Lance Leslie - Academia.edu (original) (raw)

Papers by Lance Leslie

Australian Meteorological and Oceanographic Journal, Sep 1, 2010

The passing of over three decades since tropical cyclone Tracy devastated Darwin on Christmas Day... more The passing of over three decades since tropical cyclone Tracy devastated Darwin on Christmas Day 1974 provided a reason to reflect upon the progress of numerical weather prediction over that period. Each decade was characterised by major model changes. In Australia, the 1970s saw the operational implementation of a 7-level baroclinic filtered model at a grid spacing of 254 km, with rudimentary representation of physical processes. The 1980s witnessed the operational introduction of a 12-level primitive equations model with a grid spacing of 150 km and enhanced representation of both the data assimilation approach and the model physics. Finally, in the 1990s the modeling capacity had reached the point where a 40-level model with a grid spacing of 5 km, such as the one employed here, could be run in near real-time, nested over the area of interest. Some research versions also had became available at even higher resolutions. Further improvements in the representation of physical processes and advanced data assimilation procedures, such as 4D-Var, were also available. Here, three models similar to those cited above were run out to 48 hours for the tropical cyclone Tracy case. In all models a bogussed vortex was required to provide an initial tropical cyclone circulation. However, the baroclinic model quickly lost tropical cyclone Tracy's circulation and it decayed so rapidly such that neither a circulation nor a track was discernable at 48 hours. The 1980s model was more sophisticated but still provided little gain over the 1970s model because its grid spacing remained well below that required for effective tropical cyclone modelling, especially for a storm as small as tropical cyclone Tracy. The 1990s model, which is much closer to those used operationally at present, in terms of data assimilation, resolution and physics, performed far better and provided a track for tropical cyclone Tracy that if available in 1974 could have alerted forecasters to a possible impact on Darwin. The results confirmed that major advances have taken place in tropical cyclone track forecasting, but to a lesser extent in predicting tropical cyclone intensity (especially for very small tropical cyclones like Tracy) over the past 30 years or so. They also confirmed that at present, a gap remains between research and operations. Since the development of the 1990s model, further advances in data coverage, assimilation, model formulation and resolution have produced a new generation of 2000s models. These models have advanced cloud microphysics and data assimilation schemes and can be run in research mode at resolutions well below the 1 km required to provide more realistic predictions of small storms like tropical cyclone Tracy. The computational capacity soon will exist to run these sophisticated, very high resolution models in real-time.

Weather and Forecasting, Feb 1, 2006

Over the last 50 yr, the number of tornadoes reported in the United States has doubled from about... more Over the last 50 yr, the number of tornadoes reported in the United States has doubled from about 600 per year in the 1950s to around 1200 in the 2000s. This doubling is likely not related to meteorological causes alone. To account for this increase a simple least squares linear regression was fitted to the annual number of tornado reports. A "big tornado day" is a single day when numerous tornadoes and/or many tornadoes exceeding a specified intensity threshold were reported anywhere in the country. By defining a big tornado day without considering the spatial distribution of the tornadoes, a big tornado day differs from previous definitions of outbreaks. To address the increase in the number of reports, the number of reports is compared to the expected number of reports in a year based on linear regression. In addition, the F1 and greater Fujita-scale record was used in determining a big tornado day because the F1 and greater series was more stationary over time as opposed to the F2 and greater series. Thresholds were applied to the data to determine the number and intensities of the tornadoes needed to be considered a big tornado day. Possible threshold values included fractions of the annual expected value associated with the linear regression and fixed numbers for the intensity criterion. Threshold values of 1.5% of the expected annual total number of tornadoes and/or at least 8 F1 and greater tornadoes identified about 18.1 big tornado days per year. Higher thresholds such as 2.5% and/or at least 15 F1 and greater tornadoes showed similar characteristics, yet identified approximately 6.2 big tornado days per year. Finally, probability distribution curves generated using kernel density estimation revealed that big tornado days were more likely to occur slightly earlier in the year and have a narrower distribution than any given tornado day.

