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1.
The atmospheric general circulation model ECHAM-4 is coupled to a chemistry model to calculate sulfate mass distribution
and the radiative forcing due to sulfate aerosol particles. The model simulates the main components of the hydrological cycle
and, hence, it allows an explicit treatment of cloud transformation processes and precipitation scavenging. Two experiments
are performed, one with pre-industrial and one with present-day sulfur emissions. In the pre-industrial emission scenario
SO2 is oxidized faster to sulfate and the in-cloud oxidation via the reaction with ozone is more important than in the present-day
scenario. The atmospheric sulfate mass due to anthropogenic emissions is estimated as 0.38 Tg sulfur. The radiative forcing
due to anthropogenic sulfate aerosols is calculated diagnostically. The backscattering of shortwave radiation (direct effect)
as well as the impact of sulfate aerosols on the cloud albedo (indirect effect) is estimated. The model predicts a direct
forcing of −0.35 W m-2 and an indirect forcing of −0.76 W m-2. Over the continents of the Northern Hemisphere the direct forcing amounts to −0.64 W m-2. The geographical distribution of the direct and indirect effect is very different. Whereas the direct forcing is strongest
over highly polluted continental regions, the indirect forcing over sea exceeds that over land. It is shown that forcing estimates
based on monthly averages rather than on instantaneous sulfate pattern overestimate the indirect effect but have little effect
on the direct forcing.
Received: 16 October 1996/Accepted: 24 October 1996 相似文献
2.
Impacts of Internally and Externally Mixed Anthropogenic Sulfate and Carbonaceous Aerosols on East Asian Climate 总被引:1,自引:0,他引:1 下载免费PDF全文
A coupled regional climate and aerosol-chemistry model, RIEMS 2.0 (Regional Integrated Environmental Model System for Asia),
in which anthropogenic sulfate, black carbon, and organic carbon were assumed to be externally mixed (EM), internally mixed
(IM) or partially internally mixed (IEM), was used to simulate the impacts of these anthropogenic aerosols on East Asian climate
for the entire year of 2006. The distributions of aerosol mass concentration, radiative forcing and hence the surface air
temperature and precipitation variations under three mixing assumptions of aerosols were analyzed. The results indicated that
the mass concentration of sulfate was sensitive to mixing assumptions, but carbonaceous aerosols were much less sensitive
to the mixing types. Modeled results were compared with observations in a variety of sites in East Asia. It was found that
the simulated concentrations of sulfate and carbonaceous aerosols were in accord with the observations in terms of magnitude.
The simulated aerosol concentrations in IM case were closest to observation results. The regional average column burdens of
sulfate, black carbon, and organic carbon, if internally mixed, were 11.49, 0.47, and 2.17 mg m−2, respectively. The radiative forcing of anthropogenic aerosols at the top of the atmosphere increased from −1.27 (EM) to
−1.97 W m−2 (IM) while the normalized radiative forcing (NRF) decreased from −0.145 (EM) to −0.139 W mg−1 (IM). The radiative forcing and NRF were −1.82 W m−2 and −0.141 W mg−1 for IEM, respectively. The surface air temperature changes over the domain due to the anthropogenic sulfate and carbonaceous
aerosols were −0.067, −0.078, and −0.072 K, with maxima of −0.47, −0.50, and −0.49 K, for EM, IM, and IEM, respectively. Meanwhile,
the annual precipitation variations were −8.0 (EM), −20.6 (IM), and −21.9 mm (IEM), with maxima of 148, 122, and 102 mm, respectively,
indicating that the climate effects were stronger if the sulfate and carbonaceous aerosols were internally mixed. 相似文献
3.
