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  • 1
    Language: English
    In: Climate dynamics, 2013-08, Vol.41 (3), p.735-754
    Description: An analysis is presented of an ensemble of regional climate model (RCM) experiments from the ENSEMBLES project in terms of mean winter snow water equivalent (SWE), the seasonal evolution of snow cover, and the duration of the continuous snow cover season in the European Alps. Two sets of simulations are considered, one driven by GCMs assuming the SRES A1B greenhouse gas scenario for the period 1951–2099, and the other by the ERA-40 reanalysis for the recent past. The simulated SWE for Switzerland for the winters 1971–2000 is validated against an observational data set derived from daily snow depth measurements. Model validation shows that the RCMs are capable of simulating the general spatial and seasonal variability of Alpine snow cover, but generally underestimate snow at elevations below 1,000 m and overestimate snow above 1,500 m. Model biases in snow cover can partly be related to biases in the atmospheric forcing. The analysis of climate projections for the twenty first century reveals high inter-model agreement on the following points: The strongest relative reduction in winter mean SWE is found below 1,500 m, amounting to 40–80 % by mid century relative to 1971–2000 and depending upon the model considered. At these elevations, mean winter temperatures are close to the melting point. At higher elevations the decrease of mean winter SWE is less pronounced but still a robust feature. For instance, at elevations of 2,000–2,500 m, SWE reductions amount to 10–60 % by mid century and to 30–80 % by the end of the century. The duration of the continuous snow cover season shows an asymmetric reduction with strongest shortening in springtime when ablation is the dominant factor for changes in SWE. We also find a substantial ensemble-mean reduction of snow reliability relevant to winter tourism at elevations below about 1,800 m by mid century, and at elevations below about 2,000 m by the end of the century.
    Subject(s): Atmospheric circulation ; Climate change ; Climate models ; Climatology ; Climatology. Bioclimatology. Climate change ; Earth Sciences ; Earth, ocean, space ; ENSEMBLES ; Environmental aspects ; European Alps ; Exact sciences and technology ; External geophysics ; Geophysics/Geodesy ; Meteorology ; Oceanography ; Regional climate projections ; Research ; Snow ; Snow cover duration ; Snow water equivalent ; Snow. Ice. Glaciers
    ISSN: 0930-7575
    E-ISSN: 1432-0894
    Source: Academic Search Ultimate
    Source: Alma/SFX Local Collection
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  • 2
    Language: English
    In: Climate dynamics, 2013-06, Vol.40 (11), p.3107-3134
    Description: In the framework of the global energy balance, the radiative energy exchanges between Sun, Earth and space are now accurately quantified from new satellite missions. Much less is known about the magnitude of the energy flows within the climate system and at the Earth surface, which cannot be directly measured by satellites. In addition to satellite observations, here we make extensive use of the growing number of surface observations to constrain the global energy balance not only from space, but also from the surface. We combine these observations with the latest modeling efforts performed for the 5th IPCC assessment report to infer best estimates for the global mean surface radiative components. Our analyses favor global mean downward surface solar and thermal radiation values near 185 and 342 Wm−2, respectively, which are most compatible with surface observations. Combined with an estimated surface absorbed solar radiation and thermal emission of 161 and 397 Wm−2, respectively, this leaves 106 Wm−2 of surface net radiation available globally for distribution amongst the non-radiative surface energy balance components. The climate models overestimate the downward solar and underestimate the downward thermal radiation, thereby simulating nevertheless an adequate global mean surface net radiation by error compensation. This also suggests that, globally, the simulated surface sensible and latent heat fluxes, around 20 and 85 Wm−2 on average, state realistic values. The findings of this study are compiled into a new global energy balance diagram, which may be able to reconcile currently disputed inconsistencies between energy and water cycle estimates.
