placing heavy rainfall events in context using long time series: an example from the north york...
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Placing heavy rainfall events in context using long time series: An example from the North York MoorsJonathan Hopkins, Jeff Warburton and Tim BurtUniversity of Durham
Heavy rainfall events in upland areas have
important implications for flooding, erosion,
and sediment transport. The characteristics
of many upland runoff events often result in
‘flash’ floods, which present a danger to life
due to rapidly rising floodwaters (Gruntfest
and Handmer, 2001). Recent flash floods fol-
lowing heavy rainfall have occurred in
Boscastle, Cornwall (2004), Ryedale, North
Yorkshire (2005) and Alston, Cumbria (2007).
For settlements located in upland or steep-
sided valleys, flash floods are a significant
threat due to the risk of rapid runoff in these
areas. Therefore, studies of long-term trends
in heavy rainfall, where available, are particu-
larly important in assessing this risk.
The magnitude and frequency of heavy
rainfall events (indicated using daily totals)
appear to have increased in the UK since
c. 1960, with an increase in the number of
heavy rain days in winter; however, summer
rain days appear to have declined in fre-
quency (Osborn et al., 2000; Osborn and
Hulme, 2002; Fowler and Kilsby, 2003;
Jenkins et al., 2007; Maraun et al., 2008).
Studies of rainfall in northern England show
similar trends: increases in winter rainfall
totals and heavy rainfall events, but a decline
in summer totals and heavy rainfall events,
for example in Cumbria (Malby et al., 2007).
Additionally, the long-running Durham
record shows an increase in heavy rainfall in
the latter half of the twentieth century (Burt
and Horton, 2007). The aim of this paper is
to determine whether heavy rainstorms are
becoming more frequent in upland catch-
ments by assessing the significance of the
recent 19 June 2005 event which caused a
major flash flood in upper Ryedale (North
Yorkshire), using a 93-year rainfall record.
Case study: Heavy rainfall and flash flood in upper Ryedale, 19 June 2005
In this paper, the terms ‘Ryedale’ and ‘upper
Ryedale’ refer to the valley of the River
Rye upstream of Helmsley (SE 615 845,
altitude c. 50 metres) in the western North
York Moors National Park in northeast
England (Figure 1). The River Rye and its
tributaries drain much of the western
North York Moors, an area of peat moor-
land and grassland. The catchment area of
upper Ryedale is 210 square kilometres
with a maximum altitude of 454 metres at
Round Hill (NZ 594 015, Figure 2). Average
annual rainfall varies from 950 to 1000 mil-
limetres in the highest parts of the catch-
ment to approximately 850 millimetres at
Helmsley (CEH, Wallingford, 2005).
On 19 June 2005, a heavy rainstorm in
upper Ryedale caused a major flash flood.
The day of the flood saw high temperatures
across the region: a weather station at
Carlton-in-Cleveland (NZ 508 039, Figure 2)
to the north of Ryedale recorded a maxi-
mum temperature of 28.1ºC (Cinderey,
2005). The overall synoptic situation is
shown in Figure 3. Instability in the atmos-
phere was caused by the passage of a split
cold front (Sibley, 2009) and the oro-
graphic influence of the North York Moors,
which led to rapid convection and a series
of thunderstorms, causing intense, local-
ised downpours over the western moors.
A tipping bucket rain gauge at Hawnby
(SE 542 894, Figure 2) recorded a daily
rainfall of 69.6 millimetres, of which
59.4 millimetres fell in one hour between
1600 and 1700 UTC. A second gauge at
Bilsdale, Poole House (NZ 562 000, Figure 2)
Figure 1. The location of the River Rye in the North York Moors. (Map data © Crown Copyright/
database right 2009. An Ordnance Survey/EDINA supplied service.)
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Heavy rainfall in long tim
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11 kilometres north of Hawnby recorded
a slightly higher daily total of 73.5 milli-
metres. Nearby upland rain gauges
recorded significant (but lower) accumula-
tions, including 44.1 millimetres at Church
Houses (SE 668 974, Figure 2) and 22.2 mil-
limetres at Osmotherley (SE 457 967,
Figure 2); recorded totals declined rapidly
away from the upper Ryedale area
(Cinderey, 2005). Rainfall reconstructions
based on radar predictions by Wass et al.