The direct and semi-direct radiative effects of anthropogenic aerosols on the radiative transfer and cloud fields in the Western United States (WUS) according to seasonal aerosol optical depth (AOD) and regional climate are examined using a regional climate model (RCM) in conjunction with the aerosol fields from a GEOS-Chem chemical-transport model (CTM) simulation. The two radiative effects cannot be separated within the experimental design in this study, thus the combined direct- and semi-direct effects are called radiative effects hereafter. The CTM shows that the AOD associated with the anthropogenic aerosols is chiefly due to sulfates with minor contributions from black carbon (BC) and that the AOD of the anthropogenic aerosol varies according to local emissions and the seasonal low-level winds. The RCM-simulated anthropogenic aerosol radiative effects vary according to the characteristics of regional climate, in addition to the AOD. The effects on the top of the atmosphere (TOA) outgoing shortwave radiation (OSRT) range from ?0.2?Wm?2 to ?1?Wm?2. In Northwestern US (NWUS), the maximum and minimum impact of anthropogenic aerosols on OSRT occurs in summer and winter, respectively, following the seasonal AOD. In Arizona-New Mexico (AZNM), the effect of anthropogenic sulfates on OSRT shows a bimodal distribution with winter/summer minima and spring/fall maxima, while the effect of anthropogenic BC shows a single peak in summer. The anthropogenic aerosols affect surface insolation range from ?0.6?Wm?2 to ?2.4?Wm?2, with similar variations found for the effects on OSRT except that the radiative effects of anthropogenic BC over AZNM show a bimodal distribution with spring/fall maxima and summer/winter minima. The radiative effects of anthropogenic sulfates on TOA outgoing longwave radiation (OLR) and the surface downward longwave radiation (DLRS) are notable only in summer and are characterized by strong geographical contrasts; the summer OLR in NWUS (AZNM) is reduced (enhanced) by 0.52?Wm?2 (1.14?Wm?2). The anthropogenic sulfates enhance (reduce) summer DLRS by 0.2?Wm?2 (0.65?Wm?2) in NWUS (AZNM). The anthropogenic BC affect DLRS noticeably only in AZNM during summer. The anthropogenic aerosols affect the cloud water path (CWP) and the radiative transfer noticeably only in summer when convective clouds are dominant. Primarily shortwave-reflecting anthropogenic sulfates decrease and increase CWP in AZNM and NWUS, respectively, however, the shortwave-absorbing anthropogenic BC reduces CWP in both regions. Due to strong feedback via convective clouds, the radiative effects of anthropogenic aerosols on the summer radiation field are more closely correlated with the changes in CWP than the AOD. The radiative effect of the total anthropogenic aerosols is dominated by the anthropogenic sulfates that contribute more than 80% of the total AOD associated with the anthropogenic aerosols. 相似文献
4.
Summary ?For the LITFASS-98 experiment, from June 1 until June 30, 1998, the spatially resolved insolation at surface could be computed
from NOAA-14 AVHRR data applying the modular analysis scheme SESAT (Strahlungs- und Energiebilanzen aus Satellitendaten). The satellite inferred insolation for this period shows for clear-sky regions a good agreement with surface
based observations with a rms error of 76 Wm−2. For cloudy conditions the insolation is overestimated with respect to ground based observations, with a rms error between
83 and 118 Wm−2, depending on the cloud optical thickness. This overestimation can be explained by the surface heterogeneity, leading to
underestimated cloud optical thickness, and also by a fixed relative humidity below clouds (55%, dry atmosphere) and a fixed
horizontal visibility (50 km, clear atmosphere). A detailed study of comparable scales in space and time, considering the
different observation geometries and sampling intervals, shows that a 30 min ground based observation can be compared with
a 8 × 8 km2 mean by the satellite data.
Received July 12, 2001; revised April 29, 2002; accepted June 7, 2002 相似文献
5.
