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- Lind, Petter, 1979-
(författare)
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Kilometer-scale climate modeling of precipitation in the Nordic region
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Future changes in precipitation, in particular extremes, are among the most impact-relevant consequences of a warming climate driven by increases in atmospheric greenhouse gas concentrations. Still, climate model projections of future changes in regional and local precipitation remain uncertain. This is in part due to inabilities of climate models to properly represent important atmospheric moist processes, such as convection, as well as surface properties like complex terrain, primarily since these models are typically run at relatively coarse horizontal resolution. The application of a new generation of kilometer-scale ”convection-permitting” models (CPMs), which treat deep convection explicitly, has led to a step-change improvement in simulating precipitation, especially short-duration local intense events. Therefore, CPMs have proven to be valuable tools in understanding precipitation in present climate and its response to rising global temperatures.Here, the performance of the HARMONIE-Climate (HCLIM) CPM in a regional context has been investigated as well as the added value of this model in comparison with HCLIM run with standard grid resolution of ~10 km. In the present climate, the HCLIM CPM applied over the Nordic region outperforms both the coarser-scale HCLIM and a global reanalysis data set, especially for precipitation on sub-daily time scales in summer when precipitation is often convective. This is corroborated in a study investigating how precipitation is related to large-scale atmospheric circulation, which revealed differences between the HCLIM CPM and its coarser counterpart in convection-dominated circulation types in summer. By improving the frequency and intensity distributions, the wet bias seen in the coarser HCLIM version is reduced by the CPM while also better capturing intense precipitation events, but also improvements in the partitioning between snow and rain in complex terrain.In projections of a future warmer climate, the HCLIM CPM simulates stronger increases in heavy precipitation compared to the coarser-scale HCLIM version, most notably in the warm season, sometimes in excess of the thermodynamically constrained increase in atmospheric moisture content of ~7%/oC, referred to as the Clausius-Clapeyron scaling relation (CC-relation). Applying the HCLIM CPM over European sub-regions with different temperature and humidity conditions reveal consistently stronger increase of sub-daily precipitation at the local scale compared to the scales represented by coarser models. However, the environmental conditions played an important role in the scaling of precipitation with temperature; a scaling larger than the CC-relation was found in regions with relatively moist conditions, while in dry areas the scaling was in line with or smaller than the CC-relation.It is concluded that there is a clear benefit of using HCLIM at the convection-permitting scale, a fit-for-purpose model to investigate precipitation processes and their change following global warming over the Nordic region and elsewhere.
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| 3. |
- Strandberg, Gustav, 1977-
(författare)
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Modelling regional climate-vegetation interactions in Europe : A palaeo perspective
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Studies in paleoclimate are important because they give us knowledge about how the climate system works and puts the current climate change in necessary perspective. By studying (pre)historic periods we increase our knowledge not just about these periods, but also about the processes that are important for climatic variations and changes. This thesis deals mainly with the interaction between climate and vegetation. Vegetation changes can affect climate in many different ways. These effects can be divided into two main categories: biogeochemical and biogeophysical processes. This thesis studies the biogeophysical effects of vegetation changes on climate in climate models. Climate models are a necessary tool for investigating how climate responds to changes in the climate system, as well as for making predictions of future climate. The biogeophysical processes are strongly related to characteristics of the land surface. Vegetation changes alter the land surface’s albedo (ability to reflect incoming solar radiation), roughness and evapotranspiration (the sum of evaporation and tran-spiration), which in turn affects the energy fluxes between the land surface and the atmosphere and thereby the climate. It is not, however, evident in what way; denser vegetation (e.g. forest instead of grassland) gives decreased albedo, which results in higher temperature, but also increased evapotranspiration, which contrastingly results in lower temperature. Vegetation changes are in this thesis studied in four different (pre)historic periods: two very cold periods with no human influence (c. 44,000 and 21,000 years ago), one warm period with minor human influence (c. 6,000 years ago) and a cold period with substantial human influence (c. 200 years ago). In addition to that the present climate is studied. The combination of these periods gives an estimate of the effect of both natural and anthropogenic vegetation on climate in different climatic contexts. The results show that vegetation changes can change temperature with 1–3 °C depending on season and region. The response is not the same everywhere, but depends on local properties of the land surface. During the winter half of the year, the albedo effect is usually most important as the difference in albedo between forest and open land is very large. During the summer half of the year the evapotranspiration effect is usually most important as differences in albedo between different vegetation types are smaller. A prerequisite for differences in evapotranspiration is that there is sufficient amount of water available. In dry regions, evapotranspiration does not change much with changes in vegetation, which means that the albedo effect will dominate also in summer. The conclusion of these studies is that vegetation changes can have a considerable effect on climate, comparable to the effect of increasing amounts of greenhouse gases in scenarios of future climate. Thus, it is important to have an appropriate description of the vegetation in studies of past, present and future climate. This means that vegetation has the potential to work as a feedback mechanism to natural climatic variations, but also that man can alter climate by altering the vegetation. It also means that mankind may have influenced climate before we started to use fossil fuel. Consequently, vegetation changes can be used as a means to mitigate climate change locally.
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| 4. |
- Syed, Faisal Saeed, 1971-
(författare)
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On the intra-seasonal to decadal climate variability over South-Asia
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- South Asia, a land of contrasting landscapes, seasons and climates, is highly vulnerable to climate variability over intra-seasonal to decadal time scales. In winter, precipitation over the western parts of south Asia and fog over the Indo-Gangetic (IG) plains are the two major climatic features. During summer most of the region comes under the grip of monsoon. Winter precipitation over the north-western parts of South Asia is associated with eastwards propagating ‘western disturbances’ originating mostly from Mediterranean. Both observations and regional climate-model simulations show that the winter precipitation increases/decreases during the positive/negative phases of the North Atlantic Oscillation (NAO) and the warm/cold phase of the El Niño-Southern Oscillation (ENSO). During these phases, the intensification of western disturbances results from the effect of an enhanced trough visible at sea-level as well as at higher altitudes over central Asia. The inter-annual variability of fog is coupled over IG plains with a significant trend in the fog frequencies, both in observations and ERA-Interim reanalysis data. This increase shows two distinct regime shifts in 1990 and 1998 with respect to mean and variance, this in contrast to a gradual increase of the humidity over the region. The thermodynamic analysis of the intra-seasonal summer monsoon active phases (APs) over Pakistan revealed that a few days before AP, an upper-level warm anomaly appears over the northern Hindu Kush-Himalaya region and is reinforced by surface heating. The baroclinic height anomalies, with a low-level anticyclone located east of the warming, causes a moisture convergence, strong enough to overcome the preexisting stable atmospheric conditions. The extratropical dynamics also play an important role for the inter-annual variation of the South-Asian monsoon. It is found that the two leading modes between the upper-level circulation in the Atlantic/European region and monsoon rainfall are the Circumglobal Teleconnection (CGT) and the summer NAO. The positive phase of the CGT is related to a widespread increase of monsoon rainfall, and a positive summer NAO is related to a precipitation dipole with its positive anomaly over Pakistan.
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