THE COUPLING BET WEEN CONVECTIVE VARIABILITY AND LARGE-SCALE FLOW PATTERNS OBSERVED DURING PISTON 2018-2019

Abstract

The Propagation of Intraseasonal Oscillations (PISTON) field campaign took place in the waters of the western tropical North Pacific during the late-summer and early-fall of 2018 and 2019. During both research cruises, the Colorado State University SEA-POL polarimetric C-Band weather radar obtained continuous 3D measurements of oceanic precipitation systems. This study provides an overview of the variability in convection observed during the PISTON cruises, and relates this variability to large-scale atmospheric conditions. Using an objective classification algorithm, precipitation features are identified and labeled by their size (isolated/sub-MCS/MCS) and degree of convective organization (nonlinear/linear). It is shown that although large mesoscale convective systems (MCS’s) occurred infrequently (present in 13% of radar scans), they contributed a disproportionately large portion (56%) of the total rain volume. Conversely, small isolated features were present in 91% of scans, yet these features contributed just 11% of the total rain volume, with the bulk of the rainfall owing to warm rain production. Convective rain rates and 30-dBZ echo top heights increased with feature size and degree of organization. MCS’s occurred more frequently in periods of low-level southwesterly winds, and when low-level wind shear was enhanced. By compositing radar and sounding data by phases of easterly waves (of which there were several in 2018), troughs are shown to be associated with increased precipitation and a higher relative frequency of MCS feature occurrence, while ridges are shown to be associated with decreased precipitation and a higher relative frequency of isolated convective features.