Influence of Upper Tropospheric Disturbances on the Synoptic Variability of Precipitation and Moisture Transport over Summertime East Asia and the Northwestern Pacific

Abstract

This study provides an overall understanding of the summertime synoptic variability of precipitation and moisture transport at mid-latitude from the eastern coastal region of China to the northwestern Pacific. Using satellite precipitation and reanalysis data, a clear relationship is found between upper tropospheric disturbances (Rossby waves), surface precipitation, and lower tropospheric humidity through July and August. The upper tropospheric disturbances are characterized by the undulation of the 1.5 PVU contours of potential vorticity (PV) on the 350 K isentropic surfaces. Case studies suggest that a precipitation band of several hundred kilometers wide and a thousand to several thousand kilometers long is formed very frequently on the equatorward and low-PV side of the northernmost 1.5 PVU contours, which meander together around 40°N. Lower tropospheric specific humidity is also enhanced there, and it falls sharply to the north of these contours. The synoptic situations associated with it include, but are not limited to, a common situation in which moist convection is enhanced ahead of upper-level troughs. These results are confirmed by a composite analysis over the 12 summers from 2001.A novel method of analyzing the forcing of the quasi-geostrophic potential enstrophy, in which boundary contributions are incorporated, reveals that upper tropospheric disturbances in the area are propagated predominantly from the west along the Asian jet, and that they exert a significant forcing onto near-surface levels, while the upward forcing from near-surface levels to upper tropospheric disturbances is weak. A Q-vector analysis shows that the upwelling associated with the precipitation bands is forced predominantly by confluence. This process is frontogenetic, and surface fronts are often formed therein. The upwelling is enhanced by latent heating. The latitudinal extent of humid air masses is affected not only by this circulation but by low-level flows induced by upper-level disturbances in a cooperative manner.

The Effects of an Upper-Tropospheric Trough on the Heavy Rainfall Event in July 2018 over Japan

“…Recent studies have emphasized that large-scale conditions in the middle and upper troposphere, in addition to lower-tropospheric conditions such as convective instability, are important in rainfall events over Japan. For example, over summer-time East Asia and North Western Pacific, ascent associated with the ageostrophic secondary circulation that is induced by upper-tropospheric disturbances affects synoptic variations in rainfall and lower-tropospheric moisture (Horinouchi 2014;Horinouchi and Hayashi 2017).…”

Extreme Moisture Flux Convergence over Western Japan during the Heavy Rain Event of July 2018

“…Results from a number of case studies (e.g., Kato and Goda 2001;Kato and Aranami 2005;Kato 2006; Tsuguchi and Kato 2014) have suggested primary factors behind localized heavy rainfall events over Japan, including moist airflows with high equivalent potential temperature, potential instability of the ambient atmosphere, and vertical wind shear. Meanwhile, the Baiu front and the associated rainband form under the strong influence of the subtropical jet (STJ) and disturbances propagating along it (Sampe and Xie 2010;Horinouchi 2014). Kosaka et al (2011) found that interannual variability of the Baiu-Meiyu rainband tends to accompany anomalous mid-tropospheric thermal advection by the STJ, which acts to induce anomalous ascent along the Baiu front (Sampe and Xie 2010), and also found that the anomalous Baiu rainband tends to be related to the "Silk-Road Pattern" -the dominant teleconnection pattern associated with a stationary Rossby wave train propagating along the STJ over the Eurasian continent (Enomoto et al 2003;Kosaka et al 2009).…”

Enhanced internal gravity wave activity and breaking over the northeastern Pacific–eastern Asian region

“…Tsuda et al, 2000;Schmidt et al, 2008), Formosat Satellite Mission 3 -Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT-3-COSMIC; e.g. Alexander et al, 2009;Horinouchi and Tsuda, 2009;Wang and Alexander, 2009) or using High Resolution Dynamics Limb Sounder (HIRDLS; e.g. Ern and Preusse, 2012) or Sounding of the Atmosphere using Broadband Emission Radiometry (SABER)/Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) satellite data (e.g.…”