Medium-Scale Tropopausal Waves Visualized by Upper-Level Clouds to the East of the Tibetan Plateau

Abstract

We examined upper cloud variation over East Asia and the western North Pacific using 3-hourly Geostationary Meteorological Satellite infrared (GMS-IR) observations between 1988 and 1997. Of particular focus was the relationship to medium-scale tropopausal waves (MTWs) developed near the tropopause around mid-latitude westerlies. One- to two-day variation in upper clouds was significant throughout the year, except in summer because of eastward-traveling upper cloud systems developing in the southerly of MTWs. This finding confirms a previous theoretical prediction of southerly updraft appearance. The largest 1-day variation in upper cloudiness occurred over the Tibetan Plateau. Large variation also occurred over the China Plain to the east of the Tibetan Plateau and over the East China Sea, but less variation occurred over Japan and the western North Pacific to the east. Since appearance of cloud systems of MTWs near the eastern edge of the Tibetan Plateau is diurnally regulated, the phases of 1-day variation of cloud systems of MTWs over the China Plain are determined by the eastward phase speed of MTWs and the distance from the Tibetan Plateau. When the phase speed of MTWs is uniform, systematic phase delay of diurnal variation in upper cloudiness is significant over China Plain, which has been observed as a distinct phenomenon (Asai et al. 1998). The phase-locked appearance of MTW cloud systems can be ascribed to diurnal variation in upper-tropospheric circulation around the Plateau, which is related to updrafts that develop in the afternoon over the Plateau. This suggests that thermal forcing over the Tibetan Plateau may initiate MTWs and affect their behavior over East Asia, where the magnitude of 1-day variation is larger than that of MTWs in other mid-latitude regions.