The Role of Tropical and Extratropical Waves in Rainfall Variability and Extreme Events in Vietnam

Rainfall variability and extreme precipitation events have a large socioeconomic relevance in Vietnam. The availability of freshwater is essential for the population by access to drinking water and for the rainfed agriculture. However, extreme precipitation events bear the risk of loss of life and economic losses in connection with flooding and landslides. Vietnam is located completely in the tropics and is characterized by a multitude of landscapes, that considerably influence the weather and climate of the country, and also affect the regional impacts of extreme events. Climatologically, more than 80% of total annual rainfall is observed in most regions of Vietnam during the rainy season, which lasts from May to October and is dominated by the southwest monsoon. On the contrary, the dry season from November to April is characterized by mainly northeasterly winds of the East Asian winter monsoon. During this season, substantial rainfall is predominantly limited to the narrow coastal plains on the eastern side of the Annamese Cordillera in central Vietnam. In these regions, rainfall is mainly related to orographic lifting and enhanced convection that is associated with periodical amplifications of the northeasterly flow. The aim of this thesis is to investigate the role of tropical and extratropical waves in rainfall variability and extreme precipitation events in Vietnam in both seasons. In the first part, regional phases of enhanced or suppressed moist convection of large-scale tropical wave modes are determined for the rainy season in southern Vietnam. Based on these phases, the influence of the Madden–Julian oscillation (MJO) and convectively coupled equatorial waves (CCEWs) on the modulation of daily rainfall is evaluated and quantified for the period 1979–2007. In the second part, synoptic and dynamic causes of early dry-season rainfall events in the Central Highlands region, which is Vietnam's main coffee-growing region, are analyzed. From a station rainfall time series spanning the period 1981–2007, four dynamically different rainfall cases were selected for an in-depth investigation. The third part of this thesis consists of an analysis of the causes and predictability of a recent extreme rainfall event in northeastern Vietnam. The causes of the extreme event in July/August 2015 and its predictability are investigated using ground- and space-based observations, European Centre for Medium-Range Weather Forecasts (ECMWF) (re)analyses, and forecasts from the ECMWF ensemble prediction system. Rainfall in Vietnam south of 16°N is significantly modulated by the MJO, and by convectively coupled Kelvin and equatorial Rossby (ER) waves. The MJO and ER waves exhibit the most coherent signals during the rainy season. However, the strongest ER signals occur in central Vietnam, and a pronounced influence of Kelvin waves is only discernible in the southernmost parts. The analysis also reveals that all three waves enhance the frequency of intense rainfall during wet phases, and that anomalies are significantly enhanced when wet or dry phases of the MJO occur concurrently with the respective phases of Kelvin or ER waves. In terms of thermodynamic causes of the observed rainfall modulation, at least for the MJO the depth of the moist monsoon layer and vertical wind shear are enhanced during convectively active phases. The latter provides favorable conditions for organized convection. Although the influence of tropical waves in Vietnam is most pronounced during the rainy season, tropical waves can also cause rainfall during the early dry season in the Vietnamese Central Highlands. In two of the investigated cases, tropical waves and their interaction were the causes of substantial rainfall. Nonetheless, extratropically forced events, as in the other two cases, are more common during the northeast monsoon. Overall, the synoptic-dynamic analysis of the selected cases reveals that the spatiotemporal development of the events highly depends on interactions between large-scale tropical and extratropical wave forcing, synoptic forcing, and orographic effects. The 2015 extreme precipitation event in northeastern Vietnam was caused by a tropical–extratropical interaction that was not documented for this region and season before. A surface low over the Gulf of Tonkin and northern Vietnam was related to a subtropical upper-level trough, and caused persistent moisture flux convergence and convection over the coast of the Quang Ninh province. Strong moisture flux into the region was caused by an exceptionally strong and persistent monsoon depression over the Bay of Bengal. In terms of ECMWF ensemble forecasts, predictability of the event emerged in 72-hour lead-time forecasts, and was strongly related to the correct forecasts of the intensity and location of the upper-level trough. To conclude, this link between predictability and large-scale synoptic forcing, along with improved seasonal and subseasonal forecasts of the MJO and CCEWs suggests opportunities for submonthly forecasts of dry and wet spells in Vietnam for both seasons

