摘要:

The initiation and maintenance of a continental ice sheet are investigated using the GENESIS global climate model. A necessary condition for ice sheet initiation is to have snow cover survive through the summer. Model simulations examine the sensitivity of the maintenance of summer snow cover to the prescriptions of topography and solar luminosity. The time period chosen for this study is the Late Carboniferous (306 Ma) when an extensive continental ice sheet, as large if not larger than the Pleistocene glaciations, existed on a supercontinent in the southern hemisphere. This ice sheet, persisting for over 60 m.y., was one of the most prolonged periods of continuous glaciation in Earth history. Model simulations indicate that given the geography, solar luminosity (3% less than present), and atmospheric CO2(same as present) estimated for this time period, summer snow cover remains as far equatorward as 35°S latitude. Global mean temperature is −2.4°C, 17.2°C cooler than for present. Probable regions for the initiation of the Carboniferous ice sheet are apparent in analysis of the spin‐up of the model to equilibrium. Summer snow cover first persists along the polar coastline of the supercontinent during year 4 of the simulation. Summer snow cover shows the greatest sensitivity to solar luminosity. Increasing the solar constant to its present value from the Late Carboniferous value results in a summer warming of southern hemisphere high‐latitude land areas by as much as 37°C and complete melting of any summer snow cover. Topography plays a lesser role. Reducing land elevations to sea level causes persistent summer snow cover to retreat only 8° poleward. Conversely, initializing the model with an elevated land ice sheet has no effect on summer snow co

ISSN:0148-0227    

DOI:10.1029/96JD01161

年代:1996

数据来源: WILEY

摘要:

We perform a 20‐year, global‐scale simulation with a land surface model (LSX) and compare its annual hydrologic budget with observations at continental and global scales. The model is forced with observed climate data, including temperature, relative humidity, precipitation, cloudiness, and wind. We find that LSX produces a satisfactory simulation of the annual hydrologic budget on both global and continental scales. The differences between model output and observations suggest the need for improved boundary conditions and land surface parameterizations, as well as the need for more complete data sets of global hydrologic variables. This study presents a methodology for the evaluation of land surface models for their more effective use in climate modeling stud

ISSN:0148-0227    

DOI:10.1029/96JD01164

年代:1996

数据来源: WILEY

摘要:

Experiments with a two‐dimensional model ofChao and Lin[1994] are conducted to investigate the role of WISHE (wind‐induced surface heat exchange) in the super cloud clusters. It is found that while WISHE as an instability mechanism is not responsible for the existence of the super cloud clusters, the process of surface fluxes being enhanced by surface wind has the important roles of prolonging the life span of the cloud clusters and making the super cloud clusters more robust. WISHE is also found not responsible for upstream propagation of the super cloud cluster. The latter is explained by the cloud cluster teleinduction mechanism proposed byChao and Lin[19

ISSN:0148-0227    

DOI:10.1029/96JD01224

年代:1996

数据来源: WILEY

摘要:

A simple four‐dimensional assimilation technique, called Newtonian relaxation, has been applied to the Hamburg climate model (ECHAM), to enable comparison of model output with observations for short periods of time. The prognostic model variables vorticity, divergence, temperature, and surface pressure have been relaxed toward European Center for Medium‐Range Weather Forecasts (ECMWF) global meteorological analyses. Several experiments have been carried out, in which the values of the relaxation coefficients have been varied to find out which values are most usable for our purpose. To be able to use the method for validation of model physics or chemistry, good agreement of the model simulated mass and wind field is required. In addition, the model physics should not be disturbed too strongly by the relaxation forcing itself. Both aspects have been investigated. Good agreement with basic observed quantities, like wind, temperature, and pressure is obtained for most simulations in the extratropics. Derived variables, like precipitation and evaporation, have been compared with ECMWF forecasts and observations. Agreement for these variables is smaller than for the basic observed quantities. Nevertheless, considerable improvement is obtained relative to a control run without assimilation. Differences between tropics and extratropics are smaller than for the basic observed quantities. Results also show that precipitation and evaporation are affected by a sort of continuous spin‐up which is introduced by the relaxation: the bias (ECMWF‐ECHAM) is increasing with increasing relaxation forcing. In agreement with this result we found that with increasing relaxation forcing the vertical exchange of tracers by turbulent boundary layer mixing and, in a lesser extent, by convection, is

