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Abstract
Fluctuations in the Australian summer monsoon over the period 1952–82 are described. The basis of the study is an objective definition of the major summer monsoon components based on the low-level zonal winds at Darwin; this is shown to be in good agreement with other large-scale indicators. Statistics are presented and discussed for the interannual variation in summer monsoon onset, extent, active and break conditions, circulation strength, and vertical structure.
Some relationships with the Southern Oscillation are also described. These indicate that the Southern Oscillation Index (SOI) is highly correlated with the intensity and degree of convergence in the low-level monsoonal shear zone, and with the mean daily rainfall rate over northern Australia. There is also a significant correlation between the summer monsoon onset date and the SOI in the following spring, which has implications for El Niño teleconnections.
Abstract
Fluctuations in the Australian summer monsoon over the period 1952–82 are described. The basis of the study is an objective definition of the major summer monsoon components based on the low-level zonal winds at Darwin; this is shown to be in good agreement with other large-scale indicators. Statistics are presented and discussed for the interannual variation in summer monsoon onset, extent, active and break conditions, circulation strength, and vertical structure.
Some relationships with the Southern Oscillation are also described. These indicate that the Southern Oscillation Index (SOI) is highly correlated with the intensity and degree of convergence in the low-level monsoonal shear zone, and with the mean daily rainfall rate over northern Australia. There is also a significant correlation between the summer monsoon onset date and the SOI in the following spring, which has implications for El Niño teleconnections.
Abstract
An analytic model of the radial profiles of sea level pressure and winds in a hurricane is presented. The equations contain two parameters which may be empirically estimated from observations in a hurricane or determined climatologically to define a standard hurricane; example are given. The model is shown to be generally superior to two other widely used models and is considered to be a valuable aid in operational forecasting, case studies and engineering work.
Abstract
An analytic model of the radial profiles of sea level pressure and winds in a hurricane is presented. The equations contain two parameters which may be empirically estimated from observations in a hurricane or determined climatologically to define a standard hurricane; example are given. The model is shown to be generally superior to two other widely used models and is considered to be a valuable aid in operational forecasting, case studies and engineering work.
Abstract
The boundary layer structure of Tropical Cyclone Kerry (1979) is investigated using composite analysis of research aircraft, surface ship, and automatic weather station observations. The boundary layer was moist, convective, and strongly confluent to the east of the tropical cyclone center but was dry, subsident, and diffluent to the west. The vertical momentum transport in the eastern convective sector of Kerry was around two to three times the surface frictional dissipation. In contrast, the stable boundary layer in the western sector consisted of a shallow mixed layer capped by an equivalent potential temperature minimum and a low-level jet, which underwent a marked diurnal oscillation. Three mechanisms appear to have contributed to the observed asymmetry: 1) a general, zonal distortion arose from cyclonic rotation across a gradient of earth vorticity; 2) a westerly environmental vertical shear produced forced ascent on the east side of the storm and subsidence on the west side throughout the lower and midtroposphere; and 3) the western sector boundary layer was modified by an upstream cold tongue generated by the tropical cyclone passage. The authors present evidence that substantial drying also resulted from shear-induced mixing of the subsident environmental air in the region of the low-level jet.
Thermal boundary layer budgets are derived using both a general mixing theory approach and direct flux calculations from aircraft reconnaissance data. Use of actual sea surface temperature fields are essential. The surface flux estimates of latent heat are near the average of previous studies, but the sensible heat fluxes are downward into the ocean. Since horizontal advection also cooled the boundary layer, the thermal structure was maintained by downward fluxes of sensible heat from the top of the boundary layer of around 100 W m−2. We conclude that the pattern of oceanic cooling directly determines the pattern of vertical air-sea and advective sensible heat fluxes and indirectly determines the pattern of latent heat fluxes through forcing of PBL drying at the downwind end of the SST cold pool. It further enhances the inward penetration and negative feedback resulting from an easterly trade wind surge associated with a mobile trough in the westerlies.
Abstract
The boundary layer structure of Tropical Cyclone Kerry (1979) is investigated using composite analysis of research aircraft, surface ship, and automatic weather station observations. The boundary layer was moist, convective, and strongly confluent to the east of the tropical cyclone center but was dry, subsident, and diffluent to the west. The vertical momentum transport in the eastern convective sector of Kerry was around two to three times the surface frictional dissipation. In contrast, the stable boundary layer in the western sector consisted of a shallow mixed layer capped by an equivalent potential temperature minimum and a low-level jet, which underwent a marked diurnal oscillation. Three mechanisms appear to have contributed to the observed asymmetry: 1) a general, zonal distortion arose from cyclonic rotation across a gradient of earth vorticity; 2) a westerly environmental vertical shear produced forced ascent on the east side of the storm and subsidence on the west side throughout the lower and midtroposphere; and 3) the western sector boundary layer was modified by an upstream cold tongue generated by the tropical cyclone passage. The authors present evidence that substantial drying also resulted from shear-induced mixing of the subsident environmental air in the region of the low-level jet.
