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Observations and analyses from the Variational Doppler Radar Assimilation and Analysis System (VDRAS) were used to investigate the convection initiation mechanism of a nocturnal squall line along the Meiyu Front over central East China on 11–12 July 2014. The squall line occurred on the warm side of the Meiyu Front. The main convection in the squall line was initiated along a mesoscale convergence line above a stable boundary layer at about 2330 local standard time (LST) on 11 July. The convergence line intensified due to enhancement of southerly winds. Momentum budget analyses further revealed that the enhancement of the southerly winds was mainly contributed by an increasing horizontal pressure gradient term, which was associated with the eastward movement of a preexisting mesoscale vortex above the boundary layer and the strengthening of subtropical high. In the early morning of 12 July, a jump propagation of the squall line occurred. As the squall line matured, surface cold pools strengthened due to diabatic heating (most likely evaporative cooling). Then new convection began to initiate in front of the surface cold pools. During the jump propagation of the squall line, the elevated convection in the squall line transitioned to surface‐based convection. This is the first study demonstrating the occurrence of elevated convection along the Meiyu Front and its transitioning to surface‐based convection by jump propagation.

The predictability of a dense advection fog event on 21 February 2007 over north China (NC) is investigated with ensemble simulations using the Weather Research and Forecasting Model (WRF). Members with the best and worst simulation are selected from the ensemble, and their initial condition (IC) differences are explored. To test the sensitivity of fog simulation to those differences, the model is initialized with ICs that change linearly from the worst member to the best member, and the changes in simulated results are examined. The improvement in simulations due to the linear improvement of ICs is found to be monotonic. The IC differences at lower levels are of more influence to the simulation than IC differences at higher levels. By removing the IC differences of each meteorological variable individually, it is found that improvements in potential temperature and horizontal wind are more important than that of water vapor mixing ratio in this case. Additionally, the linear improvement in each meteorological variable also contributes monotonically to the simulated results. The budget analyses of the tendency of potential temperature and water vapor mixing ratio show that turbulence mixing and advection are the major factors contributing to the formation of fog. The correct initial temperature field ensures the formation and maintenance of an inversion, and the correct initial wind field ensures the correct transport of temperature and moisture in this case. Further discussion examines the reasons for the monotonic behavior in the simulation improvement.

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