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1.
D. Lauwaet N. P. M. van Lipzig N. Kalthoff K. De Ridder 《Meteorology and Atmospheric Physics》2010,107(3-4):109-122
The evolution of precipitating convective systems in West Africa has been a research topic throughout the past three decades and is considered to be influenced by surface–atmosphere interactions. This study builds on the previous research by examining the sensitivity of a mesoscale convective system (MCS) to a change in the vegetation cover by using a regional atmospheric model with a high horizontal resolution. Vegetation cover values in the region between 10 and 15°N have increased by 10–30% over the last 20 years. The effect of both an increase and a decrease in vegetation cover by 10, 20 and 30% is investigated. The MCS case selected occurred on 11 June 2006 and was observed during the African Monsoon Multidisciplinary Analysis field campaign in Dano, Burkina Faso. The model is able to reproduce the most important characteristics of the MCS and the atmospheric environment. For the investigated case, no clear precipitation response of the MCS to the applied vegetation scenarios is found. The vegetation changes do alter the surface fluxes in the days before the MCS arrives, which have a clear effect on the modelled convective available potential energy (CAPE) values. However, a link between CAPE, mesoscale circulation and rainfall amounts could not be demonstrated as a dynamical mechanism is found to counteract the CAPE signal. By using a kilometre-scale model, a change in the cold pool dynamics of the MCS could be detected which results from alterations in boundary layer moisture. The effect of vegetation changes on the MCS is thus not straightforward and a complex interaction between different processes should be taken into account. 相似文献
2.
This study examines the role of vegetation dynamics in regional predictions of future climate change in western Africa using
a dynamic vegetation model asynchronously coupled to a regional climate model. Two experiments, one for present day and one
for future, are conducted with the linked regional climate-vegetation model, and the third with the regional climate model
standing alone that predicts future climate based on present-day vegetation. These simulations are so designed in order to
tease out the impact of structural vegetation feedback on simulated climate and hydrological processes. According to future
predictions by the regional climate-vegetation model, increase in LAI is widespread, with significant shift in vegetation
type. Over the Guinean Coast in 2084–2093, evergreen tree coverage decreases by 49% compared to 1984–1993, while drought deciduous
tree coverage increases by 56%. Over the Sahel region in the same period, grass cover increases by 31%. Such vegetation changes
are accompanied by a decrease of JJA rainfall by 2% over the Guinean Coast and an increase by 23% over the Sahel. This rather
small decrease or large increase of precipitation is largely attributable to the role of vegetation feedback. Without the
feedback effect from vegetation, the regional climate model would have predicted a 5% decrease of JJA rainfall in both the
Guinean Coast and the Sahel as a result of the radiative and physiological effects of higher atmospheric CO2 concentration. These results demonstrate that climate- and CO2-induced changes in vegetation structure modify hydrological processes and climate at magnitudes comparable to or even higher
than the radiative and physiological effects, thus evincing the importance of including vegetation feedback in future climate
predictions. 相似文献
3.
In this study we investigate the impact of large-scale oceanic forcing and local vegetation feedback on the variability of the Sahel rainfall using a global biosphere-atmosphere model, the coupled GENESIS-IBIS model, running at two different resolutions. The observed global sea surface temperature in the twentieth century is used as the primary model forcing. Using this coupled global model, we experiment on treating vegetation as a static boundary condition and as a dynamic component of the Earth climate system. When vegetation is dynamic, the R30-resolution model realistically reproduces the multi-decadal scale fluctuation of rainfall in the Sahel region; keeping vegetation static in the same model results in a rainfall regime characterized by fluctuations at much shorter time scales, indicating that vegetation dynamics act as a mechanism for persistence of the regional climate. Even when vegetation dynamics is included, the R15 model fails to capture the main characteristics of the long-term rainfall variability due to the exaggerated atmospheric internal variability in the coarse resolution model. Regardless how vegetation is treated and what model resolution is used, conditions in the last three decades of the twentieth century are always drier than normal in the Sahel, suggesting that global oceanic forcing during that period favors the occurrence of a drought. Vegetation dynamics is found to enhance the severity of this drought. However, with both the observed global SST forcing and feedback from dynamic vegetation in the model, the simulated drought is still not as persistent as that observed. This indicates that anthropogenic land cover changes, a mechanism missing in the model, may have contributed to the occurrence of the twentieth century drought in the Sahel. 相似文献
4.