Thermal boundary layer budgets are derived using both a general mixing theory approach and direct flux calculations from aircraft reconnaissance data. Use of actual sea surface temperature fields are essential. The surface flux estimates of latent heat are near the average of previous studies, but the sensible heat fluxes are downward into the ocean. Since horizontal advection also cooled the boundary layer, the thermal structure was maintained by downward fluxes of sensible heat from the top of the boundary layer of around 100 W m−2. We conclude that the pattern of oceanic cooling directly determines the pattern of vertical air-sea and advective sensible heat fluxes and indirectly determines the pattern of latent heat fluxes through forcing of PBL drying at the downwind end of the SST cold pool. It further enhances the inward penetration and negative feedback resulting from an easterly trade wind surge associated with a mobile trough in the westerlies.
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Abstract
A detailed analysis is made of the development of a region of cold cloud-top temperatures in Tropical Cyclone Hilda (1990) in the Coral Sea off eastern Australia. Observed temperatures of approximately 173 K (−100°C) from two independent satellite sources indicate that the convective turrets penetrated well into the stratosphere to an estimated height of around 19.2 km.
The analytical parcel model of Schlesinger is used, together with available observations from the cyclone vicinity, to estimate the convective updrafts required to produce the observed stratosphere penetration. Under realistic assumptions of entrainment and hydrometeor drag, an updraft speed of between 15 and 38 m s−1 at tropopause level is required to provide the observed stratospheric penetration. Independent calculations using observed anvil expansion and environmental CAPE (convective available potential energy) support these updraft findings.
Abstract
A detailed analysis is made of the development of a region of cold cloud-top temperatures in Tropical Cyclone Hilda (1990) in the Coral Sea off eastern Australia. Observed temperatures of approximately 173 K (−100°C) from two independent satellite sources indicate that the convective turrets penetrated well into the stratosphere to an estimated height of around 19.2 km.
The analytical parcel model of Schlesinger is used, together with available observations from the cyclone vicinity, to estimate the convective updrafts required to produce the observed stratosphere penetration. Under realistic assumptions of entrainment and hydrometeor drag, an updraft speed of between 15 and 38 m s−1 at tropopause level is required to provide the observed stratospheric penetration. Independent calculations using observed anvil expansion and environmental CAPE (convective available potential energy) support these updraft findings.
Abstract
Five characteristic, low-level, large-scale dynamical patterns associated with tropical cyclogenesis in the western North Pacific basin are examined along with their capacity to generate the type of mesoscale convective systems that precede genesis. An 8-yr analysis set for the region is used to identify, and create composites for, the five characteristic patterns of monsoon shear line, monsoon confluence region, monsoon gyre, easterly waves, and Rossby energy dispersion. This brings out the common processes that contribute to tropical cyclogenesis within that pattern, which are described in detail.
A 3-yr set of satellite data is then used to analyze the mesoscale convective system activity for all cases of genesis in that period and to stratify based on the above large-scale patterns. It is found that mesoscale convective systems develop in all cases of genesis except one. Seventy percent of cases developed mesoscale convective systems at more than one time during the genesis period and 44% of cases developed multiple mesoscale convective systems at a single time. Stratification by pattern type indicates some differentiation in mesoscale convective activity and it is inferred that this is due to the large-scale processes. Two of the five patterns, the monsoon shear line and the monsoon confluence region, had more than the average amount of mesoscale convective activity during the genesis period. These patterns also account for 70% of the total genesis events in the 8-yr period. The analysis for the other three patterns exhibit less mesoscale convective system activity during genesis. This may indicate either that genesis processes for these patterns are not as dominated by mesoscale convective system activity, or that genesis occurs more rapidly in these cases.
Abstract
Five characteristic, low-level, large-scale dynamical patterns associated with tropical cyclogenesis in the western North Pacific basin are examined along with their capacity to generate the type of mesoscale convective systems that precede genesis. An 8-yr analysis set for the region is used to identify, and create composites for, the five characteristic patterns of monsoon shear line, monsoon confluence region, monsoon gyre, easterly waves, and Rossby energy dispersion. This brings out the common processes that contribute to tropical cyclogenesis within that pattern, which are described in detail.