    Subject(s): Climatology ; CMIP5/IPCC-AR5 model evaluation ; Earth Radiation Budget ; Earth Sciences ; Earth, ocean, space ; Exact sciences and technology ; External geophysics ; Geophysics/Geodesy ; Global climate models ; Global energy balance ; Meteorology ; Oceanography ; Radiative transfer. Solar radiation ; Surface energy balance ; Surface/Satellite observations
    ISSN: 0930-7575
    E-ISSN: 1432-0894
    Source: Academic Search Ultimate
    Source: Alma/SFX Local Collection
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  • 3
    Language: English
    In: Journal of geophysical research. Atmospheres, 2017-05-27, Vol.122 (10), p.5237-5258
    Description: Convection‐resolving models allow to explicitly resolve deep convection at horizontal grid spacings of O(1 km). On current supercomputers, refining the grid spacing to the kilometer scale is computationally still extremely demanding, and therefore, climate simulations at this resolution have so far largely been limited to subcontinental computational domains. However, new supercomputers that mix conventional multicore CPUs and accelerators possess properties beneficial for climate codes. Exploiting these capabilities allows expansion of the size of the computational domains to continental scales. Here we present such a convection‐resolving climate simulation, using a version of the COSMO model, capable of exploiting GPU accelerators. The simulation has a grid spacing of 2.2 km, 1536 × 1536 × 60 grid points, covers the period 1999–2008, and is driven by the ERA‐Interim reanalysis. An assessment of the 10‐year‐long simulation is conducted using a wide range of data sets, including several rain gauge networks, energy balance stations, and a remotely sensed lightning data set. Substantial improvements are found for the 2 km simulation in terms of the diurnal cycles of precipitation. This confirms results found in studies using smaller computational domains. However, the continental‐scale simulations also reveal deficiencies such as substantial performance differences between regions with and without strong orographic forcing. Analysis of the statistical distribution of updrafts and downdrafts shows an increase of the amplitude in seasons with convection and a pronounced asymmetry between updrafts and downdrafts. Furthermore, the analysis of lightning data shows that the convection‐resolving simulation is able to reproduce important features of the annual cycle of deep convection in Europe. Key Points Demonstration of convection‐resolving climate simulations on a continental‐scale computational domain Robust improvements in the representation of summer convection
    Subject(s): convection resolving ; diurnal cycle ; graphics processing unit ; lightning ; moist convection ; regional climate model
    ISSN: 2169-897X
    E-ISSN: 2169-8996
    Source: Alma/SFX Local Collection
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  • 4
    Language: English
    In: Climate dynamics, 2020-07, Vol.55 (1-2), p.61-75
    Subject(s): Air pollution ; Analysis ; Climate models ; Global temperature changes ; Greenhouse gases ; Models ; Precipitation (Meteorology)
    ISSN: 0930-7575
    E-ISSN: 1432-0894
    Source: Academic Search Ultimate
    Source: Alma/SFX Local Collection
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  • 5
    Language: English
    In: Climate dynamics, 2020-07, Vol.55 (1-2), p.215-234
    Subject(s): Analysis ; Models ; Technology application ; Weather
    ISSN: 0930-7575
    E-ISSN: 1432-0894
    Source: Academic Search Ultimate
    Source: Alma/SFX Local Collection
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  • 6
    Language: English
    In: Journal of the atmospheric sciences, 2014-02-01, Vol.71 (2), p.782-799
    Description: Abstract The importance of soil moisture anomalies on airmass convection over semiarid regions has been recognized in several studies. The underlying mechanisms remain partly unclear. An open question is why wetter soils can result in either an increase or a decrease of precipitation (positive or negative soil moisture–precipitation feedback, respectively). Here an idealized cloud-resolving modeling framework is used to explore the local soil moisture–precipitation feedback. The approach is able to replicate both positive and negative feedback loops, depending on the environmental parameters. The mechanism relies on horizontal soil moisture variations, which may develop and intensify spontaneously. The positive expression of the feedback is associated with the initiation of convection over dry soil patches, but the convective cells then propagate over wet patches where they strengthen and preferentially precipitate. The negative feedback may occur when the wind profile is too weak to support the propagation of convective features from dry to wet areas. Precipitation is then generally weaker and falls preferentially over dry patches. The results highlight the role of the midtropospheric flow in determining the sign of the feedback. A key element of the positive feedback is the exploitation of both low convective inhibition (CIN) over dry patches (for the initiation of convection) and high CAPE over wet patches (for the generation of precipitation).
