Search

In this paper a revised version of Lettau's evapoclimatonomy model is used to simulate climate in West Africa. The model is applied specifically to the study sites of the HAPEX-Sahel region in Niger, an international regional experiment to study regional-scale hydrological and energy balances of the Sahel. The model uses monthly means of precipitation, potential evapotranspiration, and solar radiation from the HAPEX-Sahel observations, as well as vegetation and soil parameters adequate for the region. Evapotranspiration, runoff, and soil moisture are determined. Differences are observed between the three vegetation types (guiera, grass, and millet) and between the three supersites.

In this paper a revised version of Lettau's evapoclimatonomy model is introduced. Climatonomy is a one-dimensional representation of mean climate, which includes a complete characterization of the surface energy and water balances. The model is fundamentally forced by two input variables: incoming solar radiation and precipitation. All processes related to the ultimate distribution and exchange of the thereby available energy and moisture are parameterized. The goal of this paper is to describe the model's evolution and demonstrate its utility by sketching results of its application to several diverse climate problems in arid and semiarid Africa. Both the original and the revised evapoclimatonomy models are described, their conceptualization to that of more recent land-atmosphere interaction schemes is compared, their components in terms of the ecosystem are interpreted, and the model improvements incorporated in the current version are proposed. Finally, the use of the model in three applications to studies of climate and ecology in the Sahel and Kalahari regions of Africa is illustrated.

A global warming trend began during the late Paleocene that culminated in the early Eocene with the highest global temperatures of the Cenozoic. We have reconstructed late Paleocene surficial boundary conditions and modeled atmospheric conditions using the Goddard Institute for Space Studies general circulation model version II (GISS GCM II). These experiments were conducted to test the hypothesis that warm saline deep water formed during the late Paleocene and to understand atmospheric circulation near the beginning of a period of global warming. The warming is attributed primarily to increased sea surface temperatures at high latitudes. The sensitivity of the climate to ocean temperature was tested using two sea surface temperature distributions, each delimited latitudinally by oxygen isotope values, but with different east-west gradients. The simulations discussed here contain several features unique among warm climate experiments. The first experiment (P-1) used latitudinally constant (zonal) sea surface temperatures. The zonally distributed sea surface temperatures strengthen the general circulation of the atmosphere. In particular, Hadley Cell circulation is intensified, leading to extremes of precipitation in the equatorial region and extreme evaporation across subtropical oceans. The unusual results prompted a second experiment with modern east-west sea surface temperature gradients superimposed and referred to as P-Gradient (P-Grad). The east-west gradients in the sea surface temperature field exert a strong influence on the general atmospheric circulation, but the extreme zonality prevails. Under extreme zonal conditions it is possible to create a model where evaporation is in excess of precipitation by as much as 3 mm/day. If this occurred in restricted areas in a generally warmer ocean, such as the late Paleocene eastern Tethys Ocean and parts of the South Atlantic Ocean, it should be possible to create very saline water, which could become a component in warm saline deep water formation.

The history of climatic divisions in the contiguous United States has been pieced together from fragmentary documentation. Each of the 48 contiguous states has been subdivided into climatic divisions. Divisional boundaries are now standardized, and a set of climatic variables for time-invariant divisional boundaries has been compiled for the period of record beginning in 1895. This paper documents the origins of climatic divisions, the computational methodology of an area-invariant divisional dataset maintained by the National Climatic Data Center, and the strengths and weaknesses of divisional data.

The perception of the hypothesized greenhouse effect will differ dramatically depending upon the location on the earth at which the effect is analyzed. This is due mainly to two causes: 1) the warming signal depends upon the position on the earth, and 2) the natural variability of the warming has a strong position dependence. To demonstrate these phenomena, simulations were conducted of the surface temperature field with a simple stochastic climate model that has enough geographical resolution to see the geographic dependence. The model was tuned to reproduce the geographical distribution of the present climate, including its natural variability in both the variance and the space-time correlation structure. While such effects have been discussed elsewhere with even more realistic climate models, it is instructive to actually see simulations of time series laid side by side in order to easily compare their differences and similarities. Because of the model's simplicity, the causes of the variations are easy to analyze. Not surprisingly, some realizations of the temperature for some local areas show countertrends for a period of several decades in the presence of the greenhouse warming.

This paper, as the written version of the 1995 AMS Walter Orr Roberts Lecture, provides an overview of the current status of the inclusion of land surface processes in climate models. These processes provide fluxes of water and energy to atmospheric models and help determine surface meteorology and climate over the continents. With the increasing complexity and importance of these parameterizations and their detailed treatments of the roles of soils and vegetation have come greater demands for observational programs to evaluate their success and to provide required parameters. Intercomparisons between different land models are also becoming increasingly valuable as a means of identifying their weaknesses and limitations. The paper especially highlights the need for further emphasis on the coupling between land and the atmosphere in models. In particular, it calls for further evolution and improvement of the model treatments of precipitation, cloud effects on surface radiation, and boundary layer processes.

Traditionally, climate is defined by the properties of the averages of the meteorological fields over an appropriate time interval. In this paper the properties of the time-averaged observables of a red noise atmosphere and of a simplified model of thermal convection are investigated both analytically and numerically and are compared to those of the original finescale variables. It is shown that averaging tends to reduce the domain of variability and the attractor dimension favors persistence of initial correlations and enhances predictability. The implications of these findings in the real-world atmosphere are briefly assessed.

A shallow water model with realistic topography and idealized zonal wind forcing is used to investigate orographically forced modes in the Martian atmosphere. Locally, the model produces barotropic modes with periods within the broad range of periods observed at the sites of Viking Lander I and H (VL1 and VL2) during the fall and spring seasons. Its variability at those sites is dominated by an oscillation of 3 Martian solar days (sols) in the region of VL1 and by a 6-sol oscillation in that of VL2. These oscillations are forced by the zonal asymmetries of the Martian mountain field. Their robustness with respect to changes of the fundamental model parameters is examined. Since the exhibited periods occur for a barotropic forcing field that is highly idealized, it is difficult to say whether they have much to do with the real Mars, but their resemblance to some of the periodicities present in the observed Martian climatology deserves further investigation. The spatial variability associated with the orographically forced oscillations is studied by means of extended empirical orthogonal function (EEOF) analysis. The 3-sol VL1 oscillation corresponds to a tropical, eastward traveling, zonal wavenumber one pattern. The 6-sol VL2 oscillation is characterized by two midlatitude, eastward traveling, mixed zonal wavenumber one and two and zonal wavenumber three and four patterns, with respective periods near 6.1 and 5.5 sols. The corresponding phase speeds are in agreement with some of the conclusions drawn from the lander observations. A linear stability analysis of the zonally asymmetric climatology reveals that the two most unstable modes are associated with periods near 3 and 6 sols; with the corresponding eigenvectors showing patterns consistent with the results of the EEOF analyses.

The WKB ray-tracing formalism is extended to accommodate internal gravity waves of all frequencies in a rotating, stratified, and compressible three-dimensional atmosphere. This includes the derivation of equations governing the dispersion and refraction of the ray paths, a realistic wave amplitude equation that takes into account both radiative and turbulent damping effects, and extensions of previous wave saturation schemes to accommodate dynamical and convective instabilities along generally slanted axes. These equations have been numerically coded into a global ray-tracing model that the authors have applied to the three-dimensional CIRA 1986 reference atmosphere model in a series of preliminary experiments to investigate the impact of the newly incorporated features on synthesized wave fields in the middle atmosphere. Three main points emerge from these experiments, First, there is a striking reduction in the high-frequency cutoff with decreasing horizontal wavenumber due to a more complete dispersion relation. Second, adoption of a climatological, height-varying turbulent diffusivity profile based on measurements indicates that turbulent damping is more important than scale-dependent infrared radiative damping over a wide range of wavelengths and frequencies in all but the lower levels of the middle atmosphere. Last, the authors demonstrate that the presence of climatological planetary waves during the northern winter produces greatly varied ray paths for waves of fixed characteristics launched from different longitudes. The implications of these findings for future ray-tracing studies are discussed.

