Formation of rain layers in the Warm Pool and their feedbacks to atmospheric convection in an idealized modeling framework

在温暖的热带海洋上空，潮湿空气的供应建立了对流系统，在降雨中表现为湿润和干燥的模式，每次持续30至60天，缓慢向东传播。在这种所谓的麦登-朱利安振荡（MJO）中，对海洋对大气对流的反馈过程的理解仍然不完整，这阻碍了它在气候和预报模式中的表现。MJO对上层海洋的稳定性很敏感，因为稳定的表层海洋不容易或深度混合，这使得通过太阳加热或蒸发冷却更容易在浅层表层产生更大的温度异常。这种效应在温暖的表层中得到了更多的研究，但通常由降雨形成的相对淡水的薄层也可以增强表层的稳定性。由于这些层往往很小且短暂，因此对它们的研究还不够多。本项目将采用区域大气模式结合上层海洋简化模式，探讨雨层对大气对流的影响。除了更好地理解MJO机制外，该项目还将引入一种新的模型来研究海气相互作用，并对海洋对大气对流的反馈进行新的分析。该项目还将促进大气和海洋学科学家之间的合作，支持女性科学家和研究生教育。在过去十年的研究中，我们进一步了解了日暖层（DWLs）对MJO发生的重要性，MJO对流和风异常如何与上层海洋热含量相互作用，并记录了MJO降雨形成的大尺度海面盐度（SSS）模式的演变。然而，不太清楚的是，瞬态海洋表面淡水透镜或雨层（RLs）的形成如何调节海洋混合、海洋表面通量和MJO对流。RLs通常比暖池表面的水浮力更大，温度更低，在海洋表面产生一个冷斑。由于RLs较浅（0.5 - 5米）且寿命较短（1天），因此原位观测系统可能取样不足。对暖池中RLs的直接观测很少，记录其观测到的时空特征的努力还处于起步阶段。在整个MJO生命周期中，RLs的频率、强度和持续时间以及它们对大气对流的影响在很大程度上是未知的。广泛对流产生的雨层集合将通过海温冷却减少地表潜热通量和感热通量，但也将产生一个精细尺度的海温急剧梯度网络，这与对流起始有关。本研究将利用区域大气模拟系统（RAMS）耦合多列1维KPP海洋混合层模式1)研究表层加热和增温作用下上层海洋稳定层的形成，包括其频率、空间变异性和持续时间，以及它们对上层海洋稳定层和混合层的调节；2)研究MJO被抑制到活跃的过渡时期降雨层如何与对流相互作用。模型生成的稳定层统计数据将与DYNAMO野外活动期间收集的先前公布的海洋观测数据进行比较。将通过一系列诊断来评估海洋对对流的反馈，这些诊断旨在将上层海洋稳定性的变化与表面通量和热带对流的组织联系起来。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。

Over the warm tropical ocean, the supply of moist air sets up convective systems which manifest themselves in rainfall as a pattern of wet and dry spells, each lasting 30 to 60 days and propagating slowly Eastward. Within this so-called Madden-Julian Oscillation (MJO), the understanding of ocean feedback processes to atmospheric convection remains incomplete, and hinders its representation in climate and forecast models. The MJO is sensitive to upper ocean stability, because a stable surface ocean does not mix as readily or deeply, which makes it easier to generate larger temperature anomalies within a shallow surface layer through solar heating or evaporative cooling. This effect has been studied more with warm surface layers, but thin layers of relatively fresh water often formed by rainfall can also enhance stability of the surface layer. Because these layers tend to be small and ephemeral, they have not been studied as much. This project will use a regional atmospheric model coupled with a simplified model of the upper ocean to explore the impacts of rain layers on atmospheric convection. In addition to better understanding MJO mechanisms, the project will introduce a new model for investigating air-sea interactions, and novel analyses of ocean feedbacks to atmospheric convection. The project will also enable collaboration between atmospheric and oceanographic scientists, support for female scientists, and graduate education.Studies during the past decade have advanced our understanding of the importance of diurnal warm layers (DWLs) for the onset of the MJO, how MJO convection and wind anomalies interact with the upper ocean heat content, and have documented the evolution of large- scale sea surface salinity (SSS) patterns shaped by MJO rainfall. Less clear, however, is how the formation of transient ocean surface freshwater lenses, or rain layers (RLs), may temper ocean mixing, marine surface fluxes, and MJO convection. RLs are typically more buoyant and colder than Warm Pool surface waters, producing a cold patch on the ocean surface. Because RLs are shallow (0.5 - 5 m) and short-lived ( 1 day), they are likely undersampled by in situ observing systems. Direct observations of RLs in the Warm Pool are sparse, and efforts to document their observed spatial and temporal characteristics are in their infancy. The frequency, intensity, and duration of RLs, and their effects on atmospheric convection throughout the MJO lifecycle are largely unknown. A collection of rain layers generated by widespread convection would reduce surface latent and sensible heat fluxes via SST cooling, but would also generate a fine-scale network of sharp SST gradients, which have been linked to convective initiation. This study will use the Regional Atmospheric Modeling System (RAMS) coupled to many columns of a 1-dimensional KPP ocean mixed layer model 1) to study the formation of upper ocean stable layers by surface heating and freshening, including their frequency, spatial variability, and duration, and their regulation of upper ocean stability and mixing, and 2) to study how rain layers may interact with convection during the MJO suppressed-to-active transition period. Model-generated stable layer statistics will be compared to previously published ocean observations collected during the DYNAMO field campaign. Ocean feedbacks to convection will be assessed through a progression of diagnostics designed to link variations in upper ocean stability to surface fluxes and the organization of tropical convection.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.