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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.
Abstract
A satellite classification and climatology of propagating mesoscale cloud fines in northern Australia is presented. These cloud fines range from long, narrow lines of shallow convection to extensive deep convective squall lines with mesoscale stratiform rain areas. These lines are the predominant weather feature during periods of easterly flow over the Australian tropics.
Abstract
A satellite classification and climatology of propagating mesoscale cloud fines in northern Australia is presented. These cloud fines range from long, narrow lines of shallow convection to extensive deep convective squall lines with mesoscale stratiform rain areas. These lines are the predominant weather feature during periods of easterly flow over the Australian tropics.
Abstract
Analyses of mean sea level pressure, wind, temperature and dewpoint am used to study the life cycles of two intense, heavy-rain-producing monsoon depressions over northern Australia. Two aspects are considered: (a) the large forcing, using both synoptic flow field changes and angular momentum budgets, and (b) the role of convective and stratiform clouds using kinematic and thermodynamic budgets.
For each situation, the Northern Hemisphere circulation becomes favorable well prior to genesis. The short-term trigger for development is the strengthening of the Southern Hemisphere subtropical ridge at the surface and an amplifying upper-level trough and subtropical jetstreak to the southwest of the formation point.
The outer region structure of these monsoon depressions is remarkably similar to that of a tropical cyclone, even though the systems develop over land. During development, maximum convective heating occurs at middle levels and within a region of already high cyclonic vorticity. Evidence suggests that the cloud population is mostly comprised of deep cumulonimbus clouds, middle-level stratiform cloud and shallow cumulus. The physical significance of these findings is discussed.
Abstract
Analyses of mean sea level pressure, wind, temperature and dewpoint am used to study the life cycles of two intense, heavy-rain-producing monsoon depressions over northern Australia. Two aspects are considered: (a) the large forcing, using both synoptic flow field changes and angular momentum budgets, and (b) the role of convective and stratiform clouds using kinematic and thermodynamic budgets.
For each situation, the Northern Hemisphere circulation becomes favorable well prior to genesis. The short-term trigger for development is the strengthening of the Southern Hemisphere subtropical ridge at the surface and an amplifying upper-level trough and subtropical jetstreak to the southwest of the formation point.
The outer region structure of these monsoon depressions is remarkably similar to that of a tropical cyclone, even though the systems develop over land. During development, maximum convective heating occurs at middle levels and within a region of already high cyclonic vorticity. Evidence suggests that the cloud population is mostly comprised of deep cumulonimbus clouds, middle-level stratiform cloud and shallow cumulus. The physical significance of these findings is discussed.
Abstract
With the multitude of cloud clusters over tropical oceans, it has been perplexing that so few develop into tropical cyclones. The authors postulate that a major obstacle has been the complexity of scale interactions, particularly those on the mesoscale, which have only recently been observable. While there are well-known climatological requirements, these are by no means sufficient.
A major reason for this rarity is the essentially stochastic nature of the mesoscale interactions that precede and contribute to cyclone development. Observations exist for only a few forming cases. In these, the moist convection in the preformation environment is organized into mesoscale convective systems, each of which have associated mesoscale potential vortices in the midlevels. Interactions between these systems may lead to merger, growth to the surface, and development of both the nascent eye and inner rainbands of a tropical cyclone. The process is essentially stochastic, but the degree of stochasticity can be reduced by the continued interaction of the mesoscale systems or by environmental influences. For example a monsoon trough provides a region of reduced deformation radius, which substantially improves the efficiency of mesoscale vortex interactions and the amplitude of the merged vortices. Further, a strong monsoon trough provides a vertical wind shear that enables long-lived midlevel mesoscale vortices that are able to maintain, or even redevelop, the associated convective system.
The authors develop this hypothesis by use of a detailed case study of the formation of Tropical Cyclone Oliver observed during TOGA COARE (1993). In this case, two dominant mesoscale vortices interacted with a monsoon trough to separately produce a nascent eye and a major rainband. The eye developed on the edge of the major convective system, and the associated atmospheric warming was provided almost entirely by moist processes in the upper atmosphere, and by a combination of latent heating and adiabatic subsidence in the lower and middle atmosphere. The importance of mesoscale interactions is illustrated further by brief reference to the development of two typhoons in the western North Pacific.
Abstract
With the multitude of cloud clusters over tropical oceans, it has been perplexing that so few develop into tropical cyclones. The authors postulate that a major obstacle has been the complexity of scale interactions, particularly those on the mesoscale, which have only recently been observable. While there are well-known climatological requirements, these are by no means sufficient.
