Gravity Wave Activity In The Tropical Lower Stratosphere With High Vertical Resolution Radiosonde Data

Seven-year long high vertical resolution radiosonde data (from 1998 to 2004) from eight U.S. stations in the Northwest Pacific were used to investigate the characteristics of gravity wave activity in the tropical lower stratosphere (18-25 km height range). The prevailing zonal wind during the period and height range are westward with a period-like oscillation superimposed in the region. On average, gravity wave activities vary seasonally and weaken with increasing latitude, and exhibit interannual variations near the Equator.The spatial and temporal variations of gravity wave energy density and several key parameters were estimated. Generally speaking, the wave energy density decreases with increasing latitude and is larger in winter than in summer (Northern Hemisphere). Meanwhile, interannual variation is clear seen for estimations at latitudes near the Equator. Further detailed investigation reveals that the maxima of the GW energy density occur when equatorial zonal winds are in the transition phase of the QBO from the easterlies to the westerlies.Wave energy density is found to propagate upward, and the dominant horizontal propagation direction is toward the east. Zonal momentum fluxes exhibit a similar temporal behavior to that seen in the gravity wave energy density. Intrinsic frequencies are generally low and increase with increasing latitude, and the ratios between intrinsic frequency and the Coriolis parameter are -2.5-3.5. The ratios decrease with latitude slightly. Dominant vertical and horizontal wavelengths decrease with latitude. The vertical wavelengths are found to be longer in winter than in summer.