The effect of vegetation feedback on decadal-scale Sahel rainfall variability is analyzed using an ensemble of climate model simulations in which the atmospheric general circulation model ICTPAGCM (“SPEEDY”) is coupled to the dynamic vegetation model VEGAS to represent feedbacks from surface albedo change and evapotranspiration, forced externally by observed sea surface temperature (SST) changes. In the control experiment, where the full vegetation feedback is included, the ensemble is consistent with the observed decadal rainfall variability, with a forced component 60 % of the observed variability. In a sensitivity experiment where climatological vegetation cover and albedo are prescribed from the control experiment, the ensemble of simulations is not consistent with the observations because of strongly reduced amplitude of decadal rainfall variability, and the forced component drops to 35 % of the observed variability. The decadal rainfall variability is driven by SST forcing, but significantly enhanced by land-surface feedbacks. Both, local evaporation and moisture flux convergence changes are important for the total rainfall response. Also the internal decadal variability across the ensemble members (not SST-forced) is much stronger in the control experiment compared with the one where vegetation cover and albedo are prescribed. It is further shown that this positive vegetation feedback is physically related to the albedo feedback, supporting the Charney hypothesis. 相似文献
5.
6.
Previous studies have highlighted the crucial role of land degradation in tropical African climate. This effect urgently has
to be taken into account when predicting future African climate under enhanced greenhouse conditions. Here, we present time
slice experiments of African climate until 2025, using a high-resolution regional climate model. A supposable scenario of
future land use changes, involving vegetation loss and soil degradation, is prescribed simultaneously with increasing greenhouse-gas
concentrations in order to detect, where the different forcings counterbalance or reinforce each other. This proceeding allows
us to define the regions of highest vulnerability with respect to future freshwater availability and food security in tropical
and subtropical Africa and may provide a decision basis for political measures. The model simulates a considerable reduction
in precipitation amount until 2025 over most of tropical Africa, amounting to partly more than 500 mm (20–40% of the annual
sum), particularly in the Congo Basin and the Sahel Zone. The change is strongest in boreal summer and basically reflects
the pattern of maximum vegetation cover during the seasonal cycle. The related change in the surface energy fluxes induces
a substantial near-surface warming by up to 7°C. According to the modified temperature gradients over tropical Africa, the
summer monsoon circulation intensifies and transports more humid air masses into the southern part of West Africa. This humidifying
effect is overcompensated by a remarkable decrease in surface evaporation, leading to the overall drying tendency over most
of Africa. Extreme daily rainfall events become stronger in autumn but less intense in spring. Summer and autumn appear to
be characterized by more severe heat waves over Subsaharan West Africa. In addition, the Tropical Easterly Jet is weakening,
leading to enhanced drought conditions in the Sahel Zone. All these results suggest that the local impact of land degradation
and reduction of vegetation cover may be more important in tropical Africa than the global radiative heating, at least until
2025. This implies that vegetation protection measures at a national scale may directly lead to a mitigation of the expected
negative implications of future climate change in tropical Africa. 相似文献
7.
