An Undular Bore off the Massachussetts Coast

							An Undular Bore off the Massachusetts Coast

On April 16, 2003, the passage of a sharp cold front was associated with the development
of an undular bore off the Massachusetts coast, as noted by the GOES 12 visible satellite
loop below:

[Insert loop here, or a link -- the loop consists of 9 images with prefix g12]

Long, thin bands of low cloud form parallel to the front (probably indicated by the
leading cloud arc) and travel southward across the Cape Cod region. East of the Cape, the
pattern is better defined with 7 or 8 narrowly spaced bands. In this case, the wave train
was excited when the cold front interacted with a strong marine layer inversion caused by
the advection of unseasonably warm air out over the chilly Atlantic. Record highs of 84
and 85 were set at Boston and Blue Hill Observatory, respectively, while Chatham out on
Cape Cod (just below the northward bend of the Cape), remained in the 50's. This
marine inversion is shown nicely in the skew-T plotted from Chatham's rawindsonde
observation:

[Insert 1612.gif]

For comparison, the postfrontal skew-T at 00Z on the 17th is also shown, about 4 hours
after the frontal passage:

[Insert 1700.gif]

Following are some surface observations from Chatham during the time window of
interest (the most recent obs are at the top):

 KCQX 170152Z AUTO 02010KT 10SM FEW095 06/02 A3010 RMK AO2 SLP193
T00560017
 KCQX 170052Z AUTO 02013KT 10SM CLR 06/02 A3007 RMK AO2 SLP184
T00610022
 KCQX 162352Z AUTO 02013G18KT 10SM CLR 06/03 A3000 RMK AO2 SLP161
T00610033 10156 20061 51054
 KCQX 162252Z AUTO 02013KT 10SM CLR 07/03 A2995 RMK AO2 SLP144
T00670033
 KCQX 162152Z AUTO 02015G21KT 10SM CLR 08/04 A2990 RMK AO2 PRESRR
SLP128 T00780039
 KCQX 162052Z AUTO 01017G26KT 10SM CLR 08/05 A2984 RMK AO2 PK WND
35031/2020 WSHFT 2017 SLP107 T00830050 53037
 KCQX 162037Z AUTO 01020G24KT 10SM FEW100 09/06 A2983 RMK AO2 PK
WND 35031/2020 WSHFT 2017
 KCQX 161952Z AUTO 20009KT 8SM FEW100 12/08 A2972 RMK AO2 SLP064
T01220078
 KCQX 161852Z AUTO 19010G17KT 8SM CLR 12/08 A2972 RMK AO2 SLP066
T01220078
KCQX 161752Z AUTO 20012G15KT 8SM CLR 13/08 A2973 RMK AO2 SLP069
T01280078 10133 20089 56017
KCQX 161652Z AUTO 20012G17KT 8SM CLR 13/08 A2975 RMK AO2 SLP073
T01280083
KCQX 161552Z AUTO 19013G19KT 8SM CLR 12/08 A2975 RMK AO2 SLP076
T01220078

From the satellite images, it appears that the leading cloud arc passes through Chatham at
2015Z, which coincides nicely with a windshift observed at the surface at 2017Z and a
peak wind of 31 kts at 2020Z. Pressure was rising rapidly during the cold front passage.

This images from this event show some similarities to the undular bore observed on the
Texas coast a few years ago, as chronicled in a 'Picture of the Month' feature in Monthly
Weather Review (Clarke, 1998).

[At this juncture, you could include a link to the article:
http://ams.allenpress.com/amsonline/?request=get-document&issn=1520-
0493&volume=126&issue=04&page=1098
but this is only accessible by people with subscriptions, like in the Atmos domain]

In both cases, it appears that a sharp cold front (I forgot to mention that the sharp cold
front was associated with a 56 degree temperature drop at Blue Hill Observatory.)
interacted with a strong temperature inversion ahead of the front to cause the bore. In the
Texas case, it was a nocturnal inversion. In the Massachusetts case, it was a strong
marine boundary layer inversion. In the Texas case, the cloud system was not associated
with surface temperature or dewpoint change, just a prefrontal pressure trough and wind,
whereas the bore in the Massachusetts case appears to be nearly coincident with the
front.

For more on undular bores, see Smith (1988).


Author: Jonathan Vigh

References

Clarke, J. Christopher 1998: An Atmospheric Undular Bore along the Texas Coast. Mon.
Wea. Rev., 126, 1098-1100.

Smith, R. K. 1988: Traveling waves and bores in the lower atmosphere: The "Morning
Glory" and related phenomena. Earth Sci. Rev., 25, 267-290.



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