首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 968 毫秒
1.
Regional or local scale hydrological impact studies require high resolution climate change scenarios which should incorporate some assessment of uncertainties in future climate projections. This paper describes a method used to produce a multi-model ensemble of multivariate weather simulations including spatial–temporal rainfall scenarios and single-site temperature and potential evapotranspiration scenarios for hydrological impact assessment in the Dommel catchment (1,350 km2) in The Netherlands and Belgium. A multi-site stochastic rainfall model combined with a rainfall conditioned weather generator have been used for the first time with the change factor approach to downscale projections of change derived from eight Regional Climate Model (RCM) experiments for the SRES A2 emission scenario for the period 2071–2100. For winter, all downscaled scenarios show an increase in mean daily precipitation (catchment average change of +9% to +40%) and typically an increase in the proportion of wet days, while for summer a decrease in mean daily precipitation (−16% to −57%) and proportion of wet days is projected. The range of projected mean temperature is 7.7°C to 9.1°C for winter and 19.9°C to 23.3°C for summer, relative to means for the control period (1961–1990) of 3.8°C and 16.8°C, respectively. Mean annual potential evapotranspiration is projected to increase by between +17% and +36%. The magnitude and seasonal distribution of changes in the downscaled climate change projections are strongly influenced by the General Circulation Model (GCM) providing boundary conditions for the RCM experiments. Therefore, a multi-model ensemble of climate change scenarios based on different RCMs and GCMs provides more robust estimates of precipitation, temperature and evapotranspiration for hydrological impact assessments, at both regional and local scale.  相似文献   

2.
For the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC), the recent version of the coupled atmosphere/ocean general circulation model (GCM) of the Max Planck Institute for Meteorology has been used to conduct an ensemble of transient climate simulations These simulations comprise three control simulations for the past century covering the period 1860–2000, and nine simulations for the future climate (2001–2100) using greenhouse gas (GHG) and aerosol concentrations according to the three IPCC scenarios B1, A1B and A2. For each scenario three simulations were performed. The global simulations were dynamically downscaled over Europe using the regional climate model (RCM) REMO at 0.44° horizontal resolution (about 50 km), whereas the physics packages of the GCM and RCM largely agree. The regional simulations comprise the three control simulations (1950–2000), the three A1B simulations and one simulation for B1 as well as for A2 (2001–2100). In our study we concentrate on the climate change signals in the hydrological cycle and the 2 m temperature by comparing the mean projected climate at the end of the twenty-first century (2071–2100) to a control period representing current climate (1961–1990). The robustness of the climate change signal projected by the GCM and RCM is analysed focussing on the large European catchments of Baltic Sea (land only), Danube and Rhine. In this respect, a robust climate change signal designates a projected change that sticks out of the noise of natural climate variability. Catchments and seasons are identified where the climate change signal in the components of the hydrological cycle is robust, and where this signal has a larger uncertainty. Notable differences in the robustness of the climate change signals between the GCM and RCM simulations are related to a stronger warming projected by the GCM in the winter over the Baltic Sea catchment and in the summer over the Danube and Rhine catchments. Our results indicate that the main explanation for these differences is that the finer resolution of the RCM leads to a better representation of local scale processes at the surface that feed back to the atmosphere, i.e. an improved representation of the land sea contrast and related moisture transport processes over the Baltic Sea catchment, and an improved representation of soil moisture feedbacks to the atmosphere over the Danube and Rhine catchments.  相似文献   

