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

A decade has passed since the Hong Kong Observatory (HKO) and Government Flying Service (GFS) jointly commenced operational tropical cyclone (TC) reconnaissance over South China Sea in 2011, currently only such programme region a first USA terminated aircraft western North Pacific 1987. This paper reviews HKO–GFS flights between 2011 2020, including 24 storm-penetrating down to 2000ft 28 dropsonde missions resulting 190 profiling measurements, which have supported forecasting warning of TCs as well contributed scientific research regional collaboration. Tropical cyclones (TCs) are major weather hazard faced by many subtropical coastal cities (Gray, 1968). For Kong, is situated along southern coast hence susceptible systems originating from both (Mcgregor, 1995; Goh Chan, 2010), monitoring remained key focus its establishment 1883 (Ho, 2003). To support TC forecasting, observational data – particularly near-surface distributions winds pressure vital for analyses storms, impact assessment formulation warnings (Chavas et al., 2017; Klotzbach 2020; Knaff 2021). In recent decades, meteorological satellites rapidly gained spatial and/or temporal resolution observations, offering an ever-widening spectrum remotely-sensed derived parameters, becoming indispensable part analysis, forecast numerical prediction process (Wang 2008; Schmetz Menzel, 2015; Emanuel, 2018). Nonetheless, there certain inherent limitations information provided satellite dynamic variables under conditions. example, atmospheric motion vectors infrared images geostationary satellites, while spatio-temporally dense, tend cover upper layers when nearby deep convective clouds present (Hidehiko Kim Kim, On other hand, ocean surface scatterometers on board polar-orbiting constrained number available overpasses their relatively low horizontal (Figa-Saldana 2002; Wang 2007a; Ribal basin, lack situ wind distributions, constant challenge forecasting. It against background that HKO took forward collaboration with (GFS). Taking global perspective, long history mid-twentieth century (Simpson Starrett, 1955). Hurricane Hunters US National Oceanic Atmospheric Administration (NOAA) produced routine Atlantic eastern basins 1960s, sufficient spectral calculations, 1960s (Vonich Hakim, Furthermore, introduction positioning system (GPS) 1996, NOAA accumulated more than 10 000 observations inside or near hurricane eyewalls covering 100 2015). However, after termination 1987 (Gray 1991), had been no until officially began (Chan Tse, 2012). At present, it remains region. first, collection form low-level straight-line into stormy areas using instrumented aircraft, aim sampling distribution hazardous associated outer rainbands core TC. Such demanding undertaking made possible skill professionalism GFS, whose responsibilities include search rescue (SAR) operations Sea, often adverse These resulted unique set within boundary layer, several eyewall penetrations, first-of-its-kind Starting 2017, mode saw change following launching installed new fixed-wing GFS. Dropsonde missions, where typically up dispensable units launched troposphere Flight Information Region (HKFIR) each time, replaced flights, technical possibility 3-dimensional in-situ TCs. Operationally, domain 105–125°E, 10–30°N, issues tracks (updated every 3h) containing storm position, intensity radii. Reconnaissance ‘flavours’ provide important reference bench validating position (radii gale, force winds), otherwise based combined (NWP) together small data-sparse Sea. most NWP models, subtends few grid-points either direction; local can exhibit considerable sensitivity positional structural changes couple tens kilometres. As such, detailed observation radii valuable strategy. progress past (2011–2020). The article organised follows. First, we describe summarise operated 2016. 2017 documented discussed next, contributions literature reviewed. Finally, close brief summary outlook. section gives overview conducted Jetstream 41 used GFS 1999. twin turboprop wingspan 18.4m maximum speed 250kn. radius action about 500 nautical miles around 900km. Its main roles performing SAR providing various government departments. Figure 1 shows probe left wingtip forms dedicated measurement HKO. Aircraft Integrated Meteorological Measuring System 20Hz (AIMMS-20) manufactured Aventech Research Inc. description AIMMS-20 be found Beswick al. (2008). Here, features implementation Kong. measures true airspeed, flow angle, air temperature relative humidity. GPS module determines velocity heading aircraft. rates rotation accelerations three body axes, measured inertial unit. central processing handles real-time computations storage output data, at frequency. was use March 2016 decommissioned replacement Bombardier Challenger 605, described next section. Data initiated whenever enters northern may Normally, begins design flight profile days before planned mission. factors normally considered, actual predicted structure, projected movement storm. latest HKO's storm's evolution, model output. There also practical constraints. traffic control might prefer timeslots less International Airport (HKIA) morning early afternoon; occasionally, mission could not arranged because tightly packed arising anticipated passage. total length duration will subject fuel consumption boundaries HKFIR. always take top priority, very short notice divert should emergency occur. potential flight. terms path, order sample high surrounding concerned, straight lines cutting across quadrants preferred, minimises quality turning. altitude, level, chosen