The MJO Cloud Population as Seen by TRMM, DYNAMO, and AMIE Radars

The variation of the cloud population over the central equatorial Indian Ocean and Western Pacific with phase of the MJO is analyzed using TRMM Precipitation Radar (TRMM PR) and data from surface based radars deployed in DYNAMO and AMIE. Radar echoes classified as "shallow-isolated" by the TRMM PR 2A23 algorithm are found to be present in relatively constant amounts during all phases of the MJO in both regions. To examine the frequency of occurrence of larger convective entities, we utilize a technique that has been applied to TRMM PR data to study the extreme convective and stratiform components of precipitating systems over land. The thresholds utilized in this technique were modified to be representative of tropical oceanic convection. Contiguous three-dimensional echoes are analyzed to determine if they contain deep convective cores, wide convective cores, and/or broad stratiform regions. Deep convective cores have reflectivities in excess of 30 dBz extending over 8 km in altitude. Wide convective cores have reflectivities in excess of 30 dBz covering more than 800 square kilometers. Contiguous stratiform echo objects covering areas in excess of 50,000 square kilometers are classified as broad stratiform regions. Considering eleven years of TRMM PR data during the months of October through February, we find the frequency of the deep and wide convective cores exhibit a slight variability in both the Indian Ocean and West Pacific regions. However, radar echoes associated with broad stratiform regions have a distinct maximum of occurrence during the active phase and minimum during the suppressed phase of the MJO. In terms of the Wheeler-Hendon index, broad stratiform regions are found to maximize in the Indian Ocean during phase 3 and minimize during phase 6. Broad stratiform regions in the Western Pacific maximize during phase 6 and minimize during phase 2. These results support previous studies that also suggest that the aspect of the cloud population most strongly modulated by the MJO is large stratiform precipitation regions associated with late-stage mesoscale convective systems.

We further explore the variability of the Western Pacific cloud population through an empirical orthogonal function (EOF) analysis of contour frequency by altitude (CFAD) diagrams of reflectivity. CFADs are generated using all echo objects observed in TRMM PR swaths from 10°N to 10°S and 140°E to 170°E from 1998 to 2010 during the months of October through February. The first EOF explains the majority of the variation in the cloud population and modulates the general occurrence of rain. The principle component of the first EOF maximizes during phase 5 and minimizes during phase 8. The second EOF exhibits a dipole with the echo top height either occurring above or below the freezing level. The principle component of the second EOF exhibits a gradual increase from its minimum during phase 3 to its maximum during phase 7. The third EOF accounts for the variability in the occurrence of a bright band. The principle component of the third EOF has a slight maximum during phase 6 and minimum during phase 3.

During DYNAMO and AMIE, radar data were obtained during all phases of the MJO by several precipitation and cloud radars. Preliminary analysis of the radar echo statistics obtained by the DYNAMO and AMIE radars will be compared with the statistics based on the TRMM PR data.