OTREC: Convective Heating Profiles and the Transition from Shallow to Deep Convection over the Tropical East Pacific and Southwest Caribbean

该奖项支持热带东太平洋对流组织（OTREC）实地活动的工作。该运动旨在了解赤道东太平洋和加勒比海西南部邻近但不同的地区热带对流云的形成和发展及其相关的强降雨，以及它们在中美洲和哥伦比亚中间部分的演变。该运动还检查了每周在加勒比和东太平洋发生的东风波的起源和演变，即向西移动的大型大气扰动（波长约2000公里）。通过“OTREC”关键字搜索，可以在nsf.gov/awardsearch网页上发现全套活动奖项。热带对流在地球气候系统中起着至关重要的作用，也是极端降水和飓风形成的原因。由于东风浪有产生飓风的倾向，因此对天气预报特别感兴趣。更一般地说，热带对流是全球天气和气候的引擎，但人们对它知之甚少，也难以模拟。因此，在天气和气候模式中更好地表现热带对流的工作可以为公众和决策者带来更好的天气预报和影响评估。除了科学的社会相关性外，该运动还包括若干教育和外联活动，包括支持本科生参加实地考察和制作用于课堂教学和非正式科学教育的短纪录片视频。在该奖项下开展的工作促进了与哥伦比亚和哥斯达黎加科学家的国际科学合作，并帮助开发雨水收集和同位素分析的当地基础设施。为期八周的OTREC部署主要包括在由国家大气研究中心（NCAR）维护的湾流V （GV）研究飞机上进行20次飞行。该活动使用哥斯达黎加的一个机场，方便进入赤道太平洋和加勒比海西南部的研究区域，并使用下投探空仪和安装在机翼上的w波段雷达对这些区域的情况进行采样。下落探空仪包含与标准气象气球相同的仪器包，只是用一个小降落伞从飞机上（在这种情况下，从大约4万英尺的高度）扔下。在这里进行的工作通过收集和分析哥斯达黎加和哥伦比亚地面站点的雨水样本来增强飞机观测。尽管进行了深入的研究，但对于环境条件如何决定热带对流何时何地形成、持续多长时间、扩大和组织多少以及产生多少降雨，我们仍然缺乏一个令人满意的理论。垂直结构是热带对流的一个关键特性，它决定了热带对流的演变及其与周围大气环流和热力学的相互作用。根据对流层上层和下层上升运动的相对量，垂直结构可以简单地描述为顶重或底重。这里进行的工作旨在通过测量热带对流产生的雨水中稳定同位素的比例来确定热带对流的垂直结构。雨水几乎完全由普通的水组成，但它含有微量的水，其中氘(D)取代氢(H)形成HDO，或者氧-18取代氧-16。这些较重的同位素比普通的水更容易凝结，因此在对流层低层形成的雨滴中比在对流层高层形成的雨滴中更常见。因此，雨滴中较重同位素的相对丰度应该提供一种方法来确定它们是在头重脚轻的对流中形成的还是在底部重的对流中形成的。计划在哥伦比亚的三个地点收集雨水进行同位素分析：西海岸的努基；基布多，在更内陆的地方；以及加勒比海西南部的圣安德烈斯岛。将同位素组成估算的对流垂直剖面与飞机测量的估算值进行比较，以确定该方法是否有效。对同位素数据的进一步检查将使用来自各种大气模型的数值模拟进行，其中包括一些代表水蒸气的示踪“粒子”，当它们在云系统中上升并最终凝结时被跟踪。研究的另一个目标是了解从浅对流到深对流的转变，这是对流云生命周期的关键组成部分。分析了造成这种转变的三个因素：大气边界层的逐渐湿润；亚云层的组织，使云根在比大尺度平均湿度更大的空气中形成；而由对流下降气流形成的冷池，组织下层云产生更大的云根，而云根又不容易受到干燥空气夹带的稀释作用的影响。这些因素中的第三个可能导致正反馈，在这种反馈中，对流下降气流以一种进一步促进对流的方式组织亚云层。对加勒比海西南部的夹带进行更好的表征是特别有趣的，因为该地区的对流层中空气相当干燥，但对流仍然存在。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This award supports work on the Organization of Tropical East Pacific Convection (OTREC) field campaign. The campaign seeks to understand the formation and development of tropical convective clouds and associated heavy rainfall in the adjacent but distinct regions of the eastern equatorial Pacific and the southwest Caribbean, along with their evolution over the intervening portions of Central America and Colombia. The campaign also examines the genesis and evolution of easterly waves, large westward-moving atmospheric disturbances (wavelengths of about 2,000 km) which occur on a weekly basis in the Caribbean and eastern Pacific. The full set of campaign awards is discoverable on the nsf.gov/awardsearch webpage by a keyword search on "OTREC".Tropical convection plays a critical role in earth's climate system and is also responsible for extreme precipitation and hurricane formation. Easterly waves are of particular interest for weather forecasting given their tendency to spawn hurricanes. More generally, tropical convection is an engine of weather and climate worldwide yet poorly understood and difficult to simulate. Work leading to better representation of tropical convection in weather and climate models can thus lead to better weather forecasts and impact assessments, both for the public and decision makers. Aside from the societal relevance of the science, the campaign includes several education and outreach activities including support for undergraduates to participate in fieldwork and production of short documentary videos for use in classroom teaching and informal science education. Work performed under this award fosters international scientific collaboration with scientists in Colombia and Costa Rica and helps to develop local infrastructure for rainwater collection and isotopic analysis.The eight week OTREC deployment