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Title: The role of deep moist convective processes in western boundary currents-troposphere coupling
Author: Sheldon, Luke
ISNI:       0000 0004 7232 9972
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2015
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Previous studies have suggested that atmospheric deep convection is occurring above the oceans western boundary currents. To investigate the occurrence of deep (ocean surface to troposphere) convection in mid-latitudes, diagnostics of upright and slantwise convective instability are created and used in the ERA-interim re-analysis dataset. These diagnostics find that deep convection is restricted to the western boundary currents in winter. However, over the ocean currents deep convection can occur up to 50% of the winter. As these ocean currents, including the Gulf Stream and Kuroshio, are warm and carry large quantities of heat polewards they could influence the frequency of deep convection. A simple calculation demonstrates that decadal ocean variability can change the atmospheric stability by 80% in the core of the Kuroshio and Gulf Stream. The deep convective instability occurs in synoptic systems that pass over the ocean currents. Specifically, instability occurs on the atmospheric fronts in the extra-tropical cyclones. To investigate the convective instability and related circulation at atmospheric fronts a composite of all atmospheric fronts in the Gulf Stream region is produced. The front composites show that where slantwise instability occurred at a front an associated increase in frontal ascent throughout the troposphere and precipitation is also found. An investigation into the possible mechanisms that produced the increased ascent at slantwise unstable fronts concludes that a possible cause is a coarse resolution of slantwise circulations in the ERA-interim dataset. This increase in deep ascent associated with slantwise instability could explain, at least partially, the deep ascent found by previous studies over the Gulf Stream. This is consistent with the finding that the deep ascent in the Gulf Stream annual mean is a product of extreme events. The occurrence of deep slantwise instability and the associated circulations are also resolution dependent. A comparison between 25 km, 60 km and 135 km grid-spacing atmospheric model runs are made using the UPSCALE project dataset. The 25 km model runs indicate greater deep slantwise and upright convective instability. The 25-km set-up is also the only one where increased deep ascent at slantwise unstable fronts is observed.
Supervisor: Czaja, Arnaud Sponsor: Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral