A Case Study on the Detailed Process of the Ionospheric Responses to the Typhoon

A case study is presented to reveal the detailed process of the ionospheric responses to disturbances in the lower atmosphere when a strong typhoon lands or approaches the mainland. The ionospheric HF Doppler shift data during the periods of two strong typhoons which occurred in 1988 and 1990 respectively are analyzed in detail. The results show that except the appearance of significant wave-like disturbances (in general, medium-scale acoustic-gravity waves AGWs) in the ionosphere there are some new phenomena worth noticing: the temporal evolution of these waves in the two cases show clearly the change of both the main frequency and amplitude, the frequency was getting lower and lower as time elapses, and the amplitude enhanced gradually. After sunset, Spread-F phenomena appeared, showing the role of seeding of the AGWs for exciting of ionospheric irregularities. A comparison is made of the above phenomena with the linear theory of TIDs propagation and also with a nonlinear numerical simulation of the dynamic features of AGWs propagation in the atmosphere. All these results are in good agreement and the main features of ionospheric response to typhoon were thus explained.