ABSTRACT
Poyang Lake (PL), the biggest freshwater lake in China, is situated in the East Asian Monsoon region, and has an important impact on local convection. In general, PL can result in convection in local region when it is a heat source in nighttime of warm season. However, at around noon on 4 May 2020 (PL is a cold source), a convection was triggered about 20 km west of PL, and rapidly enhanced and resulted in lightning when approaching PL, and then quickly weakened and disappeared after entering the main body of PL. In order to explore the convection formation, several observational data and the Weather Research and Forecasting model were applied in this study. Results show that when the convection approaches PL, its rapid enhancement is induced by PL, and after entering the main body of PL, its quick weakening is also resulted from PL. However, the initiation of the convection is mainly induced by the local topography west of PL under a favorable large-scale background. Mechanism analysis indicates that the strong low-level convergence near the west shore of PL associated with lake-land breeze is responsible for the rapid enhancement of the convection, and the low-level divergence over the main body of PL associated with the lake-land breeze and the increase of the low-level stability induced by cooling of PL jointly result in the quick weakening of the convection. The prevailing southerly wind in low level passes through the local topography (Meiling Mountain) west of PL, and is divided into southwesterly wind (flow around Meiling) and southerly wind (flow over Meiling), and they converge in the north of Meiling, triggering the convection. This study is not only important to deepen the understanding of PL affecting regional weather, but also helpful for improving the refined forecasting of convection near PL.
ABSTRACT
Based on the hourly gauge-satellite merged precipitation data with the spatial resolution of 0.1° × 0.1° during 2008-2016, the characteristics of extreme precipitation (EP) diurnal cycle along the Yangtze River Basin (YRB) and their regional and sub-seasonal differences during warm season have been indicated and revealed in this study. Results show that the EP amount (EPA) over most lower reaches of YRB exhibits two diurnal peaks with one in late afternoon and the other in morning, while the EPA over most eastern Tibetan Plateau (the Sichuan Basin and the northern Yunnan-Guizhou Plateau) generally peaks during late afternoon to midnight (midnight to early morning). The afternoon (morning) EPA diurnal peaks over the areas east to 110°E is mainly resulted from the short (long) duration EP events. However, both the short and long duration EP events lead to the nocturnal diurnal peaks and eastward propagating features of EPA over the regions west to110°E. The EP events over the Sichuan Basin generally begin at midnight and mostly peak around 03:00-04:00 Beijing time, and they start earlier and end later with the duration time increased. However, the EP events with short (long) duration over the lower reaches of YRB frequently start and peak in afternoon (early morning) and typically end at around 18:00 (07:00-08:00) Beijing time, and they start later (earlier) and end later with the duration time increased. Meanwhile, the EP frequency (EPF) diurnal cycles over the lower reaches of YRB exhibit obvious sub-seasonal differences in warm season, which show only a morning peak in the pre-Meiyu period, two comparable peaks with one in afternoon and the other in morning during the Meiyu period, and a predominant afternoon peak and a secondary morning peak in the post-Meiyu period, respectively. While the EPF over Sichuan Basin characterized by only one dominant early morning peak during all periods of the warm season exhibits much smaller sub-seasonal differences in the diurnal phase relative to that over the lower reaches of YRB.
ABSTRACT
The ability of FLake, WRF-Lake, and CoLM-Lake models in simulating the thermal features of Lake Nam Co in Central Tibetan Plateau has been evaluated in this study. All the three models with default settings exhibited distinct errors in the simulated vertical temperature profile. Then model calibration was conducted by adjusting three (four) key parameters within FLake and CoLM-Lake (WRF-Lake) in a series of sensitive experiments. Results showed that each model's performance is sensitive to the key parameters and becomes much better when adjusting all the key parameters relative to tuning single parameter. Overall, setting the temperature of maximum water density to 1.1 °C instead of 4 °C in the three models consistently leads to improved vertical thermal structure simulation during cold seasons; reducing the light extinction coefficient in FLake results in much deeper mixed layer and warmer thermocline during warm seasons in better agreement with the observation. The vertical thermal structure can be clearly improved by decreasing the light extinction coefficient and increasing the turbulent mixing in WRF-Lake and CoLM-Lake during warm seasons. Meanwhile, the modeled water temperature profile in warm seasons can be significantly improved by further replacing the constant surface roughness lengths by a parameterized scheme in WRF-Lake. Further intercomparison indicates that among the three calibrated models, FLake (WRF-Lake) performs the best to simulate the temporal evolution and intensity of temperature in the layers shallower (deeper) than 10 m, while WRF-Lake is the best at simulating the amplitude and pattern of the temperature variability at all depths.
