Relative Particle Density (RPD) calculations using High Frequency Radar (HFR) observed surface currents around Palmer Deep Canyon from January to March of 2020

Relative Particle Density (RPD) reports the position of drifters at a single timestamp by normalizing the density of drifters within a gridded bin system in the study field. Relative Particle Density calculations begin with releasing virtual particles over a regular grid and tracking them through a velocity field (High Frequency Radar observed surface currents). RPD is then quantified by summing the number of drifters in each grid box, and normalizing by the median number of drifters in all grid boxes. New particles were released in a regular grid across the 80 % coverage of the HFR footprint every three hours. Particles were not counted until they had been advected in the velocity field for 6 hours (when the autocorrelation of the HFR velocities cross the e-fold), and were no longer counted when they were advected out of the HFR domain, or after they became three days old. Given the average residence time of 2 days (Kohut et al., 2018), the three-day threshold was 245 chosen to coordinate with the time phytoplankton will spend in the surface layer of the study domain. This methodology follows that used by (Oliver et al., 2019; Veatch et al., 2022, preprint: not peer reviewed). RPD reports the normalized number of drifters present in each gridded bin at each timestamp (Veatch et al., 2024 Figure 2C). Two dimensional HFR data is used to calculate RPD, creating the assumption that the integrated surface divergence is zero, and no particles are lost from the surface due to vertical velocities. Therefore, RPD will map the instantaneous concentration of surface associated particles across the entire domain given the evolving surface current fields provided by the HFR.