The Effect of Increased Horizontal Resolution on GLA Fourth Order Model Forecasts

Abstract

 A benchmark series of ten-day weather forecasts has been run with the GLA Fourth Order GCM with both a 4° latitude by 5°longitude resolution and a 2° latitude by 2.5° longitude resolution. Ensemble statistics of forecast skill and maps of systematic error fields have been generated for both resolutions. The enhanced resolution added 24 hours of useful predictive skill to the sea level pressure forecasts and 6 hours to the 500 mb height forecasts, but 5 to 6 days into the forecasts the advantage of the finer resolution was lost. The systematic error fields showed that by 8 days the "climate drift" of the 2 ° × 2.5° forecasts had become pronounced and had caused the loss of predictive skill relative to the 4° × 5° forecasts. Additional resultes indicate that a gravity wave drag parameterization scheme might alleviate the climate drift problem.
 Two cases have been studied in greater detail. A significantly improved short-range prediction of the Queen Elizabeth II storm by the 2° × 2.5° model is attributed to reduced truncation error and to more accurately simulated small-scale forcing. The useful skill of a ten-day forecast which was dominated by a quasi-stationary blocking-high pattern over Eurasia was extended by 48 h to day 8 by the 2° × 2.5°model with a realistic simulation of the blocking high and a successful prediction of a cyclone that developed rapidly off the east coast of Asia between day 6 and day 8 of the prediction.