An idealized study of African easterly waves. I: A linear view
An idealized study of African easterly waves. I: A linear view
复制标题
非洲东波的理想化研究。
DOI：
--
复制
发表时间：
1994
期刊：
影响因子：
--
通讯作者：
B. Hoskins
中科院分区：
文献类型：
--
作者：
C. Thorncroft;B. Hoskins
使用球体上的原始方程研究了非洲东风急流的线性不稳定性问题。这包括使用传统上用于中纬度地区的诊断方法（例如 Eliassen-Palm (EP) 通量和位涡）对线性增长机制和结构进行检查。研究表明，非洲东风急流的正态模态增长的特点是急流区域的 EP 通量发散，这意味着正压和斜压能量转换。线性不稳定性主要由射流水平的正负位涡梯度之间的相互作用决定，并且如之前的研究发现，简正模态主要通过正压能量转换而增长。许多与简正模态相关的天气特征与观察到的特征非常一致，但垂直速度模式除外，它在垂直方向上具有“棋盘”结构，而且要弱得多。
在保持斜压不变的情况下改变喷流纬度对增长率的影响很小，尽管正压能量转换仍然占主导地位，但当喷流更偏向极地时，最不稳定的模式变得更加斜压。最不稳定的模式在较薄的射流上生长，具有较大的生长速率、较小的波长和较强的正压能量转换。垂直速度中的“棋盘”模式与这些射流上生长的正常模式一致。
还研究了在线性问题中包含非绝热效应的效果。首先，我们发现简单的边界层方案对简正模态的影响非常小。然而，通过简单地参数化潜热释放，最不稳定模态的增长率略有增加，并且这些模态不再受正压能量转换的支配。此外，在波浪的上升和下降区域之间发现了不对称性，上升气流的长度尺度相对于下降气流的长度尺度收缩。垂直速度中不切实际的“棋盘”图案几乎被消除，并且幅度增加。使用 CISK 型方案进行潜热加热的正则模态结构与在西非观测到的结构比干模态结构有更多共同点。
有人认为，非洲东风波可能是通过混合正压/斜压不稳定机制产生的，其中潜热的作用对于增加相对于正压能量转换的斜压能量转换以及确定天气结构非常重要。
The linear instability problem of the African easterly jet has been investigated using the primitive equations on a sphere. This has included an examination of the linear growth mechanisms and structure using diagnostics traditionally employed for mid latitudes, such as Eliassen-Palm (EP) fluxes and potential vorticity. It has been shown that a growing normal mode on an African easterly jet is characterized by divergent EP fluxes in the region of the jet, implying both barotropic and baroclinic energy conversions. The linear instability is dominated by the interaction between positive and negative potential vorticity gradients at the level of the jet and, as found in previous studies, the normal modes grow mainly through barotropic energy conversions. Many of the synoptic features associated with the normal modes are in good agreement with those observed, except for the vertical-velocity pattern, which has a ‘checkerboard’ structure in the vertical and is much weaker.
Changing the jet latitude while keeping the baroclinicity constant changed the growth rates very little, and although barotropic energy conversions remained dominant, the most unstable modes became more baroclinic when the jet was more poleward. The most unstable modes, which grow on a thinner jet, have a larger growth rate, a smaller wavelength and stronger barotropic energy conversions. The ‘checkerboard’ pattern in the vertical velocity persists with the normal modes which grow on these jets.
The effect of including diabatic effects in the linear problem has also been examined. First, a simple boundary-layer scheme was found to have very little effect on the normal modes. With simply parametrized latent heat release however, the growth rates of the most unstable modes were increased slightly and the modes became less dominated by barotropic energy conversions. Also, an asymmetry is found between the ascent and descent regions in the wave, with the length scale of the updraught contracted relative to that of the downdraught. The unrealistic ‘checkerboard’ pattern in the vertical velocity is almost removed and the amplitude is increased. The structure of the normal mode, using a CISK-type scheme for the latent heating, has more in common with the observed structures over west Africa than the structure of the dry modes.
It is suggested that African easterly waves may arise through a mixed barotropic/baroclinic instability mechanism where the role of latent heating is important in increasing the baroclinic energy conversions relative to the barotropic energy conversions, and also in determining the synoptic structure.