The Response of Orographic Precipitation over Idealized Midlatitude Mountains Due to Global Increases in CO2

The sensitivity of stratiform midlatitude orographic precipitation to global mean temperature is investigated through numerical simulations. As a step toward understanding the relative influence of thermodynamic and dynamical processes on orographic precipitation, simple idealizations of Earth’s major north–south mountain chains are considered. The individual terrain elements occupy four islands equally spaced around the Northern Hemisphere of a planet otherwise covered by ocean. Although these mountains have very little influence on the sensitivity of the zonally averaged precipitation to changes in global mean surface temperature, the precipitation on the windward slopes of the ridges is highly sensitive to such changes. When the ridges run between 408 and 608N, the windward-slope hydrological sensitivity exceeds the Clausius– Clapeyron scaling of about 7%K over the northern half of the barrier, leading to substantial precipitation changes. The annual accumulated orographic precipitation is modified by changes in both the mean precipitation intensity and the changes in the number of hours during which precipitation occurs. The changes in the number of hours with significant precipitation largely results from modifications in synoptic-scale storminess associated with changes in the midlatitude storm tracks. A simple diagnostic model reveals the primary factors modifying the mean orographic precipitation intensity are variations in 1) the moist adiabatic lapse rate of saturation specific humidity, 2) the wind speed perpendicular to the mountain, and 3) the vertical displacement of saturated air parcels above the windward slope. The strong dependence of 2 and 3 on latitude further confirms that changes in midlatitude storminess are a major factor determining the response of orographic precipitation to global warming.