Evaluating the applicability of spatial precipitation concentration metrics

This study evaluates the applicability of four metrics for quantifying the spatial concentration of precipitation, namely the coefficient of variation (C_V), Gini coefficient (G), Theil index (T), and Herfindahl-Hirschman index (IHH). The performance of these metrics is examined using controlled numerical experiments and ERA5-Land monthly precipitation datasets at global land areas and the Yangtze River basin. The results indicate that all four metrics are robust to proportional variations in total precipitation, yet they exhibit distinct responses to dry-wet polarization and precipitation footprint contraction. The Theil index shows higher sensitivity to dry-wet polarization, rendering it suitable for identifying variations in the tails of precipitation distribution. By contrast, the IHH is more responsive to footprint contraction and thus effective in identifying changes in precipitation-concentrating regions. Both the C_V and G exhibit relatively stable performance across all conditions. Controlled numerical experiments based on ERA5-Land monthly precipitation fields further suggest that dry-wet polarization can be decomposed into two sub-mechanisms: one-sided enhancement of wet-tails and two-sided dry-wet differentiation. The C_V is more sensitive to one-sided wet-tail enhancement, while the Theil index is more responsive to two-sided dry-wet differentiation. Results for the Yangtze River basin are broadly consistent with those obtained for global land areas. These findings provide a methodological basis for selecting appropriate metrics in precipitation-concentration-related studies.