Impact of Moisture on the Organizational Mode and Intensity of Squall Lines Through Numerical Experiments

Composited reflectivity Doppler radar data from June to September of 2007-2010 were used to classify mesoscale convective systems (MCSs) over Central East China (CEC) into seven morphologies. The morphologies included one nonlinear mode (NL) and six linear modes: convective lines with no stratiform precipitation (NS), trailing stratiform precipitation (TS), leading stratiform precipitation (LS), parallel stratiform precipitation (PS), bow echoes (BE) and embedded lines (EL). Non-linear and linear systems composed 44.7% and 55.3% of total MCSs, respectively, but there was no primary linear mode. All MCS morphologies attained their peak occurrence in July, except BE systems which peaked in June. Significant differences in MCS-produced severe weather existed between dry and moist environments. High winds and hail events were mainly observed in dry environments, and in contrast, short-term intense precipitation occurred more frequently in moist environments. Based on the classification of convection lines, numerical experiments were conducted on a squall line occurring on June 3–4, 2009, in Henan, Anhui, and Jiangsu provinces, China, that produced high winds and hail. The impact of moisture on the entire air column and that of its vertical distribution on the intensity, development process, and morphology of mesoscale convective systems (MCSs) were investigated. Analysis revealed that the amount of moisture and its vertical distribution had a significant effect on the strength of the downdraft and cold pool, which thus affected the morphology, duration, and strength of convection. The experiments on the entire air column demonstrated that the intensity of MCSs increased with moisture and that the strengths of the cold pool and the thunderstorm high increased, which led to higher winds. Conversely, the intensities of MCSs, cold pool, and surface winds decreased with moisture. Additional moisture led to a stronger cold pool, which caused the maximum winds to strengthen at the developmental stages of the MCSs. However, the cold pool and rear inflow jet weakened more rapidly, which was unfavorable for high wind development and maintenance at mature stages. Precipitable water in entire air column remained unchanged. Linear MCSs and high winds tended to occur in environments of mid-level drier air and low-level moister air that favored the development of stronger thunderstorm highs and discouraged the persistence of MCSs.