This case study focuses on the region immediately surrounding the S-PolKa dual-polarization radar stationed on Addu Atoll (0.7oS, 73oE) in the central Indian Ocean, with environmental conditions provided by 3-hourly quality-controlled soundings and 6-hourly ERA-interim reanalysis data. The changes in precipitation areal coverage and vertical distributions of moisture and winds reveal two distinct convection periods at the beginning and end of the MJO passage over the radar. Shifts in reflectivity and dual-polarization characteristics of four precipitation types over these time periods showed a transition towards shorter bursts of more intense convective precipitation over a smaller portion of the radar domain. These convective elements displayed greater upscale organization as the surrounding environment gradually became more strongly sheared with greater instability, associated with persistent lower-level dry-air advection from the westerly wind burst. These bursts of convection, especially in the first time period, appeared with periodic dry-air intrusions into the central equatorial Indian Ocean.
The changes in the vertical distribution of ice particles indicate that graupel progressively occurred over a greater depth, associated with the overall increasing intensity of convective precipitation, while other ice particle types occurred over shallower depths with decreased reflectivity values. This change in the aggregates and ice crystals signals a transition towards shorter residence times as a result of weaker stratiform precipitation with smaller or less abundant ice particles.
High-resolution WRF simulations with an outer domain encompassing the Indian Ocean basin will be performed using hourly ERA5 reanalysis data for the initial conditions to better understand the influence of large-scale conditions on the convection in this region of the Indian Ocean. These simulations will include nested domains at convection-allowing scales centered on Addu Atoll, which allows for comparison with the S-PolKa radar and investigation into mesoscale interactions that may influence the upscale convective organization in the vicinity. Using the multi-domain numerical simulations in conjunction with DYNAMO/CINDY/AMIE observations, parcel trajectory analysis will be performed to examine the origin of the air masses pertaining to the convection near the radar.