WORLD METEOROLOGICAL ORGANIZATION
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CBS/ICT/DPFS/Doc. 5(2)
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(31.V.2002)
CBS IMPLEMENTATION/COORDINATION TEAM ON
DATA-PROCESSING AND FORECASTING
SYSTEMS
ITEM: 5
MOSCOW, RUSSIAN FEDERATION, 3-7 JUNE 2002
ENGLISH ONLY
APPLICATION OF NWP TO SEVERE WEATHER FORECASTING
(Submitted by J. Coiffier, Météo-France)
Summary and purpose of document
This document outlines what is currently termed as "severe weather events".
Action proposed
The meeting is invited to take into account the recommendations outlined in this document
and to make proposals.
CBS/ICT/DPFS/Doc. 5(2), p.2
SEVERE WEATHER FORECASTING
1 – Introduction
The aim of this preliminary paper is to give a rapid survey about what are currently called “severe
weather events.” In paragraph 2 we point out that this denomination of severe weather is often the result of a
subjective assessment linked to the consequences of the weather rather than to the meteorological phenomenon
itself. In paragraph 3 we try to classify severe weather events with their main characteristics and to give an idea
about the current possibilities of the NWP models to forecast them. In paragraph 4 we propose an approach,
which seems appropriate to take advantage of the existing meteorological centres to build a system able to
deliver early warnings in case of severe weather.
2 - About the definition of severe weather
The inspection of the web sites elaborated by numerous national meteorological services around the
world shows that in spite of different appreciations from one country to the other one, the same meteorological
phenomena are quoted as being responsible for severe weather. It is important to note that the qualification of
severe weather often depends on the observed or experienced consequences; these consequences themselves
depend on parameters external to the atmospheric machinery; thus heavy precipitation over a large inhabited
land surface can remain unobserved while a lower quantity of precipitating water concentrated over a small
basin can produce disastrous floods. Nevertheless one can try to classify the meteorological phenomena
leading to severe weather as follows:
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tropical storms and tropical cyclones (hurricanes, typhoons);
severe extratropical storms over ocean or over land;
large scale heavy precipitations (rainfall or snowfall);
active convective events with associated phenomena (heavy precipitations, hail, lightnings, gusts, tornadoes);
persistence of extreme temperatures (cold wave or heat wave)
phenomena lowering dramatically the visibility or holding up the circulation (fog, duststorm, blackice).
A report by the National Climatic Data Centrer/NESDIS/ NOAA listing the significant global weather
and climate events for the year 2000 shows that most of these phenomena and their consequences (floods,
mudslides, drought conditions, fires) are responsible for number of fatalities, perishment of many livestock
and disastrous effects on the property and way of life of millions of people.
3 - The main characteristics of the various phenomena
3.1 - Tropical storms and cyclones
Tropical storms and cyclones are always responsible for many fatalities and economic loss every year
and for this reason WMO established a global system in order to early detect such events and to issue forecasts
about their trajectory and intensity. Although tropical cyclone forecasting remains out of the scope of this study
it is interesting to examine the characteristics of this phenomenon in order to understand why the forecasting
systems works rather satisfactorily and allow people to take appropriate protection measures.
The birthplace of the tropical storms and the favourable period is well identified (low latitudes, SST
beyond a given threshold). So each forecasting centre responsible for a specific geographical area tries to
detect as soon as possible the tropical storms on the images provided by the stationary satellites and to follow
their evolution. The tracking of the cyclones is made easier because their displacement is relatively slow. It is
difficult to accurately forecast the trajectory of the tropical cyclones even with available fine mesh numerical
CBS/ICT/DPFS/Doc. 5(2), p.3
models and a small location error can have important consequences (for a small size island for example).
Nevertheless the combined use of numerical models, statistical adaptation and empirical approach allows to
forecast a set of trajectories. The main difficulty arises from the lack of accurate measurements to define the
initial state at the convenient scale and to the deficiencies of the parameterization of all the processes working
inside a tropical cyclone. The implementation by WMO of Regional Specialized Meteorological Centres for
tropical cyclone forecasting allows organizing the watch and insuring a valuable support to the National
Meteorological Services.
