Search Results

You are looking at 1 - 9 of 9 items for

  • Author or Editor: Starley L. Thompson x
  • Refine by Access: All Content x
Clear All Modify Search
Starley L. Thompson and David Pollard

Abstract

The present-day climatology of a global climate model (GENESIS Version 1.02) is described. The model includes a land-surface transfer component (LSX) that accounts for the physical effects of vegetation. The atmospheric general circulation model is derived from the NCAR CCM1 and modified to include semi-Lagrangian transport of water vapor, subgrid plume convection, PBL mixing, a more complex cloud scheme, and a diurnal cycle. The surface models consist of LSX; multilayer models of soil, snow, and sea ice; sea ice dynamics; and a slab mixed layer ocean. Brief descriptions of the current model components are included in an appendix. GENESIS is an ongoing project to develop an earth system model prototype for global change research. The Version 1.02 climate model has already proved useful in paleoclimate studies.

Results of present-day simulations are described using an atmospheric spectral resolution of RIS (∼4.5° lat×7.5° long) and a surface-model resolution of 2°×2°. In general the quality of the simulations is comparable to that of previous coarse-grid models with predicted sea-surface temperatures. Most of the errors are attributed to coarse atmospheric resolution, inaccurate cloud parameterization, large ocean roughness length, and lack of ocean dynamics.

The results are compared with those using a simplified bucket-soil model and crude parameterizations of surface albedo and roughness. Although quite similar results are obtained on global scales, significant regional differences including surface warming and drying occur in some regions of Amazonia and northern midlatitude continental interiors.

Full access
Starley L. Thompson and David Pollard

Abstract

As anthropogenic greenhouse warming occurs in the next century, changes in the mass balances of Greenland and Antarctica will probably accelerate and may have significant effects on global sea level. Recent trends and possible future changes in these mass balances have received considerable attention in the glaciological literature, but until recently relatively few general circulation modeling (GCM) studies have focused on the problem. However, there are two significant problems in using GCMs to predict mass balance distributions on ice sheets: (i) the relatively coarse GCM horizontal resolution truncates the topography of the ice-sheet flanks and smaller ice sheets such as Greenland, and (ii) the snow and ice physics in most GCMs does not include ice-sheet-specific processes such as the refreezing of meltwater.

Two techniques are described that attack these problems, involving (i) an elevation-based correction to the surface meteorology and (ii) a simple a posteriori correction for the refreezing of meltwater following Using these techniques in a new version 2 of the Global Environmental and Ecological Simulation of Interactive Systems global climate model, the authors present global climate and ice-sheet mass-balance results from two equilibrated runs for present and doubled atmospheric CO2. This GCM is well suited for ice-sheet mass-balance studies because (a) the surface can be represented at a finer resolution (2° lat × 2° long) than the atmospheric GCM, (b) the two correction techniques are included as part of the model, and (c) the model’s mass balances for present-day Greenland and Antarctica are realistic.

When atmospheric CO2 is doubled, the net annual surface mass balance decreases on Greenland from +13 to −12 cm yr−1 and increases on Antarctica from +18 to +21 cm yr−1. The corresponding changes in the ice-sheet contributions to global sea level are +1.2 and −1.3 mm yr−1, respectively, yielding a combined contribution of −0.1 mm yr−1. That would be a very minor component of the total sea level rise of ∼5 mm yr−1 expected in the next century, mainly from thermal expansion of the oceans and melting of smaller glaciers. However, biases in the GCM climate suggest a range of uncertainty in the ice-sheet contribution from about −2 to +1 mm yr−1.

Full access
Starley L. Thompson and David Pollard

Abstract

The sensitivity of the equilibrium climate to doubled atmospheric CO2 is investigated using the GENESIS global climate model version 1.02. The atmospheric general circulation model is a heavily modified version of the NCAR CCM1 and is coupled to a multicanopy land-surface model (LSX); multilayer models of soil, snow, and sea ice; and a slab ocean mixed layer. Features that are relatively new in CO2 sensitivity studies include explicit subgrid convective plumes, PBL mixing, a diurnal cycle, a complex land-surface model, sea ice dynamics and semi-Lagrangian transport of water vapor.

The global annual surface-air warming in the model is 2.1°C, with global precipitation increasing by 3.3%. Over most land areas, most of the changes in precipitation are insignificant at the 5% level compared to interannual variability. Decreases in soil moisture in summer are not as large as in most previous models and only occur poleward of ∼55°N in Siberia, northern Canada, and Alaska. Sea ice area in September recedes by 62% in the Arctic and by 43% in the Antarctic. The area of Northern Hemispheric permafrost decreases by 48%, while the total area of Northern Hemispheric snowcover in January decreases by only 13%.

The effects of several modifications to the model physics are described. Replacing LSX and the multilayer soil with a single-layer bucket model causes little change to CO2 sensitivities on global scales, and the regions of summer drying in northern high latitudes are reproduced, although with somewhat greater amplitude. Compared to convective adjustment, penetrative plume convection increases the tropical Hadley Cell response but decreases the global warming slightly by 0.1° to 0.30°, contrary to several previous GCM studies in which penetrative convection was associated with greater CO2 warming. Similarly, the use of a cruder parameterization for cloud amount changes the local patterns of cloud response but has slight effect on the global warming. The authors discuss implications of the greater global warming (3.2°C) found in an earlier version of the model and suggest that it was due to more detailed interactions that no longer occur in the current version.

