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61.
Diana Prado Garzón Christiano G.M. Brum Ezequiel Echer Néstor Aponte Michael P. Sulzer Sixto A. González Robert B. Kerr Lara Waldrop 《Journal of Atmospheric and Solar》2011,73(11-12):1568-1574
We analyze the data obtained using the Arecibo incoherent scatter radar to examine the response of the topside ionosphere to a moderate geomagnetic storm that occurred during the period March 7–11, 2008. During this time period a magnetic storm with a non-monotonic main phase decrease in the Dst index occurred. The recovery phase also exhibited a secondary Dst decrease. During the initial phase of the storm, Te and Ti increased coincident with the arrival of the solar wind. The main phase registered an increase in proton concentration proportional to Ne while temperatures reached the lowest values. Variations in O+ concentration were not significant but a reduction in helium fraction was observed. Soon after the peak of the storm, the transition height between the topside ionosphere and the protonosphere, where H+ ions dominate composition, was lower than would be expected during quiet conditions and this behavior lasted for approximately 12 h. 相似文献
62.
Soil particle-size distributions (PSD) have been used to estimate soil hydraulic properties. Various parametric PSD models
have been proposed to describe the soil PSD from sparse experimental data. It is important to determine which PSD model best
represents specific soils. Fourteen PSD models were examined in order to determine the best model for representing the deposited
soils adjacent to dams in the China Loess Plateau; these were: Skaggs (S-1, S-2, and S-3), fractal (FR), Jaky (J), Lima and
Silva (LS), Morgan (M), Gompertz (G), logarithm (L), exponential (E), log-exponential (LE), Weibull (W), van Genuchten type
(VG) as well as Fredlund (F) models. Four-hundred and eighty samples were obtained from soils deposited in the Liudaogou catchment.
The coefficient of determination (R
2), the Akaike’s information criterion (AIC), and the modified AIC (mAIC) were used. Based upon R
2 and AIC, the three- and four-parameter models were both good at describing the PSDs of deposited soils, and the LE, FR, and
E models were the poorest. However, the mAIC in conjunction with R
2 and AIC results indicated that the W model was optimum for describing PSD of the deposited soils for emphasizing the effect
of parameter number. This analysis was also helpful for finding out which model is the best one. Our results are applicable
to the China Loess Plateau. 相似文献
63.
Systematic differences between VTEC obtained by different space-geodetic techniques during CONT08 总被引:1,自引:1,他引:0
The ionosphere is a dispersive medium for microwaves, and most space-geodetic techniques using two or more signal frequencies
can be applied to extract information on ionospheric parameters, including terrestrial as well as satellite-based GNSS, DORIS,
altimetry, and VLBI. Because of their different sensitivity regarding ionization, their different spatial and temporal data
distribution, and their different signal paths, a joint analysis of all observation types seems reasonable and promises the
best results for ionosphere modeling. However, it has turned out that there exist offsets between ionospheric observations
of the diverse techniques mainly caused by calibration uncertainties or model errors. Direct comparisons of the information
from different data types are difficult because of the inhomogeneous measurement epochs and locations. In the approach presented
here, all measurements are combined into one ionosphere model of vertical total electron content (VTEC). A variance component
estimation is applied to take into account the different accuracy levels of the observations. In order to consider systematic
offsets, a constant bias term is allowed for each observation group. The investigations have been performed for the time interval
of the CONT08 campaign (2 weeks in August 2008) in a region around the Hawaiian Islands. Almost all analyzed observation techniques
show good data sensitivity and are suitable for VTEC modeling in case the systematic offsets which can reach up to 5 TECU
are taken into account. Only the Envisat DORIS data cannot provide reliable results. 相似文献
64.
