Intensity Evolution of Zonal Shear Line over the Tibetan Plateau in Summer: A Perspective of Divergent and Rotational Kinetic Energies

Based on the ERA5 reanalysis datasets during 1980–2019, a total of eleven zonal shear lines (ZSLs) that caused heavy precipitation and lasted more than 60 hours over the Tibetan Plateau in summer are selected for composite analysis. By decomposing the kinetic energy (K) near the ZSL into divergent and rotational kinetic energies (KD and KR) and the kinetic energy of interaction between the divergent wind and the rotational wind (KRD), the influence of the rotational and divergent winds on the evolution of the ZSL intensity is investigated from the perspective of KD and KR. The main results are as follows. The ZSL is a comprehensive reflection of rotation and convergence. The intensity evolution of ZSL is essentially synchronized with those of K, KR, and KRD but lags behind KD by about three hours. The enhancement of K is mainly contributed by KR, which is governed by the conversion from KD to KR. Furthermore, the increase in the conversion from KD to KR is controlled by the geostrophic effect term Af, which is determined by the joint enhancement of the zonal rotational and meridional divergent wind components (uR and vD). Therefore, the joint enhancement of uR and vD controls the increase of the ZSL intensity, leading to increased precipitation.


Introduction
As a plateau with the highest elevation in the world and the largest area in the Northern Hemisphere, the Tibetan Plateau (TP) has an average elevation of more than 4000 m and accounts for one-quarter of China's land area. It has unique weather and climate characteristics that significantly affect the atmospheric circulation, weather, and climate in China, Asia, and globally. Therefore, TP meteorology, which focuses on its dynamic and thermal forcing, was recognized early by researchers and has since become an important research field in weather and climate (Bolin, 1950;Flohn, 1957;Wu et al., 2004).
Under the unique dynamic and thermal effects of the TP, two types of typical cyclonic circulation systems with low-geopotential-height structures are isolated and named the plateau shear line and the plateau vortex. Both are formed in the near-surface layer over the TP. Research has shown that the occurrence frequency of plateau shear lines and the frequency of shear line-induced precipitation are greater than those of plateau vortexes (The Tibetan Plateau Science Research Group, 1981;He et al., 2009). Furthermore, the plateau shear lines can be categorized into the zonal shear lines with a quasi-east-west orientation and the meridional shear lines with a quasi-south-north orientation. Comparatively, zonal shear lines occur more often and have a longer duration; moreover, they experience greater difficulty to propagating out of the TP and thereby more likely to cause potential disastrous heavy precipitation over the TP tion, some scholars have obtained significant results from the perspective of kinetic energy for individual cases. For example, the main energy source during the processes of ZSL evolution is the transformation from available potential energy into kinetic energy (Yu and Luo, 1993). The conversion from eddy available potential energy to eddy kinetic energy (Luo and Li, 2019a) and the downscaled energy cascade of kinetic energy caused by the interaction between the background circulation and eddy field are conducive to the formation and development of ZSLs (Luo and Li, 2019b).
The ZSL is a discontinuous line in the cyclonic rotational wind field in the near-surface layer over the TP (Tao, 1980), featured by mid-tropospheric cyclonic rotation and horizontal wind field convergence. In addition, both the convergence (Fu et al., 2011;Zhou et al., 2014) and the rotation (Zhao et al., 2009;Fu et al., 2013;Li et al., 2019;Jin et al., 2020) of the wind field play essential roles in the development of weather systems. So, what are the respective effects of the divergent and rotational winds on the evolution of ZSLs? Research into this problem will help further promote the understanding of why ZSLs evolve. However, it is difficult to study the ZSL evolution using only the rotational and divergent winds by themselves. It is well known that kinetic energy plays a vital role in the evolution of weather systems and is directly related to the wind field. Therefore, it is helpful to study the problem mentioned above from the perspective of kinetic energy.
According to the Helmholtz theorem, the horizontal velocity can be uniquely decomposed into the rotational and diver-gent winds (Hawkins and Rosenthal, 1965), and correspondingly, the kinetic energy can be decomposed, in a similar manner, into its divergent and rotational kinetic energies and the kinetic energy of interaction between the divergent wind and the rotational wind. On this basis, numerous investigations have been conducted on weather systems from the perspective of divergent and rotational kinetic energies, such as tropical cyclones (Ding and Liu, 1986;Yu et al., 1999a, b;Zhao et al., 2009), extratropical cyclones (Pearce, 1974;Chen et al., 1978;Li et al., 2019;Jin et al., 2020), mesoscale convective complexes in North America (Fuelberg and Browning, 1983;Buechler and Fuelberg, 1986), circulation systems during the mei-yu periods (Fu et al., 2013) and the northeast cold vortex (Deng et al., 2012). The results revealed that the enhancement of weather systems is mainly caused by the increase in the rotational kinetic energy in the lower troposphere (Zhao et al., 2009;Fu et al., 2013;Li et al., 2019;Jin et al., 2020). Although the divergent kinetic energy is usually less than 10% of the kinetic energy (Pearce,1974;Ding and Liu, 1986), it is critical to the generation and conversion of kinetic energy (Fuelberg and Browning, 1983;Buechler and Fuelberg, 1986). However, there is no precedent to investigate the ZSL evolution through the perspective of divergent and rotational kinetic energies, although great achievements have been made in studying the evolution of many other weather systems through this theorem.
Therefore, it is of great significance to study the influence of the rotational and divergent winds on the evolution of the ZSL intensity from the perspective of the rotational and divergent kinetic energies, which provides a new point of view and insight for the in-depth understanding of the ZSL evolution.
The remainder of this paper is organized as follows. Section 2 introduces the data and methods. Section 3 provides the relationship of the ZSL intensity evolution with the rotational and divergent kinetic energies. Section 4 illustrates the mechanisms for the ZSL intensity evolution. Finally, section 5 presents the main conclusions and final discussion.

