Ensemble-based analysis of factors contributing to the development of a multi-day warm-season heavy rain event

This study makes use of global ensemble forecasts from the THORPEX Interactive Grand Global Ensemble (TIGGE) archive to examine the factors contributing to, or inhibiting, the development of a long-lived continental vortex and its associated rainfall. From 25--30 June 2007, a vortex developed and grew upscale over the southern Plains of the United States. It was associated with persistent heavy rainfall, with over 100 mm of rain falling in much of Texas, Oklahoma, Kansas, and Missouri, and amounts exceeding 300 mm in southeastern Kansas. Previous research has shown that, in comparison with other rainfall events of similar temporal and spatial scale, this event was particularly difficult for numerical models to predict.

Considering the ensemble members as different possible realizations of the evolution of the event, several methods will be used to examine the processes that led to the development and maintenance of the long-lived vortex and its associated rainfall, and to its apparently limited predictability. Linear statistics are calculated to identify synoptic-scale flow features that were correlated to the precipitation, and differences between composites of "dry" and "wet" ensemble members are used to pinpoint the processes that were favorable or detrimental to the system's development. The maintenance of the vortex, and its slow movement in the southern Plains, is found to be closely related to the strength of a closed midlevel anticyclone in the southwestern US and the strength of a midlevel ridge in the northern Plains. In particular, with a weaker upstream anticyclone, the shear and steering flow over the incipient vortex are relatively weak, which allows for slow movement and persistent heavy rains. On the other hand, when the upstream anticyclone is stronger, there is stronger northerly shear and steering flow, which causes the incipient vortex to move southwestward into the high terrain of Mexico. These relatively small differences in the wind and mass fields early in the ensemble forecast, in conjunction with modifications of the synoptic and mesoscale flow by deep convection, lead to very large spread in the resulting precipitation forecasts.