3.
Summary South Asian summer monsoon precipitation and its variability are examined from the outputs of the coupled climate models assessed as part of the Intergovernmental Panel on Climate Change Fourth Assessment. Out of the 22 models examined, 19 are able to capture the maximum rainfall during the summer monsoon period (June through September) with varying amplitude. While two models are unable to reproduce the annual cycle well, one model is unable to simulate the summer monsoon season. The simulated inter-annual variability from the 19 models is examined with respect to the mean precipitation, coefficient of variation, long-term trends and the biennial tendency. The model simulated mean precipitation varies from 500 mm to 900 mm and coefficient of variation from 3 to 13%. While seven models exhibit long-term trends, eight are able to simulate the biennial nature of the monsoon rainfall. Six models, which generate the most realistic 20th century monsoon climate over south Asia, are selected to examine future projections under the doubling CO2 scenario. Projections reveal a significant increase in mean monsoon precipitation of 8% and a possible extension of the monsoon period based on the multi-model ensemble technique. Extreme excess and deficient monsoons are projected to intensify. The projected increase in precipitation could be attributed to the projected intensification of the heat low over northwest India, the trough of low pressure over the Indo-Gangetic plains, and the land–ocean pressure gradient during the establishment phase of the monsoon. The intensification of these pressure systems could be attributed to the decline in winter/spring snowfall. Furthermore, a decrease of winter snowfall over western Eurasia is also projected along with an increase of winter snowfall over Siberia/eastern Eurasia. This projected dipole snow configuration during winter could imply changes in mid-latitude circulation conducive to subsequent summer monsoon precipitation activity. An increase in precipitable water of 12–16% is projected over major parts of India. A maximum increase of about 20–24% is found over the Arabian Peninsula, adjoining regions of Pakistan, northwest India and Nepal. Although the projected summer monsoon circulation appears to weaken, the projected anomalous flow over the Bay of Bengal (Arabian Sea) will support oceanic moisture convergence towards the southern parts of India and Sri Lanka (northwest India and adjoining regions). The ENSO-Monsoon relationship is also projected to weaken.  相似文献   

4.
Sensitivities to the potential impact of Climate Change on the water resources of the Athabasca River Basin (ARB) and Fraser River Basin (FRB) were investigated. The Special Report on Emissions Scenarios (SRES) of IPCC projected by seven general circulation models (GCM), namely, Japan’s CCSRNIES, Canada’s CGCM2, Australia’s CSIROMk2b, Germany’s ECHAM4, the USA’s GFDLR30, the UK’s HadCM3, and the USA’s NCARPCM, driven under four SRES climate scenarios (A1FI, A2, B1, and B2) over three 30-year time periods (2010–2039, 2040–2069, 2070–2100) were used in these studies. The change fields over these three 30-year time periods are assessed with respect to the 1961–1990, 30-year climate normal and based on the 1961–1990 European Community Mid-Weather Forecast (ECMWF) re-analysis data (ERA-40), which were adjusted with respect to the higher resolution GEM forecast archive of Environment Canada, and used to drive the Modified ISBA (MISBA) of Kerkhoven and Gan (Adv Water Resour 29(6):808–826, 2006). In the ARB, the shortened snowfall season and increased sublimation together lead to a decline in the spring snowpack, and mean annual flows are expected to decline with the runoff coefficient dropping by about 8% per °C rise in temperature. Although the wettest scenarios predict mild increases in annual runoff in the first half of the century, all GCM and emission combinations predict large declines by the end of the twenty-first century with an average change in the annual runoff, mean maximum annual flow and mean minimum annual flow of −21%, −4.4%, and −41%, respectively. The climate scenarios in the FRB present a less clear picture of streamflows in the twenty-first century. All 18 GCM projections suggest mean annual flows in the FRB should change by ±10% with eight projections suggesting increases and 10 projecting decreases in the mean annual flow. This stark contrast with the ARB results is due to the FRB’s much milder climate. Therefore under SRES scenarios, much of the FRB is projected to become warmer than 0°C for most of the calendar year, resulting in a decline in FRB’s characteristic snow fed annual hydrograph response, which also results in a large decline in the average maximum flow rate. Generalized equations relating mean annual runoff, mean annual minimum flows, and mean annual maximum flows to changes in rainfall, snowfall, winter temperature, and summer temperature show that flow rates in both basins are more sensitive to changes in winter than summer temperature.  相似文献   

