Evaluation of the NCEP Climate Forecast System and Its Downscaling for Seasonal Rainfall Prediction over Vietnam

Abstract

This study investigates the ability to apply National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) products and their downscaling by using the Regional Climate Model version 4.2 (RegCM4.2) on seasonal rainfall forecasts over Vietnam. First, the CFS hindcasts (CFS_Rfc) from 1982 to 2009 are used to assess the ability of the CFS to predict the overall circulation and precipitation patterns at forecast lead times of up to 6 months. Second, the operational CFS forecasts (CFS_Ope) and its RegCM4.2 downscaling (RegCM_CFS) for the period 2012–14 are used to derive seasonal rainfall forecasts over Vietnam. The CFS_Rfc and CFS_Ope are validated against the ECMWF interim reanalysis, the Global Precipitation Climatology Centre (GPCC) analyzed rainfall, and observations from 150 meteorological stations across Vietnam. The results show that the CFS_Rfc can capture the seasonal variability of the Asian monsoon circulation and rainfall distribution. The higher-resolution RegCM_CFS product is advantageous over the raw CFS in specific climatic subregions during the transitional, dry, and rainy seasons, particularly in the northern part of Vietnam in January and in the country’s central highlands during July.

Space–time variability of drought over Vietnam

“…The opposite signs of EOF1 and EOF2 during some periods suggest that drought events in the Northern and Southern areas are not totally in the same phase. This could be attributed to the differences of large-scale forcing associated with rainfall mechanisms and temperature regimes in these sub-regions (Wang and LI, 2004;Huang et al, 2012;Phan et al, 2009Phan et al, , 2018. A similar situation can be observed for EOF3 in comparison to EOF1 and EOF2.…”

Predictability of the rainy season onset date in Central Highlands of Vietnam

“…In the South Central and the southern part of the North Central regions, rainfall mainly occurs in late summer to early winter between approximately August and December. Contrarily, in regions farther north and farther south and in the Central Highlands (CH) region, rainfall concentrates in the summer monsoon season that usually lasts from May to October (van der Linden et al, 2016;Nguyen et al, 2014a;Phan-Van et al, 2018). Located on the western side of the Truong Son mountain range, the CH region has a complex terrain, characterized by a mixture of rough mountains of different heights, widespread forests and flat plateaus of red fertile basaltic land.…”

“…Because of this location, the interaction of different wind patterns also has a strong influence on the annual rainfall characteristics, in where the monsoon activity reveals a transitional feature of two distinct monsoons, the southwest monsoon and northeast monsoon that correspond with a wet and a dry season, respectively (Zhang et al, 2002). Over most parts of the Indochina Peninsula, the rainy season appears about 6 months from early-May to mid-October, while the period from November to late-April can be considered as a dry season (Nguyen-Le et al, 2014van der Linden et al, 2016;Ngo-Thanh et al, 2018;Phan-Van et al, 2018). The shift from dry to wet seasons is marked by an immediate increase of daily rainfall totals from 2 mm per day to 6 mm per day.…”

Extratropical Forcing of Submonthly Variations of Rainfall in Vietnam

“…1). This terrain complexity influences the tropical and extratropical monsoon components that lead to a great diversity of Vietnam's climate (Nguyen et al 2014;Phan-Van et al 2018).…”