Vertical divergence profiles as detected by two wind-profiler mesonets over East China: Implications for nowcasting convective storms

Low-level convergence is one of the most important dynamic variables in governing the convection initiation (CI) and development of storms. However, our ability to obtain high-resolution horizontal divergence profiles remains limited due to the lack of high-resolution vertical wind observations. In this study, horizontal divergence (and vertical motion) profiles are derived from horizontal winds measured by two radar wind-profiler (RWP) mesonets in East China, and then used to examine their relationship with CI occurring within a mesonet and an intense squall line moving across another mesonet. Results show that the RWP mesonets together with automated surface observations could provide realistically the lower-tropospheric profiles of horizontal divergence (and upward motion) associated with the CI in the presence of the urban heat island effects, and lake and sea breezes; and the evolution of the squall line respectively. High-resolution surface observations resolve better divergence induced by localized CI than the RWP measurements, whereas the latter capture well significant convergence below 4-km altitude that precedes the onset of squall precipitation. A statistical analysis of the RWP data during the summer months of 2018 indicates that the lower-tropospheric convergence can be detected up to about 40 min in advance of the occurrence of heavy rainfall, and the correlation is higher when it is closer to the heavy rainfall time. Nearly 40% of heavy rainfall moments (at 6-min intervals) are accompanied by strong convergence signals occurring mostly near the top of the boundary layer. This work indicates that the RWP and surface wind measurements should be integrated to aid in nowcasting the location and timing of CI and development of convective storms.