The March 2004 Chesapeake Bay Downburst Event

Analysis of RUC model data has provided new results from the study of the March 2004 Chesapeake Bay downburst event. This event was associated with the Baltimore water taxi accident that occurred during the afternoon of 6 March 2004, discussed in a previous entry. Computation and analysis of RUC-derived downdraft instability parameters, including temperature lapse rate, vertical relative humidity difference, and precipitable water, revealed local maxima in proximity to downburst occurrence about one hour prior.




Figure 1. RUC derived temperature lapse rate, vertical humidity difference (dRH, middle), and precipitable water (PW, bottom) at 2000 UTC 6 March 2004 with overlying NEXRAD radar reflectivity .

The table below outlines two strong downbursts that occurred in the upper Chesapeake Bay region between 2050 and 2120 UTC 6 March 2004 and associated RUC-derived microburst parameters from 2000 UTC:
Time (UTC)/Location/Wind Gust (kt)/Lapse Rate (K/km)/dRH (%)/PW (mm)
2050/Baltimore Harbor/35 to 45/8.6/16/25
2118/Tolchester Beach/48/8.9/17/27

The first downburst resulted in the capsize of the "Lady D" in the Baltimore Harbor. As displayed in Figure 1, and noted in the table above, the stronger downburst recorded at Tolchester Beach was associated with higher values of all the listed parameters. In general, the stronger downburst was associated with a steeper sub-cloud temperature lapse rate and a larger vertical humidity difference below 850mb, and a higher storm precipitable water content. In accordance with findings of Srivastava (1985), downbursts were associated with sub-cloud lapse rates greater than 8.5 K/km. This suggests that sub-cloud evaporational cooling in a more well-mixed boundary layer and precipitation loading were factors in the generation of downdraft instability and resulting strong downbursts. These conditions, more typically found over the Great Plains during the warm season, were effectively indicated by RUC analysis-derived parameters about one hour prior to the first downburst occurrence near the Baltimore Harbor.

References

Srivastava, R.C., 1985: A simple model of evaporatively driven downdraft: Application to microburst downdraft. J. Atmos. Sci., 42, 1004-1023.