Storm evolution.

Figures 13 and 14 show how the storm evolved with time. In particular, they show vertical scans through the core of the storm at approximate 3 minute time intervals after the observations of Figure 12. The scan at 15:23:05 (Figure 13) showed that the reflectivity core had descended and intensified in the lower part of the storm, undoubtedly due to the fall of hail within the core. This is confirmed by the $\rho _{HV}$ observations, which showed that the region of reduced correlation aloft had weakened and did not extend as high in altitude. At the same time, the correlation reductions had intensified in the lower part of the main precipitation region. Increased negative $Z_{\rm DR}$ values, approaching -1 dB, had developed in the lower part of the hailshaft, just above the level at which liquid drops started to be detected. The hail in this location was therefore elongated vertically, possibly due to melting. The strongest correlation reduction occurred in this negative $Z_{\rm DR}$ region, where $\rho _{HV}$ dropped below 0.8.

The Figure 13 data also show that the region of large liquid drops in the inferred inflow region on the close side of the storm had expanded and intensified since the earlier scan, exhibiting a maximum $Z_{\rm DR}$ value of 2.5 dB and a significantly larger extent. A weaker $+Z_{\rm DR}$ maximum was similarly situated on the far side of the core. The $K_{\rm dp}$ results show a number of regions of strong negative (red/white) and positive (blue/white) values, indicative of regions of strong $\delta_\ell$. The inferred $\delta_\ell$regions coincided with local maxima in $Z_{\rm DR}$, further confirming the identification of these regions as containing large, aligned particles. Of particular interest is the $-Z_{\rm DR}$ maximum in the lower part of the hailshaft; this was bracketed by a strong $K_{\rm dp}$ couplet of inverse polarity (positive on the near side and negative on the far side), indicating that the hail had an inverse-polarity (i.e., positive) $\delta_\ell$ value. This is consistent with the $Z_{\rm DR}$ values of the hail being negative.

The Poincaré sphere trajectory through the lower part of the storm had similar but more pronounced features as those seen in the earlier scan (Figure 2). One difference is that in the front part of the storm the polarization state changed along a ${45^\circ}$ downward path. This is typical of entering a rain region and reflects the combined effects of simultaneously increasing $Z_{\rm DR}$ and $\phi_{dp}$, whose changes are comparable at 3 cm wavelength in the Poincaré sphere space (Krehbiel and Scott, 1999). The upward meandering of the polarization state during the final part of the trajectory is due to the decrease in $Z_{\rm DR}$ on the far side of the storm, as well as accumulated differential attenuation. At the final gate the H and V powers were again nearly equal.

An additional feature of interest in the Figure 13 observations is the radial ray of slightly positive $Z_{\rm DR}$ values in the upper part of the storm. This is not an artifact of antenna sidelobes but indicates the presence of electrically aligned ice crystals. Electrical alignment observations are discussed later; the $Z_{\rm DR}$ artifact occurs because the ice crystals were oriented at an intermediate angle between horizontal and vertical, which causes the the polarization state to move downward in the projection view, in the same direction as $Z_{\rm DR}$ from liquid drops.

The radar scan at 15:26:35 (Figure 14) showed continued descent of the hail and a corresponding intensification reduction in $\rho _{HV}$ below 2 km altitude. Positive $Z_{\rm DR}$ values were no longer detected in the lower part of the hailshaft, but liquid drops continued to be present throughout much or all of the precipitation region. The latter is seen from the dramatic increase in differential attenuation along the cursor (the blue trace in the lower left panel) and by the corresponding displacement of the final part of the Poincaré trajectory well above the U axis, corresponding to negative H-V powers and values of Q. The apparent $Z_{\rm DR}$ values on the far side of the storm were about -3 dB, indicating a cumulative differential attenuation of at least this magnitude. The final polarization state was displaced almost $20^\circ$ above the equal power plane on the Poincaré sphere.

Figure 14 shows that the lower part of the hailshaft again exhibited negative $Z_{\rm DR}$ values (of about -1 dB), by now over a larger, somewhat shallower area. The fact that the negative $Z_{\rm DR}$ region did not extend lower than it did does not mean that the hail had completely melted at the lower altitudes. Rather, any negative $Z_{\rm DR}$ of the hail would have been offset by positive $Z_{\rm DR}$ from the increasing numbers of liquid drops. The $Z_{\rm DR}$values were approximately neutral in the lower part of the hailshaft, as a result of the above effect and the increasing differential attenuation. As before, an inverted polarity $K_{\rm dp}$ region existed on the far side of the melting hail, indicating that the hail produced inverse polarity (positive) $\delta_\ell$ upon backscatter.

From the above, the precipitation in the lower part of the storm was almost certainly mixed phase. This undoubtedly contributed to the large correlation reduction that was observed. The $\rho _{HV}$ values in the white regions extended dropped to as low as 0.70. Of particular significance is the fact that the correlation remained at the low values on the far side of the main precipitation region, even though the reflectivity and precipitation rates were low in this part of the storm. Such `shadowing' is good evidence of a propagation effect; in this case the radar signal developed a substantial unpolarized component while propagating through the low-$\rho _{HV}$ main precipitation region, which accumulated to the level shown. Like differential propagation phase, an unpolarized radiation component would be caused by forward scattering, in this case from particles having random shapes and/or orientations.