scholarly journals Wet and dry spells in Senegal: Evaluation of satellite-based and model re-analysis rainfall estimates

2019 ◽  
Author(s):  
Cheikh Modou Noreyni Fall ◽  
Christophe Lavaysse ◽  
Mamadou Simina Drame ◽  
Geremy Panthou ◽  
Amadou Thierno Gaye
Keyword(s):  
Spatial Scales ◽  
Spatial Coherence ◽  
West African Monsoon ◽  
Rain Gauge ◽  
West African ◽  
Dry Spells ◽  
Dry Spell ◽  
False Start ◽  
Temporal And Spatial ◽  
Rain Gauge Network

Abstract. In this study, wet and dry spells over Senegal provided by four datasets based on satellite data (TRMM-3B42 V7, TAMSAT V3, CMORPH V1.0, CHIRPS V2.0), two fully based on (re)analyses (NCEP-CFSR, ERA5) and one was fully based on gauge observations (CPC Unified V1.0/RT) are compared with respect to observation datasets derived from 65 rain gauge network. All datasets were converted to the same temporal and spatial scales with 0.25 × 0.25 as resolution. Ordinary kriging (OK) and block kriging (BK) were used for the spatial interpolation of the gauge data. Despite a spatial coherence of the seasonal rainfall accumulation between all products, more variability with intra-seasonal features are shown in this paper. The seasonal cycle of dry days shows that TRMM, CPC, ERA5, NCEP and OK record more dry days (from 45 % to 55 % of dry days in August) while TAMSAT, CHIRPS, CMORPH and BK record less dry day (from 40 % to 30 % of dry days in August). All datasets highlighted an agreement that dry spell indicator underscore often false start and early cessation of the rainy Season in Senegal. Although, it can rarely occurs during intensification of West African monsoon (August–September). The most contrast is found on the detection of wet indicators intensity. Wet spell (defined as period with precipitation higher than a certain percentile of historical precipitation) are more severe in OK and TRMM than in other datasets. However, a great similarity is shown on their temporal frequencies.

scholarly journals Quality and Value of Seasonal Precipitation Forecasts Issued by the West African Regional Climate Outlook Forum

2019 ◽  
Vol 58 (3) ◽  
pp. 621-642 ◽  
Author(s):  
J. Bliefernicht ◽  
M. Waongo ◽  
S. Salack ◽  
J. Seidel ◽  
P. Laux ◽  
...  
Keyword(s):  
Spatial Scales ◽  
Regional Climate ◽  
West African Monsoon ◽  
Rain Gauge ◽  
Small Sample ◽  
West African ◽  
African Countries ◽  
The West ◽  
Peak Period ◽  
Bootstrap Analysis

AbstractSeasonal climate forecasts for an early warning of climate anomalies are produced by regional climate outlook forums (RCOF) worldwide. This study presents a verification of one of the earliest RCOF products, the precipitation outlook for the West African monsoon peak period (July–September). The basis of this outlook is countrywide precipitation forecasts from various statistical (downscaling) models, which are subjectively reinterpreted by experts on the basis of information from observed SST pattern analysis and global forecasts. The forecast quality was analyzed from 1998 to 2013 using a novel database of rain gauge measurements established for several West African countries, among other references. The analysis indicated skill for above normal and below normal on different spatial scales but also showed typical limitations of seasonal forecasting such as lack of sharpness and poor skill for near normal. A specific feature of the RCOF product is a strong overforecasting of near normal, very likely a result of the risk aversion of experts. To better illustrate the usefulness of the outlooks, they were evaluated with respect to a binary warning system by determining the maximum economic value Vmax. This verification indicated moderate valuable precipitation warnings for dry (Vmax = 0.39) and wet (Vmax = 0.34) years for four climatological zones (Sahel, Sudan–Sahel, Sudan, and Guinean) and five river basins (Volta, Senegal, and three Niger subbasins) but with strong regional differences (0.14 < Vmax < 0.54). The bootstrap analysis illustrated large uncertainties, indicating the relevance of uncertainty margins when seasonal forecast products with small sample sizes like RCOF outlooks are evaluated.

scholarly journals Wet and dry spells in Senegal: comparison of detection based on satellite products, reanalysis, and in situ estimates

