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F. J. Robinson, M. D. Patterson, and S. C. Sherwood

1. Introduction Gravity or density currents are predominantly horizontal flows where gravity drives fluid motion because of density gradients within a fluid. Such currents are ubiquitous in the atmospheric boundary layer ( Smith and Reeder 1988 ). A common example is marine air advancing onto land as a sea breeze, which is initiated by differential solar heating of land and water surfaces (well-known examples include the Fremantle and Cape Doctors in Perth and Cape Town, respectively; Gentilli

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Robert Davies-Jones and Paul Markowski

tilted abruptly upward by a gust front, leading to strong vertical vorticity very close to the ground that can be stretched into a tornadic vortex. This process is called hereafter the gust-front mechanism. In this regard, Simpson (1982) proposed that a waterspout may form as a result of a steep density current gust front scooping up a bundle of horizontal vortex tubes from the sea surface and connecting these tubes to a mesocyclone that has extended downward to the base of an overlying convective

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Adam L. Houston

when the boundary-normal shear is in the same direction as the density gradient (negative shear). This result is consistent with the steady-state analytic density current solution of Xu (1992) , particularly in regards to the sensitivity of middepth slope of the front to the vertical shear (illustrated schematically in the inset panels of Fig. 1 ). However, the inclusion of a deep convective updraft in the H16 simulations leads to an additional explanation: strong (as well as deep) ascent is

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Philip Cunningham

1. Introduction Isolated buoyant plumes occur routinely in the atmosphere and can arise from a wide variety of sources such as wildland or structure fires and industrial stack emissions. Buoyant plumes are a canonical example of turbulent buoyant convection, and in light of their importance in the transport of heat, gases, and particulate matter, they have been studied extensively in widespread contexts (e.g., Morton et al. 1956 ; Turner 1973 ; Linden 2000 ). Density currents are also

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Robert B. Seigel and Susan C. van den Heever

1. Introduction Cold pools have long been qualitatively compared to density currents; however, Charba (1974) was the first to quantitatively relate an observed thunderstorm outflow to a laboratory-produced density current. Since then there have been numerous other studies ( Droegemeier and Wilhelmson 1985 , 1987 ; Xu 1992 ; Liu and Moncrieff 1996a , b , 2000 , hereafter LM2000 ; Simpson 1997 ; Xue 2002 ; Engerer et al. 2008 ), both observational and modeling, which have focused on the

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Mathew Wells, Claudia Cenedese, and C. P. Caulfield

1. Introduction The dynamics of density currents are of fundamental importance for understanding the transport and mixing properties of dense waters, such as the Antarctic Bottom Water or North Atlantic Deep Water. These water masses form at high latitudes and eventually fill up most of the deep regions of the world’s ocean basins. The final properties of these water masses are largely determined by the amount of interfacial mixing or “entrainment” that occurs between these density currents and

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Aviv Solodoch, Jeroen M. Molemaker, Kaushik Srinivasan, Maristella Berta, Louis Marie, and Arjun Jagannathan

1. Introduction Salinity and temperature fronts are a common occurrence in coastal regions where rivers and other waterways outflow. The corresponding high horizontal density gradients result in a baroclinic horizontal pressure gradient force, which can strongly influence the dynamics and circulation in the shelf and beyond. Several circulation patterns of the buoyant outflow typically emerge. The first is a geostrophic alongshore density current that is typically prograde, that is, in the

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W. J. Teague, H. W. Wijesekera, W. E. Avera, and Z. R. Hallock

amplitudes can be quite large, and the waves can significantly impact acoustic, optical, and biogeochemical properties of the water column. A variety of techniques have been used to observe NLIWs such as using moored temperature and conductivity sensors, current meters ( Colosi et al. 2001 ; Duda et al. 2004 ; Hallock and Field 2005 ; MacKinnon and Gregg 2003 ), and high-frequency pressure sensors ( Moum and Nash 2008 ). Shipboard measurements utilizing acoustic Doppler current profilers (ADCPs

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Jean-Yves Grandpeix and Jean-Philippe Lafore

subjects are akin to some of the challenges met by the representation of deep convection in general circulation models (GCMs), namely the representation of convection organization and propagation and of the diurnal cycle of convection over land. Fast-moving long-lasting squall lines (SLs) provide a convenient archetype of organized mesoscale convective systems (MCSs), facilitating the elaboration of a conceptual model with three components: a convective part, a stratiform part, and a density current

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Mathew Wells and Parthiban Nadarajah

1. Introduction In many parts of the World Ocean, the renewal of deep or intermediate waters is due to density currents that flow down the continental margin. For example, the combination of brine rejection and cooling in the Weddell and Ross Seas produces very cold and dense water that flows as a density current down the Antarctic continental slope to form the deep Antarctic Bottom Water. In addition to regions of the World Ocean where the major water masses are formed, there are many other

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