CONICET | Buscador de Institutos y Recursos Humanos

Cut-off lows (COLs) are synoptic scale systems that may cause extreme rainfall events producing flash floods over southern South America. The main goal of this work is to analyze how numerical weather prediction models forecast the circulation and precipitation associated to COLs events. Three case studies are carried out using two state of the art global models: WRF version 3.0 and the Global Forecast System (GFS). To study the predictability associated with these events several numerical forecasts initialised at different times are verified. An object oriented verification applied to the 500 hPa geopotential heigth anomaly is used for the quantification of the COL location and intensity errors. An object associated with a COL is defined in the forecasts and in the Global Data Assimilation System analysis using the same criteria. Once the object is defined in both data sets, differences in position, size, temperature anomaly, strength between the analysed object and the forecasted object are computed. The quality of the precipitation forecasts is examined using a score derived from the fuzzy verification techniques: the Fraction Skill Score computed using the CPC MORPHing Technique precipitation estimates. Results show that both models could represent the segregation phase of the system. However in one of these case studies WRF model shows some difficulties to represent the evolution of the cut off low, indicating a faster eastward propagation. The skill of the models in representing the main features of the life cycle of the cut off lows and its associated precipitation seems to be case dependent. The experiments also indicate that the skill in the forecasted precipitation is very sensitive to errors in the position and strength of the cut off low. The advantages and limitations of the verification strategies used in this work are also described.

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