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Chen T.-C.,Iowa State University | Yen M.-C.,National Central University | Tsay J.-D.,Iowa State University | Alpert J.,Environmental Modeling Center | Thanh N.T.T.,Aerospace Meteorological Observatory
Weather and Forecasting | Year: 2012

The formations of heavy rainfall/flood (HRF) events in Vietnam are studied from diagnostic analyses of 31 events during the period 1979-2009. HRF events develop from the cold surge vortices formed around the Philippines. These vortices' speed, size, and rainfall, which evolve intoHRF events, are enhanced distinguishably from non-HRF vortices, as they reach Vietnam. The HRF cyclone, the North Pacific anticyclone, and the northwestern Pacific explosive cyclone simultaneously reach their maximum intensities when the HRF event occurs. An HRF cyclone attains its maximum intensity by the in-phase constructive interference of three monsoon (30-60, 12-24, and 5 days) modes identified by the spectral analysis of zonal winds. The rainfall center of anHRF event is formed and maintained by the in-phase constructive interference of rainfall and convergence of water vapor flux anomalies, respectively, from three monsoon modes. Forecast times of regional models are dependent and constrained on the scale of the limited domain. For 5-day forecasts, a global or at least a hemispheric model is necessary. Using the salient features described above, a 5-day forecast advisory is introduced to supplement forecasts of HRF events made by the global model. Non-HRF vortices are filtered by threshold values for the deepening rate of explosive cyclones and basic characteristics of the HRF events predicted by the globalmodel. Anecessary condition for anHRF event is the in-phase superposition of the threemonsoonmodes. One-week forecasts for 12 HRF events issued by the NCEP Global Forecast System are tested. Results demonstrate the feasibility of the forecast advisory to predict the occurrence dates of HRF events. © 2012 American Meteorological Society. Source


Chen T.-C.,Iowa State University | Yen M.-C.,National Central University | Tsay J.-D.,Iowa State University | Thanh N.T.T.,Aerospace Meteorological Observatory | Alpert J.,Environmental Modeling Center
Monthly Weather Review | Year: 2012

The 30-31 October 2008 Hanoi, Vietnam, heavy rainfall-flood (HRF) event occurred unusually farther north than other Vietnam events. The cause of this event is explored with multiple-scale processes in the context of the midlatitude-tropical interaction. In the midlatitudes, the cold surge linked to the Hanoi event can be traced westward to the leeside cyclogenesis between the Altai Mountains and Tianshan. This cyclone developed into a Bering Sea explosive cyclone later, simultaneously with the occurrence of the Hanoi HRF event. In the tropics, a cold surge vortex formed on 26 October, south of the Philippines, through the interaction of an easterly disturbance, an already existing small surface vortex in the Celebes Sea, and the eastern Asian cold surge flow. This cold surge vortex developed into a cyclone, juxtaposed with the surface high of the cold surge flow, and established a strong moist southeasterly flow from the South China Sea to Hanoi, which helped maintain the HRF event. Spectral analysis of the zonal winds north and south of the Hanoi HRF cyclone and rainfall at Hanoi reveal the existence of three monsoon modes: 30-60, 12-24, and 5 days. The cold surge vortex developed into an HRF cyclone in conjunction with the in-phase constructive interference of the three monsoon modes, while the Hanoi HRF event was hydrologically maintained by the northwestward flux of water vapor into Hanoi by these monsoon modes. © 2012 American Meteorological Society. Source


Yen M.-C.,National Central University | Chen T.-C.,Iowa State University | Hu H.-L.,National Central University | Tzeng R.-Y.,National Central University | And 3 more authors.
Journal of the Meteorological Society of Japan | Year: 2011

