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A decade (2011–2020) of tropical cyclone reconnaissance flights over the South China Sea

K. Hon,P. Chan

Published 2022 in Weather

ABSTRACT

Introduction and background Tropical cyclones (TCs) are a major weather hazard faced by many subtropical coastal cities (Gray, 1968). For Hong Kong, which is situated along the southern coast of China and hence susceptible to weather systems originating from both the South China Sea and the western North Pacific (Mcgregor, 1995; Goh and Chan, 2010), the monitoring and warning of TCs has remained a key focus of the Hong Kong Observatory (HKO) since its establishment in 1883 (Ho, 2003). To support operational TC forecasting, observational data – particularly near-surface distributions of winds and pressure – are vital for the analyses of storms, impact assessment and formulation of warnings (Chavas et al., 2017; Klotzbach et al., 2020; Knaff et al., 2021). In recent decades, meteorological satellites have rapidly gained in spatial and/or temporal resolution of observations, offering an ever-widening spectrum of remotely-sensed and derived parameters, hence becoming an indispensable part of the TC analysis, forecast and numerical prediction process (Wang et al., 2008; Schmetz and Menzel, 2015; Emanuel, 2018). Nonetheless, there are certain inherent limitations in the information provided by satellite observations, particularly for dynamic variables under TC conditions. For example, atmospheric motion vectors derived from infrared images of geostationary satellites, while spatio-temporally dense, tend to cover the upper atmospheric layers when nearby deep convective clouds are present (Hidehiko et al., 2015; Kim and Kim, 2018). On the other hand, ocean surface winds derived from scatterometers on board polar-orbiting satellites are constrained by the number of available overpasses as well as their relatively low horizontal resolution (Figa-Saldana et al., 2002; Wang et al., 2007a; Ribal et al., 2021). For the South China Sea basin, the lack of in situ TC observations, particularly near-surface wind distributions, has remained a constant challenge to operational forecasting. It is against such a background that HKO took forward its TC reconnaissance programme in collaboration with the Government Flying Service (GFS). Taking a global perspective, TC reconnaissance flights have a long history since the mid-twentieth century (Simpson and Starrett, 1955). For example, the Hurricane Hunters of the US National Oceanic and Atmospheric Administration (NOAA) have produced routine reconnaissance data over the Atlantic and the eastern North Pacific basins since the 1960s, with sufficient temporal resolution for spectral calculations, over the Atlantic and the eastern North Pacific basins since the 1960s (Vonich and Hakim, 2018). Furthermore, since its routine introduction of the global positioning system (GPS) dropsonde in 1996, NOAA has accumulated more than 10 000 dropsonde observations inside or near hurricane eyewalls covering over 100 systems (Wang et al., 2015). However, after the termination of aircraft reconnaissance in the western North Pacific by the US in 1987 (Gray et al., 1991), there had been no routine aircraft TC observations over the South China Sea region until 2011, when the HKO–GFS reconnaissance programme officially began (Chan and Tse, 2012). At present, it remains the only such programme in the region. At first, data collection took the form of lowlevel straight-line flights into stormy areas using an instrumented aircraft, with the aim of sampling the distribution of hazardous winds associated with both outer rainbands and the core region of a TC. Such a demanding undertaking is made possible by the skill and professionalism of the GFS, whose responsibilities include search and rescue (SAR) operations over the South China Sea, often in adverse weather conditions. These low-level flights have resulted in a unique set of observational data within the TC boundary layer, including several eyewall penetrations, which are the first-of-its-kind in the region. Starting from 2017, the mode of reconnaissance saw a major change following the introduction of the dropsonde launching system installed on the new fixed-wing aircraft of the GFS. Dropsonde missions, where typically up to 10 dispensable units are launched from the upper troposphere within the Hong Kong Flight Information Region (HKFIR) each time, have replaced the low-level storm-penetrating flights, offering the new technical possibility of 3-dimensional in-situ profiling of TCs. Operationally, for TCs within the domain 105–125°E, 10–30°N, HKO issues forecast tracks (updated every 3h) containing information on storm position, intensity and hazardous wind radii. Reconnaissance observations of both ‘flavours’ provide an important reference to the forecasting bench in validating storm position and wind distribution (radii of gale, storm and hurricane force winds), which are otherwise based on a combined assessment of numerical weather prediction (NWP) and satellite data together with the small set of available surface observations over the data-sparse South China Sea. In most global NWP models, Hong Kong only subtends a few gridpoints in either horizontal direction; local winds can exhibit considerable sensitivity to positional or structural changes of only a couple of tens of kilometres. As such, detailed observation on wind radii is particularly valuable in the formulation of local TC warning strategy. This paper reviews the progress of the HKO–GFS reconnaissance programme over the past decade (2011–2020). The article is organised as follows. First, we describe and summarise the low-level storm-penetrating flights operated between 2011 and 2016. Flight missions after the introduction of the dropsonde launching system in 2017 are documented and discussed next, after which contributions to the scientific literature by the reconnaissance programme are reviewed. Finally, we close with a brief summary and outlook.

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