1,720,960 research outputs found
Precession- and Obliquity-Induced Changes in Moisture Sources for Enhanced Precipitation Over the Mediterranean Sea
Full text linkEnhanced winter precipitation over the Mediterranean Sea at times of minimum precession and maximum obliquity, that is, times of enhanced insolation seasonality, could provide freshwater required to form orbitally paced sedimentary cycles across the Mediterranean, offering a possible alternative to monsoonal runoff. We investigate the sources of the enhanced winter precipitation, by applying a moisture tracking model on the results of idealized orbital extreme experiments with a state-of-the-art climate model. Precession and obliquity enhance precipitation in fall and winter. Our study shows that the source of enhanced precipitation over the Mediterranean Sea differs during the winter half-year. In fall, the majority of the precession-induced precipitation increase originates from the Mediterranean itself. However, in late winter, the increase can be attributed to enhanced moisture advection from the Atlantic. This agrees with changes in evaporation and air-sea temperature differences over the Mediterranean. The obliquity-induced precipitation increase shows much less differences, with an equal contribution of local and Atlantic sources. The mechanism behind the Atlantic source of moisture, particularly important in late winter for precession-induced precipitation changes, is related to a weakened Azores High and slightly higher surface pressure over North Africa. The resulting anomalous circulation patterns generate enhanced Atlantic moisture transport toward the Mediterranean. These mechanisms coincide with weaker storm track activity over the North Atlantic, opposite to previous studies that often attribute enhanced Mediterranean winter precipitation to a southward shift and intensification of the Atlantic storm track. We thus provide an alternative mechanism for Atlantic sources of orbitally paced Mediterranean precipitation changes.Water Resource
A new view on the hydrological cycle over continents
No full textWhere does precipitation come from? It is not easy to answer this question because of the complex and energy-intensive processes that bring moisture to a certain location and cause moisture to precipitate highly heterogeneously in space and variable over time. Part of the precipitation comes from so-called “moisture recycling”, which is moisture from land evaporation that returns to the land surface as precipitation. It is widely accepted that land-atmosphere interactions play a crucial role in the global climate, but the importance of moisture recycling specifically had, before the research presented in this thesis, not yet been fully quantified. It is, however, important to do so as the magnitude of moisture recycling can be used as an indicator for the susceptibility of our water resources to local and remote land-use change. The main research question of this thesis is: “How important is land evaporation in the hydrological cycle over continents?” Chapter 2 presents the offline Eulerian numerical atmospheric moisture tracking model WAM-2layers (Water Accounting Model-2layers), which is being used throughout the thesis. The underlying principle of this model is simply the water balance. WAM-2layers can be used to track tagged moisture on both the regional and global scale, and both forward and backward in time. The focus of this thesis is the moisture recycling over continents and therefore a near global grid is used, which includes all continents except Antarctica. The ERA-Interim reanalysis, from which evaporation, precipitation, humidity and wind speed is used, is the main data source for input to the tracking model. WAM-2layers provides a fast computation of large scale atmospheric moisture tracking while the two layers ensure that problems such as wind shear are still adequately dealt with. Chapter 3 presents new definitions for continental moisture recycling. The continental precipitation recycling ratio identifies regions that are dependent on upwind evaporation and the continental evaporation recycling ratio identifies the importance of evaporation to sustain downwind precipitation. Global maps showing the spatial distribution of two ratios are presented and together they provide a new way to describe continental scale moisture feedback within the hydrological cycle. It is estimated that on average 40% of all terrestrial precipitation is derived from continental sources and 57% of all terrestrial evaporation returns as precipitation to continents. Mountain ranges can play an important role in continental moisture recycling by either “blocking” moisture from entering the continent or by “capturing” the moisture from the atmosphere to enhance recycling. Overall, this chapter demonstrates the important role of global wind patterns, topography