Soon-Hwan Lee Soo-Jin Hwang Chan-Su Ryu 《Asia-Pacific Journal of Atmospheric Sciences》2010,46(3):355-367
In order to clarify the characteristics of Mesoscale Convective System (MCS) development and understand the impact of the trigger effect of isolated islands, observational and numerical analysis of the heavy rainfall were carried out over the southwestern part of the Korean Peninsula on July 14, 2004. Satellite based remote sensing data and numerical model MM5 with observational data adjustment were used in this study. The MCS development, in this case, was accompanied not by directional wind shear, but by speed shear which was strongly associated with development of the updraft cloud. An inversion layer at a 750 hPa level is one of the fundamental factors in increasing instability. Effective separation of the upper and lower level atmospheric structure due to an inversion layer at a 750 hPa level creates a suitable condition to develop a MCS. According to numerical analysis it has been found that isolated islands located off the southwestern part of the Korean Peninsula are not a principal factor in causing the heavy rainfall due to the evolution of MCS in this case. Transferable topographic forcing of the downwind side can often induce the variation of MCS intensity, while associated with the precipitation amount over the lee side of the isolated islands at a mature stage of MCS development. 相似文献
8.
Using the monthly NCEP-NCAR reanalysis dataset, the monthly temperature and precipitation
at surface stations of China, and the MM5 model, we examine impacts of vegetation cover changes in
western China on the interdecadal variability of the summer climate over northwestern China during
the past 30 years. It is found that the summer atmospheric circulation, surface air temperature,
and rainfall in the 1990s were different from those in the 1970s over northwestern China, with
generally more rainfall and higher temperatures in the 1990s. Associated with these changes, an
anomalous wave train appears in the lower troposphere at the midlatitudes of East Asia and the
low-pressure system to the north of the Tibetan Plateau is weaker. Meanwhile, the South Asian
high in the upper troposphere is also located more eastward. Numerical experiments show that
change of vegetation cover in western China generally forces anomalous circulations and
temperatures and rainfall over these regions. This consistency between the observations and
simulations implies that the interdecadal variability of the summer climate over northwestern
China between the 1990s and 1970s may result from a change of vegetation cover over western
China. 相似文献
9.
Summary Within the joint research project IMPETUS (An integrated approach to the efficient management of scarce water resources in
West Africa), the effect of interactions between the Earth’s surface and the atmosphere on fresh water availability is investigated.
Explorations are conducted for a river catchment in Benin by means of simulations with a non-hydrostatic mesoscale meteorological
model. A combination of idealised ensemble simulations with a column version of the model and 3-D modelling of real precipitation
events is employed to assess the sensitivity of precipitation to variations in the land surface. Simplified ensemble studies
exhibit a dominant influence of initial soil water content and an enhanced dependence of precipitation on vegetation when
soil water availability is reduced. For wet soils, the influence of parameters that determine the intensity of near-surface
turbulence is dominant. 3-D modelling confirms that these relationships are useful to identify critical land use changes in
realistic settings, but do not comprehensively account for the effect of heterogeneous land surface changes on regional precipitation.
Instead, the interplay between surface properties, atmospheric dynamics and precipitation systems can generate intrinsic precipitation
anomaly patterns that are incongruent with the imposed surface anomalies. Hence, assessments of land use change effects on
precipitation for a specific region should be based on an integrated consideration of the interactions between surface processes,
atmospheric forcing and precipitation systems. Based on these findings, possible effects of successive land degradation are
investigated by sensitivity studies of land surface and rainfall system interaction for the Haute Vallée de l’Ouémé (HVO).
In a first series of 3-D model simulations, a successive increase of the surface fraction with adverse conditions for the
development of precipitation systems is performed. Within the scope of a second series a successive reduction of surface vegetation
and soil water at randomly distributed areas that cover half of the simulation domain is carried out. Basically, a uniform
decrease of average precipitation forced by changing conditions and a strong reduction of rainfall in some parts of the HVO
are found. As a whole, the results strongly support the hypothesis of a growing risk of rainfall decrease as a result of land
use changes.
Current affiliation: Gesellschaft für Anlagen- und Reaktorsicherheit, Cologne, Germany. 相似文献
10.