The vertical distribution of heat fluxes in the convective boundary layer may be modeled with time evolution of potential temperature profiles. In addition, time evolution of potential temperature profiles also helps in the analysis of characteristics of the boundary layer such as the depth of interfacial layer, mixed layer height, and the heat flux zero-crossing height.

The World Climate Research Programme Project for inter-comparison of Land Surface Parameterization Schemes (PILPS) is moving into its second and third phases that will exploit observational data and consider the performance of land surface schemes when coupled to their host climate models. The first stage of phase 2 will focus on an attempt to understand the large differences found during phase 1. The first site from which observations will be drawn for phase 2 intercomparisons is Cabauw, the Netherlands (51 [degrees] 58 [minutes] N, 4 [degrees] 56 [minutes] E), selected specifically to try to reduce one of the causes of the divergence among the phase 1 results: the initialization of the deep soil moisture. Cabauw's deep soil is saturated throughout the year. It also offers a quality controlled set of meteorological forcing and 160 days of flux measurements. PILPS phase 2 follows the form of the phase 1 intercomparisons: simple off-line integrations and comparisons, but in phase 2 participating schemes' results will be compared against observed fluxes. Preliminary results indicate that between model variability persists (i) in better specified experiments and (ii) in comparison with data. Although median values are consistent with observations, there is a large range among models. Phase 3, in which the intercomparison of PILPS schemes as a component of global atmospheric circulation models, is being conducted jointly with the Atmospheric Model Intercomparison Project (AMIP) as diagnostic subproject number 12. Preliminary results suggest that results differ by about the same range as in the off-line experiments in phases 1 and 2. Incomplete diagnostics suggest that bucket and canopy models differ and that variability among models can be tracked to the soil moisture parameterization. This paper offers a review of the PILPS project to date and an invitation to participate in PILPS' current and future activities.

A practical puff model, based on the Monin-Obukhov similarity theory, contains algorithms for efficiently ascertaining the factors determining air pollution diffusion. The model employs approximate solutions for the dispersion of clouds of passive pollutants from instantaneous sources.

A comparative assessment of the utility of two methodologies in determining the sameness scaling constants for wind, temperature and specific humidity, MARIAH and O'KEYPS, reveals the versatility of the former. The MARIAH methodology is simple to use as it can function in the absence of laborious, iterative schemes for ascertaining the diabatic influence functions, dimensionless lapse rate and wind shear.

An extended version of the two-way interactive grid-nesting technique in use in the Regional Atmospheric Modeling System developed at the Colorado State University includes stretched grids and a spatially variable grid-nesting ratio. This version is expected to be versatile in a wide variety of climatological assessment applications.

Usage of a time series, autoregressive order-1 model proved useful in maintaining the mean and standard deviation of temperature data sets from the Northwest Territories. This model can be of immense use in estimating winter temperatures at sites where temperature data sets are absent or inaccurate.

A spectral model was useful in computing possible solar irradiance in five broad bands including photosynthetically active, near infrared and ultraviolet-A above a green vegetation canopy. The model efficiently calculated unweighted potential, direct, diffuse and global irradiance by assessing factors such as molecular scattering and absorption by ozone.

Incorporation of differences in satellite-derived surface radiant temperature to a vegetation index while simultaneously combining this relationship to an inverse modeling scheme provides accurate estimates of regional characteristics of surface moisture availability (Mo) and fractional vegetation (Fr). Fr and Mo readings in the presence of spatially differing vegetation cover are of utility in hydrologic and atmospheric prediction models.

A matrix-free, noniterative family scheme is of utility in studying atmospheric photochemistry, as in assessments of the impacts of methane and ozone on global warming, and accurately calculating global climate changes. This modular photochemical numerics model is a marked improvement of matrix with iteration, non-matrix with iteration and non-matrix schemes of atmospheric chemical modeling.

