Partie 2_4.03 - Department of Atmospheric and Environmental ...

4.03
JLR: Too much work whereas Ernest is speaking/writing French better that I
speak/write English. I had to suppress text and rewrited the abstract which
took me more than 30 minutes....as you say, we must avoid this next time

techniques de prévision météorologique
sur la région de l'afrique de l'ouest

Ernest A. AFIESIMAMA et T. OBIDIKE

Services de la prévision météorologique, Agence Météorologique Nigériane,
Lagos, Nigeria





Résumé


Une tentative est faite dans ce papier pour fournir des applications
pratiques des techniques de prévision climatologiques, géométriques,
cinématiques et numériques qui sont les principaux outils disponibles du
météorologiste tropical et utiles pour aider les campagnes expérimentales
d'AMMA. La technique pour l'activité des précipitations étendues en été est
associée avec une divergence haute troposphère bien marquée et une
convergence bas niveau avec une humidité adéquate. Les perturbations en
Afrique de l'Ouest sont déterminées par l'instabilité barocline (présence
de cisaillement de vent vertical), l'instabilité CISK et l'instabilité
barotropique (présence de cisaillement de vent horizontal). En prenant la
ligne de la perturbation vu à l'origine aussi loin que possible à l'Est et
l'extrapolation effectuée généralement dans un mouvement vers l'Ouest, la
ligne de pertubation est prévue assez correctement. En hiver, la trace du
front d'un système extra-tropical associé avec de forts vents en surface
vers le nord-est sont des conditions favorables pour une tempête de
poussière. Le transport de la poussière en aval et la distribution sont
reliés aux vents au niveau 900m et aux indices de stabilité.
Ces techniques de prévision ont amélioré la capacité de notre société à
répondre correctement à des situations où le temps nous affecte, surtout la
production de systèmes d'alerte précoce basés sur des modèles numériques
qui sont utilisés pour estimer l'impact et donc fournir des stratégies
d'adaptation et des possibilités d'atténuer l'effet des événements de temps
extrêmes. Cependant, la capacité de faire des interprétations correctes des
analyses du temps est un attribut essentiel du prévisioniste. À cet égard,
le prévisioniste qui réussira ses prévisions possèdera une combinaison
complexe d'une bonne compréhension théorique des processus atmosphériques,
une large connaissance des phénéomènes climatiques, synoptiques et méso-
échelle, une longue expérience et un jugement adéquat.

On weather forecasting techniques over
West African region

Ernest A. AFIESIMAMA et T. OBIDIKE

Services de la prévision météorologique, Agence Météorologique Nigériane,
PMB 1215, Oshodi, Lagos, Nigeria




Abstract


Attempt has been made in this paper to provide practical applications of
the climatological, geometric, kinematic and numerical weather prediction
techniques, which are the primary tools available to the tropical
meteorologist and useful in assisting AMMA field campaigns. The technique
for widespread precipitation activity in summer is associated with well
marked upper level divergence / lower level convergence fields with
adequate moisture. The West African disturbances are determined through
baroclinic instability (reflected in the vertical shear), the CISK
instability and the barotropic instability (reflected in large lateral
shear). With the disturbance line first spotted as far east as possible and
the extrapolation taken generally in westward movement, the disturbance
line is near accurately predicted. In winter, the tracing of the cold front
from an extra-tropical system with the associated strong surface
northeasterly winds are requirements for the dust storm. The dust transport
downstream and distribution are linked to the winds at 900m level and the
stability indices. These forecast techniques have improved our society's
ability to respond properly to situations where the weather affects us,
especially the provision of early warning system from numerical models,
which are used for impact assessment so as to provide adaptation strategies
and mitigation options to extreme weather events. However, the ability to
make correct interpretation from the weather analyses is an essential
attribute of the forecaster. In this respect, the successful weather
forecaster possesses a complex combination of a good theoretical
understanding of atmospheric processes, a wide synoptic and meso-scale
climatological knowledge, long experience and sound judgement.

