**5. Conclusions**

In this paper, the seasonal variation in DSDs and microphysical parameters among the winter, premonsoon, monsoon, and postmonsoon periods were investigated using PARSIVEL disdrometer data from July 2019 to June 2020 in Mêdog, which is located in the southeast of the TP and at the entrance of the vapor channel in the YGC. In addition, EAR5 reanalysis data, FY-4A satellite products and AWS observations were used to address the possible causative factors for the distinct seasonal variation in DSDs. The conclusions of this study are outlined as follows:

(1) Precipitation mainly occurs during the monsoon period in Mêdog, contributing approximately 57% to the annual rainfall totals, and small drops are dominant in the four seasons. Weak rainfall (i.e., *R* < 1 mm <sup>h</sup>−1) with small drops (i.e., *D* m < 1 mm) and low concentrations (i.e., *N*T < 250 m<sup>−</sup>3) occurs frequently in the four seasons in Mêdog. However, taking the contributor to rainfall into account, drops with 1 ≤ *D*m < 2 mm are the largest contributor in the four seasons, and the weak rainfall with *R* < 1 mm h−<sup>1</sup> is the largest contributor in Mêdog except during the monsoon season, during which, rainfall with 2 ≤ *R* < 5 mm h−<sup>1</sup> is the largest contributor. For the average spectrum of the four seasons, the monsoon season precipitation has the narrowest spectrum width and is characterized by the highest (lowest) concentration of small (large) drops. The winter (postmonsoon) precipitation has the lowest (highest) concentration of small (large) drops. In terms of rain rate classes, a higher (lower) concentration of small (large) raindrops can be found in the monsoon season for all the considered rainfall rate classes in this study. More large drops and fewer small drops are observed in winter precipitation with *R* < 5 mm h−1. For heavy rainfall (i.e., *R* > 5 mm <sup>h</sup>−1), the premonsoon precipitation exhibits a higher concentration of large drops.

(2) Mêdog stratiform precipitation in the four seasons has a similar mean *D*m value of approximately 1.0 mm but exhibits a distinct difference in the mean value of log10(*N*w). Monsoon stratiform rain has the highest mean log10(*N*w) value of 3.75, followed by postmonsoon rain, and the winter season has the lowest mean log10(*N*w) value of 3.28. The convective rainfall during the monsoon season is characterized by the highest concentration of limited-size drops and is identified as maritime-like. Premonsoon convective rain has predominantly larger drops than other seasons. The largest mean *D*m (1.67 mm) and the lowest mean log10(*N*w) (3.70) are observed in the premonsoon convective rainfall, which could be considered a transition between maritime-like and continental-like conditions.

(3) The relationships of *μ*–Λ and *Z*–*R* corresponding to different seasons were also fitted. The *μ*–Λ relationships of the different periods show little discrepancy. The fitted *Z*–*R* relationships for stratiform precipitation exhibit little seasonal variation, and winter stratiform rain has a larger coefficient A and exponent *b*. The fitted *Z*–*R* relationships for convective precipitation show evident discrepancies among the premonsoon, monsoon, and postmonsoon periods. The *Z*–*R* relationship in monsoon convective rainfall has a smaller (larger) coefficient A (exponent b) than in other seasons, indicating a higher rain rate in monsoon convective precipitation for a given radar reflectivity. The empirical relationship of *Z* = 300*R*1.4 at midlatitudes would cause the severe underestimation of convective rain in Mêdog, especially during the monsoon period.

(4) The possible causative meteorological environments responsible for the seasonal variation in DSDs in Mêdog were discussed. Westerlies prevail over the whole TP in the premonsoon season, and rainfall is dominated by cold rain processes, resulting in the formation of large raindrops via the melting of frozen particles. In addition, less water vapor and a larger wind speed contribute to stronger evaporation, which probably leads to a lower concentration of small drops in the premonsoon precipitation. During the monsoon period, abundant warm and humid mass air intrudes from the Indian Ocean into Mêdog, and warm rain processes prevail in this period, producing many small raindrops via active collision and coalescence processes. Atmospheric conditions are characterized by humid and weak winds in the postmonsoon season, which is favorable to the production of small drops.

Notably, this work focused on the seasonal variation in DSD based on disdrometer data in Mêdog. The parameters of gamma distribution model of DSDs are trying to be used to improve the microphysical parameterization scheme of precipitation in the local numerical model. The detailed performance of the model will be evaluated later. Furthermore, disdrometer observations at more locations over the TP will be used to explore the temporal and spatial variation in DSDs. In addition, the vertical structure of DSDs in different seasons will be explored in future research by jointly using K-band Micro Rain Radar and X-band dual-polarization radar observations.

**Author Contributions:** Conceptualization, G.W. and L.L.; methodology, R.L. and G.W.; software, R.L., R.Z. and J.Z.; writing—original draft preparation, R.L.; writing—review and editing, R.L., G.W.; funding acquisition, G.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project was funded by the Second Tibetan Plateau Scientific Expedition and Research (STEP) program, gran<sup>t</sup> number 2019QZKK0105 and the National Key R & D Projects, gran<sup>t</sup> number 2018YFC1505702.

**Acknowledgments:** We thank Suolang Zhaxi and Ting Wang for their help in the maintenance of the disdrometer at Mêdog National Climate Observatory.

**Conflicts of Interest:** The authors declare no conflict of interest.