(2008) suggest peak rainfall accumulations
of c. 120 millimetres at Sutton Bank to the
west of Ryedale (SE 515 825, Figure 2);
Sibley (2009) produced an additional esti-
mate of 89 millimetres using a further
radar analysis. Walker (2008) suggested a
maximum storm rainfall of 200 millime-
tres, based upon runoff calculations at
Boltby Reservoir (SE 495 885, Figure 2). As
these estimates cannot be directly veri-
fied, the recorded Hawnby total is used for
analysis in this study. Runoff from hill-
slopes was rapid, with the River Rye peak-
ing within 1.5 hours of the rainfall event
(Wass et al., 2008). The flood caused major
damage to infrastructure and buildings
1960. The remainder of the record is com-
piled from three gauges: Hawnby (SE 569
925, altitude 123 metres), January 1961 to
March 1977; Bilsdale, Spout House (SE 575
936, altitude 143 metres), April 1977 to
November 2003 and Hawnby No. 2 (SE 542
894, altitude 112 metres), September 2004
onwards. Hawnby and Bilsdale, Spout House
are situated within 1.3 kilometres of each
other on the valley floor of Bilsdale and
Hawnby No. 2 is located four kilometres fur-
ther down the valley (Figure 2). The daily
rainfall totals from these gauges have not
been adjusted.
Minor gaps in these records were filled
using modified daily values from two nearby
rain gauges: Hambleton Greystones (SE 528
830, altitude 271 metres) was used to fill
four months and Coxwold Stores (SE 668
974, altitude 70 metres) to fill in nine
months and 14 days (Figure 2). Monthly,
seasonal and annual rainfall totals are regu-
larly used to fill gaps in series (Aguilar et al.,
2003; Burt, 2009), although no clear guide-
lines exist for daily totals (Burt and Horton,
2007). Linear regression is commonly used
to fill gaps in hydrological series where
nearby monitoring stations exist (Aron and
Rachford, 1974): this method has been used
to modify daily totals from Hambleton
Greystones and Coxwold Stores. Strong cor-
relations (coefficients >0.8) exist between
the Bilsdale/Ryedale gauges and the records
above. Remaining missing days (six in the
Ampleforth record) were replaced with
0 millimetres rainfall. Therefore, a complete
rainfall series between January 1916 and
August 2009 has been constructed.
Due to Ampleforth’s position on the
southern edge of the North York Moors, it
receives slightly lower annual rainfall totals
than the gauges which are situated in
upland valley locations (a comparison of
rainfall with Hawnby from 1961 to 1972
shows that Ampleforth received 16.6% less
rainfall than Hawnby, 14 kilometres away).
In order to account for this, annual and sea-
sonal rainfall totals are compared using nor-
malised totals (each total expressed as a
percentage of the long-term mean). Annual
totals at Ampleforth are quoted as percent-
ages of the Ampleforth mean annual total
from 1916 to 1960 (755.7 millimetres) and
annual rainfall totals at the upland gauges
are percentages of the 1961 to 2008 mean
at these stations (886.6 millimetres). A simi-
lar procedure has been used for seasonal
rainfall totals.
Analyses of heavy falls of rain follow the
methodology of Osborn et al. (2000),
Osborn and Hulme (2002) and Burt and
Horton (2007). Two thresholds of heavy
daily rainfall are used: (1) ‘category 10’
or T10 threshold defined by Osborn et al.
(2000) as the daily rainfall total above which
the heaviest 10% of rainfall has occurred (on
‘rain days’, or days with rainfall >0.25
Figure 2. Map of the upper Ryedale region, showing rain gauges used in the data analysis and
locations referred to in the text. (Map data © Crown Copyright/database right 2009. An Ordnance
Survey/EDINA supplied service.)
throughout upper Ryedale, particularly in
the villages of Hawnby and Rievaulx (SE
575 855, Figure 2) and the town of
Helmsley. The flood received extensive
local and national press coverage and
prompted an emergency debate in parlia-
ment. While the floods of 2005 were cer-
tainly a rare event, a key question that
arises is how unusual the rainstorm was in
the context of the history of the Ryedale
area.
