School of Earth Sciences - Research Publications

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Author: Lane, Todd × End date: 2013 × Start date: 2012 ×

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    Toward an understanding of vertical momentum transports in cloud-system-resolving model simulations of multiscale tropical convection
    Shaw, TA ; Lane, TP (American Meteorological Society, 2013-10-24)
    This study examines the characteristics of convective momentum transport (CMT) and gravity wave momentum transport (GWMT) in two-dimensional cloud-system-resolving model simulations, including the relationships between the two transports. A linear group velocity criterion is shown to objectively separate CMT and GWMT. TheGWMTcontribution is mostly consistent with upward-propagating gravity waves and is present in the troposphere and the stratosphere. The CMT contribution forms a large part of the residual (nonupward-propagating contribution) and dominates the fluxes in the troposphere. Additional analysis of the vertical sensible heat flux supports the physical interpretation of the two contributions, further isolating the effects of unstable convection from vertically propagating gravity waves. The role of transient and nonconservative (friction and diabatic heating) processes in generating momentum flux and their dependence on changes in convective organization was assessed using a pseudomomentum budget analysis. Nonconservative effects were found to dominate the transports; the GWMT contribution involved a diabatic source region in the troposphere and a dissipative sink region in the stratosphere. The CMT contribution was consistent with transport between the boundary layer and free troposphere via tilted convection. Transient buoyancy-vorticity correlations highlighted wave sources in the region of convective outflow and the boundary layer. These sources were akin to the previously described ''mechanical oscillator'' mechanism. Fluxes associated with this upper-level source were most sensitive to convective organization, highlighting the mechanism by which changes in organization are communicated to GWMT. The results elucidate important interactions between CMT and GWMT, adding further weight to suggestions that the two transports should be linked in parameterizations.
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    The meteorology of Black Saturday
    Engel, CB ; Lane, TP ; Reeder, MJ ; Rezny, M (Royal Meteorological Society, 2012)
    The meteorological conditions are investigated over the state of Victoria, Australia on 7 February 2009, the day of the 'Black Saturday' fires. Daytime temperatures exceeding 45°C, strong surface winds and extremely dry conditions combined to produce the worst fire weather conditions on record. A high-resolution nested simulation with the UK Met Office Unified Model and available observations are used to identify the important mesoscale features of the day. The highest resolution domain has horizontal grid spacing of 444 m and reproduces most aspects of the observed meteorological conditions. These include organized horizontal convective rolls, a strong late-afternoon cool change with many of the characteristics of an unsteady gravity current, a weaker late-evening cold front and propagating nocturnal bores. These mesoscale phenomena introduce variability in the winds, temperature and humidity at short temporal and spatial scales, which in turn lead to large spatial and temporal variability in fire danger.
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    Statistical assessment of tropical convection-permitting model simulations using a cell-tracking algorithm
    Caine, S ; Lane, TP ; May, PT ; Jakob, C ; Siems, ST ; Manton, MJ ; Pinto, J (American Meteorological Society, 2013-02)
    This study presents a method for comparing convection-permitting model simulations to radar observations using an innovative object-based approach. The method uses the automated cell-tracking algorithm, Thunderstorm Identification Tracking Analysis and Nowcasting (TITAN), to identify individual convective cells and determine their properties. Cell properties are identified in the same way for model and radar data, facilitating comparison of their statistical distributions. The method is applied to simulations of tropical convection during the Tropical Warm Pool-International Cloud Experiment (TWP-ICE) using the Weather Research and Forecasting Model, and compared to data from a ground-based radar. Simulations with different microphysics and model resolution are also conducted. Among other things, the comparisons between the model and the radar elucidate model errors in the depth and size of convective cells. On average, simulated convective cells reached higher altitudes than the observations. Also, when using a low reflectivity (25 dBZ) threshold to define convective cells, the model underestimates the size of the largest cells in the observed population. Some of these differences are alleviated with a change of microphysics scheme and higher model resolution, demonstrating the utility of this method for assessing model changes.
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    Recent Advances in the Understanding of Near-Cloud Turbulence
    Lane, TP ; Sharman, RD ; Trier, SB ; Fovell, RG ; Williams, JK (American Meteorological Society, 2012)
    Anyone who has flown in a commercial aircraft is familiar with turbulence. Unexpected encounters with turbulence pose a safety risk to airline passengers and crew, can occasionally damage aircraft, and indirectly increase the cost of air travel. Deep convective clouds are one of the most important sources of turbulence. Cloud-induced turbulence can occur both within clouds and in the surrounding clear air. Turbulence associated with but outside of clouds is of particular concern because it is more difficult to discern using standard hazard identification technologies (e.g., satellite and radar) and thus is often the source of unexpected turbulence encounters. Although operational guidelines for avoiding near-cloud turbulence exist, they are in many ways inadequate because they were developed before the governing dynamical processes were understood. Recently, there have been significant advances in the understanding of the dynamics of near-cloud turbulence. Using examples, this article demonstrates how these advances have stemmed from improved turbulence observing and reporting systems, the establishment of archives of turbulence encounters, detailed case studies, and high-resolution numerical simulations. Some of the important phenomena that have recently been identified as contributing to near-cloud turbulence include atmospheric wave breaking, unstable upper-level thunderstorm outflows, shearing instabilities, and cirrus cloud bands. The consequences of these phenomena for developing new en route turbulence avoidance guidelines and forecasting methods are discussed, along with outstanding research questions.
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    On the identification of the large-scale properties of tropical convection using cloud regimes
    Tan, J ; Jakob, C ; Lane, TP (American Meteorological Society, 2013-09)
    The use of cloud regimes in identifying tropical convection and the associated large-scale atmospheric properties is investigated. The regimes are derived by applying cluster analysis to satellite retrievals of daytime-averaged frequency distributions of cloud-top pressure and optical thickness within grids of 280km by 280km resolution from the International Satellite Cloud Climatology Project between 1983 and 2008. An investigation of atmospheric state variables as a function of cloud regime reveals that the regimes are useful indicators of the archetypal states of the tropical atmosphere ranging from a strongly convecting regime with large stratiform cloudiness to strongly suppressed conditions showing a large coverage with stratocumulus clouds. The convectively active regimes are shown to be moist and unstable with large-scale ascending motion, while convectively suppressed regimes are dry and stable with large-scale descending winds. Importantly, the cloud regimes also represent several transitional states. In particular, the cloud regime approach allows for the identification of the "building blocks" of tropical convection, namely, the regimes dominated by stratiform, deep, and congestus convection. The availability of the daily distribution of these building blocks for more than 20 years opens new avenues for the diagnosis of convective behavior as well as the evaluation of the representation of convection in global and regional models.
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    Mesoscale modelling of two 'drying events': Governing processes and implications for fire danger
    Badlan, RL ; Lane, TP ; Mills, GA ; Caine, S (Australian Bureau of Meteorology, 2012-12-01)
    This study uses mesoscale modelling to examine the processes underlying two 'drying events' and their implications for fire danger. Both events occurred in Gippsland, Victoria, Australia, in the lee of the Great Dividing Range, and the role of terrain-induced flows in causing the events is one underlying focus. The Weather Research and Forecasting (WRF) model is used in a nested configuration; the highest resolution domain has horizontal grid spacing equal to 1.5 km. The WRF model simulations identify that the first event (29 December 2001) is caused by a regime transition from blocked to unblocked flow, with the cross-mountain flow bringing warmer and drier conditions. The second event (29 May 2007) is related to enhanced downslope flow. Simulations of both events also elucidate important mesoscale processes, namely 'streamers' and lee waves, that cause significant perturbations in the fire danger in the lee of the mountains.