Tropical Cyclone Environments over the Northeastern and Northwestern Pacific Based on ERA-15 Analyses

Abstract

This study uses a 1° ? 1° lat?long dataset, extracted from ECMWF reanalyses for the 15-yr period 1979?93 (ERA-15), to composite environmental characteristics and flow features in the vicinity of named tropical cyclones (TCs) in the eastern and western North Pacific Ocean basins. Tropical cyclones are partitioned into one of four classifications as they pass by selected locations along the axes of maximum frequency and TC tracks: weak (W), strong (S), intensifying (I), or dissipating (D). Results of this study show that peak values of rising motion, within the same classification, are greater for TC composites in the western Pacific than for those in the eastern Pacific. The level of maximum rising motion was at or above the 500-hPa level for all locations and classifications, except for the Ss at our northernmost point (25°N, 130°E) in the western Pacific. Their maximum upward motion occurred at 700 hPa. It is also found that the latter systems, contrary to all other points, were located in a region of minimum large-scale convective instability. As one cause of stabilization, large-scale advection of drier air from the East China Sea into the western and southern vicinity of the composite storm is identified. Anomalies of precipitable water (PW) were found to be related to the intensity of the storm, but not to the amount of available climatological ?background? moisture. In the eastern Pacific, the monsoontype southwesterly moisture flow across 10°N was much stronger and deeper for Is than for Ws at point B (17.5°N, 112.5°W). On the other hand, the eastern Pacific Ws were more impacted by westward transports originating off the Central American coast. When integrated from the surface to 700 hPa, the net effect was a change in the direction of moisture transport vectors and, therefore, in the major source region. Such a distinct directional change between classifications was exceptional to point B, and was not found for the three points in the western Pacific and South China Sea. Finally, based on ERA-15 model-produced rain rates, it is found that, in the western Pacific, total precipitation rates for this study were compatible with those of earlier research by W. M. Frank, who used large-scale data. The fraction of ERA-15 stratiform precipitation to the total precipitation varies from 25% to 47% in composite samples used here. The representation of convective and stratiform rain in the ERA-15 model obviously favors the former when the systems are stronger and have a more intense and broader secondary circulation.