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291.
Juan-Carlos Ciscar 《Climatic change》2012,112(1):1-6
292.
The climate–population relationship has long been conceived. Although the topic has been repeatedly investigated, most of
the related works are Eurocentric or qualitative. Consequently, the relationship between climate and population remains ambiguous.
In this study, fine-grained temperature reconstructions and historical population data sets have been employed to statistically
test a hypothesized relationship between temperature change and population growth (i.e., cooling associated with below average
population growth) in China over the past millennium. The important results were: (1) Long-term temperature change significantly
determined the population growth dynamics of China. However, spatial variation existed, whilst population growth in Central
China was shown to be responsive to both long- and short-term temperature changes; in marginal areas, population growth was
only sensitive to short-term temperature fluctuations. (2) Temporally, the temperature–population relationship was obscured
in some periods, which was attributable to the factors of drought and social buffers. In summary, a temperature–population
relationship was mediated by geographic factors, the aridity threshold, and social factors. Given the upcoming threat posed
by climate change to human societies, this study seeks to improve our knowledge and understanding of the climate–society relationship. 相似文献
293.
TRMM-observed summer warm rain over the tropical and subtropical Pacific Ocean: Characteristics and regional differences 总被引:1,自引:0,他引:1
Based on the merged measurements from the TRMM Precipitation Radar and Visible and Infrared Scanner, refined characteristics (intensity, frequency, vertical structure, and diurnal variation) and regional differences of the warm rain over the tropical and subtropical Pacific Ocean (40ffiS-40ffiN, 120ffiE-70ffiW) in boreal summer are investigated for the period 1998-2012. The results reveal that three warm rain types (phased, pure, and mixed) exist over these regions. The phased warm rain, which occurs during the developing or declining stage of precipitation weather systems, is located over the central to western Intertropical Convergence Zone, South Pacific Convergence Zone, and Northwest Pacific. Its occurrence frequency peaks at midnight and minimizes during daytime with a 5.5-km maximum echo top. The frequency of this warm rain type is about 2.2%, and it contributes to 40% of the regional total rainfall. The pure warm rain is characterized by typical stable precipitation with an echo top lower than 4 km, and mostly occurs in Southeast Pacific. Although its frequency is less than 1.3%, this type of warm rain accounts for 95% of the regional total rainfall. Its occurrence peaks before dawn and it usually disappears in the afternoon. For the mixed warm rain, some may develop into deep convective precipitation, while most are similar to those of the pure type. The mixed warm rain is mainly located over the ocean east of Hawaii. Its frequency is 1.2%, but this type of warm rain could contribute to 80% of the regional total rainfall. The results also uncover that the mixed and pure types occur over the regions where SST ranges from 295 to 299 K, accompanied by relatively strong downdrafts at 500 hPa. Both the mixed and pure warm rains happen in a more unstable atmosphere, compared with the phased warm rain. 相似文献
294.
Vladimír Fuka Zheng-Tong Xie Ian P. Castro Paul Hayden Matteo Carpentieri Alan G. Robins 《Boundary-Layer Meteorology》2018,167(1):53-76
Scalar dispersion from ground-level sources in arrays of buildings is investigated using wind-tunnel measurements and large-eddy simulation (LES). An array of uniform-height buildings of equal dimensions and an array with an additional single tall building (wind tunnel) or a periodically repeated tall building (LES) are considered. The buildings in the array are aligned and form long streets. The sensitivity of the dispersion pattern to small changes in wind direction is demonstrated. Vertical scalar fluxes are decomposed into the advective and turbulent parts and the influences of wind direction and of the presence of the tall building on the scalar flux components are evaluated. In the uniform-height array turbulent scalar fluxes are dominant, whereas the tall building produces an increase of the magnitude of advective scalar fluxes that yields the largest component. The presence of the tall building causes either an increase or a decrease to the total vertical scalar flux depending on the position of the source with respect to the tall building. The results of the simulations can be used to develop parametrizations for street-canyon dispersion models and enhance their capabilities in areas with tall buildings. 相似文献
295.
Samuel D. Chamberlain Joseph Verfaillie Elke Eichelmann Kyle S. Hemes Dennis D. Baldocchi 《Boundary-Layer Meteorology》2017,165(2):197-210
Corrections accounting for air density fluctuations due to heat and water vapour fluxes must be applied to the measurement of eddy-covariance fluxes when using open-path sensors. Experimental tests and ecosystem observations have demonstrated the important role density corrections play in accurately quantifying carbon dioxide \((\hbox {CO}_{2})\) fluxes, but less attention has been paid to evaluating these corrections for methane \((\hbox {CH}_{4})\) fluxes. We measured \(\hbox {CH}_{4}\) fluxes with open-path sensors over a suite of sites with contrasting \(\hbox {CH}_{4}\) emissions and energy partitioning, including a pavement airfield, two negligible-flux ecosystems (drained alfalfa and pasture), and two high-flux ecosystems (flooded wetland and rice). We found that density corrections successfully re-zeroed fluxes in negligible-flux sites; however, slight overcorrection was observed above pavement. The primary impact of density corrections varied over negligible- and high-flux ecosystems. For negligible-flux sites, corrections led to greater than 100% adjustment in daily budgets, while these adjustments were only 3–10% in high-flux ecosystems. The primary impact to high-flux ecosystems was a change in flux diel patterns, which may affect the evaluation of relationships between biophysical drivers and fluxes if correction bias exists. Additionally, accounting for density effects to high-frequency \(\hbox {CH}_{4}\) fluctuations led to large differences in observed \(\hbox {CH}_{4}\) flux cospectra above negligible-flux sites, demonstrating that similar adjustments should be made before interpreting \(\hbox {CH}_{4}\) cospectra for comparable ecosystems. These results give us confidence in \(\hbox {CH}_{4}\) fluxes measured by open-path sensors, and demonstrate that density corrections play an important role in adjusting flux budgets and diel patterns across a range of ecosystems. 相似文献
296.
A Parameterization of Dry Thermals and Shallow Cumuli for Mesoscale Numerical Weather Prediction 总被引:2,自引:1,他引:2
Julien Pergaud Valéry Masson Sylvie Malardel Fleur Couvreux 《Boundary-Layer Meteorology》2009,132(1):83-106
For numerical weather prediction models and models resolving deep convection, shallow convective ascents are subgrid processes
that are not parameterized by classical local turbulent schemes. The mass flux formulation of convective mixing is now largely
accepted as an efficient approach for parameterizing the contribution of larger plumes in convective dry and cloudy boundary
layers. We propose a new formulation of the EDMF scheme (for Eddy Diffusivity\Mass Flux) based on a single updraft that improves
the representation of dry thermals and shallow convective clouds and conserves a correct representation of stratocumulus in
mesoscale models. The definition of entrainment and detrainment in the dry part of the updraft is original, and is specified
as proportional to the ratio of buoyancy to vertical velocity. In the cloudy part of the updraft, the classical buoyancy sorting
approach is chosen. The main closure of the scheme is based on the mass flux near the surface, which is proportional to the
sub-cloud layer convective velocity scale w
*. The link with the prognostic grid-scale cloud content and cloud cover and the projection on the non- conservative variables
is processed by the cloud scheme. The validation of this new formulation using large-eddy simulations focused on showing the
robustness of the scheme to represent three different boundary layer regimes. For dry convective cases, this parameterization
enables a correct representation of the countergradient zone where the mass flux part represents the top entrainment (IHOP
case). It can also handle the diurnal cycle of boundary-layer cumulus clouds (EUROCS\ARM) and conserve a realistic evolution
of stratocumulus (EUROCS\FIRE). 相似文献
297.
Pietro Salizzoni Massimo Marro Lionel Soulhac Nathalie Grosjean Richard J. Perkins 《Boundary-Layer Meteorology》2011,141(3):393-414
The turbulent exchange of momentum between a two-dimensional cavity and the overlying boundary layer has been studied experimentally,
using hot-wire anemometry and particle image velocimetry (PIV). Conditions within the boundary layer were varied by changing
the width of the canyons upstream of the test canyon, whilst maintaining the square geometry of the test canyon. The results
show that turbulent transfer is due to the coupling between the instabilities generated in the shear layer above the canyons
and the turbulent structures in the oncoming boundary layer. As a result, there is no single, unique velocity scale that correctly
characterizes all the processes involved in the turbulent exchange of momentum across the boundary layer. Similarly, there
is no single velocity scale that can characterize the different properties of the turbulent flow within the canyon, which
depends strongly on the way in which turbulence from the outer flow is entrained into the cavity and carried round by the
mean flow. The results from this study will be useful in developing simple parametrizations for momentum exchange in the urban
canopy, in situations where the street geometry consists principally of relatively long, uniform streets arranged in grid-like
patterns; they are unlikely to be applicable to sparse geometries composed of isolated three-dimensional obstacles. 相似文献
298.