Bulletin of the American Meteorological Society, Jul 1, 2021

InTech eBooks, May 22, 2013

Advances in Meteorology, Dec 7, 2019

e Northwest Pacific tropical cyclone (TC) intensification is classified into rapid intensificatio... more e Northwest Pacific tropical cyclone (TC) intensification is classified into rapid intensification (RI), normal intensification (NI), and slow intensification (SI) categories. e initial location and intensity, the preceding intensity change, the motion direction, the occurrence month, and the intensification duration time are all found to differ for RI cases compared with NI and SI cases. e dependence of RI, NI, and SI on environmental conditions is further examined statistically by using the intensification rates of named TCs, for the 21-year period 1995-2015, obtained from JTWC best track data, and the environmental conditions derived from the ERA-Interim reanalysis data and GODAS high-resolution global ocean analysis data. It was found that deep-layer and upper-mid vertical wind shear (VWS), upper-level outflow, sea surface temperature (SST), and ocean heat content (OHC) are statistically different among RI, NI, and SI both before and during intensification. RI is enhanced by weaker and decreasing VWS, warmer oceans, and stronger and increasing outflow. In contrast, SI typically occurs with larger and increasing VWS, cooler oceans, and weaker, static outflow. e impacts of low-level VWS and net moisture inflow are only significantly different between RI and SI and between NI and SI, but not between RI and NI. Another key finding is that increased upper-level outflow and decreased VWS are important precursors and hence are possible predictors, of RI onset. e direction of upper-level outflow affects TC intensification, with NW and NE outflow being more favorable for TC RI than SE and SW outflow.

Journal of the Atmospheric Sciences, 2013

A high-resolution nonhydrostatic numerical model, the Advanced Regional Prediction System (ARPS),... more A high-resolution nonhydrostatic numerical model, the Advanced Regional Prediction System (ARPS), was used to simulate Typhoon Morakot (2009) as it made landfall over Taiwan, producing record rainfall totals. In particular, the mesoscale structure of the typhoon was investigated, emphasizing its associated deep convection, the development of inner rainbands near the center, and the resultant intense rainfall over western Taiwan. Simulations at 15-and 3-km grid spacing revealed that, following the decay of the initial inner eyewall, a new, much larger eyewall developed as the typhoon made landfall over Taiwan. Relatively large-amplitude wave structures developed in the outer eyewall and are identified as vortex Rossby waves (VRWs), based on the wave characteristics and their similarity to VRWs identified in previous studies. Moderate to strong vertical shear over the typhoon system produced a persistent wavenumber-1 (WN1) asymmetric structure during the landfall period, with upward motion and deep convection in the downshear and downshear-left sides, consistent with earlier studies. This strong asymmetry masks the effects of WN1 VRWs. WN2 and WN3 VRWs apparently are associated with the development of deep convective bands in Morakot's southwestern quadrant. This occurs as the waves move cyclonically into the downshear side of the cyclone. Although the typhoon track and topographic enhancement contribute most to the recordbreaking rainfall totals, the location of the convective bands, and their interaction with the mountainous terrain of Taiwan, also affect the rainfall distribution. Quantitatively, the 3-km ARPS rainfall forecasts are superior to those obtained from coarser-resolution models.

