Met Office College - Course Notes
Convective cloud forecasting
Contents
1.
Introduction
2.
Forecasting Considerations
3.
Using the Tephigram
4.
Other factors to consider about convection
5.
Forecasting Showers
6.
Forecasting Thunderstorms
7.
Distribution of Convective Cloud in the UK.
8.
Convection in winter
9.
Convection in summer
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Principal, Met Office College, FitzRoy Road, Exeter, Devon, EX1 3PB. UK.
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1. Introduction
Once the basic mechanisms of convective cloud formation are
understood, we can turn our attentions to the problems associated with
forecasting such clouds and their associated weather showers and
thunderstorms.
—
When forecasting showers it is necessary to be vague because it is almost
impossible to say whether one particular location will have a shower or
not. The trick is to be not so vague that the forecast is worthless!
2. Forecasting Considerations
Decide which air mass will affect your station today.
Look at its history: did showers develop in it yesterday? If so
 Where? Only over the sea or coasts, or over hills and inland?
 When? Throughout 24 hours or only at the time of maximum
temperature?
 At what temperature?
Since yesterday what factors might have changed the stability?
 Heating or cooling from below advection over warm sea, etc.
 Warming or cooling aloft warm or cold advection.
 Increase or decrease in moisture content supplied by evaporation
from the surface or by advection of moister air aloft.
—
—
—
Study representative ascents. What factors might release latent or
potential instability?
 Low level convergence:Cyclonic curvature of the isobars.
Along sea breeze front or where two sea breeze fronts meet.
Falling surface pressure.
 Forced mass ascent
Orographic uplift.
Divergence aloft upper troughs, etc.
Along a front.
 Intense high level heating over a plateau.
—
—
3. Using the Tephigram
Choose a representative ascent, mark on the station QFE and amend for
the ‘air mass’ dew point. This can be estimated in several ways:(a)
The previous afternoon inland values.
(b)
Values at coastal stations (use with caution often too high).
(c)
Extrapolate dew point trace above cooling inversion to the
surface.
—
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(d)
Work out from a mean HMR in the mixing (boundary) layer.
Draw HMR from the surface ‘air mass’ dew point at the station QFE
until it reaches the environment temperature curve. This gives the
convective condensation level (CCL) or Normand’s Point at start of
convection.
From the CCL draw a DALR down to the station QFE. This will give the
temperature at which cloud will start to form
The time of Tcu can be found by referring to your station heating curve.
Assuming no change in low level moisture content (which means the
dew point remains constant), as the temperature continues to rise, so
Normand’s Point and the CCL will continue to rise.
At start of convection, the cloud base will be close to the CCL but as
drier air is entrained from the surrounding atmosphere the base will rise
to about 25 hPa above the CCL.
As the temperature begins to fall the CCL will fall in sympathy but the
cloud bases will generally remain as they were at the time of maximum
temperature. As convection dies away the clouds will often evaporate or
spread out into Sc and/or Ac. The base will lower in precipitating
clouds by an amount which will depend on the intensity of the shower,
the humidity of the atmosphere and other, unknown variables such as
the concentration of pollution.
Cloud tops can be estimated by following an SALR from the CCL. This
is known as the Parcel Curve (see Figure 1).
Where the SALR becomes parallel to the environment curve is the level
of Slice Tops (5). This is the level of the majority of cloud tops, especially
early in the day and if the atmosphere is dry.
The SALR reaches the environment curve at the level of the Parcel tops (P).
Only a few tops will become this high unless the atmosphere is very moist.
Note that convection through the day has the effect of increasing the cloud
base moisture content by detrainment so more tops will reach this level later in
the day. Exceptionally, tops may go above the tropopause (P). This requires
a very moist atmosphere where the parcel curve departs from the
environment curve by at least 50C over a considerable depth; this is a rare
event in the UK.
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Figure 1 Constructions on a tephigram
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4. Other factors to consider about convection

Any inversions will inhibit convection even if the parcel curve stays
to the right of the environment curve.

Moisture content of the atmosphere is very important. If a cloud
builds into a dry layer, it will tend to evaporate. The parcel curve can
follow an SALR only if condensation is taking place, otherwise the
parcel will follow a DALR.

Strong wind shear will inhibit vertical extent of convection, though
some shear is necessary for supercell storms.

Maximum convection will occur near troughs or at the time of
maximum heating.

Convection will often increase with the approach of an upper trough
but decrease rapidly after its passage.

Over the sea and on windward coasts there is no diurnal variation.7.

If the relative humidity at cloud level is 50%, suggest 2 oktas; if 75%,
5 oktas. Amounts of convective cloud are routinely over-estimated in
surface observations.

Small cumulus clouds typically have a lifetime of 20 minutes, are of
the order of 1500 ft in depth and have updraughts of 2—10 kn. The
corresponding values for large cumulus are 1 hour, 6000—15 000 ft
and 10—20 kn; and for CB more than 1 hour, 15 000-50 000 ft and 2040 kn. Updraughts well in excess of 100 kn have been observed in
supercell CB clouds using doppler radar.
5. Forecasting Showers
In general it is clear that showers are more likely to fall from deep
clouds than from shallow ones. For the Bergeron—Findeison process to
work, the temperature of the top of the cumulus should be colder than
—100C. Showers can fall from clouds warmer than this, when
precipitation forms by the coalescence process; for this process the
clouds must be at least 6000 ft thick.
If the cloud top temperature is warmer than —120C and the cloud is
shallower than 5000 ft thick there is only a 10% probability of showers;
this figure increases to 90% for clouds more than 10 000 ft deep.
However, an exception to this rule are ‘drizzly’ showers, formed by
coalescence which, while producing only small amounts of rainfall, can
seriously reduce the visibility and cloud base. These occur in maritime
environments, particularly over the tropical oceans, where there is a
high concentration of salt nuclei.
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If a CU builds up through layers in which the wind varies quite
strongly, then the cloud becomes inclined from the vertical. It has been
observed that under such conditions showers are not as frequent or as
heavy as would be expected, even though cloud has developed to a
sufficient height.
Probably the precipitation elements, after being carried up through the
clouds, fall into clear air where they evaporate, instead of falling back
through the cloud where they could grow.
As a general rule, showers occur much more readily in maritime
environments than they do over continents, due to the presence of
plentiful low-level moisture and salt nuclei.
6. Forecasting Thunderstorms
CB clouds are those convective clouds, which develop glaciated tops.
Glaciation begins at a temperature of about —120C and is general at
temperatures colder than —200C.
Forecast CB clouds if:

