Abstract: In this study, a diagnostic analysis on a rainstorm occurred in the eastern part of northwest China on July 23, 2010 was carried out using the data from the sounding, a ground encryption automatic meteorological station, the satellite cloud images and reanalyzed data of NCEP 1°×1°. The results showed that a strong convergence formed at midlow level of the low vortex shear line, a strong divergence formed on the left of north jet on the eastern side of South Asia High in the upper troposphere, and an ascend movement formed over the rainstorm area in the eastern part of northwest China. Water vapor was continuously transported to the eastern part of northwest China under the interaction of the “Chanthu” typhoon and the subtropical high. During the rainstorm, precipitable water and rainfall intensity was consistent, and the precipitable water was over 50 mm during the period of strongest rainstorm. With the ascending and strengthening of southerly wind center, the rainstorm was enhanced, contrarily, it was weakened. The variation of the strength of secondary circulation excitated by divergence of nongeostrophic moisture [WTBX]Q[WTBZ]vector was in accordance with rainstorm intensity. The distribution of negative moisture helicity in midlow troposphere could give a good indicator to the distribution and center of rainfall.

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