Doppler radar, when used in detecting large-scale intense precipitation, has its echo features differing than those from warm or cold advection alone (as an S- or anti-S- form) and than those just from large-scale convergence or divergence (as a bow-like shape) but the features of Doppler velocity resulting from the combination of warm or cold advection with convergence or divergence are called the complex windfield. Analysis of Doppler weather radar data from an extensive, persistent strong rainfall event over the Hai river basin on August 16, 2005 reveals that 1) in the presence of a low-level complex windfield related to the combination of warm advection and convergence, i.e., the Doppler velocity products show that, starting from the radar center, clockwise curvature of a zero-speed line on one side toward the positive velocity zone is more remarkable than the counterpart on the other side towards the negative velocity zone, implying that when the negative speed zone is bigger compared to the maximum positive area, rainfall is reinforced or maintained; 2) in the presence of a low-level complex windfield resulting from warm advection combined with divergence, the precipitation would be weakened or ceased. Therefore, the study on the convergence and divergence in the radar velocity field gives a good indicator of nowcasting prediction of the formation, development, maintenance and decay phases of a large-scale rainfall event.
Using the sea surface temperature (SSTA) and wind anomalies (SSWA) of the tropical Pacific from January 1970 to December 1989,main spatial patterns of tropical Pacific SSTA and SSWA coupling features in the transform course from the warm phase to the cold phase of El Nino-southern oscillation (ENSO) cycles are discussed.
The main conclusions are as follows: 1) air-sea coupling patterns at the mature stage of El Nino (La Nina) are main spatial ones of tropical Pacific SSWA and SSTA coupling; 2) at the mature stage of El Nino, the interaction of the anticgclonic anomaly wind,generated by the forcing of distinct meridional SSTA gradient in the northern Hemisphere tropical central Pacific, with the California cold current and SSTA is mainly responsible for weakening of El Nino; 3) the second sea temperature increase along the south American coast in the decaying course of El Nino results from the eastward movement of the weakened positive SSTA in the tropical central-eastern Pacific forced by anomalous west wind stress; 4) La Nina results from the joint effect of Walker circulation, Ekman drift and negative SSTA in the tropical central-eastern Pacific.