximum sustained wind (MSW) using the three- AMV data for the initial analysis (Langland et al. dimensional Doppler wind field data obtained from 2009; Yamashita 2012; Salonen and Bormann 2014; seven storms and simulated theoretical vortices. They Wu et al. 2014). For the TC analysis, Oyama et al. found that the vertical decay rate of the maximum (2016b) demonstrated the capability of MTSAT upper tangential wind normalized by the tangential wind at tropospheric AMVs to detect the changes in the upper a height of 2 km was nearly constant. The results of tropospheric wind fields within Typhoon Danas (1324) these previous studies are consistent with the fact that during its intensification, i.e., the increasing radial TC wind fields in the middle and upper troposphere outflows and tangential winds around the cloud top, are physically related to the surface wind field. which were associated with convective bursts (CBs; The improved estimates of TC intensity through Riehl and Malkus 1961; Steranka et al. 1986). research and development will be necessary to prevent AMVs have a great advantage compared to in situ and mitigate disasters caused by TCs. The Dvorak observations in terms of temporal resolution and data technique (Dvorak 1975, 1984), which is based on the coverage. In particular, the upper tropospheric AMVs, TC cloud pattern observed in satellite infrared images, which can be obtained throughout the TC lifetimes, is a method that has been used by TC forecast centers, are expected to contribute to TC analysis and mon- such as the Japan Meteorological Agency (JMA), itoring. The purpose of this study was to identify the Joint Typhoon Warning Center (JTWC), and the the characteristics of upper tropospheric winds that National Hurricane Center (NHC), to estimate the TC presage TC intensification by the analysis of the intensity. This technique is known to provide reliable upper tropospheric AMVs of TCs that have occurred 2018 R. OYAMA et al. 5 in the western North Pacific basin. For this analysis, target observations were conducted based on the we investigated the relationship between the upper requirements of this study (Table 1). That is, we tropospheric wind field and the MSW. The results are identified the differences between the MTSAT and expected to contribute to the elucidation of the TC Himawari-8 AMVs with respect to the quality and intensification process and verifications of numerical amount of data considering Typhoon Goni (1515), studies. In addition, we examined the possibility which existed in the period when both MTSAT and to extract information about TC intensification and Himawari-8 observations were made, i.e., from July
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