Validation of daily satellite rainfall estimates over africa and s

Diem, J. Hartter, S. Ryan, and M. Central equatorial Africa is deficient in long-term, ground-based measurements of rainfall; therefore, the aim of this study is to assess the accuracy of three high-resolution, satellite-based rainfall products in western Uganda for the —10 period.

Daily rainfall totals from six gauges were used to assess the accuracy of satellite-based rainfall estimates of rainfall days, daily rainfall totals, day rainfall totals, monthly rainfall totals, and seasonal rainfall totals.

The northern stations had a mean annual rainfall total of mm, while the southern stations had a mean annual rainfall total of mm. The three products tended to overestimate rainfall days at all stations and were borderline satisfactory at identifying rainfall days at the northern stations; the products did not perform satisfactorily at the southern stations.

At the northern stations, 3B42v7 performed satisfactorily at estimating monthly and seasonal rainfall totals, ARC2 was only satisfactory at estimating seasonal rainfall totals, and RFE2 did not perform satisfactorily at any time step. The satellite products performed worst at the two stations located in rain shadows, and 3B42v7 had substantial overestimates at those stations. Central equatorial Africa is in need of rainfall data. It is a region with no definitive ground-based information on long-term trends in rainfall Todd and Washington ; Trenberth et al.

On the eastern edge of this region is the Albertine Rift, which Plumptre at al. The Albertine Rift is a biodiversity hotspot Cordeiro et al. A lack of consistent, long-term rainfall data from ground-based gauges in the region makes rainfall studies very difficult e.

Location of the Albertine Rift in central equatorial Africa, the northern portion of the Albertine Rift, and the six rainfall stations within and proximate to the Uganda portion of the Albertine Rift dark line in elevation map.

Elevation is given as shading m MSL. Therefore, the aim of this study is to assess the accuracy of three high-resolution satellite-based rainfall products for the Uganda portion of the Albertine Rift from to ARC2 extends back to and is expected to be homogeneous over time; consequently, it might be useful for assessing rainfall trends Novella and Thiaw RFE2 and 3B42 are included in the analysis because these products have been found to be the most accurate satellite-based rainfall products for various river basins in tropical Africa Thiemig et al.

Daily rainfall data from to were obtained for six rainfall stations Gulu, Masindi, Ngogo, Kasese, Mweya, and Bwindi within and proximate to the Uganda portion of the Albertine Rift Fig.

validation of daily satellite rainfall estimates over africa and s

As rain gauges are rare in western Uganda and records from gauges in the region are typically old and discontinued or new with only several years of quality data, it is remarkable that daily rainfall data were available from multiple stations in the region over the most recent decade.

The yr length of the dataset enables it to capture a robust amount of the variability of daily rainfall in the region. All rainfall measurements were made at or UTC. Only one of the stations, Ngogo, was not missing any daily rainfall totals.

Gulu, Masindi, Kasese, Mweya, and Bwindi were missing 3. With the exception of Bwindi, the missing data were restricted to only several months per station.

Gulu was missing all daily totals for MarchSeptemberDecemberand August Masindi was missing all daily totals for FebruaryMayNovemberSeptemberand September—November Kasese was missing all daily totals for October and November Mweya was missing daily totals for all of and January—October Bwindi was missing one or more days of data during a majority of the months in the dataset, and months with no rainfall totals included August—DecemberAugust—DecemberJanuary—FebruaryMay—Octoberand January—February Novella, N.

This paper describes a new gridded, daily yr precipitation estimation dataset centered over Africa at 0. However, a marginal summer dry bias that occurs over West and East Africa is examined. Daily validation with independent gauge data shows RMSEs of Both daily and monthly validations suggested that ARC2 underestimations may be attributed to the unavailability of daily GTS gauge reports in real time, and deficiencies in the satellite estimate associated with precipitation processes over coastal and orographic areas.

However, ARC2 is expected to provide users with real-time monitoring of the daily evolution of precipitation, which is instrumental in improved decision making in famine early warning systems. Since the advent of remote sensing, many satellite rainfall estimate datasets have been developed to generate a more comprehensive level of global rainfall coverage. Often, the algorithms used consist of combining data from in situ rain gauge networks, and remote sensing satellite data from both geostationary and polar-orbiting platforms such as the Tropical Rainfall Measuring Mission TRMM; Huffman et al.

validation of daily satellite rainfall estimates over africa and s

The RFE has continued to provide an accurate monitoring of large-scale and regional climatic and hydrometeorological trends. It is a unique product relative to other satellite rainfall estimators because of its high, 0. The RFE2 exhibited improvement over its predecessor by reducing bias, and increasing both estimation accuracy and computational efficiency Love et al. The usefulness of ARC has been in its ability to provide daily snapshots of precipitation anomalies over various time scales e.

