Seasonal Variations of Gravity Wave Variance Inferred from CLAES

摘要

Gravity wave variances in CLAES temperature data are isolated by a 0-6 zonal wavenumber Kalman filter. Resulting vertical profiles of temperature residuals are analyzed by a combination of Maximum Entropy Method (MEM) and harmonic analysis for gravity waves (GWs). This is the same method previously employed to study GWs in CRISTA data. We obtain nearly 1.5 years of continuous GW data between 34S and 34N and good coverage at higher latitudes depending on UARS yaw maneuvers. A time series of zonal mean GW variance shows the seasonal shift of the tropical maximum of GW variance around the equator. Maximum variances are reached 1-2 months after summer solstice, consistent with the shift of the inner tropical convergence zone. Quiet summer and enhanced winter values at mid and high latitudes are due to a combination of wind filtering and wind modulation. Wind filtering occurs when GWs propagate from tropospheric west winds into the lower stratosphere. There prevailing winds reverse from west wind in winter to east wind in summer, thus causing a critical layer for low phase speed GWs during summer. The term wind modulation is used for the Doppler shift of the GW spectrum by the wind at the observation altitude shifting parts of the GW spectrum in and out the vertical-wavelength visibility limits of the instrument. We find evidence for both processes in the data and indication that GW filtering might be the more important one.