Data
The data adopted in this study includes two datasets (1) the hourly wind dataset with a horizontal resolution of 1°×1° from June to August during 1980-2019 derived from the ERA5 reanalysis dataset and (2) the daily precipitation data (1200 UTC-1200 UTC) over the same period from the daily dataset of basic meteorological elements of China's National Surface Weather Station (version 3.0) released by the National Meteorological Information Centre of China Meteorological Administration.

Case selecting for composite analysis of ZSLs
Based on the wind data from the ERA5 reanalysis data-1022 INTENSITY EVOLUTION OF ZONAL SHEAR LINE VOLUME 39 set at 500 hPa from June to August during 1980-2019, the ZSLs are identified by an objective method with three parameters: the zonal shear of the meridional wind, the relative vorticity, and the zero line of meridional wind (Ma and Yao, 2015;Zhang et al., 2016). The specific criteria are as follows.
where u is the zonal wind component, y indicates the meridional coordinate, ζ is the vertical component of the relative vorticity. When the three criteria in Eq.
(1) are met at each grid point, the line connecting these grids, with a zonal span of more than 5 degrees of longitude, is identified as a ZSL. Subsequently, in the high-frequency area of ZSLs (32°-35°N), which is located in a region with an altitude of more than 3000 m over the TP, the ZSLs with a lifetime longer than 24 hours are selected to form a dataset of ZSL cases. Furthermore, based on the daily precipitation data mentioned above, the ZSL case that causes heavy precipitation is defined by following these steps: (1) The precipitation averaged at all stations on a certain day near each ZSL case (30°-36°N) in the dataset is defined as the average precipitation of that day (x i , i=1, n. n indicates the total ZSL days in the dataset).
(2) The average daily precipitation for all cases and their standard deviation are expressed as: (2) x (3) If the x i in a certain ZSL at any time over the course of its lifetime is one σ greater than , then this case is defined as a ZSL case that caused heavy precipitation.
Consequently, 11 typical cases that caused heavy precipitation and have a similar lifetime of more than 60 hours are selected from the above cases (Table 1). The distribution of the lifetime-averaged ZSLs from the 11 cases is shown in Fig. 1a.
Finally, the arithmetic average method is performed on each physical quantity of all typical cases at individual moments, with the intent of obtaining a composite ZSL for subsequent diagnostic analysis. The composite ZSL has a life- Table 1. Elevent typical cases of zonal shear lines (ZSLs) over the Tibetan Plateau (TP) in summer (Note: ZSL mentioned in this research all refers to ZSL over the TP in summer. The avereage precipitation at all stations of each ZSL process near each ZSL case (30°-36°N) is defined as the accumulated precipitation. The LST represents the local solar time, which is six hours ahead of the coordinated universal time (UTC), that is, LST = UTC + 6 h, the same below.).  The synthetic precipitation near the ZSL during its whole lifetime, the three stages, namely 1800 LST 1st-1800 LST 2nd, 1800 LST 2nd -1800 LST 3rd, and 1800 LST 3rd -1800 LST 4th (units: mm d -1 ). The solid gray line outlines the region with an altitude of 3000 m, which indicates the major body of the TP (the same below).

