14jul2011 hail case overview 1

NWS Warning Overview

Meeting held at BOU with Eric on 7 September 2011. A replay of KFTG data and products (VIL, etc.) was done. Suspicions that storm needed a warning probably were developing during the 0104:43 volume; the decision to issue the warning was made during the 0109:21 KFTG volume. The following 3D GRAnalyst plot shows the 53-55 dBZ surface (semi-transparent), and the 60 dBZ syrface (solid purple) in the KFTG 0109:21 (warning issuance) volume:

The next plot is a similar image generated from the CHILL 0109:49 volume:

NWS Training on dual-pol hail detection

From module presented on the web, two primary areas of interest are where the PPI sweep intersects the environmental 0C and -20C levels. Basic storm structure is diagnosed using conventional reflectivity and radial velocity fields. Polarimetric moments of interest are Zdr, rhoHV and Kdp. (No mention made of Hydrometeor Classification Algorithm (HCA) products).

PPI loops during the warning decision-making phase

Reflectivity loop near 0C sounding temp level. The VCHILL base map symbols are: (h) non-severe hail diameter CoCoRaHS report; (H) severe (1 inch or larger) CoCoRaHS report; S=SPC/NCDC severe-class hail report.

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Reflectivity loop near -20C sounding temp level.

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3.6 degree PPI plots with Zh contour overlay

All plots include 3 height rings at 3, 4, and 5 km AGL. (The CHILL site elevation is 1.4 km MSL) According to the 00 UTC DNR sounding, the environmental temperatures at these three heights were ~ +3, -4, and -9 deg C. The 45, 55, and 65 dBZ reflectivity levels are shown as solid line contour overlays on each plot.

Reflectivity: The echo core associated with the Ft. Collins hailstorm starts to become evident at 0057. The three-body scattering signature appears shortly there after.

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Radial velocity (mps): Positive velocities associated with the undiluted momentum of the low level air being carried up to the PPI level in the developing updraft are apparent. The environmental airflow is seen to accelerate around the sides of the updraft. (See also the dual-Doppler plots located further down the page).

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Differential reflectivity (Zdr): A positive Zdr column exists before the intense core reflectivities appear. Zdr anomalies are also evident in the three body echo.

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H V correlation (rhoHV or NWS "cross correlation"): A localized RhoHV minimum is found in the early core development stages. A deeper minimum appears presumably as larger hail diameters reach / develop at the PPI height.

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Linear depolarization ratio (Ldr): In agreement with theory, Ldr enhancement pattern generally follows the rhoHV reductions. By the final time, high depolarization levels (above -18 dB) appear in the echo core apparently at the "root" of the three body echo.

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CSU hydrometeor ID results in the 3.6 degree elevation angle PPI scan

Color fill indicates primary hydrometeor categories for the same six sweeps shown in the previous sequence. The classifications are done by applying a fuzzy logic scheme to the polarimetric data fields. (LH and SH are large and small hail respectively). Thin contours are the 45, 55, and 65 dBZ reflectivity levels. The appearance of large hail is closely associated with the development of 55 dBZ reflectivities on this PPI surface. Based on the DNR 00 UTC sounding, the environmental temperature is ~ -4C at the central portion of the plot. (These HID plots were generated by Dr. Brenda Dolan.)

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Spotter observations and NLDN data

Spotter confirmation of severe hail production took place primarily after the storm crossed into Larimer County. The CG lightning polarity also became predominately positive during this portion of the storm's history:

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Surface verification of polarimetric signatures

(Low level polarimetric patterns are also emphasized in the NWS training materials).

Hail impacts between 0140 and 0147 UTC; largest diameter=1.25 inches; leaves shredded.

Hail impacts after pad change. Pad exposure time reported as 0147 - 0150 UTC. Impacts still sufficient to tear foil.

CAPPI plots: 7km MSL reflectivity

Based on the 00 UTC DNR sounding on 14 July, the ambient temperature at 7 km MSL was approx -11 C; the moist adiabatic temperature at this level was ~ -5 C. The following sequence of plots extends beyond the VCHILL PPI sequences shown earlier. (Those were directed primarily at the period leading up to the NWS warning issuance). During the CAPPI plot period, the echo core of interest tracked along an essentially straight line path. The red line in the CAPPI plots is located along this core motion track.

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KFTG - CHILL dual-Doppler plots

KFTG and CHILL happened to fall into reasonably good (within ~30 sec) volume start synch at 0100 and 0109 UTC. Storm-relative horizontal flow field at 7 km MSL (like the above CAPPI series) shows obstacle flow. This flow appears to be stronger and most evident on the northern flank of the storm where the horizontal flow clearly contains anti-cyclonic turning.

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The color-filled field in the following plots is the vertical air velocity field at the 7 km MSL CAPPI level at the same two dual-Doppler wind synthesis times shown in the previous plots. The vertical motions were calculated using two different integration methods: variationally adjusted and downward (only) integration. Due to the range to the storm and due the presence of ground clutter from the foothills, radial velocities at the lowest heights were not well observed. The elevation angles scanned by the two radars also resulted in incomplete sampling at the echo top level. These factors compromised the data near the areas where the boundary conditions for the vertical motion integrations were specified. Despite the resultant uncertainties, 10 - 20 mps updraft magnitudes were probably present in much of the area within the > 50 dBZ echo cores.

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Rotated vertical XC plots: reflectivity

The following vertical cross-sections were located along the red line shown in the CAPPI plots using the CEDRIC REMAP procedure. It appears that the vertical structure of the storm may have been sampled well enough to support hydrometeor identification processing.

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Rotated vertical XC plots: Zdr

The next series of plots shows the Zdr field in the same vertical cross section. The development of a hail signature reaching the surface can be seen.

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