Met Office College - Course Notes
Orographic Enhancement of Rainfall
Contents
1. Introduction
2. Processes
2.1 Forced Ascent
2.2 Differential diurnal heating
2.3 Convective release
2.4 The Seeder Feeder effect
2.4.1 What is it?
2.4.2 When does it happen?
3. A Case Study- The seeder-feeder effect in action
3.1 The Synoptic Situation
3.2 Results
3.3 Conclusions
4. Summary
5. Suggestions for further reading
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Principal, Met Office College, FitzRoy Road, Exeter, Devon. EX1 3PB. UK
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1. Introduction
If you take a look at a chart showing average annual rainfall (Fig 1), you
can quickly see an excellent correlation between areas of high ground
and high annual rainfall.
Figure 1. A chart of the distribution of annual rainfall for the U.K.
In the UK, most of the high ground is in the west and is exposed to the
prevailing moist winds from the Atlantic and perhaps the excess rainfall
might be expected but what processes are going on make it happen.
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Orographic Enhancement Of Rainfall
2. Processes
Four processes have been identified as leading to greater rainfall
in upland areas.
1. Forced ascent
2. Differential diurnal heating
3. Convective release
4. The Seeder-Feeder effect
2.1 Forced Ascent
This is perhaps the most obvious process that that goes on in
mountainous areas. Air is forced to rise as it hits an area of high ground.
As it rises it cools adiabatically and when it reaches its dew point cloud
may form close to the hillside (Fig.2). Drizzle and light rain can then fall
from this cloud. Rainfall rates may be slight but the rain will continue for
as long as it has a supply of moist air and over a long period of time there
can be significant falls on the windward side.
Figure 2. Forced ascent leading to light rain and drizzle on windward facing
slopes
2.2 Differential diurnal heating
On most occasions, temperatures fall as we rise through the troposphere.
It is therefore strange to think of excessive convection being set off on
high ground as opposed to warmer low-lying areas. It is nevertheless
true.
On a sunny day, the ground is heated by the sunshine both in low-lying
areas and on higher ground. The air away from the surface is likely to be
much colder near high ground than it is in low lying areas (Fig 3a) and
this leads to increased instability over high ground. The rising air at the
top of the hill then tends to draw air up the sides of the hill leading to
convergence at the top, further enhancing the convective activity (Fig 3b)
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Cooler
Warm
Warm
Cooler
Warm
Figure 3a. Differential heating of hilltops leads to preferential formation of
showers over high ground
Figure 3b. Convergence at hilltops due to air rising up the side of hills leads to
the same effect.
2.3 Convective release
There are several scenarios in meteorology where the uplift given by high
ground can act as the ‘final straw’, allowing the release of various forms
of instability.
For example, take a situation where convection is expected in low lying
areas but it is limited because surface temperature do not provide enough
of a trigger to set off showers. The addition uplift provided by high
ground may be enough to set things going (Fig 4a).
If an airmass is potentially unstable this can be released when passing
over high ground. A possible situation where this might occur is in a split
cold front where the cold drier air above the Shallow Moist Zone means
potential instability may be released as air rises over high ground (Fig
4b).
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Orographic Enhancement Of Rainfall
Deep
convection
Shallow
convection
Figure 4a The uplift provided by high ground provides the final push to set off
deep convection.
Cold, dry air overlying
Potential
instability
released
Shallow moist
zone
Figure 4b The uplift provided by high ground can release potential instability
in a Shallow moist zone for example.
2.4 The Seeder Feeder effect
The seeder -feeder effect is the one most closely associated with the
orographic enhancement of frontal rain. It can lead to a ten-fold
enhancement of rain from a low-lying area to an area of high ground just
a few miles downwind.
2.4.1 What is it?
Bergeron (1950 and 1965) suggested that a pre-existing upper
cloud could act with the cloud formed during ascent of the hills to
produce an increase in rainfall at the surface over and above that
observed at low levels.
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Cloud is formed by forced ascent of warm moist air this leads to a
‘capping’ cloud on hills and can produce light rain and drizzle on
its own. This is the ‘feeder’ cloud. What is now required is a preexisting upper cloud layer that is already producing rain. This rain
falls into the low moist air and washes out the cloud droplets in
the ‘capping’ cloud. This increases the drop size of rain and
increases the rainfall rate. (Fig 5)
Seede
r
Feede
r
Strong moist low level
flow
Figure 5 The seeder feeder effect
2.4.2 When does it happen?
The degree of enhancement caused the seeder-feeder effect has been
observed to depend on a number of features:
1.
