The 17 February 2010 South Atlantic Downburst Event: New Findings

During the afternoon of 17 February 2010, the Canadian Sailing Vessel (SV) Concordia sank off the coast of Brazil due to strong convective storm-generated winds. The SV Concordia was capsized in a downburst that occurred near 1722 UTC, about 290 nautical miles south-southeast of Rio de Janeiro (Capt. William Curry, SV Concordia, personal communication). Geostationary Operational Environmental Satellite (GOES)-12 Southern Hemisphere imagery was very effective in identifying the developing convective storm complex and favorable pre-conditions for downburst activity over one hour prior to the capsize of the Concordia. Satellite imagery indicated the presence of strong convective storm updrafts that resulted in heavy rainfall as well as the presence of a dry-air channel on the rear flank of the storm complex that most likely resulted in downburst generation.



Figure 1. GOES-12 infrared (IR, top) and water vapor (WV, bottom) imagery at 1709 UTC 17 February 2010. Location of the SV Concordia is indicated by an "X".

Soden and Bretherton (1996) (SB96), in their study of the relationship of water vapor radiance and layer-average relative humidity, found a strong negative correlation between 6.5μm (channel 3) brightness temperature (BT) and layer-averaged relative humidity (RH) between the 200 and 500-mb levels. Thus, in the middle to upper troposphere, decreases in BT are associated with increases in RH as illustrated in Figure 4 of SB96. In the WV image in Figure 1, a notch of warmer brightness temperatures, indicated by the "V" pattern with light green shading on the southwestern flank of the storm complex, signified the presence of lower 500-mb humidity air being channeled into the rear of the storm.


Figure 2. GOES-12 channel 3 (WV)-channel 4 (IR) brightness temperature difference (BTD) product at 1709 UTC.

The BTD image in Figure 2 at 1709 UTC 17 February marks the location of the Concordia and Rio de Janeiro.  Also shown in the image is the thunderstorm complex that produced the severe downburst.  The purple shading in the thunderstorm complex indicates the presence of intense convection and associated strong updrafts that generated heavy rainfall.  At the same time, a well-defined dry-air notch appears on the southwestern flank of the storm complex.  This dry-air notch most likely represents the 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 near 1720 UTC.  Entrainment of drier mid-tropospheric air into the precipitation core of the convective storm resulted in evaporation of precipitation, the subsequent cooling and generation of negative buoyancy (sinking air), and resultant acceleration of the downdraft.  When this intense localized downdraft reached the ocean surface, air flowed outward as a downburst.  The resulting strong winds then capsized the SV Concordia.  Note that the dry-air notch was pointing directly to the location of the Concordia, and thus, the vessel was in the direct path of downburst winds.  Ellrod (1989) noted the importance of low mid-tropospheric (500 mb) relative humidity air in the generation of the severe Dallas-Fort Worth, Texas microburst in August 1985.

Figure 3. GOES-12 BTD product animation between 1609 and 1739 UTC 17 February 2009.

The BTD image animation in Figure 3 shows that the dry-air notch was visible as early as 1609 UTC and moved east-southeastward along the rear flank of the storm between 1609 and 1739 UTC. The dry-air notch, readily apparent in both water vapor (channel 3) and BTD imagery, was an effective indicator of the immanent occurrence of a downburst. Thus, with further case studies of this phenomena to be conducted, the juxtaposition of a dry-air notch and overshooting convective storm tops, may prove to be useful in the microburst detection and forecasting process.

References

Ellrod, G. P., 1989: Environmental conditions associated with the Dallas microburst storm determined from satellite soundings. Wea. Forecasting, 4, 469-484.

Soden, B.J. and F.P. Bretherton, 1996: Interpretation of TOVS water vapor radiances in terms of layer-average relative humidities: Method and climatology for the upper, middle, and lower troposphere. J. Geophys. Res., 101, 9333-9343.