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.

Early Spring Downburst Event in Oklahoma

During the afternoon of 10 March 2010, strong convective storms, triggered by an upper-level disturbance, developed over southwestern Oklahoma and tracked northeastward. The area of convective storms produced strong downbursts in the Oklahoma City area while tracking northeastward in the wake of a cold front. Although the temperature contrast across the front was weak, a sharp decrease in boundary layer moisture and increase in low-level wind speed were apparent over central and southern Oklahoma in the wake of the cold front. Strong solar heating and gradient winds behind the cold front resulted in the development and evolution of a deep convective mixed layer, especially in the Oklahoma City area. This favorable environment, characterized by steep sub-cloud temperature lapse rate, super-adiabatic surface layer lapse rate, and large vertical humidity difference, fostered intense convective storm downdrafts. This environment was effectively illustrated as a classic "inverted V" profile in the late afternoon GOES sounding in Figure 1.

Figure 1. 2200 UTC GOES sounding profile over Oklahoma City.

This sounding profile translated into a significant risk of downbursts as shown in the GOES imager microburst products in Figure 2. About one hour later, strong downbursts were observed north of Oklahoma City, near the center of the upper-level disturbance.



Figure 2. GOES imager microburst products at 2200 UTC.

Figure 2 shows two versions of the GOES microburst product. The McIDAS-V version (bottom), displays overlying NEXRAD reflectivity from Oklahoma City (TLX) and the downburst-producing convective storms north of Oklahoma City near 2300 UTC. The microburst product indicates moderate risk (yellow to orange shading, top, green to yellow shading, bottom) associated with a wind gust potential of 35 to 40 knots. About one hour later, the two strongest downbursts of the event were recorded at Guthrie (45 knots) and Marshall (44 knots) Oklahoma Mesonet stations. This event demonstrated the effectiveness of the GOES microbust products, especially in this environment more typical of the summer season over Oklahoma.

The 17 February 2010 South Atlantic Downburst Storm

During the morning of 17 February 2010, the Canadian Sailing Vessel (SV) Concordia sank off the coast of Brazil, near Rio de Janeiro, most likely due to strong convective storm-generated winds. The SV Concordia was capsized in a downburst that occurred around 1500 UTC about 500 km off the coast. Graphical microburst potential guidance derived from a three-hour forecast of the Global Forecast System (GFS) model, valid at 1500 UTC, indicated a high risk of downbursts in the approximate location of the sinking of the Concordia. A conditionally unstable temperature lapse rate, large atmospheric precipitable water content, and the presence of mid-tropospheric dry air, as indicated by low relative humidity at the 500-mb level, contributed to strong downdraft instability that resulted in the downburst and hazardous winds on the ocean surface.

Figure 1. GFS model-derived graphical downburst potential product at 1500 UTC 17 February 2010.

A favorable downburst environment is illustrated in Figure 1, with the highest risk area outlined in red. The juxtaposition of a conditionally unstable temperature lapse rate (> 7K/km), large atmospheric precipitable water values near 50 mm (2 inches), and low mid-tropospheric (500 mb) relative humidity near the location of the sinking of the SV Concordia suggests that precipitation loading and mid-level dry air entrainment were major forcing mechanisms for strong convective winds. The presence of mid-tropospheric dry air fostered convective instability and the generation of negative buoyancy due to evaporational cooling as the drier air was entrained into the precipitation core of the convective storm. The combination of these forcing mechanisms would result in the acceleration of a downdraft to the ocean surface and the generation of a downburst that capsized the SV Concordia. For more information, please read:

http://www.cbc.ca/canada/story/2010/02/22/sinkingbrazil-shiptimeline.html