05/15/2017
Exploring Ice Crystals in Thunderstorms
Observations from the Mid-latitude Continental Convective Cloud Experiment (MC3E) show that models are missing an important microphysical pathway for ice formation in thunderstorms.
The Science
Thunderstorms play a major role in Earth’s energy and water budgets, but the processes that determine ice crystal sizes in the anvil outflow from these storms remain poorly understood and therefore poorly represented in cloud and Earth system models. Knowing ice crystal size is important for determining both a cloud’s lifetime in the atmosphere and its impact on solar and longwave radiation. A team of research scientists used ground-based and aircraft observations from the Department of Energy (DOE) and National Aeronautics and Space Administration (NASA)-sponsored Mid-latitude Continental Convective Cloud Experiment (MC3E) during a well-observed thunderstorm system to (1) provide better inputs for simulating cloud processes in models and (2) provide an observational target to establish correct simulation of outflow ice crystal size.
The Impact
The team demonstrates that the observations provide a uniquely robust combined dataset for testing and improving cloud and Earth system models. The MC3E observations indicate that the ice crystal sizes coming out of thunderstorms are commonly far too large in the models. The results are consistent with the hypothesis that model microphysics schemes are missing a key aspect of an updraft microphysics pathway that can largely determine outflow size, most likely associated with warm temperature ice multiplication. This study indicates that the MC3E observations can provide robust guidance for future model improvements of microphysical processes.
Summary
The research team derived hygroscopic aerosol size distribution input profiles from ground-based and airborne measurements for six convection case studies observed during MC3E over Oklahoma. They demonstrate use of an aerosol input profile in simulations of the only well-observed MC3E case study that produced extensive stratiform outflow, on May 20, 2011. At elevations well-sampled by aircraft between -11 and -23 degrees Celsius over widespread stratiform rain, ice crystal number concentrations were consistently dominated by a single mode near ~400 µm in randomly oriented maximum dimension (Dmax). The ice mass at -23 degrees Celsius is primarily in a closely collocated mode, whereas a mass mode near Dmax ~1000 µm becomes dominant with decreasing elevation to the -11 degree Celsius level, consistent with possible aggregation during sedimentation. However, simulations with and without observation-based aerosol inputs systematically overpredict mass peak Dmax by a factor of 3-5 and underpredict ice number concentration by a factor of 4-10. Previously reported simulations with both two-moment and size-resolved microphysics have shown biases of a similar nature. The observed ice properties are notably similar to those reported from recent tropical measurements. Based on several lines of evidence, the researchers speculate that updraft microphysical pathways determining outflow properties in the May 20 case could be similar to a tropical regime, perhaps associated with warm-temperature ice multiplication that is not well understood or well represented in models.
Principal Investigator(s)
Ann Fridlind
National Aeronautics and Space Administration Goddard Institute for Space Studies
[email protected]
Funding
This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Biological and Environmental Research (BER) under agreements DE-SC0006988, DE-SC0014065, and DE-SC0016476 (through UCAR subcontract Z17-90029); and by the National Aeronautics and Space Administration’s (NASA) Radiation Sciences Program. All measurements were obtained from DOE’s Atmospheric Radiation Measurement (ARM) Climate Research Facility. Operation of the University of North Dakota Citation aircraft was supported through NASA grant NNX10AN38G. NU-WRF is supported by the NASA Modeling, Analysis, and Prediction (MAP) program. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center and NASA’s Center for Climate Simulation (NCCS) at Goddard Space Flight Center.
References
Fridlind, A., X. Li, D. Wu, M. van Lier-Walqui, A. Ackerman, W. Tao, G. McFarquhar, W. Wu, X. Dong, J. Wang, A. Ryzhkov, P. Zhang, M. Poellot, A. Neumann, and J. Tomlinson. 2017. “Derivation of Aerosol Profiles for MC3E Convection Studies and Use in Simulations of the 20 May Squall Line Case,” Atmospheric Chemistry and Physics 17(9), 5947-72. DOI: 10.5194/acp-17-5947-2017.