| 摘要: | With radar measurements and temporally high-resolution surface observations, this study investigates surface fluctuations associated with tropical cyclone rainbands (TCRs) observed in the vicinity of Taiwan during 2000-08. A total of 263 TCRs identified from 37 typhoon events during the study period were analyzed to show the mean and common nature of perturbations of various meteorological variables associated with the passage of TCRs.
The main patterns of surface thermodynamic fluctuations, as revealed from the composite analysis of all identified TCRs, include a persistent decrease in temperature, dewpoint temperature, and equivalent potential temperature theta(e) from the outer to inner edge of the rainband. A wavelike variation of pressure perturbations associated with the rainband was evident, with a minimum coincident with the outer edge and a maximum located inside the inner edge. The kinematics of the rainband was characterized by an obvious decrease in cross-band wind component, relatively minor variations in along-band wind component, and the wind veering. Quantitative analyses indicate that the majority of the TCRs (similar to 80%-90%) exhibited variations in surface temperature, pressure, wind speed, and wind direction less than 2 degrees C, 1.5 mb, 5 m s(-1), and 20 degrees, respectively. However, a clear trend of the magnitude of TCR thermodynamic fluctuations increasing with the radial distance from the tropical cyclone center was observed.
The TCRs identified in this study were also classified into the outer and inner rainbands, which are distinguished by a radial distance of 3 times the radius of maximum wind. The composite and magnitude analyses of their surface fluctuations indicate that the outer rainbands had a higher potential than the inner rainbands to reduce the near-surface theta(e) values. This observed characteristic is likely related to more pronounced evaporative cooling taking place in drier subcloud regions and the downward transport of low-theta(e) air aloft by more vigorous convective downdrafts for the outer rainband. Fundamentally different features of surface pressure fluctuations and mean frictional vertical velocity and relative vorticity between the outer and inner rainbands were also documented. These results reflect a possibly different origin. Nevertheless, there was no dramatic difference in the pattern of kinematic fluctuations between the outer and inner rainbands, and their mean magnitudes were also found to be statistically identical, which suggests that there is not an entirely clear distinction of surface characteristics for these two types of rainbands. |