Serial number
time of 72 hours (1300 LST 1st-1200 LST 4th, short for 1300 LST on the 1st day to 1200 LST on the 4th day, the same below). Here, the LST represents the local solar time, six hours ahead of the coordinated universal time (UTC), LST = UTC + 6 h. It is important to note that the ZSL mentioned in the section below all refer to the composite ZSL from the 11 typical cases. Given that the starting time of the ZSL is close to the starting time of the 24-hour accumulated precipitation, the precipitation near the ZSL during its whole lifetime is represented by the precipitation of the three stages, namely 1800 LST 1st-1800 LST 2nd, 1800 LST 2nd -1800 LST 3rd, and 1800 LST 3rd -1800 LST 4th (Fig. 1b).

Stages of intensity evolution processes of ZSL
The arithmetic average method is performed on the ZSL over the course of its lifetime. Considering the high-frequency region for the ZSL's occurrence (Zhang et al., 2016) and the large-value region of the ζ (greater than 2 × 10 −5 s −1 ), the region of 32°-35°N, 81°-99°E is selected as the study area, as shown in the red dashed box in Fig. 2a. The ZSL represents a unique weather system in the boundary layer over the TP, generally referring to a 500-hPa convergence line with opposite wind directions at more than three stations (The Tibetan Plateau Science Research Group, 1981). Therefore, the ZSL intensity is expressed by the regional-averaged ζ in the study area at 500 hPa. According to the ZSL intensity evolution (Fig. 4), the evolutionary processes are divided into the development stage (1300 LST 1st-1200 LST 2nd), the vigorous stage (1300 LST 2nd -1200 LST 3rd), and the decay stage (1300 LST 3rd -1200 LST 4th).
The mean values of the physical quantities at each stage represent the environment of the ZSL evolution at each stage. There is an enhancement of the ZSL from the development stage to the vigorous stage and a weakening from the vigorous stage to the decay stage. The evolution of ZSL intensity is essentially synchronized with that of precipitation related to the ZSL (Fig. 1b). Note that the thermal effects of the TP exhibit significant diurnal variations, but this is not the focus of this study. To remove the influence of the diurnal variation on the ZSL evolution, the same start and end times are set for each stage to maintain consistency regarding the effect of diurnal variations.