A major reason for this rarity is the essentially stochastic nature of the mesoscale interactions that precede and contribute to cyclone development. Observations exist for only a few forming cases. In these, the moist convection in the preformation environment is organized into mesoscale convective systems, each of which have associated mesoscale potential vortices in the midlevels. Interactions between these systems may lead to merger, growth to the surface, and development of both the nascent eye and inner rainbands of a tropical cyclone. The process is essentially stochastic, but the degree of stochasticity can be reduced by the continued interaction of the mesoscale systems or by environmental influences. For example a monsoon trough provides a region of reduced deformation radius, which substantially improves the efficiency of mesoscale vortex interactions and the amplitude of the merged vortices. Further, a strong monsoon trough provides a vertical wind shear that enables long-lived midlevel mesoscale vortices that are able to maintain, or even redevelop, the associated convective system.
The authors develop this hypothesis by use of a detailed case study of the formation of Tropical Cyclone Oliver observed during TOGA COARE (1993). In this case, two dominant mesoscale vortices interacted with a monsoon trough to separately produce a nascent eye and a major rainband. The eye developed on the edge of the major convective system, and the associated atmospheric warming was provided almost entirely by moist processes in the upper atmosphere, and by a combination of latent heating and adiabatic subsidence in the lower and middle atmosphere. The importance of mesoscale interactions is illustrated further by brief reference to the development of two typhoons in the western North Pacific.
Abstract
The diurnal variations in tropical cloudiness and tropospheric winds during the Australian Monsoon Experiment (AMEX) Phase II are documented and compared to those observed elsewhere. A diurnal variation in tropical cloudiness was found to be a consistent feature of both disturbed and undisturbed conditions. The tropical cloudiness, as inferred from satellite and radar data, averaged over the entire period of AMEX Phase II, was at a maximum during the morning over the ocean and during the late afternoon over the Arnhem Land region of northern Australia. The diurnal variation in high cloud, as measured by satellite was 3:2 over the ocean and 4:1 over Arnhem Land. Radar data indicated a 1 0: 1 variation in convection over Arnhem Land, a 2:1 variation over the neighboring ocean and a 3:2 variation in the stratiform echoes over both Ambem land and the neighboring mean.
Interaction between local circulations and the large scale flow was found to he associated with the observed diurnal variations in tropical cloudiness. The large scale monsoon circulation exhibited a diurnal oscillation with maxima in both the low-level easterly and equatorial westerly flow during the early morning. Variations in the vertical motion fields were in phase with the inferred cloudiness changes, but the midlevel maximum in vertical motion did not correspond with the strongest boundary layer convergence. The precise timing upward vertical motion over oceanic regions within the primary AMEX domain and the less reliably observed region to the north of Australia varied according to the degree of convective activity; consistent features were a maximum in vertical motion at 0830 LST during disturbed conditions and an 0230 LST maximum during suppressed conditions.
Abstract
The diurnal variations in tropical cloudiness and tropospheric winds during the Australian Monsoon Experiment (AMEX) Phase II are documented and compared to those observed elsewhere. A diurnal variation in tropical cloudiness was found to be a consistent feature of both disturbed and undisturbed conditions. The tropical cloudiness, as inferred from satellite and radar data, averaged over the entire period of AMEX Phase II, was at a maximum during the morning over the ocean and during the late afternoon over the Arnhem Land region of northern Australia. The diurnal variation in high cloud, as measured by satellite was 3:2 over the ocean and 4:1 over Arnhem Land. Radar data indicated a 1 0: 1 variation in convection over Arnhem Land, a 2:1 variation over the neighboring ocean and a 3:2 variation in the stratiform echoes over both Ambem land and the neighboring mean.
Interaction between local circulations and the large scale flow was found to he associated with the observed diurnal variations in tropical cloudiness. The large scale monsoon circulation exhibited a diurnal oscillation with maxima in both the low-level easterly and equatorial westerly flow during the early morning. Variations in the vertical motion fields were in phase with the inferred cloudiness changes, but the midlevel maximum in vertical motion did not correspond with the strongest boundary layer convergence. The precise timing upward vertical motion over oceanic regions within the primary AMEX domain and the less reliably observed region to the north of Australia varied according to the degree of convective activity; consistent features were a maximum in vertical motion at 0830 LST during disturbed conditions and an 0230 LST maximum during suppressed conditions.
Abstract
The AMEX observational dataset, with its high temporal and spatial resolution, has been used to study the formation and structure of Tropical Cyclones Irma and Jason. These systems developed and evolved entirely within the experiment's special observing network. The study is mostly based upon six hourly numerical analyses of the mass and wind fields, on 11 vertical levels over a 1.25 lat/long grid. The systems are traced from prior to the formation of a resolvable closed surface circulation to when they were operationally classified as tropical cyclones.
The discussion focuses on the synoptic to cyclone scale changes during formation. Time sections of various kinematic variables, together with an index of deep convection obtained from digital satellite cloud imagery, are used to trace the development.
