Observing local and Remote Controls on Arctic Air mass evolution (ORCA2)

北极目前正以前所未有的速度变暖，同时伴随着海冰覆盖的急剧减少。这种加速变暖被称为北极放大效应。虽然已知各种气候反馈过程发挥了作用，但最近的研究强调了北极气候系统与中纬度地区之间的气团交换的类似重要性。这包括冷空气向南挤压，以及温暖潮湿的空气向北侵入北极高纬度地区。特别是强烈的水汽侵入事件，通过相关的增强的经向热湿输送，可以起到显著的作用。此外，水汽侵入物中嵌入的低空液态云增强了地表下潜长波辐射通量，显著增加了地表热收支，从而促进了海冰的融化。因此，阐明北极气团交换的确切性质，特别是水汽侵入，近年来得到了大量研究。现在已经很清楚的是，湿气团的演变是众多过程的净效应，这些过程涵盖了非常广泛的尺度，从微观尺度的湍流和云物理一直延伸到全球的罗斯比波。较小规模的过程就像对气团的局部控制，而像沉降这样的大规模过程就像远程控制。最近对北极含云湍流混合层的LES研究强调了水分侵入的两个方面；1)大尺度沉降对气团演化的强烈控制，2)湿度逆温在此过程中可能发挥重要作用。虽然这些新的见解代表了重要的进展，但对这些建模结果的观测支持仍然缺乏。为了达到这个目的，需要对北极水汽侵入期间的大规模沉降进行可靠的观测。该项目的目的是利用高空和远程研究飞机（HALO）在计划于2021年3月至4月举行的HALO-(AC)3战役期间收集的数据来填补这一数据空白。在这方面，HALO的一个明显优势是它的航程长，空气速度快，可以有效和频繁地对北极偏远地区的运动气团进行拉格朗日采样。其次，使用了一种测量大规模发散的新技术，该技术依赖于在大型圆形飞行模式中释放的下投探空仪，正如最近的NARVAL2活动所探索的那样。这些观测，结合对混合相云、辐射和湿度逆温的额外独立测量，将与跟踪气团的拉格朗日高分辨率大涡模拟相结合。这种高分辨率拉格朗日模拟和湿气侵入观测的协同作用是一种新颖的方法，因此将为增加我们对气团转变的了解创造新的机会。

The Arctic is currently warming at unprecedented rates, accompanied by dramatic loss in sea ice coverage. This accelerated warming is referred to as Arctic Amplification. While a variety of climate feedback processes is known to play a role, recent research has emphasized the similar importance of air mass exchanges between the Arctic climate system and the mid-latitudes. This involves southbound extrusions of cold air as well as northbound intrusions of warm and moist air into the high Arctic. Strong moisture intrusion events in particular can contribute significantly, through the associated enhanced meridional transport of heat and moisture. In addition, low level liquid clouds embedded in moisture intrusions enhance the downwelling long-wave radiative flux at the surface, which dramatically increases the surface heat budget and thus enhances the melt of the sea ice. Elucidating the exact nature of Arctic air mass exchanges, in particular moisture intrusions, has therefore been intensely researched in recent years. What has become clear is that the evolution of moist air masses is a net effect of a multitude of processes that covers an incredibly wide range of scales, stretching from micro-scale turbulence and cloud physics all the way to global Rossby waves. Smaller scale processes act like local controls on the air mass, while larger-scale processes like subsidence act like remote controls. Recent LES studies of cloud-bearing turbulent mixed-layers in the Arctic have highlighted two aspects of moisture intrusions; i) the strong control of large-scale subsidence on the air mass evolution, and ii) the potentially important role of humidity inversions in this process. While these new insights represent important progress, observational support for these modeling results is still lacking. What is needed to this purpose are reliable observations of large-scale subsidence during moisture intrusions in the Arctic. The aim of this project is to fill this data gap by using data collected by the High Altitude and LOng-range research aircraft (HALO) during the HALO-(AC)3 campaign, scheduled to take place in March-April 2021. A clear benefit of HALO in this respect are its long rage and high air speed, allowing the Lagrangian sampling of moving air masses in remote areas in the Arctic effectively and frequently. Second, use is made of a new technique for measuring large-scale divergence that relies on dropsondes released during large circular flight patterns, as explored during the recent NARVAL2 campaign. These observations, in combination with additional indepdendent measurements of mixed-phase clouds, radiation and humidity inversions, will be combined with Lagrangian high-resolution Large-Eddy Simulations that follow the air mass. This synergy of high-resolution Lagrangian simulation and observation of moisture intrusions is a novelty, and will thus create new opportunities for increasing our insight into air mass transformations.