CLUEing in on Spring Forecasting Experiment 2017

It’s nearly the beginning of May (even if it doesn’t feel like it in Norman, OK, with a current windchill of 38°F!) and that means that another Spring Forecasting Experiment is about to be underway. This year the Community Leveraged Unified Ensemble (CLUE) is an even more vast than last year, comprised of 81 members from organizations such as NSSL, CAPS, OU, NOAA’s Earth Systems Research Laboratory/Global Systems Division (ESRL/GSD), NCAR, and GFDL. These members will provide forecasts of 36 h to 60 h in length, depending on the subsets of the ensemble being considered.

The ensemble subsets were again designed with specific experimental goals, to test methods of ensemble design in a search for the optimal configuration. The issues examined this year are specifically relevant to the design of an operational CAM-based ensemble by NCEP/EMC, and will hopefully lead to the best possible operational CAM ensemble. Many of these subsets ask similar questions to those of SFE 2016, but with tweaks to further investigate questions that remained unanswered in SFE 2016 or to extend the results from the first experiment. Additionally, some of the members composing the subsets have been tweaked between SFE 2016 and SFE 2017, so improvement or degradation of the member performance will be noted. The subsets are:

  1. Multi-core vs. single-core, tested using a 10-member ARW-based single-physics ensemble, and a 10-member ensemble composed of 5 ARW members and 5 NMMB members. Results from 2016 produced mixed results between which ensemble was better, depending on whether QPF or severe weather parameters were examined.
  2. Physics perturbations, tested using three ensembles with perturbed initial and lateral boundary conditions. One ensemble will have single physics, one will have mixed physics, and one will have single physics with stochastic perturbations. The stochastic perturbation strategy was developed by NSSL and the Developmental Testbed Center (DTC), and is analogous to the approach used at the European Center for Medium-Range Weather Forecasts (ECMWF).
  3. GSD radar vs. CAPS radar assimilation, tested using two ensembles that are configured nearly identically but for the method used to assimilate radar data. This comparison will focus largely on the first 12 hours of forecasts and the evolution of convection present at the time of initialization.
  4. Data assimilation comparisons, tested using multiple ensembles with different 3DVAR and EnKF data assimilation strategies. While the differences between the systems make for a less controlled experiment, we hope to examine the relative performance between the five ensemble subsets.
  5. Microphysics sensitivities, tested using an ensemble of five members which are configured identically but for the microphysics schemes used. Results from 2016 showed relatively similar results between microphysical parameterization strategies.
  6. FV3, tested using two different versions of FV3 and current well-known models. This comparison will test FV3 at convection-resolving scales, looking at simulated storm structure and evolution alongside storm surrogates such as hourly maximum updraft helicity and updraft speed.

While some of these comparisons are similar to SFE2016 (specifically, the multi-core vs. single-core, data assimilation method comparisons, and microphysics comparisons), the CLUE contains many exciting new goals, with increased focus on the questions relevant to an operational CAM ensemble.

Additionally, while the CLUE is the main focus of SFE 2017 numerical guidance, the 2017 SFE encompasses much more – other NWP guidance includes ensembles such as the High Resolution Rapid Refresh Ensemble, the NSSL Experimental Warn-on-Forecast System for ensembles (NEWS-e), and the High Resolution Ensemble Forecast (HREFv2) system. Deterministic models include three from the United Kingdom Met Office that run to 120 h, the operational (v2) HRRR, and the developmental (v3) HRRR. In the realm of forecast products, SFE 2017 will explore new methods for hail and tornado guidance, high-temporal-resolution forecasts and timing guidance. For more details, check out this year’s Operations Plan.

Each of these methods of NWP is worthy of their own blog post, and many of them will be featured in the upcoming month as the spring unfolds. Stay tuned for the adventures of SFE 2017!