Search

The evident nocturnal peak dominates the summer rainfall over the Sichuan Basin (SCB), which is closely related to the nocturnal intensification of low-level winds. Based on 21-year IMERG rainfall product and reanalysis data during summertime (June–August) from 2000 to 2020, the low-level winds are classified into four groups, with strong or weak daily mean wind accompanied by a large or small diurnal amplitude, to clarify their influences on rainfall over the SCB. The results show that under strong daily mean wind conditions, the westward extension of the Western Pacific Subtropical High (WPSH) determines the southwesterly monsoon airflow to be tuned to the southerly over the eastern Yunnan-Guizhou Plateau, which provides abundant warm and moist air resources for rainfall within the basin through the southeastern side of the SCB. Strong mean winds, coupled with a large diurnal amplitude due to the acceleration of easterly, strengthen the moisture convergence at night, and contribute to the rainfall increasing remarkable over the SCB with a peak at midnight. Meanwhile, there is an apparent anomalous low-level warming over the SCB, creating more unstable atmospheric conditions. In addition, the weak upward motion associated with the afternoon heating over the eastern slope of the Tibetan Plateau (TP) enhances the easterly and facilitates the development of upward motion over the western SCB in the early night, which is responsible for the larger diurnal amplitude of rainfall. On the contrary, rainfall is suppressed and the diurnal amplitude of rainfall is gentle under the condition of weak daily wind with small diurnal amplitude, due to the weak moisture transport and cooling planetary boundary layer related to the deepened midlatitude trough, as well as more active convection over the eastern slope of TP during the day. The results imply that atmospheric conditions associated with diurnal variation of low-level winds should be considered as a key component in regulating the rainfall and moisture budget over the SCB, the strength of low-level winds in the early evening may provide a predictive signal for the development of nocturnal rainfall over the SCB. [ABSTRACT FROM AUTHOR]

Spring precipitation over the southeastern Tibetan Plateau (SETP) produces more than 34% of annual precipitation, which is comparable to summer precipitation. This pre‐monsoon rainfall phenomenon, influenced synthetically by atmospheric circulations and topography, makes the SETP an exception to its surroundings. Here, fine‐scale characteristics and typical synoptic backgrounds of this unique phenomenon have been investigated. The spring precipitation over the SETP is characterized by high frequency at hourly scale, with a single diurnal peak at night or early morning. Event‐based analysis further demonstrates that the spring precipitation is dominated by long‐lasting nocturnal rainfall events. From early to late spring, the dominant synoptic factor evolves from terrain‐perpendicular low‐level winds to atmospheric moisture, influencing the spatial heterogeneity and fine characteristics of the spring precipitation. The westerly dominated type, featured by lower geopotential height over the TP and enhanced westerlies along the Himalayas, produces limited‐area precipitation at those stations located at topography perpendicular to low‐level winds. In contrast, the moisture‐dominated type is featured by an anomalous cyclone over the Bay of Bengal and induces widespread precipitation around the SETP, which is the leading contributor to the spring precipitation there. Accumulated precipitation amount of long‐lasting nocturnal events is the largest under moisture‐dominated type, which has a large portion of weak precipitation due to weak thermal instability. Findings revealed in this study complete the picture of spring precipitation influenced by different dominant synoptic factors over the SETP, which deepen the current understanding of the joint influence of circulation and topography on the hydrological cycle of complex terrains. Plain Language Summary: The precipitation over the Tibetan Plateau (TP) is generally considered to be concentrated in summer. However, the southeastern TP (SETP) is an exception, where the spring precipitation is larger or comparable compared to summer precipitation. In this study, we analyzed the fine‐scale characteristics and typical synoptic backgrounds of this unique phenomenon. The spring precipitation over the SETP is featured by frequent nighttime rainfall events lasting more than 3 hr. In early spring (March), low‐level winds uplifted by topography produce the precipitation over the SETP, while atmospheric water vapor plays a more important role in late spring (May). Atmospheric backgrounds and complex terrain jointly influence the fine‐scale characteristics and the range of the spring precipitation over the SETP. Our analysis provides a more completed picture of the spring precipitation over the SETP, which typically reflects the joint influence of atmospheric circulations and complex terrain. Key Points: Spring rainfall over the southeastern Tibetan Plateau (SETP) is characterized by frequent nighttime events of long durationDominant synoptic factors evolve from low‐level wind in early spring to moisture in late spring, continuously producing rainfall over SETPSynoptic factors and complex terrain jointly shape fine features and spatial heterogeneity of the spring rainfall over the SETP [ABSTRACT FROM AUTHOR]