5.
A hydrologic model was driven by the climate projected by 11 GCMs under two emissions scenarios (the higher emission SRES A2 and the lower emission SRES B1) to investigate whether the projected hydrologic changes by 2071–2100 have a high statistical confidence, and to determine the confidence level that the A2 and B1 emissions scenarios produce differing impacts. There are highly significant average temperature increases by 2071–2100 of 3.7°C under A2 and 2.4°C under B1; July increases are 5°C for A2 and 3°C for B1. Two high confidence hydrologic impacts are increasing winter streamflow and decreasing late spring and summer flow. Less snow at the end of winter is a confident projection, as is earlier arrival of the annual flow volume, which has important implications on California water management. The two emissions pathways show some differing impacts with high confidence: the degree of warming expected, the amount of decline in summer low flows, the shift to earlier streamflow timing, and the decline in end-of-winter snow pack, with more extreme impacts under higher emissions in all cases. This indicates that future emissions scenarios play a significant role in the degree of impacts to water resources in California.  相似文献   

6.
Summary  The carbon dioxide exchange in arctic and subarctic terrestrial ecosystems has been measured using the eddy-covariance method at sites representing the latitudinal and longitudinal extremes of the European Arctic sea areas as part of the Land Arctic Physical Processes (LAPP) project. The sites include two fen (Kaamanen and Kevo) and one mountain birch ecosystems in subarctic northern Finland (69° N); fen, heathland, and snowbed willow ecosystems in northeastern Greenland (74° N); and a polar semidesert site in Svalbard (79° N). The measurement results, which are given as weekly average diurnal cycles, show the striking seasonal development of the net CO2 fluxes. The seasonal periods important for the net CO2 fluxes, i.e. winter, thaw, pre-leaf, summer, and autumn can be identified from measurements of the physical environment, such as temperature, albedo, and greenness. During the late winter period continuous efflux is observed at the permafrost-free Kaamanen site. At the permafrost sites, efflux begins during the thaw period, which lasts about 3–5 weeks, in contrast to the Kaamanen site where efflux continues at the same rate as during the winter. Seasonal efflux maximum is during the pre-leaf period, which lasts about 2–5 weeks. The summer period lasts 6 weeks in NE Greenland but 10–14 weeks in northern Finland. During a high summer week, the mountain birch ecosystem had the highest gross photosynthetic capacity, GP max, followed by the fen ecosystems. The polar semidesert ecosystem had the lowest GP max. By the middle of August, noon uptake fluxes start to decrease as the solar elevation angle decreases and senescence begins within the vascular plants. At the end of the autumn period, which lasts 2–5 weeks, topsoil begins to freeze at the end of August in Svalbard; at the end of September at sites in eastern Greenland; and one month later at sites in northern Finland. Received March 1, 2000 Revised October 2, 2000  相似文献   

7.
Previous measurements of urban energy balances generally have been limited to densely built, central city sites and older suburban locations with mature tree canopies that are higher than the height of the buildings. In contrast, few data are available for the extensive, open vegetated types typical of low-density residential areas that have been newly converted from rural land use. We made direct measurements of surface energy fluxes using the eddy-covariance technique at Greenwood, a recently developed exurban neighbourhood near Kansas City, Missouri, USA, during an intensive field campaign in August 2004. Energy partitioning was dominated by the latent heat flux under both cloudy and near clear-sky conditions. The mean daytime Bowen ratio (β) values were 0.46, 0.48, and 0.47 respectively for the cloudy, near clear-sky and all-sky conditions. Net radiation (R n ) increased rapidly from dawn (−34 and −58W m−2) during the night to reach a maximum (423 and 630W m−2) after midday for cloudy and near clear-sky conditions respectively. Mean daytime values were 253 and 370W m−2, respectively for the cloudy and near clear-sky conditions, while mean daily values were 114 for cloudy and 171W m−2 for near clear-sky conditions, respectively. Midday surface albedo values were 0.25 and 0.24 for the cloudy and near clear-sky conditions, respectively. The site exhibited an angular dependence on the solar elevation angle, in contrast to previous observations over urban and suburban areas, but similar to vegetated surfaces. The latent heat flux (Q E ), sensible heat flux (Q H ), and the residual heat storage ΔQ s terms accounted for between 46–58%, 21–23%, and 18–31% of R n , respectively, for all-sky conditions and time averages. The observed albedo, R n , and Q E values are higher than the values that have been reported for suburban areas with high summer evapotranspiration rates in North America. These results suggest that the rapidly growing residential areas at the exurban fringe of large metropolitan areas have a surface energy balance that is more similar to the rural areas from which they were developed than it is to the older suburbs and city centres that make up the urban fabric to which they are being joined.  相似文献   