In this study, a regional air quality model system (RAQMS) was
applied to investigate the spatial distributions and seasonal variations of
atmospheric aerosols in 2006 over East Asia. Model validations demonstrated
that RAQMS was able to reproduce the evolution processes of aerosol components
reasonably well. Ground-level PM10 (particles with aerodynamic diameter
≤10 μm) concentrations were highest in spring and lowest in
summer and were characterized by three maximum centers: the Taklimakan Desert
(~1000 μg m-3), the Gobi Desert (~400 μg m-3), and
the Huabei Plain (~300 μm-3) of China. Vertically, high
PM10 concentrations ranging from 100 μg m-3 to 250 μg
m-3 occurred from the surface to an altitude of 6000 m at
30o--45oN in spring. In winter, the vertical gradient was so
large that most aerosols were restricted in the boundary layer. Both sulfate
and ammonium reached their highest concentrations in autumn, while nitrate
reached its maximum level in winter. Black carbon and organic carbon aerosol
concentrations reached maximums in winter. Soil dust were strongest in spring,
whereas sea salt exerted the strongest influence on the coastal regions of
eastern China in summer. The estimated burden of anthropogenic aerosols was
largest in winter (1621 Gg) and smallest in summer (1040 Gg). The sulfate
burden accounted for ~42% of the total anthropogenic aerosol burden. The
dust burden was about twice the anthropogenic aerosol burden, implying the
potentially important impacts of the natural aerosols on air quality and
climate over East Asia. 相似文献
6.
UV attenuation in the cloudy atmosphere 总被引:1,自引:0,他引:1
Jianhui Bai 《Journal of Atmospheric Chemistry》2009,62(3):211-228
Ultraviolet (UV) energy absorption plays a very important role in the Earth–atmosphere system. Based on observational data
for Beijing, we suggest that some atmospheric constituents utilize or transfer UV energy in chemical and photochemical (C&P) reactions, in addition to those which absorb UV energy directly. These constituents are primarily volatile organic compounds
(VOCs) emitted from both vegetative and anthropogenic sources. The total UV energy loss in the cloudy atmosphere for Beijing
in 1990 was 78.9 Wm−2. This attenuation was caused by ozone (48.3 Wm−2), other compounds in the atmosphere (26.6 Wm−2) and a scattering factor (4.0 Wm−2). Our results for a cloudy atmosphere in the Beijing area show that the absorption due to these other compounds occurs largely
through the mediation of water vapor. This fraction of energy loss has not been fully accounted for in previous models. Observations
and previous models results suggest that 1) a cloudy atmosphere absorbs 25∼30 Wm−2 more solar shortwave radiation than models predict; and 2) aerosols can significantly decrease the downward mean UV-visible
radiation and the absorbed solar radiation at the surface by up to 28 and 23 Wm−2, respectively. Thus, quantitative study of UV and visible absorption by atmospheric constituents involved in homogeneous
and heterogeneous C&P reactions is important for atmospheric models. 相似文献
7.
H.-S. Liu 《Theoretical and Applied Climatology》1998,61(3-4):217-229
Summary For astronomical seasons, Rubincam insolation deviations at latitude 65° N varied from 218.50 Wm−2 to 225.75 Wm−2 (3%). The periodicity of the insolation cycles varied from 36.7 Kyr to 44.7 Kyr (20%) due to phase shift. Phase shift of
insolation variations can induce asymmetry of the insolation cycles, permitting rapid melting and prolonged glaciation of
ice sheets to occur. For instance, an abnormal decrease of the insolation frequency during the longer period of glacial interval
would prolong glaciation into deep ice age. In this study, we apply Rubincam’s insolation equations to investigate the phase
shift effect of insolation variations on climate change. Using complex transforms of the changing insolation, we have detected
a phase modulation signal in the insolation variations. As a result, an especially new and interesting series of the phase-related
insolation pulsation is established. The phase modulated insolation is then introduced as a forcing function into energy balance
climate models. Results of model computations shed new insights into the spectrum of the paleoclimatic proxy-data. It is shown
that phase modulation of the insolation may provide an appropriate and complete external forcing mechanism to which the climate
system would respond. The 100 Kyr cycle of the frequency modulation of the Rubincam’s insolation variations does seem adequate
to change the climate.
Received July 16, 1997 Revised May 18, 1998 相似文献
8.
S. Gonzi O. Dubovik D. Baumgartner E. Putz 《Meteorology and Atmospheric Physics》2007,96(3-4):277-291
Summary One of the great unknowns in climate research is the contribution of aerosols to climate forcing and climate perturbation.
In this study, retrievals from AERONET are used to estimate the direct clear-sky aerosol top-of-atmosphere and surface radiative
forcing effects for 12 multi-site observing stations in Europe. The radiative transfer code sdisort in the libRadtran environment is applied to accomplish these estimations. Most of the calculations in this study rely on observations which
have been made for the years 1999, 2000, and 2001. Some stations do have observations dating back to the year of 1995. The
calculations rely on a pre-compiled aerosol optical properties database for Europe. Aerosol radiative forcing effects are
calculated with monthly mean aerosol optical properties retrievals and calculations are presented for three different surface
albedo scenarios. Two of the surface albedo scenarios are generic by nature bare soil and green vegetation and the third relies on the ISCCP (International Satellite Cloud Climatology Project) data product. The ISCCP database has
also been used to obtain clear-sky weighting fractions over AERONET stations. The AERONET stations cover the area 0° to 30° E
and 42° to 52° N. AERONET retrievals are column integrated and this study does not make any seperation between the contribution
of natural and anthropogenic components. For the 12 AERONET stations, median clear-sky top-of-atmosphere aerosol radiative
forcing effect values for different surface albedo scenarios are calculated to be in the range of −4 to −2 W/m2. High median radiative forcing effect values of about −6 W/m2 were found to occur mainly in the summer months while lower values of about −1 W/m2 occur in the winter months. The aerosol surface forcing also increases in summer months and can reach values of −8 W/m2. Individual stations often have much higher values by a factor of 2. The median top-of-atmosphere aerosol radiative forcing
effect efficiency is estimated to be about −25 W/m2 and their respective surface efficiency is around −35 W/m2. The fractional absorption coefficient is estimated to be 1.7, but deviates significantly from station to station. In addition,
it is found that the well known peak of the aerosol radiative forcing effect at a solar zenith angle of about 75° is in fact
the average of the peaks occurring at shorter and longer wavelengths. According to estimations for Central Europe, based on
mean aerosol optical properties retrievals from 12 stations, the critical threshold of the aerosol single scattering albedo,
between cooling and heating in the presence of an aerosol layer, is close between 0.6 and 0.76. 相似文献
9.
Anthropogenic aerosols play an important role in the atmospheric energy balance. Anthropogenic aerosol optical depth (AOD) and its accompanying shortwave radiative forcing (RF) are usually simulated by nu- merical models. Recently, with the development of space-borne instruments and sophisticated retrieval algorithms, it has become possible to estimate aerosol radiative forcing based on satellite observations. In this study, we have estimated shortwave direct radiative forcing due to anthropogenic aerosols over oceans in all-sky conditions by combining clouds and the Single Scanner Footprint data of the Clouds and Earth’s Radiant Energy System (CERES/SSF) experiment, which provide measurements of upward shortwave fluxes at the top of atmosphere, with Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol and cloud products. We found that globally averaged aerosol radiative forcing over oceans in the clear-sky conditions and all-sky conditions were -1.03±0.48 W m-2 and -0.34 ±0.16 W m-2, respectively. Direct radiative forcing by anthropogenic aerosols shows large regional and seasonal variations. In some regions and in particular seasons, the magnitude of direct forcing by anthropogenic aerosols can be comparable to the forcing of greenhouse gases. However, it shows that aerosols caused the cooling effect, rather than warming effect from global scale, which is different from greenhouse gases. 相似文献
10.
Summary Vertical profile of surface radiative fluxes in an area of heterogeneous terrain in south-west Germany is presented. Main
data sets utilized for the study were recorded during the REgio KLIma Projekt (REKLIP). Supporting observational data were
provided by the German weather service and German geophysical consultant service. Elevation of the study sites ranged from
212 m a.s.l. to 1489 m a.s.l. From May to September, monthly mean albedo was generally low at the study sites, ranging from
19% to 24%. For the other months, monthly mean albedo lie between 22% and 25% at the lowland site but extended between 27%
and 71% at the highly elevated mountain site. Following the altitudinal increase in surface albedo, net radiative flux and
radiation efficiency declined with elevation at an annual mean of 1.15 Wm−2/100 m and 0.008/100 m respectively. Absorbed shortwave radiation and effective terrestrial radiation showed mean decline
of 1.54 Wm−2/100 m and 0.34 Wm−2/100 m, respectively, with the mean sky-to-earth radiation deficit amounting to about 52 Wm−2 for the lowland site and 73 Wm−2 for the highest elevated site.
Some empirical models which express shortwave and longwave radiative fluxes in terms of meteorological variables have been
validated for the lowland and mountain sites. Monthly mean daily total estimates of solar radiation obtained from ?ngst?m-Prescott
relation were quite consistent with observed values. Parameterisation of downward atmospheric radiation under all sky condition
was achieved by extending Brutsaert clear sky atmospheric model. Relationship between outgoing longwave radiation and screen
temperature at the study sites was best described by an exponential function unlike the linear relationship proposed by Monteith
and Unsworth. Net radiative flux for the lowland and mountain sites has been expressed in terms of absorbed shortwave radiation,
cloud amount and screen temperature.
Received March 5, 2001 Revised October 29, 2001 相似文献
11.
黄土高原半干旱区典型日吸收性气溶胶综合观测分析 总被引:2,自引:0,他引:2
利用兰州大学半干旱气候与环境观测站的太阳光度计、激光雷达、微波辐射计综合观测资料,结合辐射传输模式分析了该地区秋季典型日2012年9月3~4日、21日和28日气溶胶物理特性、垂直分布特征,及其与气象条件的关系。研究时期的气溶胶主要为局地沙尘与人为污染混合气溶胶,吸收性明显,尺度较小。其中,4日西北风增强,远距离传输沙尘气溶胶,气溶胶光学厚度最大,粒子尺度明显增大。尝试利用灰色关联度法确定参考高度,分别为7.41 km、8.47 km、7.13 km和7.66 km,反演气溶胶消光系数,由此积分得到的光学厚度与太阳光度计观测值相关性可达0.975,反演效果较好。研究时期气溶胶的抬升主要受白天热力湍流作用,边界层发展,气溶胶向上传输,每日12时(当地时间,下同)至14时传输至最大高度,气溶胶抬升的高度对应大气加热率的高值区,低层加热率可达1 K d-1。气溶胶在大气层顶和地面造成负辐射强迫,分别为-12.707 W m-2、-25.398 W m-2,大气中表现为正辐射强迫,为12.692 W m-2,大气层顶的辐射强迫对气溶胶的物理特性最为敏感,当气溶胶吸收性明显时,大气层顶的瞬时辐射强迫会出现正值。 相似文献
12.
K. D. Williams A. Jones D. L. Roberts C. A. Senior M. J. Woodage 《Climate Dynamics》2001,17(11):845-856
The indirect effects of anthropogenic sulfate aerosols on the albedo and lifetime of clouds may produce a significant impact
on the climate system. A `state of the art' general circulation model (GCM) which includes an interactive sulfur cycle and
a physically based cloud microphysics scheme is coupled to a mixed-layer ocean model in order to study the impact of the indirect
effects on the coupled climate system. The linearity of the two indirect effects on the model response is also investigated
by including each effect separately in the model. The response of the sea surface temperatures (SSTs) and sea ice is found
to provide an important feedback on the cooling at high latitudes and the change in meridional SST gradient results in a southward
shift of the inter-tropical convergence zone (ITCZ). The sensitivity of the model to the forcing from the indirect effects
of sulfate aerosol is found to be similar to, but slightly weaker than that obtained from a doubling of CO2.
Received: 30 August 2000 / Accepted: 3 January 2001 相似文献
13.