Parcel cloud tops are likely to be higher than 15000 ft (550 hPa) in
summer or 10 000-12 000 ft in winter.
Cloud top temperatures are likely to be below—200C
Steer CB clouds with the 700 hPa flow.
Turbulence and icing are usually less severe in relatively shallow
‘winter’ CB clouds than in deeper ‘summer thunderstorm’ CB.
As an aid to forecasting thunderstorm, several instability indices have
been calculated on a statistical basis:—
RACKLIFF INSTABILITY INDEX
This index is obtained by subtracting the observed 500 hPa temperature
from the 900 hPa wet bulb potential temperature. High values indicate
marked instability. In non-frontal situations a value of 30 or more
should indicate thunderstorm.
JEFFERSON MODIFIED INDEX
Jefferson amended the Rackliff index to the following form:—
Tmj = 1.60w900 -T500 - 0.5T d700 - 8
or in a simplified form:Tmj = l.6w900 -T500 -11
Thunder is probable in the UK with a value for Tmj of 29 or more.
BOYDEN INSTABILITY INDEX
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This index is obtained by taking the 1000 to 700 hPa thickness in
decametres, subtracting the 700 hPa temperature and from the result
subtracting 200. High values indicate instability and the threshold value
for thunderstorm has been calculated as 93 to 94. These values work
well for SE England but Lowndes has since suggested that values of 94
to 95 are more suited other parts of England.
7. Distribution of Convective Cloud in the UK.
Studies have been undertaken using Meteosat pictures to try to spot
areas where convective clouds develop and persist in various
airstreams.
Cloud bands originating over the land occur primarily during the
summer half year as a result of diurnal heating generating sea breezes
and convergence zones just inland from the coasts.
Bands that originate over the sea are most frequent in the winter half
year. They often result from convergence zones that develop offshore
from land breezes or frictional effects or a combination of both.
Figures 2 to 9, overleaf, show bands of cloud which form in airstreams
from 4 different compass points. They provide a fairly comprehensive
but not exhaustive picture of favoured locations for cloud. Other cloud
bands have been observed but are fairly rare.
With strong onshore winds, typically exceeding 20 kn at coastal stations,
correlation of the cloud patterns with the coastline becomes diminished.
Instead, the cloud patterns become closely correlated with the elevation
of the hills, the most persistent cloud over the higher ground
particularly the hills close to the western coasts of the UK. In the
Figures, the letter 'L' shows bands observed with light winds (less than
10 kn on windward coasts).
—
When the wind exceeded 20 kn on windward coasts, the regions of
dominant cloud areas are over high ground, near coasts. This may imply
that with winds greater than 20 kn the air is forced over the hills with a
sufficient supply of moisture to maintain the showers. With winds less
than 20 kn the air is either deflected more or has not the supply of
moisture available to prolong the showers. The stronger wind also
disrupts the summer cloud pattern, which correlates with the coast line
because the sea-breeze cannot become established.
Coastal regions enclosed by thicker lines are cloud free due to sea breeze
penetration. With the stronger wind the summer correlation between cloud
and coastline becomes disrupted, probably because the sea-breeze cannot
become established.
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Double headed arrows indicate bands that occur within a range of wind
directions, and single headed arrows where successive areas of
convective activity follow similar paths.
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8. Convection in winter
Convection in winter is generated due to air, which is unstable to sea
temperatures. The wind direction is a strong element in determining the
areas more likely to be affected by showers. Figure 2 (below) shows the
areas likely to be affected according to different wind directions.
Double-headed arrows indicate bands that occur within a range of wind
directions, and single-headed arrows where successive areas of
convective activity follow similar paths.
Figure 2 Winter convection generated due to air, which is unstable to sea
temperatures.
(a) North-westerly airflow,
(b) northerly airflow
(c) north-easterly airflow
(d) easterly airflow.
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9. Convection in summer
Convection in summer is generated due to air which is unstable to land
temperatures. The wind direction is a strong element in determining the
areas more likely to be affected by showers. Figure 3 (below) shows the
areas likely to be affected according to different wind directions. Coastal
regions enclosed by thicker lines are cloud free due to sea-breeze
penetration. With the stronger wind the summer correlation between
cloud and coastline becomes disrupted, probably because the sea breeze
cannot become established.
Figure 3 Summer convection generated due to air which is unstable to land
temperatures.
(a)
(b)
(c)
(d)
South-westerly airflow
westerly airflow
north-westerly airflow,
northerly airflow.
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10. Further Reading
Handbook of Weather Forecasting, Met. 0. 875, Chapter 19, pp. 83—90, 100-122.
Monk, G.A. 1987. Topographically Related Convection over the British Isles,
in Satellite and Radar Imagery
Interpretation, EUMETSAT, Ed. M. Bader and A.Waters.
Pike,W.S. 1990. Meteorological Magazine 119, pp. 21—32, 97—102
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