8.
Tropospheric distributions of ozone (O3) and water vapor (H2O) have been presented based on the Measurements of OZone and water vapor by Airbus In-Service AirCraft (MOZAIC) data over the metro and capital city of Delhi, India during 1996–2001. The vertical mixing ratios of both O3 and H2O show strong seasonal variations. The mixing ratios of O3 were often below 40 ppbv near the surface and higher values were observed in the free troposphere during the seasons of winter and spring. In the free troposphere, the high mixing ratio of O3 during the seasons of winter and spring are mainly due to the long-range transport of O3 and its precursors associated with the westerly-northwesterly circulation. In the lower and middle troposphere, the low mixing ratios of ∼20–30 ppbv observed during the months of July–September are mainly due to prevailing summer monsoon circulation over Indian subcontinent. The summer monsoon circulation, southwest (SW) wind flow, transports the O3-poor marine air from the Arabian Sea and Indian Ocean. The monthly averages of rainfall and mixing ratio of H2O show opposite seasonal cycles to that of O3 mixing ratio in the lower and middle troposphere. The change in the transport pattern also causes substantial seasonal variation in the mixing ratio of H2O of 3–27 g/kg in the lower troposphere over Delhi. Except for some small-scale anomalies, the similar annual patterns in the mixing ratios of O3 and H2O are repeated during the different years of 1996–2001. The case studies based on the profiles of O3, relative humidity (RH) and temperature show distinct features of vertical distribution over Delhi. The impacts of long range transport of air mass from Africa, the Middle East, Indian Ocean and intrusions of stratospheric O3 have also been demonstrated using the back trajectory model and remote sensing data for biomass burning and forest fire activities.  相似文献   

9.
The study examines future scenarios of precipitation extremes over Central Europe in an ensemble of 12 regional climate model (RCM) simulations with the 25-km resolution, carried out within the European project ENSEMBLES. We apply the region-of-influence method as a pooling scheme when estimating distributions of extremes, which consists in incorporating data from a ‘region’ (set of gridboxes) when fitting an extreme value distribution in any single gridbox. The method reduces random variations in the estimates of parameters of the extreme value distribution that result from large spatial variability of heavy precipitation. Although spatial patterns differ among the models, most RCMs simulate increases in high quantiles of precipitation amounts when averaged over the area for the late-twenty-first century (2070–2099) climate in both winter and summer. The sign as well as the magnitude of the projected change vary only little for individual parts of the distribution of daily precipitation in winter. In summer, on the other hand, the projected changes increase with the quantile of the distribution in all RCMs, and they are negative (positive) for parts of the distribution below (above) the 98% quantile if averaged over the RCMs. The increases in precipitation extremes in summer are projected in spite of a pronounced drying in most RCMs. Although a rather general qualitative agreement of the models concerning the projected changes of precipitation extremes is found in both winter and summer, the uncertainties in climate change scenarios remain large and would likely further increase considerably if a more complete ensemble of RCM simulations driven by a larger suite of global models and with a range of possible scenarios of the radiative forcing is available.  相似文献   

