Predictability and Dynamics of the Genoa Low

The rapid development and sub-synoptic scale nature of the Genoa low in the Mediterranean Sea poses a forecasting challenge for United States Air Force (USAF). The Genoa low is a high-impact event for several Department of Defense (DOD) locations located in southern Europe, especially in the Po River Valley of northern Italy. This study evaluates the predictability and dynamics of the Genoa low extending to a 4-day event lead time as is required by the mission protocols at the affected locations. Two Genoa low case studies are analyzed: 16 Feb 2015 (case 1), and 13 July 2016 (case 2), using the COnsortium for Small-scale MOdeling Limited-area Ensemble Prediction System (COSMO-LEPS). Ensemble prediction systems provide a range of possible forecast outcomes given the uncertainty in initial conditions, boundary conditions, as well as model physics. As such, ensembles are used to assess and analyze the predictability of the Genoa low. The analysis demonstrates several key findings concerning the Genoa low. The Genoa low is only weakly predictable at a lead time of 4 days. It is shown that only a small fraction of ensemble members (approximately 25%) met the Genoa low verification thresholds at this lead time. Ensemble spaghetti plots and maps of the ensemble variance show that the possibility of low formation at longer lead times is most effectively visualized using maps of ensemble variance. Traditional postage-stamp plots and minimum MSLP plots contain too much noise and variability to permit a forecaster to extract a signal indicating possible low formation. The formation of the Genoa low is associated with strong mistral winds. It is demonstrated that all ensemble simulations that were successful in identifying cyclogenesis also produce strong mistral winds, i.e., the strength of the mistral winds is anti-correlated to the minimum MSLP of the Genoa low. This linkage implies a potential dynamical connection between the two features. Further investigation shows that the mistral jet may exert an organizing influence on the Genoa low via a vorticity seeding mechanism. Time-lagged correlations show that the mistral jet amplifies several hours prior to cyclogenesis. The amplification is associated with mesoscale vorticity generation on the eastern periphery of the jet. These vorticity centers were subsequently shed into the target region where cyclogenesis occurs. Such a small-scale and rapid-developing dynamical link between the mistral winds and the Genoa low implies a limit on the predictability of the Genoa low. This study concludes that weather forecasting operations in the USAF would benefit from expansion of current ensemble prediction systems, not only for the purpose of improving the Genoa low forecast process and performance but also to better inform the mission planners of the limitations and uncertainties of predicting the Genoa low.