Large-Scale Dynamics of the Meiyu-Baiu Rainband: Environmental Forcing by the Westerly Jet*

Abstract

Meiyu-baiu is the major rainy season from central China to Japan brought by a zonally elongated rainband from June to mid-July. Large-scale characteristics and environmental forcing of this important phenomenon are investigated based on a reanalysis dataset. The meiyu-baiu rainband is accompanied by a trough of sea level pressure, horizontal shears, and sharp moisture gradients near the surface, a westerly jet tilted northward with height, and large northeastward moisture transport from the south.The analysis here reveals the westerly jet as an important culprit for meiyu-baiu. Along the rainband, mean ascending motion corresponds well with a band of warm horizontal temperature advection in the midtroposphere throughout summer. This adiabatic induction of upward motion originates from the advection of warm air by the westerlies from the eastern flank of the Tibetan Plateau. The ascending motion both induces convection and is enhanced by the resultant condensational heating. The westerly jet anchors the meiyu-baiu rainband also by steering transient eddies, creating periods conducive to convection through convective instability and adiabatic updrafts. Indeed, in meiyu-baiu, the probability distribution of convective instability shows large spreads and is strongly skewed, with a sharp cutoff on the unstable side resulting from the effective removal of instability by convection. Thus, active weather disturbances in the westerly waveguide explain a paradox that convection is active in the meiyu-baiu rainband while mean convective instability is significantly higher to the south over the subtropical North Pacific warm pool. In addition to the westerly jet, low-level southerly winds over eastern China between the heat low over Asia and the subtropical high pressure belt over the Pacific are another important environmental forcing for meiyu-baiu by supplying moisture. A conceptual model for meiyu-baiu is presented, and its implications for seasonal and interannual variations are discussed.

How East Asian westerly jet’s meridional position affects the summer rainfall in Yangtze-Huaihe River Valley?

“…Climatologically, there exists evident difference in the position of the rainband in eastern China and the pattern of circulation over East Asia between early summer and midsummer (from middle July to early August, Ding and Chan 2005;Huang et al 2008;Zhang et al 2006;Sampe and Xie 2010;Kosaka et al 2011). So as usual, in the present study the summer will be separated into early summer and midsummer, i.e., the 30-day period from June 16 to July 15 and the 30-day period from July 16 to August 14, respectively.…”

“…Ding and Wang 2005;Lu and Lin 2009;Chen and Huang 2012). Because the response of horizontal circulation to rainfall's condensational heating is primarily baroclinic and it minimizes in the midtroposphere, where the causality between the circulation and the diabatic heating/cooling anomalies is the easiest to interpret (Sampe and Xie 2010;Kosaka et al 2011), the level of 500 hPa can be chosen as a standard to verify whether EAWJ's meridional displacement has an independent effect on the rainfall anomalies in YHRV. A meridional dipole of 500-hPa zonal wind velocity anomalies associated with the average rainfall in YHRV is observed around the climatological-mean 500-hPa jet axis in both early summer and midsummer (Wang and Zuo 2016, see their Fig.…”

“…All plots were produced using Grid Analysis and Display System (GrADS) Version 2.1 where the capital letters under the equation denote terms (the sign included). Other notations are the same as those in Sampe and Xie (2010). The temperature tendency term (C) can be neglected (Sampe and Xie 2010) in seasonal scale.…”

“…Other notations are the same as those in Sampe and Xie (2010). The temperature tendency term (C) can be neglected (Sampe and Xie 2010) in seasonal scale. In both early summer and midsummer, the magnitude of standard deviation of p is an order smaller than the magnitude of the climatology of p , while the magnitude of standard deviation of vertical velocity has the same order as the magnitude of the climatological-mean vertical velocity over YHRV (figures omitted).…”

“…That means the dynamic mechanism has not been resolved yet. Recently, it is revealed that the EAWJ anchors the meiyubaiu rainband by forming warm advection and steering transient eddies, which are related to the 30-day-average temperature advection and day-to-day variability of temperature advection for the 30 days in early summer (from middle June to early July) over YHRV, respectively (Sampe and Xie 2010). Based on the perspectives of these two properties of temperature advection, the mechanisms of the impact of EAWJ's meridional displacement on summer rainfall anomaly in YHRV will be explored in this research.…”