Handout (3.6 MB)
In the case of October 2011, the eastward propagation of precipitating systems was observed along the equator every 3-4 days under a moist environment. Each precipitating system corresponded to a surge of westerly flow in the lower troposphere and was marked by a leading line of convective echoes, which were usually shallow (< 10km) but were occasionally deepened at an intersection with an upper-level cloud shield moving westward. The moving speed of these systems was in the range of 6-13 m/s, which was usually faster than environmental westerly flow. These characteristics are quite similar to those observed during the MISMO field campaign.
The eastward-propagating systems of precipitation, predicted by NICAM, also show the similar characteristics, that is, the zonal speed slower than 10 m/s and an increase in the lower-tropospheric westerly flow associated with a leading line of convection. The ray pattern of eastward-propagating precipitating systems with westward-moving cloud shields was also reproduced during the convectively active phase. The simulation results show that each of these eastward-propagating systems develops in a synoptic-scale upward motion and can sustain in a moist environment with the precipitable water greater than 50 mm. These results suggest that the ray pattern of clouds and precipitation is a dominant component of a super cloud cluster during a convectively active phase of MJO
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