**4. Discussion**

The significant seasonal variations in DSDs in Mêdog could provide a better understanding of the microphysical process of precipitation at the entrance of the vapor channel in the YGC and improve the parameterization schemes in numerical models over the TP. The possible causative mechanisms of the distinct DSD variability over seasons may be addressed from the standpoint of the meteorological environments of rainfall [13]. To explore the possible causes of seasonal variations in the DSD in Mêdog, meteorological conditions of rainy days from ERA5 reanalysis data, AWS and TBB and CTH products of the FY-4A satellite were collected. The lifting condensation level (LCL), 0 ◦C isotherm layer height, CTH, TBB probability density function, surface wind speed box diagram and the vertical integral of water vapor flux of rainy days in the four seasons are shown in Figures 13 and 14.

Due to the lack of radiosonde and ceilometer observations in Mêdog, the LCLs calculated from AWS data were approximately considered as cloud base height (CBH). The average LCL was calculated using the surface temperature ( *T*), surface dew point temperature ( *T*d) and surface pressure (*p*) according to the empirical formula (Equations (21)–(23)) given by Barnes [47]:

$$T\_{\rm LCL} = T\_d - \left(0.001296 T\_d + 0.1963\right)(T - T\_d)\_\prime \tag{21}$$

$$p\_{\rm LCL} = p[(T\_{\rm LCL} + 273.15)/(T + 273.15)]^{\frac{7}{2}},\tag{22}$$

$$\text{LCL} = 18,400(1+\text{at})\log\left(\frac{p}{p\_{\text{LCL}}}\right),\tag{23}$$

*T*LCL and *p*LCL indicate the temperature and pressure at LCL height, respectively. a = 1/273, *t* = *T*LCL-*T* (unit: ◦C). The calculated average LCL heights were 0.12 km, 0.13 km, 0.16 km and 0.20 km in the winter, premonsoon, monsoon and postmonsoon periods, respectively, exhibiting a negligible difference in the four seasons. The average heights of the 0 ◦C isotherm layer from ERA5 in the winter, premonsoon, monsoon and postmonsoon

periods were 1.53 km, 2.67 km, 4.01 km and 2.81 km, respectively, and the average CTHs were 5.13 km, 6.64 km, 6.97 km and 5.39 km, respectively.

**Figure 13.** The lifting condensation level (LCL), 0 ◦C isotherm layer height, cloud top height (CTH) and standard deviation (**a**). TBB probability density function (**b**). The dashed line in Figure 13b denotes the TBB temperature of −32 ◦C.

**Figure 14.** Surface wind speed box diagram (**a**) and the vertical integral of water vapor flux (**b**) in different seasons.

Clouds between LCL and the 0 ◦C isotherm layer level are defined as warm clouds, and those between the 0 ◦C isotherm layer level and CTH are considered cold clouds [18]. The cloud rain process is predominant during the winter precipitation period, which is evident from the significant cold cloud depth of 3.60 km compared to the relatively short warm cloud depth of 1.41 km. The microphysical and dynamic mechanisms (e.g., updraft, particle formation and particle growth processes) in the cold rain process are different from those in the warm rain process, leading to significant discrepancies in DSD characteristics [39]. Ice crystals grow quickly above the 0 ◦C isotherm level in the winter precipitation process. The higher concentration of large drops found in winter precipitation may be attributed to melted ice particles, such as low-density, large snow particles, and/or graupel (e.g., Figures 5 and 9a). In addition, wind and humidity are two important meteorological

elements affecting the evaporation process [48]. Stronger evaporation is expected in the winter season due to a larger wind speed and less water vapor (Figure 14), reducing the concentration of small raindrops (e.g., Figures 5 and 9a).

The premonsoon precipitation was characterized by a high concentration of large drops (e.g., Figures 4, 5 and 9). The average cold cloud depth (3.97 km) was much larger than the average warm cloud depth (2.54 km) in the premonsoon season, indicating that the cold rain process is also predominant in this period. The melted ice particles (e.g., graupel and/or snow particles) could result in the formation of larger drops [49]. Convective activity frequently occurs in the premonsoon season, as evidenced by the probability density function (PDF) of TBB (Figure 13b), which is often used to assess the intensity of convective activity [50]. The smaller the TBB value is, the deeper the development of convective clouds. The threshold of TBB ≤ −32 ◦C is often used to differentiate the development of convection. The probability of TBB ≤ −32 ◦C was the highest in the premonsoon season, indicating that intense convective activity occurs more frequently during this period. The westerly winds prevail over the TP during this period, and cold air masses can easily invade the middle to upper troposphere. In addition, solar radiation causes an increase in surface heating in the daytime. This destabilization of the troposphere would be beneficial to the formation of dry convection in the premonsoon season [51].

In addition, the largest surface wind speed (e.g., Figure 14a) among the four seasons may lead to relatively strong evaporation in the premonsoon season, which was partly responsible for the relatively low concentration of small drops. Thus, a higher concentration of larger raindrops and a lower concentration of small raindrops were observed for higher rainfall rate categories (e.g., *R* > 5 mm <sup>h</sup>−1) and convective rainfall types (e.g., Figures 5d–f and 9b). Therefore, the intensity increase in premonsoon precipitation was more attributed to the increase in drop diameter (e.g., Table 4).

During the monsoon season, although the CTH was highest, the average thickness of warm clouds (3.85 km) was significantly larger than that of cold clouds (2.96 km) (Figure 13a). Therefore, monsoon rainfall was dominated by warm rain processes, which tended to produce higher concentrations of small raindrops owing to collisional and coalescence processes (e.g., Figures 4, 5 and 9). A large amount of water vapor is carried to Mêdog by the Indian Ocean monsoon in this season (Figure 14b), which is conducive to the formation of warm clouds and the production of abundant small raindrops. Weak evaporation is expected in the monsoon season due to the smaller wind speed and wet environment, contributing to the production of small raindrops. The increase in precipitation intensity in the monsoon season may be mainly attributed to the significant increase in the concentration of raindrops (Table 4).

The postmonsoon precipitation had less rainfall total and was also characterized by a higher concentration of small drops (i.e., Figures 4 and 5b–d). Although the mean depth of warm clouds (2.61 km) was similar to that of cold clouds (2.58 km) in this season, Figure 14 exhibits humid and weak wind atmospheric conditions, which are favorable to the production of small drops.
