School of Earth Sciences - Theses
Permanent URI for this collection
Search Results
1 - 1 of 1
-
ItemMomentum transport by organised deep convection( 2016)Deep convection is an important process that influences the vertical redistribution of heat, moisture, and momentum. Convective momentum transport (CMT) is composed of multiscale dynamical processes, including convective-scale CMT associated with updrafts and downdrafts, and mesoscale CMT associated with the quasi-steady circulation from organised systems. This multiscale nature of CMT is an active area of research especially regarding the influence of momentum transport on the mesoscale organisation of convective systems and vice-versa; these mesoscale processes and their associated CMT are the focus of this study. Firstly, the CMT budgets of simulated idealised mesoscale convective systems are examined, and their sensitivity to horizontal resolution, domain size, and the boundary conditions, investigated. This is followed by an examination of a case study during the Tropical Warm Pool - International Cloud Experiment (TWP-ICE). The effect of horizontal resolution, time evolution, and cloud regime on the CMT were evaluated to determine the momentum profiles on both scales. These results are then compared to observational data derived from Doppler radar. The idealised simulations reveal that for relatively large domains, horizontal gradient terms are still important, including the mesoscale pressure gradients; such terms are neglected in all CMT parameterisations. Like most convection parameterisations, current CMT parameterisations only represent convective-scale processes through relatively simple plume models. Thus, they do not properly represent the transports associated with organised systems, even though the tilted circulation associated with organised convection is a fundamental aspect of CMT. The horizontal pressure gradient and the sign of the momentum flux change sign as the system develops. Small domain calculations, which have become common for radiative-convective equilibrium experiments, are shown to suppress organisation through artificially large compensating subsidence and hence provide unrealistic representations of the processes involved. Finally, examination of the cross-updraft/downdraft pressure gradients demonstrates significant errors in their representation in current schemes. The convective and mesoscale momentum transport profiles from the real cases reveal how these scales change at various stages of their evolution. Grid spacing $>$ 4 km does not capture the convective scale CMT and the coarsest resolution ($\sim$ 30 km) produces the wrong sign of CMT. Mesoscale organisation is highly influential when evaluating the CMT, as these tilted circulations are responsible for much of the momentum transport. These findings indicate that rather than just parameterising the convective or small-scale dynamics, these schemes must take into account the larger, mesoscale processes. The observational data reveals differences in the dynamical structures to those identified using the model data. Analysing momentum transport profiles identifies a downshear-tilted system, suggesting that the model is not representing the full diversity of systems that actually occur. Another simulation using a different microphysics scheme and analysis of the model's thermodynamics shows that both microphysics schemes create overly strong cold pools which preferentially generate upshear-tilted systems. The usefulness of this radar dataset has also allowed insights into the variety of systems and may be used to reproduce these systems in models.