The 26-27 January Nor'easter (Juno): Radar Analysis and Simulated Sensitivity to Riming Parameterizations
Open Access
- Author:
- Naegele, Steven M
- Graduate Program:
- Meteorology
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- December 07, 2017
- Committee Members:
- Matthew Robert Kumjian, Thesis Advisor/Co-Advisor
David Jonathan Stensrud, Thesis Advisor/Co-Advisor
Jerry Y Harrington, Committee Member - Keywords:
- winter cyclone
nor'easter
snow band
snow
riming parameterization
microphysics
dual-polarization radar
Juno
radar
graupel - Abstract:
- Northeast winter cyclones (“nor'easters”) can produce snow bands that create large amounts of snow with the potential to economically cripple coastal cities. One uncertainty in snow band prediction is the parameterization of microphysical processes used in numerical models, including riming of ice particles. In the Thompson microphysics scheme, snow mass is converted to graupel mass when the riming growth rate exceeds the depositional growth rate, where the fraction of snow converted to graupel is related to the ratio of the two growth rates. Simulations of the 26-27 January 2015 nor'easter and its associated snow bands test the sensitivity of accumulated snowfall and storm structure to this parameterization using the Weather Research and Forecasting (WRF) model with five different treatments of riming efficiency in the Thompson microphysics scheme. With an altered riming efficiency, areal coverage of heaviest snowfall varies among the simulations. All simulations have similar radar reflectivity structures, orientations, and translation speeds in the main snow band, with slightly lower reflectivity values and different but comparable snow band translation speeds compared to radar observations. Differences in snow band characteristics and accumulated snowfall among the simulations and observations are diagnosed using thermodynamic profiles, radar variables, flow structures, and two-dimensional histograms of various WRF output variables, including potential temperature budget terms. Maps of differences in simulated accumulated snow between the original snow-graupel conversion efficiency in the Thompson microphysics scheme of WRF release 3.7.1 and updated efficiencies show generally increased differences with more aggressive snow-graupel conversion efficiency. One of the simulations with Thompson microphysics that converts snow to graupel more aggressively, THO_new, appears to have the accumulated snowfall distribution closest to observations. This is based on comparing the simulated snowfall maps mentioned above and interpolated observed snowfall maps. The overall changes in simulated snow accumulation and snow band structure indicate that the parameterization of riming efficiency in the Thompson microphysics scheme impacts numerical model simulations of where nor'easter snow bands produce snowfall, thus affecting forecast accuracy of snow bands and their precipitation.