2021 ◽  
Vol 21 (3) ◽  
pp. 1051-1069
Author(s):  
Cheikh Modou Noreyni Fall ◽  
Christophe Lavaysse ◽  
Mamadou Simina Drame ◽  
Geremy Panthou ◽  
Amadou Thierno Gaye
Keyword(s):  
Measurement Techniques ◽  
West African Monsoon ◽  
Tropical Rainfall Measuring Mission ◽  
Early Warning Systems ◽  
Rain Gauge ◽  
Major Influence ◽  
Dry Spells ◽  
Dry Spell ◽  
Spatio Temporal ◽  
Wet Spells

Abstract. In this study, the detection and characteristics of dry/wet spells (defined as episodes when precipitation is abnormally low or high compared to usual climatology) drawn from several datasets are compared for Senegal. Here, four datasets are based on satellite data (TRMM-3B42 V7, CMORPH V1.0, TAMSAT V3, and CHIRPS V2. 0), two on reanalysis products (NCEP-CFSR and ERA5), and three on rain gauge observations (CPC Unified V1.0/RT and a 65-rain-gauge network regridded by using two kriging methods, namely ordinary kriging, OK, and block kriging, BK). All datasets were converted to the same spatio-temporal resolution: daily cumulative rainfall on a regular 0.25∘ grid. The BK dataset was used as a reference. Despite strong agreement between the datasets on the spatial variability in cumulative seasonal rainfall (correlations ranging from 0.94 to 0.99), there were significant disparities in dry/wet spells. The occurrence of dry spells is less in products using infrared measurement techniques than in products coupling infrared and microwave, pointing to more frequent dry spell events. All datasets show that dry spells appear to be more frequent at the start and end of rainy seasons. Thus, dry spell occurrences have a major influence on the duration of the rainy season, in particular through the “false onset” or “early cessation” of seasons. The amplitude of wet spells shows the greatest variation between datasets. Indeed, these major wet spells appear more intense in the OK and Tropical Rainfall Measuring Mission (TRMM) datasets than in the others. Lastly, the products indicate a similar wet spell frequency occurring at the height of the West African monsoon. Our findings provide guidance in choosing the most suitable datasets for implementing early warning systems (EWSs) using a multi-risk approach and integrating effective dry/wet spell indicators for monitoring and detecting extreme events.

Heavy precipitating events in satellites and rain-gauge products over the Sahel

2021 ◽  
Author(s):  
Sidiki Sanogo ◽  
Philippe Peyrillé ◽  
Romain Roehrig ◽  
Françoise Guichard ◽  
Ousmane Ouedraogo
Keyword(s):  
Burkina Faso ◽  
West African Monsoon ◽  
Extreme Rainfall ◽  
Rain Gauge ◽  
Reference Dataset ◽  
Extreme Rainfall Events ◽  
Microwave Sensor ◽  
Sahel Region ◽  
Distribution Intensity ◽  
Rain Gauge Network

&lt;p&gt;The Sahel has experienced an increase in the frequency and intensity of extreme rainfall events over the recent decades. These trends are expected to continue in the future. However the properties of these events have so far received little attention. In the present study, we define a heavy precipitating event (HPE) as the occurrence of daily-mean precipitation exceeding a given percentile (e.g., 99&lt;sup&gt;th&lt;/sup&gt; and higher) over a 1&amp;#176;x1&amp;#176; pixel and examine their spatial distribution, intensity, seasonality and interannual variability. We take advantage of an original reference dataset based on a rather high-density rain-gauge network over Burkina Faso (142 stations) to evaluate 22 precipitation gridded datasets often used in the literature, based on rain-gauge-only measurements, satellite measurements, or both. Our reference dataset documents the HPEs over Burkina Faso. The 99&lt;sup&gt;th&lt;/sup&gt; percentile identifies events greater than 26 mm d&lt;sup&gt;-1&lt;/sup&gt; with a ~2.5 mm confidence interval depending on the number of stations within a 1&amp;#176;x1&amp;#176; pixel. The HPEs occur in phase with the West African monsoon annual cycle, more frequently during the monsoon core season and during wet years. The evaluation of the gridded rainfall products reveals that only two of the datasets, namely the rain-gauge-only based products GPCC-DDv1 and REGENv1, are able to properly reproduce all of the HPE features examined in the present work. A subset of the remaining rainfall products also provide satisfying skills over Burkina Faso, but generally only for a few HPE features examined here. In particular, we notice a general better performance for rainfall products that include rain-gauge data in the calibration process, while estimates using microwave sensor measurements are prone to overestimate the HPE intensity. The agreement among the 22 datasets is also assessed over the entire Sahel region. While the meridional gradient in HPE properties is well captured by the good performance subset, the zonal direction exhibit larger inter-products spread. This advocates for the need to continue similar evaluation with the available rain-gauge network available in West Africa, both to enhance the HPE documentation and understanding at the scale of the region and to help improve the rainfall dataset quality.&lt;/p&gt;

scholarly journals A New Land Surface Hydrology within the Noah-WRF Land-Atmosphere Mesoscale Model Applied to Semiarid Environment: Evaluation over the Dantiandou Kori (Niger)