and land cover in continental moisture recycling patterns and the distribution of global water resources. Chapter 4 presents a novel approach to quantify the spatial and temporal scale of moisture recycling, independent of the size and shape of the region under study. As such, this approach overcomes the previously existing problem of scale- and shape dependency of regional moisture recycling ratios. It is shown that in the tropics or in mountainous terrain the local length scale of recycling can be as low as 500 to 2000 km. In temperate climates the length scale is typically between 3000 to 5000km whereas it amounts to more than 7000km in desert areas. The local time scale of recycling ranges from 3 to 20 days, with the exception of deserts, where it is much longer. Analysis of both the length and times scales identifies several hot spots of high local moisture recycling, in particular, in and around mountainous areas. It is also found that local moisture recycling plays a much more important role in summer than in winter. Chapter 5 present a new image of that global hydrological cycle over land, which, in contrast to traditional images of the hydrological cycle includes a quantification of moisture recycling, partitioned evaporation and the lifetime of all these processes separately. It is demonstrated that evaporated interception is more likely to return as precipitation on land than transpired moisture. On average, direct evaporation (essentially interception) is found to have an atmospheric residence time of 8 days, while transpiration typically resides 9 days in the atmosphere. Interception recycling has a much shorter local length scale than transpiration recycling, thus interception generally precipitates closer to its evaporative source than transpiration, which is particularly pronounced outside the tropics. The results suggest that the effect of land-use change on moisture recycling is very different during wet and dry seasons, and also during summer and winter, indicating that seasonality is important to consider when analysing effects of land-use change. During the wet season, increased or decreased interception could amplify or attenuate the local moisture recycling signal, but land-use change needs to be drastic to influence the evaporative fluxes in a way that this signal would have continental scale influence. During the dry season, land-use change (in particular deforestation), could lead to reduced transpiration, hence reduced moisture recycling, and therefore a drier dry season. Chapter 6 describe the concept of atmospheric watersheds. Precipitationsheds show how precipitation depends on upwind evaporation and evaporationsheds show how evaporation sustains precipitation downwind. The biggest sources and sinks are generally found close to the region of interest. However, for West Africa it is shown that, outside the rainy season, more distant sources, of in particular transpiration, are very important for the hydrological cycle as well. As such, this chapter illustrates how land-use change in one region alters evaporation and moisture recycling, and hence, influences precipitation, in a geographically separate region. It is concluded that land evaporation plays a major role in the hydrological cycle over continents as on average more than half of it returns as precipitation over land. Strong local moisture feedback is generally found in very wet regions, or in regions where it is enhanced by topography. Interception and transpiration are found to have contrasting roles in the hydrological cycle. While interception mainly works as an intensifier of the local hydrological cycle during wet spells, transpiration remains active during dry spells and is transported over much larger distances downwind where it can act as a significant source of moisture. The concepts of the precipitationshed and evaporationshed can be effectively used as tools to study the moisture recycling effect of land-use changes in specific regions of interest.Water ManagementCivil Engineering and Geoscience
Regionale veranderingen in het neerslagpatroon in Nederland
No full textHet wereldklimaat verandert en het Nederlandse klimaat verandert mee. Wat staat ons de komende eeuw te wachten? Klimaatwetenschappers schetsen het volgende toekomstbeeld: De invloed van de mens op het klimaat zal toenemen waardoor de wereldtemperatuur verder stijgt met 1 tot 6 graden. Meer en hevigere neerslag is dan het gevolg. In Nederland heeft dat verstrekkende gevolgen voor de waterhuishouding. Rioolstelsels die vaker moeten overstorten ten gevolge van hevigere neerslag. Poldersystemen die die aanhoudende neerslag niet kunnen verwerken met overstromingen en te hoge grondwaterstanden tot gevolg. Onderzoek naar trends in neerslag in Nederland heeft onder meer hetvolgende opgeleverd. Een statistisch significante toename van neerslag: 100 mm per eeuw in