Continental Convection, Rossby Wave Packets and Regime Change in the Middle Latitude and Arctic

This study continues a series of investigations into the dynamical origins of medium range (6-day) forecast busts over Europe in the ECMWF model. The goal of this research is to advance knowledge of the barriers blocking the path to improving predictive skill in the middle latitudes and the Arctic. The motivation for this study includes the Rodwell et al. (2013) investigation of forecast busts that showed these drop-outs in skill were associated with a coherent flow over North American with a trough over the Rockies and significant CAPE (Convective Available Potential Energy) to the east. A follow-on study by Lillo and Parsons (2017) revealed that dynamics of the busts varied from the mean conditions described in Rodwell et al. (2013), but were likely associated with the triggering of Rossby wave packets and subsequent Rossby wave breaking. Their study also concluded that large forecast errors associated with the busts was due to a regime change in the atmosphere that was not represented in the model. One hypothesis that could be drawn from the Lillo and Parsons study is that the busts were associated with the inability of the model to represent the triggering of Rossby waves. This hypothesis will be explored in this talk through the use of a phase independent analysis of the model error and the error in the wave activity flux (Takaya and Nakamura 2000). Our preliminary findings from several case studies are that the error source is tied to the inability of the model to represent the triggering of Rossby waves by deep convection. The wave activity flux analysis suggests that the Rossby wave packets initiated by deep convection and lacking in the model propagate to the east and subsequently break over the North Atlantic. Cases under investigation and the mean differences between bust and "good" forecasts illustrates wave break can inject lower Potential Vorticity air into the Arctic and modify the polar vortex. To date, the impact of Rossby wave breaking on the Arctic has focused on the transport of sensible and latent heat into the Arctic and the role of warm and moist air in melting sea ice. This work suggests a dynamical impact of wave breaking on the Arctic that significantly modifies the polar vortex including the likely location of both cyclogenesis in the Arctic and middle latitude weather. These findings suggest that improved representation of convection in middle latitudes and other events that trigger Rossby wave packers are a barrier to improved forecast skill in the middle latitude and the Arctic.