Roderick van der Linden

Karlsruhe Institute of Technology

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Andreas H. Fink

Karlsruhe Institute of Technology

Tan Phan‐Van

VNU University of Science

Joaquim G. Pinto

Karlsruhe Institute of Technology

Larisa Seregina

Karlsruhe Institute of Technology

Peter Oberle

Karlsruhe Institute of Technology

Frank Seidel

Karlsruhe Institute of Technology

Moritz Zemann

Disy (Germany)

Hoang Thai Duong Vu

Karlsruhe Institute of Technology

Peter Knippertz

Karlsruhe Institute of Technology

Ha Pham‐Thanh

Vietnam National University, Hanoi

Cooperative Institutions

National Oceanic and Atmospheric Administration

University of Colorado Boulder

Cooperative Institute for Research in Environmental Sciences

Goddard Space Flight Center

Centre National de la Recherche Scientifique

University of East Anglia

NOAA Earth System Research Laboratory

Royal Netherlands Meteorological Institute

National Centre for Atmospheric Science

NOAA National Centers for Environmental Information

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Statistical‐dynamical downscaling of precipitation for Vietnam: methodology and evaluation for the recent climate

International Journal of Climatology (2017)

David Schubert Roderick van der Linden Mark Reyers Andreas H. Fink Klaus Maßmeyer

ABSTRACT Precipitation over Southeast Asia is primarily controlled by the Southeast Asian monsoon system. This area features complex orography and morphology, and has limited surface precipitation observations. In this study, a statistical‐dynamical downscaling approach that combines weather typing and dynamical downscaling is developed to obtain a high‐resolution precipitation climatology for tropical Southeast Asia. A transient simulation with the regional climate model COSMO‐CLM (COnsortium for Small Scale MOdelling‐Climate Limited‐area Modelling Community) driven by ERA ‐Interim (1979–2008) is performed for the study region. Focussing on Vietnam, six weather types ( WTs ) are selected for the Indochina Peninsula during the wet season (April to October) using a k‐means cluster approach on daily 850 and 200 hPa zonal wind components from ERA ‐Interim reanalysis. The six WTs can be physically interpreted as different stages of the seasonal progression of the planetary‐scale monsoon circulation. For each WT , selected representatives from the COSMO‐CLM run are dynamically downscaled to a resolution of 0.0625° × 0.0625° (∼7 km). Using the present‐day WT frequencies, the simulated COSMO‐CLM representatives at 7 km are recombined to a high‐resolution rainfall climatology for the recent decades. The resulting high‐resolution precipitation climatology is generally able to capture the present‐day precipitation estimates derived from APHRODITE (Asian Precipitation – Highly Resolved Observational Data Integration Towards Evaluation of Water Resources) and station data. In spite of systematic biases our approach provides a valuable tool to obtain more robust regional climate change projections for the study area.

Orography

10.1002/joc.5062

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Citations (5)

Modelling ocean wave conditions at a shallow coast under scarce data availability – A case study at the western coast of the Mekong Delta, Vietnam

Moritz Zemann Roderick van der Linden Dan Trinh Cong Hoang Thai Duong Vu Nguyễn Thị Minh Nguyệt

Abstract. Against the background of the rising sea level and land subsidence, protecting the progressively eroding coast along the Vietnam Mekong Delta becomes of tremendous importance. Within the presented work, design conditions for breakwaters were derived from offshore climate reanalysis data (ERA5), which were transferred to the nearshore by two numerical approaches, i.e. SwanOne and Delft3D, for different average and extreme wave and weather conditions. Within this process, design wave heights and periods at the nearshore could be determined for 10- to 100-year recurrence intervals. Both models thereby showed sufficient accuracy according to measurements in the field. Limitations must be made regarding the available spatio-temporal resolution, where reanalysis data showed a lack of short but high peak values compared to the observed measurements. Both numerical approaches showed their capabilities, where SwanOne offers a simple and fast calculation method, while it lacks of continuous effects like wind-generated swell or bottom friction. The Delft3D software on the other hand provides a more complete representation, not only of wave but also current dynamics, while it requires a much broader amount of input parameters and more complex boundary conditions. Within this study, the advantages and disadvantages of both models could be demonstrated, whereas for the final calculation of nearshore wave characteristics, only SwanOne was applicable based on the input parameters extracted from statistical analysis of long term ERA5 data.

