The Risks and Rewards of High-Resolution and Ensemble Modeling Systems David Schultz NOAA/National Severe Storms Laboratory Paul Roebber University of Wisconsin at Milwaukee Brian Colle State University of New York at Stony Brook David Stensrud NOAA/National Severe Storms Laboratory

Objectives of this Talk n Discuss issues for operational weather forecasting in going to higher-resolution NWP. n Briefly compare advantages and disadvantages of high-resolution simulations versus lower- resolution ensembles. n Example: 3 May 1999 Oklahoma tornado outbreak. n Discuss unresolved scientific issues that will lead to improving predictability for operational forecasters.

High-Resolution NWP n High resolution (< 6 km) is now possible in real time due to increasing computer power and real-time distribution of data from National and International Modeling Centres. n Many groups have demonstrated high- resolution real-time NWP (Mass and Kuo 1998). n Small-scale weather features are able to be reproduced by high-resolution models (e.g., sea breezes, orographic precipitation, frontal circulations, convection).

But,... n The use of models to study physical processes and to make weather forecasts are two distinctly different applications of the same tool. n No guarantee that a high-resolution model will be more useful to forecasters than a model with larger grid spacing. n Model errors may increase with increasing resolution, as high-resolution models have more degrees of freedom. n High-resolution models may produce wonderfully detailed, but inaccurate, forecasts.

Ensemble Modeling Systems n Ensembles of lower-resolution models can have greater skill than a single higher- resolution forecast (e.g., Wandishin et al. 2001; Grimit and Mass 2001). n Ensemble forecasts directly express uncertainty through their inherently probabilistic nature. n But, what is the minimum resolution needed for “accurate” simulations? n How to best construct an ensemble?

The Forecast Process n Hypothesis Formation –Forecaster develops a conceptual understanding of the forecast scenario (“problem of the day”) n Hypothesis Testing –Forecaster seeks “evidence” that will confirm or refute hypothesis –observations, NWP output, conceptual models –Continuous process n Prediction –Forecaster conceptual model of forecast scenario(s) (e.g., Doswell 1986; Doswell and Maddox 1986; Hoffman 1991; Pliske et al. 2003)

Intuitive Forecasters n Defined by Pliske et al. (2003) as those who construct conceptual understanding of their forecasts on the basis of dynamic, visual images (as opposed to “rules of thumb”). n Such forecasters would benefit from both high-resolution forecasts and ensembles. –Show detailed structures/evolutions not possible in lower-resolution models –Developing alternate scenarios from ensembles –Construct probabilistic forecasts

3 May 1999 Oklahoma Outbreak (Jarboe) n 66 tornadoes, produced by 10 long-lived and violent supercell thunderstorms n 45 fatalities, 645 injuries in Oklahoma n ~2300 homes destroyed; 7400 damaged n Over $1 billion in damage, the nation’s most expensive tornado outbreak (Daily Oklahoman) (Schultz)

0131 UTC 0221 UTC 0200 UTC 0100 UTC Observed radar imagery (courtesy of Travis Smith, NSSL) 2-km MM5 simulation initialized 25 hours earlier (no data assimilation) pink: 1.5-km w (> 0.5 m/s) blue: 9-km cloud-ice mixing ratio (>0.1 g/kg) Moore Moore

Stage IV Radar/Gauge Precip. Analysis (Baldwin and Mitchell 1997) Moore Moore

Modeled Storms as Supercells n Identify updrafts(> 5 m/s) correlated with vertically coherent relative vorticity for at least 60 minutes n 22 supercells, 11 of which are on OK–TX border

Observed vs Modeled Supercells

Ensembles (Stensrud and Weiss) n 36-km MM5 simulations initialized 24 h ahead n Six members with varying model physics packages: 3 convective schemes (Kain– Fritsch, Betts–Miller–Janjic, Grell) and 2 PBL schemes (Blackadar, Burke–Thompson)

Ensemble mean convective precipitation: 2300 UTC 3 May to 0000 UTC 4 May (every 0.1 mm)

Convective Available Potential Energy (J/kg) ensemble mean ensemble minimum ensemble maximum ensemble spread 2000 J/kg 1000 J/kg 750 J/kg

Storm-Relative Helicity (m 2 s –2 ) ensemble mean ensemble minimum ensemble maximum ensemble spread

Bulk Richardson Number Shear (m 2 s –2 ) ensemble mean ensemble minimum ensemble maximum ensemble spread

Comparison n Both the high-resolution forecast and the ensemble forecasts did not put the bulk of the precipitation in the right place in central Oklahoma. n Both models indicated the potential for supercell thunderstorms with tornadoes in the Oklahoma–Texas region. n Both models were sensitive to the choice of parameterization schemes (e.g., PBL).

Remaining Scientific Issues n When should forecasters believe the model forecast as a literal forecast? n What is the role of model formulation in predictability? n What is the value of mesoscale data assimilation in the initial conditions? n What constitutes an appropriate measure of mesoscale predictability? n What is the appropriate role of postprocessing model data (e.g., neural networks, bias-correction techniques)? n Other examples and further discussion will be found in a manuscript, currently in preparation.