The DREAMS Project started off the month of July with a mission. Mission #9 took place on July 1st and a lot earlier in the day then the organizers had anticipated. Strong-to-severe thunderstorms developed early in the morning and propagated northeastward over Long Island and Connecticut. Thanks to the quick action of the DOW team (especially Rachel Humphrey who's been driving the DOW and teaching us how to operate it) the radar was set up in time for the action at Cedar Beach in Mount Sinai. A strong cell passed just to our north and some strong showers developed and moved overhead. After everything cleared up we were in a parking lot with a few inches of water to navigate around!
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| Weather Prediction Center's 12Z (8 AM) surface analysis. |
The forecast for Monday, July 1st was very similar to the previous day's forecast. Most of the ingredients for strong storms were there such as tons of moisture as evident by dew point temperatures in the 70's F and some instability that would increase if there was solar heating during the day. Another ingredient that hasn't been mentioned is called wind shear. A thunderstorm develops from rising air that grows depending on the available moisture, instability, and lifting mechanism. That rising air is concentrated into a narrow swath called an updraft. Wind shear is defined as the change in wind speed or wind direction with height. If there is no wind shear than the updraft is perfectly vertical and when the air rises and condenses its water vapor to form clouds that rain, the rain can actually "choke" the updraft by flooding it with cold, descending air. Wind shear is important to tilt the updraft so that it isn't vertical so that the storm can persist for a longer period of time before dissipating. For Monday, the lifting mechanism was predominantly a stationary front that was situated just east of the Appalachians for most of the day. The forecast was for thunderstorms to develop later in the afternoon when the daytime heating of the surface from the sun would be at a maximum and therefore the amount of instability (CAPE) would also be at a maximum. However, what actually happened was strong thunderstorms got going during the overnight hours and because of the strong wind shear (strong winds from the south near the surface and stronger winds from the southwest higher up) and the added forcing of the stationary front, they organized into a strong line of storms. The original plan for Mission #9 was to catch some late afternoon thunderstorms, but after the National Weather Service issued a tornado warning for Union County in New Jersey around 9 AM (1300 UTC), the DOW team sprang into action.
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| DOW scanning to our northwest. |
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| Pod in front of approaching storms. |
The DOW left Stony Brook University around 10:30 AM and headed to our favorite location on the North Shore- Cedar Beach in Mount Sinai. The plan was that if the line of storms held together as it swept across Long Island, then we'd have the perfect view to our north over the Long Island Sound. We started scanning the skies by 11:30 and watched the line approach our location from the west-northwest. We deployed the pod to measure conditions near the surface throughout our deployment. An interesting feature that we measured with the radar was a possible updraft core. We took vertical slices through the cells as they approached us and saw a narrow area extending up into the storm of velocity values that were opposite of the storm motion. By 1:00 PM a shelf cloud was visible just to our north associated with a strong cell. A shelf cloud forms when there is air rising just ahead of the cooler air gusting outward from a thunderstorm.
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| Shelf cloud over the Long Island Sound (photo by David Stark). |
After the strong storm passed to our north and east, we noticed there were a few smaller cells developing to our southwest. We positioned the DOW to take vertical slices of them as they grew and organized into a flash-flood inducing line of storms that passed overhead between 1:15 PM and 3:00 PM. The photo on the right is of the reflectivity data (top screen) and velocity data (bottom screen) that is showing a vertical slice through the second line of heavy rain. The yellows in the reflectivity data show a core of high value of dBZ (the unit of reflectivity) just about to approach the DOW. The velocity data shows mostly blues and purples which meant that all of those rain and cloud drops were moving towards the DOW (i.e. inbound). The students in the DOW had front row seats for the downpours that occurred and could see them approaching using the radar. Despite a leaky roof in the DOW, most of the participants in today's mission were able to keep dry! After the mission wrapped up at around 3:00 PM, water had pooled in the Cedar Beach parking lot to a depth of over four inches in spots! Dr. Kelly Lombardo's car is pictured below sitting in one such puddle. The DOW had to be backed out of a puddle to allow for its occupants to jump down to dry land.
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| Dr. Kelly Lombardo's car and the DOW sit in a few inches of post-storm water. |
Despite having planned on an afternoon/evening mission, the DREAMS Project participants made a great effort to get the DOW out and into position to sample the surprise convection that maintained itself across the Tri-State area. Although we could not have measured the confirmed tornado (with any sites that we have legal permits for), that particular storm jump-started our day. We used that storm to decide that the convection was strong and would hold together as it traveled eastward closer to Stony Brook University. We recorded some great data of the passing convection before it cleared up for the rest of the evening. Therefore, Mission #9 was a success!
- For more information on shelf clouds, please visit this site:
http://eo.ucar.edu/webweather/cloudpic1.html
- For more information on thunderstorm ingredients, please visit this site:
http://spotterguides.us/advanced/advanced03.htm
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