**1. Introduction**

The cutoff low over the northeastern China–Siberian section of the northwestern Pacific coast is usually called the northeastern China cold vortex (NCCV) [1]. At present, the widely used definition of an NCCV is as follows: (1) At 500 hPa, there is a low-pressure system with an evident cold trough or a cold core, and at least one closed geopotential height contour (4 dagpm interval). (2) The system appears in the region (35–60◦N, 115–145◦E). (3) The system in the defined area must last for at least three days [2–4]. The NCCV can be accompanied by strong convective weather such as rainstorms, tornadoes, and hail during its formation, development, persistence, and dissipation, which brings economic loss and human casualties to Northeast China. It is also worth noting that an NCCV can provide favorable conditions for the initiation of mesoscale convective systems [5], which cause asymmetries of precipitation in their interior. Obviously, it is particularly important and urgent to study the precipitation structures of the NCCV in the background of the frequent occurrence and serious impact of NCCVs on society in recent years.

During the past two decades, progress has been made in the study of macrocharacteristics and favorable conditions of precipitation inside the NCCV [6–13]. The NCCV

**Citation:** Wang, J.; Zhuge, X.; Chen, F.; Chen, X.; Wang, Y. A Preliminary Analysis of Typical Structures and Microphysical Characteristics of Precipitation in Northeastern China Cold Vortexes. *Remote Sens.* **2023**, *15*, 3399. https://doi.org/ 10.3390/rs15133399

Academic Editor: Seon Ki Park

Received: 10 May 2023 Revised: 28 June 2023 Accepted: 30 June 2023 Published: 4 July 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

precipitation has obvious asymmetry distribution, i.e., the east side of the NCCV is the main area of precipitation [6,7], and the mesoscale weather generally occurs in the southern half of the NCCV [8]. During different life stages of the NCCV, the precipitation characteristics are different. Rainstorms generally occur in the development stage of the NCCV [9]. At this stage, the cloud system at its head develops vigorously [10], and most of these systems are characterized by large-scale mixed precipitation on the south side of the NCCV. At the mature stage, the precipitation in the NCCV is mostly isolated convective precipitation [11], and the precipitation center is located closer to the NCCV center [12]. In the dissipation stage of the NCCV, the comma-shaped cloud system is not obvious, and the main form of precipitation is isolated convective precipitation [10,11], with less frequent heavy rainfall events [9]. The variations of environmental characteristics in the NCCV are responsible for the different precipitation distributions. The positive vorticity advection is located in the east side of the NCCV, and its forcing produces an updraft, which is favorable for precipitation formation [13]. The convergence of water vapor occurs in the head and tail of the NCCV comma-shaped cloud system, associated with the low-level jet and high-energy wet tongue in the tail of the NCCV, making the convection develop violently [10]. During the development period of the NCCV, the main factors affecting the precipitation area are convective available potential energy and water vapor flux. During the mature period, the convection is basically in the area with high convective available potential energy and humidity. During the dissipation period, the convective precipitation is also related to the low-level convergence line [11]. These previous studies are mostly limited to individual cases or typical periods such as summer and flood seasons. The comprehensive statistical analysis from multiple factors for different rain types in the NCCV is relatively rare.

It is worth noting that the microphysical structures and processes inside the NCCV play an important role on precipitation and clouds [14]. Studying the microphysical characteristics of the NCCV's precipitation is crucial for understanding the NCCV's structures, providing the reference for cloud and precipitation modules in NCCV modeling, and improving the accuracy of numerical predictions. Using observations from an aircraft, Qi et al. [15] found that there was a high-concentration area of ice particles in the upper part of the convective cloud band, and that ice particles increased rapidly in the areas with high supercooled water content, which plays an important role on precipitation. Zhao and Lei [16] studied the precipitation microphysics of the NCCV from the late-mature stage to the dissipation stage using aircraft observations. They found that the particle concentration in the warm layer of the clouds was larger, while in the supercooled water layer, the particle concentration was much smaller. The high concentration of ice particles in the layer between −3 and −6 ◦C is caused by the Hallett–Mossop ice-crystal multiplication process. Zhong et al. [17] analyzed an NCCV in July based on CloudSat satellite data. In the development stage of the NCCV, the convective clouds in the warm front on the east side of the NCCV were mainly composed of ice water, corresponding to the strong echo band, and the liquid water was mainly in the southeast quadrant of the NCCV. In the mature stage, the convective systems with more ice water content were mostly located on the north side of the NCCV, and the liquid water was mainly distributed below the 0 ◦C layer of the NCCV center. At present, due to the high cost of aircraft observation and the incomplete information of cloud and precipitation obtained by ground-based radar [18], our understanding of the precipitation structure and microphysical characteristics in NCCVs is still insufficient. Hence, it is necessary to introduce more refined data to study the internal microphysical structure of the NCCV systematically and comprehensively.

The Global Precipitation Measurement core observatory (GPM) satellite can detect precipitation activity in the range of 65◦S–65◦N, which provides us with an excellent opportunity to study the precipitation characteristics of the NCCV in mid- and highlatitude regions [19]. The dual-frequency precipitation radar (DPR) onboard the GPM satellite has the ability to detect microphysical characteristics, which have been widely used in the study of tropical and extratropical cyclones [20–24]. Therefore, the three-dimensional

structures and microphysical properties and process of precipitation in NCCVs will be analyzed using GPM DPR from 2014 to 2019 in the present study.
