DSD characteristics of a cool-season tornadic storm using C-band polarimetric radar and two 2D-video disdrometers

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1 DSD characteristics of a cool-season tornadic storm using C-band polarimetric radar and two 2D-video disdrometers M. Thurai 1, W. A. Petersen 2, and L. D. Carey 3 1 Colorado State University, Fort Collins, CO, USA, merhala@engr.colostate.edu 2 NASA/MSFC, Huntsville, Alabama, USA 3 University of Alabama, Huntsville, Alabama, USA (Dated: 11 July 21) M. Thurai 1. Introduction A cool-season severe storm produced an EF-2 tornado in and around downtown Huntsville, Alabama, during the early evening hours of 21 January 21. Figure 1 shows a photograph of the storm. Just prior to tornado touchdown, the storm passed directly over a bermed research area, heavily instrumented to measure precipitation properties, including two 2Dvideo disdrometers (2DVDs). One of the 2DVDs is a low profile unit and the other is a new next generation compact unit currently undergoing performance evaluation. Storm evolution was also observed by a C-band dual-polarimetric Doppler radar located about 15km southwest of the disdrometers. The radar (ARMOR, see Petersen et al. 27) sampled low-to-mid levels (-4 km) of the storm at close range (5-25 km) every 1-3 minutes leading up to and including tornado touchdown. We report here measurements from the two 2DVDs as well as polarimetric radar observations and estimates of DSD characteristics from the data. 2. The two 2D video disdrometer measurements Fall velocity, drop size and shape are three quantities (as well as drop canting and drop horizontal velocity) which are measured accurately by the 2DVD (Schönhuber et al., 28), except for very high wind cases where the de-skewing techniques for retrieving individual drop information can become difficult (Huang et al. 28). For the tornado case, the drop vertical velocities and the size distributions determined from the 2 sets of 2DVD measurements were compared with each other in order to ensure that there were no significant de-skewing problems. Figure 2 compares the drop vertical velocities from the two 2DVDs for the individual hydrometeors and Figures 3a and 3b show the 1-minute drop size distributions from the two instruments. Fig. 1: Picture of the tornadic storm Fig. 2: Vertical velocity versus drop diameter from the two 2DVDs, together with the expected fall velocities for rain From Figure 2 we note the following: (1) that the low profile and the compact 2DVD give very similar results, and (2) that both in turn agree with the expected velocity variation as given in Atlas et al. (1973), to well within 15%. Apart from a small percentage of points, the agreement implies that the hydrometeors are fully melted, at least at ground level. The measured axis ratios also showed close agreement with expected values and distributions, and moreover, the drop images from the 2 line scan cameras from each of the 2DVDs did not deviate noticeably from our reference drop shapes.

2 (a) (b) Fig. 3: DSD time series of concentration (in mm-1 m-3) as color intensity plot (log scale), from the two 2DVD (a)the low profile unit and (b) the new compact unit. The upper marks depict the mass-weighted mean diameter (Dm) while the lower marks depict the standard deviation or width of the mass spectrum (σm). The 1-minute DSDs (Figures 3a, 3b) also agree well between the two instruments. Superimposed on the color plots as black points are D m (upper points) and σ M (lower points), which corresponds to the mass weighted mean diameter and the standard deviation of the mass spectrum. The two parameters are compared separately in Figure 4, together with rainfall rates determined from the 2 2DVD measurements. In both cases, D m is seen to exceed 3 mm at around 23:5 utc and σ M reaches nearly 1.5 mm and maximum drop diameter of just over 6 mm. Between 23: and 23:1 utc, the 1-minute fitted gamma distributions gave Log 1 (N W ) to be around 2.85 and D to be around 2.25 mm, where N W and D are defined in Bringi et al. (23). The N W - D point falls within the continental cluster of points for convective rain in the global plot given in Bringi et al. (23). The touch-down time for the tornado was around 23:18 utc. 3 R (mm/h) D m (mm) σ m (mm) UTC on 21 Jan 21 Fig. 4: Time series comparisons of rainrate (upper panel), D m (middle panel) and σ M (lower panel); blue curve represents the estimates from the low-profile 2DVD measurements and the red curve represents the compact 2DVD, using 1-minute DSDs in both cases.

