Drop Size Distribution Broadening Mechanisms in a Bin Microphysics Eulerian Model
Drop Size Distribution Broadening Mechanisms in a Bin Microphysics Eulerian Model
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DOI：
10.1175/jas-d-20-0099.1
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发表时间：
2020-09
影响因子：
3.1
通讯作者：
L. H. Pardo;H. Morrison;L. Machado;J. Harrington;Z. Lebo
中科院分区：
文献类型：
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作者：
L. H. Pardo;H. Morrison;L. Machado;J. Harrington;Z. Lebo
在这项研究中，扩大滴尺寸分布（DSDs）在欧拉模型与两个时刻箱微观物理的过程进行了分析。进行了大量的测试，以隔离不同的物理机制，扩大DSD在二维和三维天气研究和预报模型模拟的理想化的无冰积云的影响。这些影响的敏感性修改水平和垂直模式网格间距也检查。正如预期的那样，碰撞合并是一个关键的过程，扩大了模拟的DSD。在云中液滴激活也大大有助于DSD加宽，而蒸发只有很小的影响和沉降有很小的影响。云稀释（无云空气和多云空气的混合）也会显著扩大DSD，无论是否伴有蒸发。这种机制涉及到液滴浓度的减少，从稀释沿着云的侧边，导致局部高过饱和度和增强的液滴生长时，这种空气随后被提升在上升气流。当DSD与来自云核心的DSD混合时，DSD加宽扩展。将水平和垂直模型网格间距从100米减少到30米对数据集定义的影响有限。然而，当这些物理增宽机制（云内激活，碰撞合并，稀释等）当垂直网格间距从100 m减小到30 m时，DSD宽度减小约20%-50%，这与垂直数值扩散的人工加宽效应一致。尽管如此，这种人为的数值加宽似乎是相对不重要的整体DSD加宽时，物理为基础的加宽机制，在模型中包括这种积云的情况下。
In this study, processes that broaden drop size distributions (DSDs) in Eulerian models with two-moment bin microphysics are analyzed. Numerous tests are performed to isolate the effects of different physical mechanisms that broaden DSDs in two- and three-dimensional Weather Research and Forecasting Model simulations of an idealized ice-free cumulus cloud. Sensitivity of these effects to modifying horizontal and vertical model grid spacings is also examined. As expected, collision–coalescence is a key process broadening the modeled DSDs. In-cloud droplet activation also contributes substantially to DSD broadening, whereas evaporation has only a minor effect and sedimentation has little effect. Cloud dilution (mixing of cloud-free and cloudy air) also broadens the DSDs considerably, whether or not it is accompanied by evaporation. This mechanism involves the reduction of droplet concentration from dilution along the cloud’s lateral edges, leading to locally high supersaturation and enhanced drop growth when this air is subsequently lifted in the updraft. DSD broadening ensues when the DSDs are mixed with those from the cloud core. Decreasing the horizontal and vertical model grid spacings from 100 to 30 m has limited impact on the DSDs. However, when these physical broadening mechanisms (in-cloud activation, collision–coalescence, dilution, etc.) are turned off, there is a reduction of DSD width by up to ~20%–50% when the vertical grid spacing is decreased from 100 to 30 m, consistent with effects of artificial broadening from vertical numerical diffusion. Nonetheless, this artificial numerical broadening appears to be relatively unimportant overall for DSD broadening when physically based broadening mechanisms in the model are included for this cumulus case.