A seeder rate of rain greater than a minimum around 0.5 mm/hr
(however a simple increase with greater seeder rates does not
occur). Release of potential instability aids formation of locally
greater seeder rates.
2.
Moisture content of the air; high θw gives more available
water.
3.
Degree of saturation of upwind air. If unsaturated less
growth is possible in the feeder cloud.
4.
Low level wind speeds – the heaviest rainfall is usually
found with low level winds (900m) greater than 30 m/s,
especially if this is associated with a warm conveyor belt jet.
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Orographic Enhancement Of Rainfall
5.
Topography – steepness of terrain begins to enhance but a
limit is attained when air goes round rather than over the
hill.
3. A Case Study- The seeder-feeder effect in action
This case is based on a paper by Hill, Browning and Bader (1981).
They carried out a number of field experiments in South Wales
during the winter of 1976/77 using a combination of rain gauges
and rainfall radar. The radar was set up to scan in three
dimensions and not just the standard two. This was to obtain data
on the vertical structure.
Figure 6 The topography of South Wales together with the position of rain
gauges used in the study.
3.1 The Synoptic Situation
A narrow warm sector crossed SW England and Wales in the early
hours of the morning of the 28th November (Fig 7). The general
conditions that prevailed are set down in Table 1
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Figure 7 The synoptic analysis at 0100 on 28 November 1976. The stippled
area represents precipitation as assessed from radar data. The arrow shows the
extent of rain included in the case.
Date
Period
Duration of rain on hills
Mean wind speed (ms-1) at
600m
3000m
Mean WBPT (C) at
600m
3000m
Mean RH in lowest 1000m (%)
28/11/76
0000 to 0515
4.5 hours
230/29
235/32
10.0
13.2
90
Table 1 General conditions for Case 1, 28th November 1976
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Orographic Enhancement Of Rainfall
3.2 Results
Figure 8. Rainfall rates recorded in the early hours of 28th November. The solid
lines are estimated from radar. The dashed line is the mean recorded at two rain
gauges in the Glamorgan Hills.
As can be seen in Fig 8, The rates rise and fall at around the same time
but the amplitude of the curves is very different with rainfall rates
increasing by up 7 times in the hills as opposed to those recorded on the
coast.
The three dimensional scanning of the radar system enabled sections to
be taken through the rain to estimate at what height above the ground the
enhancement was taking places. One such section is shown in Fig 9.
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Figure 9. A vertical section taken at 0439 on the morning of 28th November
1976.The sections were taken parallel to the direction of movement of individual
rainfall areas and crossed the hills near gauge 6 (see fig 6). It shows rainfall
rates derived from calibrated radar measurements. The dashed line corresponds
to 0.2 mmhr-1, solid contours represent 1,2,3.. etc mmhr-1 The number plotted
on the hill is rate recorded at gauge 6 at the time of the section.
3.3 Conclusions
Using evidence for this case, together the other cases investigated in this
project, Hill, Browning and Bader were able to draw the following
conclusions.
The enhancement of rainfall over high ground in this type of synoptic
situation is largely a low-level phenomenon and the washout of cloud
droplets is likely to be an important mechanism for generating it. More
than 80% of the overall enhancement was concentrated in lowest 1.5 km
above the hills when the low level winds were in excess of 20 ms-1. Over
periods of several hours, the enhancement was found to correlate well
with mean wind speed just above the friction layer (the gradient wind).
The existence of high relative humidity in lowest 1.5 km was also
important.
4. Summary
We have looked at four processes that lead to an overall increase in
annual rainfall with height. We have looked at these process as if they
are unconnected. They can and do occur simultaneously depending on
the synoptic situation.
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Orographic Enhancement Of Rainfall
5. Suggestions for further reading
Hill F.F, Browning K.A., Bader M.J. Radar and raingauge observations of
orographic rain over South Wales Quarterly Journal of the Royal Met. Soc.
July 1981 p643-670
Pedgely D.E. Heavy rainfalls over Snowdonia Weather 1970 p340-350
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