LGM paleoclimate constraints inform cloud parameterizations and equilibrium climate sensitivity in CESM2 Journal Article uri icon

Overview

abstract

  • The Community Earth System Model version 2 (CESM2) simulates a high; equilibrium climate sensitivity (ECS > 5 degC) and a Last; Glacial Maximum (LGM) that is substantially colder than proxy; temperatures. In this study, we use the LGM global temperature from; geological proxies as a benchmark to examine the role of cloud; parameterizations in simulating the LGM cooling in CESM2. Through; substituting different versions of cloud schemes in the atmosphere; model, we attribute the excessive LGM cooling to the new schemes of; cloud microphysics and ice nucleation. Further exploration suggests that; removing an inappropriate limiter on cloud ice number (NoNimax) and; decreasing the time-step size (substepping) in cloud microphysics; largely eliminate the excessive LGM cooling. NoNimax produces a more; physically consistent treatment of mixed-phase clouds, which leads to; more cloud ice content and a weaker shortwave cloud feedback over; mid-to-high latitudes and the Southern Hemisphere subtropics.; Microphysical substepping further weakens the shortwave cloud feedback.; Based on NoNimax and microphysical substepping, we have developed a; paleoclimate-calibrated CESM2 (PaleoCalibr), which simulates well the; observed 20th century warming and spatial characteristics of key cloud; and climate variables. PaleoCalibr has a lower ECS (~4; degC) and a 20% weaker aerosol-cloud interaction than CESM2.; PaleoCalibr represents a physically and numerically better treatment of; cloud microphysics and, we believe, is a more appropriate tool than; CESM2 in climate change studies, especially when a large climate forcing; is involved. Our study highlights the unique value of paleoclimate; constraints in informing the cloud parameterizations and ultimately the; future climate projection.

publication date

  • August 19, 2021

has restriction

  • hybrid

Date in CU Experts

  • September 13, 2021 1:00 AM

Full Author List

  • Zhu J; Otto-Bliesner BL; Brady EC; Poulsen C; Shaw JK; Kay JE

author count

  • 6

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