Mekong Delta

Wave height

River delta

10.5194/egusphere-2022-1447

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Citations (2)

Local‐scale rainy season onset detection: A new approach based on principal component analysis and its application to Vietnam

International Journal of Climatology (2021)

Ha Pham‐Thanh Tan Phan‐Van Andreas H. Fink Roderick van der Linden

Abstract In rain‐fed farming systems, the rainy season onset date (RSOD) is of utmost importance. RSODs are often defined by applying fixed thresholds to station data or by quantifying changes in the large‐scale atmospheric circulation between the dry and wet seasons. In this study, a new improved approach is proposed and applied to determine RSODs for Vietnam based on daily rainfall station data for the period 1979–2019. In this approach, the inter‐annual similarity between station‐based RSODs and regional‐scale RSOD signal, obtained by applying principal component analysis to multiple fields, was examined. The method allows to quantify the sensitivity of RSODs to three thresholds chosen; the rainfall amount of an initial wet spell (P), the number of wet days (N), and duration of control period (D). It is demonstrated that the sensitivity to the choice of the thresholds varies across stations and sub‐regions. In particular, P is a principal parameter to determine RSODs with the significant control at 66.4% stations over entire Vietnam. While D plays an important role at 55.7% stations, and N seems to be less important, which influences only 31.3% stations. The results also reflected that the selected optimal criteria for each station captured very well the rainy seasonal cycle of Vietnam. Trends towards earlier onset dates are found for large parts of central Vietnam, whereas several northern stations show a trend towards later onsets. The new method can be applied to other monsoon regions to define more spatially coherent thresholds to define local, agriculturally relevant rainy season onsets.

Wet season

10.1002/joc.7441

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Citations (5)

The influence of DACCIWA radiosonde data on the quality of ECMWF analysis and forecasts

EGU General Assembly Conference Abstracts (2019)

Roderick van der Linden Peter Knippertz Andreas H. Fink Angela Benedetti Bruce Ingleby

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Climatology of Low-Level Clouds over Western Equatorial Africa Based on Ground Observations and Satellites

Journal of Climate (2023)

Olivier Champagne Raffael Aellig A. H. Fink Nathalie Philippon Pierre Camberlin

Abstract The tropical cloud forest ecosystem in western equatorial Africa (WEA) is known to be sensitive to the presence of an extensive and persistent low-level stratiform cloud deck during the long dry season from June to September (JJAS). Here, we present a new climatology of the diurnal cycle of the low-level cloud cover from surface synoptic stations over WEA during JJAS 1971–2019. For the period JJAS 2008–19, we also utilized estimates of cloudiness from four satellite products, namely, the Satellite Application Facility on Support to Nowcasting and Very Short Range Forecasting (SAFNWC) cloud classification, the Day and Night Microphysical Schemes (DMS/NMS), and cross sections from CALIPSO and CloudSat (2B-GEOPROF-lidar). A comparison with surface stations reveals that the NMS at night together with SAFNWC at daytime yield the smallest biases. The climatological analysis reveals that low-level clouds persist during the day over the coastal plains and windward side of the low mountain ranges. Conversely, on their leeward sides, i.e., over the plateaus, a decrease of the low-level cloud frequency is observed in the afternoon, together with a change from stratocumulus to cumulus. At night, the low-level cloud deck reforms over this region with the largest cloud occurrence frequencies in the morning. Vertical profiles from 2B-GEOPROF-lidar reveal cloud tops below 3000 m even at daytime. The station data and the suitable satellite products form the basis to better understand the physical processes controlling the clouds and to evaluate cloudiness from reanalyses and models.

Nowcasting

Cloud fraction

Ceilometer

Cloud top

10.1175/jcli-d-22-0364.1

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Citations (2)

Synoptic-Dynamic Analysis, Predictability and Convection-Permitting Simulations of an Extreme Precipitation Event Affecting Ho Chi Minh City on 26 September 2016

33rd Conference on Hurricanes and Tropical Meteorology (2018)

Roderick van der Linden

Predictability

Ho chi minh

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The influence of DACCIWA radiosonde data on the quality of ECMWF analyses and forecasts over southern West Africa

Quarterly Journal of the Royal Meteorological Society (2020)

Roderick van der Linden Peter Knippertz Andreas H. Fink Bruce Ingleby Marlon Maranan

Abstract During the DACCIWA (Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) field campaign ∼900 radiosondes were launched from 12 stations in southern West Africa from 15 June to 31 July 2016. Subsequently, data‐denial experiments were conducted using the Integrated Forecasting System of the European Centre for Medium‐range Weather Forecasts (ECMWF) to assess the radiosondes' impact on the quality of analyses and forecasts. As observational reference, satellite‐based estimates of rainfall and outgoing long‐wave radiation (OLR) as well as the radiosonde measurements themselves are used. With regard to the analyses, the additional observations show positive impacts on winds throughout the troposphere and lower stratosphere, while large lower‐tropospheric cold and dry biases are hardly reduced. Nonetheless, downstream, that is farther inland from the radiosonde stations, we find a significant increase (decrease) in low‐level night‐time temperatures (monsoon winds) when incorporating the DACCIWA observations, suggesting a possible linkage via weaker cold air advection from the Gulf of Guinea. The associated lower relative humidity leads to reduced cloud cover in the DACCIWA analysis. Closer to the coast and over Benin and Togo, DACCIWA observations increase low‐level specific humidity and precipitable water, possibly due to changes in advection and vertical mixing. During daytime, differences between the two analyses are generally smaller at low levels. With regard to the forecasts, the impact of the additional observations is lost after a day or less. Moderate improvements occur in low‐level wind and temperature but also in rainfall over the downstream Sahel, while impacts on OLR are ambiguous. The changes in precipitation appear to also affect high‐level cloud cover and the tropical easterly jet. The overall rather small observation impact suggests that model and data assimilation deficits are the main limiting factors for better forecasts in West Africa. The new observations and physical understanding from DACCIWA can hopefully contribute to reducing these issues.

