Effects of Transient Eddies on Blocking Flows: General Circulation Model Experiments

Abstract

Prediction of blocking flows by a comprehensive general circulation model is still not satisfactory. A large portion of the unskillful forecasts can be traced to the model's inability to predict the evolution of blocking beyond a few days into the forecast. Realizing the fact that blocking is often observed to form following a series of intensive cyclogenesis activities and that the model tends to underestimate the intensity of the synoptic-scale transient eddies, a series of 10-day forecasts were conducted to assess the impact of transient eddies on the establishment of blocking flows. When the fast-propagating synoptic-scale disturbances were suppressed in the initial conditions, the subsequent forecasts completely failed to predict a blocking anticyclone. However, when the transient eddies were enhanced in the initial conditions to compensate for the deficiency of the model, blocking flows were predicted and evolved in remarkable agreement with the observations. The dynamical processes during establishment of blocking flow were then examined by a series of daily isentropic potential vorticity charts. The role played by the transient eddies can be identified through these charts, which help to explain why the transient eddies are crucial in establishing the blocking flows.