10.
Scenarios with daily time resolution are frequently used in research on the impacts of climate change. These are traditionally developed by regional climate models (RCMs). The spatial resolution, however, is usually too coarse for local climate change analysis, especially in regions with complex topography, such as Norway. The RCM used, HIRHAM, is run with lateral boundary forcing provided from two global medium resolution models; the ECHAM4/OPYC3 from MPI and the HadAM3H from the Hadley centre. The first is run with IPCC SRES emission scenario B2, the latter is run with IPCC SRES emission scenarios A2 and B2. All three scenarios represent the future time period 2071–2100. Both models have a control run, representing the present climate (1961–1990). Daily temperature scenarios are interpolated from HIRHAM to Norwegian temperature stations. The at-site HIRHAM-temperatures, both for the control and scenario runs, are adjusted to be locally representative. Mean monthly values and standard deviations based on daily values of the adjusted HIRHAM-temperatures, as well as the cumulative distribution curve of daily seasonal temperatures, are conclusive with observations for the control period. Residual kriging are used on the adjusted daily HIRHAM-temperatures to obtain high spatial temperature scenarios. Mean seasonal temperature grids are obtained. By adjusting the control runs and scenarios and improving the spatial resolution of the scenarios, the absolute temperature values are representative at a local scale. The scenarios indicate larger warming in winter than in summer in the Scandinavian regions. A marked west–east and south–north gradient is projected for Norway, where the largest increase is in eastern and northern regions. The temperature of the coldest winter days is projected to increase more than the warmer temperatures.  相似文献   

11.
Campaigns were conducted to measure Organic Carbon (OC) and Elemental Carbon (EC) in PM2.5 during winter and summer 2003 in Beijing. Modest differences of PM2.5 and PM10 mean concentrations were observed between the winter and summer campaigns. The mean PM2.5/PM10 ratio in both seasons was around 60%, indicating PM2.5 contributed significantly to PM10. The mean concentrations of OC and EC in PM2.5 were 11.2±7.5 and 6.0±5.0μg m-3 for the winter campaign, and 9.4±2.1 and 4.3±3.0 μg m-3 for the summer campaign, respectively. Diurnal concentrations of OC and EC in PM2.5 were found high at night and low during the daytime in winter, and characterized by an obvious minimum in the summer afternoon. The mean OC/EC ratio was 1.87±0.09 for winter and Z39±0.49 for summer. The higher OC/EC ratio in summer indicates some formation of Secondary Organic Carbon (SOC). The estimated SOC was 2.8 μg m-3 for winter and 4.2μg m-3 for summer.  相似文献   

12.
There is considerable interest in the potential impact of climate change on the feasibility and predictability of renewable energy sources including wind energy. This paper presents dynamically downscaled near-surface wind fields and examines the impact of climate change on near-surface flow and hence wind energy density across northern Europe. It is shown that: Simulated wind fields from the Rossby Centre coupled Regional Climate Model (RCM) (RCAO) with boundary conditions derived from ECHAM4/OPYC3 AOGCM and the HadAM3H atmosphere-only GCM exhibit reasonable and realistic features as documented in reanalysis data products during the control period (1961–1990). The near-surface wind speeds calculated for a climate change projection period of 2071–2100 are higher than during the control run for two IPCC emission scenarios (A2, B2) for simulations conducted using boundary conditions from ECHAM4/OPYC3. The RCAO simulations conducted using boundary conditions from ECHAM4/OPYC3 indicate evidence for a small increase in the annual wind energy resource over northern Europe between the control run and climate change projection period and for more substantial increases in energy density during the winter season. However, the differences between the RCAO simulations for the climate projection period and the control run are of similar magnitude to differences between the RCAO fields in the control period and the NCEP/NCAR reanalysis data. Additionally, the simulations show a high degree of sensitivity to the boundary conditions, and simulations conducted using boundary conditions from HadAM3H exhibit evidence of slight declines or no change in wind speed and energy density between 1961–1990 and 2071–2100. Hence, the uncertainty of the projected wind changes is relatively high.  相似文献   