2009 ◽  
Vol 2009 ◽  
pp. 1-13 ◽  
Author(s):  
B. Decharme ◽  
C. Ottlé ◽  
S. Saux-Picart ◽  
N. Boulain ◽  
B. Cappelaere ◽  
...  
Keyword(s):  
Land Surface ◽  
Mesoscale Model ◽  
West African Monsoon ◽  
Radar Data ◽  
Rain Gauge ◽  
Surface Energy Budget ◽  
Surface Hydrology ◽  
Land Surface Hydrology ◽  
Rain Gauge Network

Land-atmosphere feedbacks, which are particularly important over the Sahel during the West African Monsoon (WAM), partly depend on a large range of processes linked to the land surface hydrology and the vegetation heterogeneities. This study focuses on the evaluation of a new land surface hydrology within the Noah-WRF land-atmosphere-coupled mesoscale model over the Sahel. This new hydrology explicitly takes account for the Dunne runoff using topographic information, the Horton runoff using a Green-Ampt approximation, and land surface heterogeneities. The previous and new versions of Noah-WRF are compared against a unique observation dataset located over the Dantiandou Kori (Niger). This dataset includes dense rain gauge network, surfaces temperatures estimated from MSG/SEVIRI data, surface soil moisture mapping based on ASAR/ENVISAT C-band radar data and in situ observations of surface atmospheric and land surface energy budget variables. Generally, the WAM is reasonably reproduced by Noah-WRF even if some limitations appear throughout the comparison between simulations and observations. An appreciable improvement of the model results is also found when the new hydrology is used. This fact seems to emphasize the relative importance of the representation of the land surface hydrological processes on the WAM simulated by Noah-WRF over the Sahel.

scholarly journals An Early Performance Evaluation of the NEXRAD Dual-Polarization Radar Rainfall Estimates for Urban Flood Applications

2013 ◽  
Vol 28 (6) ◽  
pp. 1478-1497 ◽  
Author(s):  
Luciana K. Cunha ◽  
James A. Smith ◽  
Mary Lynn Baeck ◽  
Witold F. Krajewski
Keyword(s):  
Kansas City ◽  
Spatial Scales ◽  
Rain Gauge ◽  
Dual Polarization ◽  
Radar Rainfall ◽  
Urban Flood ◽  
Temporal And Spatial ◽  
Single Polarization ◽  
Early Performance ◽  
Rainfall Estimates

Abstract Dual-polarization radars are expected to provide better rainfall estimates than single-polarization radars because of their ability to characterize hydrometeor type. The goal of this study is to evaluate single- and dual-polarization radar rainfall fields based on two overlapping radars (Kansas City, Missouri, and Topeka, Kansas) and a dense rain gauge network in Kansas City. The study area is located at different distances from the two radars (23–72 km for Kansas City and 104–157 km for Topeka), allowing for the investigation of radar range effects. The temporal and spatial scales of radar rainfall uncertainty based on three significant rainfall events are also examined. It is concluded that the improvements in rainfall estimation achieved by polarimetric radars are not consistent for all events or radars. The nature of the improvement depends fundamentally on range-dependent sampling of the vertical structure of the storms and hydrometeor types. While polarimetric algorithms reduce range effects, they are not able to completely resolve issues associated with range-dependent sampling. Radar rainfall error is demonstrated to decrease as temporal and spatial scales increase. However, errors in the estimation of total storm accumulations based on polarimetric radars remain significant (up to 25%) for scales of approximately 650 km2.

scholarly journals Lacunarity, predictability and predictive instability of the daily pluviometric regime in the Iberian Peninsula

2007 ◽  
Vol 14 (2) ◽  
pp. 109-121 ◽  
Author(s):  
M. D. Martínez ◽  
X. Lana ◽  
A. Burgueño ◽  
C. Serra
Keyword(s):  
Spatial Distribution ◽  
Iberian Peninsula ◽  
Lyapunov Exponents ◽  
Atlantic Coast ◽  
Power Laws ◽  
Rain Gauge ◽  
Mediterranean Coast ◽  
Western Atlantic ◽  
Dry Spells ◽  
Dry Spell

