Mesoscale Features of Tropical Cyclone Formations Associated with the Trade Wind Surges in the Western North Pacific

Forty out of 531 tropical cyclones (TCs) that formed in the western North Pacific during 1986-2005 have accompanied trade wind surges located 5°-15° lat. to the north of the pre-tropical cyclones disturbance center. Composite and empirical orthogonal function analyses indicate that the trade wind surges are related to a midlatitude eastward-moving high pressure system often found during the East Asian winter monsoon. Therefore, these trade wind surge TCs tend to occur in late season (with one-third of them in December), and at lower latitudes (7° lat. lower than the climatological average formation position).

The evolution of mesoscale features during formation of trade wind surge TCs is examined. Various satellite datasets show similar mesoscale patterns during their formations. A few convective lines form by convergence between the trade wind surges and the strengthening cyclonic circulation associated with incipient vortex within the 24 hours before formation. Some mesoscale convective systems (MCSs) are embedded in the convective line with life times of about 5 hours in the begining stage, and these are illustrated through case studies. Formations usually occur when the trade winds start to decrease in magnitude and a short period after the major episodes of convection in the convective lines and mesoscale convective systems. The relationships between the temporal variability of synoptic-scale trade wind surges, the mesoscale features and associated TC formations are discussed.

The formation process is simulated using WRF. Results show that WRF model can simulate reasonably well the evolution of the mesoscale convections and the development of the pre-TC disturbance. The merging of the strong vorticity patches (of the MCSs) results in the concentration and increase of low-level vorticity as well as the development of the disturbance.