Structures of adjoint sensitivities to potential vorticity for specific initial and final norm are investigated for a short-range cyclone forecast in a three-dimensional quasigeostrophic (QG) model. Moreover, adjoint sensitivities to potential vorticity are compared with nonlinear sensitivities calculated for the same cyclogenesis case in the QG model. The adjoint sensitivities using different initial and final norms (e.g., total QG disturbance energy and potential enstrophy) show approximately similar characteristics for the horizontal and vertical structures and evolutions. Consistent with previous studies, the horizontal structure of the adjoint sensitivity is smaller for the energy norm than for the potential enstrophy norm. The dynamical mechanism of cyclone development by adjoint sensitivity coincides with that of nonlinear sensitivity, with slight differences in the region of sensitivity maxima over the upstream (nascent) low for the adjoint (nonlinear) sensitivity. The adjoint sensitivities show different vertical distributions from the nonlinear sensitivities. Consistent with the sensitive regions denoted by singular vectors and error evolution in the QG model, maxima of the adjoint sensitivities are located at both the upper and lower boundaries, with prominent secondary peaks in the lower to mid-troposphere of the domain. The level of the secondary maxima changes depending on the initial and final norm used. The secondary peak is located in the lower to mid- (mid-) troposphere for the total QG disturbance energy (potential enstrophy) as the initial and final norm. Based on the correspondence in the level of the sensitivity maxima in the interior of the domain between the adjoint and nonlinear sensitivities, adjoint sensitivities may serve as an alternative to nonlinear sensitivities given the enormous computing expenses in nonlinear sensitivity calculation. 相似文献
65.
In middle and high latitudes, climate change could impact the frequency and characteristics of frontal passages. Although transitions between air masses are significant features of the general circulation that influence human activities and other surface processes, they are much more difficult to objectively identify than single variables like temperature or even extreme events like fires, droughts, and floods. The recently developed Spatial Synoptic Classification (SSC) provides a fairly objective means of identifying frontal passages. In this research, we determine the specific meteorological patterns represented by the SSC??s Transition category, a ??catch-all?? group that attempts to identify those days that cannot be characterized as a single, homogeneous air mass type. The result is a detailed transition climatology for the continental USA. We identify four subtypes of the Transition category based on intra-day sea level pressure change and dew point temperature change. Across the contiguous USA, most transition days are identified as cold fronts and warm fronts during the winter season. Among the two less common subtypes, transition days in which the dew point temperature and pressure both rise are more frequently observed across the western states, and days in which both variables fall are more frequently observed in coastal regions. The relative frequencies of wintertime warm and cold fronts have changed over the period 1951?C2007. Relative cold front frequency has significantly increased in the Northeast and Midwest regions, and warm front frequencies have declined in the Midwest, Rocky Mountain, and Pacific Northwest regions. The overall shift toward cold fronts and away from warm fronts across the northern USA arises from a combination of an enhanced ridge over western North America and a northward shift of storm tracks throughout the mid-latitudes. These results are consistent with projections of climate change associated with elevated greenhouse gas concentrations. 相似文献
66.
Daily precipitation totals at 55 sites were used to investigate geographic variability in winter (DJF) rainfall over Cumbria, NW England, over an 11-year period. Winter is the wettest season (>800?mm in the mountainous Lake District), with rainfall mechanisms closely linked to North Atlantic forcing. The Lamb weather type catalogue was used to identify rainfall distributions under different wind directions. Precipitation magnitude over Cumbria is much more sensitive to a change in wind direction than the geographic pattern in rainfall, with southwesterly (easterly) winds producing the highest (lowest) spatially averaged daily rainfall totals of 8.2?mm (0.6?mm). S-mode principal components analysis was used to identify the main patterns of precipitation variability. Three principal components (PCs) were retained as being statistically significant (cumulative explained variance for unrotated PCs?=?84.3%), with a correlated PC structure (direct oblimin rotation) best describing the spatial variance in rainfall. PC 1 has a very high index of strength (variance measure?=?40.9), indicating that there is one dominant rainfall pattern. PC 1 shows a gradient between wetter conditions in southwest Cumbria and over the central Lake District and drier conditions in NE Cumbria, and is usually caused by active zonal west to southwest flows. Almost of equal importance to PC 1 is PC 3 (variance measure?=?39.3), which has a more uniform rainfall distribution than PC 1 and is usually caused by fronts stalling over the region. PC 2, which shows an east to west decline in rainfall totals, is much less important than PCs 1 and 3 (variance measure?=?18.6). PC 2??s rainfall pattern can be caused by easterly flows with high pressure over Scandinavia and low pressure over the Continent, or by strong southwesterly flows, with depressions often centred over Scotland. Finally, cluster analysis was carried out to identify precipitation regions for all days and for each wind direction. Clusters were found to be largely stable to changes in wind direction, with stations in the central Lake District often clustered together, thus highlighting the importance of orographic enhancement of rainfall in this region. 相似文献
67.
