摘要:

A detailed climate model is used to investigate the factors contributing to differences between the climate 9000 years ago and that at present. The climate model comprises an atmospheric general circulation model coupled to a mixed layer ocean in which heat convergence due to oceanic advection is prescribed. In the first experiment, only the Earth's orbital parameters are changed. As in earlier work, it is found that monsoon circulations are enhanced during northern summer and weakened during the northern winter and that mid‐latitudes become drier throughout the year. The simulated changes are in general agreement with paleoclimatic evidence. Various feedback mechanisms modify the local response to the changes in insolation at the top of the atmosphere. In the northern hemisphere the warming in high latitudes is greatest in autumn and winter as a result of changes in sea ice, the mid‐latitude oceans are warmer throughout the year because of changes in cloud, and the surface temperature change over land in summer is reduced in low latitudes and enhanced in mid‐latitudes because of changes in cloud and soil moisture. Thus the climatic response to a simple change in boundary conditions may be complex, a fact which should be borne in mind when interpreting paleoclimatic data. In a second experiment a representation of the Laurentide ice sheet is also included. The warming in high latitudes is substantially reduced, especially just downstream of the ice sheets. This suggests that there may have been a substantial warming in high latitudes just after the Laurentide ice sheet coll

ISSN:0148-0227    

DOI:10.1029/JD093iD07p08283

年代:1988

数据来源: WILEY

摘要:

Based upon the need to understand differences between general circulation model projections of climatic change due to increasing atmospheric carbon dioxide, the present study first categorizes reasons for these differences and presents suggestions for the design of future climate model simulations, so that these specific categories may directly be addressed and understood. Following this, and based upon tutorial use of a radiative‐convective model, it is suggested that sea surface temperature perturbations may be used, in conjunction with separation of clear and overcast regions within a model, as a surrogate climatic change for the purpose of understanding and intercomparing atmospheric climate feedback processes. This approach is illustrated through use of the Oregon State University/Lawrence Livermore National Laboratory general circulation model, with particular attention being paid to interpreting cloud/climate interactions within the mode

ISSN:0148-0227    

DOI:10.1029/JD093iD07p08305

年代:1988

数据来源: WILEY

摘要:

A tropospheric general circulation model is coupled with a Lagrangian trace species transport and removal model to determine the climatic response to continental‐scale smoke injections arising from fires generated following a hypothetical nuclear war. The hydrological response is found to be particularly sensitive, with significant reductions in precipitation over land areas occurring for smoke injections that yield only modest cooling. This sensitivity is traced to the suppression of surface evaporation caused by a smoke‐induced reduction in the convective mixing of boundary layer and upper tropospheric air. A variety of sensitivity experiments is considered, including the sensitivity to smoke injection mass, location, day, season, duration and altitude of injection, and smoke composition and size distribution. For modest injections the land surface cooling in the first few weeks is found to be proportional to the smoke loading, while for larger injections the amount of smoke affects only the duration of the surface cooling. After several weeks the northern hemisphere land surface cooling for an April injection is found to be comparable to that for a July injection. A protracted smoke injection, which produces a more homogeneous horizontal distribution of smoke than an instantaneous injection, yields a greater surface cooling. The assumed initial aerosol size distribution of the injection is found to play a particularly important role in determining the climatic response, with injections composed primarily of submicron spherical particles producing a larger and longer surface cooling than an injection composed primarily of supermicron spherical aerosols. The importance of various feedback processes is also discussed. Smoke lofting associated with solar absorption by the aerosols enhances the aerosol residence time in the atmosphere by an order of magnitude, compared to aerosol lifetimes in the control atmosphere. The ground hydrology feedback in the tropics is found to amplify the sensitivity of precipitation over land areas and to enhance surface warming after several weeks. The treatment of a stability dependent rather than neutral boundary layer enhances the surface cooling in the first few weeks by about

ISSN:0148-0227    

DOI:10.1029/JD093iD07p08315

年代:1988

数据来源: WILEY

摘要:

A Lagrangian model has been developed to treat the global‐scale transport, transformation, and removal of trace species in the atmosphere. The model uses prescribed velocity fields (either from observations or from Eulerian dynamical models) to advect constant‐mass parcels of air. These constant‐mass air parcels can contain varying amounts of aerosols, such as dust and smoke, or varying amounts of trace gases, such as ozone, methane, or nitrogen dioxide. Because advection operates on the air parcels, rather than on the trace species, the computational efficiency of the approach improves, in contrast to Eulerian approaches, as the number of trace species increases. Numerical diffusion is also much less than in typical Eulerian models, permitting a wide range of spatial distributions of species to be advected with minimal dispersion. The Lagrangian method is particularly well suited to species that are chemically active because deviations from chemical equilibrium associated with advection are greatly reduced. Additional processes presently represented in the model include precipitation scavenging, dry deposition, aerosol coagulation, gravitational settling, local mixing with nearby parcels, vertical mixing by moist convection, and horizontal and vertical mixing associated with eddy mixing on scales finer than the prescribed velocity field grid. The model can operate in either a passive or an interactive mode. In the passive mode the meteorological flow is assumed to be unaffected by the trace species. In this mode a wide variety of sensitivity experiments using the same meteorology can readily be conducted. In this paper we consider several such experiments using submicron and supermicron aerosol particles, including an assessment of the relative importance of convective versus dynamical mixing, an assessment of the relative importance of various removal mechanisms, and a sample application to investigate the deposition to the surface of locally injected radionuclides. Use of this model in the interactive mode is considered in a companion paper (Ghan et al., this i

ISSN:0148-0227    

DOI:10.1029/JD093iD07p08339

年代:1988

数据来源: WILEY

摘要:

Surface ozone measurements obtained at Brazzaville, Congo (4°20′S, 15°20′E), from 1983 to 1986 are summarized. In addition, results from aircraft surveys (1985 and 1986) from Brazzaville to the Atlantic coast and results from a shipboard expedition (1986) along the Atlantic coast at the Equator are presented. On the basis of the data, there appears to exist a seasonal cycle with the highest values in early spring (southern) which compare with the mid‐latitude values observed in the northern hemisphere. The data suggest that during the dry season, emission of precursor gases from biomass burning leads to high ozone values in the boundary layer. Ozone mixing ratios approached levels as high as 100 parts per billion by volume in a layer residing just above the monsoon. In the wet season, the data suggest that biogenic, organic emissions may act as precursors to photochemical ozone production and may contribute to the fairly high ozone burden of the lower troposphere over Equatorial

ISSN:0148-0227    

DOI:10.1029/JD093iD07p08355

年代:1988

数据来源: WILEY

摘要:

During the STRATOZ III flights in June 1984, about 120 whole‐air samples were collected and analyzed later on in the laboratory for several atmospheric trace components, including light hydro‐carbons. The STRATOZ III mission consisted of 21 flight segments and covered a latitude range from 70°N to 60°S and altitudes up to 12 km. The results of these measurements were used to construct latitude‐altitude profiles in the form of isolines for ethane, ethene, acetylene, propane, propene, n‐butane, isobutane,n‐pentane, and isopentane. These results are compared with the latitudinal cross sections obtained during a previous, very similar flight mission (STRATOZ II) in May–June 1980. Also, the few published latitudinal or vertical profiles for these nonmethane hydrocarbons (NMHCs) are used for a comparison. The two‐dimensional distributions for the longer‐lived NMHCs, especially ethane and to some extent also propane and acetylene, are reasonably representative, even on a global scale. The high variability of the short‐lived NMHCs, the C4and C5alkanes, and the light alkenes, prevents the determination of representative two‐dimensional distributions for these species. The distributions of these short‐lived compounds give at best an extremely rough idea on the distributions and should in general be considered as descriptions of a given momentary situation. These latitude‐altitude profiles indicate the existence of fast mechanisms for vertical mixing in the troposphere. The observation of high mixing ratios of short‐lived hydrocarbons in the middle and upper troposphere proves the existence of vertical mixing processes with a time scale comparable to, or even shorter than, the atmospheric lifetime of these reactive NMHCs. As a consequence, there exist several regions, even above the boundary layer, with NMHC mixing ratios high enough to make them important participants in the atmospheric ph