Assessment of a one-and-a-half-order e-epsilon closure scheme, which uses Wangara 33d-day observations as initial conditions in studying planetary boundary layer development, reveals the model to be equal in quality to first- and high-order closure models in atmospheric data simulations. Added features in this scheme include its capacity to account for turbulence states for predictive equations of turbulence kinetic energy in better representing clouds.

Brightness temperature difference (BTD) simulations provide a versatile strategy of merging radiative transfer modeling and observational data estimates of cirrus clouds. Infrared optical depth of the cloud and near infrared-infrared BTD values need to be accurately estimated for efficient cirrus cloud particle size discrimination.

This paper compares accuracy for the daily averaged reflection, transmission, and absorption of solar flux derived from the delta-four-stream approximation and a few selected two-stream approximations. In the chosen variety of two-stream approximations, a two-stream approximation suggested by Zhu produces the most accurate reflection and transmission for conservative scattering with optical thickness [[Tau].sub.1] [is less than] 3. Otherwise, the delta-Eddington approximation is superior to other two-stream approximations. The delta-four-stream approximation produces errors that are smaller than the delta-Eddington approximation by at least a factor of 2.

Thermal infrared radiation is so incorporated into the discrete ordinate method as to be usable in climate models. A source term for thermal infrared radiation contains up to the second derivative, set to be a constant, of the Planck function with respect to optical thickness, and this treatment (quadratic approximation) is indispensable for ensuring reasonable accuracy in climate models. This is because the models prognostically yield not a continuous temperature profile but a discontinuous layer-mean temperature, from which temperatures at full level and half level are diagnostically determined. Then, the temperature profile presumed from these temperatures comes to show curvature (nonlinearity) within one layer where the layer-mean temperature is minimum or maximum. Assuming that the temperature profile between adjacent full levels is linearly represented with pressure, radiative-convective equilibrium temperatures are calculated with a four-stream approximation in order to examine the accuracy of the quadratic (QD-EXT) and linear (LN-EXT) approximations of the Planck function. Their results are compared with that of an 'exact' calculation, in which the number of layers is doubled. In addition, a comparison is also made for the method of treating isothermal temperatures in the maximum and minimum temperatures with the quadratic (QD-ISO) and linear (LN-ISO) approximations. It is demonstrated that the linear approximations, LN-EXT and LN-ISO, yield prominent two-grid noises with large errors in the troposphere. The quadratic approximations, QD-EXT and QD-ISO, on the other hand, show reasonable accuracy with small errors in the troposphere. QD-EXT is found to be the most suitable method for climate models because of its accuracy and simplicity.

The character of continental-interior paleoclimate at mid-latitudes, especially the aspect of temperature, has been a major source of debate in modeling and paleontologic communities over the past few years. A recent climate modeling study provides new insight into the issue of climatic conditions of early Eocene North America. Model cases with six times the present level of C[O.sub.2], or with doubled present C[O.sub.2] and a large lake in western North America, produced results most similar to proxy paleoclimate interpretations. It is significant that the North American continental-interior climate responded as strongly to the existence of the lake as to the atmospheric C[O.sub.2] level. The large lake deflects the winter-freeze line poleward of the region containing most paleoclimate data site locations, producing above-freezing winter temperatures and providing a possible solution to the minimum-temperature difference that exists between models and data. The effect of the lake upon regional climate is significant and proves to be critical to reproducing the early Eocene climate of North America.

There is a pressing need to develop a more accurate climate model for predicting the climate system response to possible greenhouse gas scenarios. It is suggested that simultaneous observations of diameter and luminosity variations can give a reasonable description of the variations between these quantities. The solar diameter is proposed as a proxy indicator of solar luminosity for the validation of climate models.

