In order to carry out typhoon observations, three UAV flights were conducted on June 25, July 2, and July 12, 2020. These flights were used to validate and optimize the functional performance of the UAV, payload, and data transmission links. On July 19, a fourth flight obtained measurements of atmospheric temperature, humidity, wind direction, wind speed, air pressure profiles, and cloud data from the sea surface to 10 km altitude. Additionally, the flight recorded observations of the cloud structures such as cloud height and cloud thickness, as well as, microphysical characteristics (e.g. reflectivity factor) of the cloud system above the sea for the first time. During the 5 week period from June 25 to August 1, eight intelligent, reciprocating horizontal drifting radiosondes (IRHDR) were released from the Xisha Upper-air Observatory (Fig. 4). The USV and two drifting buoys were deployed in the experiment region on July 23, 2020.
At 1500 LST on July 31, 2020, a tropical disturbance was upgraded to a tropical depression by the Center Meteorological Observatory, and it was named "Sinlaku (2020)" at 1500 LST on August 1, 2020 over the South China Sea. Notification of the tropical depression (16.9°N, 113.4°E) was first issued at 0600 UTC on July 31. At that time, the USV and drifting buoys were observing continuously in the sea area affected by the low pressure system. At 1400 UTC on July 31, an IRHDR was released, which found a substantial increase in humidity at the height of 6–8 km, attributable to the tropical depression. During 0100 – 0300 UTC on August 1, data acquired by the USV and buoys were provided in real time to the Typhoon Forecast Center of the China Meteorological Administration. Furthermore, at the request of forecasters, the experiment staff navigated the USV into a designated maritime area. The closest distance of the USV to the center of the low pressure was 2.4 km. The measured sea level pressure was 996.8 hPa (see Fig. 4), and the intensity of the typhoon center was 992 hPa, (i.e., “the typhoon intensity level”), according to the Central Meteorological Observatory of CMA. The drifting buoys also observed the change of pressure. At 0600 UTC on August 1, the Typhoon Forecast Center designated Senlake a named typhoon, and the observation systems on the sea surface played an important role in determining the location and intensity of the typhoon.
During the afternoon of August 2, the UAV undertook a mission to observe the peripheral cloud system of Typhoon Sinlaku (2020). The UAV took off from Boao Airport and rose to a height of 10 km. During the flight, it successfully obtained 30 sets of dropsonde profiles (Fig. 5) and millimeter-wave cloud radar data for nearly 80 min which completed the CT-type scanning of the peripheral cloud system of Typhoon Senlake, and transmitted the dropsonde profile data back to the ground-based command system in real time. Together with the instruments on UAV, the USV, drifting buoys, horizontal drifting radiosondes, and operational Fengyun meteorological satellites, a comprehensive data set was assembled that was then used to perform model verification and evaluation. Initial data analyses indicated that the South China Sea Experiment 2020 has achieved its pre-set goals, and a comprehensive three-dimensional data set on a typhoon using unique combinations of observing platforms was obtained for the first time; such systems can help fill operational gaps in observation of typhoons near China.
The observational data set acquired during the South China Sea Experiment 2020 of the “Petrel Project” includes five types of 21 observational parameters and one UAV flying video file (which can be used for auxiliary analysis of data quality and cloud systems). The total size of the data set is 880 Gb. Much of the data was obtained for the first time, for the given settings, and is therefore scientifically very valuable. The quality of the dropsonde and horizontal drifting soundings (except the horizontal drifting temperature data) exceeds the World Meteorological Organization standards. Analyses and evaluation of the data are ongoing. Specific information regarding the data is outlined in Table 1.
Observation equipment Data time Data size Observation elements Data interval Dropping radiosonde system July19, August 2 34 sets Temperature, relative humidity, air pressure, wind direction, wind speed, geopotential height 1s Millimeterwave cloud radar July19, August 2 110 min Reflectivityfactor, speed, spectral width data 250 ms Unmanned surface vessel July22 to August 2 315 h Sea surface temperature, relative humidity, wind speed, wind direction, air pressure, shortwave radiation, sea temperature, sea surface salinity 1 min Horizontaldrift sounding July 31 7 sets Temperature, relative humidity, air pressure, wind direction, wind speed, geopotential height 1 s Drifting buoy July23 to August 2 No. 20005, 251h; No. 20006, 245h Sea surface temperature, relative humidity, wind speed, wind direction, air pressure, shortwave radiation, sea temperature, sea surface salinity 10 min UAVforesight camera data August 2 240 min During the operation of the UAV, thetakeoff-flight-landing video recorded by the foresight camera 1 s
Table 1. Specific details of the South China Sea Experiment 2020 data set.