Abstract
The impacts of initial condition errors on the forecasted intensity and location of two polar lows (PLs) occurring over the Norwegian-Barents Sea region are evaluated by applying ensemble sensitivity analysis (ESA) to 48-hour ECMWF ensemble forecasts valid at the time of the PL maximum intensity. The two PL cases represent opposite ends of the so-called PL spectrum and are chosen based on the observed cloud morphology and large-scale atmospheric conditions. The comma-cloud case is characterized by strong, deep tropospheric baroclinicity, while the spiraliform case exhibits weaker, low-level baroclinicity. Both cases develop in connection with a marine cold-air outbreak and an associated upper-level trough. Our analysis demonstrates the importance of the marine cold-air outbreak (MCAO) temperature (based on the upper-level geopotential height) to the intensity of both systems, but the comma-cloud case is more sensitive to the strength of environmental baroclinicity. The forecasted location of both systems is sensitive to the representation of the synoptic-scale circulation, with the positioning of the upper-level trough ultimately determining where the PL forms. Our analysis suggests that PL location is more predictable than PL intensity, and we show how ensemble prediction might be utilized to understand uncertainty in real-time forecasts of PLs.
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