Addressing the causes of large-scale circulation error in the Met Office Unified Model

Abstract

The Met Office Unified Model develops a systematic error consisting of a large high-pressure bias over the North Pole. The error begins to develop early in numerical weather prediction forecasts and is the leading large-scale circulation error in the model's climatological mean, with implications for principle circulation regimes and blocking. The cause of the error is investigated using a variety of diagnostic techniques, including analysis of a perturbed parameter ensemble for the model, evaluation of changes in both weather and climate simulations to minimise the risk of introducing compensating errors, and detailed comparisons with another model. A reduction in this systematic error is achieved through an overall reduction in the near-surface drag; however, to accomplish this without affecting forecast predictability adversely, a redistribution of the drag is required between the roughness from vegetation in the land-surface scheme, the near-surface drag from the orographic gravity-wave drag scheme, the turbulent orographic form drag, and the resolved drag (through changing the orographic filtering). A modest reduction in the climatological bias is achieved, along with a reduction in bias and improved predictability of near-surface winds.