How
to approach the users?
Everybody
recognizes that it is a very important point, but an equally difficult one.
The
general opinion was that the majority of today’s forecasters are not keen
using EPS information. What the majority still wants to know is whether the
model gives precipitation for tomorrow and how much.
With
a 40% probability for more than 5mm precipitation for tomorrow, they do not
feel at ease.
In
such a case, they will most probably rely on the deterministic forecasts to
know whether it rains or not tomorrow.
How
the forecasters should learn to use the EPS information?
No
clear general answer could be given to this question.
This
does not imply that EPS for the short-range cannot be used. It is much more the
consequence that Short-range EPS is much too young – it is just starting!
- to already have developed a strategy for its use.
Which
kind of Short-range EPS?
Do
we want to develop Short-range EPS for the daily weather forecasting or do we
want an EPS focused on some weather types or weather events?
The
result of the discussion is that Short-range EPS should focus on severe weather.
The
fact that two Consortia (UKMO and COSMO) have already taken this way – at
least in the definition of their respective strategy - has maybe biased the
discussion.
How to construct a Short-range EPS for severe
weather?
I
had the feeling that nobody really knows. We have today no technique to
specifically tune an EPS to emphasize potentially dangerous weather situations.
The
remark has been made that such a system would anyway be difficult to verify as
severe weather is – fortunately! – a rare event.
Focus on severe weather
There are two types of severe weather:
-
damages on the synoptic scale
Examples:
the October 1987 storm in South-England, the Danish storm (3rd December 1999),
the 26 and 27 December 1999 storms in France, Switzerland and Germany, the
August 2002 floods in Germany, Czech Republic and Austria.
-
the flash floods caused by severe convection
Examples:
Vaison-la-Romaine 1992 (F), Brig 1993 (CH), Versilia 1996 (I)
For
the flash floods, we do not have today the necessary model resolutions.
It
has clearly been said that in order to have good probabilistic forecasts of
strong convection, we must go to high model resolution. We shall attain the
necessary resolution for an operational EPS presumably not before 10 years.
What about the stochastic physics?
The use of a stochastic physics is preferable to the multi-model technique. But
in order to get in average the best results, this method requires an intensive
development phase: there are so many possibilities! Nevertheless this method
has been rated as very adequate for the EPS technique.
Two
words on the multi-model technique
As
it produces means, the multi-model technique is thought to be too conservative;
it is therefore unsuitable for an emphasis on severe weather.
An EPS for severe weather should
have biased pdf's
In
any model, each grid point has climatological pdf’s for its different
variables.
A
good model should obviously have for a given grid-point the same pdf's as at
its corresponding geographical location (if we forget the effects of the
non-resolved features).
But
pdf's of EPS trimmed to enhance severe weather must be different. Do we want
this?
If
not, it is not clear how an EPS for severe weather should be tuned.
Computation of the SV’s
Today’s
singular vectors are computed only for a dry atmosphere.
This
should be changed by computing the singular vectors of an integrated system
consisting of the moist atmosphere and the soil model. But this would raise the
following question: how do we perturb the soil variables?
Jean
QUIBY
Coordinator of the EUMETNET SRNWP
Program