**1. Introduction**

When steady wind flows around an isolated obstacle, such as a mountain or a mountainous island, atmospheric vortex streets (AVSs) can be generated on the leeward side of the obstacle under favorable meteorological conditions. The AVS pattern exhibits a double row of counter-rotating vortex pairs shedding alternately and resembles the classic von Kármán vortex street [1–3], as schematically shown in Figure 1. Vortex streets have been frequently observed in the atmosphere [4,5] and ocean [6–11]. These types of vortex streets have significant weather and climate implications. Oceanic vortex trains could enhance biological production and turbulent mixing, impacting fishing activities [12–15]. The atmospheric vortex streets may modulate the cloud and wind patterns over the downstream region [16].

The studies of AVS can be traced back to as early as the 1930s. Lettau [17] suggested that AVSs could be shed by large islands. However, it was not until the early 1960s that researchers observed AVS in cloud images taken by the first generation of earth-orbiting satellites (e.g., [18–20]). These studies also revealed the properties of AVS, such as the rate of vortex shedding eddy lifetime, eddy viscosity, and obstacle drag, from satellite

**Citation:** Liu, Q.; Wu, Z.; Tan, Z.-M.; Yang, F.; Fu, C. The Atmospheric Vortex Streets and Their Impact on Precipitation in the Wake of the Tibetan Plateau. *Atmosphere* **2023**, *14*, 1096. https://doi.org/10.3390/ atmos14071096

Academic Editor: Yoshihiro Tomikawa

Received: 29 May 2023 Revised: 26 June 2023 Accepted: 28 June 2023 Published: 30 June 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/).

imagery and suggested that the AVS on the lee side of obstacles can be interpreted as the atmospheric analog of classic von Kármán vortex streets [18,20–26]. Further studies analyzed the mechanisms and meteorological conditions under which a stable vortex on the lee side of an obstacle can develop, e.g., Etling [27] conducted laboratory experiments and found that a stable vortex street on the leeward side of an obstacle can exist when the Reynolds number (Re) is larger than a particular value and the Froude number is smaller than 0.4.

**Figure 1.** Schematic plot of a Kármán vortex street generated by wind passing a cylindrical obstacle of diameter *d*. *a* is the vortex spacing and *h* is the width of the Kármán vortex street.

In addition to observational and laboratory studies, numerical studies have been carried out to reveal more details of the formation of a vortex pair on the leeward side of the island and its separation from the island. These studies are documented in the review paper of Young and Zawislak [7]. To date, most numerical simulations of Kármán vortex shedding in the real atmosphere have focused on islands in the Northeast Atlantic [5,28–30] or in the Northwest Pacific [31]. Based on a numerical simulation model with a horizontal resolution of 2 km, Nunalee and Basu [5] revealed that the whole region in the satellite image with a cellular stratocumulus cloud pattern was disturbed by Kármán vortex shedding. Li et al. [31] simulated the observed AVS using an MM5 model and estimated a vortex-shedding rate of 1 h.

It is noted here that previous observations of AVS, including those mentioned above, focused on AVS at a spatial scale of approximately a few hundred kilometers that can be captured in a non-merged satellite image. For AVS of these spatial scales, the Reynolds number, the ratio of inertial forces to viscous forces within a fluid, is usually between 50 and 500 [25]. From a theoretical perspective, given that the Reynolds number is proportional to the spatial size of the obstacle, a stable vortex street can exist when the Reynolds number is as high as 105 [25,32], as demonstrated in laboratory experiments. However, there have been few observational studies on that scale. It is indeed this hidden possibility that is one of the reasons for us to explore whether the Tibetan Plateau, standing in strong seasonally varying westerlies, can cause AVS of similar spatial scales on its leeward side.

The Tibetan Plateau, located over South-Central Asia, is the world's highest plateau above sea level, with an average elevation of approximately 4500 m (approximately onethird of the tropospheric height). Prevailing year-round lower tropospheric westerlies flow over or flow around the Tibetan Plateau and are divided into two branches after passing by the Tibetan Plateau [33,34], with the portion of flow around being dominant in the low- to mid-tropospheric region [35–37]. Yeh noticed, as early as the 1950s, that a pair of vortices appeared frequently on the east sides of the Tibetan Plateau [33]. The southern vortex is cyclonic and associated with low surface pressure and is termed the southwest China vortex [38]. The downstream propagation of southwest China vortices can result in substantial precipitation in downstream regions [39,40]. The northern vortex is anticyclonic and called the little northwest high [41]. However, whether the downstream mesoscale vortices are indeed AVS and how they are directly tied to seasonally varying westerlies have not received much attention from researchers.

This study aims to answer the following questions by analyzing high-resolution reanalysis data: (1) Can the mesoscale systems on the leeward side of the Tibetan Plateau be interpreted as the atmospheric analog of classic von Kármán vortex streets? Do the surrounding meteorological factors and properties of the AVS satisfy conditions in which a stable vortex on the lee side of an obstacle can exist? (2) If so, does the AVS on the lee side of the Tibetan Plateau impact the precipitation and heavy rain days over the wake of the Tibetan Plateau?

The paper is organized as follows: Section 2 describes the reanalysis datasets and precipitation data used. Section 3.1 gives an overview of the seasonal variability of the meteorological situation of the Tibetan Plateau and demonstrates whether the meteorological factors satisfy the conditions under which a stable vortex AVS can exist. Section 3.2 demonstrates the similarity between the AVS on the lee side of the Tibetan Plateau and the vortex street recorded in the laboratory experiment. Section 3.3 calculates some AVS properties, such as the aspect ratio and the Strouhal number identified directly from the reanalysis datasets. Section 3.4 reveals the impact of the AVS on precipitation over the wake of the Tibetan Plateau. Finally, Section 4 summarizes our results and discusses the relationship between AVS and the meteorological systems controlling precipitation over the wake of the Tibetan Plateau.