13.
A high resolution regional climate model (RCM) is used to simulate climate of the recent past and to project future climate change across the northeastern US. Different types of uncertainties in climate simulations are examined by driving the RCM with different boundary data, applying different emissions scenarios, and running an ensemble of simulations with different initial conditions. Empirical orthogonal functions analysis and K-means clustering analysis are applied to divide the northeastern US region into four climatologically different zones based on the surface air temperature (SAT) and precipitation variability. The RCM simulations tend to overestimate SAT, especially over the northern part of the domain in winter and over the western part in summer. Statistically significant increases in seasonal SAT under both higher and lower emissions scenarios over the whole RCM domain suggest the robustness of future warming. Most parts of the northeastern US region will experience increasing winter precipitation and decreasing summer precipitation, though the changes are not statistically significant. The greater magnitude of the projected temperature increase by the end of the twenty-first century under the higher emissions scenario emphasizes the essential role of emissions choices in determining the potential future climate change.  相似文献   

14.
In order to improve the reliability of climate reconstruction, especially the climatologies outside the modern observed climate space, an improved inverse vegetation model using a recent version of BIOME4 has been designed to quantitatively reconstruct past climates, based on pollen biome scores from the BIOME6000 project. The method has been validated with surface pollen spectra from Eurasia and Africa, and applied to palaeoclimate reconstruction. At 6 cal ka BP (calendar years), the climate was generally wetter than today in southern Europe and northern Africa, especially in the summer. Winter temperatures were higher (1–5°C) than present in southern Scandinavia, northeastern Europe, and southern Africa, but cooler in southern Eurasia and in tropical Africa, especially in Mediterranean regions. Summer temperatures were generally higher than today in most of Eurasia and Africa, with a significant warming from ∼3 to 5°C over northwestern and southern Europe, southern Africa, and eastern Africa. In contrast, summers were 1–3°C cooler than present in the Mediterranean lowlands and in a band from the eastern Black Sea to Siberia. At 21 cal ka BP, a marked hydrological change can be seen in the tropical zone, where annual precipitation was ∼200–1,000 mm/year lower than today in equatorial East Africa compared to the present. A robust inverse relationship is shown between precipitation change and elevation in Africa. This relationship indicates that precipitation likely had an important role in controlling equilibrium-line altitudes (ELA) changes in the tropics during the LGM period. In Eurasia, hydrological decreases follow a longitudinal gradient from Europe to Siberia. Winter temperatures were ∼10–17°C lower than today in Eurasia with a more significant decrease in northern regions. In Africa, winter temperature was ∼10–15°C lower than present in the south, while it was only reduced by ∼0–3°C in the tropical zone. Comparison of palaeoclimate reconstructions using LGM and modern CO2 concentrations reveals that the effect of CO2 on pollen-based LGM reconstructions differs by vegetation type. Reconstructions for pollen sites in steppic vegetation in Europe show warmer winter temperatures under LGM CO2 concentrations than under modern concentrations, and reconstructions for sites in xerophytic woods/scrub in tropical high altitude regions of Africa are wetter for LGM CO2 concentrations than for modern concentrations, because our reconstructions account for decreased plant water use efficiency.  相似文献   

