Exploring the Intrinsic Predictability of an Arctic Polar Low

Polar lows are intense, mesoscale, and short-lived maritime cyclones that can develop when extremely cold polar air flows over the relatively warm sea surface, producing vigorous air-sea interactions, moist convection, and winds as strong as hurricane force. Accurately forecasting these hazardous, small-scale cyclones is a major challenge, even at relatively short 1-2-day lead times, yet predictability limits for polar lows are not well understood. In this study we investigate the intrinsic predictability (i.e., forecast limits arising from a perfect model with nearly perfect initial conditions) of an Arctic polar low from March 2024. The growth of small-amplitude initial-condition perturbations is driven by moist processes, with faster error-growth rates at smaller scales and in convectively unstable regions in the vicinity of the polar low. Differences in latent heating produce differences in the divergent wind component that spread outward in a manner resembling gravity waves, eventually producing slower growth at larger scales in the rotational wind component. This general picture of perturbation growth is consistent with previous studies of midlatitude cyclones, but in the polar low the growth tends to be slower and shallower, likely due to weaker convective instability and upper-level forcing. Also, the horizontal scale of the polar low is smaller than that of synoptic-scale cyclones, yet its intensity and location remain mostly unchanged by the perturbation growth over the course of its 1-day life cycle. Instead, the differences are primarily localized to the displacement of individual convective cells. These results suggest that, at lead times on the order of a polar-low life cycle, the convective-scale predictability can be intrinsically limited by the growth of unobservable small-amplitude initial-condition errors, but that forecasts of the polar low at large can be improved via initial-condition-error reductions at larger amplitudes and scales.