Precipitation and precipitable water: Their temporal-spatial behaviors and use in determining monsoon onset/retreat and monsoon regions

Precipitation (P) is conventionally used for determining monsoon onset/retreat. It can roughly separate monsoon regions from nonmonsoon regions, Zeng and Lu (2004) found that precipitable water (W) can also determine the monsoon onset/retreat but cannot determine the monsoon regions. Temporal-spatial behaviors of P and W are compared in this article with observed data and results of previous theoretical analyses, and the comparison is used to understand the performances of P and W in determining the monsoon onset/retreat and monsoon regions. It is shown that W increases everywhere from winter to summer, dominated by the large seasonal change of temperature. P increases from winter to summer mainly in monsoon regions; it decreases or does not change much in most of the nonmonsoon regions. Whether P increases or not from winter to summer depends on whether the increase of W is greater than the increase of temperature. Synoptic variations of P and W have positive correlations everywhere. The increases of P and W from winter to summer in monsoon regions make both able to determine the climatic monsoon onset and retreat. The positive correlations of daily P and W in monsoon onset and retreat seasons make the interannual variations of the monsoon onset and retreat determined from P able to be determined from W. The decrease or small change of P from winter to summer in most of the nonmonsoon regions and the increase of W in nonmonsoon regions make P able to roughly determine the monsoon regions while W fails to.