Radar measurements and simulations of ice crystal growth in Arctic mixed-phase clouds

Open Access
Author:
Schrom, Robert Stephen
Graduate Program:
Meteorology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 27, 2018
Committee Members:
  • Matthew Robert Kumjian, Dissertation Advisor
  • Matthew Robert Kumjian, Committee Chair
  • Johannes Verlinde, Committee Member
  • Eugene Edmund Clothiaux, Committee Member
  • Kultegin Aydin, Outside Member
  • Jerry Y Harrington, Committee Member
Keywords:
  • Radar
  • Ice crystals
  • Scattering
  • Microphysics
Abstract:
Polarimetric radar measurements offer the potential to better understand the microphysical processes in cloud and precipitation systems. The large uncertainties in current microphysical theories and corresponding numerical models of the ice growth processes suggest a natural area where these measurements can yield large benefits. However, these uncertainties make interpretations of a given set of radar observations ambiguous. Additionally, understanding the theoretical framework behind the scattering of individual ice particles, as well as the bulk signal of a population of ice particles, is necessary to fully make use of polarimetric radar observations. Given a particular set of ice crystal physical properties, there is still uncertainty in their scattering properties. The homogeneous, reduced-density simplification for the structure of branched planar crystals is shown to produce large errors in their scattering properties. The reduced-density representations underestimate the ZDR and KDP of individual ice crystals due to their inability to represent the distribution of mass within the particles, and the importance this distribution has on the electric-field interactions between dipoles comprising the particle. To more accurately map simple physical properties of branched planar crystals such as maximum dimension, aspect ratio, and effective density to their scattering properties, a set of electromagnetic scattering calculations for a range of realistic particle shapes were performed. The resulting scattering properties were then fit to the physical properties using polynomials; for long wavelengths (i.e., at frequencies above Ku-band), Rayleigh theory was used to determine representative scattering properties of the branched planar crystals through the use of equivalent-scattering solid-ice spheroids. These equivalent spheroids are therefore are valid at all wavelengths large relative to the maximum dimension. Using this mapping procedure, a forward model was then developed for the polarimetric radar signatures of populations of branched planar crystals. The ambiguity in the ice crystal structure given a particular effective density leads to an inherent uncertainty in the resulting forward-simulated radar variables. This uncertainty in the forward model was accounted for by perturbing the dimensions of the equivalent-solid ice spheroids associated with a given set of physical properties, and using these perturbations to generate a certain number of realizations of forward-simulated radar variables. These uncertainties are largest for particles with the lowest aspect ratios and lowest effective densities, and therefore the forward model uncertainty depends on the specific distribution of ice crystal physical properties. In comparison to the spatial variability of radar observations during an Arctic mixed-phase cloud case, the forward model uncertainty is relatively low, suggesting the importance in accurately characterizing the growth environment and properly constraining the assumptions about the spectra of ice particle properties associated with the given set of radar observations. In order to better interpret radar measurements of planar crystal growth in an Arctic mixed-phase cloud, where polarimetric radar observations were collected by the ARM XSAPR in Barrow, AK, the radar forward model was coupled to a bin microphysical model of vapor depositional growth. The microphysical model is driven by a simplified 2-D kinematic model with idealized updraft and downdraft cells. Given the uncertainties in the properties of these updrafts, an ensemble of simulations with different perturbations for the updraft characteristics and the ice particle concentration was created and analyzed. In addition, a new formulation for the deposition density associated with vapor growth was implemented in the model. This new deposition density is based on the shape generation procedure for the scattering calculations and therefore more realistically captures the evolution of natural branched planar crystal structures. Given the uncertainties in these ice crystal structures, the structural quantities in the deposition density formulation were also perturbed in the ensemble. The resulting simulations were able to capture the general features of the observed radar profiles, with comparable values of ZH and ZDR . However, none of the simulations produced the observed decrease in ZDR from the top to the bottom of the profile, likely due to the differential sedimentation of hydrometeors or the lack of aggregation in the model. The simulations that fell within the variability of the observations suggested two general interpretations: one with a higher concentration of ice particles, smaller sizes, and higher densities, and another with lower concentration of ice particles, larger sizes, and lower densities. These two clusters produced KDP profiles with vastly different magnitudes; lower and higher magnitudes were observed with the low- and high-concentration clusters. Given the ubiquity of aggregation and riming in Arctic clouds, the potential for radar measurements to inform the understanding of this processes is discussed. The use of observations from multiple cases and the addition of multi-frequency observations is also likely to further improve the understanding of ice growth processes in cloud and precipitation systems.