het winterhalfjaar (oktober - maart). Meteorologisch gezien het wel meer en vaker kan gaan regenen, maar dit betekent niet automatisch een toename van situaties waarbij wateroverlast optreedt. Pas als de extreme situaties, die al groter zijn dan de capaciteit waar een watersysteem is op ontworpen nog extremer worden is er mogelijk een probleem. Bovendien is het opmerkelijk dat men conclusies trekt op basis van De Bilt of een gemiddelde van meerde stations. De doelstelling van dit rapport is onderzoeken of extreme neerslag inderdaad heviger wordt en vaker voorkomt. Bovendien wordt onderzocht of er verschillen zijn tussen verschillende meetstations. De vraagstelling die hier bij hoort is: Zijn er trends in extreme neerslag in Nederland en zijn er regionale verschillen? In dit rapport zal op wetenschappelijke wijze, door middel van het analyseren van trends, getracht worden te ontdekken of het aannemelijk is dat extreme neerslag in de toekomst vaker en heviger zal voorkomen. Hierna wordt gecontroleerd of er een statistisch significante trend is te vinden in de hoeveelheid neerslag (Gaat het harder regenen?) evenals de intervalduur tussen extreme buien (Gaat het vaker extreem regenen?). Dit wordt getest met behulp van de Spearman’s Rank Test, F-test voor de stabiliteit van de variantie en de T-test voor de stabiliteit van het gemiddelde. De enige statistisch significante trends in het winterhalfjaar zijn gevonden bij de T-test voor 4-, 24- en 36-uursneerslag voor het station Groningen. Hier lijkt het vaker extreem te regenen. Met de Spearman’s Rank Test worden deze trends echter niet statistisch significant bevonden, daaruit kan geconcludeerd worden dat het toch onzeker is of het in de toekomst daadwerkelijk vaker zal gaan regenen. Naarmate er wordt gekeken naar langere duur neerslag (24-240uur) ontstaat er wel een sterkere positieve trend ten aanzien van de neerslagsommen, deze is echter niet statistisch significant. Over trends in intervalduur kan voorgaande uitspraak niet gedaan worden. In het zomerhalfjaar zijn zowel bij de testen op de neerslagsommen als op de intervalduur zijn voor de korte duur neerslagcijfers positieve trends te ontdekken, als er wordt gekeken naar langere duur neerslagcijfers slaat dit om in negatieve trends. Geen van alle neerslagcijfers zijn echter statistisch significant dus mag er niet geconcludeerd worden dat het harder of vaker gaat regenen. Er zijn aanzienlijke regionale verschillen gevonden in trends Het is aan te raden geen uitspraken te doen over heel Nederland op basis van enkel De Bilt of een gemiddelde over meerdere stations. Conclusies mogen alleen verbonden worden aan de regio van het waarnemingsstation.Water ManagementCivil Engineering and Geoscience
A new perspective on continental moisture recycling
No full textThe importance of moisture feedback between continental precipitation and evaporation, referred to as moisture recycling, is still under debate. Most of the research in the past focused on the contribution of recycling to precipitation within a certain region only. This paper clearly distinguishes between different definitions of moisture recycling. This allows us to study the complete process of continental moisture recycling. In addition to identifying how much of the precipitation originates from continental sources, a new definition is used to identify regions which are major moisture suppliers for continental precipitation. An accounting procedure based on ERA?40 reanalysis data is used to calculate moisture recycling ratios. As such, this paper derives new information from existing data. It is estimated that on average 38 % of the continental precipitation has continental origin and that 52 % of the continental evaporation returns as precipitation over continents. This paper demonstrates the important role of topography in the Andes and the Tibetan Plateau where regional moisture recycling is a key process. The Amazon and the Congo are identified as very important regions for sustaining continental precipitation. It is also demonstrated that moisture recycling from the Eurasian continent is the major supplier of the fresh water resources of China.Water ResourcesWater ManagementCivil Engineering and Geoscience
Evaluation of CMIP6 models performing on rainfall seasons and moisture tracking simulation in Yangtze River Basin
