School of Earth Sciences - Theses
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ItemOn the dynamics of orographic precipitation in idealised and realistic settings( 2013)This thesis explores the dynamics and sensitivities of orographic precipitation to highlight the complexity of precipitation generation in the Australian Alps. The thesis is arranged in two parts. First, the influence of variations in relatively simple terrain geometries and upstream conditions using idealised model simulations are examined. Second, observations and an ensemble of high-resolution model simulations are used to examine a heavy precipitation event in the Australian Alps. The first part simulates a moist flow impinging upon three alpine-scale terrain shapes: a straight ridge, a concave ridge, and a convex ridge. The single sounding environment is representative of a prefrontal flow that produced heavy precipitation in the Australian Alps. A variety of simulations were conducted to investigate the sensitivity of precipitation patterns to ridge length and upstream thermodynamic and wind conditions for an impinging flow with a nondimensional mountain height (H) of approximately unity. The concave ridge generated substantially more precipitation than the other two ridge geometries via an established precipitation-enhancing funnelling mechanism near the ridge vertex; however, for some concave ridge configurations the results featured dual-precipitation maxima, which is an important difference from previous findings. Results from a simple ensemble of idealised simulations elucidated the sensitivity of precipitation patterns to small variations in upstream conditions and how these vary for the different terrain geometries. The findings were expanded upon by examining how variations to H and the horizontal aspect ratio (β) of a straight ridge and a concave ridge influence orographic precipitation. It was hypothesised that when the approaching flow is blocked, the strength of the precipitation enhancement by the concave ridge relative to the straight ridge would be negligible; however this study revealed that when H is sufficiently large to induce flow-reversal on the windward slope, a secondary circulation develops that is strengthened by the concave ridge and precipitation is subsequently enhanced. In addition, a flow regime diagram for the straight ridge and the concave ridge was constructed to illustrate the sensitivity of the critical-H for flow regime transition to changes in the terrain geometry. The second part of the thesis examined a heavy precipitation event in the Australian Alps associated with the passage of a cold front and a prefrontal trough. Precipitation accumulated predominantly on the northwestern slopes of the Alps, and observations and results from high- resolution model simulations were used to reconstruct the event. Precipitation was produced by a variety of phenomena, including the front itself, an undular bore propagating along the stable nocturnal layer ahead of the prefrontal trough, and a mesoscale convective system. The sensitivity of a modest prefrontal precipitation event to initial condition uncertainties was then explored using an ensemble of high-resolution numerical simulations. In total, only seven of the 34 simulations produced precipitation distributions comparable to what was observed. The maximum eastward extent of the prefrontal trough and the presence of sufficient moisture upstream were found to be controlling factors. A bore was generated in every simulation suggesting the occurrence of an undular bore was highly predictable, although its position varied considerably among the members.
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ItemMesoscale circulation and variability of the Indian Ocean( 2011)The introduction of satellite altimetry in the 1990's and the increasing sophistication of ocean modelling have seen mesoscale oceanography become an increasingly prominent area of research over the last few decades. Evidence is continuing to emerge that the mesoscale variability does not average to zero over extended periods (i.e. the available observational record which is now multi-decadal) and plays an important role in the general circulation of the ocean, with implications for models from short-range through to climate-scales. This study examines mesoscale variability of the Indian Ocean based on a range of observational and model products including a new type of product, which is based on the merging of observations and high-resolution models in the form of a multi-year ocean reanalysis (BlueLink ReANalysis – BRAN2.1). Due to the computational limitations and the grid resolution of this product, we give special emphasis to the southeast Indian Ocean where the model is eddy-resolving. This study has identified quasi-zonal alignment of the mesoscale variability for surface salinity and temperature in the eddy-resolving model domain. Specifically quasi-zonal alignments are found in the orientation of spatial correlation ellipses. The statistical properties have a 2-3° meridional width that are consistently zonal in the tropics, but in the subtropics tend to incline towards the equator. Similar quasi-zonal alignments are also found in spatial anomalies of mean surface prognostic variables for salinity, temperature, as well as surface density. The statistical properties of the reanalysis for surface temperature are verified using a merged product from multiple satellites of high-resolution surface temperature observations from AVHRR. The seasonal variations of the time-mean surface quasi-zonal alignment in physical properties and its link to the coastal current systems of the southeast Indian Ocean are detailed in this study. Consistent quasi-zonal or “arterial” ocean currents of the southeast Indian Ocean are revealed using a colorwheel visualisation tool. In particular, we have identified five eastward jets in the broad surface geostrophic flow regime of the southeast Indian Ocean. Mean meridional sections of ocean currents show that the zonally coherent mean features seen at the surface extend into the abyssal ocean though decline in magnitude with depth; and are similarly quasi-zonal. The seasonality of the surface layer eastward transports in the southeast Indian Ocean is found to be positively correlated with the phase of the Leeuwin Current. Mean westward flows are seen at mid-depth with greater arterial structure. Mesoscale eddies are found to follow regularly along the seasonally persistent mean westward feature connecting the Naturaliste and Broken Plateau. Time-varying properties of quasi-zonal structures in the southeast Indian Ocean show equatorward meridional drift between 25°S-15°S and poleward meridional movements are noted between the equator and 14°S. Meridional displacements of quasi-zonal features in the southeast Indian Ocean are found to correspond with the sign and strength of the mean meridional velocity. Quasi-zonal signatures are found to be also present in the 10-year mean spatial anomalies of the overlying wind-stress field observed from the QuikSCAT satellite observations. Statistical relationships of quasi-zonal features in wind-stress and observed sea surface temperature are derived.