The disposition of radiative energy in the global climate system: GCM-calculated versus observational estimates 总被引:1,自引:1,他引:0
M. Wild A. Ohmura H. Gilgen E. Roeckner M. Giorgetta J.-J. Morcrette 《Climate Dynamics》1998,14(12):853-869
A comprehensive dataset of direct observations is used to assess the representation of surface and atmospheric radiation
budgets in general circulation models (GCMs). Based on combined measurements of surface and collocated top-of-the-atmosphere
fluxes at more than 700 sites, a lack of absorption of solar radiation within the atmosphere is identified in the ECHAM3 GCM,
indicating that the shortwave atmospheric absorption calculated in the current generation of GCMs, typically between 60 and
70 Wm-2, is too low by 10–20 Wm-2. The surface and atmospheric radiation budgets of a new version of the Max-Planck Institute GCM, the ECHAM4, differ considerably
from other GCMs in both short- and longwave ranges. The amount of solar radiation absorbed in the atmosphere (90 Wm-2) is substantially larger than typically found in current GCMs, resulting in a lower absorption at the surface (147 Wm-2). It is shown that this revised disposition of solar energy within the climate system generally reduces the biases compared
to the observational estimates of surface and atmospheric absorption. The enhanced shortwave absorption in the ECHAM4 atmosphere
is due to an increase in both simulated clear-sky and cloud absorption compared to ECHAM3. The increased absorption in the
cloud-free atmosphere is related to an enhanced absorption of water vapor, and is supported in stand-alone comparisons of
the radiation scheme with synchronous observations. The increased cloud absorption, on the other hand, is shown to be predominantly
spurious due to the coarse spectral resolution of the ECHAM4 radiation code, thus providing no physical explanation for the
“anomalous cloud absorption” phenomenon. Quantitatively, however, an additional increase of atmospheric absorption due to
clouds as in ECHAM4 is, at least at low latitudes, not in conflict with the observational estimates, though this does not
rule out the possibility that other effects, such as highly absorbing aerosols, could equally contribute to close the gap
between models and observations. At higher latitudes, however, the increased cloud absorption is not supported by the observational
dataset. Overall, this study points out that not only the clouds, but also the cloud-free atmosphere might be responsible
for the discrepancies between observational and simulated estimates of shortwave atmospheric absorption. The smaller absorption
of solar radiation at the surface in ECHAM4 is compensated by an increased downward longwave flux (344 Wm-2), which is larger than in other GCMs. The enhanced downward longwave flux is supported by surface measurements and by a stand-alone
validation of the radiation scheme for clear-sky conditions. The enhanced flux also ensures that a sufficient amount of energy
is available at the surface to maintain a realistic intensity of the global hydrological cycle. In contrast, a one-handed
revision of only the shortwave radiation budget to account for the increased shortwave absorption in GCM atmospheres may induce
a global hydrological cycle that is too weak.
Received: 26 February 1998 / Accepted: 18 May 1998 相似文献
14.
Bertrand Bessagnet Laurent Menut Gabriele Curci Alma Hodzic Bruno Guillaume Catherine Liousse Sophie Moukhtar Betty Pun Christian Seigneur Michaël Schulz 《Journal of Atmospheric Chemistry》2008,61(3):175-202
In this study, an improved and complete secondary organic aerosols (SOA) chemistry scheme was implemented in the CHIMERE model.
The implementation of isoprene chemistry for SOA significantly improves agreement between long series of simulated and observed
particulate matter concentrations. While simulated organic carbon concentrations are clearly improved at elevated sites by
adding the SOA scheme, time correlation are impaired at low level sites in Portugal, Italy and Slovakia. At several sites
a clear underestimation by the CHIMERE model is noticed in wintertime possibly due to missing wood burning emissions as shown
in previous modeling studies. In Europe, the CHIMERE model gives yearly average SOA concentrations ranging from 0.5 μg
m
− 3 in the Northern Europe to 4 μg
m
− 3 over forested regions in Spain, France, Germany and Italy. In addition, our work suggests that during the highest fire emission
periods, fires can be the dominant source of primary organic carbon over the Mediterranean Basin, but the SOA contribution
from fire emissions is low. Isoprene chemistry has a strong impact on SOA formation when using current available kinetic schemes. 相似文献
15.
