Estimating the Response of Extreme Precipitation over Midlatitude Mountains to Global Warming

Global warming-induced changes in extreme orographic precipitation are investigated using a hierarchy of models: a global climate model, a limited-area weather forecast model, and a linear mountain wave model. The authors consider precipitation changes over an idealized north-south midlatitude mountain barrier at the western margin of an otherwise flat continent. The intensities of the extreme events on the western slopes increase by approximately 4% K-1 of surface warming, close to the "thermodynamic'' sensitivity of vertically integrated condensation in those events due to temperature variations when vertical motions stay constant. In contrast, the intensities of extreme events on the eastern mountain slopes increase at about 6% K-1. This higher sensitivity is due to enhanced ascent during the eastern-slope events, which can be explained in terms of linear mountain wave theory as arising from global warming-induced changes in the upper-tropospheric static stability and the tropopause level. Similar changes to these two parameters also occur for the western-slope events, but the cross-mountain flow is much stronger in those events; as a consequence, linear theory predicts no increase in the western-slope vertical velocities. Extreme western-slope events tend to occur in winter, whereas those on the eastern side are most common in summer. Doubling CO2 not only increases the precipitation, but during extreme western slope events it shifts much of the precipitation from snow to rain, potentially increasing the risk of heavy runoff and flooding.