Open cloud cells can be described in ideal form as connected clouds that surround spots of isolated clear skies in their centers. This cloud pattern is typically associated with marine stratocumulus (MSc) that form in the oceanic boundary layer. However, it can form in deeper convective clouds as well. Here, we focus on deep-open-cells (with tops reaching up to ~5–7 km) that form in the post-frontal regions of winter Mediterranean cyclones, and examine their properties and evolution. Using a Lagrangian analysis of satellite data, we show that deep-open-cells have a larger equivalent diameter (~58 ± 18 km) and oscillate with a longer periodicity (~3.5 ± 1 h) compared to shallow MSc. A numerical simulation of one Cyprus low event reveals that precipitation-generated convergence and divergence dynamic patterns are the main driver of the open cells’ organization and oscillations. Thus, our findings generalize the mechanism attributed to the behavior of shallow marine cells to deeper convective systems.
Graupel is often parametrized as "medium-density" ice particles with a bulk density of 400 kg m −3 and corresponding fixed fall speed parameters in numerical models.In natural clouds, however, graupel has an extensive range of densities, and its fall speed is closely related to its density g . In this study, the possible responses of the microphysical and electrical structures of a simulated thunderstorm to varying g and its corresponding fall speed parameters were examined using the Advanced Research Weather and Forecasting Model (ARW-WRF) with an explicit charging and discharge lightning scheme. Six sensitivity tests were performed with different g and corresponding fall speed parameters. g ranges from 350-850 kg m −3 , to represent relatively low-(350, 450 kg m −3 ), medium-(550, 650 kg m −3 ), and high-(750, 850 kg m −3 ) density assumptions. In low-density cases, the ice water path (IWP) could be comparable with the liquid water path (LWP), while the LWP exceeds the contribution of the IWP to the total water path (TWP) in high-density cases. The results show that melting rates and precipitation were enhanced when g was increased from low to high values, resulting in a smaller size and lighter mean mass due to a shorter residence time and faster fall speed. Different assumptions about graupel density also resulted in different electrical structures in the simulated clouds.The clouds produced in the low-and medium-density cases are mainly charged with conventional tripole or positive dipole structures, while the ones formed in the high-density cases present "bottom-heavy" tripole structures. The upper positive regions become weaker, with reduced negative noninductive charging rates, as graupel falls faster and less graupel is negatively charged at higher altitude. It is also found that a faster fall speed of graupel does not necessarily mean stronger flash density, although lightning activities are correlated with higher fall speed. K E Y W O R D S electrification, graupel fall speed parameters, melting, riming Q J R Meteorol Soc. 2019;145:2404-2424.wileyonlinelibrary.com/journal/qj
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