Ensemble mean storm-scale performance in the presence of nonlinearity

Michael A. Hollan, Brian C. Ancell

Research output: Contribution to journalArticlepeer-review

4 Scopus citations


The use of ensembles in numerical weather prediction models is becoming an increasingly effective method of forecasting. Many studies have shown that using the mean of an ensemble as a deterministic solution produces the most accurate forecasts. However, the mean will eventually lose its usefulness as a deterministic forecast in the presence of nonlinearity. At synoptic scales, this appears to occur between 12-and 24-h forecast time, and on storm scales it may occur significantly faster due to stronger nonlinearity. When this does occur, the question then becomes the following: Should the mean still be adhered to, or would a different approach produce better results? This paper will investigate the usefulness of the mean within aWRFModel utilizing an ensemble Kalman filter for severe convective events. To determine when the mean becomes unrealistic, the divergence of the mean of the ensemble ("mean") and a deterministic forecast initialized from a set of mean initial conditions ("control") are examined. It is found that significant divergence between the mean and control emerges no later than 6 h into a convective event. The mean and control are each compared to observations, with the control being more accurate for nearly all forecasts studied. For the case where the mean provides a better forecast than the control, an approach is offered to identify the member or group of members that is closest to the mean. Such a forecast will contain similar forecast errors as the mean, but unlike the mean, will be on an actual forecast trajectory.

Original languageEnglish
Pages (from-to)5115-5133
Number of pages19
JournalMonthly Weather Review
Issue number12
StatePublished - 2015


  • Ensembles
  • Forecast verification/skill
  • Forecasting
  • Forecasting techniques
  • Mesoscale models
  • Models and modeling
  • Numerical weather prediction/forecasting
  • Operational forecasting


Dive into the research topics of 'Ensemble mean storm-scale performance in the presence of nonlinearity'. Together they form a unique fingerprint.

Cite this