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排序方式: 共有371条查询结果,搜索用时 203 毫秒
61.
62.
Updating Markov chain models using the ensemble Kalman filter 总被引:1,自引:0,他引:1
63.
64.
P.W. Webley B.J.B. Stunder K.G. Dean 《Journal of Volcanology and Geothermal Research》2009,186(1-2):108
Ash clouds are one of the major hazards that result from volcanic eruptions. Once an eruption is reported, volcanic ash transport and dispersion (VATD) models are used to forecast the location of the ash cloud. These models require source parameters to describe the ash column for initialization. These parameters include: eruption cloud height and vertical distribution, particle size distribution, and start and end time of the eruption. Further, if downwind concentrations are needed, the eruption mass rate and/or volume of ash need to be known. Upon notification of an eruption, few constraints are typically available on many of these source parameters. Recently, scientists have defined classes of eruption types, each with a set of pre-defined eruption source parameters (ESP). We analyze the August 18, 1992 eruption of the Crater Peak vent at Mount Spurr, Alaska, which is the example case for the Medium Silicic eruption type. We have evaluated the sensitivity of two of the ESP – the grain size distribution (GSD) and the vertical distribution of ash – on the modeled ash cloud. HYSPLIT and Puff VATD models are used to simulate the ash clouds from the different sets of source parameters. We use satellite data, processed through the reverse absorption method, as reference for computing statistics that describe the modeled-to-observed comparison. With the grain size distribution, the three options chosen, (1) an estimated distribution based on past eruption studies, (2) a distribution with finer particles and (3) the National Oceanic and Atmospheric Administration HYSPLIT GSD, have little effect on the modeled ash cloud. For the initial vertical distribution, both linear (uniform concentration throughout the vertical column) and umbrella shapes were chosen. For HYSPLIT, the defined umbrella distribution (no ash below the umbrella), apparently underestimates the lower altitude portions of the ash cloud and as a result has a worse agreement with the satellite detected ash cloud compared to that with the linear vertical distribution for this particular eruption. The Puff model, with a Poisson function to represent the umbrella cloud, gave similar results as for a linear distribution, both having reasonable agreement with the satellite detected cloud. Further sensitivity studies of this eruption, as well as studies using the other source parameters, are needed. 相似文献
65.
O. Klemm W. R. Stockwell H. Schlager M. Krautstrunk 《Journal of Atmospheric Chemistry》2000,35(1):1-18
The ozone forming potential of VOCs and NOx for plumes observed from several cities and a power plant in eastern Germany was investigated. A closed box model with a gas phase photochemical reaction mechanism was employed to simulate several scenarios based upon aircraft observations. In several of the scenarios, the initial concentrations of NOx, VOCs, and SO2, were reduced to study the factors limiting the O3 production. Ozone production was limited by the initial VOC concentrations for all of the simulated plumes. Higher O3 concentrations were produced with reduced initial NOx. In one sample with high SO2 mixing ratios (>100 ppb), SO2 was also identified as a significant contributor to the production of O3. 相似文献
66.
67.
William R. Stockwell 《Journal of Atmospheric Chemistry》1994,19(3):317-329
The rates and mechanisms of both gas and liquid phase reactions for the oxidation of sulfur dioxide play an important role in the production of atmospheric acids and aerosol particles. Rhodeet al. (1981) concluded that sulfate production rates were highly non-linear functions of sulfur dioxide emission rates. Their modelling study used an HO
x
termination mechanism for the HO—SO2 reaction in the gas-phase. Stockwell and Calvert (1983) determined that one of the products of the overall reaction of HO with sulfur dioxide was an HO2 radical. The National Research Council (1983) using a version of the Rhodeet al. (1981) model modified to include HO2 production from the HO—SO2 reaction concluded that sulfate production becomes much more linear with respect to reductions in sulfur dioxide emissions. However, the cause of this increased linearity was not explained by the National Research Council report. It is demonstrated that the increased linearity is due to the coupling of gas-phases and aqueous phase chemistry. The gas-phase sulfur dioxide oxidation mechanism has a very significant effect on hydrogen perodide production rates. 相似文献
68.
The carbon cycle and biogeochemical dynamics in lake sediments 总被引:27,自引:3,他引:27
Walter E. Dean 《Journal of Paleolimnology》1999,21(4):375-393
The concentrations of organic carbon (OC) and CaCO3 in lake sediments are often inversely related. This relation occurs in surface sediments from different locations in the same lake, surface sediments from different lakes, and with depth in Holocene sediments. Where data on accumulation rates are available, the relation holds for organic carbon and CaCO3 accumulation rates as well. An increase of several percent OC is accompanied by a decrease of several tens of percent CaCO3 indicating that the inverse relation is not due to simple dilution of one component by another. It appears from core data that once the OC concentration in the sediments becomes greater than about 12%, the CO2 produced by decomposition of that OC and production of organic acids lowers the pH of anoxic pore waters enough to dissolve any CaCO3 that reaches the sediment-water interface. In a lake with a seasonally anoxic hypolimnion, processes in the water column also can produce an inverse relation between OC and CaCO3 over time. If productivity of the lake increases, the rain rate of OC from the epilimnion increases. Biogenic removal of CO2 and accompanying increase in pH also may increase the production of CaCO3. However, the decomposition of organic matter in the hypolimnion will decrease the pH of the hypolimnion causing greater dissolution of CaCO3 and therefore a decrease in the rain rate of CaCO3 to the sediment-water interface. 相似文献
69.
Effects of mesoscale sea-surface temperature fronts on the marine atmospheric boundary layer 总被引:1,自引:0,他引:1
Eric D. Skyllingstad Dean Vickers Larry Mahrt Roger Samelson 《Boundary-Layer Meteorology》2007,123(2):219-237
A numerical modelling study is presented focusing on the effects of mesoscale sea-surface temperature (SST) variability on
surface fluxes and the marine atmospheric boundary-layer structure. A basic scenario is examined having two regions of SST
anomaly with alternating warm/cold or cold/warm water regions. Conditions upstream from the anomaly region have SST values
equal to the ambient atmosphere temperature, creating an upstream neutrally stratified boundary layer. Downstream from the
anomaly region the SST is also set to the ambient atmosphere value. When the warm anomaly is upstream from the cold anomaly,
the downstream boundary layer exhibits a more complex structure because of convective forcing and mixed layer deepening upstream
from the cold anomaly. An internal boundary layer forms over the cold anomaly in this case, generating two distinct layers
over the downstream region. When the cold anomaly is upstream from the warm anomaly, mixing over the warm anomaly quickly
destroys the shallow cold layer, yielding a more uniform downstream boundary-layer vertical structure compared with the warm-to-
cold case. Analysis of the momentum budget indicates that turbulent momentum flux divergence dominates the velocity field
tendency, with pressure forcing accounting for only about 20% of the changes in momentum. Parameterization of surface fluxes
and boundary-layer structure at these scales would be very difficult because of their dependence on subgrid-scale SST spatial
order. Simulations of similar flow over smaller scale fronts (<5 km) suggest that small-scale SST variability might be parameterized
in mesoscale models by relating the effective heat flux to the strength of the SST variance. 相似文献
70.
The portable surface capacitance insertion probe was used to measure the in situ water content of near surface soil layers. The probe readings were calibrated against gravimetric samples collected over a wide range of water contents, and were found to be very closely correlated. The capacitance probe was used repeatedly in time and space at a field site where ridges and furrows ensured a regular pattern of soil water variations. The observed spatial variations of the water content were related to topography and land mangement. The degree of variability was dependent on the average water content. 相似文献