High Temporal-Resolution Monitoring of Ice Crystal Alignment

High Temporal-Resolution Monitoring of Ice Crystal Alignment

Mark Weber¹, Dusan Zrnic², Pengfei Zhang¹

 

¹NOAA OAR National Severe Storms Laboratory and Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, Oklahoma 73072

 

²NOAA OAR National Severe Storms Laboratory, Norman, Oklahoma 73072

Abstract:  NOAA and FAA are developing concepts  and analyzing risk reduction for a next-generation Multifunction Phased Array Radar.  MPAR could subsume current national operational radar functions including surveillance for civil aviation, airport wind shear detection, severe weather observation and warning, quantitative precipitation monitoring and air-domain security.  MPAR will exploit highly digital, active electronically scanned array technology to perform these missions.

This paper discusses a previously undocumented MPAR capability:  monitoring of volumes where ice crystals are aligned by the strong electric fields inside thunderclouds, with full volumetric coverage and very high temporal resolution.  This phenomenon has been well documented in the literature (e.g. Hendry and McCormick, 1976, Krehbiel et al. 1996, Ryzhkov and Zrnic, 2007).  However, the mechanically scanned dual-polarization radars employed for these studies limited the volume of the storm that could be monitored while maintaining sufficient temporal resolution.  (The build-up and dissipation of strong electric fields in thunderclouds occurs, of course, on a time scale comparable to the interval between lightning flashes.)

MPAR’s concept of operation includes an air traffic control track-while-scan function that will provide very frequent (every 5 to 15 seconds) volumetric measurements, likely using a circular-polarization basis.  A parallel processing channel could exploit data from these scans to monitor field-induced ice-crystal alignment, with update rates comparable to the inter-flash period.  Such data could, in turn, provide operationally valuable information on microphysical conditions in the cloud.  For example, Ryzkhov (personal communication, 2015) has suggested that crystal volumes “capping” a column of enhanced differential reflectivity (Z_DR) may improve definition of the vertical extent and strength of updrafts.  Additionally,  the capability to infer the location and strength of electric fields in clouds may in some circumstances support improved warning of lightning hazards to people on the ground, or to lightning-sensitive operations such as space vehicle launch.

To evaluate this concept, we conducted measurements with the National Severe Storms Laboratory’s mobile, 3 cm wavelength, dual-polarization radar (NOXP).   Repeated RHI scans at a single azimuth were performed, thus realizing update rates comparable to what will be possible with MPAR, albeit in only one vertical plane.  A number of storm systems were sampled, including a discrete supercell and a decaying quasi-linear convective system.  Operating in simultaneous horizontal-vertical (SHV) transmission mode,  we observed volumes of negative specific differential phase (K_DP) and noiselike differential reflectivity (Z_DR) “streaks” -- both characteristic of the depolarizing effects of field-aligned crystals (see Ryzkhov and Zrnic, 2007).   These signatures were observed well above the freezing layer in portions of the clouds where strong electrification would be expected.  In the case of a nighttime storm where lightning could be reliably observed visually, the temporal development of the alignment signals correlated strongly with the lightning.  Analysis of lightning radio-noise source locations using the Oklahoma Lightning Mapping Array (LMA) is underway to more comprehensively examine correlations between the aligned crystal volumes and lightning.

In the paper, we will discuss the NOXP observations and follow-on efforts to better understand possible applications of this unique MPAR measurement capability.  Questions as to appropriate scan strategy, polarization basis, H-V isolation requirements, waveform design and processing approach will be addressed.  The planned deployment of a polarimetric MPAR “Advanced Technology Demonstrator (ATD)” at NSSL in 2018 presents a near-term opportunity to systematically study the utility of monitoring field-aligned ice crystal volumes with high spatial and temporal resolution.  We will suggest how best to accomplish this and some of the associated challenges.

 

References:

 

Hendry, A. and G.C. McCormick, Radar observations of the alignment of precipitation particles by electrostatic fields in thunderstorm, J. Geophys. Res., 81, 5353-5357, 1976.

 

Krehbiel, P., T. Chen, S. McCrary, W. Rison, G. Gray and M. Brook, The use of dual channel circular-polarization radar observations for remotely sensing storm electrification, Meteor. Atmos. Phys., 59, 65-82, 1996.

 

Ryzhkov, A.V. and D. S. Zrnic, Depolarization in ice crystals and its effect on radar polarimetric measurements, J. Atmos. Oceanic Technol.,24,1256-1267, 2007.

 

 

This abstract was prepared with funding provided by NOAA/Office of Oceanic and Atmospheric Research under NOAA-University of Oklahoma Cooperative Agreement #NA11OAR4320072, U.S. Department of Commerce. The statements, findings, conclusions, and recommendations are those of the author(s) and do not necessarily reflect the views of NOAA or the U.S. Department of Commerce.