Relating Point to Area Average Rainfall in Semiarid West Africa and the Implications for Rainfall Estimates Derived from Satellite Data

I. D. Flitcroft Department of Meteorology, University of Reading, Reading, United Kingdom

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J. R. Milford Department of Meteorology, University of Reading, Reading, United Kingdom

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G. Dugdale Department of Meteorology, University of Reading, Reading, United Kingdom

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Abstract

The variability of rainfall over small areas (100 km2) in the West African Sahel has been investigated using a dense network of raingages in the Republic of Niger. Rainfall was as well or better correlated over small distances as rainfall in other semiarid climates. A regression model was developed to specify the systematic correction required, when a raingage measurement is used to represent an area average rainfall and to quantify the errors involved. It was found that in this particular climate large point measurements are not representative of even small areas and need to be reduced by a factor which depends on the size of the area. Rainfall estimates based on satellite observations often use point rainfall to calibrate and validate estimates of areas average (pixel) rainfall. The effect of satellite location errors on the correct pairing of point measurements and pixel estimates, and the subsequent effect on the assessment of the accuracy of rainfall estimates based on satellite data are discussed.

Abstract

The variability of rainfall over small areas (100 km2) in the West African Sahel has been investigated using a dense network of raingages in the Republic of Niger. Rainfall was as well or better correlated over small distances as rainfall in other semiarid climates. A regression model was developed to specify the systematic correction required, when a raingage measurement is used to represent an area average rainfall and to quantify the errors involved. It was found that in this particular climate large point measurements are not representative of even small areas and need to be reduced by a factor which depends on the size of the area. Rainfall estimates based on satellite observations often use point rainfall to calibrate and validate estimates of areas average (pixel) rainfall. The effect of satellite location errors on the correct pairing of point measurements and pixel estimates, and the subsequent effect on the assessment of the accuracy of rainfall estimates based on satellite data are discussed.

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