Monday, 12 August 2002: 4:52 PM
Outstanding issues related to bow echoes and derechoes component of BAMEX
Bow echoes and derechoes continue to pose a significant forecast problem
over much of the nation. Although there has been much research
to date on the dynamics and severe weather potential of
these phenomena, there has never been an
observational program that has collected sufficient kinematic and
thermodynamic data to truly document the lifecycle
of these events and to test recent hypotheses as to the
critical physical processes involved. Clearly, many
outstanding issues still remain. For instance, while most of the
arguments proposed to explain the development of severe winds within bow
echoes depend on the development of a strong, deep cold pool and
associated mesohigh, severe bow echoes and derechoes can occur at night
in the presence of a stable nocturnal boundary layer that does not as
readily support the generation of the strong surface cold pool.
Observational studies also suggest the importance of pre-existing,
line-normal thermal boundaries to the formation, propagation, and severe
weather associated with bow echoes. Most numerical studies of bow
echoes to date, however, have only considered highly idealized
environments, characterized by horizontally homogeneous initial states.
Issuing appropriate warnings for bow-echo tornadoes is especially
problematic. More often than not, there is no readily identifiable
supercell signatures or mid-level mesocyclone preceding such tornadoes.
The fact that non-supercell tornadoes develop primarily northward of the
apex of the bow has not yet been explained. The identification of
Doppler radar precursors, such as MARC (Mid-Altitude Radial
Convergence), to help forecasters warn for severe wind events, are
progressing, but need more detailed datsets to better understand their
relationship with severe wind production and to refine its applications.
These, and other issues will be reviewed and discussed in
the context of planning for BAMEX operations.
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