3 3. C-band polarimetric observations Volume scans were made with the C-band polarimetric radar, ARMOR, located 15 km away form the two 2DVDs. Two PPI sweeps taken at 1.3 deg elevation are shown in Figure 5. The upper four panels were taken just before touchdown (23:15) and the lower four panels were taken almost at the touch-down time (23:18). In each case, the panels show (i) the attenuation corrected copolar reflectivity (Z h ), (ii) the attenuation corrected differential reflectivity (Z dr ), (iii) the co-polar correlation coefficient (ρ hv ) and (iv) the radial velocity. The grey star in the left hand panels denotes the 2DVD location. Note also that the co-polar correlation coefficient is given in terms of 1- ρ hv in order to show more clearly the reduction in ρ hv within the storm. The attenuation correction procedure for Z h was based on the iterative ZPHI method (Bringi et al. 21), and for Z dr, a gate-to-gate correction procedure was applied based on the specific differential phase (K dp ) estimated from the differential phase (Ф dp ) measurements. However, consistency checks made using the attenuation corrected measurements implied that for this event the K dp versus A dp (specific differential attenuation) had to follow a power-law formula representing big-drop regime (see Bringi and Chandrasekar, 21, section 7.4) at C-band. Fig. 5: PPI scans taken a few minutes prior to touchdown (top 4 panels at 23:15) and at around touchdown (23:18). The left hand panels show the attenuation corrected reflectivity, the second set shows the attenuation corrected differential reflectivity, the third set shows the correlation coefficient (1-ρ hv ) and the right hand panels show the radial velocity. The eye of the storm in Fig. 5 is evident mainly in the radial velocity. It is small in dimension and is characterized by a sharp velocity gradient at a distance of 13 km along the horizontal (east) and 6 km along the vertical (north) at 23:18. The reflectivity in the region is too low (almost below noise) because of the lack of hydrometeors in this region. Rather, the storm eye would be expected to have debris trapped inside it. In fact, the streak in the 1-ρ hv behind the eye and the corresponding streak in Z dr (which has negative values) may well be due the debris depolarizing the radar signal, which will have an impact on the radar measurements in terms of cross-coupling effects. For other radar azimuths, such effects are not noticeable. In the regions surrounding the storm eye, rain echoes are seen, with attenuation corrected Z h reaching 6 dbz, Z dr reaching 6 db and ρ hv reaching as low as.9, indicating wide DSDs and significant number of large drops. The backscatter differential phase (δ) also showed high values reaching 2 degrees or more in the cyclonically-curved region immediately surrounding the storm eye (which also had high Z dr ). The variation of δ with attenuation corrected Z dr from the radar measurements in the curved region is shown in Figure 6a. Superimposed as pink dots are calculations using the 1-minute DSD from the 2DVD data, with bulk assumptions on the drop shape and canting angle distributions. The agreement between radar estimates and the 2DVD based calculations is reasonable, given the inherent noise in Ф dp and bearing in mind that the calculations use a single formula for drop axis ratio versus drop diameter and further assume drop canting angle distributions to be Gaussian,

4 with 7 deg standard deviation. If individual drop information is used and drop-by-drop calculations are performed, the spread in the 2DVD data based calculations will be much higher. Figure 6b shows the additional points (in green) which represent calculations using 3 second DSD from the 2DVD. 4 (a) Delta (deg) 2 (b) Corrected Zdr (db) Fig. 6: Scatterplot (blue marks) of δ vs Zdr from a PPI sweep, and 2DVD data based calculations (pink and green dots) using (a) 1-minute DSDs (pink) and (b) 3-second DSD (green). Since δ determined from the radar Ф dp measurements is noisy, it has rarely been used quantitatively for DSD estimates. From our perspective, large δ implies large drops (> 6 mm due to resonance effects at C-band) and this in turn implies DSDs with large D m and σ M. Our assumption is that such large drops originate from graupel/small hail which are at the end of their melting process and as far as the Z dr is concerned appears as large oblate hydrometeors. In any case, the hail detection HDR signature was used to ensure that larger, more spherical, hail are not present in the 1.3 degree PPI scans used here. In an earlier study at a different location, Thurai et al. (28) had observed a decrease in ρ hv at C-band for several rain events with broad drop size distributions (DSD). Observational evidence came from simultaneous measurements with a C band dual-polarization radar (King city radar; see Hudak et al. 26) and a 2DVD. The possibility of utilizing the ρ hv decrease for DSD retrievals has also been considered. A preliminary method using the Z h, Z dr and ρ hv was formulated for estimating the DSD parameters. Validation was carried out by deriving the differential propagation phase (Ф dp ) from the estimated DSD parameters and comparing against radar measurements. Further assessment was performed by examining the 2D distributions of the normalized intercept parameter N W versus D m. The method had shown potential but needed to be further assessed in different rain climatologies. The same method (given in section 3a in Thurai et al. 28) was applied for the tornado case. Fig. 7 shows the resulting D m and σ m for the two sweeps considered earlier in Fig. 5. In the wall of the storm surrounding the eye (i.e. the curved region), high D m and σ m are seen, particularly at 23:18, i.e. during tornado touchdown. D m values exceed 3 mm in some regions along the swirl (probably due to drop sorting), with σ m reaching 1.5 mm or so. This is qualitatively consistent with the 2DVD data based estimates given in Fig. 3a and 3b earlier. Fig. 7: Estimates of Dm and σm for the two PPI sweeps given in Fig. 5. Finally in Figure 8, we show the attenuation corrected Z h and Z dr as well as ρ hv extracted from the volume scans at four different azimuths to show the vertical sections of the storm. At heights above 1.5 km, there are regions with high Z h and low Z dr, which can be ascribed to hail/graupel hydrometeors whereas at lower heights, there are regions with high Z h and low Z dr,

