Droughts and long dry spells, interspersed with intense rainfall events, have been characteristic... more Droughts and long dry spells, interspersed with intense rainfall events, have been characteristic of the northern Murray-Darling Basin (NMDB), a major Australian agricultural region. The NMDB precipitation results from weather systems ranging from thunderstorms to larger scale events. The larger scale events exhibit high seasonal and annual rainfall variability. To detect attributes shaping the NMDB precipitation patterns, and hence net water inflows to the vast Darling River catchment area, numerous (45) possible attributes were assessed for their influence on rainfall trends. Four periods were assessed: annual, April–May (early cool-season), June–September (remaining cool-season), and October–March (warm-season). Linear and non-linear regression machine learning (ML) methods were used to identify the dominant attributes. We show the impact of climate drivers on the increasingly dry April–May months on annual precipitation and warmer temperatures since the early 1990s. The NMDB wat...
Droughts in southeastern Australia can profoundly affect the water supply to Sydney, Australia’s ... more Droughts in southeastern Australia can profoundly affect the water supply to Sydney, Australia’s largest city. Increasing population, a warming climate, land surface changes and expanded agricultural use increase water demand and reduce catchment runoff. Studying Sydney’s water supply is necessary to manage water resources and lower the risk of severe water shortages. This study aims at understanding Sydney’s water supply by analysing precipitation and temperature trends across the catchment. A decreasing trend in annual precipitation was found across the Sydney catchment area. Annual precipitation also is significantly less variable, due to fewer years above the 80th percentile. These trends result from significant reductions in precipitation during spring and autumn, especially over the last 20 years. Wavelet analysis was applied to assess how the influence of climate drivers has changed over time. Attribute selection was carried out using linear regression and machine learning te...
As the spatial, temporal and spectral resolution of visible and infrared radiance observations fr... more As the spatial, temporal and spectral resolution of visible and infrared radiance observations from space has improved, their utility for estimating atmospheric motion, and, in particular, their benefit to Numerical Weather Prediction (NWP) has increased. The utility of these data has also been aided by increased computer power, improved NWP models and the use of improving data assimilation techniques. This review provides a brief history of the generation and application of Atmospheric Motion Vectors (AMVs), including a short description of high spatial and temporal resolution AMV processing. All AMV data types are both suitable and now used for operational regional and global NWP and the data selection and quality control issues important to NWP are discussed. A brief examination of preparations for future advanced instruments is also undertaken.
Summary Preliminary work gauging the impact of varying the spatial and temporal resolution of Clo... more Summary Preliminary work gauging the impact of varying the spatial and temporal resolution of Cloud Drift Wind (CDW) data using different assimilation techniques is presented, particularly within the framework of a generalised inverse data assimilation scheme. ...
Amongst the more important laboratory experiments which have produced concentrated vortices in ro... more Amongst the more important laboratory experiments which have produced concentrated vortices in rotating tanks are the sink experiments of Long and the bubble convection experiments of Turner & Lilly. This paper describes a numerical experiment which draws from the laboratory experiments those features which are believed to be most relevant to atmospheric vortices such as tornadoes and waterspouts.In the numerical model the mechanism driving the vortices is represented by an externally specified vertical body force field defined in a narrow neighbourhood of the axis of rotation. The body force field is applied to a tank of fluid initially in a state of rigid rotation and the subsequent flow development is obtained by solving the Navier–Stokes equations as an initial-value problem.Earlier investigations have revealed that concentrated vortices will form only for a restricted range of flow parameters, and for the numerical experiment this range was selected using an order-of-magnitude ...
The Murray-Darling Basin (MDB) is Australia’s major agricultural region. The southern MDB receive... more The Murray-Darling Basin (MDB) is Australia’s major agricultural region. The southern MDB receives most of its annual catchment runoff during the cool season (April–September). Focusing on the Murrumbidgee River measurements at Wagga Wagga and further downstream at Hay, cool season river heights are available year to year. The 27-year period April–September Hay and Wagga Wagga river heights exhibit decreases between 1965 and 1991 and 1992–2018 not matched by declining April-September catchment rainfall. However, permutation tests of means and variances of late autumn (April–May) dam catchment precipitation and net inflows, produced p-values indicating a highly significant decline since the early 1990s. Consequently, dry catchments in late autumn, even with average cool season rainfall, have reduced dam inflows and decreased river heights downstream from Wagga Wagga, before water extraction for irrigation. It is concluded that lower April–September mean river heights at Wagga Wagga a...
