边界层动力过程在Madden－Julian振荡发展中的作用

THE ROLES OF BOUNDARY-LAYER DYNAMICS ON THE DEVELOPMENT OF MADDEN-JULIAN OSCILLATION

利用包括边界层爱克曼辐合－降水加热过程在内的短期气候变化模式［１］，研究了Ｍａｄｄｅｎ－Ｊｕｌｉａｎ振荡［１１，１２］形成的机制。当经圈方向用抛物圆柱函数展开，并对赤道偶对称解取ｎ＝０，２，４三个截断模时，在长波近似下的自由波解，一为向东的Ｋｅｌｖｉｎ波，另外两支为向西的Ｒｏｓｓｂｙ波。当考虑边界层动力影响后，修正后的Ｋｅｌｖｉｎ波，其向东传播的速度约为１０ｍｓ－１，且在长波波段是不稳定的，最不稳定的波出现在纬向一波附近，不稳定增长率的量级约为Ｏ（１０ｍ－６ｓ－１），理论结果和Ｍａｄｄｅｎ－Ｊｕｌｉａｎ振荡在赤道附近主体环流东传的观测事实接近。另外两支修正后的Ｒｏｓｓｂｙ波，其中经圈尺度较小的那支波，除波长极长的波外，波是阻尼的；但另一支经圈尺度较大的波，在短的波段是不稳定增长的，这似乎可以用来解释Ｍａｄｄｅｎ－Ｊｕｌｉａｎ振荡中大尺度超级云团的西传过程。这些结果表明边界层动力过程在Ｍａｄｄｅｎ－Ｊｕｌｉａｎ振荡的形成中是一种重要的机制。

n this paper, a tropical atmospheric model of relevance to short-term climate variations (Wang and Li 1993) is utilized for study of the development of Madden-Julian oscillation. The model contains an interactive process of boundy-layer Ekman convergence and precipitation heating. The model is solved by expanding dependent variables in terms of parabolic cylindrical functions in the meridional direction and truncating three meridional modes n=0, 2, 4 for equatorial symmetric solutions. The free wave solutions obtained under long-wave approximation are induced as a Kelvin wave and two Rossby waves. After considering the effect of boundary-laryer dynamic process, the modified Kelvin wave becomes unstable in long-wave bands with a typical growth rate on an order of 10-6s s-1 and an eastward phase speed of 10 ms-1; the most unstable mode is wavenumber one. These theoretical results are consistent with the observed Madden- Julian oscillation in equatorial area. For the two modified Rossby waves. one with a smaller meridional scale (n = 4) decays except for extra long-waves; the other with a larger meridional sacale (n=2) grows in short-wave bands. This may be relevant to explaining the westward propagation of super cloud clusters in the Madden-Julian oscillation. The theory suggests that the boundary-layer dynamic process is an important mechanism in the development of the Madden-Julian oscillation.