Dual-polarimetric Radar Observations of Convection in Hawaii

Convection in the Hawaiian Islands is often driven by mesoscale rather than synoptic scale processes, and can have significant variability over small spatial and temporal scales. The WSR-88D radar network provides good coverage over the islands, but terrain blocking, large distances between islands, and single-polarization have limited the detailed observations needed to improve our understanding of how mesoscale processes play a role in tropical precipitation formation. The deployment of the Doppler On Wheels mobile X-band radar with the National Science Foundation sponsored Hawaiian Educational Radar Opportunity (HERO) project and recent WSR-88D dual-polarimetric upgrade have provided new opportunities to collect high-resolution dual-polarimetric radar data on Oahu. The HERO project lasted for three weeks in the fall of 2013 and a number of convective types were observed, including orographically forced trade wind showers and sea-breeze convection. In one particular case, an evolving trade wind shower passed directly over the radar, allowing for very high-resolution radar analysis of vertical motion and cumulus development. In another case, weakened trade winds allowed sea-breeze convection to develop in the central valley of Oahu. Observations of the entire lifecycle of a convective cell from initiation to dissipation were analyzed. After the HERO project, Hurricane Ana (2014) passed to the south of the Hawaiian islands with heavy rainfall observed by the recently upgraded WSR-88D radars. The differences in the radar echo characteristics, mesoscale convective forcing mechanisms, drop size distributions and microphysical processes deduced from the HERO and WSR-88D observations will be discussed.