Prediction of rainfall onset using a newly formulated potential vorticity intrusion index

摘要

Invertibility and conservativeness of potential vorticity (PV) make it applicable in understanding and prediction of atmospheric dynamics. Bridging of conservativeness (intrusion) of PV reveals the presence of convection and diabatic heating. Since strong PV intrusion events do not occur in summer, rainfall over West Africa has not been linked with PV intrusion. This paper shows that a new formulated PV intrusion index from the upper-level westerly duct within the Atlantic Ocean to the troposphere is applicable to rainfall onset detection at the Central Guinea Coast in West Africa. The PVI index has a significant period of 12.5 months and the highest values of PVI index are found at the beginning of the year. The 1 year periodic oscillation is associated with once in a year rainfall onset. Rainfall onset is found to be 10-15 days after the first significant recovery in PVI index following a notable reduction, mostly to a negative value. The formulated PVI index detects rainfall onset well at the CGC when the influence of El Nino Southern Oscillation is considered.