Impacts of Enhanced Cloud Condensation Nuclei (CCN) on the Organization of Convection for Monsoon Depressions

增强云凝结核（CCN）对季风低压对流组织的影响

• 批准号: 1241292
• 负责人: T. Krishnamurti
• 金额: $ 32.53万
• 依托单位: Florida State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-10-01 至 2017-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1241292&HistoricalAwards=false
• 关键词: Impacts; Enhanced; Cloud; Condensation; Nuclei

This project seeks to understand the effect of aerosol pollution on monsoon depressions. Monsoon depressions are synoptic-scale systems (2,000km to 3,000km) with baroclinic (or tilted) vertical structure which propagate westward, forming in the Bay of Bengal and producing as much as 150 to 200 mm of rainfall during their passage over any location. They occur during the monsoon season, when the vertical shear of the zonal wind is strong enough to inhibit the formation of hurricane-strength tropical cyclones, and they are distinct from weaker but similar systems called "monsoon lows", which have a barotropic (or upright) vertical structure. The Principal Investigator argues that the heavy aerosol pollution found over the Indian sector causes a reduction in the frequency of occurrence of monsoon depressions. This claim is based on several lines of evidence: first, there has been a dramatic reduction in the occurrence of monsoon depressions, from an annual average of 7 to 8 historically to 1, 0, and 2 in the years 2009, 2010, and 2011. Second, aerosol pollution has increased dramatically over the Indian sector in recent years. Third, global forecast models, which presumably do not account for the change in aerosol pollution, have overpredicted monsoon depressions in the recent years with few depressions. Fourth, preliminary experiments with the Weather Research and Forecasting (WRF) model show that the strength of monsoon depressions, and the intensity and organization of precipitation within them, can be dramatically reduced by increasing the aerosol burden from relatively pristine (5x10^8 particles per cubic meter) to heavily polluted (5x10^10 per cubic meter) values. The research is based on the hypothesis that aerosols suppress monsoon depressions because they slow the formation of rainfall, as more aerosols lead to more cloud condensation nuclei (CCN), which produce a greater number of smaller cloud droplets, which take longer to coalesce into raindrops. Monsoon depressions form through a mutual interaction between the baroclinic/barotropic instability of the large-scale flow and the latent heat of condensation accompanying precipitation within the depression. This mutual interaction can be suppressed by the excess aerosols, which increase time required for the development of precipitation by roughly a factor of two, and thus produce a mismatch between the timescales of the hydrodynamic instability and the organization of precipitation and accompanying latent heat release. Work performed under this award tests the hypothesized suppression of monsoon depressions by aerosol pollution using more sophisticated and realistic simulations, and also performs analysis of aerosol forcing, based on satellite data, during periods when monsoon depressions form and when they are absent.The suppression of monsoon depressions addressed in this research is of both scientific and societal interest, given the large population that is affected by them. If successful, the results of this work could be directly applied to operational forecasting systems used to predict these weather systems. In addition, the project provides support and training for a postdoc and a graduate student, thereby providing for the next generation of scientists in this research area.

这个项目旨在了解气溶胶污染对季风低压的影响。 季风低压是天气尺度系统（2,000公里至3,000公里），具有斜压（或倾斜）的垂直结构，向西传播，在孟加拉湾形成，并在其通过任何位置时产生多达150至200毫米的降雨。 它们发生在季风季节，当纬向风的垂直切变足够强，以抑制飓风强度的热带气旋的形成，它们不同于较弱但类似的系统，称为“季风低压”，具有正压（或直立）垂直结构。首席研究员认为，印度上空发现的重气溶胶污染导致季风低压发生频率降低。 这一说法基于几条证据：首先，季风低压的发生率急剧减少，从历史上的年均7至8次减少到2009年、2010年和2011年的1、0和2次。 第二，近年来，印度部门的气溶胶污染急剧增加。 第三，全球预测模型，大概没有考虑到气溶胶污染的变化，高估了季风低压在最近几年的低气压。 第四，使用天气研究和预报（WRF）模式进行的初步实验表明，通过将气溶胶含量从相对原始（每立方米5 × 10^8个粒子）增加到重度污染（每立方米5 × 10^10个粒子），可以显著降低季风低压的强度以及其中降水的强度和组织。这项研究是基于这样的假设，即气溶胶抑制季风低压，因为它们减缓了降雨的形成，因为更多的气溶胶导致更多的云凝结核（CCN），从而产生更多的较小云滴，这些云滴需要更长的时间才能合并成雨滴。 季风低压是由大尺度气流的斜压/正压不稳定性和低压内伴随降水的凝结潜热相互作用而形成的。 这种相互作用可以被过量的气溶胶所抑制，过量的气溶胶使降水发展所需的时间大约增加两倍，从而在水动力不稳定性的时间尺度与降水的组织和伴随的潜热释放之间产生不匹配。 在该奖项下进行的工作使用更复杂和现实的模拟来测试气溶胶污染对季风低压的假设抑制，并且还基于卫星数据在季风低压形成和不存在期间进行气溶胶强迫分析。本研究中解决的季风低压抑制具有科学和社会意义，因为受其影响的人口众多。 如果成功，这项工作的结果可以直接应用于用于预测这些天气系统的业务预报系统。 此外，该项目还为博士后和研究生提供支持和培训，从而为该研究领域的下一代科学家提供支持。