The twenty nine years (1979-2007) rain-gauge based girded precipitation data generated by the Asian Precipitation-Highly Resolved Observational Data Integration towards Evaluation of water resources (APHRODITE) are used to depict the rainfall climatology in Vietnam. The rain gauge observations of 163 stations in Vietnam for year 2007 are employed to validate the analysis results of the APHRODITE precipitation and to verify two distinct rainfall regimes: the October-November regime in central Vietnam and the May-October regime in the northern and southern part of this country identified with APHRODITE data. It appears that the Truong Son Range along the western border of Vietnam with Laos and Cambodia provides a natural separation of the October-November rainfall regime in central Vietnam from others. The inter annual variation of the October-November rainfall regime can be well depicted by a principal mode obtained from the empirical orthogonal function analysis on the 29-year-APHRODITE precipitation. Time variation of this inter annual mode is out-of-phase with the SST(NINO3.4) index. It is inferred from this negative correlated relationship that central Vietnam is drier (wetter) when the SSTs over the NINO3.4 region is warmer (colder). It is found from the water vapor transport analysis that an anomalous cyclonic (anticyclonic) circulation over south Asia is paired with an anomalous anticylonic (cyclonic) circulation over the western north Pacific during cold (warm) episodes. Water vapor is converged (diverged) by these two anomalous circulations toward (out of) the South China Sea and Philippine Sea west of 150°E during cold (warm) years. In turn, the anomalous cyclonic (anticyclonic) circulation in South Asia enhances (reduces) the water vapor supply to Indochina, particularly to Vietnam. Coupling with this pair of anomalous circulations, water vapor is converged (diverged) by the anomalous divergent circulation, coupling with the aforementioned pair of anomalous circulation, toward (out of) Southeast Asia to maintain excessive (deficient) rainfall during cold (warm) episodes. Evidently, the response of the divergent circulation to the tropical Pacific SST anomalies contributes to the interannual variation of the October-November rainfall in central Vietnam. © 2011, Meteorological Society of Japan. Source


Thompson A.M.,Pennsylvania State University | Miller S.K.,Pennsylvania State University | Tilmes S.,U.S. National Center for Atmospheric Research | Kollonige D.W.,Pennsylvania State University | And 30 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2012

We present a regional and seasonal climatology of SHADOZ ozone profiles in the troposphere and tropical tropopause layer (TTL) based on measurements taken during the first five years of Aura, 2005-2009, when new stations joined the network at Hanoi, Vietnam; Hilo, Hawaii; Alajuela/Heredia, Costa Rica; Cotonou, Benin. In all, 15 stations operated during that period. A west-to-east progression of decreasing convective influence and increasing pollution leads to distinct tropospheric ozone profiles in three regions: (1) western Pacific/eastern Indian Ocean; (2) equatorial Americas (San Cristóbal, Alajuela, Paramaribo); (3) Atlantic and Africa. Comparisons in total ozone column from soundings, the Ozone Monitoring Instrument (OMI, on Aura, 2004-) satellite and ground-based instrumentation are presented. Most stations show better agreement with OMI than they did for EP/TOMS comparisons (1998-2004; Earth-Probe/Total Ozone Mapping Spectrometer), partly due to a revised above-burst ozone climatology. Possible station biases in the stratospheric segment of the ozone measurement noted in the first 7 years of SHADOZ ozone profiles are re-examined. High stratospheric bias observed during the TOMS period appears to persist at one station. Comparisons of SHADOZ tropospheric ozone and the daily Trajectory-enhanced Tropospheric Ozone Residual (TTOR) product (based on OMI/MLS) show that the satellite-derived column amount averages 25% low. Correlations between TTOR and the SHADOZ sondes are quite good (typical r2 = 0.5-0.8), however, which may account for why some published residual-based OMI products capture tropospheric interannual variability fairly realistically. On the other hand, no clear explanations emerge for why TTOR-sonde discrepancies vary over a wide range at most SHADOZ sites. © 2012. American Geophysical Union. All Rights Reserved. Source


Ogino S.-Y.,Japan Agency for Marine - Earth Science and Technology | Ogino S.-Y.,Kobe University | Fujiwara M.,Hokkaido University | Shiotani M.,Kyoto University | And 5 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2013

Seasonal and subseasonal variations in the ozone mixing ratio (OMR) are investigated by using continuous 7 year ozonesonde data from Hanoi (21°N, 106°E), Vietnam. The mean seasonal variations for the 7 years show large amplitude at the upper troposphere and lower stratosphere (UTLS) region (10-18 km) and at the lower troposphere (around 3 km) with standard deviations normalized by the annual mean value of about 30% for both regions. In the UTLS region, the seasonal variation in the OMR shows a minimum in winter and a maximum in spring to summer. The variation seems to be caused by the seasonal change in horizontal transport. Low OMR air masses are transported from the equatorial troposphere in winter by the anticyclonic flow associated with the equatorial convections, and high OMR air masses are transported from the midlatitude stratosphere in summer possibly due to Rossby wave breakings in the UT region and anticyclonic circulation associated with the Tibetan High in the LS region. In the lower troposphere, a spring maximum is found at 3 km height. Biomass burning and tropopause foldings are suggested as possible causes of this maximum. Subseasonal variations in the OMR show large amplitude in the UTLS region (at around 15 km) and in the boundary layer (below 1 km) with the standard deviations normalized by the annual mean larger than 40%. The OMR variations in the winter UTLS region have a negative correlation with the meridional wind. This relation indicates that the low OMRs observed at Hanoi has been transported from the equatorial region. ©2013. American Geophysical Union. All Rights Reserved. Source

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