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[AAS08-P03] Role of Medium-scale traveling waves for Upper-cloud variations over East Asia
Keywords:Upper-cloud, Medium-scale traveling waves
Although most of the upper clouds in mid-latitude are accompanied by low-pressure systems, some of them have other origins. One such system is a cloud system that moves rapidly (20 m/s) from the Tibetan Plateau to Japan. Kodama et al. (2008, JMSJ, hereafter K08) showed that these cloud systems are associated with medium-scale traveling waves (hereafter MTWs). MTWs have a wavelength of about 2000-2500 km and a period of about 26 hours and are confined to the upper and middle troposphere (e.g. Sato et al. 1993, JMSJ, Yamamori et al. 1997, JMSJ). K08 also proposed that the diurnal thermal forcing over the Tibetan Plateau in the spring generates MTWs and that the waves propagate eastward and accompany upper clouds in their southern wind part. To confirm this hypothesis, we extended the analysis period to 30 years (1990-2019) and additionally analyzed the interannual variation of the wave properties and the exact position of the upper cloud in the MTWs. We used the ERA5 reanalysis dataset (Hersbach et al. 2020). We extracted MTWs with the band-pass filter (0.75-2.0 days) and regarded the variance of the v-component of wind (hereafter V) as the activity of MTWs.
In the coastal region of China (110-120E), which is a key focus of this study, the variance of the fraction of cloud cover at 300 hPa (hereafter C300) is large, whereas the variance of V is much smaller than in other regions. To examine whether the activity of the MTWs can control the variance of the C300 even in this area, we investigated the relationship between two kinds of variance each April and found a significant positive correlation. We also examined the relationship between the variance and the monthly average of C300 because the latter exhibits a spatial distribution similar to that of the variance. These two variables also show a positive correlation. The variance of upper clouds is related to both the MTWs activity and the average of the upper cloud cover.
To investigate the generation mechanism of upper clouds, we analyzed the relationship between V and upper clouds in a wave. Based on the fact that MTWs have about a 24-hour period in this region, to examine the phase relation between them, we estimated the local time difference between the maximum of V and the maximum of C300. The maximum of C300 precedes that of V by approximately 5 hours. This result differs from K08, which shows that the two maximums nearly coincide.
In the coastal region of China (110-120E), which is a key focus of this study, the variance of the fraction of cloud cover at 300 hPa (hereafter C300) is large, whereas the variance of V is much smaller than in other regions. To examine whether the activity of the MTWs can control the variance of the C300 even in this area, we investigated the relationship between two kinds of variance each April and found a significant positive correlation. We also examined the relationship between the variance and the monthly average of C300 because the latter exhibits a spatial distribution similar to that of the variance. These two variables also show a positive correlation. The variance of upper clouds is related to both the MTWs activity and the average of the upper cloud cover.
To investigate the generation mechanism of upper clouds, we analyzed the relationship between V and upper clouds in a wave. Based on the fact that MTWs have about a 24-hour period in this region, to examine the phase relation between them, we estimated the local time difference between the maximum of V and the maximum of C300. The maximum of C300 precedes that of V by approximately 5 hours. This result differs from K08, which shows that the two maximums nearly coincide.