André Lyra Pablo Imbach Daniel Rodriguez Sin Chan Chou Selena Georgiou Lucas Garofolo 《Climatic change》2017,141(1):93-105
Tropical rainforest plays an important role in the global carbon cycle, accounting for a large part of global net primary productivity and contributing to CO2 sequestration. The objective of this work is to simulate potential changes in the rainforest biome in Central America subject to anthropogenic climate change under two emissions scenarios, RCP4.5 and RCP8.5. The use of a dynamic vegetation model and climate change scenarios is an approach to investigate, assess or anticipate how biomes respond to climate change. In this work, the Inland dynamic vegetation model was driven by the Eta regional climate model simulations. These simulations accept boundary conditions from HadGEM2-ES runs in the two emissions scenarios. The possible consequences of regional climate change on vegetation properties, such as biomass, net primary production and changes in forest extent and distribution, were investigated. The Inland model projections show reductions in tropical forest cover in both scenarios. The reduction of tropical forest cover is greater in RCP8.5. The Inland model projects biomass increases where tropical forest remains due to the CO2 fertilization effect. The future distribution of predominant vegetation shows that some areas of tropical rainforest in Central America are replaced by savannah and grassland in RCP4.5. Inland projections under both RCP4.5 and RCP8.5 show a net primary productivity reduction trend due to significant tropical forest reduction, temperature increase, precipitation reduction and dry spell increments, despite the biomass increases in some areas of Costa Rica and Panama. This study may provide guidance to adaptation studies of climate change impacts on the tropical rainforests in Central America. 相似文献
299.
The turbulent characteristics of the neutral boundary layer developing over rough surfaces are not well predicted with operational weather-forecasting models. The problem is attributed to inadequate mixing-length models, to the anisotropy of the flow and to a lack of controlled experimental data against which to validate numerical studies. Therefore, in order to address directly the modelling difficulties for the development of a neutral boundary layer over rough surfaces, and to investigate the turbulent momentum transfer of such a layer, a set of hydraulic flume experiments were carried out. In the experiments, the mean and turbulent quantities were measured by a particle image velocimetry (PIV) technique. The measured velocity variances and fluxes \({(\overline{{u_{i}^{\prime}}{u_{j}^{\prime}}})}\) in longitudinal vertical planes allowed the vertical and longitudinal gradients (?/?z and ?/?x) of the mean and turbulent quantities (fluxes, variances and third-order moments) to be evaluated and the terms of the evolution equations for ?e/?t, \({\partial \overline{u^{\prime 2}}/\partial t}\), \({\partial \overline{w^{\prime 2}}/\partial t}\) and \({\partial \overline{{u^{\prime}}{w^{\prime}}}/\partial t}\) to be quantified, where e is the turbulent kinetic energy. The results show that the pressure-correlation terms allow the turbulent energy to be transferred equitably from \({\overline{{u^{\prime}}^{2}}}\) to \({\overline{{w^{\prime}}^{2}}}\). It appears that the repartition between the constitutive terms of the budget of e, \({\overline{{u^{\prime}}^{2}}}\), \({\overline{{w^{\prime}}^{2}}}\) and \({\overline{{u^{\prime}}{w^{\prime}}}}\) is not significantly affected by the development of the rough neutral boundary layer. For the whole evolution, the transfers of energy are governed by the same terms that are also very similar to the smooth-wall case. The PIV measurements also allowed the spatial integral scales to be computed directly and to be compared with the dissipative and mixing length scales, which were also computed from the data. 相似文献
300.
Adequate high-quality data on three-dimensional velocities in the atmospheric surface layer (height \(\delta \)) were acquired in the field at the Qingtu Lake Observation Array. The measurement range occupies nearly the entire logarithmic layer from approximately \(0.006\delta \)–\(0.2\delta \). The turbulence intensity and eddy structures of the velocity fluctuations in the logarithmic region were primarily analyzed, and their variations in the z (wall-normal) direction were revealed. The primary finding was that the turbulent intensity of wall-normal velocity fluctuations exhibits a sharp upswing in the logarithmic region, which differs from classic scaling law and laboratory results. The upswing of the wall-normal turbulence intensity in the logarithmic region is deemed to be linear based on an ensemble of 20 sets of data. In addition, the wall-normal extent of the correlated structures and wall-normal spectra were compared to low Reynolds number results in the laboratory. 相似文献