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- Author or Editor: Todd P. Lane x
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Abstract
Diabatic heating in the Maritime Continent region is controlled by a unique blend of mesoscale variability associated with steep topography and complex coastlines and intraseasonal variability associated with propagating planetary-scale disturbances. In this study, the diabatic heating from a 10-yr austral summer simulation over the Maritime Continent with a 4-km horizontal grid length is analyzed with respect to diurnal, spatial, and intraseasonal variations. Results are compared, where possible, to analogous estimates from the TRMM precipitation radar. We show that the heating budget is largely a balance between latent heating and vertical advection, with rays of heating and cooling extending upward and outward from the coast evident in the advection terms, consistent with the gravity wave representation of the tropical sea breeze. By classifying rainfall into convective and stratiform components, it is shown that simulated convective heating over Sumatra peaks in MJO phases 2 and 3, while simulated stratiform heating peaks in phase 4. Similarly, spectral latent heating estimates from the TRMM Precipitation Radar show that stratiform heating peaks in phases 3 and 4, while convective heating peaks in phases 2 and 3. It is also shown that stratiform precipitation plays a greater role in offshore precipitation during the night, albeit with embedded convective cores, than over the land during the day. These results emphasize the importance of achieving a realistic representation of convective and stratiform processes in high-resolution simulations in the tropics, both for total rainfall estimates and for realistic latent heating.
Abstract
Diabatic heating in the Maritime Continent region is controlled by a unique blend of mesoscale variability associated with steep topography and complex coastlines and intraseasonal variability associated with propagating planetary-scale disturbances. In this study, the diabatic heating from a 10-yr austral summer simulation over the Maritime Continent with a 4-km horizontal grid length is analyzed with respect to diurnal, spatial, and intraseasonal variations. Results are compared, where possible, to analogous estimates from the TRMM precipitation radar. We show that the heating budget is largely a balance between latent heating and vertical advection, with rays of heating and cooling extending upward and outward from the coast evident in the advection terms, consistent with the gravity wave representation of the tropical sea breeze. By classifying rainfall into convective and stratiform components, it is shown that simulated convective heating over Sumatra peaks in MJO phases 2 and 3, while simulated stratiform heating peaks in phase 4. Similarly, spectral latent heating estimates from the TRMM Precipitation Radar show that stratiform heating peaks in phases 3 and 4, while convective heating peaks in phases 2 and 3. It is also shown that stratiform precipitation plays a greater role in offshore precipitation during the night, albeit with embedded convective cores, than over the land during the day. These results emphasize the importance of achieving a realistic representation of convective and stratiform processes in high-resolution simulations in the tropics, both for total rainfall estimates and for realistic latent heating.
Abstract
Changes in the diurnal precipitation cycle as the Madden–Julian oscillation (MJO) propagates through the Maritime Continent are investigated to explore the processes behind seaward-propagating precipitation northeast of New Guinea. Satellite rainfall estimates from TRMM 3B42 and the Climate Prediction Center morphing technique (CMORPH) are combined with simulations from the Weather Research and Forecasting (WRF) Model with a horizontal resolution of 4 km.
Comparison with 24-h rain gauge measurements indicates that both satellite estimates and the WRF Model exhibit systematic biases. Despite these biases, the changing patterns of offshore precipitation with the passage of the MJO show good consistency between satellite estimates and the WRF Model. In the few days prior to the main MJO envelope, light background wind, relatively clear skies, and an increasingly moist environment promote favorable conditions for the diurnal precipitation cycle.
Two distinct processes are identified: 100–200 km from the coast, precipitation moves offshore as a squall line with a propagation speed of 3–5 m s−1. Farther offshore, precipitation propagates with a speed close to 18 m s−1and is associated with an inertia–gravity wave generated by diurnally oscillating heating from radiative and moist convective processes over the land. A gravity wave signature is evident even after the MJO active period when there is little precipitation. By correcting for the background flow perpendicular to the coast, potential temperature anomalies for the lead-up, active, and follow-on MJO periods are shown to collapse to a remarkably invariant shape for a given time of day.
Abstract
Changes in the diurnal precipitation cycle as the Madden–Julian oscillation (MJO) propagates through the Maritime Continent are investigated to explore the processes behind seaward-propagating precipitation northeast of New Guinea. Satellite rainfall estimates from TRMM 3B42 and the Climate Prediction Center morphing technique (CMORPH) are combined with simulations from the Weather Research and Forecasting (WRF) Model with a horizontal resolution of 4 km.
