The variations of the marine atmospheric boundary layer (MABL) associated with the South China Sea Summer Monsoon were examined using the Global Positioning System (GPS) sounding datasets obtained four times daily dur...The variations of the marine atmospheric boundary layer (MABL) associated with the South China Sea Summer Monsoon were examined using the Global Positioning System (GPS) sounding datasets obtained four times daily during May-June 1998 on board Research Vessels Kexue 1 and Shiyan 3. The MABL height is defined as the height at the lowest level where virtual potential temperature increases by 1 K from the surface. The results indicate that the MABL height decreased over the northern South China Sea (SCS) and remained the same over the southern SCS, as sea surface temperature (SST) fell for the northern and rose for the southern SCS after the monsoon onset. Over the northern SCS, a decrease in both the SST and the surface latent-heat flux after the onset resulted in a reduction of the MABL height as well as a decoupling of MABL from clouds. It was found that MABL height reduction corresponded to rainfall occurrence. Over the southern SCS, a probable reason for the constant increase of SST and surface heat flux was the rainfall and internal atmospheric dynamics.展开更多
文摘模块化多电平换流器(modular multilevel converter,MMC)的小信号模型是分析基于MMC的高压直流输电系统谐振问题的有效手段。MMC在稳态运行时桥臂电流和电容电压中含有各频次谐波分量,具有典型的多频率响应特征,传统建模方法难以反映其复杂的多谐波耦合特性。本文通过谐波状态空间(harmonic state space,HSS)法建立了MMC的频域小信号阻抗模型,建模过程考虑了主电路参数、控制器参数、环流抑制器及延时环节的影响,能够描述MMC内部的多谐波耦合特性,且模型阶数较低,易于编程实现。然后,在此基础上利用交流电网的等效阻抗与换流器等值阻抗特性曲线对交直流系统的宽频振荡机理和系统稳定性进行了分析,并提出了交直流系统稳定性改善的措施。最后,基于MATLAB/Simulink中的电磁暂态仿真模型对所建立的小信号阻抗模型和交直流系统稳定性分析进行了验证。
基金supported by the Chinese Academy of Sciences (Grant No. KZCX1-YW-12-01)the National Natural Science Foundation of China (Grant Nos. U0733002 and 40876009)The National Basic Research Program of China (Grant No. 2011CB403504)
文摘The variations of the marine atmospheric boundary layer (MABL) associated with the South China Sea Summer Monsoon were examined using the Global Positioning System (GPS) sounding datasets obtained four times daily during May-June 1998 on board Research Vessels Kexue 1 and Shiyan 3. The MABL height is defined as the height at the lowest level where virtual potential temperature increases by 1 K from the surface. The results indicate that the MABL height decreased over the northern South China Sea (SCS) and remained the same over the southern SCS, as sea surface temperature (SST) fell for the northern and rose for the southern SCS after the monsoon onset. Over the northern SCS, a decrease in both the SST and the surface latent-heat flux after the onset resulted in a reduction of the MABL height as well as a decoupling of MABL from clouds. It was found that MABL height reduction corresponded to rainfall occurrence. Over the southern SCS, a probable reason for the constant increase of SST and surface heat flux was the rainfall and internal atmospheric dynamics.