This process is critical to decision making in agriculture, water resources, and food security over Africa. A historical reprocessing of gauge and IR data from to was performed by Love et al. However, due to inconsistencies in the original reprocessing that led to a large bias in the original ARC1 data, this dataset no longer responds to current needs for operational climate monitoring.

Furthermore, a higher number of years was needed to make the ARC1 climatological record more stable. This has prompted us to utilize a new, long-term precipitation dataset for operational monitoring and climate analysis. The recent acquisition of historical, recalibrated IR imagery and daily summary gauge data has enabled our reconstruction of the ARC climatology dataset from to the present. A new, reconstructed Africa Rainfall Climatology ARC2 offers a number of advantages relative to other long-term climatological rainfall datasets that are widely used.

First, high-resolution historical rainfall estimates on a real-time, daily basis would help not only to monitor precipitation associated with synoptic and mesoscale disturbances, but also to undertake studies of extreme events, wet and dry spells, the number of rain days i. This will further enhance our understanding of the mechanisms associated with climate variability on shorter time scales. Second, a 0. For the FEWS-NET program, this local-scale resolution has also been instrumental in assessing the impacts of rainfall on agriculture and water resource management.

Third, the ARC2 maintains the same two inputs that remain continuous and homogeneous over time.

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This historical consistency is attributed to a single algorithm, requiring the use of calibrated IR satellite imagery with quality-controlled gauge observations. This straightforward estimation approach differs from other satellite products where there are often asynchronous rainfall inputs throughout a long-term historical dataset record.

Specifically, the utilization of two uniform inputs is expected to minimize the possibility of introducing bias associated with new satellite sensors. This homogeneity is also expected to elucidate long-term climatic trends in daily precipitation.

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Finally, because the same algorithm as the operational RFE2 is used, ARC2 precipitation estimates are also available in near—real time, allowing the dataset to be routinely updated on a daily basis. All of these features make the new ARC2 dataset unique.Nicholson, S. Klotter, L. Zhou, and W. For the evaluation the reference dataset is a newly created, gridded gauge dataset based on a gauge network that is more complete than that of GPCC in recent years. It is termed NICgridded. Gridding was achieved via a climatic reconstruction method based on principal components, so that reliable estimates of rainfall are available even in the data-sparse central basin.

The station density was notably higher in the latter region. Two time periods were also considered: —94, when station density was relatively high, and —, when station density was much lower than during the earlier period. Several products show excellent agreement with the NICgridded reference dataset.

In all cases, the performance of the 10 products evaluated is notably poorer in recent years —when the station network is sparse, than during the period —94, when the dense station network provides reliable estimates of rainfall. All products do reasonably well in reproducing the seasonal cycle and the latitudinal gradients of rainfall. Estimates of interannual variability show more scatter among the various products and are less reliable.

Overall, the most important results of the study are to demonstrate the strong impact that actual gauge data have on the various products and the need to have access to such gauge data, in order to produce reliable rainfall estimates from satellites.

Unfortunately, the well-being of the forest is being challenged by declining rainfall, potential long-term climate change, and human activities such as deforestation. An understanding of the changes and their implications is hindered by the lack of available in situ data in recent years and the lack of in-depth understanding of meteorological processes underlying climate variability in the region.

Most have centered on synoptic systems, convection, and other intraseasonal phenomena Nguyen and Duvel ; Jackson et al. Only a handful have focused on the prevailing general circulation Nicholson and Grist ; Pokam et al. Recent studies have focused on rainfall variability Samba and Nganga ; Samba et al. These studies have clearly demonstrated the importance of the equatorial Walker-type circulation over the Congo basin e.

However, there is little consensus on the factors driving this circulation. Although tropical SSTs appear to be linked to rainfall variability, there are diverse conclusions concerning the relative importance of the Atlantic e. Moreover, rainfall variability in many parts of the region shows little association with SSTs Dezfuli and Nicholson ; Nicholson and Dezfuli What does clearly emerge from the various studies is that factors in rainfall variability vary tremendously within equatorial Africa and that the regionalization of the factors and the factors themselves vary by season.

