Impacts of initial conditions on monsoon intraseasonal forecasting

Sensitivity of monsoon intraseasonal forecasting to different initial conditions is examined with the University of Hawaii Hybrid Coupled Model (UH-HCM). The target period is May to September 2004. We found that in the NCEP reanalysis the amplitudes of the convective activities associated with monsoon intraseasonal oscillation (MISO) are smaller than the observed counterparts by a factor of two to three. Motivated by this fact, we carried out a suite of forecasting experiments to explore the impacts of initial conditions on intraseasonal forecast skills. Our results reveal that with the original NCEP reanalysis as initial conditions the monsoon intraseasonal forecast skills of 850-hPa zonal winds and rainfall are only about a week over the global tropics (30°S-30°N) and Southeast Asia (10°N-30°N, 60°E-120°E). The predictability increases steadily with increased amplitudes of MISO in the initial conditions. When the MISO signals in initial conditions are recovered to a level similar to that in the observations, monsoon intraseasonal forecast skills reach 25 days for 850-hPa zonal winds and 15 days for rainfall over both the global tropics and Southeast Asia. It is also found that high-frequency weather in initial conditions generally extends rainfall predictability by about 5 days.