Divergent and rotational kinetic energies
Based on the hourly wind dataset mentioned above, an iterative method (Endlich, 1967) with high computational efficiency and high accuracy is used to decompose the original horizontal wind (V) into the rotational wind (V R ) and the divergent wind (V R ): The bias for the divergence at each grid is less than or equal to 1 × 10 −8 s −1 during the iterative process, which is less than or equal to 0.1% of the maximum divergence of the original horizontal wind field. The formulas for divergent and rotational kinetic energies are as follows.
The kinetic energy per unit mass is expressed as: The kinetic energy of an atmospheric volume in isobaric coordinates (A is the horizontal computational area) is given by: where K is the kinetic energy in a limited area (hereafter referred to as kinetic energy), K R is the rotational kinetic energy, K D the divergent kinetic energy, and K RD is the kinetic energy of the interaction between the divergent wind and the rotational wind. Note that the sign of K RD depends on the divergent and rotational wind directions.
The equation for rotational kinetic energy (Buechler and Fuelberg, 1986) is expressed as follows.
Here, u R and u D are the zonal rotational and divergent wind components, v R and v D are the meridional rotational and divergent wind components, ω is the vertical velocity (Pa s -1 ), f is the Coriolis parameter, φ is the geopotential, and F is the frictional force.
For Eq. (8), the sum of Af, Az, B, and C is hereafter denoted as C(K D , K R ). Therefore, Eq. (8) can be simplified as DK R = I R + C(K D , K R ) + G R + HF R + F R . The term on the left-hand side, DK R, is the change term of K R and denotes the local change of K R . The term I R is the change of K R caused by the nonlinear interaction between the rotational wind and divergent wind. The term C(K D , K R ) is a conversion term between K D and K R , including four terms of Af, Az, B, and C, a C(K D , K R ) greater than zero indicates a conversion from K D to K R . The term Af is the geostrophic effect term. Both terms Af and Az are affected by relative orientations and magnitudes of V R and V D . Term B describes the vertical exchange of K R , while term C is related to the configuration of V D with V D and the vertical distribution of V D . Term G R is the generation term for K R , indicating the conversion between K R and the available potential energy due to the cross-contour flow of V R . The term HF R denotes the horizontal flux divergence of K by V R . The term F R represents friction and is related to the rotational wind, denoting frictional processes and the energy transfer between resolvable and unresolvable scales of motion. As it is calculated as the residual, possible errors from other terms are also included in Eq. (8).

Relationship of the ZSL intensity evolution with the divergent and rotational kinetic energies
Previous studies have revealed that the ZSL is mainly located below 400 hPa (Tao, 1984), indicating a relatively shallow system. Therefore, this study mainly targets levels below 400 hPa.

Distribution Characteristics of the divergent and rotational winds
Figures 2b-c illustrate that V at 500 hPa is well-decom-posed into V R and V D . The magnitude and distribution of V R are similar to those of V, while the magnitude of V D is obviously smaller than that of V (Fig. 2a). Moreover, the wind speeds of V R and V D tend to be small near the ZSL but are larger on the north and south sides, in the vicinity of ZSL, especially on the south side. A cyclonic circulation of V R and the convergence of V D appears near the ZSL. In addition, the circulation center of V R is located on the ZSL, but the strongest convergence appears on the south side of the ZSL. In summary, there are cyclonic V R and convergent V D near the ZSL, and the magnitude of V R is larger than that of V D .
3.2. Spatial evolution characteristics of the kinetic energy and its components Figure 3 shows that during the entire lifetime of the ZSL at 500 hPa, K, K R , K D, the absolute value of K RD are smaller near the ZSL but larger on its north and south sides, especially in the southeast quadrant of the ZSL. The distribution characteristics of these physical quantities depend on the distribution of the horizontal wind field, as shown in Fig. 2. Specifically, there is a dense zone of K contours in the southeast quadrant of the study area at the development stage, and the maximum K reaches 25 J m −2 at this stage (Fig. 3a). In the vigorous stage (Fig. 3b), K increases throughout the entire study area, and the maximum value increases to 35 J m −2 . In addition, the increase in the gradient of K in the southeast quadrant of the study area is especially significant. Figure 3c shows that K decreases throughout the entire study area in the decay stage, noting that the gradient of K in the southeast quadrant decreases significantly, with the maximum K dropping down to 30 J m −2 . The change of gradient of K near the ZSL is basically consistent with the ZSL intensity evolution, which agrees with the results in Luo and Li (2019a). The evolution of K R (Figs. 3a1-c1) and K D (Figs. 3a2-c2) near the ZSL are similar to that of K but numerically smaller. The K RD shows uneven distributions (Figs. 3a3-c3), with positive (negative) values in the western (eastern) part of the study area, but its absolute values also increase (decrease) with the development (decay) of the ZSL. The evolution trend of the kinetic energy and its components in the vertical direction is sim-

INTENSITY EVOLUTION OF ZONAL SHEAR LINE
VOLUME 39 ilar to that in the horizontal direction (figure omitted).