Moreover, they may be changing in time as the tropical oceans warm Hoell and Funk To unravel the various factors and to potentially predict future changes, a spatially detailed picture of rainfall in the region is required. This was once available, as thousands of stations were operative in equatorial Africa in the mid-twentieth century.

However, in most countries of equatorial Africa, and especially in Angola and the Democratic Republic of the Congo DRCthe networks have continually declined since the s or s. While, in principle, satellite data now provide the needed spatial detail, the available satellite products have generally been validated only over eastern equatorial Africa, a region very different climatically from the Congo basin.

Two validations that did emphasize the Congo basin found large discrepancies between gauge and satellite data McCollum et al.Dinku, T. Ruiz, S. Connor, and P. Seven different satellite rainfall estimates are evaluated at daily and daily time scales and a spatial resolution of 0.

The reference data come from a relatively dense station network of about rain gauges over Colombia. This region of South America has a very complex terrain with mountain ranges that form the northern tip of the Andes Mountains, valleys between the mountain ranges, and a vast plain that is part of the Amazon. The climate is very diverse with an extremely wet Pacific coast, a dry region in the north, and different rainfall regimes between the two extremes.

The validation and intercomparison of these products is done for the whole as well as different parts of the country. Validation results are reasonably good for daily rainfall over such complex terrain. The best results were obtained for the eastern plain, and the performance of the products was relatively poor over the Pacific coast.

Over the past decade, a number of precipitation products with high spatial and temporal resolution and near-global coverage have been developed. These products combine precipitation information from multiple sensors and multiple algorithms to produce estimates of rainfall over the globe at spatial resolutions of 0.

Because these products are constructed from satellite data, they supply crucial rainfall information over the oceans and parts of the world where conventional surface-based observations of rainfall rain gauges and radars are very sparse or nonexistent. These products are similar in that most of them combine data from passive microwave PM and thermal infrared TIR sensors. The main differences among them are the manner in which the individual data inputs are combined.

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Other differences may include use of rain gauge observations to reduce bias and the spatial and temporal resolution of the products. These differences may lead to differences in the accuracy of these estimates over different regions of the world. Thus, the evaluation of the different satellite rainfall estimates over different climatic and geographic regions is very important.

validation of daily satellite rainfall estimates over africa and s

This will be useful in identifying specific weaknesses and strengths of the different products under different circumstances. However, the evaluation of these products, particularly over Africa and South America, has been very limited. Yet it may be argued that these regions are where the satellite products are needed most because of the sparse station networks over most parts of the two regions.

Lack of ground observations and lack of access to the available observations have been among the major limitations to validations of satellite rainfall estimates over these regions. Though the distribution of gauges is generally sparse over these regions, there are some relatively data rich areas. Dinku et al. A station network of about gauges is used here to evaluate seven satellite rainfall products over Colombia in South America.

These products are evaluated at daily and daily time scales and a spatial resolution of 0. The next sections describe the validation region, gauge, and satellite data used and present and discuss the results. The validation data come from Colombia, which is located over the northwestern part of South America Fig.

The main topographic features are the three mountain ranges that form the northern tip of the Andes Mountains, the two valleys that divide the mountain ranges, and the large plain region that covers the eastern and southern parts of the country. The elevation of the mountainous region can exceed m while the eastern half of the country is below m.

Validation of Satellite-Derived Rainfall Products over the Sahel

The change from the lowland region to the mountainous region is very dramatic Fig. The western coastal region is the wettest, with mean annual rainfall up to 11 mm, and the lowland area in the north is the driest, with mean annual rainfall of less than mm Hurtado The eastern plain receives between and mm, and the southern lowlands receive from to mm of annual rainfall. The intertropical convergence zone, which stays in the vicinity of Colombia throughout the year, is the main synoptic feature.Skip to search form Skip to main content You are currently offline.

Some features of the site may not work correctly. Novella and W. NovellaW. Thiaw Published Independent rain gauge data were collected from over local stations in Mali, Senegal, and Burkina Faso during the summer season of Save to Library. Create Alert. Launch Research Feed. Share This Paper. Chabot-Couture, Karima R. Nigmatulina, … Joseph A.

Roy, S. Hassan, Syeda Sabrina Sultana Evaluation and comparison of satellite-based rainfall products in Burkina Faso, West Africa. Zwart Figures and Tables from this paper. Figures and Tables. Citation Type. Has PDF.Always prompt, always polite and always a good price. All the tours that were booked for us were exceptional.

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