| 1. |
- Karlsson, Bodil, et al.
(författare)
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On How the Middle Atmospheric Residual Circulation Responds to the Solar Cycle Close to the Solstices
- 2018
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Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 31:1, s. 401-421
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Tidskriftsartikel (refereegranskat)abstract
- During high solar activity, the atmosphere receives more energy from the sun, particularly in the form of shortwave radiation. Most notable is the effect in the middle and upper atmosphere, which in general shows a positive temperature response due to physical and chemical processes that are intensified at high solar activity. It is thus surprising that a clear solar cycle signal is absent in the summer polar mesosphere region in spite of it being illuminated around the clock. In this study, it is investigated how the circulation in the summer mesosphere is affected by changes in the solar flux using a 30-yr run from the nudged version of the Canadian Middle Atmosphere Model (CMAM30). It is found that-in July-the solar cycle signal from direct solar heating is counteracted by an enhanced residual circulation, which adiabatically cools the region at a higher rate when the solar activity is above average. The dynamical cooling is partly initiated in the Southern Hemisphere winter stratosphere.
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| 2. |
- Kuilman, Maartje, et al.
(författare)
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Exploring noctilucent cloud variability using the nudged and extended version of the Canadian Middle Atmosphere Model
- 2017
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 164, s. 276-288
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Tidskriftsartikel (refereegranskat)abstract
- Ice particles in the summer mesosphere-such as those connected to noctilucent clouds and polar mesospheric summer echoes-have since their discovery contributed to the uncovering of atmospheric processes on various scales ranging from interactions on molecular levels to global scale circulation patterns. While there are numerous model studies on mesospheric ice microphysics and how the clouds relate to the background atmosphere, there are at this point few studies using comprehensive global climate models to investigate observed variability and climatology of noctilucent clouds. In this study it is explored to what extent the large-scale inter-annual characteristics of noctilucent clouds are captured in a 30-year run-extending from 1979 to 2009-of the nudged and extended version of the Canadian Middle Atmosphere Model (CMAM30). To construct and investigate zonal mean inter-seasonal variability in noctilucent cloud occurrence frequency and ice mass density in both hemispheres, a simple cloud model is applied in which it is assumed that the ice content is solely controlled by the local temperature and water vapor volume mixing ratio. The model results are compared to satellite observations, each having an instrument-specific sensitivity when it comes to detecting noctilucent clouds. It is found that the model is able to capture the onset dates of the NLC seasons in both hemispheres as well as the hemispheric differences in NLCs, such as weaker NLCs in the SH than in the NH and differences in cloud height. We conclude that the observed cloud climatology and zonal mean variability are well captured by the model.
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| 3. |
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| 4. |
- Kuilman, Maartje Sanne, et al.
(författare)
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The role of the winter residual circulation in the summer mesopause regions in WACCM
- 2018
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 18:6, s. 4217-4228
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Tidskriftsartikel (refereegranskat)abstract
- High winter planetary wave activity warms the summer polar mesopause via a link between the two hemispheres. Complex wave-mean-flow interactions take place on a global scale, involving sharpening and weakening of the summer zonal flow. Changes in the wind shear occasionally generate flow instabilities. Additionally, an altering zonal wind modifies the breaking of vertically propagating gravity waves. A crucial component for changes in the summer zonal flow is the equatorial temperature, as it modifies latitudinal gradients. Since several mechanisms drive variability in the summer zonal flow, it can be hard to distinguish which one is dominant. In the mechanism coined interhemispheric coupling, the mesospheric zonal flow is suggested to be a key player for how the summer polar mesosphere responds to planetary wave activity in the winter hemisphere. We here use the Whole Atmosphere Community Climate Model (WACCM) to investigate the role of the summer stratosphere in shaping the conditions of the summer polar mesosphere. Using composite analyses, we show that in the absence of an anomalous summer mesospheric temperature gradient between the equator and the polar region, weak planetary wave forcing in the winter would lead to a warming of the summer mesosphere region instead of a cooling, and vice versa. This is opposing the temperature signal of the interhemispheric coupling that takes place in the mesosphere, in which a cold and calm winter stratosphere goes together with a cold summer mesopause. We hereby strengthen the evidence that the variability in the summer mesopause region is mainly driven by changes in the summer mesosphere rather than in the summer stratosphere.
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| 5. |
- Kuilman, Maartje Sanne, et al.