Diagnosis of the dynamic and thermodynamic balances using observed climatological monthly mean data reveals that 1) anisotropic, latitude-dependent Rayleigh friction coefficients lead to much improved modeling of the monthly mean surface wind field for a given monthly mean sea level pressure field, and 2) the annual variation of the vertically averaged lapse rate is important for modeling sea level pressure. Based on the aforementioned observations, a thermodynamic equilibrium climate model for the tropical Pacific is proposed. In this model, the sea level pressure is thermodynamically determined from sea surface temperature (SST) through a vertically integrated hydrostatic equation in which the vertical mean lapse rate is a function of SST plus a time-independent correction. The surface winds are then computed from sea level pressure gradients through a linear surface momentum balance with anisotropic, latitude-dependent Rayleigh friction coefficients. The precipitation is finally obtained from a moisture budget by taking into account the effects of SST on convective instability. Despite its simplicity, the model is capable of simulating realistic annual cycles as well as interannual variations of the surface wind, sea level pressure, and precipitation over the tropical Pacific. The success of the model suggests that the tropical atmosphere on a monthly mean time scale is, to the lowest-order approximation, in a thermodynamic equilibrium state in which sea level pressure is primarily controlled by SST and the effects of dynamic feedback on sea level pressure may be parameterized by an empirical SST-lapse rate relationship. Further studies are needed to establish a firm physical basis for the proposed parameterization.

A new global atmosphere model purpose designed for climate studies is introduced. The model is solved in terms of the normal modes of the linearized primitive equations on a sphere, which allows use of long time steps without introducing computational instability or phase errors of the linear wave components. The model is tested by attempting to simulate the tropical intraseasonal oscillation using an idealized sea surface temperature distribution. Simple treatments of radiation and boundary-layer processes are used together with the much more complete Betts-Miller convection scheme. The Betts-Miller scheme maintains the atmosphere in a state of near neutrality to reversible saturated ascent. It is found that for different values of the surface evaporation time scale, either the evaporation-wind feedback mechanism postulated by Neelin et al. and Emmanuel or low-level convergence of moisture can create eastward propagating deep convective models. In general, both mechanisms seem important, but it is the latter mechanism that provides phase speeds more in line with observations. Moisture convergence in this model works to erode the low-level equivalent potential temperature inversion that is ubiquitous in nonconvecting regions, thus triggering convection. In contrast to CISK models, changes in boundary-layer equivalent potential temperature are essential in this model to create propagating modes. The primary deficiency of the model is the tendency of the model to favor horizontal scales of convective disturbances that are much smaller than the zonal wavenumber one or two disturbances observed. This is related to the absence in the model of any pulsation of convection on an intraseasonal time scale over the warmest water regions that has been observed in satellite OLR data. Possible reasons for these differences are discussed.

The Project for Intercomparison of Land-surface Parameterization Schemes (PILPS) is described and the first stage science plan outlined. PILPS is a project designed to improve the parameterization of the continental surface, especially the hydrological, energy, momentum, and carbon exchanges with the atmosphere. The PILPS Science Plan incorporates enhanced documentation, comparison, and validation of continental surface parameterization schemes by community participation. Potential participants include code developers, code users, and those who can provide datasets for validation and who have expertise of value in this exercise. PILPS is an important activity because existing intercomparisons, although piecemeal, demonstrate that there are significant differences in the formulation of individual processes in the available land surface schemes. These differences are comparable to other recognized differences among current global climate models such as cloud and convection parameterizations. It is also clear that too few sensitivity studies have been undertaken with the result that there is not yet enough information to indicate which simplifications or omissions are important for the near-surface continental climate, hydrology, and biogeochemistry. PILPS emphasizes sensitivity studies with and intercomparisons of existing land surface codes and the development of a really extensive datasets for their testing and validation.

Suarez and Duffy have noted an interesting bifurcation in a two-level gridpoint general circulation model when strong tropical heating is imposed. This bifurcation results in a model climatology with strong upper-level westerlies in the tropics. In this paper, it is argued that this bifurcation is essentially due to the dominant role played by extratropical baroclinic transients in the tropical angular momentum budget. A series of numerical experiments is analyzed with a global two-level primitive equation model, using spectral truncation in the horizontal. The model climatologies in these experiments fall into two categories: 1) conventional, that is, weak upper-level easterlies/westerlies in the tropics; and 2) superrotating, that is, strong upper-level westerlies in the tropics. An attempt is made to explain the maintenance of the general circulation in these two radically different climatologies by studying the properties of unstable normal modes for the two different time-mean states. The spectral characteristics of angular momentum transport due to transient eddies in these two climatologies are also discussed. To understand the meridional propagation of transient eddies, the notion of a 'modal' refractive index in the quasigeostrophic approximation is introduced. From this analysis it is concluded that the conventional climatology is stable to weak perturbations, with the 'restoring' force being provided primarily by extratropical baroclinic eddies. Strong perturbations completely change the propagation characteristics of these eddies, leading to a bifurcation of the general circulation. This has interesting implications for the range of validity of two-level models and the transitivity of tropospheric general circulation.