Besides the respective importance of the frequency, intensity and diurnal cycle of precipitation, the diurnal cycle of intensity–frequency structure could reveal more physical details about the evolution process of precipitation systems. Here, an objective method to quantitatively depict the diurnal cycle of intensity–frequency structure of precipitation is proposed, and applied to complex terrains of the southeastern Tibetan Plateau (TP) and its surroundings. Results show the diurnal cycle of precipitation over the high terrain has a unimodal night peak, while low terrain with a frequency‐dominated night peak and an intensity‐dominated afternoon peak. Both weak and intense precipitation are more frequent at night than those in the afternoon over unimodal regions compared to bimodal regions. Further analysis reveals different diurnal evolution process of intensity–frequency structure within unimodal and bimodal regions, respectively, which reflects different spatiotemporal traits and evolution process of precipitation. Among unimodal regions, the elevated southeastern TP is featured by the synchronous decrease in both weak and intense precipitation from the night peak to the afternoon minimum. While the decrease of total precipitation in regions with lower terrain is dominated by the decrease of intense precipitation, but with an increase in weak precipitation simultaneously. Regarding bimodal regions, the intense precipitation over southern Hengduan Mountains has a more prominent role in leading the diurnal cycle of precipitation amount, compared with that over the southeastern Yungui Plateau. The proposed method provides a new perspective with more physical details to investigate precipitation characteristics in complex terrains, which could be further used to reasonably evaluate and improve the performance of numerical models. [ABSTRACT FROM AUTHOR]

Mesoscale convective systems (MCSs) downstream of the Tibetan Plateau (TP) exhibit unique precipitation features. These MCSs can have damaging impacts and there is a critical need for improving the representation of MCSs in numerical models. However, most global climate models are typically run at resolutions that are too coarse to reasonably resolve MCSs, and it is still unclear how well higher-resolution global models can reproduce the precipitation characteristics of MCSs. In this study, the sensitivity of MCSs simulated by a global high resolution (~ 10 km), atmosphere-only climate model to different treatments of convection (with and without parametrized convection, and a hybrid representation of convection) have been investigated. The results show that explicit convection (i.e., non-parameterized) can better reproduce the observed pattern of MCS precipitation over the East Asian Summer Monsoon region. In general, explicit convection better simulates the diurnal variability of MCSs over the eastern China, and is able to represent the distinctive diurnal variations of MCS precipitation over complex terrain particularly well, such as the eastern TP and the complex terrain of central-northern China. It is shown that explicit convection is better at simulating the timing of initiation and subsequent propagating features of the MCS, resulting in better diurnal variations and further a better spatial pattern of summer mean MCS precipitation. All three experiments simulate MCS rainfall areas which are notably smaller than those in observations, but with much stronger rainfall intensities, implying that these biases in simulated MCS morphological characteristics are not sensitive to the different treatment of convection. [ABSTRACT FROM AUTHOR]