3.2 - Severe extratropical storms over ocean and over land
The main mechanism, which explains the cyclogenesis is the interaction between potential vorticity
anomalies and baroclinic areas linked to the jet-streams. The FASTEX experiment, which took place in 1997
over the northeastern part of the Atlantic was especially devoted to study the formation and the evolution of the
storms in this region. Severe storms are associated with the cyclogenesis phenomena at the midlatitudes and
are characterized by very strong winds due to the intense pressure gradients around the low pressure system.
Over the ocean these intense surface wind induce high waves, which are a real hazard for the ships. Over land
very strong gusts are responsible for important damages to the forests, to the urban infrastructures and to aerial
power transportation lines. Note also that, depending on the season, intense low pressure systems can be
accompanied by with important precipitation of rain or snow.
Typical cyclogenesis belong to synoptic meteorological phenomena, which can be detected by means
of the available satellite images and meteorological observations, when they are located at the right place.
Global or limited area models operated by many forecast centres of the National Meteorological Services with
some success currently forecast them.
Nevertheless it has to be stressed that the behaviour of the real atmosphere is often different of the
simulated atmosphere, even when using the “best models”. On one hand several models very dynamically
active have a worrying tendency to deepen some lows too much. On the other hand there are number of
examples of important storms misforecast by numerical models (the 1999’s storms over northeast Atlantic
after Christmas, for instance).
One can say that the extratropical storms can be forecast with some success by using numerical
models but it remains difficult to forecast accurately the time when the deepening becomes intense as well as
the exact trajectory. Every important storm gives the opportunity to compare various models operated by the
forecast centres in order to identify the possible sources of error. Recent studies carried out with the 1999’s
storms case, show that the ability of the numerical models to forecast accurately the cyclogenesis does not
depend on the size of the mesh (when using mesh size of the order of about 50 km) nor on the system of
equations (hydrostatic versus non-hydrostatic). In contrast there is an important dependence on the initial state
and the way to obtain it (selection of the observations, data assimilation procedures). These conclusions are
also confirmed by number of experiments consisting in modifying model initial conditions in order to better fit
several crucial observations or to improve the agreement between dynamical fields (potential vorticity) and
satellite WV images.
In order to obtain accurate forecasts it is necessary to define accurately the initial state, especially in the region
where precursors of the storm can be identified. Unfortunately these regions are often far from dense
observation networks. Improvement of the initial state comprises the development of new measurements in
connection with efficient data assimilation systems. It is also important to mention the new concept of targeted
observations, which consist in making measurements at the most sensible locations (which can be obtained
from recent “adjoint techniques”).
The early detection of storms is an important part of the current activity of the National
Meteorological Services, which make use of numerical models. Nevertheless due to the uncertainties about the
CBS/ICT/DPFS/Doc. 5(2), p.4
initial state, accurate deterministic prediction of the intense storms remains far of the perfection.
3.3 - Large scale heavy precipitations
The precipitations are artificially divided into two categories according to the process that is
responsible for the lifting of the moist air particles. One distinguishes the large-scale precipitations, which are
associated to synoptic vertical velocities, and the convective precipitations, which take place within unstable
layers of the atmosphere. Nevertheless the conceptual models, which describe the structure of the atmospheric
perturbations, show that this separation is not always very clear. In the numerical models, as and when the
resolved scales become smaller, convective circulations tend to be taken into account by the dynamics so that it
is more appropriate to speak about resolved scale precipitation (which are taken into account by the dynamics)
and unresolved scale precipitation resulting from the various parameterized moist physical processes. For sake
of clarity one speaks here about heavy large-scale precipitations associated mainly with synoptic perturbations.
Among the various ways to parametrize the processes responsible for the large scale precipitations,
there are several levels of sophistication. Simple models only use one characteristic variable for the
atmospheric water vapour and determine the precipitation amount by removing moisture to avoid
supersaturation. The effects of condensation/evaporation are taken into account to calculate the corresponding
heating/cooling; a simple relationship allows computing the cloud cover from the relative humidity. More
sophisticated models deal with the three phases of the water in the atmosphere and describe the various
interactions between them, according to microphysics processes; they are able to discriminate between
precipitating liquid water and the small droplets remaining within the clouds.
