摘要:

We use a three‐dimensional climate model, the Goddard Institute for Space Studies (GISS) model II with 8° by 10° horizontal resolution, to simulate the global climate effects of time‐dependent variations of atmospheric trace gases and aerosols. Horizontal heat transport by the ocean is fixed at values estimated for today's climate, and the uptake of heat perturbations by the ocean beneath the mixed layer is approximated as vertical diffusion. We make a 100‐year control run and perform experiments for three scenarios of atmospheric composition. These experiments begin in 1958 and include measured or estimated changes in atmospheric CO2, CH4, N2O, chlorofluorocarbons (CFCs) and stratospheric aerosols for the period from 1958 to the present. Scenario A assumes continued exponential trace gas growth, scenario B assumes a reduced linear growth of trace gases, and scenarioCassumes a rapid curtailment of trace gas emissions such that the net climate forcing ceases to increase after the year 2000. Principal results from the experiments are as follows: (1) Global warming to the level attained at the peak of the current interglacial and the previous interglacial occurs in all three scenarios; however, there are dramatic differences in the levels of future warming, depending on trace gas growth. (2) The greenhouse warming should be clearly identifiable in the 1990s; the global warming within the next several years is predicted to reach and maintain a level at least three standard deviations above the climatology of the 1950s. (3) Regions where an unambiguous warming appears earliest are low‐latitude oceans, China and interior areas in Asia, and ocean areas near Antarctica and the north pole; aspects of the spatial and temporal distribution of predicted warming are clearly model‐dependent, implying the possibility of model discrimination by the 1990s and thus improved predictions, if appropriate observations are acquired. (4) The temperature changes are sufficiently large to have major impacts on people and other parts of the biosphere, as shown by computed changes in the frequency of extreme events and by comparison with previous climate trends. (5) The model results suggest some near‐term regional climate variations, despite the fixed ocean heat transport which suppresses many possible regional climate fluctuations; for example, during Hie late 1980s and in the 1990s there is a tendency for greater than average warming in the southeastern and central United States and relatively cooler conditions or less than average warming in the western United States and much of Europe. Principal uncertainties in the predictions involve the equilibrium sensitivity of the model to climate forcing, the assumptions regarding heat uptake and transport by the ocean, and the omission of other less‐certain c

ISSN:0148-0227    

DOI:10.1029/JD093iD08p09341

年代:1988

数据来源: WILEY

摘要:

A simple climate model of Quaternary glaciations is combined with a simple isotopic model in order to study phase relationships between δ18O records in deep‐sea cores and in ice cores. Power spectra of the model's climatic and isotopic variables show a dominant peak near 100 kyr (1 kyr = 1000 years), the continuous background of all paleoclimatic spectra, smaller peaks at the orbital periodicities of 41, 23, and 19 kyr, and additional peaks at 14.7, 13, 11.5, 10.4, and 9.5 kyr. Time lags between the model's isotopic profiles are highly variable: ice core events, identified as sharp maxima or minima, can lead apparently homologous events in the marine record or lag them by a few thousand years. This time domain instability of lags is due to the complexity of the power spectrum, with its continuous component and large number of lines. Since the complex spectrum is present in both model and paleoclimatic data, we expect lag relations in the data to be quite variable as well. Independent dating of ice cores is preferable therefore to dating by comparison with marine records. Absolute dating can be based on a combination of ice flow models and of accelerator mass spectroscopy. Time‐averaged cross spectra of independently dated isotopic profiles should be stable and should help us to understand the mechanisms which lead to internal climatic variabi

ISSN:0148-0227    

DOI:10.1029/JD093iD08p09365

年代:1988

数据来源: WILEY

摘要:

Cloud amounts derived from Landsat digital data over 22 regions with various cloud types are compared to cloudiness derived from collocated, nearly simultaneous 4 × 8 km GOES visible and infrared data using a hybrid bispectral threshold method (HBTM). The mean and rms differences between the Landsat and GOES cloud amounts are −0.008 and 0.075, respectively. A sensitivity analysis showed that underestimation of approximately 0.01 in clear‐sky reflectance by the HBTM increased the mean GOES cloud amount by 0.06, more than twice the decrease in cloud amount obtained by an equivalent increase in clear‐sky reflectance. Accuracy limits were estimated for the Landsat analyses by considering the fraction of cloud‐edge pixels in a scene. For scenes having relatively homogeneous backgrounds, the Landsat cloudiness has an estimated accuracy of −0.02 to +0.04, while cloud amount in scenes with variable background reflectance is more susceptible to underestimation, having accuracy limits of −0.02 to +0.12. The lower bound for the Landsat cloud cover is reliable when considered in terms of most working definitions of cloud cover. The upper bound will remain uncertain until an absolute definition of cloud cover is established which sets the bounds on the optical properties which dictate when a cloud does or does not exist. Many of the scenes contained a substantial proportion of partially cloud‐filled GOES pixels. Landsat imagery and cloud properties derived from the Landsat data are used to explain how the partially cloud‐filled GOES pixels were treated by the HBTM. Most of the rms difference occurred over two regions: one where many of the GOES pixels were partially cloud‐filled with very bright cloud cells and one where the GOES pixels were mostly overcast with both bright (thick) and very dim (thin) clouds. It is concluded that the HBTM accounts for the effects of partially cloud‐filled fields of view in most of the cases in this study. The reflectance limits used to constrain the HBTM appear to be reasonable but cannot account for the effects of anomalously bright or dark cloud fields. Techniques are discussed for improving the HBTM's retrieval of cloud fraction in the more difficult cases. This study, though only the beginning of a validation of the HBTM, demonstrates the potential for using Landsat data to improve routine cloud parameter retrievals from the low‐resolution sensors on meteorological spacecraft. Since the scenes used in this study represent only a limited number of cloudiness conditions and viewing and illumination situations, there remains much more work ahead in order to completely validate the HBTM or any other satellite cloud retrieval algorithm. Future cloud studies using Landsat should include both visible

ISSN:0148-0227    

DOI:10.1029/JD093iD08p09385

年代:1988

数据来源: WILEY

摘要:

A parameterization of the effects of Rossby waves in the middle atmosphere is proposed for use in two‐dimensional models. By adding an equation for conservation of Rossby wave activity, closure is obtained for the meridional eddy fluxes and body force due to Rossby waves. Rossby wave activity is produced in a climatological fashion at the tropopause, is advected by a group velocity which is determined solely by model zonal winds, and is absorbed where it converges. Absorption of Rossby wave activity causes both an easterly torque and an irreversible mixing of potential vorticity, represented by the meridional eddy diffusivity,Kyy. The distribution of Rossby wave driving determines the distribution ofKyy, which is applied to all of the chemical constituents. This provides a self‐consistent coupling of the wave activity with the winds, tracer distributions, and the radiative field. Typical winter stratospheric values forKyyof 2×106m2S−1are obtained. Poleward tracer advection is enhanced and meridional tracer gradients are reduced where Rossby wave activity is absorbed in the

ISSN:0148-0227    

DOI:10.1029/JD093iD08p09405

年代:1988

数据来源: WILEY

摘要:

We report the first clear‐air observations of vertical velocities in the tropical upper troposphere and lower stratosphere (8–22 km) using the Arecibo 430‐MHz radar. Oscillations in the vertical velocity near the Brunt‐Väisälä period are observed in the lower stratosphere during the 12‐hour observation period. Frequency power spectra from the vertical velocity time series show a slope between −0.5 and −1.0. Vertical wave number spectra computed from the height profiles of vertical velocities have slopes between −1.0 and −1.5. These observed slopes do not agree well with the slopes of +1/3 and −2.5 for frequency and vertical wave number spectra, respectively, predicted by a universal gravity‐wave spectrum model. The spectral power of wave number spectra of a radial beam directed 15° off‐zenith is enhanced by an order of magnitude over the spectral power levels of the vertical beam. This enhancement suggests that other geophysical processes besides gravity waves are present in the horizontal flow. The steepening of the wave number spectrum of the off‐vertical beam in the lower stratosphere to near −2.0 is attributed to a quasi‐inertial period wave, which was present in the horizontal f

ISSN:0148-0227    

DOI:10.1029/JD093iD08p09419

年代:1988

数据来源: WILEY

摘要:

Observation and analyses of the atmospheric pressure at the surface on Mars for several Martian years has led to the discovery of transient events in the daily pressure variations of an unusual nature. These events last only a few sols (Mars days), appear to repeat on an annual basis to within a few sols, appear to cover a large portion of the hemisphere on the same sol or at least within a few sols, occur in pairs with separations of 20 sols in some instances, are coincident with the annual pressure minimum (i.e., the maximum concentration of carbon dioxide in the southern hemisphere), and consist of spectral components near to, and sometimes identical in, frequency to the diurnal and semidiurnal harmonics. It is suggested that these events are Kelvin, normal‐mode, transient, global oscillations. Examination of the initiation of the 1977A, 1977B, and 1982A global dust storms indicate that pseudo‐diurnal and some‐times pseudo‐semidiurnal oscillations exist during the initiation phase and might be involved in triggering the global dust storms, possibly in conjunction with winds due to longer‐period circulations. The diurnal harmonic disappears prior to the 1977B global dust storm and is replaced by an oscillation at 1.1 cycles/sol. If the lack of a classical diurnal tide indicates the suppression of convection, this might partially explain the slow growth of the 1977B storm. The daily average pressure during the transient season, which precedes the global dust storm season by almost one fourth of a year, exhibits a different behavior during the global dust storm years than during the nonglobal dust storm years. This implies that the global circulation must differ significantly in these cases and may be related to the presence or absence of global dust storms later in a given year. These analyses suggest that an almost diurnal and an almost semidiurnal, high‐frequency global oscillation, different from the classical solar‐driven tides, may be quite c