High-resolution meteorological datasets are urgently needed for understanding the hydrological cycle of the Tibetan Plateau (TP), where ground-based meteorological stations are sparse. Rapid advances in remote sensing create possibilities to represent spatiotemporal properties of precipitation at a high resolution. In this study, the hourly precipitation characteristics over the TP from two gridded precipitation products, one from global reanalysis (the fifth generation of the European Center for Medium-Range Weather Forecasts atmospheric reanalysis of the global climate; ERA5) and the other is simulated by Global-to-Regional Integrated forecast SysTem (GRIST) global nonhydrostatic model, are compared against satellite-gauge merged precipitation analysis (China Merged Precipitation Analysis; CMPA) from 27 July to 31 August 2014, and a satellite-retrieved precipitation estimate from the Integrated Multi-satellitE Retrievals for the Global Precipitation Measurement (IMERG) is also evolved. Two aspects are mainly focused on: the spatial distribution and the elevation dependence of hourly precipitation characteristics (including precipitation amount, frequency, intensity, diurnal variations, and frequency–intensity structure). Results indicate that: (1) The precipitation amount, frequency, and intensity of CMPA and IMERG decrease with altitude in the Yarlung Tsangpo river valley (YTRV), but increase at first and then decrease with altitude (except for intensity) in the eastern periphery of TP (EPTP). ERA5 performed well on the variation of precipitation amount with altitude (especially in EPTP), but poorly on the frequency and intensity. GRIST is the antithesis of ERA5, but they all overestimate (underestimate) the frequency (intensity) at all heights; (2) With increasing altitude, the diurnal phase of precipitation of CMPA and IMERG shifted from night to evening in the two sub-regions. IMERG's diurnal phase is 1 to 3 h earlier than CMPA's, and the discrepancy decreases (increases) as the altitude increases in YTRV (EPTP). The diurnal phase of precipitation amount and frequency in ERA5 and GRIST is significantly earlier than CMPA, and the frequency peaks around midday except in the basin. GRIST's simulation of the diurnal variation in intensity at various altitudes is consistent with CMPA; (3) ERA5 and GRIST overestimate (underestimate) the frequency of weak (intense) precipitation, with ERA5's deviance being the most severe. The deviations increased with altitude. These findings provide intensive metrics to evaluate precipitation in complex terrain and are helpful for deepening the understanding of simulated biases for further improving performance in high-resolution simulation. [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) is known as Asian Water Tower and its atmospheric water cycle has been a lasting challenge to climate modeling community. Here, we compare two sets of the Met Office Unified Model simulations—one is a convection‐parameterized version (large‐scale model; LSM) and the other is a convection‐permitting model (CPM) simulation. The added value of the CPM in terms of atmospheric water cycle process is analyzed, including external moisture transport, fraction of atmospheric water vapor converting to precipitation and the precipitation recycle ratio. Results show that the simulated TP precipitation and evaporation for the summer of 2009 is significantly improved in the CPM. First, the overestimation of atmospheric water cycle by LSM is improved in CPM due to a reasonable representation of the fraction of atmospheric water vapor converting to precipitation. The overestimation of precipitation recycle ratio also indicates the LSM generates excessive convection compared to the CPM and therefore has a larger wet bias over the TP. Second, a better simulation of local precipitation has feedback on the circulation. Compared with the LSM, the less moisture convergence in the CPM is dominated by the stronger outflow through the eastern edge of the TP rather than the weaker inflow, implying the upscale effects of the resolved moist convection on the moisture transport over the TP. Our results imply that the CPM is a useful tool in the reproduction of moisture transport and atmospheric water cycle process over the Asian Water Tower and other regions of the world with complex topography. Plain Language Summary: The atmospheric water cycle over the Tibetan Plateau (TP, also as known as the Asian Water Tower) has been a challenge to climate models with convection parameterization. Benefiting from the high enough resolution, convection‐permitting model (CPM) can explicitly resolve the deep convection and therefore reduce the bias related with the convection parameterization scheme. Here, we compare a CPM with its low resolution version that employs convection‐parameterization in simulating atmospheric water cycle process over the TP. We find the precipitation and evaporation over the TP is significantly improved in the CPM. There is more water vapor flowing out of the TP and less water vapor in the atmosphere converting into the precipitation in the CPM, resulting in a reduced wet bias. CPM is free from the constrains of the convection parameterization scheme and therefore depicts the atmospheric water cycle over the TP better. Key Points: The convection‐permitting model (CPM) better reproduces precipitation over the Tibetan Plateau than convection‐parameterized model (large‐scale model [LSM])The overestimation of atmospheric water cycle by LSM is improved in CPM simulationThe interaction between large‐scale circulation and convection further reduces moisture convergence and wet bias in the CPM [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) plays an essential role in influencing the global climate, and precipitation is one of its most important water‐cycle components. However, accurately simulating precipitation over the TP is a long‐standing challenge. In this study, a convection‐permitting model (CPM; with 4 km grid spacing) that covers the entire TP was conducted and compared to two mesoscale models (MSMs; with model horizontal resolutions of 13 and 35 km) over the course of a summer. The results showed that the two MSMs have notable wet biases over the TP and can overestimate the summer precipitation by more than 4.0 mm·day−1 in some parts of the Three Rivers Source region. Moreover, both MSMs have more frequent light rainfall; increasing horizontal resolution of the MSMs alone does not reduce the excessive precipitation. Further investigation reveals that the MSMs have a spurious early‐afternoon rainfall peak, which can be linked to a strong dependence on convective available potential energy (CAPE) that dominates the wet biases. Herein, we highlight that the sensitivity of CAPE to surface temperatures may cause the MSMs to have a spurious hydrological response to surface warming. Users of climate projections should be aware of this potential model uncertainty when investigating future hydrological changes over the TP. In comparison, the CPM removes the spurious afternoon rainfall and thus significantly reduces the wet bias simulated by the MSMs. In addition, the CPM also better depicts the precipitation frequency and intensity, and is therefore a promising tool for dynamic downscaling over the TP. [ABSTRACT FROM AUTHOR]