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Quasi-vertical profiles (QVPs) obtained from a database of U.S. WSR-88D data are used to document polarimetric characteristics of the melting layer (ML) in cold-season storms with high vertical resolution and accuracy. A polarimetric technique to define the top and bottom of the ML is first introduced. Using the QVPs, statistical relationships are developed to gain insight into the evolution of microphysical processes above, within, and below the ML, leading to a statistical polarimetric model of the ML that reveals characteristics that reflectivity data alone are not able to provide, particularly in regions of weak reflectivity factor at horizontal polarization ZH. QVP ML statistics are examined for two regimes in the ML data: ZH≥ 20 dB Z and ZH< 20 dB Z. Regions of ZH≥ 20 dB Z indicate locations of MLs collocated with enhanced differential reflectivity ZDRand reduced copolar correlation coefficient ρhv, while for ZH< 20 dB Z a well-defined ML is difficult to discern using ZHalone. Evidence of large ZDRup to 4 dB, backscatter differential phase δ up to 8°, and low ρhvdown to 0.80 associated with lower ZH(from −10 to 20 dB Z) in the ML is observed when pristine, nonaggregated ice falls through it. Positive correlation is documented between maximum specific differential phase KDPand maximum ZHin the ML; these are the first QVP observations of KDPin MLs documented at S band. Negative correlation occurs between minimum ρhvin the ML and ML depth and between minimum ρhvin the ML and the corresponding enhancement of ZH(Δ ZH= ZHmax− ZHrain).
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Analysis of Polarimetric Radar Downburst Precursors Using Automated Storm Identification and Tracking
Abstract Downbursts pose a threat to life, property, and aviation, yet they remain challenging to predict. Prior studies have found radar-based downburst signatures such as divergent and convergent velocity signatures at the surface and midlevels, respectively; descending radar reflectivity (Z) cores (DRCs); present or descending specific differential phase (KDP) cores; and troughs of decreased differential reflectivity (ZDR) collocated with decreased copolar correlation coefficient (ρhv) below the melting layer. This research expands on those studies using the multicell identification and tracking (MCIT) algorithm to automate storm detection and analyze 53 downburst cases spanning most regions of the CONUS. Individual case analysis revealed that DRCs appeared in 83% of cases, descendingKDPcores appeared in 85% of cases, andZDRtroughs and collocatedρhvdrops appeared in 89% of cases. The magnitude of low-level divergence and midlevel convergence reached a threshold of 0.0025 s−1in 68% and 83% of cases, respectively. Composite time series revealed that divergence displayed the most prominent signature near the surface; aloft,KDPat and 1 km below the freezing level, midlevel convergence,ZDRcolumn area and volume, and VIL displayed the most prominent signatures. Differences were observed between geographic regions and thermodynamic environments, with lower velocity-related and higherKDP-related values most common in the eastern United States and environments with wind index (WINDEX) < 60; conversely, higher velocity-related and lowerKDP-related values were most common in the western United States and environments with WINDEX > 60. These findings may help inform future polarimetric downburst detection and algorithm development efforts.
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- Award ID(s):
- 2110709
- PAR ID:
- 10626226
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Weather and Forecasting
- Volume:
- 40
- Issue:
- 9
- ISSN:
- 0882-8156
- Format(s):
- Medium: X Size: p. 1671-1689
- Size(s):
- p. 1671-1689
- Sponsoring Org:
- National Science Foundation
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