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- Author or Editor: Dale R. Durran x
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Abstract
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 40° and 60°N, the windward-slope hydrological sensitivity exceeds the Clausius–Clapeyron scaling of about 7% K−1 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.
Abstract
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 40° and 60°N, the windward-slope hydrological sensitivity exceeds the Clausius–Clapeyron scaling of about 7% K−1 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.
Abstract
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.
Abstract
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.
