Using Doppler radar products from three intense convective precipitation events occurring on
the night of June 12, early morning and night on July 12, 2006 over the Tianjin area, a comparative study
is made, showing that typically, the radial velocity field changes earlier than does the echo-intensity field
at the initial and mature stages of precipitation, such that much attention should be paid to the variation in
the velocity field in doing nowcasts of rainfall, and the Doppler velocity variation in conjunction with
Auto-Nowcaster predictions helps determine the change in radar echo intensity and its movement
direction, thereby improving the accuracy of nowcasts of strongly convective precipitation.
Using the 1961-2001 rainfall datasets from 25 typical stations in Fujian Province treated by means of Empirical Orthogonal Function (EOF), Fast Fourier Function (FFT), Continuous Wavelet Transform (CWT) and Orthogonal Wavelet Transform (OWT) study is made of sequences of flood/drought indices (Z index) in different seasons. Evidence suggests that 1) the regional flood/drought events have significant 2~3 year periods in 1965-1975 and the 1990s; 2) rainfall amount in the south opposite to that in the north shows pronounced 1- and 3~ 4-yearly periods after the mid 1980s; 3) quantitatively, precipitation occurs in the west to middle in an opposite way to the other parts of the province, with noticeable 1~2 yearly periods in 1985 - 1998, and more appreciable 9~13-yearly periods after the 1980s; 4) within the study period (1961-2001) the drought trend is more apparent in the south (east) than in the north (west), particularly in the 1990s; 5) the regional climate is relatively wetter (drier) in the 1960s and 1980s (1970s and 1990s).
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.
EOF analysis has been conducted of the interdecadal variability of sea temperature anomaly fields at standard levels in the subsurface, and the abrupt change feature of sea temperature has been tested by use of movable t-test technique. A possible mechanism of the ocean-air system in the tropical Pacific is investigated by using the subsurface temperature, heat storage and wind stress data, leading to the main results as follows.
The analysis indicates that around 1980 there occurs a significant interdecadal abrupt change of temperature from sea surface to different depths, of which 4 modes show the accident and their formation is closely related to the southwestward subduction route of North Pacific sea temperature anomalies. The interdecadal signal of subduction in the window region of the North Pacific propagates southwestward to the subtropics, meeting the anomalous signal which propagates northeastward from the western Pacific at ~ 160-meter level in the thermocline. Therefore, the influence of the former on ENSO interdecadal variability might be indirect while the latter plays a more important role.
The western tropical South Pacific, which displays evident interdecadal variability, is the key region of the ENSO interdecadal variability. The positive temperature anomaly will move to the mid-tropical Pacific and the atmospheric response will excite an anticyclonic wind stress to the east of Australia, which will lead to the generation of a negative temperature anomaly in the tropical southwest Pacific. A similar evolution with an opposite sign will follow subsequently. The whole cycle takes about 13 years to complete.
Based on NCEP/NCAR reanalysis data, an investigation has been carried of climatic features of East Asian subtropical summer monsoon trough (sub-trough) and its comparison to South China Sea summer monsoon trough (SCS trough). The results show that the SCS trough is stronger than the sub-trough, whether convergence or convection. The sub-trough extends to higher level and inclines northward with altitude, but the SCS trough is reversed. The SCS trough onsets early and abruptly with the positive relative vorticity appearing suddenly and retreats slowly, but the sub-trough establishes step by step with the positive relative vorticity over Yunnan and Guizhou tableland and Guangxi areas spreading northeastward and withdraws rapidly. It is an obvious indicator that the easterly reverses the westerly during the SCS trough's onset, but the sub-trough establishment is characteristic of the westerly enhancement. The sub-trough has clearly frontal property, but the SCS trough is opposite.
KEYWORDS: Doppler effect, Radar, Reflectivity, Environmental sensing, Knowledge management, Data centers, Data processing, Lutetium, Information science, Information technology
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.
Based on TBB data from GMS of Japan, NCEP/NCAR reanalysis data and precipitation data from CPC Merged Analysis of Precipitation (CMAP), an investigation is carried out of seasonal changes of precipitation and convection over Asian-Australian "land bridge" areas and its possible factors. The results show that the precipitation and convection over Sumatra take on clearly seasonal changes with abundant (less) rainfall in winter (summer). The convection over Sumatra moves northwestward rapidly along "land bridge" in the late-April and the early-May (the 25th pentad) and the rainfall shows similar variations. It is the rapid northwestward shifting of convection that affects directly the subsequent enhancement of the convection over Indo-China Peninsula (ICP) area followed by the rupture of the subtropical high (SH) bands in this region leading to South China Sea (SCS) summer monsoon establishment. The zonal wind at lower troposphere in the equatorial Indian Ocean and the cross equatorial flow in 105°E are the main factors associated with the rapid northwestward shifting of convection along "land bridge".
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