Dual-Polarimetric Challenges and Characteristics of Warm-Season Updrafts over Kennedy Space Center

The dual-polarimetric upgrade of the Weather Surveillance Radar in Melbourne, Florida, has provided the opportunity to expand the study of thunderstorms over Kennedy Space Center and Cape Canaveral Air Force Station (KSC/CCAFS). This extends a series of previous climatological and radar studies of warm-season thunderstorms (May through September) over KSC/CCAFS, with the ultimate goal of improving convective wind warnings.

With the ability now to measure relative horizontal and vertical dimensions of hydrometeors and their conforming behavior within a radar pulse volume, interpretations can be made about the type of precipitation and therefore some intra-storm dynamics. Differential reflectivity (Z_DR), correlation coefficient (ρ_HV), and specific differential phase shift (K_DP) are the dual-polarimetric products used in this study. These products are examined to identify three radar signatures within storm updrafts. Each of the three radar signatures tell in some way of the presence of liquid or mixed-phase hydrometeors above the freezing level. First, columns of Z_DR > 1 dB extending above the environmental 0°C isotherm can signify raindrops being lofted by an updraft. Second, research has shown that the 0°C isotherm can be uplifted within an updraft, resulting in a “bright band” of ρ_HV ≤ 0.93 in the melting layer, which is elevated above ambient levels. Third, K_DP > 1 °/km is a good discriminator of large raindrops in high concentrations, or water-coated hail. Large values of K_DP located above the 0°C isotherm indicate the presence of significant amounts of liquid being lofted by an updraft. With the potential for hail formation and growth in these regions of the storm, the eventual melting hailfall could enhance the negative buoyancy in a downdraft. Given the right low-level thermodynamic profile, near-surface wind gusts from the potential microburst may exceed KSC/CCAFS warning criteria.

An attempt was conducted to find correlations between the heights and magnitudes of these radar signatures versus near-surface wind gusts recorded by the high-density wind sensor network at KSC/CCAFS. Many updrafts containing Z_DR columns and large values of K_DP above the ambient freezing level were found. Using ρ_HV to find uplifted melting layer signatures was often hampered by the amount of noise within the ρ_HV field. This was likely a result of natural effects and the limitations imposed by radar scan strategies and beam broadening. Low amounts of vertical wind shear and the pulse-like behavior common to Florida warm-season convection often led to chaotic storm structure and sharp radar reflectivity gradients. As an effect, low values of ρ_HV were created by low signal-to-noise ratio (SNR) and non-uniform beam filling. To further compound the problem, low SNR and ρ_HV < 0.95 can lead to Z_DR errors > 0.3 dB, and ρ_HV < 0.90 makes K_DP unreliable. Future studies at KSC/CCAFS could benefit from the Air Force's nearby C-band radar, as well as better-tailored scan strategies, to increase spatial resolution and obtain higher values of ρ_HV. However, because the conditions dictating the strength of updrafts do not necessarily correspond with the conditions dictating the strength of downdrafts, the use of potential dual-polarimetric radar techniques likely will need to be applied within the context of the thermal, moisture, and kinematic profiles of the near-storm environment.