| 1. |
|
|
| 2. |
- Hartung, Kerstin, et al.
(author)
-
An EC-Earth coupled atmosphere-ocean single-column model (AOSCM.v1_EC-Earth3) for studying coupled marine and polar processes
- 2018
-
In: Geoscientific Model Development. - : COPERNICUS GESELLSCHAFT MBH. - 1991-959X .- 1991-9603. ; 11:10, s. 4117-4137
-
Journal article (peer-reviewed)abstract
- Single-column models (SCMs) have been used as tools to help develop numerical weather prediction and global climate models for several decades. SCMs decouple small-scale processes from large-scale forcing, which allows the testing of physical parameterisations in a controlled environment with reduced computational cost. Typically, either the ocean, sea ice or atmosphere is fully modelled and assumptions have to be made regarding the boundary conditions from other subsystems, adding a potential source of error. Here, we present a fully coupled atmosphere-ocean SCM (AOSCM), which is based on the global climate model EC-Earth3. The initial configuration of the AOSCM consists of the Nucleus for European Modelling of the Ocean (NEMO3.6) (ocean), the Louvain-la-Neuve Sea Ice Model (LIM3) (sea ice), the Open Integrated Forecasting System (OpenIFS) cycle 40r1 (atmosphere), and OASIS3-MCT (coupler). Results from the AOSCM are presented at three locations: the tropical Atlantic, the midlatitude Pacific and the Arctic. At all three locations, in situ observations are available for comparison. We find that the coupled AOSCM can capture the observed atmospheric and oceanic evolution based on comparisons with buoy data, soundings and ship-based observations. The model evolution is sensitive to the initial conditions and forcing data imposed on the column. Comparing coupled and uncoupled configurations of the model can help disentangle model feedbacks. We demonstrate that the AOSCM in the current set-up is a valuable tool to advance our understanding in marine and polar boundary layer processes and the interactions between the individual components of the system (atmosphere, sea ice and ocean).
|
|
| 3. |
|
|
| 4. |
- Hartung, Kerstin, et al.
(author)
-
Diagnosing topographic forcing in an atmospheric dataset : The case of the North American Cordillera
- 2020
-
In: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 146:726, s. 314-326
-
Journal article (peer-reviewed)abstract
- It is well known from modelling studies that surface topography influences the large-scale atmospheric circulation and that several model biases are associated with incorrect representation of topography. The textbook explanation of topographic effects on large-scale circulation appeals to the theoretical relationship between surface forcing and vortex stretching along trajectories in single-layer models. The goal of this study is to design and use a simple diagnostic of the large-scale forcing on the atmosphere when air is passing over topography, directly from atmospheric fields, based on this theoretical relationship. The study examines the interaction of the atmosphere with the North American Cordillera and samples the flow by means of trajectories during Northern Hemisphere winter. We detect a signal of topographic forcing in the atmospheric dataset, which, although much less distinct than in the theoretical relationship, nevertheless exhibits a number of expected properties. Namely, the signal increases with latitude, is usually stronger upslope than downslope, and is enhanced if the flow is more orthogonal to the mountain ridge, for example during periods of positive Pacific-North American index (PNA). Furthermore, a connection is found between an enhanced signal of topographic forcing downslope of the North American Cordillera and periods of more frequent downstream European blocking.
|
|
| 5. |
- Hartung, Kerstin, 1989-, et al.
(author)
-
Exploring the Dynamics of an Arctic Sea Ice Melt Event Using a Coupled Atmosphere-Ocean Single-Column Model (AOSCM)
- 2022
-
In: Journal of Advances in Modeling Earth Systems. - 1942-2466. ; 14:6
-
Journal article (peer-reviewed)abstract
- The Arctic climate system is host to many processes which interact vertically over the tightly coupled atmosphere, sea ice and ocean. The coupled Atmosphere-Ocean Single-Column Model (AOSCM) allows to decouple local small-scale and large-scale processes to investigate the model performance in an idealized setting. Here, an observed Arctic warm air intrusion event is used to show how to identify model deficiencies using the AOSCM. The AOSCM allows us to effectively produce a large number of perturbation simulations, around 1,000, to map sensitivities of the model results due to changes in physical and model properties as well as to the large-scale tendencies. The analysis of the summary diagnostics, that is, aggregated results from sensitivity experiments evaluated against modeled physical properties, such as surface energy budget and mean sea ice thickness, reveals sensitivities to the chosen parameters. Further, we discuss how the conclusions can be used to understand the behavior of the global host model. The simulations confirm that the horizontal advection of heat and moisture plays an important role for maintaining a low-level cloud cover, as in earlier studies. The combined cloud layers increase the energy input to the surface, which in turn enhances the ongoing melt. The clouds present an additional sensitivity in terms of how they are represented but also their interaction with the large-scale advection and the model time step. The methodology can be used for a variety of other regions, where the coupling to the ocean is important.