Abstract. This article describes a new general circula-tion model (GCM) developed jointly by The ... more Abstract. This article describes a new general circula-tion model (GCM) developed jointly by The Univer-sity of New South Wales (UNSW) and the University of Hamburg. The model is versatile in that it can be run as a medium-range (1 to 15 days) global numerical weather prediction (NWP) model; as an extended range (15 to 30 days) NWP model; and as a GCM for periods extending from seasons, through annual and decadal periods, and beyond. The model can be cou-pled with ocean models that vary in complexity from simple “swamp ” oceans to complex ocean GCMs. The atmospheric GCM also has a number of novel features, particularly in the numerical integration scheme which is a high-order, mass-conserving, semi-implicit semi-Lagrangian scheme, thereby removing the stability re-
The generation of stationary Rossby waves by sources of potential vorticity in a westerly flow is... more The generation of stationary Rossby waves by sources of potential vorticity in a westerly flow is examined here in the context of a two-layer, quasi-geostrophic, -plane model. The response in each layer consists of a combination of a barotropic Rossby wave disturbance that extends far downstream of the source, and a baroclinic disturbance which is evanescent or wave-like in character,
ABSTRACT Earlier models of fire plumes based on simple entrainment laws and neglecting dynamic pr... more ABSTRACT Earlier models of fire plumes based on simple entrainment laws and neglecting dynamic pressure have failed to produce the relatively shallow inflow over the fire perimeter known as fire wind. This inflow is of prime importance in fire modelling as it normally provides much of the air required for combustion; for this reason we have carried out a very simple numerical experiment on two-dimensional natural convection above a strip heat source with the intention of simulating those aspects of fire behaviour involved in the generation of fire wind without attempting the formidably difficult task of detailed fire modelling. Our results show clearly that fire wind is driven by the dynamic pressure field which is generated by and intimately related to the region of strong buoyant acceleration close above the ground boundary. Throughout our parametric range there is a concentrated region of large horizontal pressure gradient in a neighbourhood above the perimeter of the fire, and elsewhere the pressure gradients play a lesser role.
The Australian region seasonal tropical cyclone count (TCC) maintained a robust statistical relat... more The Australian region seasonal tropical cyclone count (TCC) maintained a robust statistical relationship with El Niño–Southern Oscillation (ENSO), with skillful forecasts of above (below) average TCC during La Niña (El Niño) years from 1969 until about 1998, weakening thereafter. The current study identifies an additional climate driver that mitigates the loss of predictive skill for Australian TCC after about 1998. It is found that the seasonal Australian TCC is strongly modulated by a southwest-to-northeast-oriented dipole in Indian Ocean sea surface temperature anomalies (SSTAs), referred to here as the transverse Indian Ocean dipole (TIOD). The TIOD emerges as the leading mode of detrended Indian Ocean SSTAs in the Southern Hemisphere during late winter and spring. Active (inactive) TC seasons are linked to positive (negative) TIOD phases, most notably during August–October immediately preceding the TC season, when SSTAs northwest of Australia, in the northeast pole of the TIOD,...
Abundant research documents a gender pay gap, such that men earn more than women, controlling for... more Abundant research documents a gender pay gap, such that men earn more than women, controlling for human capital and other drivers of pay. Against the backdrop of an overall female pay penalty, we i...
The three dimensional circulation produced by a given wind stress in a homogeneous ocean with an ... more The three dimensional circulation produced by a given wind stress in a homogeneous ocean with an arbitrary bottom topography is studied. The solution is illustrated by the use of various bottom shapes and a particular wind stress distribution. It is shown that in general a current deviates southwards in a region of decreasing depth and northwards in a region of increasing depth. However, if the wind stress has a perturbation out of phase with the bottom corrugations this result is considerably modified. Moreover, when there is a deepening of the ocean towards the north there may exist a “critical line” at which ?/?y(f/H) is zero ( f is the Coriolis parameter, H is the depth and y is the northward co-ordinate). The oceanic circulation is then divided into two parts: if the wind stress curl is positive, then for latitudes north of the “critical line” where ?/?y(f/H) is positive the Sverdrup-topographic interior flow is basically northward and there is an eastern boundary current, while for latitudes south of the critical line, where ?/?y(f/H) is negative the interior flow is basically southward and there is a western boundary current. DOI: 10.1111/j.2153-3490.1971.tb00553.x
Droughts and long dry spells, interspersed with intense rainfall events, have been characteristic... more Droughts and long dry spells, interspersed with intense rainfall events, have been characteristic of the northern Murray-Darling Basin (NMDB), a major Australian agricultural region. The NMDB precipitation results from weather systems ranging from thunderstorms to larger scale events. The larger scale events exhibit high seasonal and annual rainfall variability. To detect attributes shaping the NMDB precipitation patterns, and hence net water inflows to the vast Darling River catchment area, numerous (45) possible attributes were assessed for their influence on rainfall trends. Four periods were assessed: annual, April–May (early cool-season), June–September (remaining cool-season), and October–March (warm-season). Linear and non-linear regression machine learning (ML) methods were used to identify the dominant attributes. We show the impact of climate drivers on the increasingly dry April–May months on annual precipitation and warmer temperatures since the early 1990s. The NMDB wat...