A general circulation model is used to examine the effects of reduced atmospheric CO2, insolation changes and an updated reconstruction of the continental ice sheets at the Last Glacial Maximum (LGM). A set
of experiments is performed to estimate the radiative forcing from each of the boundary conditions. These calculations are
used to estimate a total radiative forcing for the climate of the LGM. The response of the general circulation model to the
forcing from each of the changed boundary conditions is then investigated. About two-thirds of the simulated glacial cooling
is due to the presence of the continental ice sheets. The effect of the cloud feedback is substantially modified where there
are large changes to surface albedo. Finally, the climate sensitivity is estimated based on the global mean LGM radiative
forcing and temperature response, and is compared to the climate sensitivity calculated from equilibrium experiments with
atmospheric CO2 doubled from present day concentration. The calculations here using the model and palaeodata support a climate sensitivity
of about 1 Wm-2 K-1 which is within the conventional range.
Received: 8 February 1997 / Accepted: 4 June 1997 相似文献
16.
Tropical climates at the Last Glacial Maximum: a new synthesis of terrestrial palaeoclimate data. I. Vegetation, lake-levels and geochemistry 总被引:1,自引:1,他引:0
I. Farrera S. P. Harrison I. C. Prentice G. Ramstein J. Guiot P. J. Bartlein R. Bonnefille M. Bush W. Cramer U. von Grafenstein K. Holmgren H. Hooghiemstra G. Hope D. Jolly S.-E. Lauritzen Y. Ono S. Pinot M. Stute G. Yu 《Climate Dynamics》1999,15(11):823-856
Palaeodata in synthesis form are needed as benchmarks for the Palaeoclimate Modelling Intercomparison Project (PMIP). Advances
since the last synthesis of terrestrial palaeodata from the last glacial maximum (LGM) call for a new evaluation, especially
of data from the tropics. Here pollen, plant-macrofossil, lake-level, noble gas (from groundwater) and δ18O (from speleothems) data are compiled for 18±2 ka (14C), 32 °N–33 °S. The reliability of the data was evaluated using explicit criteria and some types of data were re-analysed
using consistent methods in order to derive a set of mutually consistent palaeoclimate estimates of mean temperature of the
coldest month (MTCO), mean annual temperature (MAT), plant available moisture (PAM) and runoff (P-E). Cold-month temperature
(MAT) anomalies from plant data range from −1 to −2 K near sea level in Indonesia and the S Pacific, through −6 to −8 K at
many high-elevation sites to −8 to −15 K in S China and the SE USA. MAT anomalies from groundwater or speleothems seem more
uniform (−4 to −6 K), but the data are as yet sparse; a clear divergence between MAT and cold-month estimates from the same
region is seen only in the SE USA, where cold-air advection is expected to have enhanced cooling in winter. Regression of
all cold-month anomalies against site elevation yielded an estimated average cooling of −2.5 to −3 K at modern sea level,
increasing to ≈−6 K by 3000 m. However, Neotropical sites showed larger than the average sea-level cooling (−5 to −6 K) and
a non-significant elevation effect, whereas W and S Pacific sites showed much less sea-level cooling (−1 K) and a stronger
elevation effect. These findings support the inference that tropical sea-surface temperatures (SSTs) were lower than the CLIMAP
estimates, but they limit the plausible average tropical sea-surface cooling, and they support the existence of CLIMAP-like
geographic patterns in SST anomalies. Trends of PAM and lake levels indicate wet LGM conditions in the W USA, and at the highest
elevations, with generally dry conditions elsewhere. These results suggest a colder-than-present ocean surface producing a
weaker hydrological cycle, more arid continents, and arguably steeper-than-present terrestrial lapse rates. Such linkages
are supported by recent observations on freezing-level height and tropical SSTs; moreover, simulations of “greenhouse” and
LGM climates point to several possible feedback processes by which low-level temperature anomalies might be amplified aloft.
