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.
Kenneth, this is a nice post and a good blog.
ReplyDeleteWould it be possible to use the GOES microburst potential data to develop a microburst climatology? This would be of great interest to engineers who use microburst wind gusts as design wind speeds for structural design, and also for people (such as myself) in regions (Australia) where ground based monitoring networks are perhaps not as good as in the USA.
Matt- Thanks for the kind words and encouragement. Yes, definitely the GOES microburst products are useful for microburst climatology studies. I recently completed a validation project for two GOES microburst products over the southern Great Plains of the U.S. for 3 convective seasons, 2007-2009, in which 168 downburst events were documented. The 2009 convective season was the most active of the 3-year period over the southern plains. The results are presented in the following paper: http://knol.google.com/k/kenp/microburst-windspeed-potential/2ngphpf1g97s/1.
ReplyDeleteThe imager microburst algorithm could be easily generated using MTSAT channels IR1, IR2, and IR3. Thanks again let me know if you have any further questions.