Transcript Using Ensembles - Texas A&M University

Using Ensembles
CHAOS
• The idea that a system is very sensitive to the
initial conditions – small changes to the initial
state end up being big differences at a later
time
• This concept is fundamental to weather
forecasting – it basically says that if we can’t
observe the current state of the atmosphere
perfectly, our ability to forecast the future
weather will be limited (hint: we can’t
observe the current state perfectly!)
Chaos
• What we now do to account for this is make
one numerical forecast, tweak the initial
conditions slightly, run the model again, and
so forth.
• This is called an ensemble forecast
• We know that we can’t know the exact state
of the atmosphere in, say, 2 weeks. But an
ensemble can give an idea of the range of
possibilities
Stochastic (Probabalistic) vs.
Deterministic
• Deterministic forecasting – one model run,
one answer
• Ensembles – many model runs, many answers
• The issue is making the model runs different,
called perturbations
– Two main ways
• Perturb the initial conditions
• Perturb the Physics
• Or a little of both…
• Perturbing the Initial Conditions
– Have slightly different starting conditions based
on the same observation and forcing sets, but
within the error of those observations, etc.
• That is, nothing wacky, just reasonable variations in the
analysis conditions based on the data
– Run the model multiple times and look at the
“ensemble” of results.
• Obviously, you can’t usually run a full
resolution dynamical model 32 times, so
usually you scale back the resolution/levels,
etc.
• Perturb the physics
– Use different schemes, such as for convective
parameterization
– Or, use different models
– Guess what, you are already comfortable with
using a 2-member perturbed physics ensemble:
• GFS and NAM
– When you see them agree, how do you “feel”
about the forecast?
• Can have multi-model ensembles – can be
very powerful
Traditional Approach:
Enhancing the Long Range
• Looking for agreement, disparity, and multiple
solutions
– In general, intuitively you know that good agreement
means high confidence, and…
• Spaghetti Plots and Postage Stamps
• http://hdwx.tamu.edu/product.php?productID=90
• http://hdwx.tamu.edu/product.php?productID=92
• http://www.emc.ncep.noaa.gov/gmb/ens/fcsts/ensframe.
html
• Multiple Solutions – A new challenge for the meteorologist,
especially in conveying information/uncertainty
• The atmosphere might have a reasonable plan “B”
• Ensemble mean can be very powerful, but sometimes misleading
–
White board
A new wave of Ensemble Usage:
Short Range
• Very Helpful for QPF
– HDWX SREF:
• https://hdwx.tamu.edu/product.php?productID=110
• SREF – SPC
– Heavily used for Severe Weather Risk (“Slight Risk”
areas, etc.)
• http://www.spc.noaa.gov/exper/sref/cmm_sref.php
• Ewall SREF:
http://www.meteo.psu.edu/ewall/ewallsref.html