Comparison with 24-h rain gauge measurements indicates that both satellite estimates and the WRF Model exhibit systematic biases. Despite these biases, the changing patterns of offshore precipitation with the passage of the MJO show good consistency between satellite estimates and the WRF Model. In the few days prior to the main MJO envelope, light background wind, relatively clear skies, and an increasingly moist environment promote favorable conditions for the diurnal precipitation cycle.
Two distinct processes are identified: 100–200 km from the coast, precipitation moves offshore as a squall line with a propagation speed of 3–5 m s−1. Farther offshore, precipitation propagates with a speed close to 18 m s−1and is associated with an inertia–gravity wave generated by diurnally oscillating heating from radiative and moist convective processes over the land. A gravity wave signature is evident even after the MJO active period when there is little precipitation. By correcting for the background flow perpendicular to the coast, potential temperature anomalies for the lead-up, active, and follow-on MJO periods are shown to collapse to a remarkably invariant shape for a given time of day.
Abstract
This study explores the mesoscale processes that led to the development of two prefrontal precipitation events in the Australian Alps on 29–30 October 2010. The synoptic setting was characterized by the passage of an interacting front and prefrontal trough across southern Australia. Observations and model simulations revealed that when the prefrontal trough entered southeast Australia it resembled a density current advancing into a stable nocturnal layer, forming a bore at its leading edge. The bore detached from and propagated ahead of the prefrontal trough and became undular, supported by a wave-ducting mechanism. The undular bore was observed in the Doppler wind field of a radar, parts of which were collocated with bands of reflectivity. Strong winds coincident with this band of reflectivity suggest the undular bore triggered convection that eventually led to the bore’s demise. An ensemble of high-resolution model simulations (with perturbed initial and boundary conditions) was used to understand the key processes affecting the undular bore and two prefrontal precipitation events. While no member of the ensemble reproduced the first prefrontal precipitation event, at least six members (20%) reproduced parts of the second prefrontal precipitation event. Despite the low precipitation predictability, analysis of the ensemble suggests the undular bore was both a predictable phenomenon and integral to the initiation and/or evolution of the two prefrontal precipitation events.
Abstract
This study explores the mesoscale processes that led to the development of two prefrontal precipitation events in the Australian Alps on 29–30 October 2010. The synoptic setting was characterized by the passage of an interacting front and prefrontal trough across southern Australia. Observations and model simulations revealed that when the prefrontal trough entered southeast Australia it resembled a density current advancing into a stable nocturnal layer, forming a bore at its leading edge. The bore detached from and propagated ahead of the prefrontal trough and became undular, supported by a wave-ducting mechanism. The undular bore was observed in the Doppler wind field of a radar, parts of which were collocated with bands of reflectivity. Strong winds coincident with this band of reflectivity suggest the undular bore triggered convection that eventually led to the bore’s demise. An ensemble of high-resolution model simulations (with perturbed initial and boundary conditions) was used to understand the key processes affecting the undular bore and two prefrontal precipitation events. While no member of the ensemble reproduced the first prefrontal precipitation event, at least six members (20%) reproduced parts of the second prefrontal precipitation event. Despite the low precipitation predictability, analysis of the ensemble suggests the undular bore was both a predictable phenomenon and integral to the initiation and/or evolution of the two prefrontal precipitation events.
Abstract
The realism of convective organization in operational convection permitting model simulations is objectively assessed, with a particular focus on the mesoscale aspects, such as convective mode. A tracking and classification algorithm is applied to observed radar reflectivity and simulated radar reflectivity from the operational ACCESS-C convection permitting forecast domain over northern Australia between October 2020 and May 2022, and characteristics of real and simulated convective organization compared. Mesoscale convective systems from the operational forecast model are approximately twice as likely to be oriented parallel to the ambient wind and ambient wind shear than those observed by radar, indicating a bias toward the “training line” systems typically associated with more extreme rainfall. During highly humid active monsoon conditions, simulated convective systems have larger ground-relative speeds than systems observed in radar. Although there is less than 5% difference between the ratios of simulated and observed trailing, leading and parallel stratiform system observations, significant differences exist in other wind-shear-based classifications. For instance, in absolute terms, simulated systems are 10–35% less likely to be up-shear tilted, and 15-30% less likely to be down-shear propagating than observed systems, suggesting errors in simulated cold pool characteristics.