Isabella M. Angelini Michael Garstang Robert E. Davis Bruce Hayden David R. Fitzjarrald David R. Legates Steven Greco Stephen Macko Vickie Connors 《Theoretical and Applied Climatology》2011,105(1-2):243-261
Recent studies suggest that vegetation can drive large-scale atmospheric circulations and substantially influence the hydrologic cycle. We present observational evidence to quantify the extent of coupling between vegetation and the overlying atmosphere. Within the context of vegetation–atmospheric interactions, we reanalyze existing climatological data from springtime leaf emergence, emissivity, dew point temperatures, and historical records of precipitation and forest coverage. We construct new rainfall transects based on a robust global climatology. Using isotopic analysis of precipitation, we find that rain in Amazonia comes primarily from large-scale weather systems coupling interior regions to the ocean and is not directly driven by local evaporation. We find that changes in vegetative cover and state influence the temperature and moisture content of the surface and atmospheric boundary layer but are not reflected in observable precipitation changes. This analysis reaffirms the view that changes in precipitation over continental reaches are a product of complex processes only partly influenced but not controlled by local water sources or vegetation. 相似文献
68.
The atmospheric circulation patterns associated with the Pacific quasi-decadal oscillation (QDO) are investigated using available observational data from 1948 to 2007. Previous studies indicate that the Pacific QDO is characterized by a distinct lifecycle in the form of sea surface temperature (SST) patterns. In the warm and cool phases of the Pacific QDO, the SST patterns resemble those associated with the El Ni?o-Southern Oscillation (ENSO). During the warm?Ccool and cool?Cwarm transitions of the Pacific QDO, recurrent SST patterns are also clearly visible. The rotated empirical orthogonal function analysis on the 10?C15?year filtered data shows that the evolutions of SST and atmospheric circulation are well coupled. While the warm-/cool-phase Pacific QDO generates an ENSO-like circulation pattern, the transition phases form a distinct short-wave train emanating from Southeast Asia towards North America. This short-wave train is particularly robust in the streamfunction of water vapor flux. Diagnostic analyses of the heat budget, the stationary wave flux, and a barotropic model indicate that this short-wave train is thermodynamically maintained and is likely forced by diabatic heating near Southeast Asia. Additional modulations of forcing sources in the western and eastern tropical Pacific on this short-wave teleconnection are also discussed. 相似文献
69.
Ben P. Kirtman Edwin K. Schneider David M. Straus Dughong Min Robert Burgman 《Climate Dynamics》2011,37(11-12):2389-2416
The new interactive ensemble modeling strategy is used to diagnose how noise due to internal atmospheric dynamics impacts the forced climate response during the twentieth century (i.e., 1870?C1999). The interactive ensemble uses multiple realizations of the atmospheric component model coupled to a single realization of the land, ocean and ice component models in order to reduce the noise due to internal atmospheric dynamics in the flux exchange at the interface of the component models. A control ensemble of so-called climate of the twentieth century simulations of the Community Climate Simulation Model version 3 (CCSM3) are compared with a similar simulation with the interactive ensemble version of CCSM3. Despite substantial differences in the overall mean climate, the global mean trends in surface temperature, 500?mb geopotential and precipitation are largely indistinguishable between the control ensemble and the interactive ensemble. Large differences in the forced response; however, are detected particularly in the surface temperature of the North Atlantic. Associated with the forced North Atlantic surface temperature differences are local differences in the forced precipitation and a substantial remote rainfall response in the deep tropical Pacific. We also introduce a simple variance analysis to separately compare the variance due to noise and the forced response. We find that the noise variance is decreased when external forcing is included. In terms of the forced variance, we find that the interactive ensemble increases this variance relative to the control. 相似文献
70.