A simplified vegetation distribution prediction scheme is used in combination with the Biosphere-Atmosphere Transfer Scheme (BATS) and coupled to a version of the NCAR Community Climate Model (CCM1) which includes a mixed-layer ocean. Employed in an off-line mode as a diagnostic tool, the scheme predicts a slightly darker and slightly rougher continental surface than when BATS' prescribed vegetation classes are used. The impact of tropical deforestation on regional climates, and hence on diagnosed vegetation, differs between South America and S.E. Asia. In the Amazon, the climatic effects of removing all the tropical forest are so marked that in only one of the 18 deforested grid elements could the new climate sustain tropical forest vegetation whereas in S.E. Asia in seven of the 9 deforested elements the climate could continue to support tropical forest. Following these off-line tests, the simple vegetation scheme has been coupled to the GCM as an interactive (or two-way) submodel for a test integration lasting 5.6 yr. It is found to be a stable component of the global climate system, producing only approximately 3% (absolute) interannual changes in the predicted percentages of continental vegetation, together with globally-averaged continental temperature increases of up to +1.5 degrees C and evaporation increases of 0 to 5 W m to the -2 and no discernible trends over the 67 months of integration. On the other hand, this interactive land biosphere causes regional-scale temperature differences of + or - 10 degrees C and commensurate disturbances in other climatic parameters. Tuning, similar to the q-flux schemes used for ocean models, could improve the simulation of the present-day surface climate but, in the longer term, it will be important to focus on predicting the characteristics of the continental surface rather than simple vegetation classes. The coupling scheme will also have to allow for vegetation responses occurring over longer timescales so that the coupled system is buffered from sudden shocks.

A model for the broad assessment of the fundamental aspects of the climate problem is presented. The model, called the integrated climate assessment model, version 1(ICAM-1), consists of several submodels that take into account such processess as economic growth, population growth, energy consumption and technological change and their influence on the global climate. Combined with the excellent skills of researchers, ICAM-1 is expected to be very useful for evaluating alternative policies concerning climate change.

Long-term changes in the Indian monsoon may be attributed to alterations in boundary conditions including tectonic structure, concentration of atmospheric carbon dioxide, glacial conditions and orbital parameters. A general circulation model was employed to show that elevation and orbital changes have the greatest effects on the monsoon. In particular, orbital forcing combined with abrupt geological uplift in the Himalayas and Tibetan plateau during the Miocene may have produced a rapid climactic response in the form of a strong monsoon that is consistent with paleoclimatic observations.

A nested regional climate model is used to generate a scenario of climate change over the MINK region (Missouri, Iowa, Nebraska, Kansas) due to doubling of carbon dioxide concentration (2 x C[O.sub.2]) for use in agricultural impact assessment studies. Five-year long present day (control) and 2 x C[O.sub.2] simulations are completed at a horizontal grid point spacing of 50 km. Monthly and seasonal precipitation and surface air temperature over the MINK region are reproduced well by the model in the control run, except for an underestimation of both variables during the spring months. The performance of the nested model in the control run is greatly improved compared to a similar experiment performed with a previous version of the nested modeling system by Giorgi et al. (1994). The nested model generally improves the simulation of spatial precipitation patterns compared to the driving general circulation model (GCM), especially during the summer. Seasonal surface warming of 4 to 6 K and seasonal precipitation increases of 6 to 24% are simulated in 2 x C[O.sub.2] conditions. The control run temperature biases are smaller than the simulated changes in all seasons, while the precipitation biases are of the same order of magnitude as the simulated changes. Although the large scale patterns of change in the driving GCM and nested RegCM model are similar, significant differences between the models, and substantial spatial variability, occur within the MINK region.

