Self-aggregation of convection can be considered as the simultaneous occurrence of dry patch initiation/amplification and wet patch contraction/intensification from initially uniform moisture and temperature fields. As the twin of wet patches, dry patches play an important role in moisture and energy balance during convective self-aggregation. In this study, the WRF Model is used to study the initiation of dry patches in convective self-aggregation, especially the continuous drying in their boundary layer (BL). In the dry patch BL, increased air density leads to an enhanced high pressure anomaly, which drives an amplifying BL divergent flow and induces an amplifying BL subsidence. The virtual effect of drying by subsidence counteracts warming by subsidence and the BL process, further increasing BL air density. Our analysis indicates the existence of a dry-subsidence feedback, which leads to the initiation of dry patches in convective self-aggregation. This feedback is shown to be important even in very large-scale (3000 km × 9000 km) cloud-resolving convective self-aggregation simulations.