5 which are probably due to fully-melted hydrometeors (from hail/graupel above). The fall velocity measurements (Fig. 2) from the two 2DVDs corroborate this at ground level. The D m and σ m estimates shown in Fig. 7 (< 3 m a.g.l) are therefore representative of rain DSDs for this tornado, although it should be noted that the algorithm used for the retrievals (particularly for determining σ m ) needs to be more tuned for this special case. Fig. 8: Vertical scans at various azimuths extracted from the volume scans made at 23:18 (around the touchdown period). 4. Summary Drop size distributions in an evolving tornadic storm have been examined using C-band polarimetric radar observations and two 2D-video disdrometers. Analyses of the radar data indicate that the main region of precipitation should be treated as a big-drop regime case, in particular the variation of differential reflectivity versus differential backscatter phase. Standard attenuation-correction methods using differential propagation phase have been fine tuned to be applicable to the big drop regime. The corrected Z h and Z dr data were combined with ρ hv and specific differential propagation phase to determine the D m, and σ M, Significant areas of high D m (3-4 mm) were retrieved within the main precipitation areas of the tornadic storm, together with σ M values of mm. The values are consistent with the two sets of 2DVD measurements. The big drop regime assumption is substantiated by the two sets of 2DVD measurements. The D m values calculated from 1-minute drop size distributions reached over 3 mm, whilst the maximum drop diameters were over 6 mm. The fall velocity measurements from the 2DVD indicate almost all hydrometeors to be fully melted at ground level. Height-distance data extracted from the radar volume scans indicate the presence of hail/graupel at heights above km which subsequently melt to produce large drops at ground level.

6 The eye of the storm was mostly visible in the radial velocity measurements, although the debris trapped within it seems to have caused a streak in Z dr behind the debris region (i.e. relative to the radar) and a corresponding streak in ρ hv. This is thought to be due to radar signal depolarization and the consequent effects on radar measurements in terms of cross-coupling effects and if so, these effects can prove useful in tracking tornadic storms at C-band. Acknowledgment The NASA program NNX9AD72G enabled the data collection and subsequent analyses of the two 2DVD measurements as well as the C-band radar observations. We also wish to thank Prof. V. N. Bringi, for helpful remarks. References Atlas, D., Srivastava, R. C., and Sekkon, R. S. 1973: Doppler radar characteristics of precipitation at vertical incidence. Rev Geophys Space GE 2, 1-35 Bringi, V.N. and V. Chandrasekar, 21: Polarimetric Doppler Weather Radar: Principles and Applications, Cambridge University Press, pp 636. Bringi, V.N., Keenan, T.D. and Chandrasekar, V., 21: Correcting C-band radar reflectivity and differential reflectivity data for rain attenuation: A self-consistent method with constraints, IEEE Trans. Geosci. Remote Sens., 39, Bringi, V. N., Chandrasekar, V., Hubbert, J., Gorgucci, E., Randeu, W. L., Schoenhuber, M., 23: Raindrop size distribution in different climatic regimes from dis-drometer and dual-polarized radar analysis. J. Atmos. Sci. 6, Hudak, D., Rodreguez, P., Lee, G. W., Ryzhkov, A., Fabry F. and Donaldson, N., 26: Winter precipitation studies with a dual-polarized C-band radar, Proc. 26 European Conf. Radar Meteor. (ERAD 26), Barcelona, Spain, Sept. 26. Huang, G.J., V. N. Bringi and M. Thurai, 28: Orientation angle distributions of drops after 8 m fall using a 2D-video disdrometer, J. Atmos. Oc. Tech., 25, , DOI: /28JTECHA175.1 Petersen, W. A., K. R. Knupp, D. J. Cecil, and J. R. Mecikalski, 27: The University of Alabama Huntsville THOR Center instrumentation: Research and operational collaboration, extended abstract P. 8A.8, 33rd Conf. on Radar Meteor., Aug. 27, Cairns, Australia. Schönhuber, M., G. Lammer and W.L. Randeu, 28: The 2D-Video-Distrometer, Chapter 1 in Precipitation: Advances in Measurement, Estimation and Prediction, Michaelides, Silas (Ed.), Springer, 28. ISBN: Thurai, M., Hudak D., and Bringi, V. N., 28: On the Possible Use of co-polar correlation coefficient for improving the drop size distribution estimates at C-band, J. Atmos. Oc. Tech., 25, , DOI: /28JTECHA177.1.