Search Results
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
Historical records of aviation turbulence encounters above Greenland are examined for the period from 2000 to 2006. These data identify an important flow regime that contributes to the occurrence of aircraft turbulence encounters, associated with the passage of surface cyclones that direct easterly or southeasterly flow over Greenland’s imposing terrain. The result of this incident flow is the generation of mountain waves that may become unstable through interactions with the background directional wind shear. It is shown that this regime accounted for approximately 40% of the significant turbulent events identified in the 7-yr database. In addition, two specific cases from the database are examined in more detail using a high-resolution mesoscale model. The model simulations highlight the important role of three-dimensional gravity wave–critical level interactions and demonstrate the utility of high-resolution forecasts in the prediction of such events.
Abstract
Historical records of aviation turbulence encounters above Greenland are examined for the period from 2000 to 2006. These data identify an important flow regime that contributes to the occurrence of aircraft turbulence encounters, associated with the passage of surface cyclones that direct easterly or southeasterly flow over Greenland’s imposing terrain. The result of this incident flow is the generation of mountain waves that may become unstable through interactions with the background directional wind shear. It is shown that this regime accounted for approximately 40% of the significant turbulent events identified in the 7-yr database. In addition, two specific cases from the database are examined in more detail using a high-resolution mesoscale model. The model simulations highlight the important role of three-dimensional gravity wave–critical level interactions and demonstrate the utility of high-resolution forecasts in the prediction of such events.