No full textThis study evaluates the performance of the CMIP6 models in simulating monsoon rainfall and moisture tracking in the Yangtze River basin. The findings reveal varying degrees of accuracy across different regions of the basin during the monsoon period. Downstream and midstream regions demonstrate higher accuracy, whereas upstream areas exhibit lower precision, along with an overall trend of overestimation. The evaluation encompasses the timing of monsoon months, as well as the peak month, while analyzing the simulation’s accuracy for rainfall. It also entails an overarching examination through a Taylor Diagram and Taylor skill scores, which spotlight models with superior and inferior performance. EC-Earth3 exhibits commendable performance, whereas models like IITM-ESM showcase poorer results. Furthermore, moisture tracking assessments, utilizing the WAM2layers model, identify limitations within the CMIP6 model in terms of replicating water vapor sources and pathways, especially in proximity to geographical features such as the Himalayas and the coastline. In addition to the basin itself, the CMIP6 model simulates central Asia as the main source of evaporation, rather than the Indian Ocean, according to the results ofERA5. However, no obvious pattern differences are shown between the different CMIP6 GCMs. Persistent challenges stem from data availability and numerical inconsistencies, necessitating enhancements in both the CMIP6 models and the WAM2layers code.Civil Engineering | Hydraulic Engineerin
Climate Adaptation under Uncertainty: A novel decision scaling approach to assess climate vulnerability in the Waterberg Biosphere Reserve, South Africa
No full textManaging water resources for the future is challenging, given the wide range of climatic and hydrological uncertainty. To support decision makers in formulating robust adaptation plans and finding their way through the broad range of available climate data and models, decision scaling was introduced: an approach for bottom-up climate vulnerability assessments, informed by Global Climate Models (GCMs). This study aims to improve decision scaling as developed by Brown et al. (2012) by introducing three recent advances in climate adaptation and uncertainty science.First, the concept of environmental flows (eflows) was adopted to represent the local ecology and variable hydrology with a broad range of indicators for evaluating the impact of climate change. Second, the GCM weighting strategy of Knutti et al. (2017) was applied to account for model performance and interdependency when estimating the plausibility of future climate conditions. Lastly, climate stress testing was not only done for annual average climate changes, but also for a prolonged dry season to represent the interannually variable character of climate change. The potential application of the novel decision scaling approach was illustrated through a case study of the Mokolo River. This river is situated in the South African Waterberg Biosphere Reserve, which faces competing water demands from tourism, industry, agriculture, and ecology under a changing climate. It was found that the additions contribute to decision scaling, as eflows indicators introduced the climate impact on multiple flow components, which provides extra information on the climate vulnerability of the river during different flow conditions. In Waterberg, low and average flow conditions were found similarly sensitive to climate change. Moreover, GCM weighting increased the range of temperature uncertainty and showed high weights for both wet and dry GCM projections, which emphasizes the need for robust climate adaptation in Waterberg. Next, the additional stress test showed that prolonging the dry season by one month influences flows throughout the following year, especially in the posterior months. In this way, understanding the impact of plausible characteristics of future climate was improved. Finally, this study revealed that local activities, such as groundwater extractions and land use changes, and available knowledge challenges the application of decision scaling to a real case study as it requires models and quantification of indicators. Therefore, carefully matching models, performance indicators, local concerns and knowledge are required for formulating climate adaptation strategies with decision scaling.Water Managemen
Observed soil moisture - precipitation feedback in Illinois: A statistical analysis over different scales
No full textThe lack of soil moisture – feedback understanding remains a large source of uncertainty for land-atmosphere coupled models. Better understanding requires observational studies. Observational studies on soil moisture – precipitation feedback in Illinois have shown contradictory results based on different approaches. This paper extends earlier research by providing a more holistic approach to this feedback by considering different scales using a new 11 years hourly soil moisture dataset (2003-2013). This allows us to revisit the previously disputed hypothesis that soil moisture is positively correlated with subsequent precipitation. Statewide, this paper finds a strong positive correlation between late spring/early summer soil moisture at the root-zone depth of 50cm and summer precipitation, in particular in terms of total precipitation for which a maximum R2=0.8 is reached at the end of May. However, in terms of precipitation