Scientific Reports, Jun 29, 2020

The sequence of wildfires followed by debris flows, frequently affects southern California, refle... more The sequence of wildfires followed by debris flows, frequently affects southern California, reflecting its drought-heavy precipitation climate bipolarity. Organic debris from incomplete burning is lighter than inorganic matter, and partially inviscid. Hence lower rainfall totals can trigger downslope motion than typically required by the underlying clasts of loose inorganic granular material. After advection downslope, the pebble-laden organic debris has a higher capacity for rilling; a positive feedback process. A mechanism is proposed whereby boulders are 'rafted' by organic debris. This coordinated movement of boulders greatly enhances the debris flow erosion capacity. This climate change sensitive debris flow enhancing mechanism, through organic-inorganic granular material interaction, is supported by observations and the numerical simulations. Using a model explicitly parameterizing erosion processes, including runoff entrainment, rilling incision, and bank collapse, the lifecycle of the Montecito debris flow of January 9, 2018 is simulated, providing quantitative estimates of mass conveyed and debris sorting at the terminus. Peak rafting speeds are ~ 12.9 m/s at ~ 300 m asl. Total boulder (effective diameter > 25 cm) volume involved for the Ysidro Creek area alone is ~ 5 × 10 4 m 3 , scattered between the region 50-260 m asl. Debris flows are highly repeatable and locations prone to debris flows are identified and their likelihood of realization estimated. During 2012-2017, southern California experienced an extended drought 1-6 . The winter of 2017 (the wet season for its winter-wet Mediterranean climate) experienced unseasonal wildfires, amplified by strong Santa Ana winds and extended into densely populated Santa Barbara and Ventura counties. The Thomas Fire became the largest wildfire in California's history, as of December 18, 2017. As the wildfires subsided, storm-triggered landslides, more generally referred to as debris flows 7 , occurred on a southward facing slope of Montecito (Fig. ). Hereafter, the single term debris flow will be used to refer to the Montecito event. The Montecito debris flows were catastrophic, claiming 23 lives, and costing over US$200 million in property damage and clean up 8 . Montecito is at the intersection of numerous factors responsible for debris flows, including climate warming, drought, wildfires, seasonal storms, biological processes, and ongoing tectonic activity . Consequently, debris flows such as that of January 9, 2018 are not unexpected. Southern California is susceptible to debris flows , partly because its winter wet season (November-April) encourages plant growth. If the following dry season (May-October) extends into the wet season and strong winds occur, the soil and vegetation both dry out. Sloping terrain then becomes sufficiently unstable for debris flows to be triggered even by less intense storms in the ensuing wet season . If there are strong wind events, as has been the case for the past three years, 2017, 2018 and currently in 2019, wildfires further reduce slope stability through multiple mechanisms . Wildfires generate debris that is fuel for ensuing debris flows, as the upper 3-cm of root systems can be totally burnt, while the deeper roots remaining intact. The root web, after entrainment into the debris stream, acts as a nexus organizing the granular particles. Although the Montecitodebris flows were not unexpected 21 , their magnitudes are not yet fully understood. Towards that end, a mechanism contributing to the observed enhanced sediment detachment capability of the debris torrent, is assessed by using the Scalable, Extensible Geofluid Model for ENvironmental Tasks (SEGMENT-Landslide), a 3-D dynamical modelling system . With a positive feedback loop, the mixture of the sliding ('sliding' here is used as a general term for flowing, rolling and other forms of dislocation at granular scales) material, upon reaching

Climate, May 29, 2015

The Abdus Salam International Center for Theoretical Physics (ICTP) version 4.4 Regional Climate ... more The Abdus Salam International Center for Theoretical Physics (ICTP) version 4.4 Regional Climate Model (RegCM4) is used to investigate the rainfall response to cooler/warmer sea surface temperature anomaly (SSTA) forcing in the Indian and Atlantic Oceans. The effect of SSTA forcing in a specific ocean basin is identified by ensemble, averaging 10 individual simulations in which a constant or linearly zonally varying SSTA is prescribed in individual basins while specifying the 1971-2000 monthly varying climatological sea surface temperature (SST) across the remaining model domain. The nonlinear rainfall response to SSTA amplitude also is investigated by separately specifying +1K, +2K, and +4K SSTA forcing in the Atlantic and Indian Oceans. The simulation results show that warm SSTs over the entire Indian Ocean produce drier conditions across the larger Blue Nile catchment, whereas warming ≥ +2K generates large positive rainfall anomalies exceeding 10 mm• day -1 over drought prone regions of Northeastern Ethiopia. However, the June-September rainy season tends to be wetter (drier) when the SST warming (cooling) is limited to either the Northern or Southern Indian Ocean. Wet rainy seasons generally are characterized by deepening of the monsoon trough, east of 40°E, intensification of the Mascarene high, strengthening of the Somali low level jet and the tropical easterly jet, enhanced zonal and meridional vertically integrated moisture fluxes, and steeply vertically decreasing moist static energy. The opposite conditions hold for dry monsoon seasons.

