摘要:

Satellite measurements of the magnetic anomaly field intensity have been used to estimate components of the vector field at the Earth's surface, using a linear approximation to relate satellite scalar measurements to downward‐continued vector components. The relation between data and model was inverted using a depleted harmonic spline basis to find an almost minimum norm solution, enabling the detail present in satellite data to be preserved over continental‐sized areas. In their development of the depleted basis method,Parker and Shure(1982) advocated using a subset of the original data as points in the depleted basis. Here, it is shown that a depleted basis formed from vertical component data kernels is much more satisfactory than a subset of measured intensity data kernels; however, we do form the depleted basis at the physical locations of actual data. The downward‐continued map over Africa agrees well with its counterpart produced from Magsat orthogonal component

ISSN:0094-8276    

DOI:10.1029/95GL03092

年代:1996

数据来源: WILEY

摘要:

Accurate modeling of electromagnetic wave propagation in conducting media is important for the further development of ground‐penetrating radar technologies. Numerical stability and dispersion criteria are derived here for two common 1‐D finite‐difference solutions of Maxwell's equations. In one finite‐difference scheme one‐sided differences are used to approximate the conducting term and in the other centered differences are employed. Stability is governed by the well‐known Courant criterion. In addition there is a stability condition controlling the diffusive aspects of wave propagation for the one‐sided difference scheme. It is found that the centered difference approximation has significantly better stability and dispersion characteristics. For the centered scheme, the well‐known spatial sampling criteria for the non‐conducting case are found to be valid for conducting media. The results are tested and illustrated using 1‐D s

ISSN:0094-8276    

DOI:10.1029/95GL03515

年代:1996

数据来源: WILEY

摘要:

Eight circular geologic structures ranging from ∼3 to 17 km in diameter, showing evidence of outward‐directed radial deformation and intensive brecciation, lie within a linear swath ∼15 km wide along a straight line stretching ∼700 km across the United States from southern Illinois through Missouri to eastern Kansas. Based on their similar geological characteristics and the presence of diagnostic and/or probable evidence of shock, these structures, once classified as ‘cryptovolcanic’ or ‘cryptoexplosion’ structures, are more confidently ascribed to hypervelocity impact. No other similar occurrence of aligned features is known, and we calculate the probability of a chance alignment to be<10−9. The unusual alignment suggests that the features are coeval and related to a multiple impact event, with a best‐constrained late Mississippian‐early Pennsylvanian (∼330–310 Myr) age. Calculations suggest that the proposed impact‐crater chain is unlikely to have been formed by an incoming impactor disrupted by terrestrial or lunar tidal effects, and may have been the result of a string of asteroidal or cometary objects produced by breakup wi

ISSN:0094-8276    

DOI:10.1029/95GL03605

年代:1996

数据来源: WILEY

摘要:

Previous estimates of the partial atmospheric lifetime of CH3Br with respect to degradation in the ocean have not fully accounted for co‐variation of sea‐surface and boundary layer properties. Here we substantially reduce uncertainty in this calculation by using a coupled, ocean‐atmosphere box model and a tightly gridded data set of oceanic and atmospheric properties. The best estimate of the partial atmospheric lifetime of CH3Br with respect to the ocean is 2.7 y with a possible range, due mainly to the choice of computational procedures for critical terms, of 2.4 to 6.5 y. This range is about one‐third of that estimated previously. The total atmospheric lifetime, based upon oceanic, atmospheric, and proposed soil losses with all of their uncertainties, is 0.8 (0.6 to 1.4) y. Only 28% of this total uncertainty is attributable to the uncertainty in ocean

ISSN:0094-8276    

DOI:10.1029/95GL03022

年代:1996

数据来源: WILEY

摘要:

The tendency for more frequent El Niño events and fewer La Niña events since the late 1970's has been linked to decadal changes in climate throughout the Pacific basin. Aspects of the most recent warming in the tropical Pacific from 1990 to 1995, which are connected to but not synonymous with El Niño, are unprecedented in the climate record of the past 113 years. There is a distinction between El Niño (EN), the Southern Oscillation (SO) in the atmosphere, and ENSO, where the two are strongly linked, that emerges clearly on decadal time scales. In the traditional El Niño region, sea surface temperature anomalies (SSTAs) have waxed and waned, while SSTAs in the central equatorial Pacific, which are better linked to the SO, remained positive from 1990 to June 1995. We carry out several statistical tests to assess the likelihood that the recent behavior of the SO is part of a natural decadal‐timescale variation. One test fits an autoregressive‐moving average (ARMA) model to a measure of the SO given by the first hundred years of the pressures at Darwin, Australia, beginning in 1882. Both the recent trend for more ENSO events since 1976 and the prolonged 1990–1995 ENSO event are unexpected given the previous record, with a probability of occurrence about once in 2,000 years. This opens up the possibility that the ENSO changes may be partly caused by the observed increases in greenh