Both systems developed during active phases of the monsoon and initially were of maximum intensity in the middle troposphere. Low level spinup occurred in three stages. The first stage was associated with the establishment of a favorable large-scale environment and the development of a closed, low-level circulation. The second stage was marked by a strengthening in the low-level outer circulation and the development of a deep vortex. The final stage was the transformation of the tropical depressions into tropical cyclones, and was indicated by a large increase in low-level convergence, a burst in inner core convection, and intensification of the low-level inner circulation.
The evolution of the flow during development agrees well with the results of earlier tropical cyclogenesis studies. Large scale spinup appears to be at least partly associated with downstream Rossby-wave dispersion leading to increases in low-level horizontal wind shear and eventually the formation and strengthening of the low-level outer circulation. For the final transformation to cyclone status we suggest that the establishment of favorable patterns of vertical wind shear and inward propagation of eddy angular momentum flux convergence in the upper troposphere were important for intensification. Thermodynamic structure changes suggest that maintenance, rather than triggering of core convection, was dependent on surface evaporation.
The role of the observed structure changes, together with the processes operating during each phase of development are documented and discussed.
Abstract
The AMEX observational dataset, with its high temporal and spatial resolution, has been used to study the formation and structure of Tropical Cyclones Irma and Jason. These systems developed and evolved entirely within the experiment's special observing network. The study is mostly based upon six hourly numerical analyses of the mass and wind fields, on 11 vertical levels over a 1.25 lat/long grid. The systems are traced from prior to the formation of a resolvable closed surface circulation to when they were operationally classified as tropical cyclones.
The discussion focuses on the synoptic to cyclone scale changes during formation. Time sections of various kinematic variables, together with an index of deep convection obtained from digital satellite cloud imagery, are used to trace the development.
Both systems developed during active phases of the monsoon and initially were of maximum intensity in the middle troposphere. Low level spinup occurred in three stages. The first stage was associated with the establishment of a favorable large-scale environment and the development of a closed, low-level circulation. The second stage was marked by a strengthening in the low-level outer circulation and the development of a deep vortex. The final stage was the transformation of the tropical depressions into tropical cyclones, and was indicated by a large increase in low-level convergence, a burst in inner core convection, and intensification of the low-level inner circulation.
The evolution of the flow during development agrees well with the results of earlier tropical cyclogenesis studies. Large scale spinup appears to be at least partly associated with downstream Rossby-wave dispersion leading to increases in low-level horizontal wind shear and eventually the formation and strengthening of the low-level outer circulation. For the final transformation to cyclone status we suggest that the establishment of favorable patterns of vertical wind shear and inward propagation of eddy angular momentum flux convergence in the upper troposphere were important for intensification. Thermodynamic structure changes suggest that maintenance, rather than triggering of core convection, was dependent on surface evaporation.
The role of the observed structure changes, together with the processes operating during each phase of development are documented and discussed.
Abstract
Line integral techniques are used to calculate vertically integrated heat and moisture budgets over the Gulf of Carpentaria during Phase II of the Australian Monsoon Experiment (AMEX). The budget area is an array of six radiosondes in a monsoon environment, and the calculations are performed every 6 hours over a period of 33 days.
During convective outbreaks the integrated heating and drying of the large scale by the cumulonimbus activity has a magnitude of the order of 10°C day−1. The heat and moisture sources are dominated by the flux divergence terms, which account for over 90% of the variance. The observed warming is as large as ±1°C day−1 but is diurnally dominated and does not correspond to the latent heat release. The integrated moisture convergence has a high correlation with latent heat release but not with the measured moisture storage. The convective heat source is also highly correlated with middle tropospheric vertical velocity.
Mean budgets are presented for each of the four diurnal observation times. Also, budgets were run with each station, in turn, excluded from the sonde array to determine sensitivity of the results to the data network.
Abstract
Line integral techniques are used to calculate vertically integrated heat and moisture budgets over the Gulf of Carpentaria during Phase II of the Australian Monsoon Experiment (AMEX). The budget area is an array of six radiosondes in a monsoon environment, and the calculations are performed every 6 hours over a period of 33 days.
During convective outbreaks the integrated heating and drying of the large scale by the cumulonimbus activity has a magnitude of the order of 10°C day−1. The heat and moisture sources are dominated by the flux divergence terms, which account for over 90% of the variance. The observed warming is as large as ±1°C day−1 but is diurnally dominated and does not correspond to the latent heat release. The integrated moisture convergence has a high correlation with latent heat release but not with the measured moisture storage. The convective heat source is also highly correlated with middle tropospheric vertical velocity.
Mean budgets are presented for each of the four diurnal observation times. Also, budgets were run with each station, in turn, excluded from the sonde array to determine sensitivity of the results to the data network.
