Indirect Aerosol Effects on Tropical Convection

气溶胶对云的影响——包括自然产生的和人为产生的——早已被认识到。对于分布更广的层状云（例如，副热带层积云）和孤立的对流云，这些影响已经得到了相当广泛的研究。但在广阔的赤道地区上空的深云系统就不是这样了，因为这些地区容易受到来自大陆沙漠和城市化地区的风尘的侵袭。该基金支持的工作将在相当详细地检查热带深层对流云系统组方面具有独特性，以便更好地了解它们对微物理活性气溶胶入侵的敏感性，包括云凝结核（CCN）和冰核（in），这反过来影响降水发展和云辐射相互作用。模式运行将在大水平区域（在38个垂直高度约9600公里× 180公里）上进行，综合时间长（约100天），这将使模拟的大气达到辐射-对流平衡状态，并随后评估对不同数量和垂直分布的气溶胶的响应。具体目标包括分析不同CCN和IN浓度对以下方面的影响：(1)热带水收支，包括降水过程和潜热释放分布；(2)区分所谓的“第一”和“第二”间接气溶胶效应，即影响云层辐射传输的液滴大小和数量浓度；(3)云动力学，例如那些可能有利于先前注意到的对流云深度三模态分布的趋势，包括上升气流、下降气流、低层冷池的发展和其他影响云组织的因素；(4)热带辐射对流平衡；(5)液体和冰之间的水物质分配；(6)热带深部对流的砧-卷云特性。模拟的云统计数据还将与NASA的CloudSat卫星观测到的云特性进行比较，以更好地评估模型性能，并最终确定观测中隐含的那些物理过程。本研究的智力价值在于通过云微物理和辐射反馈的结合，改进了对控制热带云系统行为及其在全球气候系统中的作用的物理描述。它还将促进对“辐射-对流平衡”范式的理解，该范式被认为是热带大气的主要特征，并将有助于从新出现的数据流（如CloudSat提供的数据流）中提取最大价值。这项工作的更广泛影响包括改进对热带云系统（例如偶尔产生热带气旋的云系统）的中期预报所需的技术以及对地球气候的长期预报，还包括研究生教育和代表性不足的群体（即科学领域的妇女）的更多参与。

The impact of aerosols--both naturally occurring and anthropogenic--upon clouds has been long recognized. In the case of more widespread stratiform type clouds (e.g., subtropical stratocumulus) and isolated convective clouds, these impacts have been rather extensively examined. Such is not the case for systems of deep clouds over broad equatorial regions, which are subject to incursions of wind-driven dust from emanating from continental deserts and urbanized areas. Work supported by this grant will be unique in examining groups of deep tropical convective cloud systems in considerable detail in order to better understand their sensitivity to incursions of microphysically active aerosol including cloud condensation nuclei (CCN) and ice nuclei (IN), which in turn influence the precipitation development and cloud-radiation interactions. Model runs will be conducted over large horizontal regions (~9600 km x 180 km at 38 vertical levels) integrated over long periods (~100 days), which will allow the simulated atmosphere to achieve a state of radiative-convective equilibrium and subsequent assessment of responses to variable amounts and vertical distributions of aerosols. Specific goals include analysis of the impacts that variable CCN and IN concentrations have upon: (1) the tropical water budget, including precipitation processes and distribution of latent heat release; (2) differentiation between the so-called "first" and "second" indirect aerosol effect, which relate to droplet sizes and number concentrations influencing radiative transfer through cloud layers; (3) cloud dynamics, such as those that may favor a previously noted tendency toward tri-modal distribution of convective cloud depth and including updrafts, downdrafts, development of low-level cold pools and other factors influencing cloud organization; (4) radiative-convective equilibrium across the tropics; (5) partitioning of water substance between liquid and ice; and (6) the anvil-cirrus properties of deep tropical convection. Simulated cloud statistics will also be compared with observed cloud properties emerging from NASA's CloudSat satellite to better assess model performance and ultimately identify those physical processes implicit in the observations.The intellectual merit of this study rests in developing an improved description of physics governing the behavior of tropical cloud systems and their role in the global climate system via a combination of cloud microphysical and radiative feedbacks. It will also advance understanding of the "radiative-convective equilibrium" paradigm thought to characterize much of the tropical atmosphere, and will facilitate extracting maximum value from newly emerging datastreams such as provided by CloudSat.Broader impacts of this work include improvement of techniques needed for medium-range forecasts of tropical cloud systems (such as those that occasionally engender tropical cyclones) as well as longer-term prediction of the earth's climate, and include graduate student education and increased involvement by underrepresented groups (viz. women in science).