15.
Ambient respirable particles (PM10; aerodynamic diameter ≤10 μm) collected in a tropical urban environment (Delhi, India) during December 2008-November 2009 were characterized with respect to 16 US EPA priority polycyclic aromatic hydrocarbons (PAHs) and 8 major and trace metals (Fe, Mn, Cd, Cu, Ni, Pb, Zn and Cr). Concentrations of Σ16PAHs (annual mean: 74.7 ± 50.7 ng m−3, range 22.1–258.4 ng m−3) and most metallic species were at least an order of magnitude greater than values reported from similar locations worldwide. Seasonal variations in Σ16PAHs were significant (p < 0.001) with highest levels in winter while crustal and anthropogenic metals showed significant but mutually opposite seasonal dependence. Statistically significant associations were observed between chemical species and various meteorological parameters. The PAH profile was dominated by combustion-derived large-ring species (~85%) that were essentially local in origin. Principal component analysis–multiple linear regression (PCA-MLR) apportioned four sources: crustal dust (73%), vehicular emission (21%), coal combustion (4%) and industrial emission (2%) that was further validated by hierarchical cluster analysis (HCA). Temporal trend analysis showed that crustal sources were predominant in summer (p < 0.05) while the remaining sources were most active in winter. Summertime intrusions of Saharan dust were identified with the help of aerosol maps and air parcel backward trajectories. Inhalation cancer risk assessment showed that up to 3,907 excess cancer cases (357 for PAHs, 122 for Cd, 2040 for Cr (VI) and 1387 for Ni) are likely in Delhi considering lifetime inhalation exposure to these chemicals at their current concentrations.  相似文献   

16.
Based on observational meteorological data since A.D. 1864 and tree-ring records of debris-flow activity, this paper assesses changes in rainfall characteristics and their impact on the triggering of geomorphic events in a high-elevation watershed of the Swiss Alps since the end of the Little Ice Age. No trends are visible in the frequency of heavy rainfall events, but we observe a reduced number of heavy, short-lived rainfalls in summer and a concentration of advective storms is recorded in late summer and early fall since the late 1980s. These changes in triggering meteorological conditions resulted in a cluster of debris flows in the early decades of the twentieth century and a lowering of debris-flow activity since the mid 1990s, and may be mirroring the observed changes in persistent high-pressure systems over the Alps. We also observe intra-seasonal differences in debris-flow system response reflecting the state of the permafrost body in the source area of debris flows, allowing for very small debris flows to be released by limited rainfall inputs (<20 mm) in June and July. The same quantities of rain will not trigger debris flows in August or September, when a thick active layer of the permafrost body is capable of absorbing water. With the projected amplitude of climatic change, seasonality, return intervals and volumes of debris flows are likely to be altered. RCM projections based on the IPCC A2 scenario suggest a decrease in heavy summer rainfalls which will most likely result in a (further) reduction of the overall frequency of debris flows, leaving more time for sediment to accumulate in the channel. Such an increase of channel accumulation rates along with the projected destabilization of the steep rock-glacier body is likely, in turn, to exert control ultimately on sediment volumes released from the source areas during future events. Observations from adjacent catchments suggest that extremely large debris flows, beyond historical experience, could occur at the study site and in similar debris-flow systems of the Valais Alps originating from periglacial environments.  相似文献   

17.
We used a calibrated coupled climate–hydrological model to simulate Meuse discharge over the late Holocene (4000–3000 BP and 1000–2000 AD). We then used this model to simulate discharge in the twenty-first century under SRES emission scenarios A2 and B1, with and without future land use change. Mean discharge and medium-sized high-flow (e.g. Q99) frequency are higher in 1000–2000 AD than in 4000–3000 BP; almost all of this increase can be attributed to the conversion of forest to agriculture. In the twentieth century, mean discharge and the frequency of medium-sized high-flow events are higher than in the nineteenth century; this increase can be attributed to increased (winter half-year) precipitation. Between the twentieth and twenty-first centuries, anthropogenic climate change causes a further increase in discharge and medium-sized high-flow frequency; this increase is of a similar order of magnitude to the changes over the last 4,000 years. The magnitude of extreme flood events (return period 1,250-years) is higher in the twenty-first century than in any preceding period of the time-slices studied. In contrast to the long-term influence of deforestation on mean discharge, changes in forest cover have had little effect on these extreme floods, even on the millennial timescale.  相似文献   