Data sources and methodology
Many UK rainfall records are relatively short
and of limited use in deciphering long-term
trends (Lane, 2008). To overcome this prob-
lem, the rainfall record used in the following
analysis is a composite record for the period
January 1916 to August 2009. The rain
gauges used to construct this rainfall series
are shown in Figure 2 (British Atmospheric
Data Centre (UK Met Office, 2006)).
The first part of the record is composed
of a single series from Ampleforth (SE 598
789, altitude 95 metres), used from 1916 to
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millimetres); (2) DR1, a modified version of
that used by Karl and Knight (1998), rep-
resenting the rainfall total exceeded on 1%
of all days in the record. These thresholds
do not necessarily represent a rainfall
capable of generating a major flash flood,
but they are suitable for an assessment of
long-term changes in the frequency of
heavy rainfall. In order to assess whether
trends in the frequency of heavy rain days
correspond with wet years and periods,
annual and seasonal rainfall totals are
analysed.
In the following analysis decades run
from xx01 to xx00, and seasons are defined
as three-month periods: spring = March,
April and May; summer = June, July and
August; autumn = September, October and
November and winter = December, January
and February. Winter 1971 therefore
includes December 1970, January and
February 1971.
Annual and seasonal rainfall totals
The 10-year running mean of normalised
annual totals (Figure 4) shows three wet
periods with above-average rainfall: the late
1920s and 1930s, the late 1970s and 1980s
and the late 1990s to present. The earliest
of these periods includes two of the ten
wettest years on record, 1927 and 1930
(Table 1); the mean annual rainfall total in
the 1930s is the second highest of all dec-
ades (Table 2). The 1950s are the second
wettest decade in this record (Table 2): this
decade includes three of the ten wettest
years, 1951, 1956 and 1960 (Table 1). Later
periods of above-average rainfall include
1977–1981, and the mid-1980s. The late
1990s to present have been extremely wet:
from 1998 to 2008, annual rainfall totals
have been, on average, 12% above the
1916–2008 mean, and this most recent
period includes the first and second wettest
years in the Ryedale record (2000 and 2008)
and also the eighth and tenth wettest (2004
and 1999) (Table 1). Since 1961 there has
been an increase in normalised rainfall
totals which is statistically significant
(p-value = 0.0496). Periods outside those
identified above saw generally below-
average annual totals, particularly in the
1910s and early 1920s, the 1940s, 1960s and
1970s, and the early-to-mid 1990s (Table 2,
Figure 4).
For seasonal rainfall totals, there has been
a clear increase in the winter:summer rain-
fall ratio from c. 1960 to c. 2003, following
a decline from the early 1940s (Figure 5).
From 1961 to 2000, the positive trend in this
ratio lies just outside statistical significance
at the 95% confidence level (p = 0.08). Since
c. 2003, however, the ratio has declined
(Figure 5) following several summers with
above-average rainfall.
Annual trends in heavy rain days
For annual frequencies of heavy rain days,
there are no clear trends in DR1 or T10 daily
falls across the entire period, although from
the mid-1960s onwards running means
have generally increased as a result of a
particularly low number of heavy rain days
in the 1960s and a relatively high frequency
of heavy rain days in the 1980s and late
1990s to 2008 (Figure 6). This increase, from
1961 to 2008, lies just outside statistical sig-
nificance with p values of 0.06 (DR1) and
0.07 (T10). The 1960s have the lowest fre-
quency of heavy rain days of any complete
decade in the record, at both DR1 and T10
thresholds (Table 3). The 1930s have by far
the highest number of heavy rain days on
record (50) (Table 3), with the 1950s, 1970s,
1980s and 1990s also recording above-
average numbers of DR1 heavy rain days
(Figure 6, Table 3). The 2001–2008 period,
whilst not a complete decade, has wit-
nessed an above-average number of heavy
rain days per year. Heavy rain days are there-
fore particularly numerous in the periods
observing high annual rainfall totals as
Figure 3. Synoptic situation at 1200 UTC, 19 June 2005. (© Crown copyright 2005, the Met Office.)