Atmosphere, Sep 3, 2022

Assessing the impacts of both climate and land use changes on hydrologic variables is crucial for... more Assessing the impacts of both climate and land use changes on hydrologic variables is crucial for sustainable development of water resources and natural ecosystems. We conducted a case study of a catchment in southwestern Australia to assess the impacts of future climate and land use changes, both separately and in combination, on water resource availability. For this evaluation, the Soil and Water Assessment Tool (SWAT) model was first calibrated and then forced by 34 global climate models (GCMs), under two Representative Concentration Pathways (RCP4.5 and RCP8.5) and five land use scenarios (LU0-4). Our results suggested that SWAT reproduced the observed monthly streamflow well. Land use changes have impacts on all hydrologic variables, especially on runoff at the annual scale. Future runoff was projected to decrease in all seasons, especially winter and spring. For the combined effects of climate and land use changes, the results of LU1-4 were only slightly different from the response of LU0. An uncertainty analysis shows that GCMs had the greatest contribution to hydrologic variables, followed by RCPs and land use scenarios. Hence, it is advisable for impacts analysis to use an ensemble of GCMs under different RCPs to minimize the uncertainty of projected future hydrologic variables.

The Journal of the Australian Mathematical Society, May 1, 1972

Climate, Jun 11, 2020

Southeast Australia is frequently impacted by drought, requiring monitoring of how the various fa... more Southeast Australia is frequently impacted by drought, requiring monitoring of how the various factors influencing drought change over time. Precipitation and temperature trends were analysed for Canberra, Australia, revealing decreasing autumn precipitation. However, annual precipitation remains stable as summer precipitation increased and the other seasons show no trend. Further, mean temperature increases in all seasons. These results suggest that Canberra is increasingly vulnerable to drought. Wavelet analysis suggests that the El-Niño Southern Oscillation (ENSO) influences precipitation and temperature in Canberra, although its impact on precipitation has decreased since the 2000s. Linear regression (LR) and support vector regression (SVR) were applied to attribute climate drivers of annual precipitation and mean maximum temperature (TMax). Important attributes of precipitation include ENSO, the southern annular mode (SAM), Indian Ocean Dipole (DMI) and Tasman Sea SST anomalies. Drivers of TMax included DMI and global warming attributes. The SVR models achieved high correlations of 0.737 and 0.531 on prediction of precipitation and TMax, respectively, outperforming the LR models which obtained correlations of 0.516 and 0.415 for prediction of precipitation and TMax on the testing data. This highlights the importance of continued research utilising machine learning methods for prediction of atmospheric variables and weather pattens on multiple time scales.

Monthly Weather Review, Dec 1, 1987

Tellus A, Dec 1, 2013

This study examines eight microphysics schemes (Lin, WSM5, Eta, WSM6, Goddard, Thompson, WDM5, WD... more This study examines eight microphysics schemes (Lin, WSM5, Eta, WSM6, Goddard, Thompson, WDM5, WDM6) in the Advanced Research Weather Research and Forecasting Model (WRF-ARW) for their reproduction of observed strong convection over the US Southern Great Plains (SGP) for three heavy precipitation events of 27Á31 May 2001. It also assesses how observational analysis nudging (OBNUD), threedimensional (3DVAR) and four-dimensional variational (4DVAR) data assimilation (DA) affect simulated cloud properties relative to simulations with no DA (CNTRL). Primary evaluation data were cloud radar reflectivity measurements by the millimetre cloud radar (MMCR) at the Central Facility (CF) of the SGP site of the ARM Climate Research Facility (ACRF). All WRF-ARW microphysics simulations reproduce the intensity and vertical structure of the first two major MMCR-observed storms, although the first simulated storm initiates a few hours earlier than observed. Of three organised convective events, the model best identifies the timing and vertical structure of the second storm more than 50 hours into the simulation. For this wellsimulated cloud structure, simulated reflectivities are close to the observed counterparts in the mid-and upper troposphere, and only overestimate observed cloud radar reflectivity in the lower troposphere by less than 10 dBZ. Based on relative measures of skill, no single microphysics scheme excels in all aspects, although the WDM schemes show much-improved frequency bias scores (FBSs) in the lower troposphere for a range of reflectivity thresholds. The WDM6 scheme has improved FBSs and high simulated-observed reflectivity correlations in the lower troposphere, likely due to its large production of liquid water immediately below the melting level. Of all the DA experiments, 3DVAR has the lowest mean errors (MEs) and root mean-squared errors (RMSEs), although both the 3DVAR and 4DVAR simulations reduced noticeably the MEs for seven of eight microphysics schemes relative to CNTRL. Lower-tropospheric u e and convective available potential energy (CAPE) also are closer to the observations for the 4DVAR than CNTRL simulations.

