摘要:

With respect to gas transport the fim near the surface of an ice sheet can be divided into three zones. The uppermost is a “convective zone” located just below the surface of the ice sheet in which the air is rapidly flushed by convective exchange with the overlying atmosphere. Below the convective zone there is a “diffusive air column” in which diffusion is rapid but there is no convection. Between the bottom of the diffusive air column and the bubble close‐off region there may be a “nondiffusive zone” in which diffusion is so slow that negligible gas transport occurs. The diffusive air column is characterized by progressive enrichment with depth of15N in N2(and heavy isotopes of gases in general) as predicted using the barometric equation. In this paper we present data on the δ15N of N2in recently trapped air samples from 12 ice cores, along with numerous downcore samples from Byrd, Vostok, and Dome C. Bubble close‐off depths for these cores (calculated from a densification model) ranged from 51 to 114 meters below the surface (mbs). We used these data and the barometric equation to calculate the thickness of the diffusive air column, and found that it comprised 46 to 93% of the total firn thickness at our study sites. Paleo‐close‐off depths calculated from the densification model for glacial sections of Byrd, Vostok, and Dome C are 15–25 m deeper than close‐off depths today. Diffusive column heights, calculated from δ15N, varied in a more complex manner. The diffusive column height at Byrd appears to have decreased from 74 m during the last glacial period to 50 m during the Holocene. At Vostok and Dome C the diffusive column height calculated from15N increase from about 65 m during the last glacial period to about 80 m in the Holocene. We use records of surface temperature and CO2at Byrd and Vostok, along with their respective chronologies, to constrain the ice age ‐ gas age difference (Δage) throughout the section of the Vostok ice core corresponding to the last glacial termination. In principle, Δage values calculated from these data can be used to discriminate whether gas in the fim mixes to the bubble close‐off depth or to a depth equivalent to the diffusive column height. In practice, however, uncertainties in the chronology of Byrd and Vostok are too great to allow us to distinguish between these two possibilities. One can only say that at the time of the last termination, Δage for Vostok was between 3 and 10.5 kyr. Previous

ISSN:0148-0227    

DOI:10.1029/92JD01297

年代:1992

数据来源: WILEY

摘要:

The annual cycle of surface‐based inversions at nine Arctic weather stations is examined, based on a 20‐year set of daily 1200 UT significant level radiosonde data. All stations are at or near the coast. Inversions in winter months are primarily the result of strongly negative net radiation at the surface, whereas in summer, inversions more commonly result from near‐surface cooling of warm air masses. Inversion frequency is at a maximum in winter (generally>70% of days) when inversions range from ∼400 to ∼850 m in thickness. Inversion thickness and strength (temperature change across the inversion) are strongly related to surface temperature. Inversions may involve temperature changes of>30°C in6°C 100 m−1during periods of extreme warm air advection aloft. Midwinter inversions commonly persist for 2–4 days, but may remain undisturbed for several weeks, affecting lower troposp

ISSN:0148-0227    

DOI:10.1029/92JD01451

年代:1992

数据来源: WILEY

摘要:

A two‐dimensional climate model which links the northern hemisphere atmosphere, ocean mixed layer, sea ice, and continents has been asynchronously coupled to a model of the three main northern ice sheets and their underlying bedrock. The coupled model has been used to test the influence of several factors, including snow surface albedo over the ice sheets, in producing plausible ice age simulations using astronomically derived insolation and CO2data from the Vostok ice core. The impact of potentially important processes, such as the water vapor transport, clouds, and deep sea circulation, was not investigated in this study. After several sensitivity experiments designed to identify the main mechanisms governing surface temperature and ice accumulation, the model is first run with ice sheet feedback by forcing it only with the astronomical insolation over the past 122 kyr. Large variations of ice volume are simulated between 122 and 55 kyr B.P., with a rapid latitudinal extension of the North American and Eurasian ice sheets starting at 120 kyr B.P. The simulated last glacial maximum is at 19 kyr B.P. The model is able to simulate deglaciation as well. The simulated evolution of the three northern ice sheets is generally in phase with geological reconstructions. The major discrepancy between the simulation and paleoclimate reconstructions lies in the underestimation of temperature variations (linked with an underestimation of the ice sheet extent and an excess in the prescribed CO2concentration). Sensitivity experiments show that ablation is more important to the ice sheet response than snow precipitation variations. In the model a key mechanism in the deglaciation after the last glacial maximum appears to be the “aging” of snow, which decreases its albedo. The other factors which play an important role are, in decreasing level of importance, the ice sheet altitude, insolation, taiga cover, and ice sheet extent. A final set of experiments addresses the effects of CO2on the simulated climate of the last glacial maximum and on a new long term experiment covering the last 122 kyr. This last experiment is made by forcing the model with both insolation and CO2variations. This additional forcing improves the temperature and ice volume results. Despite the limitations inherent to the present modeling approach, the sensitivity experiments performed can provide insight into the relative importance of possible mechanisms responsible for the building and melting of huge ice sheets during the last glacial‐interglacia

