COAMPS Extended-range Simulations in the Tropics

The performance of 30-60 day simulations of the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®) forecast system, run over a large domain in the tropics and subtropics, is evaluated under several metrics. We have performed 60-day COAMPS integrations at 45-km and 27-km resolution for the DYNAMO (Dynamics of Madden Julian Oscillation) field experiment period, starting late October 2011. The COAMPS domain covers the tropics and subtropics from the eastern Atlantic to the eastern Pacific. Lateral boundary conditions are provided by NOGAPS (Navy Operational Global Atmospheric Prediction System) analyses. SST, provided by the NCODA (Navy Coupled Ocean Data Assimilation) system, is either held fixed to the initial value (fixed SST) or updated with analyses every 24 hours (observed SST). The extended integrations have been verified using observed OLR, TRMM precipitation estimates, and NOGAPS analyses.

Comparison of the fixed and observed SST integrations indicate that the fixed SST has a small impact on the COAMPS precipitation and wind biases for the first two weeks, but has a substantial impact after that. Comparison of the 27-km and 45-km resolution experiments indicates that while the 27-km simulation outperforms the 45-km simulation for OLR, the advantages of higher resolution for other metrics are unclear. The 45-km simulation has smaller errors for the wind and precipitation fields. Examination of time-longitude diagrams of equatorial precipitation shows that the 45-km COAMPS simulation with observed SST has a remarkably good representation of the late-November Madden Julian Oscillation (MJO). The 45-km simulation with fixed SST, and the 27-km simulations, had weaker representations of the MJO. These results demonstrate the importance of accurate SSTs for predictions beyond “weather” time scales of 1-2 weeks.