Recent and current research Topics
Representation of gravity waves in models
Gravity waves with horizontal scales between tens to few thousands of kilometers are important drivers of the circulation in the middle atmosphere. In most state-of-art art climate models, they cannot be properly resolved and need to be parameterized under simplified assumptions. How well do those parameterizations represent real-world gravity wave activity? This question is not easy to answer, because it is difficult to observe the whole breadth of gravity wave scales globally – so we don’t actually know exactly how much momentum is carried by gravity waves globally into the middle atmosphere.
Recently, (Gupta et al, 2024) we have layed out a strategy to evaluate gravity waves in global climate models based on a chain of data: local Lidar data are used to evaluate a high-resolution model that resolves gravity waves, and after this validation step we used the high-resolution model for comparison to global model data.
One obvious shortcoming of the representation of gravity waves in global models is that they can propagate only strictly upward – and not side-ward, as in the real world. This might be the reason why many models have problems to simulate for example the stratospheric winds correctly. In another recent paper (Eichinger et al, 2023), we have modified a gravity wave scheme in order to emulate the side-ways propagation of gravity wave in a climate model.
Subseasonal predictability of tropospheric weather facilitated by the stratosphere
In the Waves2Weather project (https://www.wavestoweather.de/), we work on better understanding and quantifying predictability of tropospheric weather on timescales of up to ~two months, facilitated by coupling to the stratosphere with its long inherent timescales. Among others, we have done work to tease out the role of the stratosphere for predictions of the severe cold spell in spring 2018 (Kautz et al, 2020) and of the stormy spring in 2020 (Rupp et al, 2022).
The stratospheric transport circulation in a changing climate
The global circulation in the stratosphere transports air masses from the tropics, where they enter the stratosphere, all the way to polar latitudes. The circulation is driven by momentum deposition from atmospheric waves that propagate upward from the troposphere.
Since decades, model studies predict that the stratospheric overturning circulation will accelerate due to climate change. However, it is not easy to measure this global circulation – only special tracer measurements can give us some information about how fast air is transported through the stratosphere. Those available observations do not confirm what we see in the models – rather, they seem to suggest the opposite, a slow-down of the circulation in the middle stratosphere. This apparent discrepancy puzzled scientists (including me!) for many years. In work in my group, we have therefore looked in details into uncertainties both in modeled trends and in trends derived from observations – for example, asking how well climate models of different generation represent the transport circulation (Abalos et al, 2021 ; Dietmueller et al, 2018), and how different processes contribute to trends (Eichinger et al, 2019). In particular, we looked in detail into assumptions that need to be made when calculating transport times from observed tracers, using the model as test bed (Fritsch et al, 2020 ; Loeffel et al, 2022 ). This line of work showed that there are indeed very large uncertainties in estimates of trends of the stratospheric circulation from observational data – implying that we would need more continuous high quality data, and better methods to calculate transport diagnostics from observations of tracer concentrations!
hella garny 