Estimating the Response of Mid - latitude Orographic 1 Precipitation to Global Warming

5 The sensitivity of stratiform mid-latitude orographic precipitation to global mean temper6 ature is investigated through numerical simulations with a general circulation model. The 7 individual terrain elements are idealizations of the Earth’s major north-south mountain 8 chains and occupy four islands equally spaced around the northern hemisphere of a planet 9 otherwise covered by ocean. Precipitation on the windward slopes of the ridges is much 10 more sensitive to changes in global mean surface temperature than is the zonally averaged 11 precipitation. When the ridges run between 40–60◦N, the windward-slope hydrological sen12 sitivity exceeds the Clausius-Clapeyron scaling of ∼ 7 % K−1 over the northern half of the 13 barrier. When CO2 is doubled, our simulations generate a global mean temperature increase 14 of 2.56 K, leading to substantial precipitation changes, particularly at the north end of the 15 range, where annual precipitation increases by 36%. 16 The changes in orographic precipitation are linked to northward shifts in the storm tracks, 17 which alters both the mean climatology and the frequency of synoptic-scale storm passage 18 at a given latitude. The annual accumulated precipitation is modified by changes in the 19 mean precipitation intensity and the annual mean hours during which precipitation occurs. 20 A simple diagnostic model reveals the primary factors modifying the mean orographic pre21 cipitation intensity are variations in (1) the moist adiabatic lapse rate of saturation specific 22 humidity, (2) the wind speed perpendicular to the mountain, and (3) the vertical displace23 ment of saturated air parcels above the windward slope. (2) and (3) are strongly dependent 24 on latitude, further confirming the important influence of storm track shifts on the response 25 of mid-latitude orographic precipitation to global warming. 26