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Rainfall Variability Analysis of rainfall variability and drought in the 1961-2002 period
Annual Rainfall Maps

Global analysis  Annual rainfall | National rainfall index | National rainfall index
Case studies  Burkina Faso | Cambodia | Nepal | Tanzania
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1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 movie

The choice of the data source
To improve on the methodology published as No.9 in the FAO Agrometeorology Working Paper Series and for an update of the previous work global annual rainfall grids instead of station data that become available since 1994 are compared.

Global annual rainfall grids are produced by the Global Precipitation Climatology Center (GPCC, DWD/Frankfurt), the Climate research Unit (CRU, University of east Anglia, UK) and the National Center for Environmental Prediction (NCEP, US weather service). The data set of the GPCC has 1 degree resolution and covers the 1986 to 2002 period. The second (CRU) has 0.5 degrees resolution and covers the period 1941 to 1998. The NOAA data (PRECipitation REConstruction over Land - PREC/L) produced by the National Centre for Environmental Prediction (NCEP, US Weather Service) is considered interesting due to its availability, covering the whole period from 1961 to 2002. As for the CRU data the resolution amounts 0.5 degree.

A comparison between GPCC and CRU data series , in spite of GPCC reprojection, can provide misleading information because of the different resolution. The 1 degree GPCC images have been re-projected to the same 0.5 degrees resolution. Through reprojection, pixels to be compared are given the same dimension, but every pixel originally of 1 degree is simply replaced by four pixels with the same value. Consequently, relationsship assessment between CRU and GPCC precipitation values is more difficult in non-homogneous areas and can easily lead to analysis errors, especially when rainfall differences among adjacent pixels are high.

An analysis of the relationship among the tree sources of data -CRU, GPCC and NOAA- has been carried out over a list of stations randomly chosen, situated in North, Central and South America, Africa, Asia (China), and characterized by different climatological conditions. The correlations have confirmed a better comparability between NOAA station data (0.5 degree) and CRU (0.5 degree), rather than between NOAA and GPCC (1 degree).
Additionaly, 51 arid and semi-arid African countries have been examined in order to carry out a comparison between GPCC and CRU national rainfall averages of 1993. It has been considered the year 1993 since data published on the FAO Agrometeorology Workling Paper go until this year. For the same African countries, a comparison between NOAA and GPCC/CRU has been carried out. The result is a good comparibility among NOAA and CRU series.

Due also to its availability since 1961 to 2002, NOAA series reliability has been further evaluated. An analyses of its relationsship with the National Rainfall Index (NRI) of the African countries shows, that except for those countries where rainfall values are lower - i.e. Botswana, Maritana, Niger, Chad and Mali- NOAA data can be considered quite similar with NRI calculated data. Therefore, the comparison among the tree sources of data led to the selection of the NOAA series.

Global rainfall maps in equal area projection

The NOAA data set, produced by the National Climatology Centre for Environmental Prediction (NCEP, US Weather Service), has been used to create maps of global average rainfall for the period 1961 to 2002. As most commonly used, the annual rainfall amounts, expressed in mm/month, are presented by calendar year. It is worth mentioning that the images are converted into Hammer-Aitoff projection. This projection is an equal-area map projection which displays the world on an ellipse. The menu above allows to scroll between the different years.

Annual rainfall maps (and the reference rainfall 1986-2000) are also used to create global annual maps of the national rainfall index (NRI). Consequently, corrections to account for N-S hemisphere differences are applied. The rainfall images used to compute NRI are composite images. In the northern hemisphere, where the agricultural season coincides with the calendar year, accumulated rainfall totals from January to December are illustrated for every year. But in the southern hemisphere, July rainfall of one year to June rainfall of the following year corresponds to the same agricultural season. So, the composite image of the year 1982 for example, displays in the northern hemisphere the annual rainfall totals of the calendar year 1982, but in the southern hemiphere the July 1981 to June 1982 rainfall total.

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