SOLARIS Coordinated Experiments I
A) Coordinated Model Runs to Investigate Aliasing of Different Factors in the Tropical Lower Stratosphere
Recently, the discrepancy between modelling studies and observations regarding the vertical structure in the tropical solar signal, as shown by the WMO (2007), has been reduced in both CCMVal-1 reference simulations (Austin et al., 2008) and CCMVal-2 simulations (SPARC CCMVal, 2010, chapter 8). Similarly, other recent simulations with CCMs reproduce the observed vertical structure in the tropical stratosphere, but only with a (prescribed) QBO, time-varying solar cycle conditions and constant SSTs (Matthes et al., 2007; Matthes et al., 2010), or in a CCM with fixed solar cycle conditions, with or without an internally-generated QBO (Schmidt et al., 2010). It is still unclear why a vertical structure in the solar signal appears, and whether it is related to non-linear interactions or arises from contamination by other signals (QBO, tropical SSTs). To eliminate possible aliasing between the solar cycle and the QBO, as well as between the solar cycle and the SSTs, and/or the QBO and the SSTs, the REF-B1 CCMVal experiments (Eyring et al., 2008, SPARC CCMVal 2010) were repeated with filtered SST and/or QBO data:
- Filtered SST data:
The QBO signal (2-3 years) and solar cycle signals (larger than 10 years) have been filtered out of the SST data set used as a lower boundary for the REF-B1 simulations.
- Filtered QBO data:
Similarly, the QBO data were filtered to retain only periods between 9-48 months and exclude signals related to ENSO or the solar cycle.
Currently, two CCMs with a prescribed QBO (EMAC, WACCM), and one with internally generated QBO (MRI) have finished one ensemble of the modified REF-B1 experiments.
B) Coordinated Model Runs to Study the Uncertainty in Solar Forcing
Uncertainties in the solar irradiance could have a large impact on the simulation of the climate system. The solar irradiance data compiled by J. Lean (Lean, 2000) are most frequently used for model simulations. However, in the wavelength range important for ozone chemistry (200-400 nm), there are differences of up to 20% to the estimate of Krivova et al. (2006). Further uncertainties arise from new measurements from the SIM instrument onboard the SORCE satellite, which shows a completely different spectral distribution than expected, with possible implications for solar heating and ozone chemistry (e.g., Haigh et al., 2010).
The proposed coordinated CCM experiments include:
- A control (time slice) experiment with either Lean (standard) or SIM solar irradiance data,
- Idealized experiments with enhanced solar UV forcing in certain spectral ranges, i.e. an increase of 5% between 200 and 300 nm, and an increase of 1% between 300 and 400nm.
It is important to investigate the reliability of current SOLARIS irradiance data recommendations for CMIP5 and SPARC-CCMVal, and test the sensitivity of different radiation and photochemistry models to different spectral irradiance data sets.