Significant environmental changes such as generation of heat, use and storage of water, land use and fossil fuel combustion could alter the balance of radiation on Earth thus causing global warming and associated climate change. Computer models called climate models have been devised to make projections of future climate however they cause systematic errors which affect the results. Experts are working hard to produce a climate system model which is more than the compilation of the components.

Recent studies show that the radiative-transfer models used in simulating the Earth's climate do not give accurate values of cloud absorption rates. The findings indicate that the clouds' capacity to absorb shortwave radiation is much higher than the value predicted by the radiative-transfer models.

While 300 scientists gathered in a former casino in Ensenada, Mexico, in late January to talk about global change, bulldozers outside were clearing debris from roads and river channels after [...]

The dynamical processes associated with block evolution are investigated by analyzing a GCM run, forced with perpetual January conditions. The core of the analysis lies on the temporal evolution of the blocks and on vorticity budget terms obtained from appropriate compositing procedures on a 350-mb model output. The results from the budget analysis are examined with barotropic model experiments, which allow the investigation of the influence of an individual dynamical process on block evolution. Results are presented for two composite blocks, one close to the Atlantic storm track and the other farther downstream. Although these two blocks are found to develop differently, they share the following characteristics. During the decay linear processes dominate, and the high- and low-frequency eddy fluxes contribute equally toward prolonging the lifetime of the blocks by 2 to 3 days. While the time average of the budget yields results that are consistent with previous diagnostic studies, it is shown that such an approach exaggerates the role played by high-frequency eddies. The barotropic model experiments show that the nonlinear self-interaction of the composite block anomaly plays a minimal role in the block evolution. It is the remaining part of the composite low-frequency eddy flux that contributes significantly toward the block evolution, indicating that case-to-case variability of the individual blocking events can be substantial, and that the nonlinearity of a slowly moving, nonsteady component of the flow plays an important role for the individual blocking events. The model experiments also demonstrate that the effect of divergence is crucial for correctly reproducing the structure of the blocking high. The implications of these results, as they apply to some of the prominent blocking theories, are also discussed.

Coupled general circulation models (CGCMs) integrate our knowledge about atmospheric and oceanic circulation. Different versions of CGCMs are used to provide a better understanding of natural climate variability on interannual and decadal time scales, for extended weather forecasting, and for making seasonal climate scenario projections, They also help to reconstruct past climates, especially abrupt climate change processes, Model intercomparisons, new test data (mainly from satellites), more powerful computers, and parameterizations of atmospheric and oceanic processes have improved CGCM performance to such a degree that the model results are now used by many decision-makers, including governments, They are also fundamental for the detection and attribution of climate change., Numerical models integrate our knowledge of certain fields of science, but they can only be as good as our understanding of all the processes involved. For weather and climate models, [...]

The quasi-equilibrium tropical circulation model (QTCM1) is implemented and tested. The formulation, described by Neelin and Zeng, uses a Galerkin framework in the vertical, but with basis functions tailored to quasi-equilibrium deep convective physics via analytical solutions. QTCM1 retains a single vertical structure of temperature and humidity. For a balanced treatment of dynamics and subgrid-scale physics, a physics parameterization package of intermediate complexity is developed. This includes a linearized longwave radiation scheme, a simple cloud prediction method, simple shortwave radiation schemes, and an intermediate land surface model. The simulated climatology has a reasonable spatial pattern and seasonal evolution of the tropical convergence zones, including over land regions. Outgoing longwave radiation and net surface heat flux both appear satisfactory. The Asian monsoon is slightly weak but depicts the northward progression of the monsoon onset, and a monsoon wind shear index exhibits interannual variability associated with observed SST that is similar to general circulation model (GCM) results. The extent and position of the main El Nino-Southern Oscillation rainfall anomalies are simulated, as well as a number of the observed tropical and subtropical teleconnections. The seasonal cycle and interannual variability of the Amazon water budget, including evapotranspiration, interception loss, and surface and subsurface runoff, illustrate reasonable simulation of the hydrologic cycle. Sensitivity studies on effects of topography, evaporation formulation, and land surface processes are also conducted. While the results are imperfect with respect to observations, many aspects are comparable to or better than GCMs of the previous generation. Considering the complexity of these simulated phenomena, the model is computationally light and easy to diagnose. It thus provides a useful tool filling the niche between GCMs and simpler models.

