Physically Based Global Downscaling: Climate Change Projections for a Full Century

Steven J. Ghan Pacific Northwest National Laboratory, Richland, Washington

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Timothy Shippert Pacific Northwest National Laboratory, Richland, Washington

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Abstract

A global atmosphere–land model with an embedded subgrid orography scheme is used to simulate the period 1977–2100 using ocean surface conditions and radiative constituent concentrations for a climate change scenario. Climate variables simulated for multiple elevation classes are mapped according to a high-resolution elevation dataset in 10 regions with complex terrain. Analysis of changes in the simulated climate leads to the following conclusions. Changes in surface air temperature and precipitation differ from region to region in a manner similar to simulations without the subgrid scheme. Subgrid elevation contributes little to spatial variability of the change in temperature and the relative change in precipitation. In some regions somewhat greater warming occurs at higher elevations because of the same tendency in the free troposphere, but in others greater warming occurs near the melting level where snow albedo feedback amplifies the warming. Changes in snow water are highly dependent on altitude because of its nonlinear dependence on changes in the melting level. Absolute changes usually increase with altitude because more snow is currently available for depletion, but for extremely cold conditions the simulated warming is insufficient to increase melting. Relative changes in snow water always decrease with altitude as the likelihood that a warming will enhance melting or change the phase of precipitation decreases with decreasing temperature at higher altitudes. In places where snow accumulates, an artificial upper bound on snow water (which is required in any climate model that does not treat lateral snow transport) limits the sensitivity of snow water to climate change considerably. The simulated impact of climate change on regional mean snow water varies widely, with little impact in regions in which the upper bound on snow water is the dominant snow-water sink, moderate impact in regions with a mixture of seasonal and pemanent snow, and profound relative impacts on regions with little permanent snow.

Corresponding author address: Dr. Steven Ghan, Pacific Northwest National Laboratory, Mail Stop K9-30, P.O. Box 999, Richland, WA 99352. Email: steve.ghan@pnl.gov

Abstract

A global atmosphere–land model with an embedded subgrid orography scheme is used to simulate the period 1977–2100 using ocean surface conditions and radiative constituent concentrations for a climate change scenario. Climate variables simulated for multiple elevation classes are mapped according to a high-resolution elevation dataset in 10 regions with complex terrain. Analysis of changes in the simulated climate leads to the following conclusions. Changes in surface air temperature and precipitation differ from region to region in a manner similar to simulations without the subgrid scheme. Subgrid elevation contributes little to spatial variability of the change in temperature and the relative change in precipitation. In some regions somewhat greater warming occurs at higher elevations because of the same tendency in the free troposphere, but in others greater warming occurs near the melting level where snow albedo feedback amplifies the warming. Changes in snow water are highly dependent on altitude because of its nonlinear dependence on changes in the melting level. Absolute changes usually increase with altitude because more snow is currently available for depletion, but for extremely cold conditions the simulated warming is insufficient to increase melting. Relative changes in snow water always decrease with altitude as the likelihood that a warming will enhance melting or change the phase of precipitation decreases with decreasing temperature at higher altitudes. In places where snow accumulates, an artificial upper bound on snow water (which is required in any climate model that does not treat lateral snow transport) limits the sensitivity of snow water to climate change considerably. The simulated impact of climate change on regional mean snow water varies widely, with little impact in regions in which the upper bound on snow water is the dominant snow-water sink, moderate impact in regions with a mixture of seasonal and pemanent snow, and profound relative impacts on regions with little permanent snow.

Corresponding author address: Dr. Steven Ghan, Pacific Northwest National Laboratory, Mail Stop K9-30, P.O. Box 999, Richland, WA 99352. Email: steve.ghan@pnl.gov

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