Impacts of Moisture on the Organizational Modes of Mesoscale Convective Systems Associated with Warm-Sector Heavy Rainfall

Moisture is a key factor that affects heavy rainfall. The impacts of moisture on MCSs associated heavy rainfall have been mainly examined from two aspects. One is the direct impact of moisture especially lower-level moisture on the amount and locations of heavy rainfalls produced by MCSs. The other is the impact of moisture on the intensity of the MCSs that are associated with heavy rainfall. Relative to the widely examined impacts of moisture on the intensity of MCSs, the impacts of moisture on the organizational mode of MCSs have been quite limited.

This work examines sensitivities of organizational modes of warm-sector heavy-rainfall-associated mesoscale convective systems (MCSs) in South China to relative humidity based on an MCS event with a typical linear organizational mode of trailing stratiform precipitation (TS). This TS system with two arc-shaped structures in its convective region induced by two pairs of bookend vortices was simulated as the control experiment (CTRL), based on which sensitivity experiments were performed by perturbing relative humidities in different layers.

Results showed that sensitivities of organizational modes of the MCS to moisture decrease with height, with the highest sensitivity to lower-level moisture. Increasing lower-level moisture produces a bow echo (BE) mode with more stratiform precipitation than that in CTRL. Increasing lower-level moisture results in the increase of CAPE (convective available potential energy) and the decrease of LCL (lifting condensation level) thus favoring convection enhancement with strong cold pool and cold-pool-outflow-associated convergence, which lead to faster movements and larger rearward storm-relative winds that produce more stratiform precipitation. The occurrence of the BE mode is owning to the strengthening of rear inflows resulting from the merging of bookend vortices. Conversely, decreasing lower-level moisture produces no stratiform precipitation (NS) mode, possibly resulting from the lack of rearward storm-relative winds owning to the weak convection, the lack of cold pool and slow movements as a result of the decrease of CAPE and the increase of LCL. In addition, decreasing midlevel moisture may increase stratiform rainfall and the curvature of the convective line owning to the cold pool intensification resulting from the strong midlevel evaporation.