As the network of raingauges measures the total precipitation it is not possible to assess the ability of
the numerical models to forecast large-scale precipitations only. The comparison between accumulated
precipitation forecast by the models with the estimates deduced from the observation network exhibit generally
similar patterns. Nevertheless day to day controls show that there can be many discrepancies, which can be
caused by deficiencies in the evaluation of the various forcings on the resolved vertical velocity and in the
calculation of the unresolved scale precipitation resulting mainly from the release of convective instability.
Improvement of the quality of precipitation forecasts is expected to result from the use of a finer mesh
allowing to better represent the orographic forcing and from a more accurate description of the microphysics
processes. A correct initialization of the moisture content in the atmosphere is also essential for avoiding the
spin-up effect, which deteriorates the quality of the very short-range forecasts.
At present time forecasters cannot completely rely on precipitation fields given by the model to
evaluate the expected amounts. They have to examine them in parallel with the dynamical fields to recognize
the structures responsible for the forcing on the vertical velocity.
It is important to note that the consequences of heavy precipitation in terms of floods do not only
depend on the accumulated amount of water reaching the surface but also on the properties of the basin
collecting the water (shape, soil water content). So it is clear that atmospheric numerical model have to be
coupled with hydrological models in order to provide useful information. From the forecaster’s point of view it
is necessary to be informed about the current state of the surface. In several cases a small increase of the
precipitation can result in disastrous floods ; this is the reason why it is difficult to base a warning system on
fixes threshold values.
3.4 - Convective events
Convection is the main physical mechanism leading to the development of the severe thunderstorms
observed in many places around the world, which are responsible of many fatalities and economic loss. Typical
hazardous phenomena are associated with thunderstorms.
CBS/ICT/DPFS/Doc. 5(2), p.5
Hail is at the origin of important damages not only for agricultural areas but also for urban areas when
the diameter of the hailstones is larger than 2 cm. The formation of hail is very complex and depends on many
facts such as the updraft strength, the wind structure and the cloud microphysics. This explains why accurate
prediction of hail is difficult by using numerical models.
Strong wind gusts are often associated with the thunderstorm can induce fall of trees and cause
important damages to the habitations. They can occur in a large variety of convective situations and are
generally associated with the cold air outflow as the downdraft reaches the surface.
Flash floods are frequently associated with severe thunderstorms as a consequence of intense rainfall
occurring at the same place during several hours and strongly depend on the nature of the surface (especially
over saturated soils). Thus relatively minor rainfall can cause damaging flash flood. Flash flood is the
phenomenon, which is responsible for important loss of life induced by strong convective events.
Tornadoes are phenomena, which are associated with convective events, especially over several parts of the
world (the Great Plain in Northern America for example). The very intense wind that takes place inside these small scale
vortices and their sucking effect is the reason why the tornadoes are listed as the meteorological phenomena having the
most destructive effect on life and property. They are often associated with supercell thunderstorms but can also occur in
other convective events.
The hazardous convective events are mesoscale phenomena, which are difficult to properly simulate
with numerical models. Nevertheless numerical models provide relevant information about the synoptic
environment propitious to the development of severe thunderstorms. Low tropopause anomalies, wet bulb
potential temperature and wind convergence in the lowest layers, wind shear, helicity, convective available
potential energy (CAPE) allow to know the regions where intense thunderstorms are likely to occur and to alert
the services in charge of the public safety. The National Meteorological Services which implemented
nowcasting systems based on satellite images, radar networks, lightning detection systems are the able to
detect the location of growing up convective cells and to try to extrapolate their displacement one or two hours
ahead and to calculate estimated water accumulation at the surface.
