Oscillating Deep Open Cells in Cyprus Low Events

Abstract:

Rayleigh-Bénard convection patterns can be frequently recognized in the atmosphere by roll-like and hexagonal-cells cloud patterns. Commonly, heat from the warm sea surface is transferred by convection to the colder boundary layer top, and therefore force a marine stratocumulus (MSc) organization into cloud streets (roll-like, aligned with the background flow), closed cells (with isolated clouds in the center), or open cells (with clear center surrounded by clouds). Typically, open MSc cells are characterized by 10 to 50 km diameters, and depth of 1 or 2 km. However, in post-frontal regions of Cyprus lows (in the Eastern Mediterranean), open cell structures can form in much deeper convective clouds that produce strong precipitation. To study these deep open cells’ properties and evolution, both observations and modeling data are used in this work. Lagrangian analysis of satellite data from a few case studies reveals that deep open cells (top height of ~5.5 km) have a large equivalent diameter (~50 km) and they oscillate, with longer oscillating periodicity (3-4 h) compared to shallow MSc. While shallower cloud streets (top of ~2 km) located upstream, maintain a rigid structure for at least 8 hours, which can be seen as an analogue behavior of closed MSc cells. Numerical simulation of a case study of a Cyprus Low event (25-26 January 2018), further reveals that precipitation-generated divergence and convergence patterns are the main driver of the open cells’ organization and oscillations. Thus, our findings generalize the mechanism behind shallow marine cells’ behavior to deep convective systems.