Cloud-Top Motion Variation of a Landfall Typhoon Observed by Geostationary Satellite Imagery

Typhoons' rapid variation makes effective hazard prevention difficult, which remains a challenging research topic requiring novel observation technology and analysis methodology. We developed an automatic methodology for mining the information on the rapid variation of typhoon cloud-top motion from high-spatial-temporal-resolution satellite cloud images, which has steps of estimating and decomposing the typhoon cloud-top motion, locating the typhoon cloud-top center, and analyzing the motion and center position variations. The Gaofen-4 satellite acquired 50-m-1-min-resolution cloud images of Typhoon Megi's (2016) landfall over mountainous Taiwan Island. The data provide an excellent chance to explore what such high-spatial-temporal-resolution successive observations can tell about the cloud-top motion variation of a typhoon landfall using the methodology. Before its landfall, Typhoon Megi's cloud-top center dramatically increased the northwestward migration speed from ∼22 to ∼58 km/h in only ∼2.5 h. It also presented small-scale oscillation along its migration path with a 1.6-to-2.4-km amplitude, a 31-to-33-km spatial period, and a 1.46-to-1.48-h temporal period. We averaged the motion speeds (velocity magnitudes) in each motion field to assess the field strength. The so-calculated cloud-top average rotation speed of Typhoon Megi decreased quickly during landfall. Meanwhile, its average divergence speed increased dramatically by ∼30% in a short period of ∼2.5 h, reaching ∼2 times the rotation speed. This result means that Megi's cloud-top motion rapidly changed from a typical rotation motion-dominated status to a divergence motion-dominated status. This rapid transformation from typical rotation motion dominance to divergence motion dominance implies intense upward warm and moist airflows that cause severe precipitation.