CSI Notes for thesis Traditionally, ES were thought to exist only in statically unstable environments, (Moore et. al. 2003, Trier and Parsons 1993, Horgan et. al. 2007), or in potentially unstable environments, where layer lifting destabilises those layers to vertical motions (Moore et. al. 1998). Interestingly, Colman (1990a) found the majority of elevated storms occurred in weakly stable to neutral environments (850hPa Lifted Index mean of 0.6). Correcting the Lifted Index for conditional symmetric instability (CSI - saturated parcels are unstable to slantwise motions, but stable to vertical motions) Colman (1990a) yielded a mean 850-hPa lifted index of -0.41 (i.e., slightly unstable), however he may have obtained a different set of ES stability results had he calculated Lifted Indices below 850hPa (Williams 1991). In addition, observed events with either thermodynamic environments (a) evolving from statically unstable to conditionally-symetriclly unstableI (Colman 1990b), or (b) statically unstable environments co-existing at the same time over mesoscale distances with CSI environments, (Browning et. al. 2010, Schultz and Schumacher 1999, Nicosia and Grumm 1999. Melick et. al. 2008) show CSI as another of the many thermodynamic environments ES exist in.

To assess if CSI is present in pre-storm environments when static instability is not observed, much of the current literature and text books use two-dimensional cross-sections (vertical and cross-baroclinic) of various fields (Nicosia 1999, Browning 2010). In a comprehensive review of the use of CSI in single and multiple banded precipitation, Schultz and Schumacher (1999) suggest any two dimensional field used to assess the presence of CSI is too simplistic and results of CSI rely on various geostrophic flow assumptions, and the orientation of the cross section chosen. Schultz and Schumacher (1999) instead recommend using a three dimensional field of negative saturated geostrophic potential vorticity, which is shown not to have the same adherence to the two dimensional flow assumptions. Additionally, “CSI diagnostics always should be employed in conjunction with tests for moist gravitational and inertial instabilities” (Schultz and Schumacher 1999, p.2715). This Masters aims to investigate the presence of those instabilities mentioned above in the environments where ES have been identified to exist. In particular:

• Which types of instabilities are associated with Australian elevated thunderstorms?

A common diagnostic in forecasting ES in Australia has been the use of observed potentially unstable layers in atmospheric soundings. There are many precursors assumed with this diagnostic, such as (a) there will be broad-scale layer lifting of those potentially unstable layers to create gravitationally unstable layers over the forecast period, and (b) a trigger is forecast to lift air parcels in the newly created gravitationally unstable layers to saturation and to the level of free convection (LFC). Schultz and Schumacher (1999) similarly define the existence of Potential Symmetric Instability (PSI) as layers potentially unstable to lifting along constant geostrophic absolute momentum surfaces. Whilst it would be insightful to investigate if potentially/potentially-symmetrically unstable layers existed in the pre-ES environments, there will not be time to conduct this line of research in this Masters.