Microphysical characterisation of West African MCS anvils
Microphysical characterisation of West African MCS anvils
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西非 MCS 砧的微观物理表征
DOI：
10.1002/qj.557
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发表时间：
2010
影响因子：
8.9
通讯作者：
Bouniol D
中科院分区：
文献类型：
--
作者：
Bouniol D
热带地区的深层对流是大型对流层延展云的来源，通常被称为针状云团。这些砧板可能产生降水(深对流风暴的层状区)，也可能产生持续数小时至数天的卷云屏蔽。针状物影响辐射收支，在整个水收支中产生一个储存期，但量化程度较差，在对流层上层可能会产生动力反馈。2006年雨季期间在西非上空举行的AMMA实地活动提供了一个独特的机会，从前所未有的空中观测记录这些砧骨的微物理特性。利用空中冰晶测量和95 GHz多普勒雷达观测来描述热带砧骨的微物理特性。数据集被分成层状和环状两个区域。层状区域中的一些数据可能是在距离对流核心足够近的地方获得的，因此粒子可能主要是在这些核心内生长的。在大陆和海洋上获得的数据也是分开描述的。颗粒习性和生长过程是通过对收集的颗粒图像的检查、从现场微观物理观测计算的95 GHz反射率与测量的雷达反射率的定量比较以及通过对二维颗粒图像的统计分析来推断的。层状砧区0°C等温线以上的主要降水粒子为环状集合体。当系统向后向环形区域移动时，这些有边框的聚集体似乎变得不那么致密，直径也更小。反演的密度定律(假定为幂定律)与Locatelli和Hobbs的边缘粒子的关系很接近，也研究了热带斜坡中的粒度分布。指数形状似乎是这些颗粒大小分布的一个很好的近似值。环状区浓度随直径减小的速度也快于层状区。归一化这些颗粒尺寸分布产生一个相对不变的形状(与先前的研究一致)，但归一化直径的最小值和最大值的可变性增加。利用现场微观物理观测对大块微观物理性质的表征表明，冰水含量、有效半径和反射率加权下落速度通常随着气温的升高而增加，这与先前的研究一致。研究发现，环状区域的这些参数总体上比层状区域小，而且在所有温度下都是如此。然后将这些值与云解析模型和一般环流模型中使用的统计关系进行比较，因为模型中微物理的真实表示不仅对于理解工作中的过程，而且对于了解动态反馈和对气候的影响非常重要。发现了很大的差异，当前的参数化不能再现所考虑的微物理参数的大值。版权所有(C)2010皇家气象学会
Deep convection in the Tropics is the source of large tropospheric extended clouds usually called anvils. These anvils may produce precipitation (stratiform region of deep convective storms), and also cirrus shields persisting from several hours to several days. Anvils impact the radiation budget, they induce a storage term in the whole water budget which is still relatively poorly quantified, and dynamical feedbacks may be induced in the upper troposphere. The AMMA field campaign which was held over West Africa during the 2006 wet season provided a unique opportunity to document the microphysics of these anvils from unprecedented airborne observations.Airbornein situmeasurements of ice crystals and observations from a 95 GHz Doppler radar are used to characterize the microphysical properties of tropical anvils. The dataset is binned into stratiform and cirriform regions. Some data in the stratiform regions were likely obtained close enough to convective cores that the particles may have grown primarily within those cores. The data obtained over the continent and over the ocean are also characterized separately. Particle habit and growth processes are inferred from an examination of the collected particle images, from quantitative comparisons of 95 GHz reflectivities calculated from thein situmicrophysical observations with the measured radar reflectivities, and from a statistical analysis of the two‐dimensional particle images. The predominant precipitation particles above the 0 °C isotherm in the stratiform anvil region are rimed aggregates. These rimed aggregates seem to get less dense and of smaller diameter when moving rearward of the system towards the cirriform region. The retrieved density laws (assumed to be power laws) lie close to the relationship for rimed particles of Locatelli and Hobbs.Particle size distributions in tropical anvils are also studied. The exponential shape seems to be a good approximation for these particle size distributions overall. The decrease in concentration with diameter is also found to be faster for cirriform regions than for stratiform regions. Normalising these particle size distributions produces a relatively invariant shape (in agreement with earlier studies), with however an increased variability for the smallest and largest values of the normalised diameter.The characterisation of the bulk microphysical properties using thesein situmicrophysical observations shows that the ice water content, the effective radius and the reflectivity‐weighted fall velocity generally increase with air temperature, in agreement with earlier studies. These parameters are found to be systematically smaller on average in cirriform regions than in stratiform regions, and this is true at all temperatures. These values are then compared with statistical relationships used in cloud‐resolving models and general circulation models, since a realistic representation of microphysics in models is very important to understand not only the processes at work, but the dynamical feedbacks and effects on climate. Large differences are found, the current parametrizations being unable to reproduce the large values of the considered microphysical parameters. Copyright © 2010 Royal Meteorological Society
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