From Tony Bannister
Paul,
I understand the analysis
part but I am still stuck on the prognosis times.
I am currently looking at
the mesolaps which has a windchange west of Melbourne
at 0z tomorrow, presently
do I take this as the meso thinks it is 11am (AEDT) or
10am or 9am? Talking
to my NSW collegues they are punting for 9am, I would like it
to be 11am (to make our
forecast for Melbourne temp look half reasonable). Whatever
the time is, is this the
same for GASP and LAPS?
Do you then know how EC, US,UK prognosis times fit into the picture?
Yours in anticipation,
Paul Stewart wrote:
> RE: BASE TIMES IN NMOC'S
OPERATIONAL NWP SYSTEMS
>
> Dear Colleagues,
>
> The following is an attempt
to answer Bob Moore's query as to "What is the real
> time on the models?"
>
> The times for the various
operational NWP systems in NMOC are still based on 11
> and 23 UTC (and also 05
and 17UTC).
>
> However, this does not
mean that only observational data for these specific
> times are used. The current
operational assimilation schemes (LAPS, TLAPS,
> including TC_LAPS, and
GASP) use 6-hourly data insertions. This effectively
> means that 6 hours of
observational data is used in generating an analysis at a
> given time (ie at 05,
11, 17 or 23 UTC).
>
> Currently for a given
analysis time, the time window used is from -2 to +4
> hours. Thus, for example,
observations from 09 UTC to 15 UTC are used in the
> analysis for 11 UTC. (It
is further noted that the observations, within the 6
> hour window, are effectively
weighted according to their time difference from
> the nominal analysis time.)
>
> Over the years, a number
of changes have been implemented with the aim of
> eventually using the base
times: 00, 06, 12 and 18 UTC consistently throughout
> the operational system
in NMOC. These have included:
>
> . the adjustment of chart
labels so that base times for analyses were rounded
> off to 00, 06, 12 and
18 UTC and valid forecast times adjusted accordingly;
> . the use of 00, 06, 12
and 18 UTC in the actual data set names within the real
> time data base (which
were introduced for GASP several years ago and have, just
> in the last few months,
been used for the various LAPS systems).
>
> However, the observational
extraction procedure and the internal files
> (including those in NetCDF
form), used within the operational NWP systems,
still have base times of
05, 11, 17 and 23 UTC.
>
> It is only when these
latter procedures and file structures are changed that we
> can say that the base
times are 00, 06, 12 and 18 UTC. (It is planned that this
> will happen during 2002.)
From Robin Bowen
hi Tony et al
> However, the observational
extraction procedure and the internal files
> (including those in NetCDF
form), used within the operational NWP systems, still
> have base times of 05,
11, 17 and 23 UTC.
so where your chart says
00Z the actual time is 23Z, ie one hour prior !
and the AEDT time is 10am
( if not daylight saving then it would be 9am )
this is the same for LAPS
and GASP.
I will leave Paul or someone
else to answer concerning EC, US,UK prognosis
times !
cheers
Robin
From Phil Davill
Hi all,
Surely if the model
time is for 23Z it should be labelled for 23Z not one
hour later. If we want 00Z
why not output the 00Z data (not rename the
23Z to 00Z) and run the
model out to 49 hours instead of 48 hours.
What we are currently
doing sounds very unscientific. It would be
interesting to see what
a smart lawyer would do to us in a court
case.
Phil Davill
From
Tony Bannister
Thanks Robin,
So is the same for all the
hourly progs, eg is 07z really 5pm AEDT? Phil Davil
raises a good point, also
if you took a straw poll amongst forecasters I bet they took
the time to be correct (ie
07z being 6pm AEDT). With the models getting better at
timing of windchanges we
look carefully at what the model is saying, and at what time
it is saying it. I
was using hourly meso5k heavily in Sydney last week forecasting
for the NSW bushfires. The
prog times should indicate the correct time.
Graham Mills
Regrettably, I'm forced to comment -
As described by others, the
base time for the LAPS, meso etc
models is 23Z (or 11Z etc).
Thus only data at that time gets full
weight, and obs 1 or 2 hours
off get less weight. However, its more
complex than that - the
analysis is a correction of a background
field, which is the short-term
(6-hour) model forecast, and for a 23Z
analyis is valid at 23Z
(this means 23Z here). If the data is at 22Z
(eg the synops) then the
increment is the difference between a 23Z
guess and a 22Z obs, downweighted
by the time de-correlation so
that the amplitude of any
correction is down-weighted. Its pretty
easy to see than a progressive
trough will also introduce a phase
error here too, with subsequent
unknown consequences. Thus, as
I've said at training sessions
many times in the past, the numerical
analysis is produced as
an initial state for the forecast model, not as
a substitute for a meticulous
manual analysis.
Also critical for the meso-scale
models is the fact that the model's
radiation, and thus its
diurnal heating cycle, is tied to the 23Z base
time. This can be critical
in situations like today's forecast for
Victoria, as the rate of
movement and evolving structures of these
changes is strongly modified
by the diurnal cycle of land-sea
heating contrast. Sea-breeze
circulation timings are also obviously
affected, as would be even
tempereature forecasts for a given time.
Thus a forecast saying 00Z
on NMC's web site, and I think on the
MCIDAS files if I'm interpreting
the plots on VRO's web site
correctly, really means
23Z. Just to be really confusing, if you are
looking at the pretty coloured
5km meso-LAPS charts that come
through BMRC's web site,
then the times are not incremented by
1hour, so a 23Z-valid forecast
there is the same time as the 00Z-
valid (labelled) chart on
NMOc's web site, and really means 23Z.
Confusing eh? Its hard enough
out there without having to do this
sort of mental arithmetic,
as well as the extra calculations for daylight
savings time, so be careful.
As a challenge, who'd care
to explain all this to the media, or to a
coroner if you had to?
Graham (not really much of
a guru) Mills
David Thomas
Dear all,
Terry Hart is away at the
moment so I will make the following comment on his
behalf. He may put out a
more current and informed view on his return on 15
January. Paul Stewart is
also on leave.
Firstly, let me clarify;
all the models we receive from outside of Australia
are, and have been for as
long as I know, based on 00 & 12z. Some of the
newer models also run on
06 adn 18z. International observing times are also
built around the 00 &
12z. Australia really has been the odd one out in the
global modelling community.
NMOC has been driving the
push to move from the traditional use of 23/11z in
Australian models to 00/12z
to be inline with the rest of the world. This
has been a mammoth project
and an incremental one. Some of the steps have
been taken based on opportunity
(new RTDS2) , others based on need. Paul
Stewart's email earlier
describes the plan quite well, and you can see from
Robin and Graham's comments,
just how complex this is. However, most things
are now in place to take
the final step to be fully converted to 00/12z
early this year.
regards
David Thomas
Manager Central Operations
(SRCO) & a/STAN