Multi-sensor observations of anvil mammatus are analyzed in order to gain a more detailed understanding of their spatio-temporal structure and microphysical characterization. Remarkable polarimetric radar signatures are detected for the Pentecost 2014 supercell in Germany and severe storms in Oklahoma, U.S.A., along their mammatus-bearing anvil bases. Radar reflectivity at horizontal polarization ZH and cross-correlation coefficient ρHV decrease downwards towards the bottom of the anvil while differential reflectivity ZDR rapidly increases, consistent with the signature of dendritic growth (Figure 1).
Figure 1: Cross section through the 3D composite along the most intense mammatus lobes showing the polarimetric variables ZH (top left), ZDR (top right), ρHV (bottom left), and KDP (bottom right).
ZDR within mammatus exceeds 2 dB in the Pentecost storm and in several Oklahoma severe convective storms examined for this paper. Observations from a zenith pointing Ka-band cloud radar and a Doppler wind lidar during the Pentecost storm indicate the presence of a supercooled liquid layer of at least 200–300 m depth near the anvil base at temperatures between -15oC and -30oC (Figure 2). These liquid drops, which are presumably generated in localized updrafts of up to 1.5 m s-1, coexist with ice particles identified by cloud radar. We hypothesize that pristine crystals nucleate and grow rapidly within these layers of supercooled water, and that oriented planar ice crystals falling from the liquid layers lead to high ZDR at precipitation radar frequencies. A mammatus detection strategy using precipitation radar observations is presented, based on a methodology so far mainly used for the detection of updrafts in convective storms. Owing to the presence of a supercooled liquid layer detected above the mammatus lobes, the new detection strategy might also be relevant for aviation safety.
Figure 2: Detailed zoom in the underside of the anvil with the most intense mammatus lobes. The different panels show effective reflectivity factor (top), mean Doppler velocity (middle, positive values show updrafts), and spectral width (bottom) measured at JOYCE between 1733 and 1745 UTC on 9 June 2014 with the JOYRAD-35 Ka-band cloud radar. Overlaid black (top two panels) and white (bottom panel) dots represent the location of a liquid layer as identified with the co-located 1.5-µm Doppler lidar measurements (Fig. 9); the red line at the bottom of each panel indicate times where the Doppler lidar was not in vertically pointing mode.