Double Warm-Core Structure of Typhoon Lan (2017) as Observed through Upper-Tropospheric Aircraft Reconnaissance during T-PARCII

A jet airplane with two newly-developed GPS dropsonde receivers was used to examine the inner core of Typhoon Lan on 21 and 22 October 2017, as part of the Tropical Cyclones-Pacific Asian Research Campaign for the Improvement of Intensity Estimations/Forecasts (T-PARCII) program. During the first flight with several eye soundings in 0500-0700 UTC on the first day, this typhoon was at its maximum intensity with a central pressure of 925 hPa. In satellite imagery, the typhoon exhibited a large eye roughly 90 km wide with well-established, axisymmetric eyewall convection. The flight route was initially designed to go around the eyewall within the 200-km radius, but was modified to go into the eye. This decision was made in flight using information of the nose radar. The Gulfstream-II jet flew at 43,000 feet (~13.7 km) of altitude, and totally 8 and 10 of the Meisei iMDS-17 dropsondes were deployed in the eye and around the eyewall, respectively. The deployment from this altitude enabled us to examine thermodynamic features of the eye in the lower and middle troposphere and marginally in the upper troposphere.

Vertical profile of the eye soundings showed the warm core structure extending from the lower through upper troposphere, with two peaks near 3 km (~ 700 hPa) and above 12 km MSL (< 200 hPa). Potential temperature anomaly was estimated using a local reference sounding obtained at 0600UTC at the JMA 47945 site, which was at 400-km north of the eye. The estimated anomalies were approximately +12 K on average and +16 K at both the lower and upper peaks. The height and strength of the upper peak was basically consistent with those reported in the previous observational studies. In contrast, the height of lower peak was about 2 km lower than that reported in many of the previous studies.