Downburst Applications of the Haines Index

During the afternoon of 5 August 2010, strong convective storms developed over western Maryland and Virginia ahead of a cold front and produced a severe downburst that was observed at Washington, DC National Airport. The 1900 UTC Geostationary Operational Environmental Satellite (GOES) Microburst Windspeed Potential Index (MWPI) product on 5 August effectively indicated the potential for strong downbursts over the greater Washington, DC metropolitan area during the late afternoon. The MWPI product indicated wind gust potential of 35 to 49 knots where wind gusts of 42 to 51 knots were recorded in the Washington, DC area between 1940 and 2000 UTC. The pre-convective environment over the Washington area was characterized by an "inverted-V" profile with strong lower atmospheric instability that resulted from intense solar heating during the afternoon hours. Large convective available potential energy (CAPE) and a steep temperature lapse rate, especially below the 850-mb level, were forcing factors for strong convective downdrafts.

It has been found recently that the Haines Index (Haines 1988) can provide beneficial information for assessing downburst potential. The 5 August downburst event very effectively highlighted the application of the Haines Index especially when compared to corresponding MWPI product imagery and RUC sounding profiles. The Haines Index characterizes the potential impact of dry, unstable air on wildfire behavior and growth (Haines 1988) and is driven by classic atmospheric stability indicators such as temperature lapse rate (DT) and near-surface dewpoint depression (DD). Similar to the MWPI, the Haines Index is composed of both a stability (A) and moisture (B) component. The A component represents the environmental lapse rate, while the B component is the dewpoint depression for a specific pressure level. For both components, the calculated temperature and dewpoint depression are categorized into three groups that are assigned an ordinal value of 1, 2, or 3, and then summed. The resulting index has a range from 2 (very low risk) to 6 (high risk). A Haines Index product that consists of composite of the vertical temperature difference and dewpoint depression has been implemented in Man computer Interactive Data Access System (McIDAS)-V.

Figure 1. RUC sounding at 1900 UTC 5 August 2010 (top) compared to Haines Index product based on the RUC one-hour forecast valid at 1900 UTC (bottom). White cross marks the location of Washington National Airport and the above sounding. Note that the severe downburst, indicated by the spearhead echo apparent in radar imagery, occurred in a region of maximum temperature lapse rate and dewpoint depression.

The Haines Index product in Figure 1 was generated and visualized by McIDAS-V. Figure 1 shows that the severe downburst, associated with a wind gust of 51 knots, recorded at Washington National Airport at 1952 UTC occurred in a region characterized by strong low-level instability with maxima in temperature lapse rate and dewpoint depression:

DT (950-850mb): 9C

DD (950mb): 13C

A value: 3

B value: 3

Haines Index: 6 (high)

The index value of six is the highest for the Haines Index. Consequently, this index value indicated that sub-cloud evaporational cooling in a highly unstable lower atmospheric layer fostered intense downdraft generation that resulted in the observed downburst winds. The high index value corresponded well with the unstable, "inverted-V" sounding profile displayed in Figure 1. This case shows tremendous potential for the Haines Index, originally conceived as a wildfire threat product, in downburst potential assessment.

References

Haines, D.A. 1988. A lower atmospheric severity index for wildland fire. National Weather Digest, 13, 23-27.