Date:
Meeting:
28 November 2001
RSSPWG
Document Filename:
Paper Number:
116097969
RSSP (05-01)/42
A rain-rate map for the UK
To improve the accuracy of rain-rate data, for rates exceeded for 0.01% time in the UK, it is
proposed to use high-resolution UK data where available, with the ITU-R data associated with
P.837-2 elsewhere. This document records the process of building this data-set.
1. The ITU data
Data from P.837-2 was extracted for
a regular grid of points at 5 km
intervals of the UK grid. The
geographic area covered by the
table is given by the coordinates of
its NW and SW corners as follows:
Corner
E km
N km
NE
800
1300
SW
0
-100
This is plotted on the right. The two
areas of highest rate, both exceeded
45 mm/h, are in W. Scotland and
along the coast of Norway.
The 'X' and 'Y' scales are UK grid
coordinates E and N of SV000000
respectively (apologies for the inept
N scaling). The contours are at
intervals of 5 mm/h.
2. The BADC data
A table of rain rates for the same UK grid points was extracted from a file supplied by Bath
University in which the data were given at intervals of 0.025 degrees of latitude and
longitude. A 361  361 array of data was provided, covering a geographic area given by the
longitude and latitude of the
SW and NE corners as follows:
Corner lon deg lat deg
NE
2.0
59.0
SW
-7.0
50.0
This is plotted on the right.
Where the BADC data was not
available the rate has been set
to zero.
Compared to the ITU data, the
BADC data provides
considerable additional detail
in the south and midlands of
the UK mainland. It does not
show any evidence of the
rainfall peak in W Scotland.
There are indications on this
plot of the artifacts (more
evident in the Bath report)
roughly along the SE and E
coasts.
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3. Difference between data
The plot on the right shows the
difference BADC Minus ITU
data where BADC data exists.
The BADC data gives as much
as 20 mm/h less than the ITU
data in W Scotland, but up to
20 mm/h more in the SE of
England, although this latter
maximum is rather close to part
of the artifacts mentioned
above.
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4. Combining the data
The data were combined
by the following
processes:
The BADC data is used
where it exists and is free
of artifacts. An
interpolation procedure is
used to produce a
transition region across
which the relative
proportions of BADC and
ITU data are varied
linearly. The interpolation
zones are defined by a set
of straight lines in UK grid
coordinates, as illustrated.
The lines forming the
western limits have been
chosen to permit the ITU
data showing high rainfall
in W.Scotland, which is
believed to be realistic, to
appear in the final results.
The BADC component of
the final data will 100%
inside the blue lines, and
0% outside the red lines,
with a linear transition
between them.
Where the interpolation zones overlap the smaller of the two BADC proportions is used.
The coordinates of the lines are:
 45 100 

 200 1300 
W0  
 400
E1   600

 420
 0
S0  
 800
100 
260 

1300 
20 

20 
 0 950 

 800 950 
N1  
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 95 100 

 400 1300 
W1  
 500
E0   650

 470
 0
S1  
 800
100 


1300 
70 

70 
260
 0 1000 

 800 1000 
N0  
Lines on the west side of the map,
defining 0 BADC and 1 BADC
Lines on the east side of the map
South side
North side
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5. The UK rain-rate map:
The final result shows that the region of high rainfall rate in W Scotland most of the detail in
S.E. England have been preserved. Within central UK mainland the BADC data shows its
finer structure than the ITU data, and there is now only slight evidence of the BADC artifacts.
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The difference made by the Bath data
This diagram shows the difference UK-map minus ITU data, and is thus zero outside the area
covered by the BADC data. The two main trends, in addition to the fine detail in the BADC
data not found in the ITU data, is that lower rain rates (5 mm/h less in a few places) will be
obtained in the W Scotland, and higher rain rates (up to 15 mm/h more) in S.E. England.
(The straight-line boundaries of the contour plot on the W and N sides are not discontinuities.
They show where linear interpolation towards the BADC data starts. The plot colour changes
as soon as zero is exceeded.)
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