ABSTRACT
Landfalling and transitioning tropical cyclones (TCs) over the northeast U.S.
often result in high-impact weather events with heavy rainfall and dangerous flash
flooding. Mesoscale features found in these storms not only exacerbate flooding on
smaller scales, but also provide an important forecast challenge. The purpose of this
thesis is to document how the observed mesoscale distribution of heavy rainfall in
landfalling and transitioning TCs is modulated by interactions between synoptic and
mesoscale disturbances in the presence of the complex physiography of the northeast
U.S.
A 52-storm (1950–2006) dataset of landfalling and transitioning TCs that
produced storm-total rainfall amounts of at least 100 mm has been assembled to identify
candidate storms for a climatology study. Three representative storms from this dataset
were selected for detailed study in order to document and illustrate the important
dynamical processes that modulate the precipitation distribution relative to the TC track.
The large-scale distribution of rainfall and embedded mesoscale precipitation structures
accompanying TCs Ivan (2004), Jeanne (2004), and Ernesto (2006) are identified using
WSI Corporation radar reflectivity data and by constructing analyses of storm-total and
6-h rainfall from the 10-km National Weather Service River Forecast Center quantitative
precipitation estimates dataset. Gridded datasets from the National Centers for
Environmental Prediction 1.0° Global Forecast System are used to investigate how
synoptic and mesoscale processes determine the precipitation distribution. A natural
coordinate partitioning of the Q vector into along-isentrope (Qs) and cross-isentrope (Qn)
components, along with the divergence of the respective components, is used to help
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identify areas where quasigeostrophic forcing for ascent could provide a favorable
environment for mesoscale frontogenesis and heavy rainfall.
Characteristic lower- and upper-level flow signatures were identified from case
studies of heavy precipitation events that occurred in conjunction with TCs Ivan, Jeanne,
and Ernesto, including the occurrence of TC-induced upper-level downstream ridge
development over the eastern U.S. and western North Atlantic. Each TC interacted with
a midlatitude upper-level trough and associated upper-level jet streak, and a lower-level
warm-frontal boundary positioned to the north of the TC to produce conditions favorable
for heavy precipitation in advance of the TC. The location of the upper-level trough/jet
streak and the vertical structure of the warm-frontal boundary were correlated with
distinct precipitation distributions relative to the TC track. The warm-frontal boundary
associated with a left-of-track precipitation distribution (Ivan and Jeanne) corresponded
to a tropospheric-deep frontogenesis and ascent pattern that tilted toward cooler air with
height, while the warm-frontal boundary associated with a right-of-track precipitation
distribution (Ernesto) corresponded to strong frontogenesis that was focused near the
surface and strong, upright ascent that was maximized immediately above the boundary
layer.
Q vector diagnostics for TCs Ivan, Jeanne, and Ernesto revealed that heavy
precipitation accompanying each TC occurred in the presence of both low-level Qs and
Qn forcing for ascent. In all three cases, strong frontogenesis produced a banded pattern
of Qn forcing for descent–ascent positioned within a low-level warm-frontal boundary
beneath the equatorward entrance region of an upper-level jet streak. The strong cyclonic
circulation associated with each TC generated a dipole pattern of Qs forcing for ascent
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(descent) located between the TC and a downshear (upshear) low-level thermal ridge
(trough) over the eastern U.S. A cyclonically curved low-level jet located on the eastern
side of the TC enhanced the northward flux of tropical air from the Atlantic into the lowlevel warm-frontal boundary. All three TCs were accompanied by mesoscale bands of
enhanced rainfall that occurred along and on the cool side of the surface warm-frontal
boundary. Finally, conceptual models are provided to assist forecasters in identifying the
key ingredients for heavy rainfall from landfalling and transitioning TCs.
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