The sensitivity of a GCM's marine stratocumulus to cloud‐top entrainment
The sensitivity of a GCM's marine stratocumulus to cloud‐top entrainment
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GCM 海洋层积云对云顶夹带的敏感性
DOI：
10.1256/qj.03.114
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发表时间：
2004
影响因子：
8.9
通讯作者：
A. Lock
中科院分区：
文献类型：
--
作者：
A. Lock
结果表明，气象局大气环流模式(GCM)对东北副热带太平洋平流层积云的模拟比较准确，平均云量较高，较好地反映了观测到的水-液路径的平均和昼夜循环，逆温盖层高度合理。这表明边界层热力收支也必须得到很好的模拟。然而，这个预算中占主导地位的术语之一是云顶夹带，它在这个模型中被明确地参数化，并且仍然是文献中许多争论的主题。为了量化GCM对这种参数化的敏感性，给出了应用三种卷吸公式扰动的模拟结果。一个变化，即增强浮力反转区域中的夹带，被发现几乎没有影响。另外两个导致层积云区域的云量显著减少，与观测结果的一致性较差。第一项措施是将参数化夹带率提高两倍。第二是消除夹带通量与辐射平流通量和分解平流通量的耦合，这是以前发现的防止产生虚假附加夹带的严重数值误差所必需的一个实施方面。进一步假设，数值误差可能与东北太平洋平流层积区地面热通量中的一个信号有关。这一信号可以在许多其他的GCM中看到，这表明与急剧反转中的混合有关的数值问题可能很普遍。©皇冠版权所有，2004年。
It appears that the Met Office general‐circulation model (GCM) represents stratocumulus over the northeast subtropical Pacific Ocean quite accurately—the mean cloud cover is realistically high, the observed mean and diurnal cycle of liquid‐water path is well represented and the capping inversion is at a reasonable height. This suggests that the boundary‐layer thermodynamic budget must also be well modelled. However, one of the dominant terms in this budget is cloud‐top entrainment, which is parametrized explicitly in this model, and is still the subject of much debate in the literature. In order to quantify the sensitivity of the GCM to this parametrization, the results of simulations applying three perturbations of entrainment formulation are presented. One change, to the enhancement of entrainment in the buoyancy‐reversal regime, is found to have little impact. The other two led to a significant reduction of the cloudiness in the stratocumulus area, giving worse agreement with observations. The first of these was to increase the parametrized entrainment rate by a factor of two. The second was to remove the coupling of the entrainment fluxes to the radiative and resolved advective fluxes, an aspect of the implementation previously found necessary to prevent serious numerical errors which generated spurious additional entrainment. It is further hypothesized that the numerical errors can be associated with a signal in the surface heat fluxes in the north‐east Pacific stratocumulus area. This signal can be seen in many other GCMs, suggesting that numerical problems associated with mixing across sharp inversions may be prevalent. © Crown copyright, 2004.