7. Conclusions and Future Work
7.1 Conclusions
The influence of ENSO and the MJO on 500-hPa cutoff cyclone frequency in the
Northeast US for 2000 through 2007 has been examined. A list of cutoff cyclones,
defined as cyclones that maintain a 30-m geopotential height rise in all directions at 500
hPa for at least three consecutive analysis times (i.e., a 12-h period), was obtained from a
dataset compiled objectively by Scalora (2009). The separate and combined ENSO and
MJO phases for the date of the first appearance of the cutoff cyclone in the Northeast
cutoff cyclone domain were examined. It was shown that statistically significant results
were obtained only when considering the combined influence of ENSO and the MJO on
500-hPa cutoff cyclone frequency in the Northeast US. Cutoff cyclone frequency in the
Northeast US was maximized when enhanced convection associated with the MJO was
over the Maritime Continent during ENSO cooling or over the Western Hemisphere
during ENSO warming. The results of the current study suggest that consideration of the
combined influence of ENSO and the MJO on 500-hPa cutoff cyclone frequency in the
Northeast US could have operational forecast value.
A total of 384 cutoff cyclone days were indentified in the Northeast US for the
2004/05–2008/09 cool seasons and were placed into 15 composite categories according
to precipitation amount (HP, LP, NP), the tilt of the cutoff cyclone at 500 hPa (negative,
neutral, or positive), and the structure of the cutoff cyclone at 500 hPa (“cutoff” or
“trough”). The average location of all 500-hPa cutoff cyclones within each of the 15
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composite categories indicates that there was a distinct difference in location of cutoff
cyclones between precipitation amount categories, with HP, LP, and NP cutoff cyclones
typically located west, north, and east of the Northeast precipitation domain, respectively.
Examination of cyclone-relative composites of the synoptic-scale patterns at upper levels,
midlevels, and low levels revealed that the difference in location of cutoff cyclones likely
contributed to variations in observed precipitation amount by affecting the location of
upper-level and low-level jet streaks, midlevel absolute vorticity maxima, and low-level
temperature advection, all of which play a role in forcing for ascent. Schematic diagrams
depicting the key synoptic-scale features that affect precipitation distributions were
presented for the 15 composite categories. In general, heavy precipitation was supported
by: (1) ascent favored in the exit and entrance regions of upper-level jet streaks; (2) QG
forcing for ascent in regions of inferred differential cyclonic absolute vorticity advection
in the lower troposphere and warm air advection at low levels; (3) advection of PW into
the Northeast US by a low-level jet; and (4) enhanced forcing for ascent in the vicinity of
surface fronts.
Case study analyses of three cutoff cyclone events that that were associated with
precipitation forecasting challenges and varying precipitation distributions were
conducted.
The 2–3 February 2009 cutoff cyclone event was associated with light
precipitation, with less than 5 mm of precipitation observed throughout the Northeast US,
and was investigated to determine factors that contributed to the lack of observed
precipitation in order to help improve future precipitation forecasts associated with 500hPa cutoff cyclones. In general, the lack of heavy precipitation associated with this
cutoff cyclone event was attributed to the lack of low-level forcing for ascent and low
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PW values throughout the Northeast US. The 1–4 January 2010 and the 12–16 March
2010 cutoff cyclone events were both long-duration events chosen for this study because
of their association with varying daily precipitation distributions that posed forecasting
challenges. Analyses of these two cutoff cyclone events indicated that the synoptic-scale
features that contributed to heavy precipitation were similar with those presented in the
composite schematic diagrams, with the key features including upper-level jet streaks,
midlevel absolute vorticity maxima, regions of warm air advection, regions of
frontogenesis, and advection of anomalous PW into the Northeast US.
In addition, it
was shown that modification of the low-level flow by the topography of the Northeast US
and lake-effect precipitation played a role in altering the mesoscale distributions of
precipitation observed with 500-hPa cutoff cyclones in the Northeast US.
7.2 Suggestions for Future Work
The current study is a continuation of research pertaining to precipitation
associated with 500-hPa cutoff cyclones performed by AA02, Fracasso (2004), Najuch
(2004), and Scalora (2009). The results arising from the current study aim to increase
forecaster situational awareness by identifying key synoptic-scale and mesoscale features
that affect precipitation distributions associated with cool-season 500-hPa cutoff cyclones
in the Northeast US. Future work on cutoff cyclones and precipitation may include:
1) Examination of the influence of other modes, such as the Pacific–North American
pattern, the Arctic Oscillation, and the North Atlantic Oscillation, on cutoff
cyclone frequency in the Northeast US.
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2) Investigation of meteorological conditions that determine whether mesoscale
enhancement of precipitation will or will not occur in association with upslope
flow or lake effect.
3) Additional analyses of cutoff cyclone events to include cutoff cyclones from each
of the cutoff cyclone composite categories.
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