Assessment of GOES Sounder Microburst Product

During the afternoon of 4 May 2009, supercell convective storms developed over the coastal plain regions of Virginia and North Carolina. A supercell produced strong downbursts in the Norfolk, Virginia area near 1800 UTC 4 May. Measured downburst wind gusts ranged from 42 to 48 knots. Elevated index values were indicated by the new Geostationary Operational Environmental Satellite (GOES) sounder microburst product in proximity to the downbursts in the Norfolk area about one hour prior (1700 UTC). The GOES sounder microburst product was introduced in the previous entry, and was found to perform effectively for the 4 May event in a similar manner to the 26 April event. National Ocean Service (NOS) Physical Oceanographic Real-Time System (PORTS) observations were utilized as verification for this downburst event.


The images above are a Geostationary Operational Environmental Satellite (GOES) sounder microburst products at 1700 UTC: McIDAS-V version with overlying radar reflectivity imagery from Wakefield, Virginia NEXRAD (KAKQ)(top) and McIDAS-X version (bottom), available on the microburst product web page, with the location of the downburst wind gust observation of 48 knots plotted on the image. The microburst product images display elevated risk values over southeastern Virginia, with a local maximum over the Norfolk area. Overlying radar reflectivity imagery displayed scattered supercell storms over eastern Virginia and North Carolina. A particularly large supercell with a well defined hook echo was indicated over the city of Norfolk. This storm produced strong downbursts between 1800 and 1810 UTC, with peak wind gusts of 45 and 48 knots observed at South Craney Island PORTS station (below) and Norfolk Naval Air Station (NAS), respectively. The wind histogram at South Craney Island clearly indicates the occurrence of a downburst as a sharp peak in wind speed near 1800 UTC.



Elevated output BTD greater than 40K (orange shading) was indicated by the GOES sounder microburst product in proximity to the downbursts about one hour prior to downburst occurrence. Previous validation has identified that BTD greater than 40K is strongly correlated to wind gust potential of 40 knots or greater (Pryor 2009). The microburst products and the 1700 UTC Rapid Update Cycle (RUC) model analysis sounding profile displayed below show that the preconvective environment over southeastern Virginia was characterized by a steep temperature lapse rate and a well-mixed boundary layer that favored the development of intense convective downdrafts and resultant downburst generation. This sounding profile can be identified as a modified "inverted V" profile.


The relatively deep mixed layer (depth near 6000 feet) present over southeastern Virginia resulted in outflow-dominated supercell storms that produced strong downbursts rather than tornadoes. McCaul and Cohen (2002) noted that the combination of a relatively high Level of Free Convection (LFC) and Lifting Condensation Level (LCL) favors outflow dominated storms that could produce strong convective downdrafts and subsequent downbursts. Precipitation loading and downward momentum transport from near the top of the boundary layer to the surface were also factors in strong downburst generation.

References

McCaul, E.W. and C. Cohen, 2002: The Impact on Simulated Storm Structure and Intensity of Variations in the Mixed Layer and Moist Layer Depths. Mon. Wea. Rev., 130, 1722-1748.

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