Intensity and Amplification of Tropical Deep Convection

本研究试图了解控制热带地区深对流强度和聚集的基本机制。深对流是指在积云中发现的与阵雨和强降水有关的倾覆运动。这项研究涉及的一个中心问题是陆地和海洋区域对流强度的差异，这可以从热带卫星观测到的单位降雨量闪电率的差异来推断。较高的闪光率被普遍认为是陆地对流降水的上升气流和下沉气流速度高于融化水平的指示，因此闪光率可作为对流强度的替代指标。由于对流可用势能(CAPE)值较高，有时人们认为陆地上的对流更强烈，但这一说法的证据有些含糊。还提出了其他假说，其中一种假说认为，这种差异是由于地面干燥导致的相同温度和日照条件下，陆地上的表面加热比海洋上发生的更强烈，另一种假说是陆地表面的不均一性造成的，第三种假说是陆地上更多的云和冰凝结核造成的。这里的工作假设是，陆地上更大的不稳定实际上是强度差异的原因，但需要系统地检查开普值的全部分布，而不是简单地比较陆地和海洋的平均值。进一步的研究将解决热带对流的聚集问题，这意味着热带降水对流云的趋势是以大的有组织的聚集的形式出现。这项工作始于观察到，有组织的对流事件通常始于一种“底部重”结构，在这种结构中，大部分凝结潜热(随着水蒸气凝结成云滴而释放)发生在对流层的中低层。随着对流的加强，云层加深，呈现出“顶重”的结构，最终在对流层顶附近形成一个宽阔的层状区域。PI假设具有底部重结构的云能够有效地将水汽和能量吸收到该区域，从而加强对流并导致深对流云的发展。PI认为，这种自我放大可以用“干燥效率”来理解，“干燥效率”决定了大气的总体湿度稳定性是高于还是低于临界水平。这项研究还考虑了短期和长期对流聚集之间这种放大效果的差异。这项工作是通过观测数据分析和数值模式模拟相结合的方式进行的。模拟使用云解析模型，通常在通道形状的周期性区域中，一端有一个岛屿，但其他情况下被海洋表面覆盖。观测数据来自再分析产品和气象气球探测，包括对陆地和海洋上的CAPE的检查。由于对热带对流降水的强度和组织的理解、模拟和预测的价值，该研究具有更广泛的影响。这显然是热带地区当地关注的问题，热带对流的大范围也会影响中高纬度的天气。因此，更好地了解对流的聚集和组织可以改善全球大部分地区的天气预报。这项工作还具有更广泛的教育影响，因为该项目将支持和培训两名研究生，从而为该领域的下一代科学家提供支持。PI还与威斯康星州麦迪逊的扩展您的视野(EYH)计划合作。EYH举办了一个为期一天的会议，为中学年龄的女孩提供机会，探索使用数学和科学的职业。

This research seeks to understand the basic mechanisms which control the intensity and aggregation of deep convection in the tropics. Deep convection refers to the overturning motions found in cumulus clouds and associated with showers and intense precipitation. A central issue addressed by this research is the contrast in convective intensity between land and ocean regions, which can be inferred from the difference in lightning flash rate per unit rainfall seen in satellite observations from the tropics. The higher flash rate is generally accepted as an indication that the updraft and downdraft speeds above the melting level are are greater for convective precipitation over land, thus the flash rate serves as a proxy for the intensity of convection. It is sometimes assumed that convection is more intense over land due to higher values of Convective Available Potentially Energy (CAPE), but evidence for this claim is somewhat ambiguous. Other hypotheses have been proposed, including one that the difference is due to the more intense surface heating that occurs over land than ocean for the same temperature and insolation due to the dryness of the surface, another that the heterogeneity of land surfaces is at fault, and a third that the greater abundance of cloud and ice condensation nuclei over land are responsible. The working hypothesis here is that greater instability over land is in fact the cause of the intensity difference, but that a systematic examination of the full distribution of CAPE values is required, rather than a simple comparison of land versus ocean mean values.Further research would address the aggregation of convection in the tropics, meaning the tendency of precipitating convective clouds in the tropics to occur in large organized aggregates. The work begins from the observation that episodes of organized convection typically start out with a "bottom heavy" structure, in which most of the latent heat of condensation (released as water vapor condenses into cloud droplets) occurs at low to middle levels of the troposphere. As the convection intensifies the clouds deepen and take on a "top heavy" structure, eventually forming a broad stratiform region near the tropopause. The PI hypothesizes that clouds with a bottom heavy structure are effective in drawing moisture and energy into the region, thereby intensifying convection and leading to the development of deep convective clouds. The PI argues that this self-amplification can be understood in terms of a "drying efficiency" which determines the gross moist stability of the atmosphere is above or below a critical level. The research also considers variations in the effectiveness of this amplification between short-lived and longer-lasting convective aggregates.The work is conducted through a combination of observational data analysis and numerical model simulations. Simulations use a cloud resolving model, often in a channel-shaped periodic domain with an island on one end but otherwise covered by an ocean surface. Observational data comes from reanalysis products and weather balloon soundings, and includes an examination of CAPE over land and oceans.The research has broader impacts due to the value of understanding, simulating, and forecasting the intensity and organization of tropical convective precipitation. This is clearly an issue of local concern in the tropics, and large regions of tropical convection can also influence weather in middle and high latitudes. A better understanding of the aggregation and organization of convection could thus lead to improvements in weather prediction over much of the globe. The work also has educational broader impacts, as the project will support and train two graduate students, thereby providing for the next generation of scientists in this field. The PI also works with the Expanding Your Horizons (EYH) program in Madison, WI. EYH holds a one-day conference for that provides middle school-aged girls the opportunity to explore careers that use math and science.