Abstract
The realism of convective organization in operational convection permitting model simulations is objectively assessed, with a particular focus on the mesoscale aspects, such as convective mode. A tracking and classification algorithm is applied to observed radar reflectivity and simulated radar reflectivity from the operational ACCESS-C convection permitting forecast domain over northern Australia between October 2020 and May 2022, and characteristics of real and simulated convective organization compared. Mesoscale convective systems from the operational forecast model are approximately twice as likely to be oriented parallel to the ambient wind and ambient wind shear than those observed by radar, indicating a bias toward the “training line” systems typically associated with more extreme rainfall. During highly humid active monsoon conditions, simulated convective systems have larger ground-relative speeds than systems observed in radar. Although there is less than 5% difference between the ratios of simulated and observed trailing, leading and parallel stratiform system observations, significant differences exist in other wind-shear-based classifications. For instance, in absolute terms, simulated systems are 10–35% less likely to be up-shear tilted, and 15-30% less likely to be down-shear propagating than observed systems, suggesting errors in simulated cold pool characteristics.
Abstract
This study uses a two-dimensional cloud-resolving model to examine how convectively generated gravity waves modify the environment of an isolated convective cloud. The model is initialized with an idealized sounding, and the cloud is initiated by adding a locally buoyant perturbation. The modeled convection generates a spectrum of gravity waves with vertical wavelengths that are harmonics of the depth of the troposphere. It is shown that the first three wave modes significantly modify the cloud environment.
The modification of the cloud environment is quantified in terms of the convective available potential energy (CAPE) and convective inhibition (CIN). The first two wave modes travel fastest away from the cloud and are responsible for the changes in CAPE, whereas the third wave mode causes low-level lifting and hence a reduction in CIN. The maximum far-field perturbations in CAPE and CIN are approximately 15% and 33% of the initial background values, respectively. These results agree with previous studies of more organized convection, predicting the existence of a region surrounding the convective system that favors the development of new convection.
Abstract
This study uses a two-dimensional cloud-resolving model to examine how convectively generated gravity waves modify the environment of an isolated convective cloud. The model is initialized with an idealized sounding, and the cloud is initiated by adding a locally buoyant perturbation. The modeled convection generates a spectrum of gravity waves with vertical wavelengths that are harmonics of the depth of the troposphere. It is shown that the first three wave modes significantly modify the cloud environment.
The modification of the cloud environment is quantified in terms of the convective available potential energy (CAPE) and convective inhibition (CIN). The first two wave modes travel fastest away from the cloud and are responsible for the changes in CAPE, whereas the third wave mode causes low-level lifting and hence a reduction in CIN. The maximum far-field perturbations in CAPE and CIN are approximately 15% and 33% of the initial background values, respectively. These results agree with previous studies of more organized convection, predicting the existence of a region surrounding the convective system that favors the development of new convection.
Abstract
This study examines how variations to the nondimensional mountain height Ĥ and the horizontal aspect ratio β of a straight ridge and a concave ridge influence orographic precipitation. An idealized three-dimensional model is used to simulate a moist flow impinging upon these two ridges with Ĥ = 0.66–2.0 and β = 1.0–8.0. The concave ridge generates substantially more precipitation than the straight ridge via an established precipitation-enhancing funneling mechanism near the ridge vertex when the flow is unblocked. Based on previous work, it was hypothesized that when the approaching flow becomes blocked, the strength of the precipitation enhancement by the concave ridge relative to the straight ridge becomes negligible. This study reveals that, if Ĥ is sufficiently large to induce flow reversal on the windward slope, then a secondary circulation develops that is strengthened by the concave ridge with a subsequent enhancement of precipitation. It is also shown that the competing effects of the ridge length and width render the strength of the precipitation enhancement largely insensitive to β. A flow regime diagram for the straight ridge and the concave ridge is also constructed to illustrate the sensitivity of the critical Ĥ value for flow regime transition to changes in the terrain geometry; variations to the low-level relative humidity are also explored.