The middle atmosphere version of NCAR Community Climate Model (MACCM2) and the GEOS-STRATAN data assimilation system (DAS) of NASA/GSFC have been used to generate a zonally averaged set of transport parameters (meridional circulation and resolved eddy mixing) for use in two-dimensional (2D) chemistry transport models (CTMs). This was done by performing 3D transport numerical experiments with two orthogonal tracers. Some transport diagnostics from the MACCM2 and DAS are compared with each other, and with other approaches and estimates of horizontal mixing ([K.sub.yy]) obtained from satellite data and GCMs. Some differences in the derived [K.sub.yy] values for these passive tracer experiments and using the potential vorticity (PV) methods are discussed, as well as the annual and interannual variations, and the interhemispherical asymmetries in [K.sub.yy] structures. The set of 2D parameters obtained from the MACCM2 transport diagnostics has been used in a 2D CTM to simulate the distribution of [CH.sub.4], [N.sub.2]O, [O.sub.3], and the age of air (AOA). These 2D simulations compare well with the zonally averaged results obtained from 3D simulations of these constituents and for AOA in previous studies. The derived 2D transport parameters from the MACCM2 gave an annual cycle of constituent structure and values of mean age that compares well with those of the parent 3D model.

It is a fundamental tenet of the scientific method that theories must be consistent with observations. To test our understanding of the climate system, we must evaluate how accurately climate [...]

An examination is presented on the various processes and methodologies that will be used to obtain and compare data for the Third Assessment Report of the Intergovernmental Panel on Climate Change. Topics include mapping, the relevance of absolute values and the importance of conducting observations at different frequencies.

Some scientists attending the American Geophysical Union's second Chapman Conference on water vapour in Oct 1999 expressed doubts about the science on which climate modelling is based. Within the debate about modelling the climate system, water vapour plays a key role. Richard Lindzen of the Massachusetts Institute of Technology believes that the climate models are not correctly assessing the distribution and physical processes that occur when water vapour is transferred to the upper troposphere.

A simple four-cell model of the tropical atmosphere in equilibrium with its boundaries is introduced, which can support a variable diabatic circulation and prognostic temperature and humidity profiles. The model is used to predict atmospheric perturbations away from the observed base state. Prognostic variables include radiation, surface fluxes, and dynamic transports, with temperature and water vapor levels determined by conservation constraints. The model includes a specially developed water vapor scheme that performs favorably compared with observations. The model is used to simulate the local and nonlocal sensitivity of the tropical maritime atmosphere to changes in surface temperature and other boundary conditions at very large horizontal scales. The main findings are as follows. (i) The sensitivity of boundary layer convergence to sea surface temperature (SST) variations depends on the behavior of convective heating over cooler regions and may be overestimated by heuristic models that ignore or oversimplify thermodynamic and radiative constraints. (ii) The maintenance of humidity equilibrium over weakly convective areas is modulated by local radiative feedback. (iii) Evaporation feedbacks on SST may be overestimated by heuristic arguments that do not carefully treat atmospheric water transport. An explanation for the constant-relative humidity behavior of general circulation models under climate changes is also offered based on the results.

A workshop jointly sponsored by the International Institute of Theoretical and Applied Physics and University of Iowa's Center for Global and Regional Environmental Research endorsed to engage scientists from developing countries in researches focusing on the basic and applied sciences. Participants supported the move to provide these scientists with training and support to expand their knowledge and experience in dealing with issues concerned with the global climate and the environment.