| 154 | | Tuning strategy[[BR]] |
| 155 | | |
| 156 | | Apart from obtaining better computing performances, one aim of setting up IPSL-CM5A2 was to overcome the cold bias depicted in global surface air temperature (t2m) in IPSL-CM5A -explained by the lack of tuning for this latter version (Dufresne et al., 2013)- while trying not to worsen the long standing biases of the model (especially the warm bias of the ocean surface over equatorial upwelling regions and the presence of a double ITCZ in the equatorial eastern Pacific). Therefore we define a tuning strategy that responds to one single target: increasing the global t2m to reach the value of 13.5°C at equilibrium with pre-industrial boundary conditions.[[BR]] |
| 157 | | |
| 158 | | First we ran a first simulation of CM5A2 forced by CMIP5 pre-industrial boundary conditions, the ocean component initiated by the routinely-used levitus climatologies. |
| 159 | | DECRIRE simul CM6.VLR.4.0.1 ici.[[BR]] |
| 160 | | |
| 161 | | Prior to tuning, CM5A2 pre-industrial run depicts annual t2m values lower than CM5A, stabilizing at ca. 11.29°C after 1000 years of simulation, whereas CM5A depicted t2m stabilized at ca. 12.06°C. This ca. 0.8°C cooling between the 2 versions is associated with a stronger negative radiative forcing in CM5A2 at mid-latitudes and along the equator and a negative anomaly in both surface and top of atmosphere (TOA) radiative balance between CM5A2 (-0.28 W.m-2) and CM5A (+0.18 W.m-2). The reason for these differences between two rather close versions of the ISPL model remains to be explained.[[BR]] |
| 162 | | |
| 163 | | Choice was made to act on cloud microphysics to alter their radiative effect and in turn the global temperature. According to Sundqvist (1978), the rate of precipitation formation is related to the amount of water in the cloud. As described in Hourdin et al. (2013), a threshold for condensed water (0.418 g/kg before tuning) needs to be reached for rainfall to start precipitating. , with a time constant τ_conv for auto-conversion (set at 1800 s):[[BR]] |
| 164 | | |
| 165 | | (dq_lw)/dt= - q_lw/τ_conv [ 1-e^-(qlw/clw)**2 ][[BR]] |
| 166 | | |
| 167 | | where qlw is the mixing ratio, clw is the in-cloud water threshold for autoconversion, τ_conv is a time constant for auto-conversion (here set at 1800 s).[[BR]] |
| 168 | | |
| 169 | | Decreasing clw is expected to lower cloud density and reduce the net cloud radiative forcing, as depicted in sensitivity experiment CLDLC in Hourdin et al. (2013). Here we carried out forced-by-SSTs LMDZ simulations, keeping in mind that a change by 1 W.m-2 in the net radiative balance shifts global t2m by 1K (Hourdin et al. 2013). |
| 170 | | Two simulations were run with clw set at 0.316 and 0.250 g/kg, respectively, to define the sensitivity of surface and TOA radiative budget to this parameter.[[BR]] |
| 171 | | |
| 172 | | || ||=Control=||=Exp 1=||=Exp 2=|| |
| 173 | | ||=clw (g/kg)=||0.418||0.316||0.250|| |
| 174 | | ||=CRF (W/m-2)=||-21.56||-18.94||-17.10|| |
| 175 | | ||=BILS (W/m2)=||0.176||2.737||4.544|| |
| 176 | | |
| 177 | | [[BR]] |
| 178 | | |
| 179 | | [[Image(CRE_3sim+data.png, 500px)]] |
| 180 | | |
| 181 | | Setting clw at 0.316 g/kg provides a slightly too strong increase in the cloud radiative forcing (+2.61 W.m-2) that echoes in surface heat budget (+2.56 W.m-2) in the atmosphere-only simulation. These figures are confirmed in the coupled simulation, that depicts annual t2m reaching 13.75°C and BILS stabilizing at 0.19 W/m2 after 500 years of simulation. From these experiments we hypothesize that setting clw at 3.25 g/kg would be the right choice to reach the +2.2 target in t2m and bils. Thus we branched a new coupled experiment on the previous one and let the model run for 1000 years for all the slow components to reach equilibrium. We obtain a net surface heat flux of 0.11 W.m-2 and a global air temperature at surface of 13.56°C. |
| 182 | | |
| 183 | | [[Image(Evo_t2m_preind02.png, 500px)]] |
| 184 | | |
