High Concentrations of Ice: Investigations Using Polarimetric Radar Observations Combined With In Situ Measurements and Cloud Modeling

Principal Investigator(s):
Alexander Ryzhkov, University of Oklahoma

Co-Investigator(s):
Alexander Khain, The Hebrew University of Jerusalem, Israel
Vaughan Phillips, Lund University, Sweden

The concentration of ice particles in clouds typically exceeds the concentration of ice nucleating particles (INPs) by several orders of magnitude. Numerical cloud models generally fail to reproduce very high concentrations of small-size ice persistently observed in the periphery of deep convective storms such as thunderstorm anvils, stratiform shields of mesoscale convective systems, or outer areas of tropical cyclones. Because of the large spatial dimensions of these areas, this type of ice plays an important role in Earth’s radiative balance. The albedo of ice clouds is influenced by the environment in which the deep convection forms and critically depends on the number concentration of very small ice particles. In addition, commercial airplanes are exposed to a serious risk in the high ice water content (HIWC) regions due to possible engine power loss and damage.

Rapid proliferation of research and operational dual-polarization weather radars around the world provides unique opportunities for better quantification of ice in deep clouds compared to conventional single-polarization radars. A novel method for retrieval of ice size distributions and ice water content using radar reflectivity Z and two additional polarimetric variables will be utilized as opposed to the use of a single variable Z. The retrieval results will be evaluated by the comparison with in situ aircraft measurements using simultaneous polarimetric radar and aircraft data collected during past DOE-sponsored field campaigns.

Different microphysical processes potentially responsible for high concentrations of small-size ice will be explicitly treated in a sophisticated Weather Research and Forecasting – Spectral Bin Microphysics (WRF-SBM) model and constrained with the results of novel ice radar retrievals in various types of ice clouds with high IWC in different parts of the world. The primary mechanisms include (1) homogeneous freezing of cloud droplets facilitated by in-cloud nucleation of ultra-fine aerosols and (2) ice multiplication due to ice-ice collisions and drop freezing. Cloud modeling will be performed for a number of observed events ranging from tropical mesoscale convective systems with warm cloud base to continental midlatitude storms with colder cloud base.

Three major research objectives will be addressed in the proposed study

  1. Utilize a novel polarimetric radar technique to retrieve size distributions and amounts of ice from radar data collected during the previous DOE ARM field campaigns.
  2. Validate the results of ice microphysical retrievals using available in situ aircraft measurements and other remote sensors.
  3. Improve the treatment of microphysical processes leading to cloud glaciation and ice multiplication in the numerical cloud models.

Better understanding of the microphysical processes leading to the formation of clouds with high concentration of ice and the role of aerosols involved in these processes will be the primary benefit of the proposed study. Such clouds with large dimensions and high albedo play very important role in the Earth’s radiative balance. The improved microphysical schemes will eventually be utilized in the models of global circulation. Broader impact of the study includes utilization of a new radar methodology to detect the areas of high IWC which pose serious threat to commercial aviation.