Estimates of Rear-Flank Downdraft Buoyancy as a Predictor of Tornadogenesis
William A. Gallus Jr., Christopher J. Anderson, and Adam E. Frederick
Iowa State University, Ames, IA
Traditional methods used in tornado forecasting rely on analyses of vertical wind shear and gravitational instability. Research shows that such methods highlight conditions that are favorable for rotating storms, i.e. supercells, but not necessarily tornadic supercells (Brooks et al. 1995, Rasmussen and Blanchard 1998). Since tornadoes do not form in a large fraction of supercells (maybe as many as 80%, Don Burgess, personal communication 2000), high false alarm rates result for tornado forecasts (Rasmussen and Blanchard 1998).
Observations from the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) have revealed that equivalent potential temperature of air in the rear flank downdraft (RFD) for tornadic supercells is warmer than for non-tornadic supercells (Erik Rasmussen, personal communication 1999). Thus, RFD buoyancy is hypothesized to be a critical factor for tornadic supercells. We present a method to estimate RFD buoyancy from soundings that represent ambient conditions.
We have analyzed 74 radiosonde launches in 1998 and 1999, which are archived at the National Climatic Data Center under the title Rawinsonde Data for North America and available online at raob.fsl.noaa.gov. A sounding was selected if it was located within 100 miles and 2 hours of the reported severe weather and appeared to represent inflow air. Soundings were screened manually to eliminate those that were affected by precipitation or frontal passages and outflow. Each sounding was identified with a supercell storm that was monitored by the Storm Prediction Center (storm discussions available online at www.spc.noaa.gov/archive). Maximum tornado intensity was taken from Storm Data.
We examined several different methods for estimating RFD buoyancy. We stratified our methods by RFD air source (either inflow or mid-level ambient), level at which descent began (300, 500, or 700 mb), entrainment, rainwater content, and dry/moist adiabatic descent. Generally, we found that moist adiabatic descent from the LCL of inflow air worked best at separating soundings associated with strong (F2-5) and weak (F0-1) tornadoes. This parameter is nearly a linear function of LCL height, a discriminating parameter in other studies (Davies 1998, Rasmussen and Blanchard 1998). Brooks et al. (1993) and Rasmussen and Blanchard (1998) hypothesize that low LCL height might reduce evaporative cooling below cloud base. A cold RFD might surge away from the storm, cutting off inflow and reducing updraft strength (and vorticity stretching).
In our data exploration, we have estimated storm motion from each sounding by the algorithm in Bunkers et al. (2000). In so doing, we have found that the magnitude of both the shear term and estimated storm motion differ for F2-5 and F0-1 soundings. The shear term is related to the development of storm rotation and is hypothesized to influence entrainment in two ways. First, Gilmore and Wicker (1998) have found that downdraft intensity in simulated supercells decreases with increasing ambient shear. Second, Droegemeier et al. (1993) has hypothesized that increasing helical flow decreases turbulence (and mixing) in the updraft. In addition, higher storm speed reduces the likelihood that outflow will move ahead of the storm.
We have modified our parameter to include adjustments for entrainment above cloud base and storm speed (referred to as the LCL-Entrainment method). At the LCL, we decrease the parcel’s mixing ratio by a fraction of the difference between the parcel’s mixing ratio and the mixing ratio of the average wet bulb temperature in the layer from the LCL to 4 km. The proportionality constant is determined from terms that represent the storm speed and 0-6 km ambient wind shear. Specifically:
where
The new parcel mixing ratio is given by:
The parcel is then cooled to saturation and lowered to the surface moist adiabatically. RFD buoyancy is the difference between the final and initial surface parcel temperatures.
The results for the LCL-Entrainment method show that RFD buoyancy in each category has a nearly symmetric distribution with the largest range for the F0-1 category (Figure 1). In this small sample, a significant difference is evident between the means of the F0-1 and F2-5 categories. However, this difference is less than within category spread, so that overlap occurs between –3 and –11.
The adjustment tends to be less for cases in the F2-5 category (Figure 2), for which the shear and storm speed terms compensate (not shown). This non-linear response in the RFD buoyancy enhances between category separation, as can be demonstrated by projecting the data onto each axis in Figure 2 and noting that a larger gap exists between circles and squares near –6 on the y-axis than near –4 on the x-axis.
Future work will focus on expanding our data set and examining the behavior of RFD buoyancy estimates near fronts. Another area of potentially useful research might be to use soundings from model analysis and forecasts.
This research was partially funded by an Iowa State University Liberal Arts and Sciences Undergraduate Poster Award.
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