Gravity waves observed as a causal mechanism for transition from closed to open cellular convection in the remote South East Pacific

This case study discusses an important observed role for atmospheric gravity waves (AGWs) in the South East Pacific (SEP) region by virtue of their ability to evidently modulate cloud radiative and dynamical properties over a wide area. The case study presented here employs satellite imagery together with measurements of satellite-retrieved cloud bulk properties during the period 7 October to 11 October 2008 over the SEP, which together serves to illustrate the horizontal propagation of a series of AGW trains, manifest by their influence on a large-scale stratocumulus cloud deck capping the local marine boundary layer (see white ellipses in Figure 1). The AGWs were observed as a periodic modulation of retrieved cloud top height by up to 500 metres peak-to-trough, whilst the horizontal direction of wave propagation was perpendicular to the synoptic boundary layer flow. The AGWs of interest appear to originate near 30 S, 85 W and were initiated for a 24-hour period beginning at midday on 7 October 2008, propagating along a vector directed approximately northeastward toward the Peruvian Coast (15 S, 70 W) over the following 48 hours as illustrated in Figure 1. During this time, the gravity waves were seen to affect both reversible and non-reversible changes in cloud radiative properties and cloud dynamics such that persistent areas of clear sky developed in the troughs of passing gravity wave fronts. (see purple ellipses in Figure 1). The formative mechanisms for such cloud-free regions, or so-called Pockets of Open Cells (POCs), observed as cell-like (openly-convecting) cloud-free areas embedded in remote marine stratocumulus sheets, are currently the subject of intense speculation and scientific interest due to their ability to allow solar radiation to reach the ocean surface in regions where atmospheric models predict dense cloud cover. In this case study, we will investigate the effects of gravity waves on the SEP stratocumulus cloud deck and investigate the synoptic scale disturbances which initiate gravity waves in the area.