A study on the effects of convective momentum transport asociated with rain bands within the Madden-Julian Oscillation

Convective momentum transport (CMT) properties of rain bands within the Madden-Julian Oscillation (MJO) case reproduced by the global cloud-system resolving Non-hydrostatic Icosahedral Atmospheric Model (NICAM) are examined. In the eastern Indian and western Pacific tropical oceans, 60,997 rainband cases are identified, 15,221 cases of which come along with complete 3-D dataset necessary to calculate CMT. The distribution structure of upscale acceleration vectors due to CMT associated with each of the identified rainbands are composed according to their respective locations relative to the center of the MJO convective region. Strong upscale CMT acceleration are most frequently found between 0 to 20 degree west to the MJO center. Zonal components of the CMT acceleration vectors form a well organized three storied structure: positive near the surface (below 1.6km); negative at low to mid troposphere (2km - 6.5km); positive at upper troposphere (above 11km). Over the equator, the upscale acceleration due to CMT account for -160% on the 2km - 6.5km averaged zonal wind transition that occur along with the eastward propagation of the MJO convective region. The ensemble CMT effect operates to delay the eastward propagation of the westerly wind at the low-mid troposphere, thereby delays the eastward shift of convectively favorable region, and slows down the propagation speed of the entire MJO. Through a simplified estimation, it is suggested that the impact of omitting CMT effects may be large enough to modify the eastward phase speed to 10m/s - 15m/s.