    Subject(s): Clouds ; Convection (Meteorology) ; Environmental aspects ; Research ; Soil moisture
    ISSN: 0022-4928
    E-ISSN: 1520-0469
    Source: Academic Search Ultimate
    Source: Alma/SFX Local Collection
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  • 7
    Language: English
    In: Climate dynamics, 2015-06, Vol.44 (11-12), p.3393-3429
    ISSN: 0930-7575
    E-ISSN: 1432-0894
    Source: Academic Search Ultimate
    Source: Alma/SFX Local Collection
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  • 8
    Language: English
    In: Journal of climate, 2009-10-01, Vol.22 (19), p.5003-5020
    Description: Moist convection is a key aspect of the extratropical summer climate and strongly affects the delicate balance of processes that determines the surface climate in response to larger-scale forcings. Previous studies using parameterized convection have found that the feedback between soil moisture and precipitation is predominantly positive (more precipitation over wet soils) over Europe. Here this feedback is investigated for one full month (July 2006) over the Alpine region using two different model configurations. The first one employs regional climate simulations performed with the Consortium for Small-Scale Modeling Model in Climate Mode (CCLM) on a grid spacing of 25 km. The second one uses the same model but integrated on a cloud-resolving grid of 2.2 km, allowing an explicit treatment of convection. Each configuration comprises one control and two sensitivity experiments. The latter start from perturbed soil moisture initial conditions. Comparison of the simulated soil moisture–precipitation feedback reveals significant differences between the two systems. The 25-km simulations sustain a strong positive feedback, while those at 2.2-km resolution are associated with a predominantly negative feedback. Thus the two systems yield not only different strengths of this key feedback but also different signs. This has important implications, with the cloud-resolving model exhibiting a shorter soil moisture memory and a smaller soil moisture–temperature feedback. Analysis shows that the different feedback signs relate to the sensitivity of the simulated convective development to the presence of a stable layer sitting on top of the planetary boundary layer. In the 2.2-km integrations, dry initial soil moisture conditions yield more vigorous thermals (owing to stronger daytime heating), which can more easily break through the stable air barrier, thereby leading to deep convection and ultimately to a negative soil moisture–precipitation feedback loop. In the 25-km integrations, deep convection is much less sensitive to the stable layer because of the design of the employed convective parameterization. The authors also show that there are considerable differences in the simulated soil moisture–precipitation feedback between low-resolution modeling frameworks using different cloud convection schemes.
