Sponge layer feedbacks in middle‐atmosphere models
Sponge layer feedbacks in middle‐atmosphere models
复制标题
中层大气模型中的海绵层反馈
DOI：
10.1029/96jd01994
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发表时间：
1996
影响因子：
--
通讯作者：
J. Koshyk
中科院分区：
文献类型：
--
作者：
T. Shepherd;K. Semeniuk;J. Koshyk
中大气模式通常在其区域的上部使用海绵层。结果表明，海绵的松弛特性使其能够以人工方式与较低水平的动力学耦合。特别是，对强加的温带局地力或非热加热的长期纬向对称响应被证明会在海绵中引起阻力，从而修正Haynes等人[1991]的“向下控制”论点所预期的响应。在施加局部力的情况下，海绵的作用是将平均子午线质量通量的一部分向上偏转，对于实际参数值，其近似等于exp(−Δz/H)，其中Δz是强迫区与海绵层之间的距离，H是密度标度高度。这种由海绵引起的上层细胞引起的温度变化，在海绵层以下，与在强迫区以下的温度变化幅度相当。在施加局部非绝热加热的情况下，海绵引起了贯穿整个大气深度的经向环流。这种循环引起的温度变化，在海绵层以下，与加热区域的变化具有相反的符号和相当的幅度。在这两种情况下，海绵引起的温度变化基本上与施加的力或非绝热加热的高度无关，只要后者位于海绵外部，但随着从海绵向下移动，温度变化呈指数下降。因此，在给定的高度，通过将海绵放置在足够高的地方，可以使海绵的效果任意小；例如，如果施加的力或绝热加热位于海绵外部，则其对低于两个标度高度的温度的影响大致为10%。然而，当在海绵层内施加一个作用力时（对于参数化的中间层重力波阻力来说，这是一种非常可信的情况），其作用几乎完全被海绵层反馈所抵消，其对下面温度的预期影响在很大程度上未能实现。本文描述了利用大气中层环流模式进行的模拟，结果表明这种海绵层反馈可以在物理感兴趣的参数体系中发挥重要作用。纬向对称（二维）中层大气模式通常在整个模式域中使用瑞利阻力。结果表明，在这种情况下，对施加的温带局地力或非绝热加热的长期纬向对称响应明显地改变了对向下控制的预期，即使阻力系数很弱。
Middle-atmosphere models commonly employ a sponge layer in the upper portion of their domain. It is shown that the relaxational nature of the sponge allows it to couple to the dynamics at lower levels in an artificial manner. In particular, the long-term zonally symmetric response to an imposed extratropical local force or diabatic heating is shown to induce a drag force in the sponge that modifies the response expected from the “downward control” arguments of Haynes et al. [1991]. In the case of an imposed local force the sponge acts to divert a fraction of the mean meridional mass flux upward, which for realistic parameter values is approximately equal to exp(−Δz/H), where Δz is the distance between the forcing region and the sponge layer and H is the density scale height. This sponge-induced upper cell causes temperature changes that, just below the sponge layer, are of comparable magnitude to those just below the forcing region. In the case of an imposed local diabatic heating, the sponge induces a meridional circulation extending through the entire depth of the atmosphere. This circulation causes temperature changes that, just below the sponge layer, are of opposite sign and comparable in magnitude to those at the heating region. In both cases, the sponge-induced temperature changes are essentially independent of the height of the imposed force or diabatic heating, provided the latter is located outside the sponge, but decrease exponentially as one moves down from the sponge. Thus the effect of the sponge can be made arbitrarily small at a given altitude by placing the sponge sufficiently high; e.g., its effect on temperatures two scale heights below is roughly at the 10% level, provided the imposed force or diabatic heating is located outside the sponge. When, however, an imposed force is applied within the sponge layer (a highly plausible situation for parameterized mesospheric gravity-wave drag), its effect is almost entirely nullified by the sponge-layer feedback and its expected impact on temperatures below largely fails to materialize. Simulations using a middle-atmosphere general circulation model are described, which demonstrate that this sponge-layer feedback can be a significant effect in parameter regimes of physical interest. Zonally symmetric (two dimensional) middle-atmosphere models commonly employ a Rayleigh drag throughout the model domain. It is shown that the long-term zonally symmetric response to an imposed extratropical local force or diabatic heating, in this case, is noticeably modified from that expected from downward control, even for a very weak drag coefficient.