Why We Tune Models

Why We Tune Models

Interactive graphic showing relationship between radiative balance and cloud cover

When we simulate the climate system, we often want to run models for a very long time. For meaningful results, we want no intrinsic drift in global climate. In other words, with constant boundary conditions, the simulated atmosphere/ocean/land system should neither warm nor cool over a long period (except for internal variability). If the amount of energy coming in equals that going out globally, there will be no tendency for the model to drift, to warm or cool. It will be in a steady state, resulting in a stable global control climate.

In a process akin to calibrating laboratory instruments to reduce measurement errors, modelers "tune" the model to achieve a steady state under constant boundary conditions. Once a stable control is established, only then can they design experiments to answer questions about the effects of changing those boundary conditions.

This interactive figure shows one way in which climate models can be tuned to achieve a stable climate. Adjusting the relative humidity threshold for cloud formation, one can increase or decrease the incoming solar radiation that is reflected back to space. If there is too little cloud cover, less incoming solar radiation will be reflected back to space, allowing too much solar radiation reaching the surface. As a result, the model climate will warm. With a long enough simulation, that system will eventually reach a new balance, but the resulting climate will be very warm.

NWP modelers investigating new operational NWP models also test, tune, and retest the models until the skill score of a new model is the same as or better than that of the old model.