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

Grant Allen, Geraint Vaughan, Paul Connolly, Peter Cook, Patrick Minnis, Thomas Toniazzo, Hugh Coe

    Research output: Contribution to conferencePoster

    Abstract

    The formative mechanisms of 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 speculationand scientific interest. These structures can act to modulate the thermodynamic and radiative properties of largeareas of the Marine Boundary Layer (MBL), as well as modifying the composition of the remote MBL throughthe washout of already scarce particulate matter (CCN). Moreover, the important potential climate impact of thesestructures, through their radiative properties, is not represented in climate or regional-scale models.The necessary conditions, or “tipping point” for the transition between the metastable closed and open cell dynamicstates is also the subject of much speculation and has been observed by aircraft to be linked to the scavenging ofavailable CCN by drizzle, with consequent feedback on the underlying convective dynamic of the cloud.This work discusses observations of satellite-retrieved cloud bulk properties during October 2008, which clearlyillustrate the propagation of several gravity waves in the MBL, emanating from a trough at 30 degrees South, offthe coast of Chile. The waves are manifest by their modulation of cloud top height by up to 500 metres peak-totrough,with propagation perpendicular to the mean flow. Analysis of satellite imagery indicates the waves have aperiod of approximately one hour and a wavelength of around 55 km.The “opening up” or formation of POCs in the wake is observed as wave trains traverse the Pacific Ocean. ThePOCs formed appear stable and subsequently advect with the mean flow. We demonstrate here, with the aid ofa parcel model, that simulated gravity waves are able to effectively induce drizzle through their effect on MBLand cloud dynamics, thus scavenging available CCN and initiating the transition to open cell convection. We donot suggest that gravity waves are a ubiquitous mechanism for all POCs observed in the South East Pacific (orelsewhere), but that gravity waves are one way to induce drizzle formation; with drizzle formation being the likelynecessary action for POC formation through CCN scavenging. Furthermore, the period, wavelength and amplitudeof the gravity waves are shown here to be critical to whether cloud droplets are precipitated.
    Original languageEnglish
    Publication statusPublished - Apr 2010
    EventEuropean Geophysical Union Conference 2010 - Vienna, Austria
    Duration: 19 Apr 201023 Apr 2010

    Conference

    ConferenceEuropean Geophysical Union Conference 2010
    CityVienna, Austria
    Period19/04/1023/04/10

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