Morphodynamic response of an embayed beach to different typhoon events with varying intensities

Beach erosion has occurred globally in recent decades due to frequent and severe storms.Dongsha beach,located in Zhujiajian Island,Zhejiang Province,China,is a typical embayed sandy beach.This study focused on the morphodynamic response of Dongsha beach to typhoon events,based on beach topographies and surficial sediment characteristics acquired before and after four typhoon events with varying intensities.The four typhoons had different effects on the topography and sediment characteristics of Dongsha beach.Typhoons Ampil and Danas caused the largest(-51.72 m3/m)and the smallest erosion(-8.01 m3/m),respectively.Remarkable alongshore patterns of beach profile volumetric changes were found after the four typhoon events,with more erosion in the southern and central parts of the beach and few changes in the northern part.Grain size coarsening and poor sorting were the main sediment patterns on the beach influenced by different typhoons.Typhoons that occurred in the same year after another typhoon enhanced the effect of the previous typhoon on sediment coarsening and sorting variability,but this cumulative effect was not found between typhoons that occurred during different years.A comparison of the collected data revealed that the topographic state of the beach before the typhoon,typhoon characteristics,and tidal conditions were possible reasons for the difference in the responses of Dongsha beach to typhoon events.More severe beach erosion was caused by typhoons with higher intensity levels and longer durations,and high tide levels during typhoons can determine the upper limit of the beach profile erosion site.Taken together,these results can be used to improve beach management for storm prevention.

DIAGNOSIS OF WAVE ACTIVITY OVER RAINBAND OF LANDFALL TYPHOON

A generalized wave-activity density, which is defined as an absolute value of production of three-dimensional vorticity vector perturbation and gradient of general potential temperature perturbation, is introduced and its wave-activity law is derived in Cartesian coordinates. Constructed in an agoestrophic and nonhydrostatie dynamical framework, the generalized wave-activity law may be applicable to diagnose mesoscale weather systems leading to heavy rainfall. The generalized wave-activity density and wave-activity flux divergence were calculated with the objective analysis data to investigate the character of wave activity over heavy-rainfall regions. The primary dynamical processes responsible for disturbance associated with heavy rainfall were also analyzed. It was shown that the generalized wave-activity density was closely correlated to the observed 6-h accumulative rainfall. This indicated that the wave activity or disturbance was evident over the frontal and landfall-typhoon heavy-rainfall regions in middle and lower troposphere. For the landfall-typhoon rainband, the portion of generalized wave-activity flux divergence, denoting the interaction between the basic-state cyclonic circulation of landfall typhoon and mesoscale waves, was the primary dynamic process responsible for the evolution of generalized wave-activity density.

Reconstruction of typhoons in the Yangtze River Delta during 1644-1949AD based on historical chorographies

The proxy records on typhoons in the Yangtze River Delta from 1644 to 1949AD were extracted from historical chorographies in the Qing Dynasty and the Republic of China Period. In reference to the basic principles for identifying historical typhoons, time series on the Yangtze River Delta over a period of 306 years were developed. The conclusions are as follows. （1） There were a total of 241 typhoons from 1644 to 1949AD. Using the historical chorographies from 1884 to 1949AD, the number of typhoons was 65, equal to 87.8% re- corded by meteorological observation. The number of years with differences in typhoon ac- tivities reconstructed using two ways no more than once is 55, reaching 83.3% in the period from 1884 to 1949AD. This result means the series of historical typhoons reconstructed using historical chorographies can represent the change of typhoon activities over years. （2） The average number of typhoon activities is 0.79 times per year from 1644 to 1949AD, and they show an increasing trend. These 306 years can be divided into three periods by the average number of typhoon activities： it is low from 1644 to 1784AD, and more typhoon activities are found from 1785 to 1904AD. It is worth noting that the number of typhoon activities reaches the summit in the last period, which is 1.2 times per year from 1905 to 1949AD. （3） Before the 20th century, the number of typhoon activities in warm periods is less than the number of cold periods. However, the number of typhoon activities increased dramatically in the early 20th century. Comparing the typhoon activities with El Nifio events, the data show that the number of typhoon activities did not increase when El Nifio occurred.