A new earth system climate model of intermediate complexity has been developed and its climatology compared to observations. The UVic Earth System Climate Model consists of a three-dimensional ocean general circulation model coupled to a thermodynamic/dynamic sea-ice model, an energy-moisture balance atmospheric model with dynamical feedbacks, and a thermomechanical land-ice model. In order to keep the model computationally efficient a reduced complexity atmosphere model is used. Atmospheric heat and freshwater transports are parametrized through Fickian diffusion, and precipitation is assumed to occur when the relative humidity is greater than 85%. Moisture transport can also be accomplished through advection if desired. Precipitation over land is assumed to return instantaneously to the ocean via one of 33 observed river drainage basins. Ice and snow albedo feedbacks are included in the coupled model by locally increasing the prescribed latitudinal profile of the planetary albedo. The atmospheric model includes a parametrization of water vapour/ planetary longwave feedbacks, although the radiative forcing associated with changes in atmospheric CO 2 is prescribed as a modification of the planetary longwave radiative flux. A specified lapse rate is used to reduce the surface temperature over land where there is topography. The model uses prescribed present-day winds in its climatology, although a dynamical wind feedback is included which exploits a latitudinally-varying empirical relationship between atmospheric surface temperature and density. The ocean component of the coupled model is based on the Geophysical Fluid Dynamics Laboratory (GFDL) Modular Ocean Model 2.2, with a global resolution of 3.6°(zonal) by 1.8°(meridional) and 19 vertical levels, and includes an option for brine-rejection parametrization. The sea-ice component incorporates an elastic-viscous-plastic rheology to represent sea-ice dynamics and various options for the representation of sea-ice thermodynamics and thickness distribution. The systematic comparison of the coupled model with observations reveals good agreement, especially when moisture transport is accomplished through advection. Global warming simulations conducted using the model to explore the role of moisture advection reveal a climate sensitivity of 3.0°C for a doubling of CO 2 , in line with other more comprehensive coupled models. Moisture advection, together with the wind feedback, leads to a transient simulation in which the meridional overturning in the North Atlantic initially weakens, but is eventually re-established to its initial strength once the radiative forcing is held fixed, as found in many coupled atmosphere General Circulation Models (GCMs). This is in contrast to experiments in which moisture transport is accomplished through diffusion whereby the overturning is reestablished to a strength that is greater than its initial condition. When applied to the climate of the Last Glacial Maximum (LGM), the model obtains tropical cooling (30°N-30°S), relative to the pr...

We investigate the ability of a global atmospheric general circulation model (AGCM) to reproduce observed 20 year return values of the annual maximum daily precipitation totals over the continental United States as a function of horizontal resolution. We find that at the high resolutions enabled by contemporary supercomputers, the AGCM can produce values of comparable magnitude to high quality observations. However, at the resolutions typical of the coupled general circulation models used in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, the precipitation return values are severely underestimated.

No abstract

Recently available satellite observations from the Atmospheric Infrared Sounder (AIRS) are used to calculate relative humidity in the troposphere. The observations illustrate many scales of variability in the atmosphere from the seasonal overturning Hadley-Walker circulation to high-frequency transient variability associated with baroclinic storms with high vertical resolution. The Asian monsoon circulation has a strong impact on upper-tropospheric humidity, with large humidity gradients to the west of the monsoon. The vertical structure of humidity is generally bimodal, with high humidity in the upper and lower troposphere, and a dry middle troposphere. The highest variances in humidity are seen around the midlatitude tropopause. AIRS data are compared to a simulation from a state-of-the-art climate model. The model does a good job of reproducing the mean humidity distribution but is slightly moister than the observations in the middle and upper troposphere. The model has difficultly reproducing many scales of observed variability, particularly in the Tropics. Differences in humidity imply global differences in the top of atmosphere fluxes of ϳ1 W m Ϫ2 .

Using existing measurements of streamflow data, we present a new data set of fiver discharges to the ocean, for use in validating climate models. This data set includes annual mean discharges for 981 rivers, representing approximately 824,500 m3/s of global river discharge. This is about 65% of contemporary estimates of continental precipitation minus continental evaporation (P-E). Using our new data set, we present a self-consistent water budget over continents. We find that rivers with annual mean discharges between about 250 m3/s and about 20,000 m3/s closely follow a power law size distribution. We estimate the total flow from rivers smaller than 250 m3/s by assuming they obey the same power law size distribution. Under this assumption, the estimated total flow from these rivers is about 33% of P-E. Including an existing estimate of soil moisture entering the ocean underground, our estimated total flow into the ocean is within about 2% of contemporary estimates of continental P-E. Our data set is available in digital format. flux of fresh water into the oceans affects the ocean thermohaline circulation. Land-surface hydrology partially controls the rates of growth and decomposition of most vegetation species, and hence affects the global carbon cycle. Despite its importance, large-scale hydrology is typically treated with less sophistication than other aspects of global climate models. One reason for this is that validation of largescale land-surface hydrologic calculations is difficult. Most of the physical processes being modeled (evapotranspiration, soil moisture content, surface runoff, etc.) are extremely heterogeneous, and hence are difficult to measure on the large scale; thus, it is difficult to determine if model calculations agree with the observed hydrologic properties.The hydrologic cycle in climate models can be validated in part by using observed river discharge data. Computed continental river discharge can be compared to measurements of river discharge at gauge stations near significant river outlets. If the simulated flows agree closely with the observed river discharges, then confidence in the simulated hydrologic system is increased.