Search Results

You are looking at 1 - 3 of 3 items for

  • Author or Editor: Nguyen Thi Tan Thanh x
  • Refine by Access: All Content x
Clear All Modify Search
Tsing-Chang Chen
,
Ming-Cheng Yen
,
Jenq-Dar Tsay
,
Nguyen Thi Tan Thanh
, and
Jordan Alpert

Abstract

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.

Full access
Tsing-Chang Chen
,
Ming-Cheng Yen
,
Jenq-Dar Tsay
,
Jordan Alpert
, and
Nguyen Thi Tan Thanh

Abstract

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 into HRF 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 an HRF 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 global model. A necessary condition for an HRF event is the in-phase superposition of the three monsoon modes. 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.

Full access
Tan Phan-Van
,
Phuong Nguyen-Ngoc-Bich
,
Thanh Ngo-Duc
,
Tue Vu-Minh
,
Phong V. V. Le
,
Long Trinh-Tuan
,
Tuyet Nguyen-Thi
,
Ha Pham-Thanh
, and
Duc Tran-Quang

Abstract

In this study, the spatiotemporal variability of drought over the entire Southeast Asia (SEA) region and its associations with the large-scale climate drivers during the period 1960–2019 are investigated for the first time. The 12-month Standardized Precipitation Evapotranspiration Index (SPEI) was computed based on the monthly Global Precipitation Climatology Centre (GPCC) precipitation and the monthly Climate Research Unit (CRU) 2-m temperature. The relationships between drought and large-scale climate drivers were examined using the principal component analysis (PCA) and maximum covariance analysis (MCA) techniques. Results showed that the spatiotemporal variability of drought characteristics over SEA is significantly different between mainland Indochina and the Maritime Continent and the difference has been increased substantially in recent decades. Moreover, the entire SEA is divided into four homogeneous drought subregions. Drought over SEA is strongly associated with oceanic and atmospheric large-scale drivers, particularly El Niño–Southern Oscillation (ENSO), following by other remote factors such as the variability of sea surface temperature (SST) over the tropical Atlantic, the Pacific decadal oscillation (PDO), and the Indian Ocean dipole mode (IOD). In addition, there exists an SST anomaly dipole over the Pacific Ocean, which modulates the atmospheric circulations and consequently precipitation over SEA, affecting drought conditions in the study region.

Full access