January 11,2000:
Downburst from Shallow Convection
BACKGROUND

Overnight on January 10-11, 2000 an occluded cyclone left the NE United States in a relatively cool air mass (temperatures at 12Z on 1/11/2000 around 5-8C in NJ.) The January 11, 2000 12Z surface analysis showed a cold front following the cyclone, and was located in western PA. This front moved quickly through the area and was well offshore by 2045Z.
12Z Surface Analysis for January 10, 2000
GOES-8 IR image for January 11, 2000 0245Z showing front has moved well offshore by the evening of January 10, 2000
12Z Surface Analysis for January 11, 2000

Map Credit
NOAA Central Library Data Imaging Project
This sequence of GOES-8 visible images documents the progress of the front as it brought rainfall to the East Coast. By 1545 EST (2045Z), shallow convection formed in the cold air behind the front and was approaching central NJ.
1445Z
1745Z
2045Z
~1620 EST (~2120Z)

Stop 1
While leaving NEC Research Institute: observed low-topped CB with anvil silhouetted by setting sun at Independence Way and U.S. Route 1, just north of Princeton, NJ. No opportunity for photography presented itself until able to pull off Scudders Mill Road near the J & J facility and took this picture. From this vantage point, however, a stream of scud obsures the easternmost extent of the anvil, which is about 35km to the SW. At this point the sun is about 6˚ above the horizon. Direction of motion of the storm is eastward.

The location of Stop#1 is marked with a magenta "star" in the images at right.

View of shallow convective system from Plainsboro, NJ, taken with a Pentax K-1000 (50mm f/2.0 lens; Kodak ASA 200 color film)
2117Z 0.48˚ (lowest) Reflectivity scan from Mt. Holly (Fort Dix) 2117Z 6˚ Reflectivity scan locating approximate level of anvil in photograph above, between 5km and about 6km in altitude.
~1640 EST (~2140Z)
Stop 2
The next location with good visibility of the system is a local rise on South Lane overlooking the Miry Run tributary, with a travel time of about 15-20 minutes under normal conditions from the location of Stop 1. During the drive occasional glimpses of mammatus were made. However, by the time of the second stop, the mammatus had developed into more significant features, as seen in the photograph at right.

We are very close to the storm at this point, so it is difficult to gain perspective on it. The cloud base is very low--nearly at tree-top level (see close-up, middle panel.) The convection appears to be very shallow, with an updraft feeding a long plume being carried away by strong westerly winds.

The sinking mamma-like cloud masses suggest the development of mid-air downbursts as described by Fujita in as "Type-A" or "Anvil Downbursts". (Thanks to Dr. Karl Schulze of the College of DuPage for providing this figure from Fujita's report.) However, since the convection is so shallow, these mammatus have the potential to reach the ground. Observations of mammatus which exhibited midair downburst features as they dissipated may be found here. The measured vertical velocities in mammatus clouds are on the order of a few m/s (Martner, B.E. Doppler Radar Observations of Mammatus, Bull. Am. Met. Soc. 123, 3115-3121 (1995)), so if these are low-hanging mammatus it is unlikely they resulted in significant downburst winds if they reached the ground before dissipating.
Looking west towards Edinburg, NJ. The inflow is clearly visible, as are mamma-like cloud masses which were observed to be sinking. Another view, shifted to the right (northwards) a bit is here.
Close-up of the cloud base, looking west towards Edinbutg, NJ. The elevation of this rise is about 95 ft above MSL.
2140Z 0.48˚ KDIX Reflectivity scan. Stops are numbered and marked with filled circles, 1-4.
~1645EST (~2145Z)
"Stop 3"
While making a stop at the Sharon School, sudden onset of gale force winds (whole trees in motion, experienced difficulty in walking) signal the possible arrival of downburst winds. Westerly gusts with virtually no precipitation (few drops) passed by within five minutes.
KDIX 0.48˚ Base Reflectivity scan at 2146Z, about the time gale force winds were experienced at the Sharon School (Marker 3)
KDIX 0.48˚ Radial Velocity scan at 2146Z, about the time gale force winds were experienced at the Sharon School (marked by white circle.) There is nothing remarkable in this scan, indicating the high winds occurred very close to ground level.
~1650 EST (~2150Z)
Stop 4
Stop on shoulder of West Manor Way, just south of the NJ turnpike overpass. Sunset is about 2156Z at this latitude and longitude. We are west and south of the core of the storm, which has moved rapidly to the east. There is no longer any evidence of a well-defined cloud base, as was noted at the South Lane stop, but from this perspective it may not be obvious.
View from West Manor Way, looking N and E towards storm as it passes eastward. High-tension lines which lie along the New Jersey Turnpike (I-95) are visible on the middle-left edge of the image.
In this picture, taken shortly after the one above, the cell is seen to have a lower and more diffuse base than surrounding clouds.
KDIX 0.48˚ Base reflectivity scan at 2152Z showing that the cell of interest is moving off towards the coast and another cell has developed in its wake (just east of KTTN.)
2152Z Radial Velocity scans show low-level divergence; this downburst developed since the 2146Z scan, and is accompanied by upper-level convergence. The scan elevation at "A" in the 4.22˚ scan is about 1.5km.
2152Z 0.48˚ KDIX Radial Velocity scan showing low-level divergent flow characteristic of downburst.
2152Z 4.22˚ KDIX Radial Velocity scan showing reversal of the radial velocity pattern--convergence just north of I-195.
Next: Reflectivity and Radial Velocity Image Loops