The Performance of ECMWF sub-seasonal forecasts to predict the Rainy Season Onset Dates in Vietnam

The onset of the rainy season is an important date for the mostly rain-fed agricultural practices in Vietnam. Sub-seasonal to seasonal (S2S) ensemble hindcasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) are used to evaluate the predictability of the rainy season onset dates (RSODs) over five climatic sub-regions of Vietnam. The results show that the ECMWF model reproduces well the observed inter-annual variability of RSODs, with a high correlation ranging from 0.60 to 0.99 over all sub-regions at all lead times (up to 40 days) using five different RSOD definitions. For increasing lead times, forecasted RSODs tend to be earlier than the observed ones. Positive skill score values for almost all cases examined in all sub-regions indicate that the model outperforms the observed climatology in predicting the RSOD at sub-seasonal lead times (~28–35 days). However, the model is overall more skilful at shorter lead times. The choice of the RSOD criterion should be considered because it can significantly influence the model performance. The result of analysing the highest skill score for each sub-region at each lead time shows that criteria with higher 5-day rainfall thresholds tend to be more suitable for the forecasts at long lead times. However, the values of mean absolute error are approximately the same as the absolute values of the mean error, indicating that the prediction could be improved by a simple bias correction. The present study shows a large potential to use S2S forecasts to provide meaningful predictions of RSODs for farmers

An IMERG-Based Optimal Extended Probabilistic Climatology (EPC) as a Benchmark Ensemble Forecast for Precipitation in the Tropics and Subtropics

Current numerical weather prediction models show limited skill in predicting low-latitude precipitation. To aid future improvements, be it with better dynamical or statistical models, we propose a well-defined benchmark forecast. We use the arguably best available high-resolution, gauge-calibrated, gridded precipitation product, the Integrated Multisatellite Retrievals for GPM (IMERG) “final run” in a ±15-day window around the date of interest to build an empirical climatological ensemble forecast. This window size is an optimal compromise between statistical robustness and flexibility to represent seasonal changes. We refer to this benchmark as extended probabilistic climatology (EPC) and compute it on a 0.1° × 0.1° grid for 40°S–40°N and the period 2001–19. To reduce and standardize information, a mixed Bernoulli–Gamma distribution is fitted to the empirical EPC, which hardly affects predictive performance. The EPC is then compared to 1-day ensemble predictions from the European Centre for Medium-Range Weather Forecasts (ECMWF) using standard verification scores. With respect to rainfall amount, ECMWF performs only slightly better than EPS over most of the low latitudes and worse over high-mountain and dry oceanic areas as well as over tropical Africa, where the lack of skill is also evident in independent station data. For rainfall occurrence, EPC is superior over most oceanic, coastal, and mountain regions, although the better potential predictive ability of ECMWF indicates that this is mostly due to calibration problems. To encourage the use of the new benchmark, we provide the data, scripts, and an interactive web tool to the scientific community

Impact of Climate Change on Water Resources in the Kilombero Catchment in Tanzania

This article illustrates the impact of potential future climate scenarios on water quantity in time and space for an East African floodplain catchment surrounded by mountainous areas. In East Africa, agricultural intensification is shifting from upland cultivation into the wetlands due to year-round water availability and fertile soils. These advantageous agricultural conditions might be hampered through climate change impacts. Additionally, water-related risks, like droughts and flooding events, are likely to increase. Hence, this study investigates future climate patterns and their impact on water resources in one production cluster in Tanzania. To account for these changes, a regional climate model ensemble of the Coordinated Regional Downscaling Experiment (CORDEX) Africa project was analyzed to investigate changes in climatic patterns until 2060, according to the RCP4.5 (representative concentration pathways) and RCP8.5 scenarios. The semi-distributed Soil and Water Assessment Tool (SWAT) was utilized to analyze the impacts on water resources according to all scenarios. Modeling results indicate increasing temperatures, especially in the hot dry season, intensifying the distinctive features of the dry and rainy season. This consequently aggravates hydrological extremes, such as more-pronounced flooding and decreasing low flows. Overall, annual averages of water yield and surface runoff increase up to 61.6% and 67.8%, respectively, within the bias-corrected scenario simulations, compared to the historical simulations. However, changes in precipitation among the analyzed scenarios vary between −8.3% and +22.5% of the annual averages. Hydrological modeling results also show heterogeneous spatial patterns inside the catchment. These spatio-temporal patterns indicate the possibility of an aggravation for severe floods in wet seasons, as well as an increasing drought risk in dry seasons across the scenario simulations. Apart from that, the discharge peak, which is crucial for the flood recession agriculture in the floodplain, is likely to shift from April to May from the 2020s onwards

