The current generation of land-surface models used in GCMs view the soil column as the fundamental hydrologic unit. While this may be effective in simulating such processes as the evolution of ground temperatures and the growth/ablation of a snowpack at the soil plot scale, it effectively ignores the role topography plays in the development of soil moisture heterogeneity and the subsequent impacts of this soil moisture heterogeneity on watershed evapotranspiration and the partitioning of surface fluxes. This view also ignores the role topography plays in the timing of discharge and the partitioning of discharge into surface runoff and baseflow. In this paper an approach to land-surface modeling is presented that allows us to view the watershed as the fundamental hydrologic unit. The analytic form of TOPMODEL equations are incorporated into the soil column framework and the resulting model is used to predict the saturated fraction of the watershed and baseflow in a consistent fashion. Soil moisture heterogeneity represented by saturated lowlands subsequently impacts the partitioning of surface fluxes, including evapotranspiration and runoff. The approach is computationally efficient, allows for a greatly improved simulation of the hydrologic cycle, and is easily coupled into the existing framework of the current generation of single column land-surface models. Because this approach uses the statistics of the topography rather than the details of the topography, it is compatible with the large spatial scales of today’s regional and global climate models. Five years of meteorological and hydrological data from the Sleepers River watershed located in the northeastern United States where winter snow cover is significant were used to drive the new model. Site validation data were sufficient to evaluate model performance with regard to various aspects of the watershed water balance, including snowpack growth/ablation, the spring snowmelt hydrograph, storm hydrographs, and the seasonal development of watershed evapotranspiration and soil moisture.

The impact of doubled atmospheric CO₂ on the climate of the middle atmosphere is investigated using the GISS global climate/middle atmosphere model. In the standard experiment, the CO₂ concentration is doubled both in the stratosphere and troposphere, and the sea surface temperatures are increased to match those of the doubled CO₂ run of the GISS 9 level climate model. Additional experiments are run to determine how the middle atmospheric effects are influenced by tropospheric changes, and to separate the dynamic and radiative influences. These include the use of the greater high latitude/low latitude surface warming ratio generated by the Geophysical Fluid Dynamics Laboratory doubled CO₂experiments, doubling the CO² only in either the troposphere or stratosphere, and allowing the middle atmosphere to react only radiatively.

As expected, doubled CO² produces warmer temperatures in the troposphere, and generally cooler temperatures in the stratosphere. The net result is a decrease of static stability for the atmosphere as a whole. In addition, the 100 mb warming maximizes in the tropics, leading to improved propagation conditions for planetary waves, and increased potential energy in the lower stratosphere. These processes generate increased eddy energy in the middle atmosphere in most seasons. With greater eddy energy comes greater eddy forcing of the mean flow and an increase in the intensity of the residual circulation from the equator to the pole, which tends to warm high latitudes. Increased gravity wave drag in some of the experiments also helps to intensify the circulation. The middle atmosphere dynamical differences are on the order of 10%–20% of the model values for the current climate, and, along with the calculated temperature differences of up to some 10°C, may have a significant impact on the chemistry of the future atmosphere including that of stratospheric ozone, the polar ozone “hole,” and basic atmospheric composition.

The stratosphere accumulates the bulk of meteoric and cometary material that impacts the Earth and provides a measure of that flux. Recent models and measurements of the effective age of stratospheric air demonstrate our ability to simulate this buildup. Current observations of H₂O in the stratosphere and mesosphere are consistent with an extraterrestrial source of H₂O, but limit its influx to less than 2 Tg/yr. Such a flux is consistent with standard estimates but is 100 times less than the small-comet hypothesis.