This study explores the effects of the mesoscale descent associated with a convective burst during the genesis of typhoon Hagupit (2008), based on a high-resolution cloud-resolving numerical simulation. The simulation result captures the synoptic-scale circulation surrounding the pre-Hagupit depression and the evolution of the storm. A burst of intense deep convection occurs about 1 day before the genesis of Hagupit. After the convective burst, temperature deviation near the center of the depression increases in the lower troposphere. This warming contributes to a drop in the central pressure of the depression and hence to the beginning of the so-called system-scale intensification. In addition, the low-level warming tends to inhibit vertical motion by acting as a lid. Horizontal flow is therefore dominant in the boundary layer; thereby, the air can efficiently gain energy from the sea surface. Increased energy in the boundary layer air feeds intense deep convection near the center of the depression just before the genesis time. These results are consistent with a previous observational study. Tangential momentum budget analysis demonstrates that, just before the genesis time, actual tendency of tangential velocity has larger values throughout the depth of the troposphere, indicating the importance of the deep-layer spin-up of the depression. These large values are attributed to the upward transport of tangential momentum by intense deep convection. In contrast, when the convective burst occurs about 1 day before the genesis time, positive actual tendency is confined to the lower troposphere because of smaller upward transport of tangential momentum.