Figure 4. Normalised annual rainfall totals for the Ryedale record, 1916–2008, showing 10-year
running mean.
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Heavy rainfall in long tim
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defined above. The year with the highest
number of heavy rain days at both thresh-
olds is 2000 (9 DR1, 8 T10); however, the late
1920s–1930s period has three years which
record eight DR1 rain days each (1927, 1930,
1935) and two years which record seven
(1931, 1933).
In terms of the proportion of annual rain-
fall falling on days with heavy rainfall, a simi-
lar pattern to the heavy rain day frequency
emerges (Figure 7), with the 1930s having
the highest heavy rain day (DR1 threshold)
contribution to annual rainfall of any dec-
ade (Table 3). Notably, the peak in the
10-year running mean in the early-to-mid
1950s is higher and more pronounced than
it is for heavy rain day frequencies (Figure 7):
DR1 rain days produced over 25% of annual
rainfall in 1948, 1951 and 1956. Recent years
have also seen a high proportion of rainfall
falling as heavy rainfall, as the years 2000
and 2002 were two of only seven years
across the 1916–2008 period to have a con-
tribution to annual rainfall from T10 rain
days greater than 20%.
Considering the maximum daily rainfall
recorded annually, it is interesting to note
that the rainstorm which caused the flood
in Ryedale in 2005 is only the fifth heaviest
daily rainfall on record (Table 4), as days in
2002 (87 millimetres), 1948 (81.8 millime-
tres), 1976 (75 millimetres) and 1940 (73.4
millimetres) recorded more rainfall. The
2002 and 2005 falls are part of a set of very
high annual daily maxima at the end of the
record, as 1997 and 2000 also recorded daily
falls of 67.6 millimetres and 53.7 millimetres,
respectively. By contrast, the highest maxi-
mum recorded from 1990 to 1996 was only
39.9 millimetres. Figure 8 indicates a general
increase in the maximum daily rainfall
recorded annually since the 1960s: this, how-
ever, is not statistically significant between
1961 and 2008 (p = 0.2).
Seasonal trends in heavy rain days
There are no statistically significant trends
in seasonal frequencies of heavy rain days
(DR1 threshold), or the proportion of sea-
sonal rainfall contributed by heavy rain
days, across either the whole record (January
1916–August 2009) or the shorter January
1961–August 2009 period. The contribution
of heavy rainfall to winter rainfall totals,
however, has generally increased from the
1940s to the late 1990s. Meanwhile, the
summer running mean has remained well
below its 1950s peak (Figure 9). Since
c. 2000, there has been a marked decline in
the contribution of heavy rain days in winter
(the 10-year running mean is at its lowest
value), and a sharp rise in the proportion of
summer rainfall contributed by heavy rain
days (Figure 9).
Over the 1916–2008 period, heavy daily
rainfalls tend to occur predominantly in
summer (36% of all DR1 heavy rain days)
and autumn (33%); 17% of DR1 rain days
have fallen in winter and just 14% have
fallen in spring. Summer rain days tend to
be heavier than those in other seasons: six
of the highest ten daily rainfall totals in the
Ryedale record have occurred in summer
(Table 4). Additionally, the mean of the high-
est 100 daily totals recorded in summer is
33.7 millimetres: comparative averages for
winter, spring and autumn are 24.5 millime-
tres, 25.1 millimetres and 29.7 millimetres
respectively. Furthermore, the largest daily
Table 1
The 10 years with the highest and lowest normalised annual rainfall totals, respectively, in
the Ryedale record, 1916–2008.
