An Observational Investigation on Gravity Wave Characteristics and Propagation in the Lower Stratosphere and Mesopause

Gravity Wave;Atmospheric Dynamics;Airglow Imager

Li, Zhenhua

关键词: Gravity Wave;    Atmospheric Dynamics;    Airglow Imager;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/26160/Li_Zhenhua.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

In this thesis, investigations on gravity waves are conducted in tworegions of the middle atmosphere: the lower stratosphere usinghigh-resolution radiosonde at South Pole and the mesopause regionusing OH airglow imager at Maui, Hawaii and Cerro Pachon, Chile. Wavecharacteristics at these regions are deduced and the seasonalvariation of wave activity, wave sources, and propagation effect arestudied. The study of gravity waves in the lower stratosphere at South Polereveals that sources other than topography are important even for thelower part of middle atmosphere. Horizontal propagation must beincluded in parameterization schemes to reflect the fact that wavesderived from radiosondes have slant propagation paths. They travellong distance horizontally before they reach higher altitudes.Long term gravity wave characteristics over Maui from 2002 to 2007 arededuced from OH airglow imager. Wave parameters from the long termimager observation provide robust statistics of high-frequency gravitywave in the midlatitudes. Poleward wave propagation preference duringsummer and equatorward wave propagation preference during winter areobserved over Maui. They are also opposite to the seasonal meanmeridional wind direction which are always pointing toward winterpole. Momentum fluxes deduced from OH imager are also highlyanti-correlated with background winds. At least for the part ofspectrum observed by airglow imager, gravity waves act as dampingmechanism for diurnal tide. Gravity wave occurrence frequency does notfollow the variation of local convective sources and convectivesources in a large domain when ducted waves are considered. In fact,with a constant wave source and monthly mean background atmosphericcondition, the simulated wave transmission resembles the waveoccurrence frequency observed by OH airglow imager at Maui. Thus, atMaui the propagation effect dominates the seasonal variation in waveactivity.Gravity wave momentum fluxes deduced from airglow imager provideimportant observation constraint for gravity wave parameterization forthe mesopause region. To explain the cause of seasonal change onmeridional propagation preference, three mechanisms are investigated:critical-layer filtering, wave ducting, and Doppler-shifting by localmean wind. Critical-layer filtering failed to explain the propagationpreference. Observed gravity wave propagation directions are largelyrelated to the background wind in the airglow layer. This is caused byDoppler-shifting of gravity waves by background wind. Background windDoppler shifts gravity waves propagating against (along) backgroundwind to higher (lower) frequency and larger (smaller) verticalwavelength. Thus, the observed gravity waves tend to propagate againstbackground wind. The apparent against background wind propagation islargely caused by the contrast in cancellation factor for wavespropagate in different direction. To a lesser degree, the differencein dissipation for waves propagate in different direction alsocontributes to the observed against background wind propagation.The results from this work show gravity wave's propagation in middleatmosphere is strongly affected by atmospheric field. For lowfrequency waves, their propagation paths are slant and can travelhundreds of kilometers before they reach the middle atmosphere. Forhigh frequency gravity waves, though their propagation paths aremostly vertical, they are subject to ducting and reflection. Due tothe large contribution of momentum flux in the Mesosphere and LowerThermosphere (MLT) by high-frequency, short-horizontal-scale waves,these propagation effects must be included in gravity waveparameterizations.