V. Conclusions and Suggestions for Future Work
5.1 Conclusions
The study of tropical cyclones impacting the northeastern US has occurred for
approximately 65 years, beginning with the analysis of synoptic-scale parameters during
the northward propagation of the great New England Hurricane of 1938 (Pierce 1939).
The subsequent years of research have produced results that have progressively revealed
synoptic and mesoscale features responsible for observed precipitation distributions in a
very small subset of storms. The goals of the current research effort were to investigate
a larger set of storms, from which the importance of synoptic and mesoscale features to
observed precipitation distributions could be individually and collectively examined.
Working in conjunction with the NWS, it is hoped that integration of these research
results into an operational forecast setting (schematic diagram, Fig. 4.1) will ultimately
lead to more accurate and timely forecasts during situations of catastrophic inland
flooding associated with landfalling and transitioning tropical cyclones over the
northeastern US.
The results of in-depth case studies (i.e., Hurricane Bob 1991, Hurricane Gloria
1985, and Hurricane Belle 1976), as well as cursory case studies for the catastrophic
flooding events of Hurricanes Connie and Diane in 1955, have identified characteristic
synoptic and mesoscale features responsible for the regions of heaviest precipitation
during these events. The most pronounced and influential feature present during all of
the cases studied was a downstream ridge located over the western Atlantic Ocean.
Regardless of the initial location, orientation and strength of the ridge, significant
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reinforcement of this feature by way of latent heat release was hypothesized to occur as a
result of convection associated with the tropical cyclone. The reinforcement of the ridge
then led to intensification of the northward-displaced geopotential height gradient,
subsequently enhancing and anchoring the jet maximum over areas of the Northeast, and
producing a vigorous area of upward motion through an implied thermally direct
ageostrophic circulation in the equatorward entrance region. The enhanced jet
maximum then can become coupled to the northward-propagating tropical cyclone,
thereby positioning favorable dynamical support for heavy precipitation over the
Northeast for an extended period of time.
Ridge amplification in the upper levels also extends to lower levels, resulting in
an enhanced geopotential height gradient and southerly/southeasterly low-level flow
between the ridge and the northward-propagating tropical cyclone. The formation of the
low-level jet has numerous consequences: (1) transport of warm, moist tropical air well
in advance of the cyclone occurs as a result of an increase in southerly flow, which can
act to inject the vertical branch of an implied thermally direct circulation with high θe
air, thus establishing conditions conducive to heavy precipitation, (2) proper
configuration of significant low-level jet winds around the north side of the tropical
cyclone promotes the formation and maintenance of a coastal front that enhances
precipitation closer to the time of landfall, and (3) reorientation of the strongest lowlevel jet winds to a position unfavorable for coastal front development on the north side
of the tropical cyclone, combined with the degree of inland penetration of the system,
can allow enhanced easterly/southeasterly flow to impinge on the windward-facing
slopes of the terrain, thereby enhancing precipitation through orographic processes. The
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role that the low-level jet plays in any given case is dependent on the strength and
orientation attained when it is positioned between the ridge and the tropical cyclone, and
can vary from case to case.
The movement of the tropical cyclone to higher latitudes most often results in
some sort of interaction with a midlatitude extratropical system. When this interaction
occurs, ascent becomes increasingly driven by midlatitude forcing functions such as
differential CVA, the Laplacian of WAA, and outflow jet dynamics. The midlatitude
forcing functions observed in the case studies previously presented exhibited a
secondary influence on the observed heavy precipitation in each event. The vast
majority of the heaviest precipitation in the cases studied here was not due to a direct
interaction with an extratropical system and its associated midlatitude forcing functions,
rather it was significantly dependant on the positioning, orientation, and strength of the
cyclone-induced upper- and lower-level jets within a tropical environment. The
interaction with a midlatitude trough did, however, modulate the overall precipitation
distribution relative to the storm track. Tropical cyclones interacting with an
extratropical system exhibited a preferential left-of-track shift in the precipitation
distribution, while cyclones solely under the influence of outflow jet dynamics exhibited
a preferential right-of-track shift in the overall precipitation distribution. These shifts are
consistent with the previous findings of Darr (2002) and Atallah (2003).
Mesoscale features also played a significant role in producing the heaviest
precipitation within the broad precipitation shield. The formation of cyclone-induced
coastal fronts in a number of the events examined showed direct enhancement of
precipitation. The coastal fronts formed near the intersection of inland northerly flow
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induced by the cyclone and enhanced easterly flow associated with the low-level jet
around the northern side of the system. Although the coastal front was found to be a
much less pronounced feature when compared to its wintertime counterpart, the presence
of a warm, moist tropical environment likely enabled upward motion associated with the
coastal front circulation to produce a significant enhancement within the broad
dynamically forced precipitation shield. Dissipation, or a lack of formation of the
coastal front, together with significant inland propagation of the tropical cyclone,
resulted in penetration of enhanced easterly/southeasterly flow that impinged on the
windward-facing slopes of the terrain features throughout the Northeast. This
orographic process resulted in significant enhancement of the precipitation on relatively
small scales, providing further evidence supporting the importance of mesoscale features
during such events.
5.2 Suggestions for Future Work
This research entailed applying a synoptic-dynamic approach to investigate the
mechanisms responsible for the observed precipitation distributions in landfalling and
transitioning tropical cyclones over the northeastern US. Expanding upon the work
conducted for this research effort, a comprehensive synoptic-dynamic examination of the
complete 41-storm dataset could yield an even better understanding of the significant
mechanisms at work. This type of analysis may yield more fruitful findings in the
coming years, as a result of the recent introduction of finer-resolution datasets such as
the NARR (North American Regional Reanalysis) gridded datasets based on the NCEP
Eta model. Once a large number of storms have been examined, compositing of the
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sample may lead to a more precise schematic diagram of the situation when compared to
the initial results presented in this thesis (see Fig. 4.1). Past research results have shown
that numerical model performance during landfalling and transitioning tropical cyclones
is poor at best. A detailed examination of the various model biases surrounding a large
number of these events may ultimately lead to improved model performance during
these events, enabling the forecaster to issue timely and accurate forecasts to the general
public.
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