Abstract. The complexity of the daily pluviometric regime of the Iberian Peninsula is analysed from the point of view of its lacunarity, predictability and predictive instability. The database consists of daily pluviometric records obtained from 43 rain gauges in Spain and Portugal for the period 1950–1990. Five different series are generated for every rain gauge. The first series is constituted by the consecutive daily amounts. The other four consist of dry spell lengths with respect to daily amount thresholds of 0.1, 1.0, 5.0 and 10.0 mm/day. A dry spell length is defined as the number of consecutive days with rainfall amounts below one of these thresholds. The empirical lacunarity for every rain gauge is well reproduced by two power laws, the exponents varying notably from one gauge to another. The spatial distribution of the lacunarity is characterised by a north to south or southeast gradient, thus suggesting that this parameter can be a useful tool to distinguish between different pluviometric regimes. The predictability of the five series is quantified by means of the rescaled analysis and the interpretation of the Hurst exponent. Its patterns reveal that most part of the Iberian Peninsula shows signs of persistence for the daily rainfall and the dry spell series, although persistence is only clearly manifested in some small domains. The instability of possible predictive algorithms is analysed through the Lyapunov exponents. They are only computed for the series of daily amounts and for dry lengths respect to the threshold level of 0.1 mm/day due to the short number of dry spells for larger threshold levels. The series of daily amounts depict the highest instability along the Mediterranean coast. The series of dry spells show an increasing instability from NE to SW Spain, with a relevant nucleus of high Lyapunov values in the south-western Atlantic coast. As a summary, lacunarity and Hurst and Lyapunov exponents depict a relevant spatial variation, which is in agreement with well known patterns of the pluviometric regime, such as annual amount spatial distribution and return periods of dry spells.

scholarly journals Differences in Draft Core Statistics from the Wet to Dry Spell over Gadanki, India (13.5°N, 79.2°E)

2009 ◽  
Vol 137 (12) ◽  
pp. 4293-4306 ◽  
Author(s):  
T. Narayana Rao ◽  
K. N. Uma ◽  
T. Mohan Satyanarayana ◽  
D. Narayana Rao
Keyword(s):  
Vertical Structure ◽  
Elevation Angle ◽  
Lower Troposphere ◽  
Bimodal Distribution ◽  
Rain Gauge ◽  
Core Size ◽  
The Core ◽  
Surface Rainfall ◽  
Dry Spells ◽  
Dry Spell

Abstract The Indian mesosphere–stratosphere–troposphere (MST) radar observations during the passage of 37 convective systems are utilized to investigate the characteristics of vertical air velocity w in different convection categories (shallow, deep, and decaying) and also the differences in draft core statistics from the wet to dry spell. The radar and optical rain gauge measurements show pronounced differences in core statistics (in terms of their vertical structure, draft strength, size, number, and the elevation angle) and surface rainfall characteristics from the dry to wet spell. The shallow convective cores are preponderant in the dry spell. Composite w profiles, retrieved from all deep cases and also from individual convection cases, depict an upper-tropospheric peak in the wet spell and a bimodal distribution (peaks at 5 and 11–13 km) in the dry spell, illustrating that they are characteristic features of wet and dry spells. The average vertical extents of the cores are nearly equal (about 8 km) in both spells of the monsoon; however, the core-base (and top) altitudes are different. In both wet and dry spells, the composite w profile for all cores show similar vertical variation to that of for updraft cores, while the composite w for downdraft cores do not show much variation with altitude, indicating that the updraft cores dictate the vertical structure of composite w. The core size varies considerably (a factor of 2) with altitude in both spells of the monsoon. Although nearly equal in the lower troposphere in both phases of the monsoon, the core size is larger by 1–2 km in the dry spell in the middle and upper troposphere. Consistent with the longer lifetime (bigger core size) of cores in the dry spell, the cores are more inclined (with a mean elevation angle of 30°) in the dry spell. The surface rainfall distribution is wider and has large number of intense rainfall rates in the wet spell. The mean rainfall rate for the wet spell is also larger by a factor of 2, consistent with earlier studies.

scholarly journals A Stochastic Approach for the Analysis of Long Dry Spells with Different Threshold Values in Southern Italy

