Simulation Of The Stratospheric Wind Quasi-biennial Oscillation In The Whole Atmosphere Community Climate Model

The quasi-biennial oscillation of tropical stratospheric zonal wind is an important process in the middle and upper atmosphere.It is convinced that the equatorial trapped waves,such as the Kelvin waves and mixed Rossby-gravity waves,the inertia gravity waves(IGWs)and the small scale convective gravity waves are the dominant source of momentum flux for QBO.The activity of these waves are very frequent near the equator with different wave features.The influence of QBO is not limited to the equator as the oscillation can affect the polar votex at high latitude and can coup with the semi-annual oscillation in the mesosphere and lower thermosphere through the global circulation.What's more,the QBO phenomenon also has influences on the states of equatorial ionosphere.The local observation of QBO had last for a long time,but we are still absent of an effective global observation.As a result,the numerical modeling show great importance in studying the QBO.Until now,many global climate models(GCMs)can simulate the QBO with similar features compared with the observations.But the spontaneous formation of QBO is still a challenge in Whole Atmosphere Community Climate Model(WACCM)version 4.0.This may be caused by several reasons as below.One possible reason preventing the formation of QBO in WACCM4.0 is the lacking of necessary parameterization of gravity waves.In previous work by Xue et al.(2012),the application of inertial gravity wave parameterization has allowed for the spontaneous generation of QBO,basing on Lindzen's linear saturation theory modified to describe inertia-gravity waves(IGW)by considering the Coriolis effect.In this work,we improve the parameterization by importing a more realistic IGW phase speed spectrum that exhibits a double peak Gaussian distribution calculated from tropical radiosonde observations.A series of numeric simulations are performed to test the sensitivity of QBO-like oscillation features to the phase speed spectrum and the settings of parameterized IGW.All of these simulations are capable of generating equatorial wind oscillations in the stratosphere based on standard spatial resolution settings.Central phase speeds of the "double-Gaussian parameterization" affect QBO magnitudes and periods,and the momentum flux of IGW determines the acceleration rate of zonal wind.Furthermore,stronger IGW forcing can lead to a propagation of the QBO-like oscillation to lower altitude.The intermittency factor of the parameterization also prominently affects the QBO period.Stratospheric QBO-like oscillation are generated using the new IGW parameterization in a long time simulation with obvious improvements,including the proper period,the significant altitude of QBO eastward phase and the reliability of parameterization.The second possible reason is the coarse vertical resolution of larger than 1 km delays the upward propagation of gravity waves.The GWs would release their momentum flux through saturated breaking effect layer by layer in the atmosphere,until meeting the critical level and stop to propagate.The coarse vertical resolution may not have a proper layered atmosphere structure,causing the lack of mean-flow interaction between the gravity waves and the background wind.In our study,we have concluded that the simple enhance of convective GWs has little effect on the formation of QBO,but higher vertical resolution can increase the upward propagating efficiency of GW's momentum flux.The three major sources of QBO drag are the equatorial waves,the convective gravity waves and the inertia gravity waves,which cannot be replaced be each other.The third reason is that the basic core of WACCM4.0 is likely the finite volume(FV)core.When compared with the spectral element(SE)dynamical core in CAM5,the SE core can avoid the polar effect of horizontal grid and can reflect the atmospheric dynamic processes better.Based on tens of years of observations and simulations,we are clearly convinced of the QBO mechanism,and the three main purposes of the paper is as below:])study the relationship between QBO features and IGWs settings;2)simulate a realistic QBO using proper IGW parameterization settings;3),make sure the necessity of kinds of gravity wave parameterizations and higher model resolution to simulate a QBO in WACCM4.0.