13 Jul 2022
13 Jul 2022

Bimodality in Ensemble Forecasts of 2-Meter Temperature: Event Aggregation

Cameron Bertossa1, Peter Hitchcock1, Arthur DeGaetano1, and Riwal Plougonven2 Cameron Bertossa et al.
  • 1Dept. Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, USA
  • 2LMD-IPSL, Ecole Polytechnique, Institut Polytechnique de Paris, ENS, PSL Research University, Sorbonne Université, CNRS, Paris, France

Abstract. A previous study has shown that a large portion of subseasonal-to-seasonal European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble forecasts for 2-meter temperature exhibit properties of bimodality, in some locations reaching occurrence rates of over 30 %. This study introduces novel methodology to help identify `bimodal events', meteorological events which trigger the development of widespread bimodality in forecasts. Understanding such events not only provides insight into the dynamics of the meteorological phenomena causing bimodal events, but also has drastic implications for the skill of forecasts affected. The methodology that is developed allows one to systematically characterize the spatial and temporal scales of the derived bimodal events, and thus uncover the flow states that lead to them. Three distinct regions that exhibit high occurrence rates of bimodality are studied: one in South America, one in the Southern Ocean and one in the North Atlantic. It is found that each region's bimodality appears to be triggered by large-scale atmospheric processes interacting with geographically specific processes: In South America bimodality is related to an atmospheric wave interacting with the Andes, in the Southern Ocean bimodality is related to an atmospheric wave interacting with sea ice, and in the North Atlantic bimodality is connected to an atmospheric wave deforming near the Gulf Stream. This common pattern of large-scale circulation anomalies interacting with local boundary conditions suggests that any deeper dynamical understanding of these events should incorporate such interactions.

Cameron Bertossa et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Cameron Bertossa et al.

Cameron Bertossa et al.


Total article views: 216 (including HTML, PDF, and XML)
HTML PDF XML Total BibTeX EndNote
156 51 9 216 2 1
  • HTML: 156
  • PDF: 51
  • XML: 9
  • Total: 216
  • BibTeX: 2
  • EndNote: 1
Views and downloads (calculated since 13 Jul 2022)
Cumulative views and downloads (calculated since 13 Jul 2022)

Viewed (geographical distribution)

Total article views: 189 (including HTML, PDF, and XML) Thereof 189 with geography defined and 0 with unknown origin.
Country # Views %
  • 1
Latest update: 29 Nov 2022
Short summary
This work has identified characteristic spatial and temporal scales for non-Gaussian outbreaks in forecasts, specifically, bimodality. Methodology is introduced which allows one to connect meteorological phenomena to bimodal outbreaks. Large-scale circulation interacting with local processes is uncovered as a frequent ingredient to such outbreaks. These insights not only provide a deeper understanding of the dynamical processes involved, but also have drastic implications for forecast skill.