Highest annual rainfall totals Lowest annual rainfall totals
Rank Year Normalised rainfall (%) Rank Year Normalised rainfall (%)
1 2000 135.6 1 1964 58.9
2 2008 128.8 2 1921 64.7
3 1960 126.1 3 1959 71.7
4 1930 123.3 4 1973 73.8
5 1927 123.0 5 1949 74.5
6 1956 120.6 6 1975 75.8
7 1951 120.6 7 1989 75.8
8 2004 120.5 8 1991 78.5
9 1978 119.7 9 1996 80.1
10 1999 118.3 10 1971 80.7
Table 2
Decadal averages of normalised annual and seasonal rainfall totals, Ryedale, 1916–2008.
Period Annual Winter Spring Summer Autumn
1916–1920 93.8 90.0 111.2 96.8 83.5
1921–1930 97.7 98.6 92.3 97.9 100.0
1931–1940 105.9 109.6 107.0 95.3 112.0
1941–1950 98.5 98.3 103.4 96.4 96.0
1951–1960 101.1 97.6 91.7 112.0 100.2
1961–1970 96.2 91.0 95.0 101.7 97.2
1971–1980 96.9 106.0 94.7 90.9 95.7
1981–1990 100.8 94.2 107.1 100.8 99.8
1991–2000 99.8 102.3 103.0 82.6 109.7
2001–2008 107.7 108.0 100.2 129.9 96.8
Figure 5. Winter:summer rainfall ratio, Ryedale, January 1916–August 2009, showing 10-year
running mean.
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rainfall in winter across the whole record
(43.2 millimetres in 1979) is exceeded on 15
summer days overall, as well as eight days
in both spring and autumn.
Discussion
Despite large variability in heavy rainfall, it
has been possible to identify certain periods
when a large number of heavy rainfall
events have taken place in upper Ryedale
since 1916: the late 1920s and 1930s, the
1950s, 1980s and the late 1990s to the
present. In terms of heavy rain day fre-
quency, the late 1920s to 1930s is the wet-
test period in the record. Since 1997, there
have been a high number of heavy rain days
as well as a number of very high annual
maximum falls. Intervening periods (espe-
cially the 1960s, the bulk of the 1970s and
early-to-mid 1990s) have largely seen fewer
heavy rain days, lending support to the find-
ings of Lane (2008), who argued that,
nationwide, the recent ‘flood-rich period’
from the late 1990s to the present followed
a relatively flood-poor period from 1960,
which itself followed a period of higher
flood frequencies.
In the context of the increase in the winter:
summer rainfall ratio over the latter half of
the twentieth century (Figure 5), recent heavy
summer rainfalls in Ryedale (2005, 2002,
1997) appear unusual. Trends to wetter win-
ters (with more intense precipitation) and
drier summers have been found by other
rainfall studies in northern England (Burt and
Horton, 2007; Malby et al., 2007). Climate
models also suggest future increases in heavy
winter rainfall (Murphy et al., 2009). However,
since c. 2000 the frequency of heavy rain
days in winter (and winter rainfall totals) has
declined and summer rainfall totals, and the
proportion of summer rainfall falling on
heavy rain days, has increased. Nationwide,
the picture is similar, as recent summer
flooding is at odds with the dominant rainfall
trend of the latter part of the twentieth cen-
tury and climate change predictions which
suggest increasing frequencies of heavy
rainfall in winter and a decrease in summer
(Marsh and Hannaford, 2007; Lane, 2008).
However, over the 1916–2008 long-term
record, the heavy rainfall recorded on 19
June 2005 is less unusual, as four other daily
totals have exceeded it, and summer rain
days in Ryedale have been typically heavier
than their counterparts in other seasons.
Nationally, heavy and extreme rainfall events
(as defined at various daily thresholds above
50 millimetres) typically occur predominantly
in summer (Burt, 2005; Rodda et al., 2009)
due to higher temperatures leading to heavy
convectional rainfall: a mechanism frequently
associated with flash floods in upland areas,
including that in Ryedale in 2005.
