An Undular Bore off the Massachusetts Coast
Published: April 23, 2003
On April 16, 2003, the passage of a sharp cold front (temperature dropped 56 deg F at Blue Hill Observatory in less than 18 h) was associated with the development of an undular bore off the Massachusetts coast, as noted by the GOES 12 visible satellite loop below:
GOES-12 Visible Loop
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:
Chatham sounding for 12Z April 16 (before frontal passage)
Chatham sounding for 00Z April 17 (after frontal passage)
For comparison, the postfrontal skew-T at 00Z on the 17th is also shown above, about 4 hours after the frontal passage.
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).
In both cases, it appears that a sharp cold front 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).
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.