The kinetic energy variations of mean flow and turbulence at three levels in the surface layer were calculated by using eddy covariance data from observations at Jinta oasis in 2005 summer.It is found that when the mean horizontal flow was stronger,the turbulent kinetic energy was increased at all levels,as well as the downward mean wind at the middle level.Since the mean vertical flow on the top and bottom were both negligible at that time,there was a secondary circulation with convergence in the upper half and divergence in the lower half of the column.After consideration of energy conversion,it was found that the interaction between turbulence and the secondary circulation caused the intensification of each other.The interaction reflected positive feedback between turbulence and the vertical shear of the mean flow.Turbulent sensible and latent heat flux anomaly were also analyzed.The results show that in both daytime and at night,when the surface layer turbulence was intensified as a result of strengthened mean flow,the sensible heat flux was decreased while the latent heat flux was increased.Both anomalous fluxes contributed to the cold island effect and the moisture island effect of the oasis.
Using the monthly mean NCEP/NCAR reanalysis data and the monthly rainfall observations at 160 rain gauge stations of China during 1961-1999,and based on major characteristics of the atmospheric circulation over East Asia and the western Pacific,a simple index for the East Asian subtropical summer monsoon (EASSM) is defined.The relationship between this index and summer rainfall in China and associated circulation features are examined.A comparison is made between this index and other monsoon indices. The results indicate that the index defined herein is reflective of variations of both the thermal low pressure centered in Siberia and the subtropical ridge over the western Pacific.It epitomizes the intensity of the EASSM and the variability of summer rainfall along the Yangtze River.Analysis shows that the Siberian low has a greater effect on the rainfall than the subtropical ridge,suggesting that the summer rainfall variability over the eastern parts of China is to a large extent affected by anomalies of the atmospheric circulation and cold air development in the midlatitudes.Taking into account of the effects of both the Siberian low and the subtropical ridge can better capture the summer rainfall anomalies of China. The index exhibits interannual and decadal variabilities,with high-index values occurring mainly in the 1960s and 1970s and low-index values in the 1980s and 1990s.When the EASSM index is low,the Siberian low and the subtropical ridge are weaker,and northerly wind anomalies appear at low levels over the midlatitudes and subtropics of East Asia,whereas southwesterly wind anomalies dominate in the upper troposphere over the tropics and subtropics of Asia and the western Pacific.The northerly wind anomalies bring about frequent cold air disturbances from the midlatitudes of East Asia,strengthening the convergence and ascending motions along the Meiyu front,and result in an increase of summer rainfall over the Yangtze River.
本文以大气边界层物理、大气动力学、土壤物理、水文学和生物物理等理论为基础,研发了一系列适用于不同时空尺度的大气边界层与陆面物理过程模式:(1)基于地表能量和水分平衡方程,构建了新一代北京大学陆面物理过程模式(Land Surface Physics Process Model of Peking University,LSMPKU);(2)基于城市街谷冠层结构以及相应物理过程,构建了北京大学单层城市冠层模式(Modified Single Layer Urban Canopy Model of Peking University,UCMPKU);(3)基于计算流体力学理论,构建了北京大学街区尺度模式(Block Scale Model of Peking University,BSMPKU).通过模拟研究表明:上述模式在各自的时空适用尺度上均能很好地模拟出相应尺度上的主要大气物理过程,在与其他同类模式的对比实验中表现出明显的改进和提高.