occurrence this is less clear, except for extreme dry conditions. When considering different climatological regions within Illinois, the correlation between soil moisture and precipitation varies spatially. No significant R2 is found for precipitation in the northwest and southeast explained by soil moisture of the various regions. This might be due to local effects of Lake Michigan in the north and the hills in the south. On the other hand, precipitation variability over central Illinois can be explained by soil moisture variability in the northwest, reaching a maximum of R2=0.81. This can be of practical use for farmers and water resources managers. In addition, this paper separates precipitation induced by soil moisture and precipitation as a mere consequence of precipitation persistence due to non-local effects such as sea surface temperature driving large scale atmospheric circulations. However, no influence of teleconnections on precipitation generation in Illinois is found based on linear correlation tests between climate indices (SOI, MJO(6), MJO(7), NAO, EP/NP, AMO, EA and PNA) and subsequent precipitation. This suggests that precipitation is poorly explained by large scale atmospheric circulation. Hence, this paper demonstrates the observational evidence of a soil moisture - precipitation feedback in Illinois.Water ResourcesWater ManagementCivil Engineering and Geoscience
Impact of climate change on precipitation in Suriname
No full textSuriname is highly vulnerable to hazards that are climate change related, such as droughts and floods. Knowledge about future precipitation in Suriname is needed for the population of Suriname in order to adapt to climate change effects. By analysing 32 CMIP6 models, this study investigates the impact of climate change on precipitation in Suriname. The intermodel spread of the projected change in precipitation in 2100 compared to the reference period (1991-2020) is large, ranging from -45% (-2.0 mm/day) to +10% (+0.5 mm/day). Drivers of climate change in Suriname are the Intertropical Convergence Zone (ITCZ), Atlantic Meridional Overturning Circulation (AMOC) and EL Niño Southern Oscillation (ENSO). A weaker AMOC strength leads to warming of the south Atlantic Ocean and cooling of the north Atlantic Ocean, indicating a southward shift of the ITCZ. More El Niño like conditions lead to weakening of the mean zonal circulation along the equator and an eastward migration of the Walker Circulation in the Pacific. The occurrence of an increase in upward motion over the Pacific ITCZ and an increase in downward motion over Suriname also indicate an eastward migration of the Walker Circulation. With an eastward migrated Walker circulation an upward motion of moisture is strengthened and deep convection is increased over the Pacific. At the same time the opposite happens in Suriname where deep convection is decreased due to downward motion of air.Models with relatively high future drying project the ITCZ at a more southward position, leading to low precipitation amounts in Suriname. Next to that, these models project a relatively stronger southward shift of the ITCZ compared to wet models, leading to an even stronger drying effect in Suriname. The climate models show a mean weakening of the AMOC strength, especially the dry models. More El Niño like conditions, with a decrease in deep convection in projections from dry models, are another reason for lower precipitation projections for dry models than for wet models.After a bias-correction with Quantile Delta Mapping (QDM) the intermodel spread decreases significantly from 1-8 mm/day to 4-7.5 mm/day. Next to that, QDM correction has shifted the historical multimodel mean precipitation upwards by approximately 3 mm/day, almost doubling it. The doubling of the average precipitation indicates that climate models fail to accurately simulate the climate in Suriname. The 10% most extreme 1-day and 5-day cumulative precipitation values decrease according to climate projections. This is probably due to a decrease in the average projected precipitation in Suriname throughout the year. Extreme precipitation increases for the 0.1% most extreme 1-day and 5-day cumulative precipitation values. Despite the projected decrease in average precipitation, the models project more intense extreme precipitation events. The higher values for 0.1% extreme events can be explained by future warming, giving rise to a higher air capacity for water vapor.Civil Engineerin
Oceanic sources of continental precipitation and the correlation with sea surface temperature
