Igbp/gaim Report Series Report #4 Atmospheric Tracer Transport Model Intercomparison Project (transcom) a Special Project of Igbp-gaim G L B a L O Change

2 Overview The goal of Atmospheric Tracer Transport Model Intercomparison Project (TransCom) is to quantify and diagnose the uncertainty in inversion calculations of the global carbon budget that result from errors in the simulated transport. The specific objectives of the TransCom project are (1) quantify the degree of uncertainty in current carbon budget estimates that results from uncertainty in model transport; (2) identify the specific sources of uncertainties in the models; and (3) identify key areas to focus future transport model development and improvements in the global observing system that will reduce the uncertainty in carbon budget inversion calculations. Our initial intercomparison of global transport models used in the CO 2 inversion problem revealed that inversion estimates of some carbon budget components may currently be uncertain by about a factor of two due to transport alone [Law et al., 1996; Rayner and Law, 1995]. Because the models used in this exercise also form the dynamical core of many models of reactive chemical species, this problem is also of serious concern to those in the global atmospheric chemistry community. The project is part of a larger GAIM research program which aims to develop coupled ecosystem-atmosphere models that describe time evolution of trace gases with changing climate and changes in anthropogenic forcing. Atmospheric chemical tracer transport models (CTMs) serve three crucial functions in the development, testing, and validation of global Earth system models: 1) predictions of trace gas fluxes at the Earth's surface may be used to drive CTMs and the resulting simulations of atmospheric concentrations may be compared to observations to test Earth system models; 2) trace gas fluxes at the surface may be calculated from observations of atmospheric concentration "inversion" of the data with a CTM, improving process-level understanding and directly validating Earth system models; and 3) simulation of the fate and temporal evolution of reactive trace gases such as methane (CH 4) and nitrous oxide (N 2 O) requires a detailed atmospheric chemistry module in Earth system models, which includes both transport and chemical transformation. An important source of uncertainty in these calculations is the simulated transport itself, which varies among the many transport models used by the community. TransCom investigators have conducted a series of 3-dimensional tracer model intercomparison experiments with leading transport codes which are intended to (1) quantify the degree of uncertainty in current carbon budget estimates that results from uncertainty in model transport; (2) identify …