Dynamical conditions of ice supersaturation and ice nucleation in convective systems: A comparative analysis between in situ aircraft observations and WRF simulations

Authors:

John J. D’Alessandro¹, Minghui Diao¹, Chenglai Wu^2,3, Xiaohong Liu², Ming Chen⁴, Hugh Morrison⁴, Trude Eidhammer⁵, Jorgen B. Jensen⁶, Aaron Bansemer⁴, Mark A. Zondlo⁷, and Josh P. DiGangi⁸

¹Department of Meteorology and Climate Science, San Jose State University, San Jose, California, USA, ²Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming, USA, ³International Center for Climate and Environment Sciences, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China, ⁴Mesoscale and Microscale Meteorology Division, National Center for Atmospheric Research, Boulder, Colorado, USA, ⁵Research Applications Laboratory, National Center for Atmospheric Research, Boulder, Colorado, USA, ⁶Research Aviation Facility, National Center for Atmospheric Research, Broomfield, Colorado, USA, ⁷Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey, USA, ⁸Chemistry and Dynamics Branch, NASA Langley Research Center, Hampton, Virginia, USA

Abstract:

Occurrence frequency and dynamical conditions of ice supersaturation (ISS, where relative humidity with respect to ice (RHi)>100%) are examined in the upper troposphere around convective activity. Comparisons are conducted between in situ airborne observations and the Weather Research and Forecasting model simulations using four double-moment microphysical schemes at temperatures≤-40°C. All four schemes capture both clear-sky and in-cloud ISS conditions. However, the clear-sky (in-cloud) ISS conditions are completely (significantly) limited to the RHi thresholds of the Cooper parameterization. In all of the simulations, ISS occurrence frequencies are higher by ~3–4 orders of magnitude at higher updraft speeds (>1ms^-1) than those at the lower updraft speeds when ice water content (IWC)>0.01gm^-3, while observations show smaller differences up to ~1–2 orders of magnitude. The simulated ISS also occurs less frequently at weaker updrafts and downdrafts than observed. These results indicate that the simulations have a greater dependence on stronger updrafts to maintain/generate ISS at higher IWC. At lower IWC (≤0.01gm^-3), simulations unexpectedly show lower ISS frequencies at stronger updrafts. Overall, the Thompson aerosol-aware scheme has the closest magnitudes and frequencies of ISS>20% to the observations, and the modified Morrison has the closest correlations between ISS frequencies and vertical velocity at higher IWC and number density. The Cooper parameterization often generates excessive ice crystals and therefore suppresses the frequency and magnitude of ISS, indicating that it should be initiated at higher ISS (e.g.,≥25%).

Key Points:

？ Ice supersaturation (ISS) occurrence frequencies in simulations show greater dependence on vertical velocity than observations

？ Cooper parameterization of ice crystal formation suppresses ISS magnitude and frequency

？ Model-observation comparison results show improvements by limiting ice nucleation at lower IS

Citation:

D’Alessandro, J. J., et al. (2017), Dynamical conditions of ice supersaturation and ice nucleation in convective systems: A comparative analysis between in situ aircraft observations and WRF simulations, J. Geophys. Res. Atmos., 122, doi: 10.1002/2016JD025994.