No full textIdentifying the sources of continental precipitation has received increasing attention in recent years. With the use of various numerical methods, sources of precipitation have been identified from local to global scales. In this paper we identify the oceanic sources based on an atmospheric backtracking analysis of continental precipitation. We find that the strongest source areas are located close to the continents. In general, we define an oceanic area as a significant source when on average more than 20% of the total evaporation, and at least 250 mm/yr of evaporation ends up as continental precipitation. We grouped these identified source areas into 15 regions and performed a forward tracking analysis of oceanic evaporation. We identified the areas on the adjacent continents that receive this oceanic moisture and whether this is nearby or remote. Moreover, we showed how the oceanic sources vary over the year in time and space. Furthermore, we correlated sea surface temperatures (SSTs) in the 15 source regions and the Niño 3.4 region with precipitation on all continents. For South America, we found that the El Niño Southern Oscillation (altering wind patterns) has a larger effect on precipitation than local SSTs. For West Africa, however, we show that SST in the source regions is strongly correlated with precipitation in the rainy season. In Australia, both local SST and the Niño 3.4 region appear to have a big influence on precipitation. As such this research provides new insight in the ocean-atmosphere-land coupling, which can be useful for studying seasonal weather predictions as well as climate change impact.Water ManagementCivil Engineering and Geoscience
The performance of Earth System Models in simulating droughts
No full textThis research evaluated the performance of Land Surface Models (LSMs) in simulating droughts, examining Land-Hist offline simulations from the Land, Surface, Snow and Soil Moisture Intercomparison Project (LS3MIP). It is well known that LSMs possess uncertainties and biases due to oversimplifications or the absence of certain physical processes (e.g., groundwater interactions and lateral connectivity). Therefore, the objective of this research was to identify the strengths and weaknesses of various LSMs and how this relates to the performances in simulating soil moisture droughts. To address this objective, eight LSMs were evaluated: CESM2, CMCC-ESM2, E3SM-1-1, EC-Earth3-Veg, HadGEM3-GC31-LL, IPSL-CM6A-LR, MIROC6, and UKESM1-0-LL. Two reference evaporation data sets (DOLCE V3 and an ensemble of FLUXCOM-RS, BESS and PML) and a reference soil moisture data set (SoMo.ml) were utilized for the evaluation. After a global analysis on the LSM evaporation characteristics, six climate diverse study areas were selected for further investigation. A long-term analysis was performed by examining the water balance and implementing the LSMs into the Budyko Framework. Subsequently, soil moisture deficits were calculated for the driest periods in time, and the resulting accumulated deficits were compared with the reference evaporation data. The timing and progression of the deficits were evaluated utilizing the reference soil moisture data. Finally, the sensitivity of the model was evaluated by examining the response of evaporation anomalies to precipitation anomalies and comparing this with the reference evaporation data. The results showed that there was a large spread in output and performance among the LSMs across all parts of the evaluation. The greatest contrasts among the LSMs were found in the dry to wet transition zones within the tropics. In this latitudinal range, the worst performing LSMs overestimated the accumulation of soil moisture deficits and the severity of droughts, while the opposite was found for the extratropical regions. Additionally, the models showed, in general, to be overly sensitive to precipitation anomalies. When ranking the implemented model bases in the LSMs based on their performance during droughts, the findings showed that the Community Land Model (implemented in CMCC-ESM2, E3SM-1-1 and CESM2) was predominantly the best performing, followed by ORCHIDEE (IPSL-CM6A-LR) and HTESSEL (EC-Earth3-Veg). MATSIRO (MIROC6) and JULES (HadGEM3-GC31-LL and UKESM1-0-LL) were the least performing model bases. From a hydrological perspective, the findings of this research could be linked to some known limitations of LSMs. Oversimplified soil and vegetation dynamics could contribute to the LSMs being overly sensitive to precipitation anomalies while the contrasts between the tropical and extratropical regions could be attributed to the representation of the soil moisture-evaporation coupling, which plays a greater role in the tropical study areas. Ultimately, this research could contribute to LS3MIP and the Land Surface Modeling community, as the results highlight the strengths and weaknesses of the LSMs in simulating soil moisture droughts. From there, this research could contribute to improving LSMs, understanding drought mechanisms, and addressing climate change impacts, especially in drought-prone regions.Water Managemen
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