A 3-yr set of satellite data is then used to analyze the mesoscale convective system activity for all cases of genesis in that period and to stratify based on the above large-scale patterns. It is found that mesoscale convective systems develop in all cases of genesis except one. Seventy percent of cases developed mesoscale convective systems at more than one time during the genesis period and 44% of cases developed multiple mesoscale convective systems at a single time. Stratification by pattern type indicates some differentiation in mesoscale convective activity and it is inferred that this is due to the large-scale processes. Two of the five patterns, the monsoon shear line and the monsoon confluence region, had more than the average amount of mesoscale convective activity during the genesis period. These patterns also account for 70% of the total genesis events in the 8-yr period. The analysis for the other three patterns exhibit less mesoscale convective system activity during genesis. This may indicate either that genesis processes for these patterns are not as dominated by mesoscale convective system activity, or that genesis occurs more rapidly in these cases.
Abstract
The development of Typhoon Irving is investigated using a variety of data, including special research aircraft data from the Tropical Cyclone Motion (TCM-92) experiment, objective analyses, satellite data, and traditional surface and sounding data. The development process is treated as a dry-adiabatic vortex dynamics problem, and it is found that environmental and mesoscale dynamics mutually enhance each other in a cooperative interaction during cyclone formation. Synoptic-scale interactions result in the evolution of the hostile environment toward more favorable conditions for storm development. Mesoscale interactions with the low-level, large-scale circulations and with other midlevel, mesoscale features result in development of vorticity in the midlevels and enhancement of the low-level vorticity associated with the developing surface cyclone.
Multiple developments of mesoscale convective systems after the storm reaches tropical depression strength suggests both an increase in low-level confluence and a tendency toward recurrent development of associated mesoscale convective vortices. This is observed in both aircraft data and satellite imagery where subsequent interactions, including mergers with the low-level, tropical depression vortex, are observed. A contour dynamics experiment suggests that the movement of mesoscale convective systems in satellite imagery corresponds well to the movement of their associated midlevel vortices. Results from a simple baroclinic experiment show that the midlevel vortices affect the large-scale, low-level circulation in two ways: 1) initially, interactions between midlevel vortices produce a combined vortex of greater depth; 2) interaction between midlevel vortices and the low-level circulation produces a development downward of the midlevel vorticity. This strengthens the surface vortex and develops a more cohesive vortex that extends from the surface through the midtroposphere.
Abstract
The development of Typhoon Irving is investigated using a variety of data, including special research aircraft data from the Tropical Cyclone Motion (TCM-92) experiment, objective analyses, satellite data, and traditional surface and sounding data. The development process is treated as a dry-adiabatic vortex dynamics problem, and it is found that environmental and mesoscale dynamics mutually enhance each other in a cooperative interaction during cyclone formation. Synoptic-scale interactions result in the evolution of the hostile environment toward more favorable conditions for storm development. Mesoscale interactions with the low-level, large-scale circulations and with other midlevel, mesoscale features result in development of vorticity in the midlevels and enhancement of the low-level vorticity associated with the developing surface cyclone.
Multiple developments of mesoscale convective systems after the storm reaches tropical depression strength suggests both an increase in low-level confluence and a tendency toward recurrent development of associated mesoscale convective vortices. This is observed in both aircraft data and satellite imagery where subsequent interactions, including mergers with the low-level, tropical depression vortex, are observed. A contour dynamics experiment suggests that the movement of mesoscale convective systems in satellite imagery corresponds well to the movement of their associated midlevel vortices. Results from a simple baroclinic experiment show that the midlevel vortices affect the large-scale, low-level circulation in two ways: 1) initially, interactions between midlevel vortices produce a combined vortex of greater depth; 2) interaction between midlevel vortices and the low-level circulation produces a development downward of the midlevel vorticity. This strengthens the surface vortex and develops a more cohesive vortex that extends from the surface through the midtroposphere.
Abstract
The potential for predicting the skill of 36-h forecasts from the Australian region limited area model is investigated using three predictors of model forecast error (MFE) for mean sea level pressure. Two of the predictors utilize single forecasts: one is based on statistical regression of the MFE against the initial analysis and the forecast; and the other uses a measure of the degree of persistence in the forecast. The third predictor utilizes the divergence, or spread, of an ensemble of forecasts from other NWP centers.
