Drivers of Precipitation Change: An Energetic Understanding

Abstract

The response of the hydrological cycle to climate forcings can be understood within the atmospheric energy budget framework. In this study precipitation and energy budget responses to five forcing agents are analyzed using 10 climate models from the Precipitation Driver Response Model Intercomparison Project (PDRMIP). Precipitation changes are split into a forcing-dependent fast response and a temperature-driven hydrological sensitivity. Globally, when normalized by top-of-atmosphere (TOA) forcing, fast precipitation changes are most sensitive to strongly absorbing drivers (CO2, black carbon). However, over land fast precipitation changes are most sensitive to weakly absorbing drivers (sulfate, solar) and are linked to rapid circulation changes. Despite this, land-mean fast responses to CO2 and black carbon exhibit more intermodel spread. Globally, the hydrological sensitivity is consistent across forcings, mainly associated with increased longwave cooling, which is highly correlated with intermodel spread. The land-mean hydrological sensitivity is weaker, consistent with limited moisture availability. The PDRMIP results are used to construct a simple model for land-mean and sea-mean precipitation change based on sea surface temperature change and TOA forcing. The model matches well with CMIP5 ensemble mean historical and future projections, and is used to understand the contributions of different drivers. During the twentieth century, temperature-driven intensification of land-mean precipitation has been masked by fast precipitation responses to anthropogenic sulfate and volcanic forcing, consistent with the small observed trend. However, as projected sulfate forcing decreases, and warming continues, land-mean precipitation is expected to increase more rapidly, and may become clearly observable by the mid-twenty-first century.

Effects of Anthropogenic Activity on Global Terrestrial Gross Primary Production

“…The difference in 10‐m wind speed between two simulations is nearly constant over time. The differences in climate variables, which increase with warming, are consistent with previous studies that reported increased surface temperatures and an intensified water cycle with warming (Boucher et al, ; Richardson et al, ). The anthropogenic effect on surface radiation is less obvious, although model‐based studies have reported a decrease in shortwave radiation at the surface with warming (Richardson et al, ).…”

“…The differences in climate variables, which increase with warming, are consistent with previous studies that reported increased surface temperatures and an intensified water cycle with warming (Boucher et al, ; Richardson et al, ). The anthropogenic effect on surface radiation is less obvious, although model‐based studies have reported a decrease in shortwave radiation at the surface with warming (Richardson et al, ). To the best of our knowledge, there is no observational evidence of a long‐term decrease in surface shortwave radiation with large decadal variability (Wild, ).…”

Are Changes in Atmospheric Circulation Important for Black Carbon Aerosol Impacts on Clouds, Precipitation, and Radiation?

“…This matches the reduction in the surface latent heat flux associated with evaporation (Table ) and is consistent with the stabilizing influence of BC on the atmosphere. The reductions in global precipitation are also consistent with the requirement to rebalance the atmosphere's heat budget (e.g., Myhre et al, ; Richardson et al, ), which is explored later in section . The global‐mean and local changes in precipitation are almost entirely due to changes in convective (parameterized) precipitation, with weak or mostly insignificant changes in stratiform precipitation (Figure S5).…”

“…This matches the reduction in the surface latent heat flux associated with evaporation (Table 2) and is consistent with the stabilizing influence of BC on the atmosphere. The reductions in global precipitation are also consistent with the requirement to rebalance the atmosphere's heat budget (e.g., Myhre et al, 2018;Richardson et al, 2018),…”

“…In this study, changes in heat transport were derived as a residual assuming that the global multiannual mean heat content of the atmosphere does not change with time (ΔH + ΔLP + ΔQ = 0). As shown in Richardson et al (2018), the H term is typically dominated by changes in vertical motion, especially in the tropics. As potential temperature generally increases with height, ascent generally causes dynamical cooling, and vice versa for descent.…”