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The Role of Wave Breaking, Linear Instability, and PV Transports in Model Block Onset *

The Role of Wave Breaking, Linear Instability, and PV Transports in Model Block Onset * To understand mechanisms responsible for the onset of atmospheric blocks, the authors study model blocks that form in a two-layer isentropic primitive equation model. The latter includes diabatic heating, parameterized as a Newtonian relaxation of the actual interface toward an equilibrium interface, and a zonal wavenumber-2 orography. The study concentrates on four different blocking events. One of the blocks is present in the control run, while the remaining three are excited by appropriate perturbation of the model’s state vector at preselected times when the prevailing flows are classified as zonal. With the parameter calibration chosen in this investigation two phases in the formation of the blocks are conveniently identified: The first phase consists of the formation of cutoff or nearly cutoff cyclones in the upper layer at low latitudes, and the second phase features a rapid intensification of the upper-layer blocking ridge, accompanied by advection of high potential vorticity (PV) beneath it. While the initiation of the first phase may be perceived as far back in time as 6 days before the second phase, the latter occurs on a timescale of 1 to 2 days, giving rise to a well-defined blocking pattern. The first phase features the Simmons–Hoskins basic baroclinic life cycle in the total PV field that acts as a conditioner of the large-scale flow for the second phase to occur. The authors hypothesize that the second phase consists of (intense) instability of normal mode form, very much as in the theory of barotropic and baroclinic instability of three-dimensional basic-state flows for the onset of blocks. From a different perspective, based on the concept of interaction between different scales of motion, both phases predominantly involve the transport of synoptic-scale potential vorticity by the planetary waves. Planetary–planetary interactions are, however, nonnegligible. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of the Atmospheric Sciences American Meteorological Society

The Role of Wave Breaking, Linear Instability, and PV Transports in Model Block Onset *

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Publisher
American Meteorological Society
Copyright
Copyright © 1997 American Meteorological Society
ISSN
1520-0469
DOI
10.1175/1520-0469(1998)055<2852:TROWBL>2.0.CO;2
Publisher site
See Article on Publisher Site

Abstract

To understand mechanisms responsible for the onset of atmospheric blocks, the authors study model blocks that form in a two-layer isentropic primitive equation model. The latter includes diabatic heating, parameterized as a Newtonian relaxation of the actual interface toward an equilibrium interface, and a zonal wavenumber-2 orography. The study concentrates on four different blocking events. One of the blocks is present in the control run, while the remaining three are excited by appropriate perturbation of the model’s state vector at preselected times when the prevailing flows are classified as zonal. With the parameter calibration chosen in this investigation two phases in the formation of the blocks are conveniently identified: The first phase consists of the formation of cutoff or nearly cutoff cyclones in the upper layer at low latitudes, and the second phase features a rapid intensification of the upper-layer blocking ridge, accompanied by advection of high potential vorticity (PV) beneath it. While the initiation of the first phase may be perceived as far back in time as 6 days before the second phase, the latter occurs on a timescale of 1 to 2 days, giving rise to a well-defined blocking pattern. The first phase features the Simmons–Hoskins basic baroclinic life cycle in the total PV field that acts as a conditioner of the large-scale flow for the second phase to occur. The authors hypothesize that the second phase consists of (intense) instability of normal mode form, very much as in the theory of barotropic and baroclinic instability of three-dimensional basic-state flows for the onset of blocks. From a different perspective, based on the concept of interaction between different scales of motion, both phases predominantly involve the transport of synoptic-scale potential vorticity by the planetary waves. Planetary–planetary interactions are, however, nonnegligible.

Journal

Journal of the Atmospheric Sciences – American Meteorological Society

Published: Jun 23, 1997

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