Weather and Forecasting, Mar 1, 1990

Tellus A, 1970

In an earlier paper, a momentum integral method was developed by one of the authors (Smith, 1968)... more In an earlier paper, a momentum integral method was developed by one of the authors (Smith, 1968) to investigate the gross features of the surface friction layer of a steady, axisymmetric hurricane, which is specified by its radial pressure variation near the sea surface. Thus, by choosing suitable vertical profiles of inflow and swirling velocity components in the boundary layer, the technique provides estimates for the radial distribution of mean inflow, of boundary layer thickness and of mean upflow through the top of the layer. It therefore gives a measure of the constraint imposed by the inflow layer on the vortex which produces it. I n the present work, the method is used to investigate the effects of turbulent structure on the boundary layer characteristics. The turbulence is represented by an eddy viscosity K,, and solutions corresponding to a variety of models for the variation of KM, together with an appropriate surface boundary condition, are compared. These models range from an eddy viscosity which is everywhere constant and with the condition of no-slip at the surface, to a K , which has both radial and vertical structure and which varies linearly with height in the first few tens of metres above the sea surface. I n the latter case, one is able to parameterize the roughness of the sea surface. The solutions indicate that in actual hurricanes, an increase of KM towards the region of maximum winds produces a significant increase in the upflow compared with a similar layer in which K , has no radial variation. Moreover, the radial profile of boundary layer thickness differs markedly between the two cases. Solutions for three surface boundary conditions are compared and the volume inflow and upflow rates at a given radius are also found to increase with an increase in the constraint at the sea surface, that is, with an increase in surface stress. An error in one of the calculations of the first paper is also resolved.

Comments on “Statistical Single-Station Short-Term Forecasting of Temperature and Probability of Precipitation: Area Interpolation and NWP Combination”

Weather and Forecasting, 2001

We offer the following comments on the recent paper by Raible et al. (1999), hereafter referred t... more We offer the following comments on the recent paper by Raible et al. (1999), hereafter referred to as RB99. Their paper argues the case for implementing short-term (defined by RB99 as 0–24 h) statistical methods for single-station and areal forecasting. RB99 describe how they set up the statistical models, combine them with the relevant output from a regional NWP model, and how they introduce an areal interpolation procedure for operational usage on a Web site. Before beginning our comments, we wish to state that although we will be making some criticisms of RB99, we strongly endorse the general approach and recognize the pioneering efforts over the past two decades in this field of at least one of the authors (KF). Our motivation for writing these comments is to use RB99 as a springboard. We wish to generate an exchange of ideas in the literature, as an essential part of the health and vitality of this important area of applied research. We begin with a point that often seems to be ignored by the advocates of purely deterministic procedures. Statistical techniques have a necessary role in very short-term forecasting and in our opinion will continue to do so for the foreseeable future. At whatever resolution, or grid scale, a deterministic model is run at, there is always a subgrid-scale. The ultimate subgridscale is, of course, a point and a point forecast can only be attained from a deterministic model by employing either an intelligent downscaling technique (e.g., one of the various forms of regression) or by interpolating in some manner. All procedures for obtaining subgridscale, or point, forecasts are statistical methods. Focusing now on RB99, we make a series of com-

A numerical study of boundary effects on concentrated vortices with application to tornadoes and waterspouts