Impact of Climate and Land Use/Land Cover Change on the Water Resources of a Tropical Inland Valley Catchment in Uganda, East Africa

The impact of climate and land use/land cover (LULC) change continues to threaten water resources availability for the agriculturally used inland valley wetlands and their catchments in East Africa. This study assessed climate and LULC change impacts on the hydrological processes of a tropical headwater inland valley catchment in Uganda. The hydrological model Soil and Water Assessment Tool (SWAT) was applied to analyze climate and LULC change impacts on the hydrological processes. An ensemble of six regional climate models (RCMs) from the Coordinated Regional Downscaling Experiment for two Representative Concentration Pathways (RCPs), RCP4.5 and RCP8.5, were used for climate change assessment for historical (1976-2005) and future climate (2021-2050). Four LULC scenarios defined as exploitation, total conservation, slope conservation, and protection of headwater catchment were considered. The results indicate an increase in precipitation by 7.4% and 21.8% of the annual averages in the future under RCP4.5 and RCP8.5, respectively. Future wet conditions are more pronounced in the short rainy season than in the long rainy season. Flooding intensity is likely to increase during the rainy season with low flows more pronounced in the dry season. Increases in future annual averages of water yield (29.0% and 42.7% under RCP4.5 and RCP8.5, respectively) and surface runoff (37.6% and 51.8% under RCP4.5 and RCP8.5, respectively) relative to the historical simulations are projected. LULC and climate change individually will cause changes in the inland valley hydrological processes, but more pronounced changes are expected if the drivers are combined, although LULC changes will have a dominant influence. Adoption of total conservation, slope conservation and protection of headwater catchment LULC scenarios will significantly reduce climate change impacts on water resources in the inland valley. Thus, if sustainable climate-smart management practices are adopted, the availability of water resources for human consumption and agricultural production will increase

Hydrological Modeling in Data-Scarce Catchments: The Kilombero Floodplain in Tanzania

Deterioration of upland soils, demographic growth, and climate change all lead to an increased utilization of wetlands in East Africa. This considerable pressure on wetland resources results in trade-offs between those resources and their related ecosystem services. Furthermore, relationships between catchment attributes and available wetland water resources are one of the key drivers that might lead to wetland degradation. To investigate the impacts of these developments on catchment-wetland water resources, the Soil and Water Assessment Tool (SWAT) was applied to the Kilombero Catchment in Tanzania, which is like many other East African catchments, as it is characterized by overall data scarcity. Due to the lack of recent discharge data, the model was calibrated for the period from 1958–1965 (R2 = 0.86, NSE = 0.85, KGE = 0.93) and validated from 1966–1970 (R2 = 0.80, NSE = 0.80, KGE = 0.89) with the sequential uncertainty fitting algorithm (SUFI-2) on a daily resolution. Results show the dependency of the wetland on baseflow contribution from the enclosing catchment, especially in dry season. Main contributions with regard to overall water yield arise from the northern mountains and the southeastern highlands, which are characterized by steep slopes and a high share of forest and savanna vegetation, respectively. Simulations of land use change effects, generated with Landsat images from the 1970s up to 2014, show severe shifts in the water balance components on the subcatchment scale due to anthropogenic activities. Sustainable management of the investigated catchment should therefore account for the catchment–wetland interaction concerning water resources, with a special emphasis on groundwater fluxes to ensure future food production as well as the preservation of the wetland ecosyste