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The relationship of precipitation in the western United States to variations in the outgoing long-wave radiation field over the tropical Pacific : the role of the mid-latitude circulation



Graduation date: 1995The relationship between the outgoing long-wave radiation (OLR) field\ud over the tropical Pacific and wintertime monthly precipitation for the\ud western U. S. is investigated, using the mid-latitude upper-air circulation\ud as an intermediary. Principal components (PC's) of the 500mb monthly\ud averaged height field over the NE Pacific and western North America are\ud compared with those of the monthly tropical Pacific OLR field. It is found\ud that, of the first 6 PC's of the height field, five are correlated significantly\ud with the first 3 OLR field PC's at lags of between two and six months.\ud Canonical correlations between the two sets of PC's are greatest at a lag of\ud four months and are highly significant. When stratified by different levels\ud of the OLR field PC's, the separations between means of the height field\ud PC's are highly significant as well. Differing distributions of the height\ud field PC ensemble are also found to be associated with different OLR field\ud PC levels.\ud The relationship between the 500mb height field and concurrent\ud western district precipitation is examined. Using a hybrid model\ud including both linear statistical and non-linear physical components it is\ud found that considerably more of the variance in the precipitation can be\ud explained by that of the height field alone than when the precipitation is\ud inferred directly from a linear statistical model.\ud A set of reconstructed height field PC's is predicted from OLR values\ud based on the height field/OLR stratification associations compiled for a\ud period separate from that of the forecast. Applied to the precipitation\ud model, this results in predicted western district precipitation which is\ud better correlated with the observations than is the equivalent precipitation\ud forecast from the linear statistical relationship of precipitation to the\ud Southern Oscillation Index

Year: 1995
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