It can be expected that the progresses in numerical modelling (with horizontal resolution of the order
of 1 kilometer) and the improvements in the simulation of cloud microphysics will allow better simulating the
life cycle of the thunderstorms. Nevertheless it is important to point out that numerical models start from an
initial state, which has to be carefully analyzed especially for the small-scale distribution of humidity and
liquid water.ppp
3.5 - Extended periods of extreme temperature
Among the climate events cold waves and heat waves in many places around the world are regularly
reported. Cold waves and heat waves are characterized by the persistence during several days of extreme
temperatures going beyond the climatological values. When such events take place in regions where the
population, do not use to cope with such conditions, they can cause many fatalities. It is important to note also
that prolonged periods of heat wave are propitious to the increase of tropospheric ozone concentration in urban
areas and to propagation fire over forested areas. The origin of such events is linked with the global
atmospheric circulation and the distribution of the large-scale pressure centres around the Earth. It is
recognized that the interannual perturbations of the atmospheric circulation like the ENSO in southern Pacific
or the NAO for the northern Atlantic have an important impact on the occurrence of such events in several
parts of the globe (So negative values of the NAO index are associated with rather cold winters over Europe,
while active El Niño induces heat waves over south-east Asia and northern America. These large-scale
phenomena are rather well predicted by the current global models several days in advance. Even seasonal
forecast have some skill for predicting these kinds of events (the last El Niño forecast in 1998, for example).
Nevertheless despite of good meteorological forecasts, it seems that is difficult to determine precisely the time
CBS/ICT/DPFS/Doc. 5(2), p.6
when the situation becomes really dangerous for the human life if protection means are lacking. Forecasting
such events clearly lies in the realm of the possibilities of meteorological centres operating global models for
medium range or seasonal outlook purposes.
3.6 - Other events affecting the economical life
There are other meteorological phenomena, which are able to disturb the economical life of a country:
reduced visibility due to sand storms or dense generalized fog can considerably hinder the communication by
air and on the roads. Similarly the occurrence of blackice over large parts of a country can be responsible of
serious accidents. This kind of events are not always easy to forecast with help of numerical models because
their formation and their disappearing depend on threshold effect. Nevertheless if such events can have
temporarily important consequences for the economical life they represent a less threatening danger than
severe thunderstorms.
4 - How to use numerical models for severe weather forecasting.
After this rapid survey of the severe weather phenomena events with their main characteristics and the
efficiency of the numerical models to forecast them one can formulate the following points.
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NWP global models are generally able to forecast large-scale circulation of the atmosphere and the
position of the pressure centres responsible for the establishment during several days of airflows
leading to cold waves during the winter (resp. heat wave during the winter).
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NWP models are able to produce realistic lifecycle of extratropical cyclones leading to strong wind
or/and heavy precipitations but in the details (trajectory, chronology) the reality often differ from the
numerical simulation because of the incomplete knowledge of the initial state. So it is risky to want to
directly use only one model output in order to issue warnings.
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NWP gives important information about the synoptic environment favourable for the development of severe
thunderstorms but are unable to give with accuracy the right location and intensity of the most intense
phenomena.
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The development of warning systems for severe convective storms is essentially based on nowcasting techniques
involving radar networks and lightning detection systems. The example of the United States where such systems
have been developed show that they allow to better inform the public thus to reduce the number of fatalities.
If mesoscale models are able to simulate the behaviour of severe local storms, it is not possible at the
moment to use them as operational tools for several reasons.
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efficient mesoscale models require a very important computing powers (they are likely to deal with the same
number of degrees of freedom as larger scale global models) ;
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dynamic and moisture variable have to be carefully initialised by using mesoscale measurements collected as
rapidly as possible ;
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finally, one can expect that mesoscale model integrations will be very sensitive to the initial conditions so that it
will be too restrictive to trust in purely deterministic forecasts.
Taking into account the fact that accurate forecasts of severe weather events are difficult to achieve with help of
individual models it is essential to try to take advantage of the variety of NWP models, which are currently operated by
number of National Meteorological Centres. The realization of ensemble forecasts and the production of probabilistic
forecasts is the best way to take into account the uncertainty about the specification of the initial state and to reap the
benefits from the differences between the numerical models. It is important to note that ECMWF developed by means of
CBS/ICT/DPFS/Doc. 5(2), p.7
the Ensemble Prediction System (EFS) a “severe weather forecast index” (by measuring a distance between the
distribution function of the forecasts with the climatological) which is currently evaluated by forecasters. Such an
approach would be quite convenient in the view of the implementation of a coordinated system aiming to issue
pre-warnings for severe weather events.
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