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Factors contributing to the onset of the Australian summer monsoon

2018-08-11
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Hung, C. W. and M. Yanai (2004). “Factors contributing to the onset of the Australian summer monsoon.” Quarterly Journal of the Royal Meteorological Society 130(597): 739-758.
论文原文链接

  1. Introduction
  2. Data and the Definition of the onset
  3. Composite features of heat sources and moisture sinks
  4. Factors contributing to the onset
    4.1 land-sea thermal contrast
    4.2 Barotropic instability 正压不稳定
    4.3 The arrival of the MJO
    4.4 The intrusion of the midlatitude trough
  5. Conclusion
    5.1 Heating processes in the Australian region
    5.2 Major factors contributing to the onset

1. Introduction

一般来说地形对大尺度大气环流的影响主要体现在两个方面——热力作用(海陆热力对比)和动力作用(绕流)。为将两者区分开来研究地形对季风的作用,需要找一个没有高地但有季风的区域作为研究对象,以关注地形的热力作用。因此,澳大利亚季风是一个理想的研究对象。

The Australian summer monsoon usually begins in December and ends in March.Its mature stage is characterized by heavy precipitation and low-level westerly wind over northern Australia and the Arafura Sea.

In the 1960s, several studies (e.g. Berson 1961) showed that the Australian summer monsoon has similarities to the Asian summer monsoon, such as its abrupt poleward shift, sudden onset over a large region and intraseasonal active–break periods

Despite these studies, the Australian summer monsoon has received relatively little attention compared to the Asian summer monsoon. Several gaps in our existing knowledge need to be addressed:
接着作者列举了四个该研究的必要性,但主要还是集中于说以往研究所用数据较短且刚好遇上EI Nino年,使用的都是站点数据,没有研究Australian的热力和水汽收支情况,且以往研究Australian monsoon onset时关注的过程都不一样,有必要用统一的标准来研究四个因子对onset的影响。
因此该文使用1979-1993年(15年)再分析资料(ECMWF,ERA,CPCC)计算了热力和水汽收支情况,综合分析了四个因子的影响。

2. Data and the Definition of the onset

介绍了apparent heat source Q1和apparent moisture sink Q2的计算方法及判断Asutralian monsoon onset的标准——2-15S,115-150E区域(主要是Australian北部一小块陆地及阿拉弗拉海)平均的850hPa西风及OLR、precipitation。

The first day with average 850 hPa zonal wind exceeding 2 m/s over NAU is chosen as the onset day when the westerly wind is sustained for longer than 10 days and the OLR is lower than 210 W/m2 for at least several days during the 10-day period.

3. Composite features of heat sources and moisture sinks

在北半球春季夏季过渡期间,由于TP的加热作用使TP南部500-200hPa温度梯度反转,使大气环流发生改变,这与亚洲季风的爆发同时发生。但在Australian则不存在这种现象,该地区的地面感热加热主要集中在底层,750hPa以下。

  • 前人研究表明northern Australia Q1在1月的最大值为2.3K/day,峰值位于500到400hPa。
  • 在对流系统的不同阶段,Q1随高度的变化不大
  • The well-separated Q1 and Q2 peaks in the vertical are typical features of a deep convective atmosphere (Yanai et al. 1973)
  • This second peak of Q2 is located at about the same pressure level as the Q1 peak. It indicates the presence of rain from stratiform cloud in NAU during the monsoon season.
  • The negative Q2 value implies moistening due to net evaporation near the surface
  • The maximum of Q1 in the ITCZ is always at 500–400 hPa
  • Australia主体陆地上空Q1和Q2的加热情况与Northern Australia完全不同

4. Factors contributing to the onset

利用合成方法分析Australian monsoon爆发前后的底层径向温度梯度分布、径向垂直剖面温度分布、径向垂直剖面风场分布、绝对涡度的径向梯度

  1. 海陆热力差异对比使得北半球秋季时,澳大利亚与赤道海洋海温的温度梯度反转(即南高北低),但该反战的温度梯度只局限于800hPa以下。
  2. Australian monsoon爆发于15S以北,大气接近正压分布。纬向流正压不稳定的环境是——绝对涡度的径向梯度符号变化。而14个样本中,有9个样本在季风爆发前三天在850hPa有绝对涡度径向梯度的符号变化
  3. 利用OLR纬向时间分布图,判断在季风爆发时,有来自Indian Ocean的MJO达到Australia并加强。
  4. 利用底层风场合成图,说明南半球中纬度槽对季风爆发的重要性

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