The downstream impact of tropical cyclones on a developing baroclinic wave in idealized scenarios of extratropical transition

The interaction of a tropical cyclone with a developing baroclinic wave is investigated in an idealized scenario of extratropical transition (ET). The impact of ET is examined by comparing and analyzing two numerical baroclinic-wave experiments: a traditional experiment in which baroclinic development is excited by a localized upper-level perturbation on a realistic jet profile and an experiment in which, additionally, a model tropical cyclone is inserted south of the jet at the initial time. ET occurs in a wavy upper-level flow while baroclinic surface systems are still weak. The characteristic direct impact of ET on the midlatitude flow is the formation of a distinct jet streak and the amplification of a ridge trough couplet in the adjacent downstream region. The subsequent rapid cyclogenesis downstream is a direct consequence of these upper-level flow modifications. This faster and stronger development constitutes the amplification of the leading edge of downstream development. Both the upper-level wave pattern and the surface development are subsequently amplified in the region further downstream. The formation of the ridge adjacent to the intensified downstream cyclone is analyzed in detail to elucidate the next stage in the downstream dispersion of the ET impact. Ridge-building in the ET scenario exhibits characteristics distinct from those in the life-cycle experiment. Wave breaking, feedback from the modified low-level frontal structure and diabatic processes all contribute to the high-amplitude wave pattern downstream of ET. The downstream impact of ET is highly sensitive to the initial storm location and intensity. The considerable amplification of the leading edge found in the reference experiment is the most widespread and rapidly propagating impact. We thus speculate that this leading edge represents an optimal location of the midlatitude circulation, where ET can lead to the most significant impact on the downstream flow. Copyright (C) 2010 Royal Meteorological Society