Effects of resolution and model physics on tracer transports in the NASA Goddard Institute for Space Studies general circulation models

Abstract

We explore the dependency of general circulation model tracer transports on model physics and horizontal and vertical resolution. We use NASA Goddard Institute for Space Studies (GISS) Model E at 4° × 5° with 20 and 23 layers and the GISS Global Climate Middle Atmosphere Model 3 at 4° × 5° with 23 and 53 layers and at 2° × 2.5° with 53 and 102 layers. The online tracers employed are CO2, CH4, N2O, CFC‐11, SF6, 222Rn, bomb 14C, and O3. Model experiments are done two ways: with specified stratospheric ozone or with the stratospheric ozone tracer used for atmospheric radiation calculations. The results show that when model physics produces greater precipitation over land in the Northern Hemisphere summer monsoon region, as occurs in Model 3, the associated dynamics (stronger Hadley cell) and subgrid‐scale transports lead to faster and more realistic interhemispheric transport. Increased vertical resolution results in some increase in vertical mixing between the boundary layer and upper troposphere, due to both convective and synoptic‐scale influences. A better resolved boundary layer does not result in higher surface concentrations, as the influence of various processes (convection, turbulence, rainfall) contribute in different ways. Transport into, within, and out of the stratosphere is faster (less realistic) with the coarser resolution models as wave forcing generates a stronger residual circulation. It is also faster in Model E as a result of its larger parameterized orographic gravity wave drag; the latter also results in a more “leaky” stratospheric tropical pipe. Horizontal resolution in this range by itself has minimal impact on most transports (although for active chemical tracers, photochemistry has been shown to be resolution‐dependent). In contrast, finer vertical resolution leads to faster interhemispheric transport, slower mixing into and out of the stratosphere, and greater age of stratospheric air. When both resolutions are increased, the largest changes are seen. The interactive stratospheric ozone tracer, without an ozone hole parameterization, produced (as expected) greater ozone values than observations in the lower stratosphere. The associated temperature warming of a few degrees Celsius increased atmospheric stability and altered the tropospheric wave forcing of the Brewer Dobson circulation such that the stratospheric age of air increased by some 30%. This large sensitivity has implications for past and future stratospheric circulations and for the ability of climate perturbations to affect the stratosphere.