Decadal Sahelian rainfall variability was mainly driven by sea surface temperatures (SSTs) during the twentieth century. At the same time SSTs showed a marked long-term global warming (GW) trend. Superimposed on this long-term trend decadal and multi-decadal variability patterns are observed like the Atlantic Multidecadal Oscillation (AMO) and the inter-decadal Pacific Oscillation (IPO). Using an atmospheric general circulation model we investigate the relative contribution of each component to the Sahelian precipitation variability. To take into account the uncertainty related to the use of different SST data sets, we perform the experiments using HadISST1 and ERSSTv3 reconstructed sets. The simulations show that all three SST signals have a significant impact over West Africa: the positive phases of the GW and the IPO lead to drought over the Sahel, while a positive AMO enhances Sahel rainfall. The tropical SST warming is the main cause for the GW impact on Sahel rainfall. Regarding the AMO, the pattern of anomalous precipitation is established by the SSTs in the Atlantic and Mediterranean basins. In turn, the tropical SST anomalies control the impact of the IPO component on West Africa. Our results suggest that the low-frequency evolution of Sahel rainfall can be interpreted as the competition of three factors: the effect of the GW, the AMO and the IPO. Following this interpretation, our results show that 50% of the SST-driven Sahel drought in the 1980s is explained by the change to a negative phase of the AMO, and that the GW contribution was 10%. In addition, the partial recovery of Sahel rainfall in recent years was mainly driven by the AMO. 相似文献
11.
利用各种观测资料和NCEP/NCAR 1×1°再分析资料,对2012年7月30日夜间和31日夜间鲁西北连续两天强降雨天气进行诊断和对比分析。结果表明:强降水产生在西风槽前和副热带高压边缘的偏南暖湿气流中,西风槽稳定少动,台风在东南沿海北上,副高加强北抬,为鲁西北连续两天的强降水提供了天气尺度背景。925hPa及以下的低层,来自于渤海的偏东气流和来自于华东沿海的东南气流同时向鲁西北强降水区输送水汽,低层比湿大,CAPE和K指数较高。第1次强降水产生在偏南气流的暖区中,降水强度大,维持时间短。第2次强降水期间,低层有冷空气锲入,把暖湿气流抬升,前期为对流性降水,中后期转为稳定性降水,降水强度小,维持时间较长。850hPa及以下倒槽式切变线和中尺度低涡环流是造成强降水的中尺度影响系统,近地面层来自于渤海的东北气流与来自于东南沿海的东南暖湿气流形成中尺度涡旋,产生气旋式辐合上升,触发对流不稳定能量释放。对流云团在鲁西北形成长形的中尺度对流系统(MCS),稳定少动,有明显的列车效应和后向传播特征。强降水具有较强的日变化,夜间发展增强,白天减弱。 相似文献
12.
The south-west of Western Australia has experienced significant land-cover change as well as a decline in rainfall. Given
that most precipitation in the region results from frontal passages, the impact of land-cover change on the dynamics of cold
fronts is explored using the Regional Atmospheric Modeling System version 6.0. Frontal simulations are evaluated against high
resolution atmospheric soundings, station observations, and gridded rainfall analyses and shown to reproduce the qualitative
features of cold fronts. Land-cover change results in a decrease in total frontal precipitation through a decrease in boundary-layer
turbulent kinetic energy and vertically integrated moisture convergence, and an increase in wind speed within the lower boundary
layer. Such processes contribute to reduced convective rainfall under current vegetation cover. 相似文献
13.
