A 3D stochastic cloud model for investigating the radiative properties of inhomogeneous cirrus clouds
A 3D stochastic cloud model for investigating the radiative properties of inhomogeneous cirrus clouds
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DOI：
10.1256/qj.04.144
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发表时间：
2005-10
影响因子：
8.9
通讯作者：
R. Hogan;S. Kew
中科院分区：
文献类型：
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作者：
R. Hogan;S. Kew
已知云的三维结构对于确定其辐射效应非常重要，但很难直接从观测中获得这种结构。在本文中，描述了一个随机模型，能够模拟卷云独特的结构特性：瀑布的几何形状和剪切诱导的混合。我们首先提出了一个分析的时间高度云雷达部分从英格兰南部提取卷云参数的利益。研究发现，冰水含量对数的水平功率谱（根据雷达反射率因子和温度估计）在云顶附近通常表现出约-5/3的谱斜率，该斜率随着进入云中的深度而减小，在某些情况下低至-3.5。这种减少可以解释为风切变加上粒子下落速度的传播，导致均匀化，优先在较小的尺度。功率谱表现出明显的尺度突变，在大于约50公里的尺度（“外尺度”）上变得平坦。根据平均风速廓线和平均冰降速度廓线可以预测冰迹的走向。然后，我们描述了随机模型，该模型将冰水含量、光谱斜率、外尺度和风速的平均值和分数标准差作为输入剖面。它首先通过对模拟傅立叶系数矩阵执行逆3-D傅立叶变换来生成各向同性3-D分形场，该矩阵的振幅与观察到的1-D光谱一致。系数的随机相位允许生成具有相同统计特性的云的多个实现。然后依次操作域中的水平切片，以模拟水平位移和光谱随高度的变化。最后，对场进行缩放，以产生观测到的冰水含量的平均值和分数标准差。从该域提取的垂直二维切片在外观上与云雷达观测非常相似。使用独立柱近似进行的辐射传输计算表明，不同的风切变导致的不同的落痕方向可以使大气层顶的平均辐射通量在短波中改变超过45 W m−2，在长波中改变超过15 W m−2。风切变诱导水平混合的作用引起了附加的但较小的辐射效应。我们还研究了在大气环流模式（GCM）的辐射方案中所做的假设所预期的偏差。结果发现，水平均匀性和最大随机重叠假设产生的误差之间存在一些补偿;如果GCM要改进重叠假设，但仍然假设云是水平均匀的，那么云辐射效应的总误差可能会增加。版权所有© 2005皇家气象学会。
The three‐dimensional structure of clouds is known to be important for determining their radiative effects, but it is difficult to obtain this structure directly from observations. In this paper a stochastic model is described that is capable of simulating the structural properties unique to cirrus: fallstreak geometry and shear‐induced mixing. We first present an analysis of time–height cloud radar sections from southern England to extract the cirrus parameters of interest. It is found that horizontal power spectra of the logarithm of ice‐water content (estimated from radar reflectivity factor and temperature) typically exhibit a spectral slope of around −5/3 near cloud top that decreases with depth into the cloud, to values as low as −3.5 in some cases. This decrease can be explained by wind shear coupled with a spread of particle fall speeds leading to a homogenization that acts preferentially at smaller scales. The power spectra exhibit a distinct scale break, becoming flat at scales larger than around 50 km (the ‘outer scale’). The orientation of the fallstreaks may be predicted from the profile of mean wind and mean ice fall speed. We then describe the stochastic model, which takes as input profiles of the mean and fractional standard deviation of ice‐water content, spectral slope, outer scale and wind speed. It first generates an isotropic 3‐D fractal field by performing an inverse 3‐D Fourier transform on a matrix of simulated Fourier coefficients with amplitudes consistent with the observed 1‐D spectra. Random phases for the coefficients allow multiple realizations of a cloud with the same statistical properties to be generated. Then horizontal slices from the domain are manipulated in turn to simulate horizontal displacement and changes to the spectra with height. Finally the field is scaled to produce the observed mean and fractional standard deviation of ice‐water content. Vertical 2‐D slices extracted from the domain are very similar in appearance to cloud radar observations. Radiative‐transfer calculations using the independent column approximation are used to show that the different fallstreak orientation resulting from different wind shears can change mean top‐of‐atmosphere radiative fluxes by in excess of 45 W m−2 in the short‐wave and 15 W m−2 in the long‐wave. The effect of wind shear to induce horizontal mixing causes an additional but smaller radiative effect. We also investigate the biases that would be expected from the assumptions made in the radiation schemes of general‐circulation models (GCMs). It is found that there is some compensation between the errors arising from the assumptions of horizontal homogeneity and maximum‐random overlap; if a GCM were to improve the overlap assumption but still assume clouds to be horizontally homogeneous then the total error in cloud radiative effect would be likely to increase. Copyright © 2005 Royal Meteorological Society.