ISSN:0148-0227    

DOI:10.1029/92JD01256

年代:1992

数据来源: WILEY

摘要:

Surface radiation budget data are presented for the midseasonal months of July and October of 1983 and January and April of 1984. These data allow examination for the first time of geographical and seasonal variations of the entire surface radiation budget from pole to pole. The latest flux estimation techniques have been used along with data from the International Satellite Cloud Climatology Project (ISCCP) and the Earth Radiation Budget Experiment (ERBE). Regional, zonal, and hemispheric distributions of the downward and net components of both shortwave and longwave fluxes and of the net total surface fluxes are determined. Seasonal flux variation per region, expressed as flux range, is illustrated for these components also. The estimated fluxes appear to be accurate to about 16 W m−2on a global average, based on sensitivity analyses and comparisons with ground data. An analysis of flux errors showed that most of the error was attributable to errors in input dat

ISSN:0148-0227    

DOI:10.1029/92JD00675

年代:1992

数据来源: WILEY

摘要:

A model for the accelerated calculation of clear sky fluxes based on the line‐by‐line radiance model FASCODE has been developed and applied to the calculation of cooling rates for atmospheric water vapor. The model achieves computational accuracies for the longwave upwelling and downwelling fluxes of the order of 0.2%, an accuracy well within current limitations imposed by uncertainties in the spectral parameters, the line shape, and the associated continua. For the same treatment of line shape, the Voigt profile with a 10 cm−1cutoff and no continuum, the results from the present model are in acceptable agreement with those from two other line‐by‐line models reported as part of the intercomparison of radiation codes used in climate models (ICRCCM). For this line profile and the mid‐latitude summer atmosphere, the largest difference between the results from our model and the Goddard Laboratory for Atmospheres (GLA) model occurs for the downwelling flux at the surface, with the present model providing a value greater than that from GLA. The differences are generally consistent with greater atmospheric opacity from the present model, attributable to the inclusion of a self‐broadening component for the half width for water and to finer spectral sampling in the lower‐pressure regime. Utilization of the line shape and associated continuum model included in FASCODE gives results that are significantly different from those provided by the other two models. This radiance model, including contributions from the foreign continuum as well as from a modified self‐continuum, has received extensive validation against measured radiance spectra, an example of which is provided. For the mid‐latitude summer atmosphere the principal contribution from the foreign continuum occurs in the upper troposphere in the 250–350 cm−1spectral region, whereas the contribution from the self‐continuum, dependent on the square of the water vapor density, is greatest in the lower troposphere. For the mid‐latitude summer atmosphere the foreign continuum contributes 0.4 Kd−1or 20% to the cooling in the upper troposphere and the self‐continuum contributes 1.9 K d−1to the cooling rate at the surface due to water vapor. The latter is 0.17 K d−1less than the cooling rate from the GLA model which is principally due to a modification of the self‐continuum. A significant result that has developed from the present work is the insight into atmospheric radiative processes provided by spectral profiles of the cooling rate. In the spectral domain there exists a mapping between the altitude and the molecular absorption strength as weighted by the Planck function. The extremely high correlation between the outgoing spectral radiance at the top of the atmosphere and the spectral cooling rate profile suggests that measurement of the outgoing spectral radiance can provide important information about atmospheric state that is not available from spectrally integrated quantities. Our results also indicate the critical importance of the spectral region from 100 to 600 cm−1for the radiative transfer

ISSN:0148-0227    

DOI:10.1029/92JD01419

年代:1992

数据来源: WILEY

摘要:

We present a new approach to remote sensing of water vapor based on the global positioning system (GPS). Geodesists and geophysicists have devised methods for estimating the extent to which signals propagating from GPS satellites to ground‐based GPS receivers are delayed by atmospheric water vapor. This delay is parameterized in terms of a time‐varying zenith wet delay (ZWD) which is retrieved by stochastic filtering of the GPS data. Given surface temperature and pressure readings at the GPS receiver, the retrieved ZWD can be transformed with very little additional uncertainty into an estimate of the integrated water vapor (IWV) overlying that receiver. Networks of continuously operating GPS receivers are being constructed by geodesists, geophysicists, government and military agencies, and others in order to implement a wide range of positioning capabilities. These emerging GPS networks offer the possibility of observing the horizontal distribution of IWV or, equivalently, precipitable water with unprecedented coverage and a temporal resolution of the order of 10 min. These measurements could be utilized in operational weather forecasting and in fundamental research into atmospheric storm systems, the hydrologic cycle, atmospheric chemistry, and global climate change. Specially designed, dense GPS networks could be used to sense the vertical distribution of water vapor in their immediate vicinity. Data from ground‐based GPS networks could be analyzed in concert with observations of GPS satellite occultations by GPS receivers in low Earth orbit to characterize the atmosphere at planetary