Received: 7 September 1998 / Accepted: 18 March 1999 相似文献
17.
On summing the components of radiative forcing of climate change 总被引:1,自引:0,他引:1
Radiative forcing is a useful concept in determining the potential influence of a particular mechanism of climate change.
However, due to the increased number of forcing agents identified over the past decade, the total radiative forcing is difficult
to assess. By assigning a range of probability distribution functions to the individual radiative forcings and using a Monte-Carlo
approach, we estimate the total radiative forcing since pre-industrial times including all quantitative radiative forcing
estimates to date. The resulting total radiative forcing has a 75–97% probability of being positive (or similarly a 3–25%
probability of being negative), with mean radiative forcing ranging from +0.68 to +1.34 W m−2, and median radiative forcing ranging from +0.94 to +1.39 W m−2.
Received: 14 March 2001 / Accepted: 1 June 2001 相似文献
18.
Quantifying the radiative forcing due to aerosol–cloud interactions especially through cirrus clouds remains challenging because of our limited understanding of aerosol and cloud processes. In this study, we investigate the anthropogenic aerosol indirect forcing (AIF) through cirrus clouds using the Community Atmosphere Model version 5 (CAM5) with a state-of-the-art treatment of ice nucleation. We adopt a new approach to isolate anthropogenic AIF through cirrus clouds in which ice nucleation parameterization is driven by prescribed pre-industrial (PI) and presentday (PD) aerosols, respectively. Sensitivities of anthropogenic ice AIF (i.e., anthropogenic AIF through cirrus clouds) to different ice nucleation parameterizations, homogeneous freezing occurrence, and uncertainties in the cloud microphysics scheme are investigated. Results of sensitivity experiments show that the change (PD minus PI) in global annual mean longwave cloud forcing (i.e., longwave anthropogenic ice AIF) ranges from 0.14 to 0.35 W m–2, the change in global annual mean shortwave cloud forcing (i.e., shortwave anthropogenic ice AIF) from–0.47 to–0.20 W m–2, and the change in net cloud forcing from–0.12 to 0.05 W m–2. Our results suggest that different ice nucleation parameterizations are an important factor for the large uncertainty of anthropogenic ice AIF. Furthermore, improved understanding of the spatial and temporal occurrence characteristics of homogeneous freezing events and the mean states of cirrus cloud properties are also important for constraining anthropogenic ice AIF. 相似文献
19.
B. C. Weare 《Climate Dynamics》2001,17(2-3):143-150
20.
A group of twenty-four leading atmospheric and climate scientists provided subjective probability distributions that represent
their current judgment about the value of planetary average direct and indirect radiative forcing from anthropogenic aerosols
at the top of the atmosphere. Separate estimates were obtained for the direct aerosol effect, the semi-direct aerosol effect,
cloud brightness (first aerosol indirect effect), and cloud lifetime/distribution (second aerosol indirect effect). Estimates
were also obtained for total planetary average forcing at the top of the atmosphere and for surface forcing. Consensus was
strongest among the experts in their assessments of the direct aerosol effect and the cloud brightness indirect effect. Forcing
from the semi-direct effect was thought to be small (absolute values of all but one of the experts' best estimates were ≤0.5
W/m2). There was not agreement about the sign of the best estimate of the semi-direct effect, and the uncertainty ranges some
experts gave for this effect did not overlap those given by others. All best estimates of total aerosol forcing were negative,
with values ranging between −0.25 W/m2 and −2.1 W/m2. The range of uncertainty that a number of experts associated with their estimates, especially those for total aerosol forcing
and for surface forcing, was often much larger than that suggested in 2001 by the IPCC Working Group 1 summary figure (IPCC,
2001). 相似文献