(författare)
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Using the climate feedback response analysis method to quantify climate feedbacks in the middle atmosphere
- 2020
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 20:21, s. 12409-12430
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Tidskriftsartikel (refereegranskat)abstract
- Over recent decades it has become clear that the middle atmosphere has a significant impact on surface and tropospheric climate. A better understanding of the middle atmosphere and how it reacts to the current increase in the concentration of carbon dioxide (CO2) is therefore necessary. In this study, we investigate the response of the middle atmosphere to a doubling of the CO2 concentration, and the associated changes in sea surface temperatures (SSTs), using the Whole Atmosphere Community Climate Model (WACCM). We use the climate feedback response analysis method (CFRAM) to calculate the partial temperature changes due to an external forcing and climate feedbacks in the atmosphere. As this method has the unique feature of additivity, these partial temperature changes are linearly addable. In this study, we discuss the direct forcing of CO2 and the effects of the ozone, water vapour, cloud, albedo and dynamical feedbacks. As expected, our results show that the direct forcing of CO2 cools the middle atmosphere. This cooling becomes stronger with increasing height; the cooling in the upper stratosphere is about three times as strong as the cooling in the lower stratosphere. The ozone feedback yields a radiative feedback that mitigates this cooling in most regions of the middle atmosphere. However, in the tropical lower stratosphere, and in some regions of the mesosphere, the ozone feedback has a cooling effect. The increase in the CO2 concentration causes the dynamics to change. The temperature response due to this dynamical feedback is small in terms of the global average, although there are large temperature changes due to this feedback locally. The temperature change in the lower stratosphere is influenced by the water vapour feedback and, to a lesser degree, by the cloud and albedo feedback. These feedbacks play no role in the upper stratosphere and the mesosphere. We find that the effects of the changed SSTs on the middle atmosphere are relatively small compared to the effects of changing the CO2. However, the changes in SSTs are responsible for dynamical feedbacks that cause large temperature changes. Moreover, the temperature response to the water vapour feedback in the lower stratosphere is almost solely due to changes in the SSTs. As CFRAM has not been applied to the middle atmosphere in this way before, this study also serves to investigate the applicability and the limitations of this method. This work shows that CFRAM is a very powerful tool for studying climate feedbacks in the middle atmosphere. However, it should be noted that there is a relatively large error term associated with the current method in the middle atmosphere, which can, to a large extent, be explained by the linearization in the method.
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| 6. |
- Kuilman, Maartje, 1989-
(författare)
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Variability and feedbacks in the middle atmosphere
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The importance of the middle atmosphere for the weather and climate on Earth is increasingly realized. Variability and feedback processes in the middle atmosphere need to be better understood and form the subject of this thesis. Initially, the focus has been on the variability of the summer polar mesopause, which is the coldest place in the Earth's system. The variability of this region is driven by a variety of atmospheric processes, such as atmospheric waves and the solar cycle and is even coupled to the atmosphere on other side of the globe through interhemispheric coupling. The low temperatures in the summer polar mesopause allow for thin ice clouds to form: noctilucent clouds (NLCs). It is investigated how well the Canadian Middle Atmosphere Model (CMAM30), in which the NLCs are represented in terms of a simple model, can be used to study zonal mean NLC variability. Comparing to satellite data, it is shown that the basic NLC characteristics, such as seasonal onsets and development, interannual variability and interhemispheric differences, are well captured by the model. The role of the winter residual circulation in shaping the conditions of the summer polar mesopause is also investigated, using the Whole Atmosphere Community Climate Model (WACCM). It is found that without the gravity waves in winter, the summer mesopause region would be significantly warmer. This means that the interhemispheric coupling mechanism has a net cooling effect on the summer mesopause regions. In addition, the effect of the solar cycle on the summer polar mesopause is studied. In CMAM30, there is no substantial temperature change due to the solar cycle. It is shown that there is an enhanced circulation in this region during solar maximum as compared to solar minimum, which causes adiabatic cooling counteracting the direct effect of the solar cycle. Finally, feedbacks in the middle atmosphere are studied using WACCM. The Climate Feedback Response Analysis Method (CFRAM) is used to examine the middle atmosphere response to a doubling of the CO2-concentration with respect the pre-industrial state. It was found that the temperature response to direct CO2 forcing would be approximately -9 K in the middle atmosphere. This cooling is being mitigated by the combined effect of the different feedbacks processes, the strongest of which being the ozone feedback. The dynamical feedback has large effects on the temperatures locally, while the role of the cloud, albedo and water vapor feedback are small in the middle atmosphere.
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| 7. |
- Wang, Tongmei, et al.
(författare)
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Response of stratospheric water vapour to CO2 doubling in WACCM
- 2020
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Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 54, s. 4877-4889
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Tidskriftsartikel (refereegranskat)abstract
- Stratospheric water vapour (SWV), as a greenhouse gas, modulates the radiative energy budget of the climate system. It is sensitive to, and plays a significant role in the climate change. In this study, we investigate the SWV response to CO2 increase with the Whole Atmosphere Community Climate Model (WACCM). In addition, we study its possible feedback on stratospheric temperature and relevant mechanisms. In our model experiments, the CO2 concentration and sea surface temperature (SSTs) are changed at the same time, as well as separately, to enable separating the radiative-photochemical and dynamical response to CO2 doubling scenarios. The model results show that the response of SWV to CO2 doubling is dominated by the changes in the SSTs, with an increase of the SWV concentration by similar to 6 to 10% in most of the stratosphere and more than 10% in the lower stratosphere, except for winter pole in the lower stratosphere, where the CO2 doubling decreases water vapour. The increase of SWV is mostly due to a dynamical response to the warm SSTs. Doubled CO2 induces warm SSTs globally and further leads to moist troposphere and a warmer tropical and subtropical tropopause, resulting in more water vapour entering stratosphere from below. As a greenhouse gas, large increase of SWV in the lower stratosphere, in turn, affects the stratospheric temperature, resulting in a warming of the tropical and subtropical lower stratosphere, offsetting the cooling caused by CO2 doubling.
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| 8. |
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