    Subject(s): Climate models ; Cloud cover ; Clouds ; Convection ; Convection clouds ; Earth, ocean, space ; Exact sciences and technology ; External geophysics ; Meteorology ; Negative feedback ; Parameterization ; Precipitation ; Soil water ; Soil water content
    ISSN: 0894-8755
    E-ISSN: 1520-0442
    Source: Academic Search Ultimate
    Source: Alma/SFX Local Collection
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  • 9
    Language: English
    In: Climate dynamics, 2017-05-01, Vol.48 (9-10), p.3425-3440
    Description: Climate models robustly project a strong overall summer warming across Europe showing a characteristic north-south gradient with enhanced warming and drying in southern Europe. However, the processes that are responsible for this pattern are not fully understood. We here employ an extended surrogate or pseudo-warming approach to disentangle the contribution of different mechanisms to this response pattern. The basic idea of the surrogate technique is to use a regional climate model and apply a large-scale warming to the lateral boundary conditions of a present-day reference simulation, while maintaining the relative humidity (and thus implicitly increasing the specific moisture content). In comparison to previous studies, our approach includes two important extensions: first, different vertical warming profiles are applied in order to separate the effects of a mean warming from lapse-rate effects. Second, a twin-design is used, in which the climate change signals are not only added to present-day conditions, but also subtracted from a scenario experiment. We demonstrate that these extensions provide an elegant way to separate the full climate change signal into contributions from large-scale thermodynamic (TD), lapse-rate (LR), and circulation and other remaining effects (CO). The latter in particular include changes in land-ocean contrast and spatial variations of the SST warming patterns. We find that the TD effect yields a large-scale warming across Europe with no distinct latitudinal gradient. The LR effect, which is quantified for the first time in our study, leads to a stronger warming and some drying in southern Europe. It explains about 50 % of the warming amplification over the Iberian Peninsula, thus demonstrating the important role of lapse-rate changes. The effect is linked to an extending Hadley circulation. The CO effect as inherited from the driving GCM is shown to further amplify the north-south temperature change gradient. In terms of mean summer precipitation the TD effect leads to a significant overall increase in precipitation all across Europe, which is compensated and regionally reversed by the LR and CO effects in particular in southern Europe.
    Subject(s): Atmospheric circulation ; Atmospheric temperature ; Climatic changes ; Environmental aspects ; Observations ; Thermodynamics
    ISSN: 0930-7575
    E-ISSN: 1432-0894
    Source: Academic Search Ultimate
    Source: Alma/SFX Local Collection
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  • 10
    Language: English
    In: Journal of climate, 2009-06-01, Vol.22 (11), p.2940-2957
    Description: In this study, 40-yr ECMWF Re-Analysis (ERA-40) data are used for the description of the seasonal cycle and the interannual variability of the westerly jet in the Tibetan Plateau region. To complement results based on the analysis of monthly mean horizontal wind speeds, an occurrence-based jet climatology is constructed by identifying the locations of the jet axes at 6-hourly intervals throughout 1958–2001. Thus, a dataset describing the highly transient and localized features of jet variability is obtained. During winter and summer the westerly jet is located, respectively, to the south and north of the Tibetan Plateau. During the spring and autumn seasons there are jet transitions from south to north and vice versa. The median dates for these transitions are 28 April and 12 October. The spring transition is associated with large interannual variations, while the fall transition occurs more reliably within a 3-week period. The strength of the jet exhibits a peculiar seasonal cycle. During northward migration in April/May, the jet intensity weakens and its latitudinal position varies largely. In some springs, there are several transitions and split configurations occur before the jet settles in its northern summer position. In June, a well-defined and unusually strong jet reappears at the northern flanks of the Tibetan Plateau. In autumn, the jet gradually but reliably recedes to the south and is typically more intense than in spring. The jet transitions between the two preferred locations follow the seasonal latitudinal migration of the jet in the Northern Hemisphere. An analysis of interannual variations shows the statistical relationship between the strength of the summer jet, the tropospheric meridional temperature gradient, and the all-India rainfall series. Both this analysis and results from previous studies point to the particular dynamical relevance of the onsetting Indian summer monsoon precipitation and the associated diabatic heating for the formation of the strong summer jet. Finally, an example is provided that illustrates the climatological significance of the jet in terms of the covariation between the jet location and the spatial precipitation distribution in central Asia.
    Subject(s): Atmospheric circulation ; Climate cycles ; Climate models ; Climatic zones ; Climatology ; Earth, ocean, space ; Exact sciences and technology ; External geophysics ; Geodetic position ; Jet streams ; Meteorology ; Monsoons ; Precipitation ; Seasons ; Winds and their effects
    ISSN: 0894-8755
    E-ISSN: 1520-0442
    Source: Academic Search Ultimate
    Source: Alma/SFX Local Collection
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