Based on a 5-month period of daily 36-h forecasts, correlations were found between the above predictors and the MFE of 0.58, 0.18, and 0.40, respectively. Combining the three predictors in an optimal linear manner increased the correlation to 0.71. Further testing of the combined predictors on a 2-month independent dataset produced a correlation of 0.67. Thus, application of the technique to both dependent and independent datasets explained approximately 50% of the variance in the MFE. This demonstrates that the technique has operational utility for differentiating overall poor and good model forecasts. Using case studies concentrating on southeastern Australia, it is further demonstrated that the predictors can provide excellent differentiation of forecast skill across the forecast domain.
Abstract
The potential for predicting the skill of 36-h forecasts from the Australian region limited area model is investigated using three predictors of model forecast error (MFE) for mean sea level pressure. Two of the predictors utilize single forecasts: one is based on statistical regression of the MFE against the initial analysis and the forecast; and the other uses a measure of the degree of persistence in the forecast. The third predictor utilizes the divergence, or spread, of an ensemble of forecasts from other NWP centers.
Based on a 5-month period of daily 36-h forecasts, correlations were found between the above predictors and the MFE of 0.58, 0.18, and 0.40, respectively. Combining the three predictors in an optimal linear manner increased the correlation to 0.71. Further testing of the combined predictors on a 2-month independent dataset produced a correlation of 0.67. Thus, application of the technique to both dependent and independent datasets explained approximately 50% of the variance in the MFE. This demonstrates that the technique has operational utility for differentiating overall poor and good model forecasts. Using case studies concentrating on southeastern Australia, it is further demonstrated that the predictors can provide excellent differentiation of forecast skill across the forecast domain.
Abstract
North Australian Clouds Lines are distinctive, squall-line phenomena that occur in easterly flow across northern Australia. Three basic types have been identified, ranging from a long, narrow line of convective clouds (Type 1) to a severe squall line (Type 3). In this paper we examine a group of Type 1 lines, which occurred during the first phase of the Australian Monsoon Experiment (AMEX). The lines occurred in an ambient easterly flow with a distinct maximum near 850 hPa. Most of the lines developed on the western side of deep convective cells along the sea-breeze front in a manner that had substantial similarities to the African squall-line development described by Bolton. The resolvable structure resembled a shallow version of the Moncrieff–Miller squall line.
Abstract
North Australian Clouds Lines are distinctive, squall-line phenomena that occur in easterly flow across northern Australia. Three basic types have been identified, ranging from a long, narrow line of convective clouds (Type 1) to a severe squall line (Type 3). In this paper we examine a group of Type 1 lines, which occurred during the first phase of the Australian Monsoon Experiment (AMEX). The lines occurred in an ambient easterly flow with a distinct maximum near 850 hPa. Most of the lines developed on the western side of deep convective cells along the sea-breeze front in a manner that had substantial similarities to the African squall-line development described by Bolton. The resolvable structure resembled a shallow version of the Moncrieff–Miller squall line.
Abstract
The meteorological conditions for the development of Australian east-coast cyclones are described. The main synoptic precursor is a trough (or “dip”) in the easterly wind regime over eastern Australia. The cyclones are a mesoscale development which occurs on the coast in this synoptic environment. They form preferentially at night, in the vicinity of a marked low-level baroclinic zone, and just equatorward of a region of enhanced convection resulting from flow over the coastal ranges.
Three different types of east-coast cyclone have been identified. Types 1 and 3 are very small systems which can have lifetimes as short as 16 hours, during which hurricane force winds have been observed to develop. The other, type 2, system is a meso/synoptic-scale cyclone that can bring sustained strong winds and flood rainfall over several days. Because of their intensity, rapid development, and occasional tiny size, these systems are a major forecast problem.
Abstract
The meteorological conditions for the development of Australian east-coast cyclones are described. The main synoptic precursor is a trough (or “dip”) in the easterly wind regime over eastern Australia. The cyclones are a mesoscale development which occurs on the coast in this synoptic environment. They form preferentially at night, in the vicinity of a marked low-level baroclinic zone, and just equatorward of a region of enhanced convection resulting from flow over the coastal ranges.
Three different types of east-coast cyclone have been identified. Types 1 and 3 are very small systems which can have lifetimes as short as 16 hours, during which hurricane force winds have been observed to develop. The other, type 2, system is a meso/synoptic-scale cyclone that can bring sustained strong winds and flood rainfall over several days. Because of their intensity, rapid development, and occasional tiny size, these systems are a major forecast problem.
