The physical basis for increases in precipitation extremes in simulations of 21st-century climate change

Abstract

Global warming is expected to lead to a large increase in atmospheric water vapor content and to changes in the hydrological cycle, which include an intensification of precipitation extremes. The intensity of precipitation extremes is widely held to increase proportionately to the increase in atmospheric water vapor content. Here, we show that this is not the case in 21st-century climate change scenarios simulated with climate models. In the tropics, precipitation extremes are not simulated reliably and do not change consistently among climate models; in the extratropics, they consistently increase more slowly than atmospheric water vapor content. We give a physical basis for how precipitation extremes change with climate and show that their changes depend on changes in the moist-adiabatic temperature lapse rate, in the upward velocity, and in the temperature when precipitation extremes occur. For the tropics, the theory suggests that improving the simulation of upward velocities in climate models is essential for improving predictions of precipitation extremes; for the extratropics, agreement with theory and the consistency among climate models increase confidence in the robustness of predictions of precipitation extremes under climate change.

Fig. 1.

The 99.9th percentile of daily precipitation (millimeters per day) for the periods 1981–2000 (blue) and 2081–2100 (red) in the SRES A1B scenario (multimodel median), and based on Global Precipitation Climatology Project (GPCP) data for the period 1997–2006 (black). Model scatter (shading) for the period 1981–2000 is shown using the interquartile range (50% of models lie within the shaded region). The spatial resolution of the GPCP data were degraded from 1° to 3°, which is comparable with climate model resolutions. A Gaussian smoothing filter of standard deviation 6° latitude was applied to reduce noise in all plots showing variations with latitude.

Fig. 2.

Fractional changes in the 99.9th percentile of daily precipitation (blue), zonally averaged atmospheric water vapor content (green), saturation water vapor content of the troposphere (black dotted), full precipitation extremes scaling (Eq. 2) (red dashed), and thermodynamic scaling for precipitation extremes (black dashed). The lines show multimodel medians of the fractional changes relative to 20th-century values, normalized by the global-mean change in surface air temperature for each model. Model scatter is shown for the fractional change in precipitation extremes using the interquartile range (shading). The saturation water vapor content is calculated using an average of the climatological monthly-mean temperature over all times and longitudes at which the extreme precipitation occurs.

Fig. 3.

Fractional changes in the 99.9th percentile of daily precipitation for each model versus changes in atmospheric water vapor content and scalings for precipitation extremes. (A) Atmospheric water vapor content (open symbols) and the thermodynamic scaling that neglects changes in upward velocity (solid symbols). (B) Full scaling for precipitation extremes. The fractional change are relative to 20th-century values, averaged over the extratropics (Left) or tropics (Right) and normalized by the change in surface air temperature averaged over the extratropics or tropics. Solid lines correspond to one-to-one relationships. The extratropics are defined as the regions poleward of 30° latitude, and the tropics are defined as the region equatorward of 30° latitude.