A "Ray" Pattern of Clouds and Precipitation in a Super Cloud Cluster Observed and Simulated during the CINDY2011/DYNAMO Field Campaign

This study provides a description of the multiscale structure of a super cloud cluster, accompanied by eastward-propagating precipitating systems and westward-moving upper-tropospheric cloud shields, during a convectively active phase of MJO over the equatorial Indian Ocean. This “ray” pattern of clouds and precipitation was discovered during the previous field campaign (MISMO) in 2006, and was observed again by Doppler radars and satellite microwave sensors during the field campaign of the Cooperative Indian Ocean experiment on intraseasonal variability in the Year 2011 (CINDY2011) / the Dynamics of the Madden-Julian Oscillation (DYNAMO). Moreover, the eastward-propagation of precipitating systems was captured by a week-long near real-time forecast system using a regionally stretched version of Nonhydrostatic Icosahedral Atmospheric Model (NICAM). Based on both the observational data and simulation results, the evolution and structure of these precipitating systems were examined.

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