The single (or pulse) cell microburst has been one of the most difficult damaging wind events to detect and warn for. This type of microburst has been a factor in several major aircraft accidents over the years. In the past, warnings were based primarily on descending strong reflectivity cores in a favorable microburst environment.
According to Roberts et al. (1989), there are actually four features that can be used as microburst predictors. These are (1) a descending reflectivity core aloft; (2) increasing convergence within and near the cloud base; (3) a reflectivity notch; and (4) rotation. Rotation and reflectivity notches do not appear to be reliable microburst predictors unless the first two features are also present.
Operational research has suggested the following signatures from Doppler weather radar provide good indications of potential microburst conditions:
1. Descending high reflectivity core.
2. Mid-level velocity convergence just above the cloud base.
3. Divergent winds near the surface taken directly from the base velocity or SRM products.
The first two signatures can lead to warnings issued prior to the occurrence of microburst winds. The third signature would only be present as the microburst is occurring (at close range).
On July 23, 1996, the NWS Doppler radar at Shreveport, Louisiana, detected all three of these Doppler radar signatures in a single cell thunderstorm. This storm produced a microburst of 60 mph at Waskom, Texas, 15 mi west of Shreveport (Fig. 1).
The radar images associated with this event are shown in Fig. 2. Figure 2(a) shows the low level (surface) reflectivity of a strong single cell thunderstorm on July 23, 1996, at 2149 UTC from the KSHV Doppler radar. On the same volume scan (at 2153 UTC) in Fig. 2(b), the storm relative velocity product (SRM) at 7000-ft AGL elevation (just above cloud base), shows converging winds just west of Waskom. This was the maximum magnitude of converging winds detected at any level, at any time with this storm (62 kt). This velocity convergence above the cloud base appears to be a precursor of a microburst (see signature 2 above).
Eilts et al. (1996) suggested storm velocity convergence of 44 kt or greater may produce damaging winds at the surface.
Three Doppler radar volume scans later at 2207 UTC in Fig. 2(c), the low level (surface) reflectivity of the thunderstorm is shown. Between these volume scans, a maximum reflectivity core of 61 dBZ was observed to descend. The expansion in the reflectivity pattern of this thunderstorm from 2149 UTC, Fig. 2(a), to 2207 UTC, Fig. 2(c), also indicates a descending core. Figure 2(d) shows the low level (from 0.5 deg SRM) maximum diverging winds associated with the microburst at Waskom. Comparing Fig. 2(b) and Fig. 2(d) shows the maximum converging winds aloft (7000 ft AGL) were seen about 14 minutes before the maximum diverging winds at the low level (surface) were detected. A spotter in Waskom, reported 60 mph winds and large trees down with this thunderstorm at 2215 UTC.
This Doppler radar detection of converging winds in the low to mid levels of the storm (above cloud base) could be a significant advancement in the detection and warning of single cell microbursts. The detection of velocity convergence just above cloud base (at 7000-ft elevation AGL in this case) leads one to suspect that this signature may be detectable at longer ranges, and at an earlier time, than the low level diverging winds microburst signature. Beyond the detectable range of a low level divergent winds signature, the velocity convergence above cloud base may be the best, and sometimes the only, indicator of a potential microburst event.
Additional documentation of actual cases of these Doppler radar signatures could lead to improvement in the lead time and accuracy of warnings for single cell microbursts.
Eilts, M.D., J. T. Johnson, E. D. Mitchell, R. J. Lynn, P. Spencer, S. Cobb, and T. M. Smith, 1996: Damaging Downburst Prediction and Detection Algorithm for the WSR-88D. Preprints, 18th Conference on Severe Local Storms, Amer. Meteor. Soc., 541-545.
Roberts, R.D., and J. W. Wilson, 1989: A Proposed Microburst Nowcasting Procedure Using Single-Doppler Radar. JAM, 28, 285-303.