Quarterly Journal of the Royal Meteorological Society, 1975

This paper extends the numerical study of the structure and development of a concentrated vortex ... more This paper extends the numerical study of the structure and development of a concentrated vortex by Leslie (1971), in which a vortex is simulated by suddenly imposing an upwards body force along a section of the vertical axis of a contained rotating fluid, initially in a state of uniform rotation. Whereas the former paper was concerned primarily with demonstrating the prediction of Morton (1969) that a concentrated vortex may be generated only for a restricted range of the flow parameters, the present paper investigates the important role of boundaries on vortex behaviour.Particular interest is focused on the boundary which is normal to the vortex core and ‘behind’ the body force. On this boundary the surface stress is related to the surface velocity by a drag coefficient CD and experiments are performed in which CD is infinite, unity and zero corresponding with a no‐slip, a partially yielding and a free‐slip boundary respectively. These calculations are motivated by the desire to a...

Tornadogenesis

Quarterly Journal of the Royal Meteorological Society, 1978

ABSTRACT In this paper we use a simple numerical model to study vortex growth in a flow configura... more ABSTRACT In this paper we use a simple numerical model to study vortex growth in a flow configuration which broadly simulates the principal characteristics of a severe tornadic storm system, i.e. strong vertical forcing by intense cumulus or cumulonimbus convection in the presence of an organized field of rotation on the cloud scale. The model is similar to one described by L. M. Leslie in which the updraught in the main cloud cell is modelled by an imposed body force, but differs in that the rotation field is determined by specifying the vertical profile of swirling velocity at the lateral boundary of the flow domain and air is allowed to enter or leave the computational region through its radial and upper boundaries. In particular, we compare situations in which the imposed swirl is concentrated aloft, primarily above cloud base, and when it extends to lower levels. In the former case, solutions exhibit genesis to a steady suspended vortex provided that the forcing strength lies within a certain range, depending on the level of rotation. However, if the imposed swirl extends sufficiently far below cloud base, the vortex continues its downward growth and establishes contact with the ground.We believe our results provide a plausible and consistent picture of the growth of a tornado beneath the main updraught of a severe thunderstorm and indicate why only a relatively small proportion of such clouds spawn pendant funnel clouds and why only a fraction of these develop into tornadoes. Moreover, they appear to be consistent with the observed development of circulation patterns deduced from single-pulse Doppler radar measurements of a tornado-producing storm system by Burgess, Lemon and Brown.

Thermally driven vortices: A numerical study with application to dust-devil dynamics

Quarterly Journal of the Royal Meteorological Society, 1976

ABSTRACT This paper describes a series of numerical experiments designed to explore the close int... more ABSTRACT This paper describes a series of numerical experiments designed to explore the close interplay between the rotational, the pressure gradient, and the buoyancy, force fields in concentrated vortex flows driven thermally by heating from below. The calculations are motivated by a desire to understand the dynamics of dust-devils and to provide a theoretical framework in terms of which both past and future observations of these vortices may be interpreted.In the model, a vortex is generated along the vertical axis of a cylindrical region of fluid bounded by a rigid (no-slip) lower boundary, and a sidewall consisting of a rigid, impermeable upper portion and a rotating, porous, lower portion through which fluid at ambient temperature enters the cylinder and acquires rotation. The flow is driven by maintaining a circular portion of the lower boundary at a fixed temperature above ambient and fluid is allowed to enter or leave the cylinder normally through the upper boundary. For a cylinder of given size, the flow behaviour depends on the magnitudes of two nondimensional parameters which characterize the strength of the thermal forcing and of the imposed rotation. According to the sizes of these, flows with axial downflow on part or all of the axis of rotation are possible and, in some cases, a closed cell of reversed flow is possible. In each case it is possible to give a complete description of the force field balance which combines to produce the particular flow pattern.The calculations go a long way to providing an understanding of the main features of dust-devils as observed in a careful and detailed study by Sinclair and corroborate well with the experimental results of Fitzgarrald in relation to laboratory vortices. In particular, we show how rotationally induced axial pressure gradients can supplement buoyancy forces in accelerating the vertical flow in dust-devils as appears necessary to explain the large vertical accelerations observed near the ground in these vortices.