balance representativeness safety crew. FL020 (‘Flight Level 20’, 610m above sea level) commonly used, layer. An example shown 2. taken Severe Typhoon Utor 13 August 2013 2014). panel path overlaid concurrent image schematic diagram profile. right shows, bottom, time series flight-level winds, 10m reduction values reduced mean sea-level pressure. list given Table 1. conducted, 19 different intensities ranging area (LPA) severe typhoon (STY) during respective missions. (The LPA 16 would subsequently develop Dianmu; hence, they count one system.) particular, seven flying (TY) STY, four (STS), eight (TS) two depression (TD) . performed 20 2015, were 605. paths covered these 3. Each represented solid, coloured line. grey dotted line marks approximate seen 3(a) essentially spanned all corners HKFIR, highest density towards southeastern sides. because, exception forming rare approach south southwest 2007b), affecting follow northwest west-northwest track Pacific. 3(b) offers alternative view storm-relative coordinate system. achieved subtracting corresponding position. analysis positions storms concerned three-hourly intervals. No interpolation applied, closest particular used. predominantly sampled northeast quadrants, again reflecting climatology. 12 entered 1° latitude-longitude analysed centre Among these, counted five penetrations above: Tembin, Utor, Kalmaegi, Linfa Nida. best knowledge authors, layer penetration 605 (Figure 4). declared 2017. twin-turbofan multi-purpose cruise 470kn range 3500 miles. 700 8h endurance. addition equipment, equipped Airborne Vertical Profiling (AVAPS-II), GPS-enabled dropsondes. parachuted, expendable electronic device contains sensors pressure, humidity sensor three-dimensional components derived. continuously state atmosphere descent, slowed stabilised parachute. 2Hz frequency, transmitted back unit radio communications. mission, dropsondes launched, usually released FL300 (an altitude 30 000ft, 11 800m). constraints arise need avoid busiest airspaces world. Similar TC, evolution structure. Drop locations, separation 0.5 latitude–longitude, selected extent those expected affect Where possible, vertical profiles help estimate intensity. WMO BUFR format, internationally exchanged Global Telecommunication (GTS) starting late (WMO, Up end 2020 season, completed, systems. Around whole spectrum, LPA, nine TS, STS, TY, STY SuperTY. From collected. summarised 5. panels 5, separate markers. 5(a) that, parts operable HKFIR already tip. blank half reflects 150-nautical mile (approximately 300km) semi-circle centred drops made, per regulatory requirements. If switch similar manner 3(b), so far obtained TD (Wipha), TS (Aere, Barijat, Wipha Higos), STS (Mawar) (Khanun). case Khanun possibly decades (highlighted 5). 6, show Mangkhut 2018. among destructive weather-related disaster year 2018, causing widespread damage countries regions (Choy 2020). After tracking Philippines, SuperTY 15 September extensive circulation, same evening. measuring quadrant distance 350 400km barbs level. DOTSTAR (Wu 2007) Luzon Strait area, northeastern Mangkhut. occasion occurring programmes span hours. While primarily aimed supporting produces research, briefly paragraphs. Both send adverse, extreme, environmental therefore arises enhance robustness data. joint work manufacturer, Foster Chan (2012) reported refinements algorithms known hardware response characteristics, improvements accuracy 20% 30%, benefits subsequent users agencies institutes. We note measurements turbulence ocean, rare. (2011) presented study demonstrating feasibility calculating turbulent kinetic energy (TKE), longitudinal power eddy dissipation rate challenging operating environment posed opened door further studies accumulated. Zhao (2019) suite parameters TKE, diffusivity (K) mixing lengths six confirmed law behaviour K speeds 40ms?1 levelling off, inflow insensitive speed. Another Sparks 2014 2016, super-linear relationship momentum region, contrast linear relationships Atlantic. results benefit development helping constrain parameterisation schemes extreme speeds. Liao angular Nida, conjunction ground-based Nida significant Guangdong Province, China. presence supergradient radial enhancement, configuration, play role intensification Most recently, Tang (2021) in-depth Kalmaegi direct evidence helical rolls out 150–250km typhoon, modelling side, variational assimilation studied Wong (2014) mesoscale HKO, Gao (2018) Global/Regional Assimilation Prediction (GRAPES). Despite differences software, positive impacts aspects fields demonstrated. Aere Zhang (2020) Mangkhut, GRAPES software. studies, demonstrated, latter reporting precipitation, validation retrieval remote-sensing algorithm brightness input microwave sounders, verified favourably even potentially benefitting participated collaborative projects Southern Monsoon Rainfall Experiment (Luo 2017) Landfalling Cyclone Project (Duan 2019), contributing exchange risk scale. writing, effort has, despite constraints, 10-year period dataset, benefitted meteorologists researchers. Between altitudes level system, Following collectively STY. dozen peer-reviewed topics turbulence, structure NWP. Looking ahead, aware accumulation still stage, being orders magnitude fewer what achieved, basin. coming years, continue building dataset eventually establishing statistical suitable comparison basins. Analytical continue, explorations advanced methods. leverage technology enhancing monitoring, understanding TCs, primary areas. colleagues acknowledged years. Satellite MTSAT Himawari-8 courtesy Japan Agency (JMA).