Convectively Coupled Equatorial Wave Skill in the Unified Forecast System

Tropical precipitation and circulation are often coupled and span a vast spectrum of

scales from a few to several thousands of kilometers and from hours to weeks. Current

operational numerical weather prediction (NWP) models struggle with representing the

full range of scales of tropical phenomena. Synoptic to planetary scales are of particular

importance because improved skill in the representation of tropical larger scale features

such as convectively coupled equatorial waves (CCEWs) have the potential of reducing

forecast error propagation from the tropics to the midlatitudes.

Here we evaluate CCEW skill in two sets of model forecasts. First, two recent

versions of NOAA’s Unified Forecast System (UFS): operational GFSv15 forecasts and

experimental GFSv16 forecasts from April through October 2020. And second, several

versions of the subseasonal-seasonal (S2S) component of the UFS: coupled prototypes

5, 7 and 8.

Results show overall better initial CCEW skill in the coupled prototypes than in

operational forecasts, indicating a positive impact from coupling to an ocean model.

Kelvin and Mixed-Rossby Gravity wave skill is below 0.5 by lead time 48 h, while

Equatorial Rossby waves and the Madden-Julian Oscillation forecasts have skill until

lead time 96-144 h in some cases. In general, CCEW precipitation skill increases

somewhat for newer model versions, however the increase is not statistically significant,

leaving room for further improvement.