Weather Systems Surrounding The Pacific

Sensitivity of the midlatitude flow to latent heat release in weather systems surrounding Western North Pacific extratropical transition: a representative case study

Student: Yves Karrer

Supervisor: Dr Christian Grams, Jacopo Riboldi (PhD), IAC


Tropical cyclones (TCs) that undergo extratropical transition (ET) may pose serious threat to the surrounding regions and far downstream by triggering high impact weather. Due to the various processes involved in ET forecast accuracy in the affected regions is reduced. Better understanding of the physical processes could improve the quality of numerical weather pre-diction of ET situations. This study investigates the role of latent heat release (LHR) of different weather systems (a Predecessor Rain Event (PRE), the ET system, downstream cyclones and warm conveyor belts) in an ET scenario. Composite fields of twelve North Pacific ET cases are used as initial and boundary conditions for numerical simulations with the COSMO model. In different runs, LHR is switched off either during specific time periods and regions or in the entire domain. Findings reveal that the LHR in each weather system is crucial for the modification of the midltatitude flow in different ways. By the LHR-driven outflows of the weather systems a net transport of low-PV air into the upper troposphere is observed, leading to ridgebuilding directly downstream and thereby to an amplification of the Rossby wave train. Furthermore, the results demonstrate the importance of the ridgebuilding by the PRE ahead of the transitioning TC: It postpones the ET process and further re-intensifies the transitioning cyclone.