Observations of Negative ZDR in Sandstorms and Storm Outflows

Polarimetric observations from a great variety of weather events became available upon the upgrade of the national WSR-88D radar network to this new technology. Herein are reported unusual negative regions of differential reflectivity within storm outflow in continental and desert areas. Three outflows that created sandstorms in Phoenix are observed to have large negative differential reflectivity ZDR regions of about -5 dB. Negative ZDR in one of three sandstorms was also observed simultaneously by the NSSL X-band dual polarimetric mobile radar. Our hypothesis is that scatterers such as pieces of grass, sticks, insects, and chaffs lofted by outflow were vertically oriented in the charged-dusts induced electric field within the sandstorm. These scatterers produced negative ZDR in the S and X-band dual polarimetric radar observations. Previous studies documented significant vertical electric fields caused by charged dust in sandstorms. These electric fields were more than an order of magnitude larger than fair weather electric fields. Thus such large negative ZDR measurements can be considered as an indicator of the location and movement of sandstorms. Observations of ZDR in gust fronts over vegetated areas reveal very different signatures of ZDR. For most part the values in the outflow are relatively large and positive suggesting that insects are prime contributors. Occasional negative values are also observed. Preferential location appears to be at the leading edge of outflow region. It could be that the electric field is also causing vertical orientation, or it could be that the shear at the leading edge is responsible.