ISSN:0148-0227    

DOI:10.1029/JD093iD08p09433

年代:1988

数据来源: WILEY

摘要:

Short‐period, apparently global oscillations are clearly present in the multiyear time series of surface pressure variations recorded at the two Viking Lander sites on Mars. Previous work has emphasized the contributions of solar‐driven thermal tides to the surface pressure field. Recent analyses of single‐station data on Earth and on Mars suggest that the planetary normal modes (i.e., free modes of planetary scale) may also contribute to the short‐term, seasonally modulated variability in surface pressure. In this paper the periods of free modes in the Martian atmosphere are computed, with due consideration to the short atmospheric radiative damping time and to the seasonal and interannual variation of globally representative temperatures, both of which differ by an order of magnitude from their terrestrial counterparts. The likelihood of atmospheric resonance (i.e., free modes) or of efficient excitation and possibly resonant amplification of forced modes is then discussed. In summary, a thermally forced diurnal Kelvin wave still seems the best explanation for the short‐period, transient mode which appears just prior to or during the episodic great dust storms on Mars. Resonant modes seem most likely to occur during northern spring and summer, as previously suggested by Tillman. However, the limited atmospheric temperature data appear to have the wrong seasonal trend for resonance alone to account for the transient surface pressure oscillations observed in mid to late northern summer at the Viking Landers. One possibility is that during this period, unobserved changes in the atmospheric static stability near the surface have temporarily amplified the resonant modes or have moved the resonant frequency through the frequencies of the solar thermotida

ISSN:0148-0227    

DOI:10.1029/JD093iD08p09452

年代:1988

数据来源: WILEY

摘要:

The measurement of trace constituents in the middle atmosphere by ground‐based microwave spectroscopy has emerged as an important tool for probing that region of the atmosphere. However, the vertical resolution attainable with the technique has remained somewhat ambiguous and is worthy of further study. In order to explore this question, we first applied the Backus‐Gilbert inversion technique, which automatically yields quantitative estimates of the inversion spatial resolution, to this particular inverse problem. This indicated that the optimum resolution of Backus‐Gilbert inversions of microwave spectroscopic measurements is about 10 km. We then developed a general technique, based on inversion of delta function simulated profiles, for evaluating the resolution of any inversion technique. This was applied to the Chahine inversion technique, which is representative of inversion methods typically used in microwave spectroscopy. These results indicated that the optimum resolution of the Chahine technique is about 6 or 7 km, or nearly a factor of 2 better than the equivalent Backus‐Gilbert results. With the addition of simulated measurement error this resolution is degraded somewhat, especially near the inversion boundaries. However, for random measurement errors up to about the 1% level, 6‐ or 7‐km resolution is still attainable in the interior of the inver

ISSN:0148-0227    

DOI:10.1029/JD093iD08p09463

年代:1988

数据来源: WILEY

摘要:

An experiment is described in which cloud droplets are sampled as a function of their size using a counterflow virtual impactor, thereby revealing the degree to which the different sized droplets in a cloud are chemically different. The chief result is that the larger droplets sampled from a marine stratus cloud had a solute mass concentration that was a factor of 3 greater than in the smaller droplets.

ISSN:0148-0227    

DOI:10.1029/JD093iD08p09477

年代:1988

数据来源: WILEY

摘要:

The one‐dimensional vertical model of radiation transfer in a plant canopy described by Verstraete (1987) is extended to account for the transfer of diffuse radiation. This improved model computes the absorption and scattering of both visible and near‐infrared radiation in a multilayer canopy as a function of solar position and leaf orientation distribution. Multiple scattering is allowed, and the spectral reflectance of the vegetation stand is predicted. The results of the model are compared to those of other models and actual observati

ISSN:0148-0227    

DOI:10.1029/JD093iD08p09483

年代:1988

数据来源: WILEY