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61.
Greenhouse warming due to carbon dioxide atmospheres may be responsible for maintaining the early Earth's surface temperature above freezing and may even have allowed for liquid water on early Mars. However, the high levels of CO2 required for such warming should have also resulted in the formation of CO2 clouds. These clouds, depending on their particle size, could lead to either warming or cooling. The particle size in turn is determined by the nucleation and growth conditions. Here we present laboratory studies of the nucleation and growth of carbon dioxide on water ice under martian atmospheric conditions. We find that a critical saturation, S=1.34, is required for nucleation, corresponding to a contact parameter between solid water and solid carbon dioxide of m=0.95. We also find that after nucleation occurs, growth of CO2 is very rapid, and we report the growth rates at a number of supersaturations. Because growth would be expected to continue until the CO2 pressure is lowered to its vapor pressure, we expect particles larger than those being currently suggested for the present and past martian atmospheres. Using this information in a microphysical model described in a companion paper, we find that CO2 clouds are best described as “snow,” having a relatively small number of large particles. 相似文献
62.
Sunspots are strong absorbers of f and p modes. A possible absorption mechanism is direct conversion to slow magnetoacoustic waves. Calculations based on vertical magnetic field models show that this works well for f modes, but is inadequate for p modes. Using a very simple shallow spot model, in which the effects of the magnetic field are accounted for solely by a surface condition, we investigate the possibility that p modes first scatter into f modes inside the spot, which are then more susceptible to conversion to slow modes. We find that the coupling between an incident p mode and the internal f mode is unlikely to be strong enough to account for the observed absorption, but that the incident modes do couple strongly to the acoustic jacket in some cases, leading to a region immediately around the sunspot where a significant fraction of the surface velocity is due to the jacket modes. 相似文献
63.
64.
Vertical turbulent transports of latent and sensible heat through the tropical marine boundary layer were measured with ship and airborne instruments in the GATE experimental area. The measurements from the two systems are compared for cases of simultaneous sampling in the same locations during undisturbed weather and during the wakes of convective disturbances. The paired average fluxes from the airborne eddy correlation measurements and the shipboard bulk aerodynamic measurements, for undisturbed weather, are related with correlation coefficients in the range of 0.6 to 0.9; the correlations depend primarily on stratifications of the aircraft data according to sampling altitude (15–153 m msl) and flight pattern relative to the mean wind. The agreement between the two types of measurements is best for ship data paired with aircraft samples from the lowest altitudes (15 and 30 m) and from alongwind rather than crosswind flights, as is appropriate since the stationary ships monitor the alongwind characteristics of the turbulence. The use of long (1–3 hr) versus short (10 min) ship samples did not significantly affect the comparability with the aircraft samples during undisturbed periods.The good baseline comparability demonstrated by the undisturbed-weather comparisons was applied to interpret the results from the wakes behind showers. Although these paired measurements were few, not only the sensible heat, but also the latent heat flux was shown to decrease substantially with altitude from 10 m to at least 150 m in the wakes, where the mixing was relatively intense. Variations in the fluxes were much greater at the higher level than nearer the sea surface. 相似文献
65.
H.L. Barnes 《Geochimica et cosmochimica acta》1981,45(7):1255-1256
66.
67.
Margaret Leinen 《Geochimica et cosmochimica acta》1977,41(5):671-676
The opal content of deep-sea sediment can be estimated by subtracting non-biogenic silica, calculated from the aluminum and magnesium concentrations in the sediment, from the total silica content of the samples. Unlike most previously described methods, this calculation is capable of yielding reliable estimates of opal in pre-Pleistocene sediments because it is unaffected by structural changes that take place in opal as it ages. 相似文献
68.
Nick Porch Gregory J. Jordan David M. Price Richard W. Barnes Mike K. Macphail Mike Pemberton 《Quaternary Science Reviews》2009,28(27-28):3197-3210
This paper explores the palaeoclimatic significance of a fossil plant and insect record from Yarra Creek, on King Island, between Tasmania and the Australian mainland. The record dates, based upon a thermoluminescence chronology and other evidence, to Marine Isotope Stage 5 (MIS 5); the exact timing is impossible to ascertain given the resolution of the thermoluminescence results and the presence of an unconformity in the dated section. The presence of a cool-temperate rainforest flora, outside its modern range, and other independent evidence, suggest the sequence may represent the last interglacial (MIS 5e) rather than a later MIS 5 substage. Using coexistence methods that compare modern climatic ranges of the taxa in the assemblage we reconstruct independent beetle and plant based annual and seasonal temperate and precipitation parameters. The results imply the assemblage was deposited under a wetter summer climate and suggest conditions of enhanced temperature seasonality. It is probable that enhanced temperature seasonality is a methodological artefact reflecting the rarity of extremely equable climates (like King Island) in modern climate space. This would indicate a limitation of most methods of palaeoclimatic reconstruction that rely on modern datasets – it is only possible to reconstruct past climates as being within the range of values in that currently exist in modern climate space. 相似文献
69.
Carolyn Barnes Simon Jennings Jon. T. Barry 《Estuarine, Coastal and Shelf Science》2009,81(3):368-374
Carbon stable isotopes can be used to trace the sources of energy supporting food chains and to estimate the contribution of different sources to a consumer's diet. However, the δ13C signature of a consumer is not sufficient to infer source without an appropriate isotopic baseline, because there is no way to determine if differences in consumer δ13C reflect source changes or baseline variation. Describing isotopic baselines is a considerable challenge when applying stable isotope techniques at large spatial scales and/or to interconnected food chains in open marine environments. One approach is to use filter-feeding consumers to integrate the high frequency and small-scale variation in the isotopic signature of phytoplankton and provide a surrogate baseline, but it can be difficult to sample a single consumer species at large spatial scales owing to rarity and/or discontinuous distribution. Here, we use the isotopic signature of a widely distributed filter-feeder (the queen scallop Aequipecten opercularis) in the north-eastern Atlantic to develop a model linking base δ13C to environmental variables. Remarkably, a single variable model based on bottom temperature has good predictive power and predicts scallop δ13C with mean error of only 0.6‰ (3%). When the model was used to predict an isotopic baseline in parts of the overall study region where scallop were not consistently sampled, the model accounted for 76% and 79% of the large-scale spatial variability (101–104 km) of the δ13C of two fish species (dab Limanda limanda and whiting Merlangus merlangius) and 44% of the δ13C variability in a mixed fish community. The results show that source studies would be significantly biased if a single baseline were applied to food webs at larger scales. Further, when baseline δ13C cannot be directly measured, a calculated baseline value can eliminate a large proportion of the unexplained variation in δ13C at higher trophic levels. 相似文献
70.
Magnetohydrostatic models of the solar atmosphere are often based on idealized analytic solutions because the underlying equations are too difficult to solve in full generality. Numerical approaches, too, are often limited in scope and have tended to focus on the two-dimensional problem. In this article we develop a numerical method for solving the nonlinear magnetohydrostatic equations in three dimensions. Our method is a fixed-point iteration scheme that extends the method of Grad and Rubin (Proc. 2nd Int. Conf. on Peaceful Uses of Atomic Energy 31, 190, 1958) to include a finite gravity force. We apply the method to a test case to demonstrate the method in general and our implementation in code in particular. 相似文献