ISSN:0148-0227    

DOI:10.1029/96JD01218

年代:1996

数据来源: WILEY

摘要:

A new approach to constructing angular distribution models (ADMs) from satellite data has been developed. The ADMs model the radiance anisotropy and are used to convert satellite measured radiance to flux at the top of the atmosphere. The radiance pairs method (RPM) processes radiance pairs that view approximately the same area at the same time. By ratioing the paired radiances, the flux or field strength is eliminated, producing ratios of anisotropies which are taken as the data source for the ADMs. The ADMs are modeled as random functions, and the RPM estimates the mean of the ADM. The RPM is compared to the standard method of sorting by angular bins (SAB) and is shown to remove questionable assumptions, converge faster, and give better accuracy than the SAB method. Both methods were applied to the same Nimbus 7 ERB data and resulted in statistically different longwave ADMs.

ISSN:0148-0227    

DOI:10.1029/96JD00368

年代:1996

数据来源: WILEY

摘要:

To investigate the excess shortwave absorption by clouds, a numerical cloud generation model has been coupled to a plane‐parallel discrete ordinates radiative transfer model. The former was used in a time‐dependent fashion to generate a cumulonimbus turret and three types of cirrus anvil (precipitating, extended, detached) representing three stages of cloud evolution outward from the turret. The cloud particle size distributions, as a function of altitude, were used as input to the radiative transfer model using indices of refraction for pure water and pure ice and equivalent sphere Mie theory. The radiative transfer model was used to calculate the ratio of cloud forcing at the surface to cloud forcing at the top of the atmosphere, both for the broadband shortwave and as a function of wavelength. Recent empirical studies have placed this cloud forcing ratio at around 1.5, and our coupled model results approach this value for small solar zenith angles, when the cloud contains large (>100 μm) ice particles that absorb significantly in the near infrared (primarily the 1.6‐μm window). However, the empirical studies are based on diurnal averages, and our plane‐parallel radiative transfer model yields an area and diurnally averaged cloud forcing ratio of only 1.18 for a tropical cumulonimbus and cirrus anvil system, primarily because of the rapid decrease of the ratio with solar zenith angle. The ratio decreases because of the increase in albedo with solar zenith angle, which is a characteristic feature of plane‐parallel clouds. Adding dust or aerosol to the cloud layers, to make them absorb at visible wavelengths, makes the instantaneous cloud forcing ratio larger for an overhead Sun but also makes the solar zenith angle dependence in the cloud forcing ratio more pronounced. These two effects cancel, eliminating interstitial aerosol as a possible explanation for the excess cloud absorption in plane‐parallel radiative transfer modeling. The strong dependence of the surface/top of the atmosphere cloud forcing ratio on solar zenith angle may be a fundamental defect with the plane‐parallel approach to solar radiative transfer in a cl

ISSN:0148-0227    

DOI:10.1029/96JD01154

年代:1996

数据来源: WILEY

摘要:

We examine the sensitivity of cirrus cloud albedo and bidirectional reflection function to particle shape using the phase functions of liquid water spheres, regular hexagonal ice crystals, and random‐fractal ice particles calculated at a nonabsorbing visible wavelength of 0.63 μm. Accurate multiple‐scattering calculations for plane‐parallel clouds show that hexagonal ice crystal clouds have systematically larger planetary and global albedos than liquid water clouds of the same optical thickness. There is accumulating evidence that the idealized phase function of regular hexagonal crystals, which causes pronounced halos, is not necessarily the best representation of the range of reflectance characteristics of the majority of ice clouds. A more typical representation of the scattering phase function for ice clouds that are composed of a complex set of crystal shapes and sizes may be obtained using a model of randomly shaped irregular particles. Even larger cloud albedos are obtained for the random‐fractal particle model because of its smaller asymmetry parameter. Our computations also show that a larger planetary albedo does not always imply a larger reflectance and that the relative brightness of ice versus liquid water clouds is highly scattering‐geometry dependent. Use of the wrong particle shape model (crystal instead of water droplet and vice versa) in retrieving cloud optical thickness from bidirectional reflectance measurements can result in an underestimation or overestimation of the true optical thickness by a factor that can exceed 3. At some scattering geometries, use of the wrong model can give an unrealistically large optical thickness or no solution at all. Overall, bidirectional reflectance differences between random‐fractal and regular hexagonal particle shapes are significantly smaller than those between either ice crystal and liquid water spheres, except at the back scatteri

