Northeast Cool-Season Cyclones Associated with Significant Upper-Level Easterly Wind Anomalies

Adrian N. Mitchell, Lance F. Bosart, and Kristen L. Corbosiero Department of Atmospheric and Environmental Sciences, University at Albany, SUNY

!e-mail: amitchell@albany.edu

Motivation Data and Methodology Event Classification

593!

•  Two significant snowstorms in early 2010 (2-3 January and 25-28 February) were associated with upper-level (UL) easterly jet streaks

•  Both cyclones: !  produced upwards of 30 in. (76 cm) of snowfall over parts of

the Northeast !  resulted in long duration snowfall events (> 30 h) !  led to significant orographically enhanced precipitation !  presented a forecast challenge as a result of anomalous

synoptic and mesoscale forcing •  The opportunity exists to explore cyclones of a similar nature

through a climatological and composite analysis, with a concentration on synoptic scale forcing features

•  Data ! NCEP/NCAR 2.5° Reanalysis Data

•  Methodology ! Cyclone domain: 37.5-51 °N, 83-60 °W !  Time span: 1 Jan 1948 through 31 Dec 2010 ! Cyclone events were defined as a domain average 300-hPa

zonal wind anomaly < -3σ, and surface pressure < 1000 hPa for at least 12 h

!  78 cool-season cyclone events were identified and classified based on synoptic structure for compositing purposes

! Grids for each cyclone were averaged and shifted to the centroid of all cyclones to create cyclone-relative composites

•  Cyclone events displaying a 300-hPa easterly jet streak (≥ 30 m s-1) poleward of the surface low and 850-hPa northerly warm-air advection west of the surface low were classified as Easterly Jet Streak (EJS) events

•  To be considered an EJS event: 1) the 300-hPa jet streak core must have a primary easterly component (see schematic), and 2) the 850-hPa wind must have a primary northerly component and display warm-air advection (WAA) (see schematic)

•  Remaining events were classified as either open or cutoff based on the configuration of the 300-hPa height field

•  To be considered a ‘cutoff’, there must be a 60 m height rise in all directions from center of the 300-hPa circulation

Conclusions

EJS (32)

Open (25)

Cutoff (21)

500-hPa composite mean geo. height (dam, solid contours) and composite anomaly (σ, shaded)"!

Lag = -5!Lag = 0!

Lag = 0!

Lag = 0! Lag = -5!

Lag = -5!

Results Earth-relative (E-R) Composites

925-850 hPa RH (%, shaded), MSLP (hPa, black contours), 850-hPa potential temperature (K, red contours), 850-hPa wind (kt, barbs >10 kt)

500-hPa geo. height (dam, black contour) and abs. vort (10-5 s-1, shaded), 300-hPa wind (kt, barbs), and PW (mm, red contours). The blue contours outline regions of 300-hPa wind greater than 50, 70 and 90 kt

Cyclone-relative (C-R) Composites

EJS (32)

EJS (32)

Cutoff (21)

Cutoff (21)

Open (25)

Open (25)

Monthly Frequency Decadal Frequency Cyclone Intensity

Events relative to Arctic Oscillation (AO) value EJS Event

Open Event

Cutoff Event

•  At lag = -5, 500-hPa height anomalies resemble the negative phase of the AO in all three E-R composites •  At lag = 0, a positive Pacific/North American (PNA) pattern is present in all three E-R composites •  At lag = 0, a deep trough is in place over the eastern U.S. with anomalous ridging over eastern Canada •  C-R composites reveal variations in the location of major forcing features in each of the three types of events

•  Cyclone events occur throughout the cool-season with a maximum in December and April •  EJS events tend to be associated with stronger cyclones than open and cutoff events •  The majority of cyclone events occur when the AO is negative (blocking over the North Atlantic)

•  All three types of cyclone events are favored during a +PNA, -AO pattern, and clustered when the AO is significantly negative (< -1). Enhanced predictability is possible by analyzing lagged composites

! EJS events are the strongest, with a closed 500-hPa circulation over New England. Forcing for ascent is favored in the equatorward exit region of an easterly jet streak, collocated with wrap around northerly WAA. Low-level northwesterly flow and high RH values favor significant terrain enhancement west of the sfc. low

! Cutoff events are slightly weaker, with a negatively tilted 500-hPa long wave trough and cutoff circulation over NJ. Forcing for ascent is favored across New England where the left exit region of a 300-hPa jet streak is collocated with 500-hPa PVA and 850-hPa WAA. Terrain enhancement is likely in moist low-level NW flow

! Open events are the weakest, with a 300-hPa jet streak rounding the base of a negatively tilted 500-hPa trough. Forcing for ascent is favored over northeast New England where 500-hPa PVA is collocated with 850-hPa WAA. Terrain enhancement is not as likely as in EJS and cutoff events

Results Composite low tracks and

precipitation area >.5” •  EJS cyclone events tend to

retrograde westward as they encounter strong blocking downstream

•  Cutoff cyclone events tend to remain around the same longitude while drifting northward

•  Open cyclone events tend to track northeastward

•  The heaviest precipitation is concentrated to the west of EJS and cutoff cyclone events, and is orographically enhanced Precipitation Data: NCEP daily (1200–1200 UTC) Unified

Precipitation Dataset (UPD) "!

This research was supported by NOAA CSTAR grant #NA01NWS4680002

•  The three types of events identified can also be regarded as three stages of cyclone development

!  First stage: no wrap-around low-level (LL) warm-air advection, no UL easterly jet streak

! Second stage: more significant LL wrap-around warm-air advection, developing UL easterly jet streak

!  Third stage: significant wrap-around LL warm-air advection, well defined UL easterly jet streak