**1. Introduction**

The Tibetan Plateau (TP) (26◦00–39◦47N, 73◦19–104◦47E) is often referred to as 'the roof of the world' owing to its average elevation exceeding 4000 m, or 'the Asian water tower' [1] because several of Asia's major rivers such as the Yangtze, Yellow, and Lancang originate from the region. The precipitation over the TP is important to these rivers, and the heat supply of the TP serves as an important energy source of the atmosphere [1–3]. The TP has also a profound impact on the precipitation over its surrounding and downstream areas [4–7]. The Sichuan Basin (SB) is a deep basin with an elevation on average 2500 m lower than that of the TP. Remote sensing and ground observations and modeling of the diurnal cycle of cloud and precipitation over the TP are important to understand the weather and climate processes over the TP and surrounding areas. Recently, the second (TIPEX2) and third (TIPEX3) Tibetan Plateau Atmospheric Experiments were carried out [8] and obtained large quantities of observational data from the TP to the SB, especially ground

**Citation:** Cao, B.; Yang, X.; Li, B.; Lu, Y.; Wen, J. Diurnal Variation in Cloud and Precipitation Characteristics in Summer over the Tibetan Plateau and Sichuan Basin. *Remote Sens.* **2022**, *14*, 2711. https://doi.org/10.3390/ rs14112711

Academic Editors: Jing Wei and Kai Qin

Received: 15 April 2022 Accepted: 30 May 2022 Published: 5 June 2022

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radar observations of cloud and precipitation. These observational data and analysis provide a basis for further studies on the mechanisms of cloud and precipitation and the improvement of parameterization schemes for cloud and precipitation physical processes.

According to ground- and satellite-based observations as well as simulation results in previous research, the total precipitation is <400 mm during the entire summer (June–August) over the TP, which is smaller than that over the surrounding areas [9]. Moreover, the monthly averaged precipitation rate is ~0.3 mm <sup>h</sup>−1; the precipitation rate in precipitation events is 1~20 mm <sup>h</sup>−1; the averaged daily precipitation is <15 mm over the TP [10,11]. The cloud top height usually exceeds 12 km above ground level (AGL), and sometimes exceeds 16 km AGL over the TP, mainly including mixed-phase and ice-phase processes, in which super-cold water may be contained [9–11]. The average cloud base height (CBH) during summer over the TP exceeds 1.5 km AGL, which is larger than that over the plains and SB.

The summer precipitation in China can be divided into two stages: the mei-yu period and the midsummer period [12–16]. Precipitation during summer shows an obvious diurnal cycle, peaking in the evening, with the greatest change occurring over the central TP [17–22]. The eastern foothills of the TP are dominated by nocturnal rainfall before midsummer [23]. The diurnal cycle of precipitation from the TP to downstream areas shows diurnal propagation during the pre-mei-yu period. However, this diurnal propagation from the TP to downstream areas disappears during midsummer. The zonal wind weakens from the pre-mei-yu period to midsummer. In addition, the precipitation over the valleys of the Himalaya mainly occurs from midnight to the sunrise [24]. High quantities of cloud cover occur mainly over the ridges and then move to the valleys. Convective activity mainly occurs at night over the valleys [10]. Turbulence and convective cloud over the TP develop more easily than in the surrounding areas because the air density is lower. A convective cloud develops after sunrise, reaching a maximum from late afternoon to early morning the following day, as does convective available potential energy (CAPE) [6,14]. The diurnal cycle in monsoonal flow, the sea–land breeze, boundary-layer flow, low-level jet, aerosols, and inertial oscillation in the mid-level horizontal wind field in the mid-troposphere (~500 hPa) are the key factors that influence the diurnal variation in precipitation over the EASM region [25–27].

Satellite precipitation observation can obtain the precipitation on a global scale, which is better than conventional measurements made by rain (and snow) gauges and surfacebased weather radar observations. Many advanced satellite algorithms have been released that make use of infrared and passive microwave data, for example, the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks-Climate Data Record (PERSIANN-CDR) [28], the Integrated Multi-Satellite Retrievals for GPM (IMERG) [29], the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) [30], the Climate Prediction Center Morphing technique (CMORPH) [31], and the Hydro-Estimator (HE) Satellite Rainfall Estimates [32]. Mountainous regions represent a major challenge for these satellite data products. Over highly complex topography such as the Andes area, HE provides the most stable result, which could be associated with the best performance of HE on the development of precipitation from warmer and relatively lower clouds [33]. The HE with orographic correction to some extent captures the spatial distribution and timing of diurnal convective events over a mountainous region [34]. In addition, spatial distribution in cloud optical thickness and the cloud water path derived from satellite retrievals over the TP were closely associated with the increase in water-vapor transport flux divergence [35].

However, there is little research on the differences in the diurnal cycle of precipitation between HE satellite products and ground observations datasets, as well as the differences between the TP and SB. Moreover, the macroscopic properties of clouds such as cloud cover and cloud liquid or ice water content (LWC and IWC, respectively) and their relationships with surface thermal effects have received minimal attention. HE satellite products do not provide cloud microphysical parameters, while reanalysis data can provide cloud microphysical parameters. Therefore, it is a good choice to combine reanalysis data and high-precision satellite data such as HE satellite products to analyze the distribution characteristics of cloud microphysical parameters over the TP and SB. Focusing on the diurnal cycle of cloud physical parameters, this paper seeks to answer the following questions on the basis of satellite, ground, and cloud radar observations and a reanalysis dataset: (1) What are the phase differences of water within clouds during the diurnal cycles over the TP and SB, and how do these change diurnally? (2) What are the possible mechanisms responsible for the phase differences of precipitation and cloud? Following this introduction and a description of the data (Section 2) and methods (Section 3) employed in this study, the diurnal variation in precipitation and cloud parameters (CBH AGL, cloud cover, cloud IWC and LWC) over the TP and SB using ECMWF Reanalysis v5 (ERA5) and the ground and HE are investigated in Section 4. Additionally, the relationships of the precipitation rate in HE, and cloud cover and cloud IWC and LWC from ERA5 with surface thermal effects are investigated. This research is important for addressing the bias of precipitation in the diurnal cycle during ground and satellite observations.
