Characteristics on intensity and precipitation of super typhoon Rammasun (1409) andreason why it rapidly intensified offshore

Using conventional observation data, observations from automatic weather station at Hainan, Doppler radar products at Haikou,FY satellite cloud images and NCEP/NCAR reanalysis data with 1°×1° resolution, we discuss the characteristics of intensity and precipitationof super typhoon Rammasun (1409) before and after landfall in Hainan Island and, the reason of its rapidly intensification offshore. Theresults indicate that before landfall in Wengtian town of Wenchang in Hainan, the maximum wind speed near the center of Rammasun exceeded74 m·s-1 and the minimum sea-level pressure reached to 899.2 hPa. At landfall in Wengtian, Rammasun’s intensity was maintained orslightly decreased, for which it became to the strongest typhoon at landfall in China mainland since 1949. Near the center of Rammasun, themaximum wind speed increased by 5 m·s-1 and the minimum sea-level pressure decreased by 20 hPa form 10:00 BT to 15:00 BT 18 July2014; its super typhoon grade has been maintained for 17 hours from 11:00 BT the same day. In the north of Hainan the heavy precipitationcaused by eyewall echoes of Rammasun is characterized by mixed precipitation with high precipitation efficiency and convective activity beingnot very strong. However, in the west of Hainan extreme precipitation caused by spiral rain bands around Rammasun is characterized bytypical convective precipitation. Rammasun continuously strengthens offshore under the influence of a wide variety of systems such as WesternPacific subtropical high, low level jet, westerly trough and South Asia high. It rapidly intensifies offshore due to the convergence in the lowlevel and divergence in the upper level, weak vertical shear of environment wind, appropriate sea surface temperature and deep warm vortex.