West African monsoon response to greenhouse gas and sulphate aerosol forcing under two emission scenarios 总被引：1，自引：0，他引：1
The impact of increased greenhouse gases (GHG) and aerosols concentrations upon the West African monsoon (WAM) is investigated
for the late twenty-first century period using the Météo-France ARPEGE-IFS high-resolution atmospheric model. Perturbed (2070–2100)
and current (1961–2000) climates are compared using the model in time-slice mode. The model is forced by global sea surface
temperatures provided by two transient scenarios performed with low-resolution coupled models and by two GHG evolution scenarios,
SRES-A2 and SRES-B2. Comparing to reanalysis and observed data sets, the model is able to reproduce a realistic seasonal cycle
of WAM despite a clear underestimation of the African Easterly Jet (AEJ) during the boreal summer. Mean temperature change
indicates a global warming over the continent (stronger over North and South Africa). Simulated precipitation change at the
end of the twenty-first century shows an increase in precipitation over Sudan-Sahel linked to a strong positive feedback with
surface evaporation. Along Guinea Gulf coast, rainfall regimes are driven by large-scale moisture advection. Moreover, results
show a mean precipitation decrease (increase) in the most (less) enhanced GHG atmosphere over this region. Modification of
the seasonal hydrological cycle consists in a rain increase during the monsoon onset. There is a significant increase in rainfall
variance over the Sahel, which extends over the Guinea coast region in the moderate emission scenario. Enhanced precipitation
over Sahel is linked to large-scale circulation changes, namely a weakening of the AEJ and an intensification of the Tropical
Easterly Jet. 相似文献
14.
Recent trends in vegetation dynamics in the African Sahel and their relationship to climate 总被引：26，自引：1，他引：26
Stefanie M. Herrmann Assaf Anyamba Compton J. Tucker 《Global Environmental Change》2005,15(4):394-404
Contrary to assertions of widespread irreversible desertification in the African Sahel, a recent increase in seasonal greenness over large areas of the Sahel has been observed, which has been interpreted as a recovery from the great Sahelian droughts. This research investigates temporal and spatial patterns of vegetation greenness and rainfall variability in the African Sahel and their interrelationships based on analyses of Normalized Difference Vegetation Index (NDVI) time series for the period 1982–2003 and gridded satellite rainfall estimates. While rainfall emerges as the dominant causative factor for the increase in vegetation greenness, there is evidence of another causative factor, hypothetically a human-induced change superimposed on the climate trend. 相似文献
15.
Severe weather has important social and economic impacts. Some studies have indicated that its intensity may increase over this century as a consequence of climate change induced by greenhouse gases. This study aims to investigate the possibility of a future increase in deep convective events over North America resulting from the evolution of favourable atmospheric conditions. Our analysis is based on an ensemble of projections performed using the Canadian Regional Climate Model (CRCM) at 45?km resolution, driven by different Global Climate Models (GCMs) and reanalyses. We concentrate our study on Convective Available Potential Energy (CAPE), vertical wind shear, and convective precipitation. Based on two different approaches to linking atmospheric conditions and severe weather, we find that the number of extreme weather events is expected to increase during the twenty-first century. In agreement with other studies on this subject, we find that CAPE is expected to increase, whereas wind shear is expected to decrease slightly. Through the analysis of the CRCM's convective precipitation outputs, we show that severe convective liquid precipitation events may become both more frequent and slightly more intense. Sensitivity experiments show that results depend on the driving GCM although they confirm the general conclusions. Additional experiments conducted with reduced humidity input at the lateral boundaries show the significant role that the humidity level of the driving GCMs has on simulated extreme regional events. At the regional level results are, in general, consistent with those found at the continental scale, but large inter-regional variations exist. 相似文献
16.
L. M. Druyan 《Climatic change》1991,18(1):17-36
The climate model of the Goddard Institute for Space Studies (Hansen et al., 1983) is used to study the sensitivity of sub-Saharan rainfall to Atlantic Ocean SST. Initial changes of SST in the South Atlantic Ocean on March 1st are shown to reduce the June–August sub-Saharan precipitation totals using the model version with an interactive ocean that updates SST. Evidence is offered in support of theories that link Sahel drought with anomalously warm SST in the eastern South Atlantic and the study compares the model's response to spatially coherent SST anomalies with the response to random SST perturbations. The physical processes whereby SST and sea-level pressure synoptics influence the African summer monsoon are discussed in reference to the simulations. Predictibility of Sahel summer rainfall based on spring SST patterns or spring atmospheric circulation patterns is implied by the results. The SST/Sahel drought links are discussed for projections of future climate characteristics. 相似文献
17.
