DPWX/Strong attenuation and differential attenuation at X-Band: 14 July 2015

From CHILL

Jump to: navigation, search

Author: Patrick C. Kennedy

14jul2015 2142 XB Zdr contours anot.png

CSU-CHILL X-band reflectivity data in a low elevation PPI scan through a thunderstorm. Solid white contours enclose the -4 and -8 dB Zdr values marking radials with significant differential attenuation. Time lapse loops showing the evolution of the X-band attenuation effects as this thunderstorm intensified have been prepared.



Contents

Overview

On 14 July 2015 the S-band component of the CSU-CHILL radar was out of service, so data collection was limited to the X-band portion of the radar. 360 degree low level elevation angle (1.5 deg) surveillance scans were started near 2132 UTC as thunderstorms developed within 30 km range. One storm located ~18 km east of the radar showed a well-defined increase in maximum reflectivity between 2132 and 2142 UTC. As this echo core intensified, and rain rates increased, signatures of attenuation and differential attenuation became increasingly apparent in the down-range portions of the echo.

Reflectivity loop

The following loop shows the reflectivity data in a sequence of eight PPI scans at ~1 minute and 40 second time intervals. In the final three frames, radially-aligned areas of reduced reflectivity begin to appear at ranges beyond the high reflectivity regions. These signal reductions are due to the attenuation that the X-band radar waves experience as they propagate through the heavy rain areas. This propagation loss diminishes the intensity of the radar illumination available in the areas beyond the intense cores, producing the radially-aligned reduced-reflectivity "shadows".


Click play to begin animation

Delay: ms

Linear
Rock

Phidp loop

The next loop shows the differential propagation phase data from the same series of PPI scans. As the colors move from green through white towards yellow (i.e., upwards along the color bar) along a radial, the received phase of the horizontally (H) polarized received signal is becoming increasingly lagged with respect to the vertically (V) polarized received phase. This phase lag develops when the radar waves travel through heavy rain areas where high concentrations of oblate raindrops exist. Large phidp accumulations with increasing range, indicated by the attainment of hues near the top of the color bar, occur during the later time periods when heavy rain is more widespread. These differential phase accumulations can be used to develop corrections for attenuation at X-band [Gorgucci and Chandrasekar, JTEC 2005]


Click play to begin animation

Delay: ms

Linear
Rock

Kdp loop

The range derivative of phidp (Kdp) is proportional to rain rate. The next loop shows the Kdp values derived from the preceding phidp data. Peak one-way Kdp values of at least 9 deg / km appear by the end of the loop. At X-band, a 9 deg / km Kdp value corresponds to a rain rate of ~100 mm / hr.


Click play to begin animation

Delay: ms

Linear
Rock

Zdr loop

The same high concentrations of oblate rain drops that generate large phidp shifts / significant Kdp levels also cause greater attenuation to occur for H polarized waves relative to V polarized waves. Since differential reflectivity (Zdr) is formed from the logarithm of the H/V received signal power ratio, negative Zdr values will occur when the H received power become less then the V received power level. This effect can be seen in the following Zdr image loop. Significantly negative (dark blue / ~ -8 dB) Zdr values appear down-range of the heavy rain areas during the final three loop frames.


Click play to begin animation

Delay: ms

Linear
Rock

KFTG loop

To obtain a sense of the time evolution of the heavy rain cores that were producing the X-band attenuation effects, vertical cross sections were constructed from a sequence of three NWS KFTG radar volume scans (2134 - 2145 UTC). The KFTG radar is located near Denver, ~75 km south of the storm of interest; this long range prevented the filling of grid points in the cross section plane at heights below 3 km AGL. The vertical cross section was constructed essentially along the 086 degree azimuth from the CSU-CHILL radar. This azimuth was chosen to intercept one of the significant attenuation areas contained in the final X-band PPI scan (~2142 UTC). The high (> 60 dBZ) reflectivities associated with the X-band attenuation effects appear to be due to an echo core that was descending towards the surface.


Click play to begin animation

Delay: ms

Linear
Rock

2142 UTC X-band plots

The final pair of plots show selected X-band data observed in the last PPI scan through the storm at 2142 UTC. For reference, the reflectivity plot is repeated with example attenuation regions marked:

14jul2015 2142 XB Z V2 anot.png

The final plot shows the corresponding Zdr field with the 5 deg / km Kdp contours added as an overlay. The occurrence of significant differential attenuation (evidenced by highly negative Zdr values) down-range from the Kdp maxima is apparent. Corrections for differential attenuation effects need to be applied to X-band Zdr data when oblate rain drops are present along appreciable portions of the beam path [Matrosov et al, 2014]


14jul2015 2142 XB DR KD anot.png

References