consists primarily of a set of 20 flights in the Gulfstream V (GV) research aircraft maintained by the National Center for Atmospheric Research (NCAR). The campaign uses an airport in Costa Rica for easy access to the equatorial Pacific and southwest Caribbean study regions, and conditions in these regions are sampled using dropsondes and a wing-mounted W-band radar. Dropsondes contain the same instrument package as standard weather balloons, only dropped from an aircraft (in this case from about 40,000 feet) with a small parachute. Work performed here augments the aircraft observations through the collection and analysis of rainwater samples from ground sites in Costa Rica and Colombia.Despite intensive study we lack a satisfactory theory for how environmental conditions determine when and where tropical convection will form, how long it will last, how much it will expand and organize, and how much rain it will produce. Vertical structure is a key property of tropical convection that determines its evolution and its interactions with the circulation and thermodynamics of the surrounding atmosphere. The vertical structure can be simply characterized as top- or bottom-heavy, according to the relative amounts of rising motion that occur in the upper and lower troposphere.Work conducted here seeks to determine the vertical structure of tropical convection by measuring the ratios of stable isotopes in rainwater produced by it. Rainwater is almost entirely composed of ordinary H2O, but it contains trace amounts of water in which deuterium (D) replaces hydrogen (H) to form HDO, or oxygen-18 replaces oxygen-16. These heavier isotopes condense more readily than ordinary H2O and are thus more common in raindrops formed lower rather than higher in the troposphere. The relative abundance of heavier isotopes in raindrops should thus provide a means to determine if they formed in top-heavy or bottom-heavy convection. Rainwater collection for isotopic analysis is planned at three sites in Colombia: Nuqui, on the west coast; Quibdo, further inland; and Isla San Andres, in the southwest Caribbean. Estimates of convective vertical profiles from isotopic composition are compared with estimates from aircraft measurements to determine if the method works. Further examination of the isotopic data would be conducted using numerical simulations from a variety of atmospheric models including some in which tracer "particles" representing water vapor are tracked as they rise through cloud systems and ultimately condense.A further goal of the research is to understand the transition from shallow to deep convection, a critical component of the lifecycle of convective clouds. Three contributing factors in the transition are examined: the gradual moistening of the atmospheric boundary layer; organization of the subcloud layer, which allows cloud roots to form in air which has greater moisture than the large-scale mean; and cold pools formed by convective downdrafts, which organize the subcloud layer to produce larger cloud roots which are in turn less susceptible to the diluting effect of dry air entrainment. The third of these factors may lead to a positive feedback, in which convective downdrafts organize the subcloud layer in a way that further promotes convection. Better characterization of entrainment is of particular interest over the southwest Caribbean, as midtropospheric air in this region is quite dry yet convection still occurs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.