Precipitable water

10.1002/qj.3763

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Citations (24)

Modulation of Daily Rainfall in Southern Vietnam by the Madden–Julian Oscillation and Convectively Coupled Equatorial Waves

Journal of Climate (2016)

Roderick van der Linden Andreas H. Fink Joaquim G. Pinto Tan Phan‐Van George N. Kiladis

Abstract Rainfall extremes have a large socioeconomic relevance for southern Vietnam. More than 30 million people live in this low-lying, flood-prone region in Southeast Asia. In this study the influence of the Madden–Julian oscillation (MJO) and convectively coupled equatorial waves on the modulation of daily rainfall during the rainy season (May–October) is evaluated and quantified using an extensive station database and the gridded Asian Precipitation–Highly Resolved Observational Data Integration Toward Evaluation of Water Resources (APHRODITE) product for different phases of the equatorial waves. The MJO, Kelvin, and equatorial Rossby (ER) waves significantly modulate daily rainfall in Vietnam south of 16°N. The MJO shows the most coherent signals across the region, followed by ER waves, whose influence is strongest in central Vietnam; Kelvin waves only affect the southern parts of Vietnam. For all waves, the frequency of occurrence of intense daily rainfall larger than 25 mm is significantly enhanced during wet phases, whereas the magnitude of rainfall anomalies is related to the wave’s amplitude only in the MJO and ER cases. A novel wave interference diagram reveals strong positive interferences of dry and wet anomalies when the MJO occurs concurrently with Kelvin and ER waves. In terms of causes of rainfall anomalies, the waves modulate tropospheric moisture convergence over the region, but a strong influence on the depth of the monsoon flow and the vertical wind shear is discernible from radiosonde data only for the MJO. The results suggest new opportunities for submonthly prediction of dry and wet spells in Indochina.

Madden–Julian oscillation

Kelvin wave

Equatorial waves

Wet season

10.1175/jcli-d-15-0911.1

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Citations (28)

The Performance of ECMWF Subseasonal Forecasts to Predict the Rainy Season Onset Dates in Vietnam

Weather and Forecasting (2021)

Ha Pham‐Thanh Tan Phan‐Van Roderick van der Linden Andreas H. Fink

Abstract The onset of the rainy season is an important date for the mostly rain-fed agricultural practices in Vietnam. Subseasonal 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 subregions of Vietnam. The results show that the ECMWF model reproduces well the observed interannual variability of RSODs, with a high correlation ranging from 0.60 to 0.99 over all subregions 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 subregions indicate that the model outperforms the observed climatology in predicting the RSOD at subseasonal lead times (∼28–35 days). However, the model is overall more skillful at shorter lead times. The choice of the RSOD criterion should be considered because it can significantly influence the model performance. The result of analyzing the highest skill score for each subregion 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.

Predictability

Lead time

Ensemble average

Wet season

Lead (geology)

10.1175/waf-d-21-0144.1

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Citations (3)

A new and flexible rainy season definition: Validation for the Greater Horn of Africa and application to rainfall trends

International Journal of Climatology (2018)

Larisa Seregina Andreas H. Fink Roderick van der Linden Nadir Ahmed Elagib Joaquim G. Pinto

Previous studies on observed or projected rainfall trends for the Greater Horn of Africa (GHA) generally focus on calendric 3‐month periods, and thus partly neglect the complexity of rainfall seasonality in this topographically heterogeneous region. This study introduces a novel and flexible methodology to identify the rainfall seasonality, the onset, cessation and duration of the rainy seasons and the associated uncertainties from rainfall time series. The definition is applied to the Climate Hazards Group InfraRed Precipitation with Stations (CHIRPS) satellite product and an extensive rain gauge data set. A strong agreement with known seasonal dynamics in the region and the commonly used calendric rainy seasons is demonstrated. Compared to the latter definition, a clear added value is found for the new approach as it captures the local rainfall features (associated with, for example, the sea breeze), thus facilitating evaluations across rainfall seasonality borders. While previously known trends are qualitatively confirmed, trends are amplified in some regions using the flexible definition method. Notably, a drying trend in Tanzania and Democratic Republic of Congo and a wetting trend in central Sudan and parts of eastern Ethiopia and Kenya can be detected. The trends are regionally associated with changes in rainy season cessation. CHIRPS and station trend patterns are consistent over larger regions of the GHA, but differ in regions with known rainfall contributions from warmer cloud tops. Discrepancies are found in coastal and topographically complex areas, and regions with an unstable seasonality of rainfall. As expected, CHIRPS shows spatially more homogeneous trends compared to station data. The more precise definition of the rainy season facilitates the assessment of rainfall characteristics like intensity, rainfall amounts or temporal shifts of rainy seasons. This novel methodology could also provide a more adequate calibration of climate model simulations thus potentially enabling more realistic climate change projections for the GHA.

Wet season

Seasonality

Dry season

10.1002/joc.5856

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Citations (44)