Droughts in southeastern Australia can profoundly affect the water supply to Sydney, Australia’s ... more Droughts in southeastern Australia can profoundly affect the water supply to Sydney, Australia’s largest city. Increasing population, a warming climate, land surface changes and expanded agricultural use increase water demand and reduce catchment runoff. Studying Sydney’s water supply is necessary to manage water resources and lower the risk of severe water shortages. This study aims at understanding Sydney’s water supply by analysing precipitation and temperature trends across the catchment. A decreasing trend in annual precipitation was found across the Sydney catchment area. Annual precipitation also is significantly less variable, due to fewer years above the 80th percentile. These trends result from significant reductions in precipitation during spring and autumn, especially over the last 20 years. Wavelet analysis was applied to assess how the influence of climate drivers has changed over time. Attribute selection was carried out using linear regression and machine learning te...
As the spatial, temporal and spectral resolution of visible and infrared radiance observations fr... more As the spatial, temporal and spectral resolution of visible and infrared radiance observations from space has improved, their utility for estimating atmospheric motion, and, in particular, their benefit to Numerical Weather Prediction (NWP) has increased. The utility of these data has also been aided by increased computer power, improved NWP models and the use of improving data assimilation techniques. This review provides a brief history of the generation and application of Atmospheric Motion Vectors (AMVs), including a short description of high spatial and temporal resolution AMV processing. All AMV data types are both suitable and now used for operational regional and global NWP and the data selection and quality control issues important to NWP are discussed. A brief examination of preparations for future advanced instruments is also undertaken.
Summary Preliminary work gauging the impact of varying the spatial and temporal resolution of Clo... more Summary Preliminary work gauging the impact of varying the spatial and temporal resolution of Cloud Drift Wind (CDW) data using different assimilation techniques is presented, particularly within the framework of a generalised inverse data assimilation scheme. ...
Amongst the more important laboratory experiments which have produced concentrated vortices in ro... more Amongst the more important laboratory experiments which have produced concentrated vortices in rotating tanks are the sink experiments of Long and the bubble convection experiments of Turner & Lilly. This paper describes a numerical experiment which draws from the laboratory experiments those features which are believed to be most relevant to atmospheric vortices such as tornadoes and waterspouts.In the numerical model the mechanism driving the vortices is represented by an externally specified vertical body force field defined in a narrow neighbourhood of the axis of rotation. The body force field is applied to a tank of fluid initially in a state of rigid rotation and the subsequent flow development is obtained by solving the Navier–Stokes equations as an initial-value problem.Earlier investigations have revealed that concentrated vortices will form only for a restricted range of flow parameters, and for the numerical experiment this range was selected using an order-of-magnitude ...
The Murray-Darling Basin (MDB) is Australia’s major agricultural region. The southern MDB receive... more The Murray-Darling Basin (MDB) is Australia’s major agricultural region. The southern MDB receives most of its annual catchment runoff during the cool season (April–September). Focusing on the Murrumbidgee River measurements at Wagga Wagga and further downstream at Hay, cool season river heights are available year to year. The 27-year period April–September Hay and Wagga Wagga river heights exhibit decreases between 1965 and 1991 and 1992–2018 not matched by declining April-September catchment rainfall. However, permutation tests of means and variances of late autumn (April–May) dam catchment precipitation and net inflows, produced p-values indicating a highly significant decline since the early 1990s. Consequently, dry catchments in late autumn, even with average cool season rainfall, have reduced dam inflows and decreased river heights downstream from Wagga Wagga, before water extraction for irrigation. It is concluded that lower April–September mean river heights at Wagga Wagga a...
