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The model used here is the System of Atmospheric Modeling (Khairoutdinov and Randall, 2003), version 6.8. All simulations are run in a three-dimensional domain of 128 km × 128 km in the horizontal direction, with a resolution of 1 km. The vertical grid has 64 levels, with the grid spacing gradually increasing from 37.5 m at the surface to 500 m above 7 km. There are six water species in the microphysics scheme: water vapor, cloud liquid, cloud ice, snow, rain, and graupel. The interactive radiation scheme is from the National Center for Atmospheric Research Community Climate Model (Kiehl et al., 1998). A slab ocean of 0.5 m is used as the surface condition. The surface fluxes are interactively computed using Monin–Obukhov similarity theory.
Eight kinds of climate forcing are investigated in this study (Table 1), each with a pair of control and perturbed runs. The control runs of these experiments share the same settings (except the SST case): the CO2 concentration is 355 ppm, and the insolation is set as annual mean values at 0°, without seasonal or diurnal variation, with a solar constant of 1367 W m−2. All the aerosol species have zero concentration. With the above insolation condition, the equilibrium SST will be much higher than our desired reference climate state. Thus, similar to the treatment in Romps (2011), a downward heat flux of 95 W m−2 is applied at the bottom of the slab ocean to cool the equilibrium SST to be 299.6 K. The imposed flux sink may be viewed as heat moving into the deeper ocean and then being transported to higher latitudes by ocean circulation. In the SST experiment, the control run has a prescribed SST of 299.6 K.
Experiment short name Climate forcing Change in SST Mean precipitation sensitivity (% K−1) Extreme precipitation sensitivity (% K−1) Ranges of forcing magnitudes
in realitySST Changing prescribed SST +3.0 K 4.7 6.7 ~2 K at end of the century under global warming (Myhre et al., 2013). CO2 Doubling CO2 +2.7 K 3.9 5.4 An increase of ~200 ppm in the last century (Myhre et al., 2013). Ins Solar constant changes from 1367 to 1385 W m−2 +3.1 K 4.6 6.2 A variation of ~3 W m−2 for the 11-year solar cycle (Myhre et al., 2013); an increase of 270–350 W m−2 from the Archean Eon (2.7 billion years ago) to today (Kasting, 2010). Alb Surface albedo changes from 0.045 to 0.025. +2.5 K 4.8 6 The seasonal change of sea-ice albedo in the Arctic region is ~0.1, equivalent to a global albedo change of ~0.004 (Hummel and Reck, 1979). Aer_str Volcanic ash is added in the lower stratosphere (Fig. S1). −2.5 K 4.2 4.7 A global annual-mean total volcanic aerosol amount of ~4 Tg; a major eruption, such as the Samalas eruption in 1257–58, reaching ~160 Tg. (Liu et al., 2016). Aer_tro Tropospheric aerosol (11 species) is added in the troposphere (Fig. S2, with concentration multiplied by 20). −3.1 K 5.8 6.1 Annual anthropogenic aerosol emissions of 100–400 Tg yr−1 in the early 1970s and 300 Tg yr−1 in the 1990s (Gras, 2003). Aer_sul Sulfate added in the troposphere (Fig. S2, with concentration multiplied by 30). −2.7 K 5 5.9 Global annual-mean emissions of ~200 Tg yr−1 (Penner et al., 2001). Aer_bc BC added in the troposphere (Fig. S2, with concentration multiplied by 200). +3.1 K −1.2 1.8 Global annual-mean emissions of about 4–8 Tg yr−1 (Bond et al., 2004). Table 1. The experimental designs in this study. The vertical profile of the volcanic aerosol added in the Aer_str experiment is shown in Fig. S1. The vertical profiles of the tropospheric aerosols added in the Aer_tro, Aer_sul, and Aer_bc experiments are shown in Fig. S2. In the Aer_tro experiment, the amplitude of the added aerosols is 20 times those in Fig. S2. In the Aer_sul experiment, the amplitude of the added sulfate aerosol is 30 times that in Fig. S2. In the Aer_bc experiment, the amplitude of the added black carbon aerosol is 200 times that in Fig. S2.
Next, we introduce the perturbed runs. In the SST experiment, the perturbed run has a prescribed SST of 302.6 K. Note that the prescribed SST experiment does not conserve energy for the surface. For the other experiments, the SST is calculated interactively with the slab ocean. In these experiments (except the SST case), the magnitude of forcing in the perturbed runs is chosen so that the changes of equilibrium SST are around 3 K. In the perturbed run of the CO2 experiment, the CO2 concentration is doubled, leading to a warming of SST by 2.7 K. In the perturbed run of the Ins experiment, the solar constant is increased to 1385 W m−2. In the perturbed run of the Alb experiment, the surface albedo is decreased to 0.025 (compared with 0.045 in the control run). There are nine aerosol species (eight tropospheric aerosols and one stratospheric aerosol) in System of Atmospheric Modeling (Table S1 in Electronic Supplementary Material, ESM). All the tropospheric aerosols only have shortwave radiative effects, and the stratospheric (volcano) aerosol (75% sulfuric acid and 25% water) has both shortwave and longwave effects. In the perturbed run of the Aer_str experiment, volcanic aerosol is added in the lower stratosphere (Fig. S1 in ESM) to mimic the effects of major volcano eruptions. The magnitude of the added volcanic aerosol is 1 × 105 Tg, which is about 600 times that of a major volcanic eruption such as Samalas (Liu et al., 2016). The Aer_tro experiment is designed to examine the effects of tropospheric aerosols. The vertical distributions of the tropospheric aerosols are the global and annual-mean profiles from a GCM study (Fig. S2; Kipling et al., 2016). To reach a large SST difference of 3.1 K, the added aerosol concentrations are 20 times larger than the GCM results shown in Fig. S2. Additional experiments are performed to examine the individual effects of two representative tropospheric aerosols: sulfate aerosol (the Aer_sul experiment), which has a strong scattering effect on shortwave radiation; and BC (the Aer_bc experiment), which has a strong absorbing effect on shortwave radiation. Note that the magnitude of forcing in the two runs is also tuned accordingly, to have an SST change of about 3 K. All the runs are integrated for more than 1000 days, so that a statistical equilibrium is reached. As an example, several key domain-mean variables and cloud variables of the control and perturbed runs of the CO2 case are shown in Fig. S3 and Fig. S4. The domain-mean variables are sampled every 5 min and the 3D snapshots are sampled every 6 h for the last 250 days (total of 72 000 samples of means and 1000 samples of 3D snapshots).
The responses of the hydrological cycle are calculated by taking the differences between the perturbed run and the control run in each pair of experiments. Extreme precipitation is defined as the surface precipitation rate at the 99.9th percentiles with all grid points considered (including grids with zero precipitation). For the five cases of SST, CO2, Ins, Alb, and Aer_bc (called group 1 hereafter), the perturbed runs have warmer SST than the control runs. For the other three cases of Aer_str, Aer_tro, and Aer_sul (called group 2), adding these aerosols cools the SST. For better comparison, all the results are normalized by the SST changes. Thus, the results of cases in group 2 should be interpreted as removing aerosols from a background state of zero (this is justified if the hydrological response is nearly linear for small perturbations).