The flash flood which occurred in upper
Ryedale in 2005 was an extreme hydrological
event, with an extraordinarily extreme esti-
mated return period (in excess of 2400 years)
for peak river flow, far exceeding the com-
parative rarity of the causative rainfall event
(Wass et al., 2008). The rainstorm which
occurred in Hawnby on 19 June 2005 was
clearly an unusual rainfall event in terms of
its intensity and magnitude. It is, however,
not as rare as might have initially been
thought when viewed in the context of a
93-year rainfall record for upper Ryedale.
Although the 1997–2008 period has seen a
large number of heavy rainfall events, larger
daily rainfall totals have been recorded than
those in 2005. In addition to the Ryedale area,
the North York Moors uplands have experi-
enced rainfalls of greater intensity and overall
rainfall accumulation than the 19 June 2005
Table 4
Largest daily rainfalls recorded in Ryedale,
1916–2008.
Date
Rainfall
(millimetres)
1 August 2002 87.0
2 June 1948 81.8
11 September 1976 75.0
17 July 1940 73.4
19 June 2005 69.6
5 November 1967 67.8
31 August 1997 67.6
28 March 1979 60.1
30 June 1988 58.8
21 May 1918 54.3
Table 3
Decadal means of heavy rain day frequency and the proportion of annual rainfall falling
on heavy rain days in Ryedale, 1916–2008. Figures in brackets indicate the average
frequency of events per year in ‘incomplete’ decades (1916–1920, 2001–2008). Additionally
the mean frequency of events per decade does not include these incomplete decades.
Frequency of heavy rain days
Proportion of annual rainfall
falling on heavy rain days (%)
Period DR1 T10 DR1 T10
1916–1920 15 (3) 11 (2.2) 11.4 9.1
1921–1930 35 19 11.3 6.8
1931–1940 50 33 17.1 12.7
1941–1950 33 23 12.7 9.9
1951–1960 38 28 13.7 11.0
1961–1970 27 18 10.0 7.5
1971–1980 38 22 14.1 9.6
1981–1990 40 32 14.7 12.6
1991–2000 41 26 14.4 10.2
2001–2008 33 (4.1) 24 (3) 14.1 11.3
Mean 37.8 25.1 13.5 10.0
Figure 6. The frequency of DR1 and T10 rainfall events recorded per year for Ryedale, 1916–2008,
showing 10-year running means.
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event, including an extraordinarily intense
thunderstorm on 10 August 2003 at Carlton-
in-Cleveland (NZ 508 039, Figure 2), where
47.4 millimetres of rain fell in 20 minutes,
including 30 millimetres in five minutes, a
national record (Cinderey, 2003). The maxi-
mum 15-minute accumulation during this
storm (49.1 millimetres) greatly exceeds the
comparative figure at Hawnby on 19 June
2005 (26.8 millimetres). Furthermore, the
69.6 millimetres daily accumulation at
Hawnby has been exceeded twice during the
period 2002–2007 at Fylingdales (SE 864 967,
c. 32 kilometres east of Ryedale), on 1 August
2002 (114.6 millimetres) and 25 June 2007
(74.4 millimetres). Furthermore, a prolonged
rainstorm on 11 September 1976 gave a
maximum fall of 145 millimetres at Kildale
East Green Beck (NZ 620 097, c. 20 kilometres
north of Ryedale), with other gauges in the
western North York Moors recording daily
totals above 100 millimetres.
Conclusions
An analysis of annual rainfall totals and
heavy rainfall events in upper Ryedale
between 1916 and 2008 indicates that there
is no overall trend in the frequency of heavy
rainfall or the proportion of annual rainfall
falling on heavy rain days. There are four
main periods of high annual rainfall totals
and a large overall proportion and fre-
quency of heavy rainfall: 1927–1940, the
1950s, 1980s and 1997–present. Since the
1960s, heavy rain days appear to have
increased in frequency and in their overall
contribution to annual rainfall. However, the
1930s has the highest decadal frequency of
heavy rainfall events on record. Therefore,
increases in heavy rainfall since 1961 should
be placed in a longer-term context which
recognises the extremely low number of
heavy rainfall events recorded in the 1960s,
and the greater numbers of heavy rain days
which have occurred in decades prior to
1960.