Response of the kinetic energy and its components to the evolution of the ZSL intensity
The above analysis is based on the mean ZSL averaged at each stage, through which how much time the response of K and its components (K R , K D , K RD ) to the evolution of the ZSL intensity takes cannot be determined. As mentioned above, the 500 hPa relative vorticity ζ represents the ZSL intensity. Therefore, the following section compares the regional-averaged hourly K, K R , K D , and K RD with ζ at 500 hPa to investigate the response relationship of K, K R , K D , and K RD with the evolution of the ZSL intensity. ζ As revealed by Deng et al. (2012), Fig. 4 also shows that the variations of K, K R , and ζ are generally synchronous, all with three obvious peaks. The peak values of K, K R, and in the three stages generally appear at 0000 LST of each day, with the maximums occurring at the vigorous stage, followed by the development stage. The minimums occur in the decay stage. The divergent kinetic energy K D also exhibits three peaks simultaneous to when ζ has the most significant positive change at each stage of the ZSL evolution, i.e., when the ZSL develops most rapidly at each stage. The peak value of K D is largest in the development stage, slightly smaller in the vigorous stage, and rapidly decreases in the decay stage. Note that the variation of K D occurs roughly three hours earlier than that of ζ. One possible reason why the kinetic energy of divergent wind increases earlier than that of rotational wind is that the upper-level divergence and the low-level convergence produce ascending motion, precipitation, and consequent latent heat release, which leads to strengthened rotation. The reason for the three-hour delay may be attributed to the accuracy of the ERA5 reanalysis dataset. In addition, the evolution of K RD is essentially consistent with that of ζ. However, due to the opposite signs of K RD in the eastern and western parts of the ZSL (figure omitted), the hourly evolutionary characteristics of the regional-averaged K RD are weaker than those of K, K R, and K D , as shown in Fig. 4.
In summary, the evolution of the ZSL intensity is related to K, K R , K D, and K RD . The ZSL intensity enhances (weakens) when K, K R , K D , and the absolute value of K RD increase (decrease), which leads to increased (decreased) precipitation. Particularly, the evolution of K and K R are basic-ally synchronous with the ZSL intensity, and the variation of K D is about three hours earlier than that of the ZSL intensity.

Evolutionary mechanisms of the ZSL intensity
It can be seen from the above analysis that the evolution of K and K R are basically synchronous with the ZSL intensity, and the variations in K D occur about three hours earlier than that of the ZSL intensity. Therefore, it is reasonable to explore the evolution mechanism of the ZSL intensity by discussing the evolution of K and its components.

Preliminary analysis of the effect of divergent and rotational winds on the kinetic energy
The preliminary analysis of the effects of V D and V R on K will be discussed through the relative contributions of K R , K D , and K RD to K, Table 2 reveals that K, K R , K D , and K RD increase (decrease) with the development (decay) of the ZSL, which agrees with previous conclusions. For the entire lifetime-averaged ZSL, K reaches 13.6 ×10 3 J m −2 , where K R makes the largest contribution, accounting for 79.1%, followed by K D (14.9%) and the smallest by K RD (only 5.9%). Noting that Table 2. Area-time averaged vertical integrals of K, K R , K D, and K RD from the surface up to 450 hPa (units: 10 3 J m −2 ).  ) averaged in the study area near the ZSL at 500 hPa. The black coordinate axis is for K, the purple coordinate axis for K D , the blue coordinate axis for K R , the green coordinate axis for K RD , and the red coordinate axis for ζ.
although the relative contributions of K R , K D , and K RD to K vary during different stages of the ZSL evolution, K R always contributes the most (above 76%) at each stage, followed by K D (more than 11%) and the smallest by K RD (about 6%). During the evolutionary processes of K, K R is much larger than K D and K RD , and it contributes the most to K, accounting for about 79%, followed by K D (about 15%) and the smallest by K RD (only about 6%). Therefore, K R plays a leading role in the evolutionary process of K, while the effects of K D and K RD are rather small.

