Rapid ice aggregation process revealed through triple-wavelength Doppler spectrum radar analysis
Rapid ice aggregation process revealed through triple-wavelength Doppler spectrum radar analysis
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DOI：
10.5194/acp-19-5753-2019
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发表时间：
2019-05-02
影响因子：
6.3
通讯作者：
Stein, Thorwald H. M.
中科院分区：
文献类型：
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作者：
Barrett, Andrew I.;Westbrook, Christopher D.;Stein, Thorwald H. M.
我们已经确定了一个区域的冰云，双波长比的急剧过渡发生在一个固定的高度超过20分钟。在本文中，我们提供的证据表明，快速聚集的冰粒发生在这一地区，创造大颗粒。这一证据来自偶然收集的三波长多普勒光谱雷达数据。通过定量比较三个雷达的多普勒频谱，我们能够估计冰粒子的大小分布（大于0.75毫米的粒子）在云中的不同高度。这使我们能够调查冰颗粒尺寸分布的演变，并确定演变是否与聚集，起霜或气相沉积相一致。新开发的方法使我们能够将信号与分布中的较大（非瑞利散射）颗粒隔离开来。因此，在双波长比值法因体积双波长比值太接近于零而失效的云区域，可以反演颗粒尺寸分布。冰粒在10分钟内从最大尺寸0.75迅速增长到5 mm，同时下降到500 m以下。这种快速增长与聚集的理论估计吻合得很好，聚集效率约为0.7，与其它生长过程不一致，e. G.通过气相沉积或成膜生长。聚合发生在云的中间，并不存在于云的整个生命周期中。然而，快速聚集层在恒定的高度上非常明确，温度为-15摄氏度，持续至少20分钟（近似水平距离：24公里）。就在这一层之上，雷达多普勒频谱是双峰的，这表明在该高度有新的小冰粒形成。我们认为，这些新形成的颗粒在大约-15摄氏度的温度下生长出树枝状臂，使它们能够容易地互锁并加速聚集过程。在这个云的大的估计聚集效率是一致的，最近的实验室研究在这个温度下的树突。
We have identified a region of an ice cloud where a sharp transition of dual-wavelength ratio occurs at a fixed height for longer than 20 min. In this paper we provide evidence that rapid aggregation of ice particles occurred in this region, creating large particles. This evidence comes from triple-wavelength Doppler spectrum radar data that were fortuitously being collected. Through quantitative comparison of the Doppler spectra from the three radars we are able to estimate the ice particle size distribution (of particles larger than 0.75 mm) at different heights in the cloud. This allows us to investigate the evolution of the ice particle size distribution and determine whether the evolution is consistent with aggregation, riming or vapour deposition. The newly developed method allows us to isolate the signal from the larger (non-Rayleigh scattering) particles in the distribution. Therefore, a particle size distribution retrieval is possible in areas of the cloud where the dual-wavelength ratio method would fail because the bulk dual-wavelength ratio value is too close to zero.The ice particles grow rapidly from a maximum size of 0.75 to 5mm while falling less than 500m in under 10 min. This rapid growth is shown to agree well with theoretical estimates of aggregation, with aggregation efficiency being approximately 0.7, and is inconsistent with other growth processes, e. g. growth by vapour deposition or riming. The aggregation occurs in the middle of the cloud and is not present throughout the entire lifetime of the cloud. However, the layer of rapid aggregation is very well defined at a constant height, where the temperature is -15 degrees C and lasts for at least 20 min (approximate horizontal distance: 24 km). Immediately above this layer, the radar Doppler spectrum is bimodal, which signals the formation of new small ice parti-cles at that height. We suggest that these newly formed particles, at approximately -15 degrees C, grow dendritic arms, enabling them to easily interlock and accelerate the aggregation process. The large estimated aggregation efficiency in this cloud is consistent with recent laboratory studies for dendrites at this temperature.