Ice formation and development in aged, wintertime cumulus over the UK: observations and modelling
Ice formation and development in aged, wintertime cumulus over the UK: observations and modelling
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DOI：
10.5194/acp-12-4963-2012
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发表时间：
2012-01-01
影响因子：
6.3
通讯作者：
Blyth, A.
中科院分区：
文献类型：
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作者：
Crawford, I.;Bower, K. N.;Blyth, A.
作为气溶胶特性、过程和对地球气候的影响（APPRAISE）项目的一部分，使用雷达和激光雷达对云微物理特性进行了现场高分辨率飞机测量，并与地面遥感观测相协调，对一系列小积云进行了测量。对英国南部上空的一条狭窄但广泛的浅对流云线（大约 100 公里长）进行了研究。观察到云顶温度高于-8°C，但云层由过冷的水滴和不同浓度的冰粒组成。没有观察到冰粒从上方落入云顶。当前冰核 (IN) 数量的参数化预测，作为冰核活跃的颗粒太少，无法解释观测到的云顶附近温度 (-7.5 A 摄氏度) 下的冰颗粒浓度。矿物尘埃颗粒的作用与在地表附近观察到的浓度一致，在这种情况下，充当高温 IN 被认为很重要。研究发现，如果存在观察到的少量初级冰（约 0.01 L-1）来启动二次冰粒，则二次冰粒的生成可以产生非常高浓度的冰粒（高达 100 L-1）。这强调了了解轻微过冷云中初级冰形成的必要性。通过简单的计算表明，哈雷特-莫索普过程（HM）可能是次生冰的来源。案例研究的模型模拟是使用气溶胶云和降水相互作用模型（ACPIM）进行的。这些地块模型研究证实，HM 过程是产生观察到的高冰浓度的一个非常重要的机制。产生高浓度的关键步骤是雨滴的碰撞和聚结过程，雨滴一旦形成，就会迅速穿过云层，收集冰粒，导致它们冻结并立即形成大的边缘颗粒。因此，通过碰撞聚结扩大液滴尺寸分布是该过程中至关重要的一步，因为这是在可用时间内产生观察到的大量冰晶所必需的。还使用 WRF（天气、研究和预报）模型进行了模拟。结果表明，虽然 HM 在这些模型模拟中确实起到了增加冰粒质量和数量浓度的作用，但并未发现它对于降水的形成至关重要。然而，WRF 模拟产生的云顶太冷，再加上冰晶形成所去除的冰核持续补充的假设，导致与观测和地块建模相比，通过初级成核形成了太多的冰晶。
In situ high resolution aircraft measurements of cloud microphysical properties were made in coordination with ground based remote sensing observations of a line of small cumulus clouds, using Radar and Lidar, as part of the Aerosol Properties, PRocesses And InfluenceS on the Earth's climate (APPRAISE) project. A narrow but extensive line (similar to 100 km long) of shallow convective clouds over the southern UK was studied. Cloud top temperatures were observed to be higher than -8 A degrees C, but the clouds were seen to consist of supercooled droplets and varying concentrations of ice particles. No ice particles were observed to be falling into the cloud tops from above. Current parameterisations of ice nuclei (IN) numbers predict too few particles will be active as ice nuclei to account for ice particle concentrations at the observed, near cloud top, temperatures (-7.5 A degrees C).The role of mineral dust particles, consistent with concentrations observed near the surface, acting as high temperature IN is considered important in this case. It was found that very high concentrations of ice particles (up to 100 L-1) could be produced by secondary ice particle production providing the observed small amount of primary ice (about 0.01 L-1) was present to initiate it. This emphasises the need to understand primary ice formation in slightly supercooled clouds. It is shown using simple calculations that the Hallett-Mossop process (HM) is the likely source of the secondary ice.Model simulations of the case study were performed with the Aerosol Cloud and Precipitation Interactions Model (ACPIM). These parcel model investigations confirmed the HM process to be a very important mechanism for producing the observed high ice concentrations. A key step in generating the high concentrations was the process of collision and coalescence of rain drops, which once formed fell rapidly through the cloud, collecting ice particles which caused them to freeze and form instant large riming particles. The broadening of the droplet size-distribution by collision-coalescence was, therefore, a vital step in this process as this was required to generate the large number of ice crystals observed in the time available.Simulations were also performed with the WRF (Weather, Research and Forecasting) model. The results showed that while HM does act to increase the mass and number concentration of ice particles in these model simulations it was not found to be critical for the formation of precipitation. However, the WRF simulations produced a cloud top that was too cold and this, combined with the assumption of continual replenishing of ice nuclei removed by ice crystal formation, resulted in too many ice crystals forming by primary nucleation compared to the observations and parcel modelling.