*2.4. NCCV Coordinate System and Dynamic Composite Analysis*

For the convenience of statistics, an NCCV coordinate system is defined in this study. In this coordinate system, the origin represents the center of the NCCV, the *x* axis indicates the east–west direction, and the *y* axis indicates the north–south direction.

The near-surface rain rate and other physical variables for precipitation were analyzed using dynamic composite analysis [7,32] in the NCCV coordinate system. The formula of dynamic composite analysis is

$$\overline{S}(\mathbf{x}, \mathbf{y}) = \frac{1}{N} \sum\_{t=1}^{N} S\_t(\mathbf{x}, \mathbf{y}), \tag{1}$$

where *St*(*x*, *y*) are the physical variables at time *t*, and (*x*, *y*) are the coordinates in the composite area, which moves with the position of the NCCV center. *N* is the total number of samples, and *S*(*x*, *y*) is the average value of the samples obtained after the dynamic composite analysis. In the composite analysis, the NCCV center is used as the composite center; we took 2000 km in each direction around the NCCV center [24] to perform dynamic composite analysis in a square area with a side length of 4000 km.

In total, 6432 NCCV cases from 2014 to 2019 are used in the dynamic composite analysis. These things considered, the samples with near-surface rain rate greater than 0.5 mm h−<sup>1</sup> are defined as precipitation samples.
