Field measurements of Tropical Storm Aere (1619) via airborne GPS-dropsondes over the South China Sea Fu, J. Y.; Shu, Z. R.; Li, Q. S.; Chan, P. W.; Hon, K. K.; He, Y. C. Published in: Meteorological Applications Published: 01/09/2020 Document Version: Final Published version, also known as Publisher’s PDF, Publisher’s Final version or Version of Record License: CC BY Publication record in CityU Scholars: Go to record Published version (DOI): 10.1002/met.1958 Publication details: Fu, J. Y., Shu, Z. R., Li, Q. S., Chan, P. W., Hon, K. K., & He, Y. C. (2020). Field measurements of Tropical Storm Aere (1619) via airborne GPS-dropsondes over the South China Sea. Meteorological Applications, 27(5), [e1958]. https://doi.org/10.1002/met.1958 Citing this paper Please note that where the full-text provided on CityU Scholars is the Post-print version (also known as Accepted Author Manuscript, Peer-reviewed or Author Final version), it may differ from the Final Published version. 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Download date: 10/10/2021 Received: 3 May 2020 Revised: 23 September 2020 Accepted: 25 September 2020 DOI: 10.1002/met.1958 RESEARCH ARTICLE Field measurements of Tropical Storm Aere (1619) via airborne GPS-dropsondes over the South China Sea J. Y. Fu1 | Z.R.Shu2 |Q.S.Li3 | P. W. Chan4 | K. K. Hon4 |Y.C.He1 1Joint Research Center for Engineering Structure Disaster Prevention and Abstract Control, Guangzhou University, In the past two decades, Global Positioning System (GPS)-dropsonde has been Guangzhou, China developed as an effective tool to explore the internal structures of tropical 2 Department of Civil Engineering, cyclones (TCs). To facilitate the short-term forecasting of the TCs' intensity University of Birmingham, Birmingham, UK and track over the Hong Kong Flight Information Region (HKFIR), Hong 3City University of Hong Kong, Kong Observatory (HKO) launched in 2016 reconnaissance campaigns by Hong Kong (SAR), China using airborne GPS-dropsondes. On October 7, 2016, when Tropical Storm 4 Hong Kong Observatory, Hong Kong Aere (1619) got close to Hong Kong, 10 dropsondes were released from a (SAR), China reconnaissance aircraft at 10 km altitude to sample the TC at different storm- Correspondence relative positions. This offers a valuable opportunity to examine the functional Y. C. He, Joint Research Center for performance of the dropsonde system. In the study, the validity of dropsonde Engineering Structure Disaster Prevention and Control, Guangzhou University, measurements is first examined, mainly by comparing the results with those Guangzhou, China. collected contemporaneously by radiosonde balloons from a nearby station. Email: [email protected] Observational results of the structural characteristics are then discussed. Evi- Funding information dence was observed for the invasion of the storm's inner structure by back- National Natural Science Foundation of ground atmosphere, which was unfavourable for the intensification of Aere in China, Grant/Award Numbers: 11972123, 51878194, 51925802 the following time. Meanwhile, a height-resolving model of the TC's pressure field is proposed. It is also observed that the parachutes of two dropsondes failed to deploy during the descending period. The results presented here are expected to provide useful information for a better understanding of the in-situ measurements from dropsondes deployed over the HKFIR, and for advancing the knowledge on TC's inner structure. KEYWORDS airborne reconnaissance, GPS-dropsonde, Hong Kong Observatory (HKO), tropical cyclone 1 | INTRODUCTION possible. During the last decades, continuous efforts have been made on this topic, based on ever-developing instru- Tropical cyclones (TCs) are one of most destructive natu- ments. A majority of these studies rely on conventional ral phenomena in the world. To better facilitate the pre- observation equipment, such as ground-based meteoro- vention and control of TC-related disasters, it is of great logical masts/towers (Cao et al., 2009; Song et al., 2012). importance to explore TC characteristics as far as Researchers have also investigated TC features and This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2020 The Authors. Meteorological Applications published by John Wiley & Sons Ltd on behalf of the Royal Meteorological Society. Meteorol Appl. 2020;27:e1958. wileyonlinelibrary.com/journal/met 1of14 https://doi.org/10.1002/met.1958 2of14 FU ET AL. associated effects on civil structures with the help of limited (Chan et al., 2011, 