Influence factors of the rotational kinetic energy
The rotational kinetic energy K R , which contributes the most to K, is far greater than K D and K RD . Besides, the intensity evolution trend of K and K R at 500 hPa is consistent with its evolution in the layer near the ZSL. Hence, the source of K R at 500 hPa is investigated to reveal factors that trigger the evolution of K, with the intent of exploring the evolutionary mechanism of the ZSL intensity.

Generation and transportation of the rotational kinetic energy
At 500 hPa, the term DK R in the development stage is only positive in the eastern part of the study area , and this positive-value area generally extends to the entire study area in the vigorous stage. In contrast, the entire study area is basically dominated by negative values in the decay stage. This indicates that the term K R increases (decreases) with the strengthening (weakening) of the ZSL intensity, consistent with the results discussed above. In Figs. 5a1-c1, the term G R is negative as the work done by the pressure gradient force consumes K R at 500 hPa around 85°-90°E near the ZSL (figure omitted). While in the eastern and western parts near the ZSL, the pressure gradient force does positive work to produce K R (figure omitted), which induces positive G R (Figs. 5a1-c1). For the term HF R , the southward V R on the south side of the ZSL transports K to the vicinity of the ZSL , mainly causing positive HF R near the ZSL (Figs. 5a3-a3). However, on the north side of the ZSL, the term HF R presents an alternative distribution of positive and negative values along the latitudinal direction. Furthermore, the values of G R and HF R basically increase (decrease) with the enhancement (decay) of the ZSL intensity. Therefore, from the development stage to the vigorous stage of the ZSL, the term G R in the eastern and western parts of the ZSL and the term HF R in most areas near the ZSL are conducive to the increase of K R , especially the K R to the south of the ZSL. However, the terms G R and HF R in the abovementioned regions remain positive from the vigorous stage to the decay stage, which is not favorable for the decrease of K R . Figure 6 shows that the term C(K D , K R ) is positive throughout the whole lifetime of the ZSL, with a similar hori-zontal distribution to those of K, K R , and K D at 500 hPa. In addition, the values of C(K D , K R ) are significantly larger than those of G R and HF R (Fig. 5), basically above 4 × 10 −4 W m −2 Pa −1 . This indicates a conversion from K D to K R throughout the whole lifetime of the ZSL, and the influence of C(K D , K R ) is more significant than those of G R and HF R on K R . At the development and vigorous stages, the term C(K D , K R ) near the ZSL is positive and increases with the development of the ZSL (same as Table 3), indicating that the term C(K D , K R ) is favorable for the increase of K R . However, the term C(K D , K R ) is still positive at the decay stage but smaller than that at the vigorous stage. In addition, the values of I R are relatively small (figure omitted). Combined with the distributions of G R and HF R (Fig. 5), it can be concluded that the decrease of K R is mainly caused by F R (figure omitted).

Conversion between the divergent and rotational kinetic energies
As shown in Table 3, during the evolution process of the ZSL, the four terms in C(K D , K R ) are all positive, and the geostrophic effect term Af always contributes the most to C(K D , K R ). For the lifetime-averaged ZSL, the contribution rate of Af to C(K D , K R ) reaches 59.6%, followed by B (26.4%), while the contributions of Az and C are relatively smaller, being 11.3% and 2.7%, respectively. In addition, the four terms Af, Az, B, and C also increase (decrease) with the strengthening (weakening) of the ZSL intensity. Therefore, the geostrophic effect term Af among the four terms in C(K D , K R ) affects K R the most.