ISSN:0148-0227    

DOI:10.1029/JD093iD07p08367

年代:1988

数据来源: WILEY

摘要:

We have measured insoluble particle size distributions covering the radius range 0.05–1.31 μm. for six sections of ice core from Dome C, Antarctica. Two of the sections are from the Holocene, two are from the last glacial maximum (LGM), and another two are from the period that preceded it. Our measurements lead us to the conclusion that the southern hemisphere insoluble background aerosol size distribution, in the range of our measurements, has not changed significantly over the 26,000‐year period that we studied. We also compared the concentration of diatoms in a sample of Holocene ice with that in two samples of LGM ice and found that the concentration of diatoms whose largest dimension was equal to or greater than 10 μm. was 20 times larger during the LGM, the same as the ratio we measured for the concentration of insoluble particles. We interpret this to mean that the higher dust levels were mainly due to an increase in wind strength rather than to increased continental ar

ISSN:0148-0227    

DOI:10.1029/JD093iD07p08378

年代:1988

数据来源: WILEY

摘要:

The shapes and sizes of photochemically produced aerosol particles of polyacetylene, polyethylene, and polyhydrogen cyanide were studied experimentally. All of the single particles were found to be perfectly spherical and semiliquid. However, they aggregate readily, with a sticking coefficient near unity, to form nonspherical particles, which could give rise to the observed polarization from Titan's and Jupiter's upper haze layers. The absorbance of polyacetylene was remeasured and corrected, and it is now much closer to that of polyethylene. The measured real and imaginary indices of refraction of the two materials make them both suitable material for Titan's and Jupiter's upper haze layers. However, the larger abundance and higher rate of polymerization of acetylene would make it the dominant aerosol‐forming material in both atmosphere

ISSN:0148-0227    

DOI:10.1029/JD093iD07p08383

年代:1988

数据来源: WILEY

摘要:

The Ames airborne, autotracking sunphotometer has been operated aboard a Sandia Laboratories research aircraft to measure magnitudes, temporal/spatial variabilities, and wavelength dependence of optical depths in the near‐ultraviolet to near‐infrared spectrum of smoke from two forest fires and one jet fuel fire and of background air. The results were corrected for Rayleigh scattering and for estimated absorption by ozone and nitrogen dioxide. Characteristic differences in the aerosol optical depths of background atmospheres and of different types of smokes are the following: (1) the magnitude and wavelength dependence of “background” optical depths vary with the geographic location at which the measurements are performed; (2) the wavelength dependence of smoke optical depths depends on the fuels that feed the fires and on the residence time of the smoke cloud in the atmosphere. In general, the jet fuel smoke optical depths tended to be less wavelength dependent (near‐ultraviolet to near‐infrared) than background aerosol optical depths. Forest fire smoke optical depths showed a wide range of wavelength dependences, including incidents of wavelength‐independe

ISSN:0148-0227    

DOI:10.1029/JD093iD07p08388

年代:1988

数据来源: WILEY

摘要:

A radiative transfer model is developed to illustrate the processes which determine the spectral albedo and transmission of lake ice. The calculated spectral albedo is dominated by specular reflection from the ice surface in the near infrared, whereas multiple scattering by bubbles below the surface dominates the visible albedo. Adding a snow cover to lake ice will normally increase the visible albedo, but may reduce the albedo in some regions of the near infrared if the sun is low, by reducing the specular reflection. In a preliminary test of the model, spectral albedo was measured on the natural ice cover of a frozen lake. The measurements are explained using the radiative transfer model applied to the air bubble size distribution measured in the same ice. The uncertainty in measurement of bubble size distribution leads to uncertainty in the theoretical albedo which is much larger than the error due to approximations used in derivation of the model.

ISSN:0148-0227    

DOI:10.1029/JD093iD07p08403

年代:1988

数据来源: WILEY