ISSN:0094-8276    

DOI:10.1029/95GL03602

年代:1996

数据来源: WILEY

摘要:

Precipitation and temperature trends in nine dryland regions are analysed for the period 1900 to 1994. No widespread desiccation in dryland climates is found, the African Sahel being the only region to demonstrate a significant drying trend. All dryland regions have warmed, however, with the majority of the warming probably being unrelated to regional dryland effects. This warming may have contributed to a worsening of the P/PE ratio in many of these dryland regions. Three African dryland regions are alone in showing a significant negative correlation between precipitation and temperature, contrasting with the relationship for global land areas of warmer years/decades being also wetter years/decades. The relative lack of warming in the Sahel is noted and increased atmospheric dust resulting from the increased desiccation of the region is suggested as a possible explanation.

ISSN:0094-8276    

DOI:10.1029/95GL03586

年代:1996

数据来源: WILEY

摘要:

The direct radiative forcing of climate by carbon monoxide (CO) is generally considered to be negligible. However, a recent study of localised clear‐sky surface irradiances asserts that the forcing by CO may be comparable to that by CFC‐11. Nevertheless no detailed comparison of CO and CFC climate forcings has yet been made. Thus the present study estimates the radiative impact of the increases in CO, CFC‐11 and CFC‐12 that have occurred since industrialisation. A radiative transfer model is used to reproduce the results of the earlier study. Clouds are then added, and the stratosphere‐adjusted forcing at the tropopause (the “climate forcing”) calculated. Global‐mean anthropogenic climate forcing by CO is determined to be 32% of that by CFC‐11, 12% of that by CFC‐12, and 9% of that by the CFCs combined. Even if the contemporary global‐mean CO concentration is increased by a factor of three, the climate forcing by CO is still only 75% of that due to CFC‐11. Regarding instantaneous clear‐sky forcings, further analysis shows that surface measurements can give a misleading impression of effects at the tropopause. While the indirect effects of CO on climate change are not yet properly quantified, the direct radiative effects appear, as previously thought, to be o

ISSN:0094-8276    

DOI:10.1029/95GL03593

年代:1996

数据来源: WILEY

摘要:

We present here a case study of cloud‐mediated production of cloud condensation nuclei (CCN) recorded at Palmer Station (64°46′S, 64°05′W), Antarctica on 20 Jan 1994. Four instances of CCN bursts occurred on Jan 17, 19, 20 and Feb 7, 1994 when cloud base descended to the surface and dissipated under prevailing meteorological conditions. The most spectacular event occurred on Jan 20 when the CCN concentration was enhanced by a factor of four at 0.25% supersaturation (with respect to water) compared to the pre‐event concentration. At 1.25% supersaturation, the corresponding enhancement was by a factor of seven. This indicated a larger production of aerosol particles in smaller size ranges. The elevated CCN concentrations were measured for over fifteen hours. The CCN activity spectrum during the event resembled the ones that are typical of previous measurements in the urban plumes of St. Louis a

ISSN:0094-8276    

DOI:10.1029/95GL03588

年代:1996

数据来源: WILEY

摘要:

The Lidar In‐Space Technology Experiment (LITE) was launched aboard the Space Shuttle Discovery on September 9, 1994. It employed a three‐wavelength Nd:YAG lidar system. The validation of LITE involved ground truth experiments which were carried out by over 50 international groups consisting of both airborne and ground‐based lidar platforms. Our experiment involved the operation of an airborne Nd:YAG lidar system, at the fundamental wavelength of 1064 nm, aboard National Research Council's Convair 580 (CV580) aircraft, operating in a nadir orientation. Two flight tracks aligned with orbital tracks over California's coast provided an interesting study. First, comparisons with preliminary LITE data and CV580 results will illustrate the transport of urban aerosols from Los Angeles and San Francisco. Secondly, a correlative study of cloud top heights, obtained from a marine stratus cloud deck off the coast of California showed agreement with the 10s averaged LITE data of

ISSN:0094-8276    

DOI:10.1029/95GL03338

年代:1996

数据来源: WILEY

摘要:

Surface ozone is measured at 70°N and 78°N with analysers having response time of 1 and 20 sec, and characteristic ozone depletions due to polluted air are revealed. Using the rapid instrumental time response, significant variations are found to occur within a few seconds. It is hard to find any ozone enhancements due to polluted air, and local ozone production seems to have minor importance. Therefore no significantly enhanced ozone levels due to local and long‐range transported car exhaust are likely to occur at high latitudes. Additional depletions due to dry and wet depositions are probably occurr

ISSN:0094-8276    

DOI:10.1029/95GL03581

年代:1996

数据来源: WILEY