The Key Role of Diabatic Outflow in Amplifying the Midlatitude Flow: a Representative Case Study of Weather Systems Surrounding Western North Pacific Extratropical Transition

Recurving tropical cyclones (TCs) undergoing extratropical transition (ET) may substantially modify the large-scale midlatitude flow pattern. This study highlights the role of diabatic outflow in midlatitude flow amplification in the context of a review of the physical and dynamical processes involved in ET. Composite fields of 12 western North Pacific ET cases are used as initial and boundary conditions for high-resolution numerical simulations of the North Pacific-North American sector with and without the TC present.

It is demonstrated that a three-stage sequence of diabatic outflow associated with different weather systems is involved in triggering a highly amplified midlatitude flow pattern: (1) Preconditioning by a Predecessor Rain Event, (2) TC-extratropical flow interaction, and (3) downstream flow amplification by a downstream warm conveyor belt (WCB). An ensemble of perturbed simulations demonstrates the robustness of these stages.

Beyond earlier studies investigating PREs, recurving TCs, and WCBs individually, here we highlight that each impacts the midlatitude flow through a similar sequence of processes surrounding ET: Latent heat release in rapidly ascending air leads to a net transport of low-PV air into the upper troposphere. Negative PV advection by the diabatically driven outflow initiates ridge building, accelerates and anchors a midlatitude jet streak, and overall amplifies the upper-level Rossby wave pattern. However, the three weather systems markedly differ in terms of the character of diabatic heating and associated outflow height, with the TC outflow reaching highest and the downstream WCB outflow producing the strongest negative PV anomaly.

In a second step simulations are performed suppressing latent heat release and associated diabatic outflow for the three weather system categories. These simulations corroborate the incremental impact of the different diabatic outflow on the overall midlatitude flow amplification during ET: A simulation that mimics the dry-dynamical evolution is very close to the simulation without the TC present which corroborates that diabatic outflow of the transitioning TC is essential for downstream development and downstream flow amplification. Diabatic outflow during the Preconditioning stage establishes a favorable environment for the subsequent reinstensification of the transitioning TC. Diabatic outflow by the downstream WCB results in a moderate further flow amplification.

Grams, C.M. and H.M. Archambault, 2016: The key role of diabatic outflow in amplifying the midlatitude flow: a representative case study of weather systems surrounding western North Pacific extratropical transition. Mon. Wea. Rev., in press, doi:10.1175/MWR-D-15-0419.1 .