18.
The distributions and daily variations of the apparent heat source (Q1) and the apparent moisture sink (Q2) in East China in the early summer of 1984 have been estimated with the budget calculation method. It has been found that during this time period, there occurred three significant episodes of strong heating that corresponded to the three events of heavy rainfalls prior to, during and post to the onset of mei-yu (plum rains). The peaks of Q1 were generally found at 200 hPa, with the heating rate of 6°－10°C/day observed, while the peaks of Q2 were located at about 700 hPa, with their magnitudes being 12o-20°C/day. The vertical distribution of Q1 and Q2 indicates the importance of eddy vertical flux. In other words, the convective activity plays a very important role in the processes of precipitation in East Asia in the early summer. This result is different from the finding obtained by Luo and Yanai (1984) in their calculation of the case of 1979. They pointed out that in the early summer of 1979 the continuous precipitation dominated the region of East China.Among the three terms of Q1 and Q2, the maximum contribution was made from the adiabatic term, which was caused by strong ascending motion. The adiabatic cooling produced by this term may compensate for the heating created by the condensation process.In addition, it has been revealed that the three significant heating processes were closely related to the seasonal transition from spring to summer in East China. One major synoptic event associated with it showed up in the sudden jump of the upper tropospheric, subtropical jet-stream from 30°N to 40°N. So did the plane-tary frontal zone in East China.  相似文献   

19.
Summary ?Using the data of 6 automatic heat balance observation (AWS) stations and a data set of 52 surface observation stations over the Qinghai-Tibetan Plateau (“the Plateau”) and surroundings, the horizontal distribution is studied of “apparent atmospheric heat sources” 〈Q 1〉 and of “apparent atmospheric moisture sinks” 〈Q 2〉. The AWS stations were established during the period May to August 1998 of the Tibetan Plateau Meteorological Experiment (second TIPEX) by a cooperation of China and Japan. For this period the Plateau mean of 〈Q 1〉 is positive. Its value of 74 W/m2 is a little greater than a climate value and than values from MONEX and the first TIPEX in 1979, respectively. Also the corresponding 〈Q 2〉 is positive. Hence during that time the Plateau is a heat source and a moisture sink. A day-to-day change of 〈Q 1〉 and 〈Q 2〉 is more pronounced over the middle and east part of the Plateau than over the west part. Diagnostics accompanied by numerical simulations are used to study the daily relationship between 〈Q 1〉 over the Plateau and the weather over China and Asia for this summer. The results suggest that 〈Q 1〉 may affect precipitation over northern China and position of the west Pacific subtropical high. Abnormal southward retreat of this Pacific high seems to have caused the second flood over the middle and lower Yangtse river basin in July. Received May 20, 2001; revised February 2, 2002  相似文献   

20.
The scaled-decomposed atmospheric water budget over North America is investigated through the analysis of 25 years of simulation by the Canadian Regional Climate Model (CRCM) driven by the NCEP–NCAR reanalyses for the period 1975–1999. The time average and time variability of the atmospheric water budget for the winter and summer seasons are decomposed into their large-scale and small-scale components to identify the added value of the regional model. For the winter season, the intra-seasonal transient-eddy variance is the main temporal variability. The large- and small-scale terms are of the same order of magnitude, and are large over both coasts and weak over the continent. For the summer season, the time–mean atmospheric water budget is rather different to that of winter, with maximum values over the south-eastern part of the continent. The summer intra-seasonal variance is about twice stronger than in winter and also dominates the variability, but the inter-monthly variance is non-negligible and can be in part associated to North American Monsoon System. Over the continent, the intra-seasonal climatological variance is dominated by the variability of the small scales. The small scales, that is those scales that are only resolved in the regional model but not in the reanalyses, contribute to the added value in a regional climate simulation. In the winter season, the added value of the CRCM is large and dominated by oceanic forcing, while in summer, it is dominant (larger than the large scales) and controlled mainly by convective processes.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号