Large scale environmental wind patterns and the intensification rates of western north Pacific tropical storms

Large scale environmental flow patterns around western north Pacific tropical storms are identified using NCEP/NCAR reanalysis data (1975--2001), with the goal of finding a signal for those most favorable for rapid intensification, based on the hypothesis that aspects of the horizontal flow influence tropical cyclone intensification at an early stage of development. Use of the JTWC best track data over the same period reveals that intensification rate is a strong function of initial intensity. Very rapid, rapid and slow 24 hr intensification periods from weak and strong tropical storm stages (35 kts and 60 kts) are defined. Analysis reveals that over 70% of the most extreme rapid intensifiers from 35 kts have anomalously strong cyclonic horizontal shear and/or confluence of the background flow. Given the strength of the signal, it may provide a new rapid intensification predictor for operational use. At 200 mb the importance of the location of the incipient tropical storm directly under a region of flow splitting into the mid-latitude westerlies to the north and the sub-equatorial trough to the south is identified as a common criterion for the onset of rapid intensification.;Using an anomaly version of a tropical cyclone model, storms are simulated in the presence of simplified horizontally sheared background flows with specific interest in rapid intensification periods. Through examining the dry dynamics of the interactions and full moist simulations it is found that a mechanism attendant to the dry dynamics sets of asymmetric patterns of convergence, generating asymmetric potential vorticity outside the center of the developing storms. The axisymmetrization of the asymmetries results in a spin up of the tangential flow inward of the initial radius of maximum winds and a resultant intensification of the vortex. This process does not occur for a model run with no background flow. It is concluded that asymmetries developed by background horizontally sheared flows act as a positive factor for tropical cyclone intensification at an early stage.