April Bow Echo and Downbursts

During the evening of 8 April 2010, a line of strong convective storms developed over northern Virginia and tracked northeastward over the Washington, DC and Baltimore, Maryland metropolitan areas between 2345 UTC 8 April and 0045 UTC 9 April 2010. The convective storm line evolved into a bow echo and produced widespread damaging winds in the Washington, DC area near 0000 UTC. Geostationary Operational Environmental Satellite (GOES)-12 imagery was very effective in identifying the evolution of the convective storm system and favorable pre-conditions for downburst activity at least fifteen minutes prior to the observation of severe winds in Washington, DC. Satellite and radar imagery indicated the presence of a dry-air channel on the rear flank of the bow echo (Przybylinski 1995) that most likely resulted in downburst generation. It has been found recently that the BTD between GOES infrared channel 3 (water vapor at 6.5μm) and channel 4 (thermal infrared at 11μm) can highlight regions where severe outflow wind generation (i.e. downbursts, microbursts) is likely due to the channeling of dry mid-tropospheric air into the precipitation core of a deep, moist convective storm.


Figure 1. GOES-12 channel 3 (WV)-channel 4 (IR) brightness temperature difference (BTD) product at 2345 UTC April 8, 2010 with overlying radar reflectivity from Washington Terminal Doppler Weather Radar (TDWR) at 2355 UTC. White line represents the co-located dry-air notch and rear-inflow notch pointing to Washington, DC National Ocean Service observing station (location “X”).


Figure 2. GOES-12 channel 3 (WV)-channel 4 (IR) brightness temperature difference (BTD) product at 0045 UTC April 9, 2010 with overlying radar reflectivity from Baltimore-Washington International Airport TDWR at 0034 UTC. White line represents the co-located dry-air notch and rear-inflow notch pointing to Francis Scott Key Bridge, Maryland National Ocean Service observing station (location “X”).

The BTD images in Figures 1 and 2 mark the location of downbursts recorded at Washington, DC and Francis Scott Key (FSK) Bridge, Maryland National Ocean Service (NOS) observation stations. A well-defined southwest-to-northeast oriented dry-air notch appears on the southwestern flank of the bow echo that is co-located with a broad rear-inflow notch (RIN) as identified in radar imagery. This dry-air notch satellite feature, co-located with the RIN, most likely represents drier (lower relative humidity) air that was channeled into the rear of the storm and provided the energy for intense downdrafts and the resulting downburst winds at Washington (50 knots) near 0000 UTC and at FSK Bridge (42 knots) at 0036 UTC April 9, 2010. Note that the dry-air notch was pointing directly to the location of the observed downburst winds in both images.



Figure 3. GOES-12 channel 3 (WV)-channel 4 (IR) brightness temperature difference (BTD) product at 0015 UTC 9 April 2010 (top) and a plot of transmittance weighting functions for channels 3 and 4 compared to the temperature and mixing ratio curves of the 0000 UTC RAOB at Dulles Airport (bottom). Location of Dulles Airport is marked by an "X".



Figure 4. Conceptual model of a strong bow echo evolution showing bookend vortices and development of a Rear-Inflow Notch (RIN) (top) and schematic of a vertical cross-section through a mature bow echo (bottom). Courtesy COMET (1999).

Comparison of product images to transmittance weighting functions and a radiosonde observation at Dulles International Airport, Virginia at 0000 UTC 9 April emphasizes the importance of the dry air in the rear inflow region in the generation of intense downdrafts. Figure 3 shows a steep, near dry-adiabatic temperature lapse rate in the mid-troposphere, between the 300 and 500-mb levels, and a co-located dry-air layer with a mixing ratio near zero. The channel 3 weighting function peak near the 400-mb level and channel 4 weighting function peak near the surface is associated with an overall steep temperature lapse rate between the 300-mb level and the surface. More information pertaining to transmittance weighting functions is available at this site. Thus, the maximum BTD value near 30K observed at 0015 UTC near Dulles Airport can be related to a convectively unstable environment with sufficient mid-level dry air to generate strong downdrafts as the dry air is channeled into the rear flank of the convective storm line and heavy precipitation core. A correlation has been found between maximum BTD in the dry air notch and corresponding downburst wind gust magnitude: 39K over northern Virginia at 2345 UTC associated with the 50-knot gust observed at Washingtion, DC at 0000 UTC; and 30K southwest of Baltimore associated with the 42-knot gust observed at FSK Bridge at 0036 UTC. This entrained dry air results in evaporation of descending precipitation and cooling within the downdraft, and the subsequent acceleration of the downdraft toward the surface. When this intense downdraft impacts the surface, air flows outward as a downburst. A graphical description of the physical process of downburst generation within a bow echo is presented in Figure 4.

References

COMET, 1999: Mesoscale Convective Systems: Squall Lines and Bow Echoes. Online training module, http://www.meted.ucar.edu/convectn/mcs/index.htm.

Przybylinski, R.W., 1995: The bow echo. Observations, numerical simulations, and severe weather detection methods. Wea. Forecasting, 10, 203-218.