Integrated relative humidity as a layer-resolved indicator for moisture-precipitation interactions

We introduce Integrated Relative Humidity (IRH), a layer-resolved diagnostic of tropospheric saturation, to investigate moisture-precipitation interactions in the subtropical coastal environment (Hong Kong). Using 16 years (2005-2020) of co-located radiosonde and rain gauge data, we classify observations into four precipitation lifecycle stages: No Rain (NR), Before Rain (BR), During Rain (DR), and After Rain (AR). IRH is computed from the surface to nine standard pressure levels (1000-150 hPa), enabling layer-specific saturation diagnostics across these stages. IRH probability density functions (PDFs) reveal stage-specific vertical structures with saturation peaks consistently near 850 hPa. BR shows increasing lower-tropospheric saturation, DR exhibits the highest deep-column saturation, and AR displays reduced mid-level saturation, reflecting dynamic redistribution across the precipitation lifecycle. Bimodal PDF structures in BR, DR, and AR stages are consistent with transitions between shallow and deep convective regimes. Reversed cumulative distribution function (RCDF) for IRH = 0.90 captures low/mid-level buildup in BR and DR, while RCDF for IRH = 0.80 highlights upper-level dissipation in AR. Crucially, IRH thresholds of 0.82 at 700 hPa and 0.70 at 150 hPa correspond to 84% and 82% of DR events, respectively, while both exclude 76% of NR cases. Furthermore, the threshold of 0.70 at 150 hPa is also exceeded in 72% of BR events with the same NR exclusion. These findings demonstrate IRH as a layer-resolved metric for moisture-precipitation interactions, providing a quantitative diagnostic framework that may inform the development of nowcasting tools and aid in model evaluation in subtropical environments.