Friday, June 21, 2013

Mission #1: Embedded Convection Within Stratiform Precipitation

The first day out in the field with the Doppler on Wheels (DOW) was a wet one for the students and faculty involved. Tuesday, June 18th was Mission #1 and  the second day of the field campaign. The forecast was for precipitation to form somewhere along a frontal boundary to the north of the Island and somewhere to the west of the Island. The weather models discussed during the planning meeting the previous day were in pretty good agreement that there should be some form of precipitation for the DOW to measure. The question was where would it be? The decision was made to station the DOW at Jones Beach where it would have the ability to scan south and west, and even to the north if needed.

The students partitioned into a few teams. There were students stationed with the DOW at Jones Beach where they quality-controlled the data as it came in and adjusted where the radar was scanning if an interesting feature developed. There were students given hand-held instruments to take down observations of the environment that included temperature, dew point, relative humidity, wind speed (max gust and averaged) and wind direction at the DOW site and along Jones Beach. The project also included extra radiosonde, or weather balloon, launches from the NWS NYC Forecast Office in Upton, NY to provide a vertical profile of atmospheric conditions in-between times when the NWS launches their own balloons at 8 AM EDT and 8 PM EDT. There were even a couple students remaining at Stony Brook University with real-time observations including regional radar, satellite, and high-resolution model data to act as real-time forecasters (i.e "nowcasters") should they be needed.

The18Z (2 PM) radiosonde.
The students stationed with the DOW got to scan embedded convection within stratiform precipitation, as the title of this post states. That means that there was a broad area of light rain developing west and south of Long Island and within that light rain, there were smaller cells of heavier rain. The students got to take some vertical slices of these embedded cells with the DOW and analyze reflectivity and velocity data. The students not in the cabin of the DOW got a little wet once the rain moved into Jones Beach, but that's part of the fun of field work! The radiosonde group successfully launched one balloon at 18Z (2 PM EDT) which showed that the environment was moist from middle to upper-levels and the wind profile was veering, indicating that they were south of a warm frontal boundary (see plot).
The Nowcasters were never needed thanks to the availability of smart phones and the great data being measured by the DOW!

You may ask, "Why weren't there more thunderstorms? Isn't that sort of boring?" There weren't that many thunderstorms (the data showed very infrequent measured lightning strikes) because in order for storms to form you need three things: moisture, instability, and lift. There was plenty of moisture in the atmosphere as the morning's sounding and extra 2 PM sounding both showed. Another way of knowing where there is moisture is to look at satellite imagery and see where there are clouds. The lift was provided by the convergence boundary. This means that along central NJ and points west there were low-level westerly (from the west) winds colliding with low-level southeasterly (from the southeast) winds. When low-level winds collide, there is rising motion and therefore air can be lifted. The missing ingredient was the instability or the idea that if you have a bubble of air, if it is lighter (hotter) than the surrounding air then it will rise and it will keep rising until its temperature is equal to that of the surrounding air. A measure of instability is Convective Available Potential Energy (CAPE) and is the go-to quantity for forecasting instability for convection. On the day of Mission #1 there was marginal CAPE that didn't impress any of the student or faculty forecasters at the planning meeting. This was likely due to the abundant moisture that was allowing for clouds persist and block out most of the sun. This kept the sun from having its energy reach the ground to warm it up and raise the temperature so that air bubbles near the ground would want to rise. Despite the lack of instability, there was plenty of moisture and lift that the students got to see and measure first-hand which was certainly far from boring.

If you didn't get a chance to see the DOW at Jones Beach, don't worry! We'll likely be back if there's a chance for storms to fire up to our southwest. The first day out in the field was a success with an interesting and complex weather system. Hopefully there will be some more classic thunderstorms while the DOW is still in town-- so please keep your fingers crossed!

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