overview

Coastal Gravity Wave Event of 24 April 2010National Weather Association Annual National Conference, Tucson, AZ2-7 October 2010Scott Overpeck, Chris McKinney, and Kent ProchazkaNOAA/National Weather Service Houston/Galveston, TX

OverviewThe goal of this study is to document the passing of mesoscale gravity waves that propagated along the Upper Texas Coast during the early morning hours of 24 April 2010. Pressure changes of 5-7 hPa were observed with the passing of the waves, and high winds of 60 knots caused damage to house roofs and snapped power poles.

Synoptic Environment for Gravity Waves.Possible source region of the Gravity Waves on April 24.IR and Water Vapor Satellite Imagery, Radar reflectivity and base velocity showing the Gravity Wave passage.Corresponding pressure and wind data from 5 observation sites along the Upper Texas Coast.Summary of the event.

Synoptic Environment Koch and OHandley (1997)

Synoptic Environment 24 April 2010

IR Satellite Image 0315 UTC, 24 April 2010

Water Vapor Satellite Image 0345 UTC, 24 April 2010

Radar Image 0525 UTC, 24 April 2010

Pressure and Wind Observation

Gravity Wave SummaryThe gravity wave most likely had a period of about 3 hours, a wavelength between 155-215 NM, and a velocity of 52-65 knots.The average pressure change was 6.7 hPa with a minimum pressure of 995.6 hPa and wind speeds of 37 knots (gusts of 47 knots).

SitePeriod (hours)Pressure Change (MB)Minimum Pressure (MB)Maximum Sustained Wind (KNOTS)Peak Wind Gust (KNOTS)

Angleton (KLBX)32.4998.32026Freeport (USCG 8772447)3.35.9996.13543Scholes Field (KGLS)3.257.1995.14560Pleasure Pier (GPST2, 8771510)3.26.7994.53643Sabine Pass North (SBPT2, 8770570)2.77996.63242Average3.16.7995.63747

Gravity Wave CharacteristicsFreeport to KGLSKGLS to Sabine PassFreeport to Sabine Pass

Average Period (Hours)3.2752.9753Wave Velocity (Knots)655255Wavelength (Nautical Miles)213155165

Questions? Comments?

Coastal Gravity Wave Event of 24 April 2010National Weather Association Annual National Conference, Tucson, AZ2-7 October 2010Scott Overpeck, Chris McKinney, and Kent ProchazkaNOAA/National Weather Service Houston/Galveston, TX

E-mail: [email protected]

