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 This methodology follows that used by (Oliver et al., 2019; Veatch et al., 2022, preprint: not peer reviewed).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.相对粒子密度（Relative Particle Density, RPD）通过对研究区域内网格化分箱系统中的粒子密度进行归一化处理，用以表征单一时戳下漂流粒子的空间分布位置。相对粒子密度的计算流程始于在规则网格上投放虚拟粒子，并通过由高频雷达（High Frequency Radar, HFR）观测得到的流场对粒子进行追踪。随后通过统计每个网格箱内的粒子总数，并以所有网格箱内粒子数的中位数进行归一化，完成相对粒子密度的量化计算。每3小时便会在覆盖HFR观测区域80%范围的规则网格上投放新的虚拟粒子。虚拟粒子仅在流场中平流输送满6小时（此时HFR观测海流的自相关系数达到e折叠阈值）后才会被计入统计；当粒子被平流输送出HFR观测区域，或存活时长达到3天时，则不再纳入统计。结合浮游植物在研究区域表层海域的平均停留时长（Kohut等，2018），本次研究设定3天的统计阈值以匹配浮游植物在表层水体中的滞留时间。本方法沿用了Oliver等（2019）以及Veatch等（2022，预印本，未经过同行评议）所采用的研究范式。相对粒子密度表征的是各时戳下每个网格化分箱内的归一化粒子数量（Veatch等，2024，图2C）。本次计算采用二维HFR观测数据，其前提假设为：表层海域的积分散度为零，且不存在因垂直运动导致粒子脱离表层水体的情况。因此，结合HFR提供的逐时演化表层流场，相对粒子密度可映射出整个研究区域内与表层相关联粒子的瞬时浓度分布。