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.

Reanalysis of 26 August Oklahoma Downburst Event

During the afternoon of 26 August 2009, strong convective storms developed along a weak, slow-moving cold front as it was tracking eastward over Oklahoma. Although there was very little temperature contrast across the front, the front acted as a convergence zone and a trigger for deep, moist convection. The pre-convective environment downstream of the cold front over western Oklahoma was dominated by vertical mixing that fostered the development and evolution of a convective boundary layer. Strong downbursts that were recorded by Oklahoma Mesonet stations between 0000 and 0100 UTC 27 August resulted from a combination of precipitation loading and sub-cloud evaporation of precipitation as described in a previous entry. New RUC model graphical guidance effectively indicated the potential for strong downbursts near and west of Oklahoma City. Parameters calculated include 850-1000mb temperature lapse rate (LR), 850-1000mb relative humidity difference (dRH), precipitable water (PW), and surface dewpoint depression (DD).



Figure 1. RUC derived temperature lapse rate and radar reflectivity with overlying surface dewpoint depression (DD,top), precipitable water (PW, middle), and vertical humidity difference (dRH, bottom) at 2200 UTC 26 August.

The table below lists three strong downbursts that occurred over central and western Oklahoma between 0020 and 0040 UTC 27 August and associated RUC-derived microburst parameters from 2200 UTC 26 August:
Time (UTC)-Station-Wind Gust (kt)-Lapse Rate (K/km)-dRH (%)-DD (K)-PW (mm)
0020-Kingfisher (K)-43-8.2-9-13-40
0030-Weatherford (W)-41-8.6-13-18-35
0040-El Reno (E)-50-8.5-17-16-40

As displayed in Figure 1, and noted in the table above, the strongest downburst recorded at El Reno, overall, was associated with local maxima in all of the listed parameters. In general, stronger downbursts were associated with steeper sub-cloud temperature lapse rates, higher storm precipitable water content, and larger surface dewpoint depressions. This suggests that a combination of sub-cloud evaporational cooling in a more well-mixed boundary layer and precipitation loading was a factor in the generation of downdraft instability and resulting strong downbursts. These conditions were effectively indicated by RUC analysis-derived parameters over two hours prior to downburst occurrence.

New Graphical Microburst Guidance Product

A new microburst graphical guidance product has been developed that employs data from the Rapid Update Cycle (RUC) model. Prototypical conditions for microbursts include a steep temperature lapse rate and decreasing humidity with decreasing height in the boundary layer. Thus, the graphical guidance product incorporates boundary layer temperature lapse rate and vertical relative humidity difference, important factors in initiating and sustaining a convective downdraft. The new guidance product demonstrated effectiveness in indicating favorable conditions for downbursts over the northern Chesapeake Bay region during the afternoon of 5 November 2009 in which a convective storm produced a strong wind gust of 38 knots at Tolchester Beach, Maryland (See 6 November blog entry) . The 1800 UTC RUC graphical microburst product indicated high downburst risk in proximity to Tolchester Beach.

Figure 1. RUC graphical microburst product at 1800 UTC November 5, 2009 with radar reflectivity from Dover Air Force Base NEXRAD at 2237 UTC overlying the image (top) and RUC model analysis sounding over Tolchester Beach, Maryland at 1800 UTC (bottom).

Figure 1 compares the new Rapid Update Cycle (RUC) graphical guidance microburst product to a corresponding RUC sounding over Tolchester Beach, Maryland at 1800 UTC, 5 November 2009. At 1800 UTC, the image product showed a large area with steep boundary layer lapse rates, greater than 8.5 K/km, extending from eastern Pennsylvania and New Jersey to Virginia. The product image also displayed a local maximum in vertical humidity difference over northern Maryland and southeastern Pennsylvania. The highest microburst risk was indicated where the highest vertical humidity difference was co-located with steep lapse rates in the 850-1000mb layer. Srivastava (1985) noted that microbursts are likely with lapse rates greater than 8.5 K/km. Overlying radar reflectivity imagery from Dover Air Force Base NEXRAD at 2237 UTC displayed a downburst-producing convective storm as a spearhead echo over Tolchester Beach. At 2242 UTC, the Tolchester Beach PORTS station recorded a wind gust of 38 knots. The corresponding RUC sounding profile echoed favorable conditions for downbursts in the Tolchester Beach area with the presence of a 5000-foot deep mixed layer and steep temperature lapse rate below 850mb. Although radar reflectivities with this storm were not impressive (25-40 dBZ), the steep lapse rate was a major contributor to downdraft instability. As demonstrated in this case study, the RUC graphical guidance product, visualized by McIDAS-V software, effectively highlighted a region favored for strong convective winds. The ability to overly radar reflectivity on the microburst product shows the utility of the product in the downburst nowcasting process.

December Downburst over the Chesapeake Bay

During the early morning of 3 December 2009, strong convective storms developed over the lower Chesapeake Bay region ahead of a cold front. A supercell storm produced a strong downburst over the lower Chesapeake Bay, with a wind gust of 44 knots recorded at York River East Rear Range Light PORTS station at 0636 UTC as shown in Figure 1.


Figure 1. Wind histogram from York River East Rear Range Light PORTS station.

Although radar reflectivity displayed in Figure 2 was modest with the supercell (35-40 dBZ), downward horizontal momentum transport within a shallow mixed layer still resulted in the generation of strong surface winds.


Figure 2. Radar reflectivity image from Wakefield, Virginia NEXRAD.

The 0600 UTC RUC analysis sounding near the mouth of the York River, displayed in Figure 3, indicated wind speeds between 40 and 45 knots about 1000 feet AGL. This supercell downburst was a typical cold-season low CAPE, strong shear-forced event that is not well-anticipated by the GOES microburst products. In this case, downburst generation was driven by a combination of precipitation loading and downward momentum transport processes.


Figure 3. RUC analysis sounding at 0600 UTC 3 December 2009.