**1. Introduction**

The Tibetan Plateau (TP) occupies about a quarter of the land area of China, with an average elevation of more than 4000 m. It is well known as "the roof of the world" and "the third pole of the Earth" [1]. Surface sensible heat flux (SH), as one of the significant parameters to characterize the strength of the interaction between surface and atmosphere, accelerates the updraft in spring and summer, which can directly act on the middle troposphere and modulate the atmospheric circulations to affect the Asian summer monsoon variabilities [2–5] and even the development of tropical ENSO and the air–sea interaction in the mid-latitude Pacific [6]. Duan et al. [7] have pointed out that the SH from April to June over the TP can be used as an effective predictor of precipitation in July in the Jianghuai valley of China. Moreover, the abnormal spatial distribution and temporal evolution of SH will also lead to more abnormal characteristics of climate in China [8,9]. Ma et al. [10] demonstrated that the SH over the TP shows obvious change rules in multiple time scales of interannual, interdecadal, seasonal, and diurnal variations, which further jointly act on the weather and climate in local and remote regions [11–14].

**Citation:** Zhu, Z.; Wang, M.; Wang, J.; Ma, X.; Luo, J.; Yao, X. Diurnal Variation Characteristics of the Surface Sensible Heat Flux over the Tibetan Plateau. *Atmosphere* **2023**, *14*, 128. https://doi.org/10.3390/ atmos14010128

Academic Editor: Jonathan E. Pleim

Received: 1 November 2022 Revised: 26 December 2022 Accepted: 3 January 2023 Published: 6 January 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/).

In terms of the diurnal variations, Ma et al. [15] suggested that the meadow surface over the northern TP was a strong heating source for the atmosphere in the day and a weak cold source at night. Liu et al. [16] noted that the exchange of the SH between land and atmosphere on the slopes of Mt. Everest mainly occurred in the afternoon. Duan et al. [14] revealed that the SH of dry and wet phases at Shiquanhe and Linzhi station changed with a single peak in the day, but the specific variation characteristics were different. As one of the vital parameters for calculating SH, the heat transfer coefficient CDH also has obvious diurnal variation because it is largely affected by the stability of the atmosphere. Li et al. [17] calculated the CDH at Gaize and Shiquanhe over the western TP by using the profile-flux method, and the CDH showed diurnal variation characteristics with different amplitudes and phases in winter and summer, respectively. Generally, the CDH is large and stable in the day, but small and fluctuating at night [18].

However, due to the lack of observational data over the TP, especially those with high temporal resolutions, more studies are still needed to investigate the diurnal variation of the SH over the TP. Furthermore, an accurate estimate of SH is always a challenge. In this study, based on a long-time dataset with a high temporal resolution (hourly) from 2006 to 2016, the diurnal variation characteristics of the observed SH over the TP are quantitatively analyzed. Furthermore, it is worth noting that numerous studies related to TP SH have been conducted, generally based on the calculated SH, in which the magnitude of the heat transfer coefficient CDH is vital and has not been determined. Wang et al. [19] pointed out that the parameterization of the regional CDH ranged from 2.5 × <sup>10</sup>−<sup>3</sup> to 5 × <sup>10</sup>−<sup>3</sup> over the TP based on the GIMMS-NDVI dataset. The CDH estimated by the CHEN-WONG scheme [20] is about 3.6 × <sup>10</sup>−3, depending on the averaged wind speed, which was applied to reveal the TP SH interannual variation and its response to climate change [12]. However, in the majority of previous studies [21–26], they basically recommended to take CDH as 4 × <sup>10</sup>−<sup>3</sup> when calculating the TP SH. Therefore, we will additionally discuss the differences between the calculated SH and the directly observed SH on a daily scale, and potentially provide a more suitable CDH for improving the accuracy of the SH calculations.

This paper is organized as follows: Section 2 presents the data and methods; Section 3 describes the annual and seasonal mean of the SH diurnal variations over the TP; Section 4 investigates the monthly changes of the SH diurnal variation over the TP; and Section 5 introduces the effect of the CDH on SH diurnal variation. A Conclusions and Discussion are provided in Section 6.

#### **2. Data and methods**