|
|
| 6. |
- Hartung, Kerstin, 1989-
(author)
-
Paths to improving atmospheric models across scales : The importance of the unresolved scales
- 2018
-
Doctoral thesis (other academic/artistic)abstract
- Correct representation of physical processes, the parametrizations, and their interaction with the resolved circulation is crucial for the performance of numerical models. Here, focus is put on understanding model biases and developing tools to alleviate existing biases. Atmospheric blocking can divert the typical atmospheric flow for several days up to weeks and thereby impacts the mean climate of the region experiencing blocking. Models typically underestimate the frequency of atmospheric blocking. Based on results from the global climate model EC-Earth, it is found that the atmospheric model resolution is not strongly influencing the representation of atmospheric blocking once the grid reaches about 80 km grid length in the horizontal. Updating several physical parametrizations, and thereby the model version, is the largest contributor to advancements in simulating atmospheric blocking. The importance of the topography for the large-scale atmospheric flow is further investigated with the reanalysis ERA-Interim by applying a simplified theoretical analysis. It is found that the idealized topographic forcing theory can explain some part of the observed large-scale properties of the flow, though the method does mainly produce relative results. The explained part of the large-scale structure is increased during periods of northwesterly flow and when the flow impinges the mountain ridge almost orthogonally.Small-scale processes acting in air masses transported from midlatitudes to the Arctic are also discussed. Numerical models often struggle with representing the stable conditions in the Arctic and tend to underestimate the downward longwave impact during cloudy conditions. A comparison of single-column models (SCMs) indicates that most models can capture the bimodal longwave distribution which develops from alternating cloudy and clear-sky conditions. SCMs are often used for model development as they allow to decouple the parametrized physical processes from the large-scale environment and enable many parameter sensitivity tests. A new tool is presented which can be used for the development of physical parametrizations in marine and polar conditions. It combines one-dimensional models of the atmosphere and ocean, including sea-ice, into a coupled atmosphere-ocean SCM (AOSCM). The presented setup constitutes an advantage compared to SCMs of one component because the coupling is directly modelled and the interaction between the respective boundary layers does not dependent on prescribed boundary conditions.
|
|
| 7. |
- Hartung, Kerstin, et al.
(author)
-
Resolution, physics and atmosphere–ocean interaction – How do they influence climate model representation of Euro-Atlantic atmospheric blocking?
- 2017
-
In: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 69:1
-
Journal article (peer-reviewed)abstract
- Atmospheric blocking events are known to locally explain a large part of climate variability. However, despite their relevance, many current climate models still struggle to represent the observed blocking statistics. In this study, simulations of the global climate model EC-Earth are analysed with respect to atmospheric blocking. Seventeen simulations map the uncertainty space defined by the three-model characteristics: atmospheric resolution, physical parameterization and complexity of atmosphere–ocean interaction, namely an atmosphere coupled to an ocean model or forced by surface data. Representation of the real-world statistics is obtained from reanalyses ERA-20C, JRA-55 and ERA-Interim which agree on Northern Hemisphere blocking characteristics. Blocking events are detected on a central blocking latitude which is individually determined for each simulation. The frequency of blocking events tends to be underestimated relative to ERA-Interim over the Atlantic and western Eurasia in winter and overestimated during spring months. However, only few model setups show statistically significant differences compared to ERA-Interim which can be explained by the large inter-annual variability of blocking. Results indicate slightly larger biases relative to ERA-Interim in coupled than in atmosphere-only models but differences between the two are not statistically significant. Although some resolution dependence is present in spring, the signal is weak and only statistically significant if the physical parameterizations of the model are improved simultaneously. Winter blocking is relatively more sensitive to physical parameterizations, and this signal is robust in both atmosphere-only and coupled simulations, although stronger in the latter. Overall, the model can capture blocking frequency well despite biases in representing the mean state of geopotential height over this area. Blocking signatures of geopotential height are represented more similar to ERA-Interim and only weak sensitivities to model characteristics remain.
|
|
| 8. |
- Ortega, Pablo, et al.
