Maryland Squall Line and Downbursts

During the evening of 25 April 2010, an intense linear mesoscale convective system (MCS) (or squall line) tracked through central Maryland producing strong outflow winds. The squall line produced strong downburst winds in the Baltimore area between 0200 and 0230 UTC 26 April. The data buoy at Francis Scott Key Bridge recorded a wind gust of 35 knots at 0224 UTC. Both GOES-13 channel 3-4 brightness temperature difference (BTD) imagery (Pryor 2010) and radar reflectivity imagery from BWI TDWR indicated the presence of a strong rear-inflow jet (RIJ)(Weisman 1992), illustrated in Figure 1, that enhanced downdraft strength due to entrainment of mid-tropospheric dry air into the leading convective storm line of the MCS. Figure 2 shows the dry-air notch, as indicated in the BTD image, in line with the RIN in the trailing stratiform region identified in radar imagery. The juxtaposition of these two features signified the presence of a strong RIJ that was enhanced by 50 knot-winds in the elevated dry-air layer near the 500-mb level.

Figure 1. A schematic of a RIJ (from Weisman (1992)).


Figure 2. BTD image with overlying radar image from BWI TDWR (top) and RAOB from Dulles Airport (bottom). Location of FSK data buoy indicated by an "X".

Interaction of the RIJ with the leading convective storm cells in the MCS was an important forcing factor in generating intense downdrafts and subsequent downbursts in the vicinity of the Baltimore Harbor. The direction of the downburst wind gust recorded at FSK Bridge, from the west-northwest as illustrated in Figure 3, corresponded closely to the orientation of the RIJ shown in Figure 1. This provides evidence that downward momentum transport and evaporational cooling in the convective precipitation cores fostered downburst generation with this linear MCS.


Figure 3. Wind histogram from FSK Bridge data buoy.

References

Pryor, K. L., 2010: Microburst applications of brightness temperature difference between GOES Imager channels 3 and 4. arXiv:1004.3506v1 [physics.ao-ph]

Weisman, M. L., 1992: The Role of Convectively Generated Rear-Inflow Jets in the Evolution of Long-Lived Mesoconvective Systems. J. Atmos. Sci., 49, 1826–1847.