Cloud structure and evolution of Mesoscale Convective Systems(MCSs) retrieved from the Tropical Rainfall Measuring Mission Microwave Imager(TRMM TMI) and Precipitation Radar(PR) were investigated and compared with some pioneer studies based on soundings and models over the northern South China Sea(SCS).The impacts of Convective Available Potential Energy(CAPE) and environmental vertical wind shear on MCSs were also explored.The main features of MCSs over the SCS were captured well by both TRMM PR and TMI.However,the PR-retrieved surface rainfall in May was less than that in June,and the reverse for TMI.TRMM-retrieved rainfall amounts were generally consistent with those estimated from sounding and models.However,rainfall amounts from sounding-based and PR-based estimates were relatively higher than those retrieved from TRMM-TMI data.The Weather Research and Forecasting(WRF) modeling simulation underestimated the maximum rain rate by 22% compared to that derived from TRMM-PR,and underestimated mean rainfall by 10.4% compared to the TRMM-TMI estimate,and by 12.5% compared to the sounding-based estimate.The warm microphysical processes modeled from both the WRF and the Goddard Cumulus Ensemble(GCE) models were quite close to those based on TMI,but the ice water contents in the models were relatively less compared to that derived from TMI.The CAPE and wind shear induced by the monsoon circulation were found to play critical roles in maintaining and developing the intense convective clouds over SCS.The latent heating rate increased more than twofold during the monsoon period and provided favorable conditions for the upward transportation of energy from the ocean,giving rise to the possibility of inducing large-scale interactions. 相似文献
18.
利用江苏省13个气象观测站历史上短时强降水观测资料,用遗传算法进行特征选择,选定影响短时强降水的950 hPa假相当位温、700 hPa比湿、500 hPa比湿、对流有效势能(Convective Available Potential Energy,CAPE)等14个特征为主要因素,将是否为短时强降水抽象成二元分类问题。借助机器学习中CART决策树算法进行分类分析,构建便于使用的短时强降水预报规则集。实验部分,随机选择5816条样本进行训练模型,得到适合江苏地区的短时强降水规则集,利用剩余的1454条数据进行实际检验,模型的短时强降水预报准确率为91.35%,非强降水预报准确率为97.11%,较特征选择之前分别提升了8.66%和1.05%。 相似文献
19.
20.
The influence of the land surface on the transition from dry to wet season in Amazonia 总被引：1，自引：0，他引：1
Summary Analysis of the fifteen years of European Centre for Medium Range Weather Forecasts (ECMWF) reanalysis suggests that the transition from dry to wet season in Southern Amazonia is initially driven by increases of surface latent heat flux. These fluxes rapidly reduce Convective Inhibition Energy (CINE) and increase Convective Available Potential Energy (CAPE), consequently providing favourable conditions for increased rainfall even before the large-scale circulation has changed. The increase of rainfall presumably initiates the reversal of the cross-equatorial flow, leading to large-scale net moisture convergence over Southern Amazonia. An analysis of early and late wet season onsets on an interannual scale shows that a longer dry season with lower rainfall reduces surface latent heat flux in the dry and earlier transition periods compared to that of a normal wet season onset. These conditions result in a higher CINE and a lower CAPE, causing a delay in the increase of local rainfall in the initiating phase of the transition and consequently in the wet season onset. Conversely, a wetter dry season leads to a higher surface latent heat flux and weaker CINE, providing a necessary condition for an earlier increase of local rainfall and an earlier wet season onset. Our results imply that if land use change in Amazonia reduces rainfall during dry and transition seasons, it could significantly delay the wet season onset and prolong the dry season. 相似文献