Abstract. This article describes a new general circula-tion model (GCM) developed jointly by The ... more Abstract. This article describes a new general circula-tion model (GCM) developed jointly by The Univer-sity of New South Wales (UNSW) and the University of Hamburg. The model is versatile in that it can be run as a medium-range (1 to 15 days) global numerical weather prediction (NWP) model; as an extended range (15 to 30 days) NWP model; and as a GCM for periods extending from seasons, through annual and decadal periods, and beyond. The model can be cou-pled with ocean models that vary in complexity from simple “swamp ” oceans to complex ocean GCMs. The atmospheric GCM also has a number of novel features, particularly in the numerical integration scheme which is a high-order, mass-conserving, semi-implicit semi-Lagrangian scheme, thereby removing the stability re-
The generation of stationary Rossby waves by sources of potential vorticity in a westerly flow is... more The generation of stationary Rossby waves by sources of potential vorticity in a westerly flow is examined here in the context of a two-layer, quasi-geostrophic, -plane model. The response in each layer consists of a combination of a barotropic Rossby wave disturbance that extends far downstream of the source, and a baroclinic disturbance which is evanescent or wave-like in character,
ABSTRACT Earlier models of fire plumes based on simple entrainment laws and neglecting dynamic pr... more ABSTRACT Earlier models of fire plumes based on simple entrainment laws and neglecting dynamic pressure have failed to produce the relatively shallow inflow over the fire perimeter known as fire wind. This inflow is of prime importance in fire modelling as it normally provides much of the air required for combustion; for this reason we have carried out a very simple numerical experiment on two-dimensional natural convection above a strip heat source with the intention of simulating those aspects of fire behaviour involved in the generation of fire wind without attempting the formidably difficult task of detailed fire modelling. Our results show clearly that fire wind is driven by the dynamic pressure field which is generated by and intimately related to the region of strong buoyant acceleration close above the ground boundary. Throughout our parametric range there is a concentrated region of large horizontal pressure gradient in a neighbourhood above the perimeter of the fire, and elsewhere the pressure gradients play a lesser role.
The Australian region seasonal tropical cyclone count (TCC) maintained a robust statistical relat... more The Australian region seasonal tropical cyclone count (TCC) maintained a robust statistical relationship with El Niño–Southern Oscillation (ENSO), with skillful forecasts of above (below) average TCC during La Niña (El Niño) years from 1969 until about 1998, weakening thereafter. The current study identifies an additional climate driver that mitigates the loss of predictive skill for Australian TCC after about 1998. It is found that the seasonal Australian TCC is strongly modulated by a southwest-to-northeast-oriented dipole in Indian Ocean sea surface temperature anomalies (SSTAs), referred to here as the transverse Indian Ocean dipole (TIOD). The TIOD emerges as the leading mode of detrended Indian Ocean SSTAs in the Southern Hemisphere during late winter and spring. Active (inactive) TC seasons are linked to positive (negative) TIOD phases, most notably during August–October immediately preceding the TC season, when SSTAs northwest of Australia, in the northeast pole of the TIOD,...
Abundant research documents a gender pay gap, such that men earn more than women, controlling for... more Abundant research documents a gender pay gap, such that men earn more than women, controlling for human capital and other drivers of pay. Against the backdrop of an overall female pay penalty, we i...
The three dimensional circulation produced by a given wind stress in a homogeneous ocean with an ... more The three dimensional circulation produced by a given wind stress in a homogeneous ocean with an arbitrary bottom topography is studied. The solution is illustrated by the use of various bottom shapes and a particular wind stress distribution. It is shown that in general a current deviates southwards in a region of decreasing depth and northwards in a region of increasing depth. However, if the wind stress has a perturbation out of phase with the bottom corrugations this result is considerably modified. Moreover, when there is a deepening of the ocean towards the north there may exist a “critical line” at which ?/?y(f/H) is zero ( f is the Coriolis parameter, H is the depth and y is the northward co-ordinate). The oceanic circulation is then divided into two parts: if the wind stress curl is positive, then for latitudes north of the “critical line” where ?/?y(f/H) is positive the Sverdrup-topographic interior flow is basically northward and there is an eastern boundary current, while for latitudes south of the critical line, where ?/?y(f/H) is negative the interior flow is basically southward and there is a western boundary current. DOI: 10.1111/j.2153-3490.1971.tb00553.x
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Papers by Lance M Leslie