Full access
Starley L. Thompson and Dennis L. Hartmann

Abstract

abstract not available.

Full access
Starley L. Thompson and Dennis L. Hartmann

Abstract

A simple diagnostic study is performed in order to examine the influence of parameterized westerly momentum exchange by cumulus convection on the observed Hadley circulation. Observed precipitation and 200 mb winds are used to diagnose the mean 100–200 mb tropical cumulus friction using the parameterization proposed by Schneider and Lindzen (1976). By using seasonal data on a latitude-longitude grid, we estimate not only the annual (standing) zonally symmetric cumulus friction but the seasonal (transient) and zonally asymmetric contributions as well. In general, the standing zonally symmetric contribution dominates the total cumulus friction, but other components are significant at certain latitudes. The meridional mass flux induced by cumulus friction is estimated through the use of an approximate u-momentum equation. The diagnosed meridional mass transports are about an order of magnitude smaller than those which are observed by direct measurements. Therefore it appears unlikely that cumulus friction plays a dominant role in maintaining the tropical mean meridional circulation.

Full access
Starley L. Thompson and Stephen G. Warren

Abstract

Correction to Volume 39, Issue 12, Article 2667.

Full access
Starley L. Thompson and Stephen G. Warren

Abstract

State-of-the-art radiative transfer models can calculate outgoing infrared (IR) irradiance at the top of the atmosphere (F) to an accuracy suitable for climate modeling given the proper atmospheric profiles of temperature and absorbing gases and aerosols. However, such sophisticated methods are computationally time consuming and ill-suited for simple vertically-averaged models or diagnostic studies. The alternative of empirical expressions for F is plagued by observational uncertainty which forces the functional forms to be very simple. We develop, a parameterization of climatological F by curve-fitting the results of a detailed radiative transfer model. The parameterization comprises clear-sky and cloudy-sky terms. Only two parameters are used to predict clear-sky outgoing IR irradiance: surface air temperature (T s) and 0–12 km height-mean relative humidity (^RH). With this choice of parameters (in particular, the use of ^RH instead of precipitable water) the outgoing IR irradiance can be estimated without knowledge of the detailed temperature profile or average lapse rate. Comparisons between the clear-sky parameterization and detailed model show maximum errors of ∼10 W m−2 with average errors of only a few watts per square meter. Single-layer “black” clouds are found to reduce the outgoing IR irradiance (relative to clear-sky values) as a function of T sT c, T c and ^RH, where T c is the cloud-top temperature. Errors in the parameterization of the cloudy-sky term are comparable to those of the clear-sky term.

Full access
H. Nuzhet Dalfes, S. H. Schneider, and Starley L. Thompson

Abstract

A zonally averaged energy balance climate model is used to generate zonal temperature variability through fluctuating meridional energy transports. In the base model, stochastic transport fluctuations are introduced by multiplying the eddy diffusion coefficients by Gaussian random deviates. For eddy coefficient variability of 50%, the base model generates an interannual temperature variability of 0.03 K for the global temperature, and 0.04 and 0.05 K for the Northern and Southern Hemispheric temperatures, respectively. The sensitivity to modeling assumptions of the model generated variability and its meridional distribution are investigated through a series of numerical experiments. For the range studied, the temperature variability level generated is linearly related to the transport variability level introduced. Switching from the multiplicative noise model of the base to an additive noise model results in an increase in the level of model generated temperature variability and a change in the shape of variance spectra of temperature anomaly time series. These model results are compared with a time series of central England temperatures as well as GCM generated climate variability. Because the level of variability is so dependent on the form of stochastic forcing parameterization, we conclude that great caution is needed before ascribing physical reality to such stochastic fluctuations.

Full access
Gordon B. Bonan, David Pollard, and Starley L. Thompson

Abstract

The statistical representation of multiple land surfaces within a grid cell has received attention as a means to parameterize the nonlinear effects of subgrid-scale heterogeneity on land-atmosphere energy exchange. However, previous analyses have not identified the critical land-surface parameters to which energy exchanges are sensitive; the appropriate number of within-grid-cell classes for a particular parameter, or the effects of interactions among several parameters on the nonlinearity of energy exchanges. The analyses reported here used a land-surface scheme for climate models to examine the effects of subgrid variability in leaf area index, minimum and maximum stomatal resistances, and soil moisture on grid-scale fluxes. Comparisons between energy fluxes obtained using parameter values for the average of 100 subgrid points and the average fluxes for the 100 subgrid points showed minor differences for emitted infrared radiation and reflected solar radiation, but large differences for sensible heat and evapotranspiration. Leaf area index was the most important parameter; stomatal resistances were only important on wet soils. Interactions among parameters increased the nonlinearity of land-atmosphere energy exchange. When considered separately, six to ten values of each parameter greatly reduced the deviation between the two flux estimates. However, this approach became cumbersome when all four parameters varied independently. These analyses suggest that the debate over how to best parameterize the nonlinear effects of subgrid-scale heterogeneity on land-atmosphere interactions will continue.

Full access