From 1961 to 2000, winter rainfall totals
increased relative to summer totals.
Additionally, the proportion of winter rain-
fall falling as heavy rainfall increased over
the latter half of the twentieth century,
while heavy rain days in summer broadly
declined from the 1950s to 2000. Since
2000, however, this trend has reversed, as
winter has seen few heavy rainfall events
and summer has had higher seasonal rain-
fall totals and a relatively high frequency of
heavy daily rainfall events and maxima.
In the context of the 1916–2008 rainfall
record, the rainstorm which occurred in
upper Ryedale on 19 June 2005 is unusual
but not unprecedented. Higher daily rainfall
totals have been recorded in Ryedale and
over the North York Moors in the past. There
is a tendency for summer rainfall events to
be heavier than those in other seasons
Figure 7. The proportion of annual rainfall falling on heavy rain days in Ryedale, 1916–2008. Lines
indicate 10-year running means.
Figure 8. The maximum daily rainfall recorded annually in Ryedale, 1916–2008. Bold line indicates
10-year running mean.
Figure 9. Changes in the proportion of seasonal rainfall falling on heavy rain days for winter and
summer, Ryedale, January 1916–August 2009. Heavy rain days defined using the DR1 threshold.
Lines show 10-year running means.
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throughout the Ryedale record, and for more
heavy rain days to occur in summer than in
other seasons. When compared with the
most recent period (2001–2008), higher fre-
quencies of heavy rain days and a greater
proportion of annual rainfall falling on heavy
rain days occurred in the 1930s.
The composite record which has been con-
structed for Ryedale in this study has enabled
heavy rainfalls and seasonal trends to be
assessed over a 93-year period in an upland
area. The conclusion that recent heavy rain-
fall events, particularly in summer, are not as
rare as may first have been thought clearly
illustrates the importance of using long-term
climatic monitoring and associated records
to put recent weather events into context.
Acknowledgements
We gratefully acknowledge use of the British
Atmospheric Data Centre and also the use of
a synoptic chart (both UK Met Office), and
the use of the Digimap service and Strategi®
map data. Jeff Warburton also acknowledges
support from NERC (Award NE/D005744/1).
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Dynamicists versus modellers: a growing divide?Meeting reportThis meeting, on 18 November 2009, was
organised by the Society’s History Group
and was held at the University of Reading.
It was advertised with what was described
during the meeting as a deliberately pro-
vocative abstract, asking whether the mete-
orological community has forgotten the
pioneering principles of Rossby and Charney
and whether today’s modellers are really
computer scientists who ‘tweak’ models
without fully understanding them.
In his introduction, Malcolm Walker (Chair
of the History Group) introduced the cast of
characters (Rossby, Charney, Sutcliffe et al.)
whose classic papers in the 1940s laid the
foundations of dynamical meteorology, and
whose experiments with numerical weather
prediction (NWP) led to the first computer-
based weather forecasting systems. The sec-
ond speaker, John Methven (University of
Reading), gave an introduction to the early
theories of the development of extratropical
weather systems, beginning with the Bergen
School and the development of the polar
front theory of cyclones and ending with
the independent development of quasi-
geostrophic theory by Rossby and Charney.
The point was stressed that what both
Rossby and Charney sought, and indeed
found, was a mathematically closed theory
for the evolution of synoptic-scale cyclones.
The initial NWP systems were based on
quasi-geostrophic equations, keeping a
close link with theory. With increases in
computing power, and the realisation that
accurate prediction at finer resolution
required greater knowledge of the unbal-
anced flow, NWP moved to the primitive
equations and the link to theory began to
weaken. The talk ended with a Monty
Python inspired question: what has quasi-
geostrophic theory ever done for us? John’s
answer: It has given us the ability to under-
stand the evolving flow and develop intel-
ligent diagnostics for analysing the output
from numerical simulations.
Next, Lennart Bengtsson (University of
Reading) gave a summary of the initial
development of NWP. His first point fol-
lowed directly from John Methven’s talk: the
fundamental requirement for the inception