2014). To facilitate the short- structural health-monitoring systems established on term forecasting of the TCs' intensity and track over the high-rise buildings or long-span bridges (Li et al., 2020; Hong Kong Flight Information Region (HKFIR), Hong Wang et al., 2020). However, these instruments can only Kong Observatory (HKO) in 2016 started to launch sur- detect a considerably limited portion of a TC. IN this veillance experiments by using airborne GPS-dropsondes, regard, remote-sensing devices (e.g. Doppler radar/lidar/ which was the first time a member of the Typhoon Com- sodar wind profilers) and radiosonde balloons have been mittee of the World Meteorological Organization (WMO) increasingly adopted (He et al., 2018, 2020). had conducted routine typhoon surveillance using GPS- Global Positioning System (GPS)-dropwindsonde dropsonde over the South China Sea. On October 7, 2016, (or dropsonde for short) has been developed as an effective when Tropical Storm Aere (1619) got close to Hong Kong, tool to explore the internal structures of TCs since its a surveillance experiment was conducted on the storm. debut in the 1990s (Hock and Franklin, 1999). It has some This offered a valuable opportunity to examine the func- overwhelming advantages against the aforementioned tional performance of the GPS-dropsonde system. instruments. First, it allows flexible deployment at targeted In the study, the validity of collected GPS-dropsonde storm-relative positions and different evolution stages of a measurements is first examined. Observational results of TC, making it feasible to detect TCs strategically and sys- Aere's inner structure are then discussed. The results tematically. Second, GPS-dropsondes can offer high- presented are expected to provide useful information for fidelity and high-resolution (along height) samplings of a better understanding of the in-situ measurements from the inner structure of a TC (Hock and Franklin, 1999), GPS-dropsondes deployed over the HKFIR, and for which facilitates the resolving of detailed TC characteris- advancing knowledge about the TC's inner structure, tics within a wide portion of the TC's depth. especially at middle and upper TC depths. Owing to the above features, GPS-dropsondes have been used extensively in TC surveillance campaigns to support operational weather forecasting and meteorologi- 2 | EXPERIMENT AND DATA SETS cal research. The earliest routine deployment of GPS- dropsondes started in 1997 from US hurricane reconnais- 2.1 | Tropical Storm Aere and the sance aircraft (Aberson, 2010), with the vast majority of positions of the released dropsondes the devices released above the Atlantic TC basin and a small number above the eastern and central North Pacific Aere is the 19th TC storm that has developed over the basins. In 2002, researchers in Taiwan initiated the western North Pacific (WNP). As shown in Figure 1, it Dropwindsonde Observations for Typhoon Surveillance formed as a tropical depression to the east of Dongsha on near the Taiwan Region (DOTSAR) experiment the afternoon of October 5, 2016. It then moved across (Wu et al., 2007). Several reconnaissance missions were the Luzon Strait and entered the northeastern South also conducted in the west Pacific Ocean (Aberson, 2011). China Sea the next day, where it intensified into a tropi- Based on the collected data sets, typical TC characteristics cal storm. After crossing the sea south of Dongsha in the have been analysed, including the surface wind factor early morning of October 7 Aere slowed down and drifted (Franklin et al., 2003), air–sea interaction (Powell et al., 2003), low-level-jet of wind profiles (Kepert, 2006a, 2006b; Vickery et al., 2009; Giammanco et al., 2013), TC boundary layer depth (Zhang et al., 2011), TC outflow and warm core structures (Komaromi and Doyle, 2017), and so on. GPS-dropsonde data have been also used to improve the forecasting accuracy of TC track and intensity (Rappaport et al., 2009; Chen et al., 2013; Doyle et al., 2017). Comparison studies conducted by Wu et al. (2007) showed that the assimilation of GPS- dropsonde data could lead to an improvement of 22% in 72 hr average track forecasts. Southeast China is located in a TC-prone region. The TCs can result in severe casualties and economic losses every year in this area. Currently, detections of the TCs in this region have been conducted mostly via ground-based FIGURE 1 Best crack of Aere (1619) issued by Hong Kong instruments, while TC surveillance via aircraft is still Observatory (HKO) FU ET AL.