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<br />12 <br /> <br />The concept of a smooth orographic cloud does not apply here; <br /> <br />convective instability in the low level moist layer is released as <br /> <br />~ <br />~ <br /> <br />air is forced to rise up the barrier. Aircraft flights during winter <br /> <br />storms of 1971-72 and 1972-73 were taken to measure cloud liquid water <br /> <br />~ <br />. <br /> <br />across the Sierra (Lamb et al., 1976). A strongly convective region, <br /> <br />as revealed by turbulence, with high liquid water content was found <br /> <br />from 40 to 75 km upwind of the main crest. Surface precipitation was <br /> <br />greatest at a distance of 10 to 30 km upwind of the crest, with par- <br /> <br />tic1es growing in the convective region and falling to the ground some <br /> <br />distance nearer the crest. Downwind of the convective zone and beyond <br /> <br />the crest, much less cloud activity and little surface precipitation <br /> <br />was found. Convective overturning was found to occur in both frontal <br /> <br />and post-frontal situations, as the barrier restricts low level flow <br /> <br />~ <br /> <br />more than at higher levels. <br /> <br />Marwitz et ale (1978) have found that a post-frontal convective <br /> <br />~ <br /> <br />cloud wall, where upwind and downwind regions are mostly clear, is <br /> <br />common after a cold front. Located upwind of the crest over the foot- <br /> <br />hills, this cloud wall is truly orographic in the sense that its <br /> <br />existence is due solely to the presence of the Sierra. Their aircraft <br /> <br />flights have shown these clouds to have the most supercooled water and <br /> <br />least ice crystals of all clouds during a storm's lifetime over the <br /> <br />Sierra. <br /> <br />~ <br /> <br />~ <br />