Predecessor Rain Events ahead of recurving tropical cyclones: Using numerical simulations and ensemble forecasts to quantify the rainfall enhancement

Recent research has identified "predecessor rain events'' (PREs), in which mesoscale regions of heavy rainfall occur ~1000 km poleward and eastward of recurving tropical cyclones (TCs). PREs typically occur 24--36 h prior to the arrival of the main rainshield associated with the TC, and frequently result in significant flooding. A distinguishing feature of a PRE is that it is enhanced by deep tropical moisture directly associated with the TC that is transported well poleward ahead of the TC. Although this connection between tropical moisture and the remote rainfall is quite apparent from visual inspection of available observations and analyses, the actual contribution of the tropical moisture to the total rainfall is less clear.

This study will present two different approaches to quantifying the enhancement of rainfall associated with recurving TCs. First, the results from convection-permitting WRF-ARW simulations of the PRE ahead of TC Erin (2007) will be presented. The control simulation accurately represents the broad swath of precipitation that was observed in the upper Midwest, and the extreme local rainfall totals (>350 mm) that were observed in Minnesota and Wisconsin. A simulation in which the moisture attributable to TS Erin was removed shows a 25% reduction in the total precipitation and a 50% decrease in the maximum precipitation over the PRE region compared to the control simulation. The extent of this decrease in precipitation in the reduced-moisture run underscores the importance of moisture originating from TS Erin in transforming a heavy rain event into a high-impact, record-breaking rain event.

The second approach is to use global ensemble forecasts that have been made available via the THORPEX Interactive Grand Global Ensemble (TIGGE) project. Ensemble forecasts initialized several days before landfall, which include some recurving members and some that do not recurve, are compared to quantify the overall transport of moisture into midlatitudes by the TC. The implications of these results for uncertainties in medium-range forecasts of heavy midlatitude rainfall will also be addressed.