Annual rainfall |
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
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.