ISSN:0148-0227    

DOI:10.1029/96JD01155

年代:1996

数据来源: WILEY

摘要:

A 30‐year record of subjectively‐analyzed daily temperature and geopotential height data from the lower and middle stratosphere (50, 30, and 10 hPa) is used to derive a climatology of planetary waves, with zonal wavenumberss= 1, 2, and 3, of temperatureTand geopotential heightZ. This should be useful as (1) a reference data set for intercomparison with shorter records and validation of general circulation models and (2) for fundamental understanding of the nature of stratospheric wave disturbances. First, seasonal (December, January and February) statistics are presented. The steady, standing waves are strongest at 60°–70°N and generally lead to northward eddy heat and momentum fluxes consistent with vertically propagating, dissipated disturbances, in broad agreement with previous studies based on shorter data sets. At 30 hPa the stationary eddies provide about 50–60% of the net northward heat flux, but the transients contribute a similar proportion to the net northward momentum flux. Statistical analysis of the transient wave amplitudes reveals unimodal distributions, which are skew for higher wavenumbers. Transient variations in the eddy fluxes are shown to be mainly due to amplitude variations rather than changes in the barotropic or baroclinic structure of the eddies. It is shown that changes in the geographical location of the waves contribute a considerable amount to the transience, so that a composite mean based on the wave amplitude maximum contributes a much larger fraction of the net eddy fluxes than the stationary waves. The importance of the long data record for these statistical distributions is emphasized by examining a 10‐year subsample. Finally, the occurrence of “preferred flow regimes” through the 30‐year data record is examined; neither trends nor definite links with possible external forcing mechanisms are found, despite considerable low‐frequency (biennial to d

ISSN:0148-0227    

DOI:10.1029/96JD01226

年代:1996

数据来源: WILEY

摘要:

The morphology of westerly wind bursts (WWBs) during the 1980s is investigated using 1000‐mbar winds analyzed by the European Center for Medium‐Range Weather Forecasts (ECMWF). The variety of synoptic situations in which WWBs occur are explored so that the dynamics of WWB generation and maintenance can be studied and so that those interested in short‐ and long‐term forecasts of conditions over the equatorial Pacific will have an increased knowledge of those elements associated with WWBs. WWBs are identified by searching for episodes in which the zonal wind was at least 5 m s−1over at least 10° longitude for two or more days. This definition, based on surface winds rather than elevated winds or wind anomalies, reflects the impact WWBs have on air/sea interaction processes. The spatial and temporal criteria provide a synoptic‐scale envelope for such interactions and mitigate against erroneous analyses. According to these criteria, 131 distinct WWBs occurred during the 1980s. A new two‐dimensional classification scheme has been developed; the nine patterns in it describe the near‐surface flow during 90% of these WWBs. A single cyclone or a series of cyclones in one or both hemispheres and several different types of cross‐equatorial flow are the major components of the patterns. Only 8% of the bursts were associated with concurrent cyclones in both the northern and southern hemispheres. Four bursts involving cross‐equatorial flow are subjected to further study. The flow is decomposed into barotropic and baroclinic parts and into rotational and divergent baroclinic components. One of the bursts was predominantly barotropic and another predominantly baroclinic; the cross‐equatorial flow in all four cases was predominantly divergent in nature. The information from these cases, coupled with the typical vertical structures and small meridional length scales associated with the various synoptic patterns, suggests that simple baroclinic models of circulations driven by near‐equatorial heating are not adequate to describe

ISSN:0148-0227    

DOI:10.1029/96JD00030

年代:1996

数据来源: WILEY

摘要:

The results of this study demonstrate that the surface wind velocity and pressure fields derived from spaceborne scatterometers are useful in monitoring the location and intensity of tropical cyclones. Satellite‐borne microwave scatterometers can penetrate the cloudy core regions of tropical cyclones to resolve the circulation in detail over data sparse regions. The location of the cyclone observed by the ERS‐1 scatterometer is very close to that revealed in Geostationary Meteorological Satellite images. The surface winds provided by the ERS‐1 scatterometer are used here with a modified two‐layer planetary boundary layer model which includes effects of curvature, stability, and secondary flow to derive surface pressures near tropical cyclone Oliver. The curvature effect is found to be more significant than stability and secondary flow, which are crucial in deriving accurate surface pressure fields in midla

ISSN:0148-0227    

DOI:10.1029/96JD01229

年代:1996

数据来源: WILEY