Abstract
This study examines how variations to the nondimensional mountain height Ĥ and the horizontal aspect ratio β of a straight ridge and a concave ridge influence orographic precipitation. An idealized three-dimensional model is used to simulate a moist flow impinging upon these two ridges with Ĥ = 0.66–2.0 and β = 1.0–8.0. The concave ridge generates substantially more precipitation than the straight ridge via an established precipitation-enhancing funneling mechanism near the ridge vertex when the flow is unblocked. Based on previous work, it was hypothesized that when the approaching flow becomes blocked, the strength of the precipitation enhancement by the concave ridge relative to the straight ridge becomes negligible. This study reveals that, if Ĥ is sufficiently large to induce flow reversal on the windward slope, then a secondary circulation develops that is strengthened by the concave ridge with a subsequent enhancement of precipitation. It is also shown that the competing effects of the ridge length and width render the strength of the precipitation enhancement largely insensitive to β. A flow regime diagram for the straight ridge and the concave ridge is also constructed to illustrate the sensitivity of the critical Ĥ value for flow regime transition to changes in the terrain geometry; variations to the low-level relative humidity are also explored.
Abstract
An idealized cloud-system-resolving model simulation is used to examine the coupling between a tropical cloud population and the mesoscale gravity waves that it generates. Spectral analyses of the cloud and gravity wave fields identify a clear signal of coupling between the clouds and a deep tropospheric gravity wave mode with a vertical wavelength that matches the depth of the convection, which is about two-thirds of the tropospheric depth. This vertical wavelength and the period of the waves, defined by a characteristic convective time scale, means that the horizontal wavelength is constrained through the dispersion relation. Indeed, the wave–convection coupling manifests at the appropriate wavelength, with the emergence of quasi-regular cloud-system spacing of order 100 km. It is shown that cloud systems at this spacing achieve a quasi-resonant state, at least for a few convective life cycles. Such regular spacing is a key component of cloud organization and is likely a contributor to the processes controlling the upscale growth of convective systems. Other gravity wave processes are also elucidated, including their apparent role in the maintenance of convective systems by providing a mechanism for renewed convective activity and system longevity.
Abstract
An idealized cloud-system-resolving model simulation is used to examine the coupling between a tropical cloud population and the mesoscale gravity waves that it generates. Spectral analyses of the cloud and gravity wave fields identify a clear signal of coupling between the clouds and a deep tropospheric gravity wave mode with a vertical wavelength that matches the depth of the convection, which is about two-thirds of the tropospheric depth. This vertical wavelength and the period of the waves, defined by a characteristic convective time scale, means that the horizontal wavelength is constrained through the dispersion relation. Indeed, the wave–convection coupling manifests at the appropriate wavelength, with the emergence of quasi-regular cloud-system spacing of order 100 km. It is shown that cloud systems at this spacing achieve a quasi-resonant state, at least for a few convective life cycles. Such regular spacing is a key component of cloud organization and is likely a contributor to the processes controlling the upscale growth of convective systems. Other gravity wave processes are also elucidated, including their apparent role in the maintenance of convective systems by providing a mechanism for renewed convective activity and system longevity.
Abstract
Over the past decade, numerous numerical modeling studies have shown that deep convective clouds can produce gravity waves that induce a significant vertical flux of horizontal momentum. Such studies used models with horizontal grid spacings of O(1 km) and produced strong gravity waves with horizontal wavelengths greater than about 20 km. This paper is an examination of how simulated gravity waves and their momentum flux are sensitive to model resolution. It is shown that increases in horizontal resolution produce more power in waves with shorter horizontal wavelengths. This change in the gravity waves’ spectra influences their vertical propagation. In some cases, gravity waves that were vertically propagating in coarse simulations become vertically trapped in fine simulations, which strongly influences the vertical flux of horizontal momentum.
Abstract
Over the past decade, numerous numerical modeling studies have shown that deep convective clouds can produce gravity waves that induce a significant vertical flux of horizontal momentum. Such studies used models with horizontal grid spacings of O(1 km) and produced strong gravity waves with horizontal wavelengths greater than about 20 km. This paper is an examination of how simulated gravity waves and their momentum flux are sensitive to model resolution. It is shown that increases in horizontal resolution produce more power in waves with shorter horizontal wavelengths. This change in the gravity waves’ spectra influences their vertical propagation. In some cases, gravity waves that were vertically propagating in coarse simulations become vertically trapped in fine simulations, which strongly influences the vertical flux of horizontal momentum.