Water ◽  
2019 ◽  
Vol 11 (10) ◽  
pp. 2026 ◽  
Author(s):  
Sirangelo ◽  
Caloiero ◽  
Coscarelli ◽  
Ferrari
Keyword(s):  
Return Period ◽  
Southern Italy ◽  
Simulation Analysis ◽  
Statistical Significance ◽  
Synthetic Data ◽  
Poisson Model ◽  
Rain Gauge ◽  
Mean Values ◽  
Dry Spells ◽  
Dry Spell

A non-homogeneous Poisson model was proposed to analyze the sequences of dry spells below prefixed thresholds as an upgrade of a stochastic procedure previously used to describe long periods of no rainfall. Its application concerned the daily precipitation series in a 60-year time span at four rain gauges (Calabria, southern Italy), aiming at testing the different behaviors of the dry spells below prefixed thresholds in two paired periods (1951–1980 and 1981–2010). A simulation analysis performed through a Monte Carlo approach assessed the statistical significance of the variation of the mean values of dry spells observed at an annual scale in the two 30-year periods. The results evidenced that the dry spells durations increased passing from the first 30-year period to the second one for all the thresholds analyzed. For instance, for the Cassano station, an increase of about 10% of the maximum dry spell duration was detected for a threshold of 5 mm. Moreover, the return periods evaluated for fixed long dry spells through the synthetic data of the period 1981–2010 were lower than the corresponding ones evaluated with the data generated for the previous 30-year period. Specifically, the difference between the two 30-year periods in terms of the return period of long dry spells occurrence increased with the growing thresholds. As an example, for the Cosenza rain gauge with a threshold of 1 mm, the return period for a dry spell length of 70 days decreased from 20 years (in 1951–1980) to about 10 years (in 1981–2010), while for a threshold of 5 mm, the return period for the dry spell lengths of 120 days decreases from 70 years to about 20 years. These results show a higher probability of the occurrence of long dry spells in the more recent period than in the past.

scholarly journals Characteristics of wet and dry spells in the West African monsoon system

2017 ◽  
Vol 18 (3) ◽  
pp. 125-131 ◽  
Author(s):  
Stéphanie Froidurot ◽  
Arona Diedhiou
Keyword(s):  
West African Monsoon ◽  
West African ◽  
The West ◽  
African Monsoon ◽  
Dry Spells ◽  
Monsoon System

scholarly journals Hydrological evaluation of open-access precipitation data using SWAT at multiple temporal and spatial scales

2020 ◽  
Author(s):  
Jianzhuang Pang ◽  
Huilan Zhang ◽  
Quanxi Xu ◽  
Yujie Wang ◽  
Yunqi Wang ◽  
...  
Keyword(s):  
Open Access ◽  
Water Balance ◽  
Spatial Scales ◽  
Rain Gauge ◽  
Surface Flow ◽  
Base Flow ◽  
Detection Methods ◽  
Hydrological Process ◽  
Water Balance Components ◽  
Temporal And Spatial

Abstract. Temporal and spatial precipitation information is key to conducting effective hydrological process simulation and forecasting. Herein, we implemented a comprehensive evaluation of three selected precipitation products in the Jiang River Watershed (JRW) located in southwest China. A number of indices were used to statistically analyze the differences between two open-access precipitation products (OPPs), i.e. Climate Hazards Group Infra-Red Precipitation with Station (CHIRPS) and CPC-Global (CPC), and the rain gauge (Gauge). The three products were then categorized into sub-basins to drive SWAT simulations. The results show: (1) the three products are highly consistent in temporal variation on a monthly scale, yet distinct on a daily scale. CHIRPS is characterized by overestimation of light rain, underestimation of heavy rain, and a high probability of false alarm. CPC generally underestimates rainfall of all magnitudes; (2) All three products satisfactorily reproduce the stream discharges at the JRW outlet with better performance than the Gauge model. On a temporal scale, the OPPs are inferior with respect to capturing flood peak, yet superior at describing other hydrograph features, e.g. rising and falling processes and base flow. On a spatial scale, CHIRPS offers the advantage of deriving smooth, distributed precipitation and runoff due to its high resolution; (3) The water balance components derived from SWAT models with equal simulated streamflow discharges are remarkably different between the three precipitation inputs. The precipitation spatial pattern results in an increasing surface flow trend from upstream to downstream. The results of this study demonstrate that evaluating precipitation products using only streamflow simulation accuracy will conceal the dissimilarities between these products. Hydrological models alter hydrologic mechanisms by adjusting calibrated parameters. Specifically, different precipitation detection methods lead to temporal and spatial variation of water balance components, demonstrating the complexity in describing natural hydrologic processes.

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