ISSN:0148-0227    

DOI:10.1029/92JD01517

年代:1992

数据来源: WILEY

摘要:

Low‐frequency climate oscillations of the Colorado Plateau region are investigated with evolutionary spectral analysis to observe temporal changes in the power spectra of 12 drought‐sensitive tree ring chronologies. The power spectra show large temporal and spatial variability. Several oscillations are simultaneously present at the majority of sites, indicating a high probability that the oscillations are related to regional climate phenomena. Most of the oscillations are of short duration, with periods of 20 to 80 years. The large temporal and spatial variability of the oscillations limits their usefulness in predictive models. The observed abrupt changes in power spectra suggest chaotic behavior and may represent a reordering of the processes influencing climate. A solar‐climate interaction is suggested by a close correspondence between times of low sunspot activity and abrupt changes in the power spectrum observed in the chronol

ISSN:0148-0227    

DOI:10.1029/92JD01301

年代:1992

数据来源: WILEY

摘要:

Accurate measurements of total ozone at high latitudes during winter have become increasingly important for studies of polar ozone depletion. The direct Sun measurements are the most direct and physically well defined type of measurement of total ozone with Dobson and Brewer spectrophotometers along with the focused Sun measurements. The direct Sun and focused Sun method are influenced by many sources of error at low solar elevations. A large error arises from the addition of radiation scattered from air along the path of the solar beam. A method to correct for this error using the Brewer instrument is described. The method gives reasonably accurate values down to solar elevations of 5° compared to 10°–20° without corrections. This is especially important for high‐latitude stations, where the low solar elevations are a limiting factor for accurate measurements of the total ozone particularly during the

ISSN:0148-0227    

DOI:10.1029/92JD01030

年代:1992

数据来源: WILEY

摘要:

The nature of the interaction of HCl vapor with ice has been investigated, using thermal analysis and FTIR spectroscopy to characterize the ice substrate, and mass spectrometry to measure the concentration of HCl and H2O vapors in the gas phase. The results indicate that a liquid layer is formed rapidly at the ice surface for ice exposed to HCl vapor at partial pressures above those characteristic of the ice‐liquid (aqueous HCl solution) equilibrium system. This liquid layer also forms below the eutectic temperature (186 K); that is, it forms even at temperatures at which the liquid is metastable with respect to the formation of HCl trihydrate. For smaller HCl partial pressures such as those prevailing in the stratosphere, the HCl is taken up by the ice surface in amounts corresponding to a large fraction of a monolayer. The chemical reactivity of this surface HCl is very large: Chlorine activation by type II polar stratospheric clouds (consisting of ice particles) should occur efficiently by reaction of the HCl with ClONO

ISSN:0148-0227    

DOI:10.1029/92JD01220

年代:1992

数据来源: WILEY

摘要:

A three‐dimensional global chemical tracer model (CTM) with a multiyear circulation provided from the Los Alamos general circulation model (GCM) is developed to simulate the distributions and trends of global CFC 11 concentrations. Realistic geographic emission patterns and parameterized photolysis rates are included in the model. The initial condition corresponds to the observed values in July 1978. The results of six model years from July 1979 to June 1985 are presented. Comparisons with the Atmospheric Lifetime Experiment (ALE) observations show reasonable agreement. The intrinsic features of the five ALE sites can be summarized as follows: Adrigole is sensitive to horizontal diffusion because it is located between the European and North American sources; Samoa and Tasmania are heavily influenced by the interhemispheric transport; and Oregon and Barbados require a better resolved flow pattern to receive the transport from their nearest sources. The modeled global budget in terms of interhemispheric burden difference, stratospheric loss rate, interhemispheric transport, and its associated time constant is comparable with that derived from a previous study. A model sensitivity test using a large (∼3 times) horizontal diffusion shows that it modestly improves certain aspects of the simulations at southern hemisphere ALE sites, however, at the cost of a greatly reduced interhemispheric burden difference and exchange time const

ISSN:0148-0227    

DOI:10.1029/92JD01262

年代:1992

数据来源: WILEY