Abstract
Deep moist convection generates turbulence in the clear air above and around developing clouds, penetrating convective updrafts and mature thunderstorms. This turbulence can be due to shearing instabilities caused by strong flow deformations near the cloud top, and also to breaking gravity waves generated by cloud–environment interactions. Turbulence above and around deep convection is an important safety issue for aviation, and improved understanding of the conditions that lead to out-of-cloud turbulence formation may result in better turbulence avoidance guidelines or forecasting capabilities. In this study, a series of high-resolution two- and three-dimensional model simulations of a severe thunderstorm are conducted to examine the sensitivity of above-cloud turbulence to a variety of background flow conditions—in particular, the above-cloud wind shear and static stability. Shortly after the initial convective overshoot, the above-cloud turbulence and mixing are caused by local instabilities in the vicinity of the cloud interfacial boundary. At later times, when the convection is more mature, gravity wave breaking farther aloft dominates the turbulence generation. This wave breaking is caused by critical-level interactions, where the height of the critical level is controlled by the above-cloud wind shear. The strength of the above-cloud wind shear has a strong influence on the occurrence and intensity of above-cloud turbulence, with intermediate shears generating more extensive regions of turbulence, and strong shear conditions producing the most intense turbulence. Also, more stable above-cloud environments are less prone to turbulence than less stable situations. Among other things, these results highlight deficiencies in current turbulence avoidance guidelines in use by the aviation industry.
Abstract
Deep moist convection generates turbulence in the clear air above and around developing clouds, penetrating convective updrafts and mature thunderstorms. This turbulence can be due to shearing instabilities caused by strong flow deformations near the cloud top, and also to breaking gravity waves generated by cloud–environment interactions. Turbulence above and around deep convection is an important safety issue for aviation, and improved understanding of the conditions that lead to out-of-cloud turbulence formation may result in better turbulence avoidance guidelines or forecasting capabilities. In this study, a series of high-resolution two- and three-dimensional model simulations of a severe thunderstorm are conducted to examine the sensitivity of above-cloud turbulence to a variety of background flow conditions—in particular, the above-cloud wind shear and static stability. Shortly after the initial convective overshoot, the above-cloud turbulence and mixing are caused by local instabilities in the vicinity of the cloud interfacial boundary. At later times, when the convection is more mature, gravity wave breaking farther aloft dominates the turbulence generation. This wave breaking is caused by critical-level interactions, where the height of the critical level is controlled by the above-cloud wind shear. The strength of the above-cloud wind shear has a strong influence on the occurrence and intensity of above-cloud turbulence, with intermediate shears generating more extensive regions of turbulence, and strong shear conditions producing the most intense turbulence. Also, more stable above-cloud environments are less prone to turbulence than less stable situations. Among other things, these results highlight deficiencies in current turbulence avoidance guidelines in use by the aviation industry.
Abstract
This study explores the generation of turbulence in the upper outflow regions of simulated idealized mesoscale convective systems. The simulated storms are shown to generate parameterized turbulence that occurs significant distances (>100 km) from the main convective regions, in both the clear air surrounding the convection and low simulated reflectivity regions with cloud ice but negligible amounts of graupel and snow. The source of the turbulence is related to Kelvin–Helmholtz instabilities that occur in the shear zones above and below the storm-induced upper-level outflow jet that is centered near the tropopause; the model produces resolved-scale billows within regions of low gradient Richardson number. Short-scale gravity waves are also coincident with the regions of turbulence, become trapped within the jet core, and appear to be generated by the shear instability. Additional experiments with different initial upper-level wind shear show similar mechanisms to those simulations with no initial upper-level shear. These results help elucidate the dynamics of turbulence generation near convection, which has important implications for the aviation industry and the fundamental understanding of how convective clouds interact with their environment.
Abstract
This study explores the generation of turbulence in the upper outflow regions of simulated idealized mesoscale convective systems. The simulated storms are shown to generate parameterized turbulence that occurs significant distances (>100 km) from the main convective regions, in both the clear air surrounding the convection and low simulated reflectivity regions with cloud ice but negligible amounts of graupel and snow. The source of the turbulence is related to Kelvin–Helmholtz instabilities that occur in the shear zones above and below the storm-induced upper-level outflow jet that is centered near the tropopause; the model produces resolved-scale billows within regions of low gradient Richardson number. Short-scale gravity waves are also coincident with the regions of turbulence, become trapped within the jet core, and appear to be generated by the shear instability. Additional experiments with different initial upper-level wind shear show similar mechanisms to those simulations with no initial upper-level shear. These results help elucidate the dynamics of turbulence generation near convection, which has important implications for the aviation industry and the fundamental understanding of how convective clouds interact with their environment.