1. Introduction


Weather forecasting is concerned with extrapolation of future conditions of
the atmosphere on the basis of the present and past conditions. Forecasts
are usually referred to as very short-range (or nowcast) (less than 1 day),
short-range (1-2 days), extended range (3-5 days), medium-range (up to 6-10
days) and long-range (monthly or seasonal) outlooks. Severe weather
systems are typically short-lived (usually less than 2 hours) and due to
their meso-scale characters (less than 100km), they affect local or
regional areas necessitating site-specific forecasts. Examples of such
severe weather systems in West Africa include thunderstorms, squall lines,
high wind waves along the coasts, flash floods, dust storms or any other
such phenomena. However, the development of radar networks, new instruments
and high-speed communication links has now provided a means of issuing
early warnings of severe weather.

It must be noted that forecasting is a very complex activity which require,
not only proper understanding of the movement of a particular weather
system, but also understanding its position and intensity both intensity at
the surface and at all other levels. Unfortunately, the laws governing the
complex interactions of the various factors controlling the weather systems
are not completely understood. To the extent that they are understood, they
can be represented by mathematical equations, but even then, it turns out
that the solution of the equations in their full complexity would require
computers of much greater capacity and speed than the largest computer so
far available. In spite of the problems and deficiencies associated with
weather forecasting in West Africa, forecasts have been made at useful
levels of accuracy for several decades.

2. Techniques of Weather Forecasting over the West African region

The two major approaches are (i) the subjective approach and (ii) the
objective approach.


2.1 The subjective approach


The subjective approach involves the use of (a) empirical forecasting model
(b) extrapolation (c) forecast by similarity (d) climatology (e) local
weather forecasting techniques.

2.1.1 Empirical forecasting models: The empirical forecasting model
involves a sequence of weather charts covering long periods, which shows
that the weather does not change in a completely random way.

2.1.2 Forecast by Extrapolation: A very useful and widely practised
technique of forecasting in West Africa is to predict the movement of
existing weather by extrapolating from the trends of the immediate past.
The motion of the weather is extended into the future on the assumption
that changes will continue to occur in a similar way.




2.1.3 Forecast by similarity: One way of forecasting the future
developments that are likely to follow a particular situation is to search
for similar situations (or analogues) in the past and see what happened
then.





2.1.4 Forecast based on Climatology: Climatology provides an extremely
valuable tool in any forecasting technique. To issue the forecast of some
meteorological element such as pressure or temperature or even the ITD, an
idea of the normal value of the element as deduced from climatological
charts, together with the departure of the present value of this element
from this normal value, i.e. the anomaly, will give some indication whether
some normal or abnormal weather conditions are to be expected from the
observed values.





2.1.5 Local Weather Forecasting Techniques: Many features of the weather
are subject to wide variations over quite short distances. Fog is a
particularly good example of this. It is frequently very patchy, and often
one place will be in thick fog while others nearby have quite good
visibility. Forecasting of such features therefore require a good knowledge
of the local terrain and its influence on weather elements.


2.2 The objective approach

The objective approach of prediction where a number of non-linear equations
have to be solved using fast computers in order to predict numerical values
at a grid of points is known as numerical prediction methods.

2.2.1 Direct Use from Model Output products: The availability of gridded
NWP output for National Meteorological Services in West Africa presents
field forecasters with a new source of guidance information for improving
their services. As a means of expanding the utility of digital NWP guidance
data beyond conventional map preparations, several National Meteorological
Services in the subregion have acquired a variety of computer software
tools to display and manipulate gridded data, such as RETIM, SYNERGIE, etc.
The West African forecaster using a wide variety of diagnostic products is
essentially applying a technique based on physical and dynamical evolution
of synoptic systems. The systems include lows, highs, troughs, ridges,
convergence zones, jets, etc.

2.