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Constraining the aerosol ERF in UKESM1 through in-depth process-based assessment of underlying physical model parameterizations and aerosol processes

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Thursday, 08 March 2018, 2:00

Thursday, March 8, 2018. Constraining the aerosol ERF in UKESM1 through in-depth process-based assessment of underlying physical model parameterizations and aerosol processes. Jane Mulcahy, UK MET Office.  More information here.

Abstract: The latest generation UK Earth System model, UKESM1, is built on top of core physical climate model, HadGEM3-GC3.1. Simulations from both the physical and ES models will be submitted to CMIP6. In this seminar I will briefly introduce these models and the synergistic model development approach of the Met Office and wider UK academic community as well as CMIP6 plans. Aerosol processes and, in particular, aerosol-cloud interactions cut across the traditional physical-Earth system boundary of coupled Earth system models and remain one of the key uncertainties in estimating anthropogenic radiative forcing on climate. An improved representation of tropospheric chemistry-aerosol processes was therefore an integral part of the development of UKESM1 which incorporates the UKCA stratospheric-tropospheric chemistry (Telford et al. 2014) and GLOMAP-mode aerosol microphysics (Mann et al. 2010, 2012) schemes. We quantify the effective radiative forcing (ERF) due to aerosols in the prototype physical and ES models. The aerosol ERF was found to be large and negative due to underlying assumptions in both the physical model and aerosol parameterizations – the result is an unrealistically weak total anthropogenic forcing over the 20th century. A number of model improvements are investigated to assess their impact on the aerosol ERF. These include; an improved representation of cloud droplet spectral dispersion, updates to the aerosol activation scheme and black carbon optical properties. Furthermore, we examine the role of the largely unconstrained preindustrial aerosol climate by evaluating the contribution of uncertainties in the natural marine emissions of dimethyl sulphide (DMS) and organic aerosol to the ERF. The combination of model improvements derived from these studies weaken the aerosol ERF by more than 40% of the original value but more importantly do not degrade the present-day climate model performance. The improvements are expected to lead to a total anthropogenic historical forcing more in-line with assessed values.

 

Location  NOAA GFDL, Smagorinsky Seminar Room, Princeton, NJ.