Composite Analyses of Low-Skill Arctic Cyclones during Summer

Abstract:

Arctic cyclones (ACs) are synoptic-scale low pressure systems that frequently form over the Arctic or move into the Arctic from Eurasia during summer. Interactions between ACs and the synoptic-scale flow over the Arctic may be associated with alterations to the evolution of ACs and the synoptic-scale flow over the Arctic. It is anticipated that relatively low forecast skill of ACs and the synoptic-scale flow over the Arctic may be attributed in part to forecast error growth accompanying interactions between ACs and the synoptic-scale flow over the Arctic and the associated alterations to the evolution of ACs and the synoptic-scale flow over the Arctic. The purpose of this study is to examine various features and processes governing the evolution of ACs that are characterized by low forecast skill and that occur during periods of low forecast skill of the synoptic-scale flow over the Arctic during summer.

Forecast skill of the synoptic-scale flow over the Arctic is evaluated by calculating a standardized anomaly of area-averaged root mean square error (RMSE) of day-5 forecasts of 500-hPa geopotential height over the Arctic during summer (June–August) for the 2007–2017 period in the 11-member NOAA GEFS reforecast dataset version 2. Periods characterized by exceptionally high values of standardized anomaly of area-averaged RMSE are referred to as low-skill periods. ACs occurring during low-skill periods are identified using an ERA-Interim AC climatology. Forecast skill of ACs occurring during low-skill periods is evaluated by calculating the RMSE of the intensity and position of the ACs for forecasts in the GEFS reforecast dataset v2 that are of 1–7 day forecast lead time and valid at the time of the lowest sea level pressure of the ACs when located in the Arctic during low-skill periods. ACs characterized by relatively high values of RMSE of AC intensity and position are referred to as low-skill ACs. Composite analyses will be performed using the ERA5 on selected categories of low-skill ACs in order to examine various features (e.g., upper-tropospheric troughs and vortices, lower-tropospheric baroclinic zones, and warm conveyor belts) and processes (e.g., baroclinic processes, surface boundary processes, and latent heating) governing the evolution of low-skill ACs occurring during low-skill periods.