Geostrophic effect
The following discussion will shed light on how Af works. The Coriolis parameter f does not change with time, so the evolution of Af is determined by the members in the term . It follows: and have different effects on Af, as shown below.
It can be seen from Fig. 7 that the values of (Figs. 7a1-c1) and  are basically positive near the ZSL, and the values of are greater than those of during the entire lifetime of the ZSL. In the development stage (Fig. 7a1), there is a dense zone of the values to the south of the study area, and the maximum reaches 10 m 2 s -2 . In the vigorous stage, the values of ( Fig. 7b1) increase rapidly, with the maximum reaching 20 m 2 s -2 . There is also a significant reduction of from the vigorous stage to the decay stage, whose magnitude is less than the observed increase from the development stage to the vigorous stage (Figs. 7b1-c1). For the values of , there is no obvious change from the development stage to the decay stage (Figs. 7a2-c2). Thus, the evolution of Af is essentially governed by the values of . The mechanism of shear line evolution is summarized as follows. In the vicinity of the ZSL, the terms of C(K D , K R ) near the ZSL, G R in the eastern and western parts, and HF R in most areas are all favorable factors for increasing K R . Specifically, the term C(K D , K R ) is far larger than G R and HF R and thus represents the dominant factor for increas-ing K R , while the decrease of K R is mainly caused by F R . The plausible reason for this is that energy transfer between resolvable and unresolvable scales of motion is possible because of the multi-scale ZSL in this study and that the friction near the ground is considerable because of the complex topography of the TP. Furthermore, the most important part of the conversion term is the geostrophic effect  Fig. 3, but for (a-c) the local change of K R (DK R ), (d-f) the generation term of K R (G R ), and (g-i) the horizontal flux divergence of K by V R (HF R ) (contours; units: 10 −4 W m −2 Pa −1 ). u R v D term Af, and the joint action of and determines the evolution of Af.

Conclusions and discussion
By using the ERA5 reanalysis dataset and the daily precipitation data from the daily meteorological dataset of basic meteorological elements of China National Surface Weather Station (version 3.0) from June to August during 1980-2019, 11 cases of ZSLs that cause heavy precipitation and have a lifetime of more than 60 hours in a high-frequency region for ZSL occurrence (32°-35°N) are selected for composite analysis. By decomposing K into the terms of K D , K R , and K RD , the relationship of K D , K R , and K RD with the evolution of the ZSL intensity is investigated. In addition, the evolution mechanisms of the ZSL intensity are also preliminarily explored from the perspective of K D and K R . The main conclusions are as follows.
(1) There are cyclonic V R and convergent V D near the ZSL, and the magnitude of V R is generally larger than that of V D .
(2) The ZSL intensity evolution is basically synchronous with that of K, K R., and K RD but lags behind that of K D by about three hours. Therefore, the precipitation caused by ZSLs could be predicted by monitoring the evolution of K D .
(3) During the processes of the ZSL intensity evolution, K R contributes the most to K, accounting for about 79%, while the effects of K D and K RD to K are rather small.
(4) The rotational kinetic energy budget shows that the increase in K R is governed by the conversion from K D to K R . Furthermore, the most important part of the conversion term is the geostrophic effect term Af, and the evolution of Af is determined by the joint action of the zonal rotational and meridional divergent wind components. Therefore, the enhancement of the ZSL intensity is mainly controlled by the joint increase of the zonal rotational and meridional divergent wind components. In contrast, the attenuation of the ZSL intensity is primarily attributed to friction and transfers of energy between resolvable and unresolvable scales of motion.
In this research, from a large sample of ZSLs over the TP during a long summertime period, 11 typical cases of ZSLs that cause heavy precipitation and maintain for a long time are systematically selected for composite analysis, aiming to overcome the limitations in analysis with individual case and thus get more representative results. From the per- Table 3. Same as Table 2, but for C(K D , K R ) (units: 10 3 J m −2 ).  6. Same as Fig. 3, but for conversion term between K D and K R (C(K D , K R )) (contours; units: 10 −4 W m −2 Pa −1 ).
spective of the divergent and rotational kinetic energies (K D and K R ), the relationship of K D and K R with the evolution of the ZSL intensity is obtained, which further improves the understanding of the ZSL evolution. In addition, during the evolutionary process of ZSLs, the source of K D and the kinetic energy balance between K R and K D require further study. It should be noted that the results in this study are based on the composite analysis of 11 cases of ZSLs of a specific type, and the evolutionary mechanisms of other types of zonal shear lines over the TP need to be further explored.