Primary Study Of Gravity Waves Generated By Tropical Cyclones With Lidar At Hainan Island

Gravity wave(GW)activity is the most common dynamic phenomenon in the atmosphere.Air micelles leave their initial positions due to external disturbances(i.e.,wave sources),and oscillations are generated while the air micelles return their original positions under the combined action of gravity and buoyancy.These oscillations spread away in the atmosphere and form a series of waves,which called atmospheric GWs.In general,GWs observed in the middle and upper atmosphere mainly originate from lower atmosphere disturbances below 20 km,and the topography,convection and wind shear are the most important sources.The energy and momentum coupling and exchange between different atmosphere layers are realized through the wave saturation,dissipation and breaking during gravity waves propagate from the lower atmosphere to the upper atmosphere.Tropical cyclone(TC)is a kind of low-pressure vortex that typically occur over tropical or subtropical oceans.It is a tropical weather system containing large amounts of unstable energy and is an important regulatory mechanism for the global equilibrium distribution of heat and momentum.TCs frequently occur in the South China Sea and the Northwest Pacific Ocean.While bringing significant precipitation to the inland of China,they also bring the sun’s tropical energy to the middle and higher latitudes.As a kind of strong vortex disturbance(special wave source)in the troposphere,tropical cyclone can trigger a large number of atmospheric GWs.During the upward propagations,GWs transfer the energy and momentum of the lower atmosphere to the middle and upper atmosphere via wave saturation,dissipation and breaking.It is helpful to carry out observations of atmospheric GWs triggered by TCs to analyzing the coupling mechanism of energy and momentum between the lower atmosphere and the middle and upper atmosphere.Hainan Island locates in the South China Sea and mainly has a tropical monsoon climate,and it is a natural laboratory for the observation and research of atmospheric GWs induced by TCs because a large number of TCs occur in the South China Sea and the Western Pacific Ocean every year.Comprehensive observations of atmospheric GWs triggered by TCs in this region with ground-based and space-based facilities are conducive to scientific research on the energy and momentum coupling mechanism between the lower atmosphere and the middle and upper atmosphere.Meanwhile,in the global atmospheric circulation system,the active atmospheric activities over tropics also play an important role in the global climate change and the space weather environment change at low latitudes.However,due to the limitation of special geographical location or the lack of observation facilities,few scientific reports on GWs triggered by TCs have been reported till now,and most of them are focusing on a single event.Lidar is an active optical detection equipment which widely used in the field of atmospheric remote sensing,and lidar observation has the advantages of high spatial and temporal resolution,high detection sensitivity and large vertical span.Hainan lidar station(19.5°N,109.1°E)was built in 2010 with the support from the Chinese Meridian project,and the long-term observation of atmospheric activities in the middle and upper atmosphere has been realized at Haikou.Based on the Rayleigh scattering mechanism of atmospheric molecules,the density and temperature structure of the middle atmosphere(30-65 km)can be effectively detected by Rayleigh lidar.By analyzing the power spectrum of atmospheric temperature disturbance caused by GWs,the parameters such as vertical wavelength and observation period activity can be extracted.During the June 16 to July 3 in 2013,tropical storm Leepi,Bebinca and severe tropical storm Rumbia sequentially occurred in the South China Sea and the western Pacific Ocean.Based on the observation data from Hainan lidar during this time period,researches on the characteristics of atmospheric GWs induced by TCs and their propagation properties have been carried out in present study,and the main contents are as follows:(1)Atmospheric temperature structure was retrieved with the method introduced by Hauchecorne and Chanin.Through the data processing steps such as background noise removal,low-pass filtering,reference height selection,and satellite observation data comparison analysis,the temperature structure was retrieved according to Hainan lidar observation data,and maximum relative measurement errors in temperature at different altitudes were controlled to less than 5%.(2)Based on the wavelet analysis of atmospheric temperature perturbation,the power spectral density(PSD)of temperature perturbation caused by GW was calculated by Fourier transform using the autocorrelation function.After the atmospheric temperature perturbation was obtained,wavelet analysis was applied to each temperature perturbation profile,and the vertical wavenumber power spectrum and time-frequency temporal frequency spectrum are obtained for the dominant wave components with one-dimensional Fourier transform.The power spectral density of GWs disturbance was calculated with two-dimensional fast Fourier transform.Finally,the information on the scale and propagation direction of gravity waves were obtained.(3)Parameters of lidar-observed GWs were extracted during the time period of TCs.By comparing the observation data,gravity wave activities observed by lidar were analyzed during the occurring of tropical cyclones Leepi、Bebinca and Rumbia.Characteristics such as vertical wavelength,observation period and propagation direction were also extracted for every GW.These researches on atmospheric GW activities stimulated by TCs at Hainan Island can enhance the understanding of the special properties of GW activities induced by TC and their influences on the energy and momentum changes in the middle and upper atmosphere.Moreover,it may broaden people’s understanding of the activities and dynamic processes of the middle and upper atmosphere in the near-equatorial Marine climate region,and also have a certain reference value for the researches of global climate change.