November 2008 Chesapeake Bay Downburst Event

During the afternoon of 15 November 2008, a squall line developed ahead of a strong cold front over the western Piedmont region of Maryland and Virginia. Temperatures and dewpoints were much above normal for mid-November ahead of the cold front, especially over the Delmarva Peninsula where solar heating of the boundary layer during the afternoon resulted in strong static instability. The the squall line produced strong convective winds as it tracked from central Maryland through the Delmarva Peninsula during afternoon and evening. The GOES imager microburst product, derived from brightness temperature difference (BTD) between sounder band 11 (mid-level water vapor, 7μm), band 8 (longwave infrared window, 11μm), and split window band 7 (12μm), indicated favorable conditions for downbursts two to three hours prior to each event.

The first strong convective winds were observed in eastern Frederick County, Maryland between 1845 and 1900 UTC 15 November 2009. Downburst wind gusts were estimated near 35 knots based on radar velocity measurements by Sterling, Virginia NEXRAD (KLWX). Figure 1 shows the convective storm line moving through Frederick and western Montgomery Counties at 1847 UTC. Apparent is a spearhead echo embedded in the storm line east of Frederick. Immediately downstream of the line are NEXRAD radial velocities near 35 knots, as indicated by the red shading. Corresponding storm relative (outflow) wind (not shown) was measured near 30 knots. Two hours prior, the GOES imager microburst product indicated wind gust potential near 30 knots over nearby Carroll County.


Figure 1. Geostationary Operational Environmental Satellite (GOES) imager microburst product at 1647 UTC 15 November 2008, with overlying radar velocity data from Sterling, Virginia NEXRAD at 1847 UTC.

As the squall line moved east of the Chesapeake Bay, it encountered a more unstable boundary layer due to a greater amount of surface heating through the afternoon. This was reflected in figure 2, the GOES microburst product at 1846 UTC with overlying radar reflectivity imagery from Sterling NEXRAD at 2203 UTC. The microburst product indicated elevated risk (yellow to orange shading) with output BTD of 32K, corresponding to wind gust potential near 32 knots. At 2203 UTC, the storm line produced a downburst wind gust of 32 knots, very well marked in the wind speed trace at Cambridge National Ocean Service (NOS) observing station (location "X"), also shown in Figure 2. High reflectivities (>45 dBZ) were indicated over Cambridge at the time of downburst occurrence.



Figure 2. GOES imager microburst product at 1846 UTC 15 November 2008, with overlying radar reflectivity data from Sterling, Virginia NEXRAD at 2203 UTC (top); wind histogram from Cambridge NOS observing station (bottom).

About two hours later, near 0000 UTC 16 November, a stronger downburst was observed farther south on the Chesapeake Bay Bridge. The 2200 UTC (15 November) imager microburst product, shown in Figure 3, indicated slightly higher risk values, with corresponding wind gust potential of 32 to 35 knots, in proximity to the Bay Bridge. A downburst wind gust of 40 knots was recorded at the Chesapeake Bay Bridge NOS observing station (location "X") at 0006 UTC. Similar to the Cambridge downburst, the downburst-producing segment of the line exhibited high radar reflectivity near 50 dBZ. As shown in Figure 3, this downburst event was also well-marked with a sharp peak in wind speed near 0000 UTC.



Figure 3. GOES imager microburst product at 2146 UTC 15 November 2008, with overlying radar reflectivity data from Wakefield, Virginia NEXRAD (KAKQ) at 0006 UTC 16 November(top); wind histogram from Chesapeake Bay Bridge NOS observing station (bottom).

This event demonstrated the usefulness of NOS meteorological observations in the microburst product validation process over the Chesapeake Bay region. The temporal resolution of NOS data, six minutes, is well-suited for downburst observation. In addition, the environment over the Bay region was favorable for a late Fall convective high wind event as inferred by the GOES imager microburst product, characterized by steep low to mid-level temperature lapse rates. Steep lapse rates in conjunction with heavy precipitation produced by the squall line fostered strong convective downdraft development.

Downbursts in Alaska?

Yes- downbursts do occur in Alaska- especially in the Central Interior region during the summer. During the afternoon of 23 May 2009, strong convective storms developed over the Central Interior region of Alaska when temperatures were much above normal for the season. Two strong downbursts were recorded in the Fairbanks area during the late afternoon and early evening. The first downburst wind gust of 40 knots was recorded at the Alaska Climate Research Center (ACRC) weather station at 0015 UTC 24 May (1615 AKDT). A weaker downburst (35 knots) was recorded at Eielson Air Force Base (AFB) near Fairbanks about three and a half hours later at 0350 UTC. The pre-convective environment during the afternoon was more typical for the interior northwestern United States. Strong solar heating resulted in the development of a deep convective mixed layer (ML) that was favorable for downbursts. This favorable downburst environment was effectively indicated by the Geostationary Operational Environmental Satellite (GOES)-11 imager microburst product, in which output brightness temperature difference is strongly correlated with microburst risk. The 2300 UTC GOES imager product did indicate elevated microburst risk with wind gust potential of 40 to 45 knots 75 minutes prior to the downburst at ACRC. Further explanation of the GOES-11 imager microburst product is available in Pryor (2009).


Figure 1. Geostationary Operational Environmental Satellite (GOES) imager microburst product at 2300 UTC 23 May 2009 with overlying radar reflectivity from Pedro Dome, Alaska NEXRAD (PAPD) at 0015 UTC 24 May.


Figure 2. Geostationary Operational Environmental Satellite (GOES) imager microburst product at 2300 UTC 23 May 2009 with overlying radar reflectivity from Pedro Dome, Alaska NEXRAD (PAPD) at 0350 UTC 24 May.

Figures 1 and 2 displayed scattered convective storms developing over the central interior region of Alaska. Apparent in both images are downburst-producing convective storms occurring in a region of elevated microburst risk as indicated by the orange shading near ACRC ("X" in Figure 1) and the yellow shading near Eielson AFB ("X" in Figure 2). In general, output brightness temperature difference (BTD) of 35 to 45K corresponded to wind gust potential of 35 to 45 knots. A downburst wind gust of 40 knots was recorded at the Alaska Climate Research Center (ACRC) weather station at 0015 UTC 24 May (1615 AKDT) followed by a weaker downburst (35 knots) recorded at Eielson Air Force Base (AFB) near Fairbanks at 0350 UTC.


Figure 3. Radiosonde observation (RAOB) at Fairbanks, Alaska at 0000 UTC 24 May 2009.

The pre-downburst environment in the Fairbanks area is most effectively illustrated by the sounding profile shown in Figure 3. Similar to favorable downburst environments over the interior northwestern U.S., the boundary layer was well-mixed with low relative humidity near the surface and a high cloud base near the 700-mb level. This environment fosters strong convective downdrafts through evaporational cooling and negative buoyancy generation as precipitation descends in the sub-cloud layer. Thus, the downbursts that occurred in the Fairbanks area were "dry" type associated with relatively light precipitation (radar reflectivity near 30 dBZ) and an "inverted-v" sounding profile with a deep mixed layer (ML) as shown in Figure 3. Noting the elevated output BTD associated with each downburst, it is evident that the GOES-11 imager microburst product effectively captured favorable conditions for strong convective winds in the Fairbanks area during the afternoon and evening of 23 May.

References

Pryor, K.L., 2009: Microburst windspeed potential assessment: progress and developments. Preprints, 16th Conf. on Satellite Meteorology and Oceanography, Phoenix, AZ, Amer. Meteor. Soc.