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 html 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: SREF – SPC – Heavily used for Severe Weather Risk (“Slight Risk” areas, etc.) Ewall SREF: