A CHARACTERISTIC LIFE-CYCLE OF UPPER-TROPOSPHERIC CYCLOGENETIC PRECURSORS DURING THE EXPERIMENT ON RAPIDLY INTENSIFYING CYCLONES OVER THE ATLANTIC (ERICA)
Gm. Lackmann et al., A CHARACTERISTIC LIFE-CYCLE OF UPPER-TROPOSPHERIC CYCLOGENETIC PRECURSORS DURING THE EXPERIMENT ON RAPIDLY INTENSIFYING CYCLONES OVER THE ATLANTIC (ERICA), Monthly weather review, 125(11), 1997, pp. 2729-2758
This paper documents a characteristic life cycle of upper-tropospheric
precursors to surface cyclogenesis observed during the field phase of
the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ER
ICA, December 1988-February 1989). This life cycle begins with the dev
elopment of an elongated region of lower dynamic tropopause that forms
in association with an intensifying midtropospheric jet/front over ce
ntral North America. The elongated disturbance subsequently compacts i
nto a more circular configuration prior to crossing the east coast of
North America and frequently is associated with rapid surface cyclogen
esis offshore. A representative example of the life cycle outlined abo
ve is documented through a detailed case study of the upper precursor
associated with the second ERICA intensive observation period (IOP 2)
cyclone. Emphasis is placed upon (i) description of the tropopause str
ucture and evolution during the upper-precursor life cycle, (ii) diagn
osis of mechanisms leading to the development and intensification of a
midtropospheric cyclonic vorticity maximum and frontal zone, (iii) an
alysis of the role of transverse jet-front circulations in deforming t
he dynamic tropopause, (iv) documentation of the influence of the low-
and high-frequency flow components on the upper-precursor life cycle,
and (v) isolation of dynamic and thermodynamic factors that render thi
s life cycle especially conducive to rapid surface cyclogenesis. Confl
uence downstream of the axis of a low-frequency (i.e., periods greater
than 120 h), troposphere-deep ridge over western North America facili
tates the organization of a midtropospheric jet/front over central Nor
th America. As this precursor disturbance approaches the inflection be
tween the western ridge and a downstream trough, tilting, in the prese
nce of cold advection along the midtropospheric frontal zone, becomes
an important vorticity generation and frontogenesis mechanism in the u
pper precursor. Transverse circulations accompanying the jet/front ste
epen and lower the dynamic tropopause prior to surface cyclogenesis. C
ompaction of the initially elongated upper precursor is shown to invol
ve deformation in the high-frequency component of the upper-tropospher
ic flow. The compacted upper-precursor configuration, lowered tropopau
se, and reduced static stability in the offshore environment lead to s
trong vertical coupling and vigorous surface cyclogenesis as the upper
precursor passes offshore. The foregoing results suggest that the lif
e cycle of a common class of cyclogenetic precursors is closely relate
d to midtropospheric frontogenesis. A favored location for the develop
ment of midtropospheric jet/fronts is over central North America durin
g northwesterly flow episodes. Production of vorticity in the midtropo
spheric jet/front and subsequent compaction of this vorticity feature
suggest a link between midtropospheric frontogenesis and mobile upper-
trough genesis. This link may explain the existence of a maximum in th
e upper-trough-genesis distribution over central North America documen
ted by Sanders.