STUDY OF DOUBLE RAIN BANDS IN A PERSISTENT RAINSTORM OVER SOUTH CHINA

Using the U.S. National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data at 1°× 1° o resolution, analysis is performed on a persistent heavy rainfall event with two rain bands to the south of the Yangtze River during 17-22 June 2005. The northern rain band was related to the atmospheric mass adjustment of cold front precipitation and the associated ageostrophic feature to the rear right of subtropical westerly jets, while the southern counterpart formed under the joint influence of easterly/westerly jets and the South Asian high （SAH）. The ageostrophic wind field to the rear right of the easterly jet center gives rise to an anti-circulation that favors the genesis of the southern belt. The feature ofdu/dt 〈0 around the SAH ridge line and to the rear right of the easterly jet streak results in a strong v - vg〈O field in the vicinity of the rain region as well as to its south. When westerly jets move southward, an intense v Vg〉0 feature appears to the north of the rain region, i.e., behind the center of the westerly jets. The associated mass adjustment leads to vigorous divergence over the rain region, which is responsible for the strong precipitation from the warm sector of the front. Also, a θe front at the middle level of the southern rain band and the cold front favor the release of instable energy to enhance the rainstorm. The southern and northern fronts approach each other and the two rain belts merge into one.

COMPARATIVE ANALYSIS OF THE INTENSIFYING AND WEAKENING LANDFALL TROPICAL CYCLONES DURING EXTRATROPICAL TRANSITION OVER CHINA

Based on the Tropical Cyclone(TC) Yearbooks data and JRA-25 reanalysis data from the Japan Meteorological Agency(JMA) during 1979-2008, dynamic composite analysis and computation of kinetic energy budget are used to study the intensifying and weakening TCs during Extratropical Transition over China. The TCI shows strong upper-level divergence, strengthened low-level convergence and significantly enhanced upward motion under the influence of strong upper-level troughs and high-level jets. The TCI is correspondingly intensified after Extratropical Transition(ET); TCW exhibits strong upper-level divergence, subdued low-level convergence and slightly enhanced upward motion under the influence of weak upper-level troughs and high-level jets. It then weakens after ET. The increase(decrease) of the generation of kinetic energy by divergence wind in TCI(TCW) at low level is one of the major reasons for TCI's intensification(TCW's weakening) after transformation. The generation of kinetic energy by divergence wind is closely related to the development of a low-level baroclinic frontal zone. The growth of the generation of kinetic energy by rotational wind in TCI at upper level is favorable for TCI's maintenance, which is affected by strong upper-level troughs. The dissipation of the generation of kinetic energy by rotational wind in TCW at upper level is unfavorable for TCW's maintenance, which is affected by weak upper-level troughs.

ANALYSIS AND CASE STUDY ON THE GENERATION OF RADAR ECHO BRIGHT BAND

In this study, the vertical profiles of radar refractive factor(Z) observed with an X-band Doppler radar in Jurong on July 13, 2012 in different periods of a stratiform cloud precipitation process were simulated using the Sim RAD software, and the contributions of each impact resulting in the bright band were analyzed quantitatively. In the simulation, the parameters inputted into Sim RAD were updated until the output Z profile was nearly consistent with the observation. The input parameters were then deemed to reflect real conditions of the cloud and precipitation. The results showed that a wider(narrower) and brighter(darker) bright band corresponded to a larger(smaller) amount, wider(narrower) vertical distribution, and larger(smaller) mean diameter of melting particles in the melting layer. Besides this,radar reflectivity factors under the wider(narrower) melting layer were larger(smaller). This may be contributed to the adequate growth of larger rain drops in the upper melting layer. Sensitivity experiments of the generation of the radar bright band showed that a drastic increasing of the complex refractive index due to melting led to the largest impact,making the radar reflectivity factor increase by about 15 d BZ. Fragmentation of large particles was the second most important influence, making the value decrease by 10 d BZ. The collision-coalescence between melting particles, volumetric shrinking due to melting, and the falling speed of raindrops made the radar reflectivity factor change by about 3-7d BZ. Shape transformation from spheres to oblate ellipsoids resulted in only a slight increase in the radar reflectivity factors(about 0.2 d BZ), which might be due to the fact that there are few large particles in stratiform cloud.