The title slide has a background image of the Graphicast we issued to show the higher winds along the coast. The color scheme used was poor but one could still see the higher radial velocities from the radar.*This was a rare event in which there were high impacts with wind damage along Galveston Island. It was localized along the coast and not forecasted at all. We believe the gravity wave originated over the mountains in Mexico. While there was some ongoing convection in Texas, regional radar loops showed that the wave did not propagate from a decaying MCS.*This study is presented here to show a typical gravity wave environment. The gravity wave on 24 April 2010 did not fit this case although there are some similarities between the temperature profiles.*In this slide animation, the first image on the upper left is the 300 hPa map at 00Z 24 April 2010 which depicts a strong polar jet coupled with a strong sub-tropical jet just to its south over Mexico and S Texas. The jet is quite strong and since it was located over the mountains, we suspect that the topography and/or geostrophic adjustment in this region was responsible for generating the gravity wave. At the upper right hand is the 700 hPa map for the same time. There seems to be a weak shortwave trough through S Texas. This may be a reflection of the gravity wave or where the wave was ducted. The bottom image is the 06Z 24 April 2010 surface map. This shows that the gravity wave occurred in the warm sector ahead of a developing meso-low, a departure from the typical synoptic environment presented in the previous slide.*In this slide animation, the first image in the upper left is the 00Z 24 April 2010 upper air sounding at KCRP. There is a hint at an inversion at around 850 hPa, but weak and may not be enough to duct the gravity wave. ACARS soundings prior to the event near Houston Hobby and Intercontinental Airports (KHOU and KIAH respectively) so show a more substantial inversion that likely ducted the gravity wave. The first ACARS sounding at the right is from KHOU and there is a stronger inversion at 850-800 hPa. The second ACARS sounding at the bottom left is from KIAH and again shows an inversion around 900-850 hPa. It is not as pronounced and more similar to the KCRP 00Z sounding. In the end, there was enough of an inversion similar to the Koch and OHandley 1997 study that would support ducting of gravity waves.*This image gives a basic overview of the movement of the gravity wave and when it reached certain locations.*This is an IR Satellite Image at 0315 UTC or just as the gravity wave was about to move off the TX coast. The black arrow points to the gravity wave.*This is an IR Satellite Image at 0631 UTC or just as the gravity wave has moved into Galveston Island. The black arrow points to the gravity wave. About 20 minutes later, there is a 60 knot wind gusts at Scholes Field (KGLS).*This is an IR Satellite Image at 0915 UTC or just as the gravity wave has moved into Louisiana. The black arrow points to the gravity wave. There seems to be a banded cloud feature with the wave at this point.*This is a Water Vapor Satellite Image at 0345 UTC or just as the gravity wave has moved off the coast into the NW Gulf of Mexico. The black arrow points to the gravity wave.*This is a Water Vapor Satellite Image at 0631 UTC or just as the gravity wave has moved into Galveston Island. The black arrow points to the gravity wave.*This is a Water Vapor Satellite Image at 0915 UTC or as the gravity wave moved into Louisiana. The black arrow points to the gravity wave.*This radar image at 525 UTC shows the approaching gravity wave. You can seem some shower activity near Freeport (center of image) which may be associated with a weak wave. The second much stronger gravity wave was back to the SW where this is more substantial convection associated with ascent within the gravity wave. The radar is also indicating higher radial velocities with this second wave (light blue colors in base velocity image to the right).*This is a pressure trace and wind observation from the Angleton, TX (KLBX) ASOS. The previous radar image matches up of when the first wave moved through KLBX. There was a then a pressure rise then followed by only a 2 MB drop over the next couple of hours. This corresponds well with the passing of the second wave.*This is another pressure trace and wind speed observations from the Freeport, TX USCG gage. The data is at 6 minute intervals which helps capture the pressure falls/rise with the passing of the gravity wave. While only about 20-25 miles south of Angleton, Freeport experienced a much more significant pressure fall with the gravity wave (~6MB total drop, ~3MB in 12 minutes from 600-612UTC), and this resulted in much higher wind speeds and gusts (sustained 25-30 knots, gusts to 43 knots).*This is a radar image at 609 UTC showing the gravity wave approaching Galveston. You can see some arching reflectivity and some shower activity with the beginning of the wave. The base velocity data shows higher velocities back to the SW which would approach Galveston.*This radar image about 40 minutes later at 652 UTC shows at least 2 or 3 perturbations in the gravity wave where there are more scatters in the base reflecitivty. The base velocity image shows these perturbations better and shows some higher velocity bands associated with the perturbations. This was also when Galveston Scholes Field (KGLS) experienced wind gusts of 56 and 60 knots. Another observation is that there is some type of boundary SE of the radar near Texas City that stretches WSW/ENE and into Galveston Bay. We suspect this may be a marine layer boundary but really do not have any other data to back it up. If so, south of this boundary would possibly be a stable marine layer which could further help duct the gravity wave or possibly allowed for better mixing of the winds so that the 60 knot wind gusts could occur.*This graph shows the 1 minute pressure and wind observations from Scholes Field (KGLS). Note the initial pressure drop around 600 UTC and then a pressure rise. The next pressure fall occurs around 615 UTC with about a 3 MB pressure drop from 630-645 UTC. Another 2 MB pressure drop occurs from 645-655 UTC which corresponded with the sustained winds of 45 knots and wind gusts of 56 and 60 knots. There were 3-4 more pressure rise/falls or perturbations in the gravity wave. Winds remain around 30 knots sustained with gusts to 40 knots.*This radar image at 723 UTC again shows some of the perturbations in the gravity wave and higher velocities associated with them. Some of the radial velocities are reaching 70 knots just a couple thousand feet AGL. The suspicious marine layer boundary was still evident SE of the radar but less definite.*Just to show the how localized the effects of the gravity wave were, here is the pressure and wind observations from the Pleasure Pier gage just 5 miles from KGLS. Again the pressure fall of about 3 MB shows up well from 642-654 UTC and this corresponds with winds around 30-35 knots and gusts to 41 and 42 knots. The minimum pressure was 994.5 MB which was the lowest pressure measured during the event.*This last radar image at 812 UTC shows the back edge of the gravity wave and a larger area of higher winds in the base velocity data. It appears that the supposed marine layer boundary has pushed a little more north but became more diffuse by this time. The gravity wave should also be coming at a peak for the Sabine Pass area, one of the last good pressure and wind observation of the gravity wave.*This is the pressure and wind observation from Sabine Pass, TX. Again there were a couple of pressure drops that corresponded with increases in wind speeds. The pressure fall had the shortest duration of all the observations possibly indicating that the wave had weakened or became less defined. Still there was a 4 MB pressure fall from 830-900 UTC, and this was when the highest wind speeds were observed.*This slide shows some basic statistics about the gravity wave as it moves along the Upper Texas Coast. Overall the gravity wave persisted for about 3 hours with overall pressure changes of about 6-7 MB. Wind speeds were generally around 30-35 knots with gusts to around 40 knots. The peak winds were at Galveston Scholes Field (KGLS) with a maximum wind of 45 knots and gust of 60 knots. Based on radar imagery and surface obs, the wave moved at about 55-60 knots, lasting about 3 hours with a wave length of anywhere from 155-213 NM.

From a NWS operations standpoint, we noted that the night before there was a gravity wave in WFO Corpus Christis area. I think they had a gust of 50 knots at either KCRP or KNGP. We had seen wave like figures in the radar imagery so we were looking for that type of signature. As the wave moved along, we did not see any observations that indicated high winds. Just in case, we prepared a high wind warning and special marine warnings for quick issuance. Once we got reports of tree damage and roof damage on the west end of Galveston Island, we issued the warnings. This was also about the same time KGLS reported its wind gusts of 56 and 60 knots. This resulted in no lead time on the warnings, but we expected this might end up being a prolonged wind event. We did not get any other reports of wind damage, although there could have been more. It was a localized event, but could have had more of an impact if it occurred during the day and not at night.*Thank you for reading through this presentation. If you have any questions, please e-mail me at [email protected].*

overview

Documents

wind observationradar

ir satellite image

background image

gravity wave passage

wind damage

coastal gravity wave

wind data

wind observationpressure