**1. Introduction**

As the "Third Pole of the Earth" and the "atmospheric water tower", the Tibetan Plateau (TP) has an area of about 2.5 million square kilometers and an average altitude greater than 4000 m. The strong solar radiation on this elevated surface makes the plateau a heating source for the atmosphere in the middle troposphere. This surface heating not only impacts the local weather and synoptic situations over the plateau, but also affects the climate and environment of East Asia and the Northern Hemisphere [1–4]. For example, Yanai et al. revealed that Tibetan surface heating has a grea<sup>t</sup> influence on the onset of the South Asian summer monsoon [2], and Zhou et al. noted that the possible impacts of the plateau surface heating could be expanded to the Northern Atlantic Ocean [4]. Zou showed the role of surface heating in the formation of the Tibetan ozone low [5].

To understand the surface heating over the Tibetan Plateau, many scientific experiments have been carried out, in particular, the First and Second Tibetan Plateau Atmospheric Scientific Experiments. Using the observational data, Ye and Gao studied the

**Citation:** Li, H.; Zhou, L.; Wang, G. The Observed Impact of the South Asian Summer Monsoon on Land-Atmosphere Heat Transfers and Its Inhomogeneity over the Tibetan Plateau. *Remote Sens.* **2022**, *14*, 3236. https://doi.org/10.3390/ rs14133236

Academic Editors: Massimo Menenti, Yaoming Ma, Li Jia and Lei Zhong

Received: 9 May 2022 Accepted: 30 June 2022 Published: 5 July 2022

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land-air heat exchange and noted that sensible heat transfer dominates the surface heating over the plateau, especially in summer [1]. Gao et al., Bian et al., Li et al. and Zou et al. observed the land-air heat exchange at Nagqu (Central Tibet), Qamdo (East Tibet), Gerze (West Tibet), and the northern slope of Mt. Everest (South Tibet), respectively, in spring and early summer [6–9]. They showed that the total turbulent heat fluxes (defined as the sum of sensible and latent heat flux) over East, West, and Central Tibet were in the range of 80.0–144.0 Wm−2, with a higher sensible heat transfer in the range of 43.0–86.0 Wm−2, and a lower latent heat transfer with the range of 28.0–59.0 Wm−2. However, Zou et al. found that the near-surface heat transfer in the Southeast Tibet in early summer was significantly different from that in the other Tibetan regions [10], with a total heat flux of 86.3 Wm−2, a sensible heat flux of 22.9 Wm−<sup>2</sup> and a latent heat flux of 63.4 Wm−2. The latent heat transfer dominates the land-air heat exchange in Southeast Tibet.

The South Asian summer monsoon (SASM) is an important component of the Asian monsoon, which has a grea<sup>t</sup> influence on the atmospheric processes over Asia [11–15]. The SASM usually starts in late May or early June, which is characterized by the formation of cyclonic vortices in the Bay of Bengal or in the Southeast Arabian Sea [11,16,17]. After its onset, the SASM develops during the summer and autumn, with several active and break periods observed, which are characterized by the heavy and light rainfalls associated with the different monsoon troughs over South Asia [12,18–20]. The SASM decays in late September or early October. The SASM mainly affects the Indian Peninsula and Indo-China Peninsula, and the affected areas could extend northwards to the Qinghai-Tibet Plateau and Southwest China [13,16]. Gao et al. showed that the precipitation over Southeast Tibet can be affected by the monsoon [21]. Zhou et al. [17,22–25] and Li et al. [26] found that the local atmospheric properties in the Himalayas and Southeast Tibet region are closely related to the SASM evolutions. Most recently, Zou et al. [9,10] and Zhou et al. [27] revealed that the land-air heat transfer in the Himalayas and Yarlung Zangbo River Valley in Nyingchi is strongly affected by the SASM. Due to the lack of observation data, the above studies mainly focused on the analysis of a single site or a typical underlying surface, but there was a lack of studies on the influence of the South Asian summer monsoon on the land-air exchange at other different sites in the Tibetan Plateau.

The network of plateau observation stations is sparse; the representativeness of observation stations is limited by the complex topography and underlying surface characteristics. The study of land-air interaction under the complex terrain of the plateau is much more difficult than that in other areas, the observation time, space, and physical-property variables are very limited. Because of this limitation, the third Tibetan Plateau Experiment for atmospheric science (TIPEX III) has been organized by China Meteorological Administration since June 2014, with nine boundary-layer observation stations established in the central, western, and southeastern parts of the plateau [28]. The observation sites are more widely distributed, and the data are the latest and most comprehensive, which provides an important data basis for the study of land-air energy exchange over the Tibetan Plateau. With this observational data, Wang et al. analyzed the surface parameters and near-surface turbulent fluxes over TP [29]. Most previous studies on the SASM impacts are from one or two in-situ stations in the south or Southeast Tibet, while the SASM impacts may be the largest. In this study, a total of eight stations covering different regions of TP were applied, aiming to study the different phases of SASM (active/break periods) impacts on land-atmosphere heat transfer over different plateau regions. In addition, previous studies suggested the grea<sup>t</sup> impacts of SASM on the local TP heating, as well as the regional climate over the Southeast Tibet and South Tibet; however, whether these impacts extend northwards was not clear until now. Thus, one of our purposes was to understand the SASM-affected area and extension. In this paper, data and methods are introduced in Section 2, and the SASM evolution (transition of active and break phases), and its possible influences on the land-air heat transfers over different plateau regions are presented in Section 3. The discussion and conclusions are given in Sections 4 and 5, respectively.

### **2. Data and Methods**

The data used in this paper are from the third Tibetan Plateau (TP) Experiment for atmospheric science (TIPEX III) from late July to early September, 2014. During the experimental period, 9 observation stations were installed over the plateau regions. These stations are Ali, Nagqu, Amdo, Nyainrong, Biru, Baingoin, Lhari, Nyingchi, and Namco (see Figure 1 for the topography and Table 1 for the detailed station locations). At each station, the radiation fluxes (downward shortwave radiation flux and net radiation flux) were measured by a 4-component net radiometer (NR01, Hukseflux Thermal Sensors, Delftechpark, The Netherlands), and the land-atmosphere heat transfers (sensible and latent heat fluxes) were measured by a 3-D ultrasonic anemometer (CSAT3, Campbell Scientific, Inc., Logan, UT, USA). These raw data were calculated as the averaging interval of 30 min for analysis in this paper. In this paper, the total heat flux is defined as the sum of sensible and latent heat flux.

**Figure 1.** Topography of the Tibetan Plateau, with 9 plateau stations being denoted by red dots. It should be noted that the Nyingchi station was excluded from our study after the data quality control steps were completed.

**Table 1.** Station locations over the plateau regions.


EDDYPRO (version 5.1) software (from Li-COR Corporation) is also used for turbulent flux data quality control [29–31]. After quality control steps were performed, Nyingchi station was excluded from our study due to the missing data of more than 30%. For the following analysis, 29 July–26 August was selected as the observational period when data were available from all 8 stations.

In addition to the observational data, the large-scale reanalysis data from ECMWF (European Centre for Medium-Range Weather Forecasts) Interim were also used, including wind and specific humidity, with a horizontal resolution of 0.75◦ × 0.75◦. The interpolated outgoing long-wave radiation (OLR) data from NOAA (National Oceanic and Atmospheric Administration) were applied to illustrate the convection conditions, with a horizontal resolution of 1.0◦ × 1.0◦.