(author)
-
Improving Arctic Weather and Seasonal Climate Prediction : Recommendations for Future Forecast Systems Evolution from the European Project APPLICATE
- 2022
-
In: Bulletin of The American Meteorological Society - (BAMS). - 0003-0007 .- 1520-0477. ; 103:10, s. E2203-E2213
-
Journal article (peer-reviewed)abstract
- The Arctic environment is changing, increasing the vulnerability of local communities and ecosystems, and impacting its socio-economic landscape. In this context, weather and climate prediction systems can be powerful tools to support strategic planning and decision-making at different time horizons. This article presents several success stories from the H2020 project APPLICATE on how to advance Arctic weather and seasonal climate prediction, synthesizing the key lessons learned throughout the project and providing recommendations for future model and forecast system development.
|
|
| 9. |
- Pithan, Felix, et al.
(author)
-
Select strengths and biases of models in representing the Arctic winter boundary layer over sea ice : the Larcform 1 single column model intercomparison
- 2016
-
In: Journal of Advances in Modeling Earth Systems. - 1942-2466. ; 8:3, s. 1345-1357
-
Journal article (peer-reviewed)abstract
- Weather and climate models struggle to represent lower tropospheric temperature and moisture profiles and surface fluxes in Arctic winter, partly because they lack or misrepresent physical processes that are specific to high latitudes. Observations have revealed two preferred states of the Arctic winter boundary layer. In the cloudy state, cloud liquid water limits surface radiative cooling, and temperature inversions are weak and elevated. In the radiatively clear state, strong surface radiative cooling leads to the build-up of surface-based temperature inversions. Many large-scale models lack the cloudy state, and some substantially underestimate inversion strength in the clear state. Here, the transformation from a moist to a cold dry air mass is modeled using an idealized Lagrangian perspective. The trajectory includes both boundary layer states, and the single-column experiment is the first Lagrangian Arctic air formation experiment (Larcform 1) organized within GEWEX GASS (Global atmospheric system studies). The intercomparison reproduces the typical biases of large-scale models: some models lack the cloudy state of the boundary layer due to the representation of mixed-phase microphysics or to the interaction between micro- and macrophysics. In some models, high emissivities of ice clouds or the lack of an insulating snow layer prevent the build-up of surface-based inversions in the radiatively clear state. Models substantially disagree on the amount of cloud liquid water in the cloudy state and on turbulent heat fluxes under clear skies. Observations of air mass transformations including both boundary layer states would allow for a tighter constraint of model behavior.
|
|
| 10. |
- Sotiropoulou, Georgia, et al.
(author)
-
Large-eddy simulation of a warm-air advection episode in the summer Arctic
- 2018
-
In: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 144:717, s. 2449-2462
-
Journal article (peer-reviewed)abstract
- While there is an increasing scientific interest in the role of advection of warm and moist air into the Arctic, there is little understanding of the interactive processes between the advected air, boundary-layer clouds and turbulence during such events and almost all studies refer to winter conditions. We use large-eddy simulation (LES) to investigate these processes for an extreme warm-air advection episode observed during summer 2014. The results indicate that moisture advection is the critical factor for cloud formation; shutting off this supply resulted in cloud dissipation, regardless of heat advection being present or not. The dissipation of the cloud reduced the surface energy budget by up to 37W/m(2). Advection of heat suppresses cloud-driven mixing through enhancement of the atmospheric stability. Turning off the large-scale heat transport therefore resulted in a somewhat optically thicker cloud, on average increasing the liquid water path by approximate to 10g/m(2). The results showed little sensitivity to a number of assumptions and simplifications in the LES set-up, such as the prescribed cloud condensation nuclei concentration, friction velocity, surface albedo and the available moisture above the cloud layer.
|
|
