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ABSTRACT The relationship between daily large-scale atmospheric circulation types (CTs) and wintertime daily precipitation over Iran during the period 1965-2000 is investigated. Twelve atmospheric CTs identified in a previous study, which applied the K-means clustering technique to the rotated principal components (RPCs) of the 500 hPa geopotential height fields, are also considered in this study. The leading spatial modes of daily precipitation variability over Iran are extracted by a PC analysis, with Varimax rotation, applied to the APHRODITE (Asian Precipitation-Highly Resolved Observational Data Integration Towards Evaluation of the Water Resources) gridded precipitation dataset; six Iranian sub-regions with independent precipitation variability are identified. The relationships between the CTs and the daily precipitation are investigated by computing: (1) the spatial patterns of the performance index (PI) for each CT and (2) the cross tabulations between the frequencies of occurrence of the CTs and the RPC scores of the daily precipitation, associated with each of the six sub-regions. Results suggest that two particular CTs affect the precipitation occurrence over most of the country, while the remaining ten provide more regional or negligible contributions to precipitation. The more (less) influencing CTs in each precipitation sub-region are then identified and a characterization of the main large-scale atmospheric features governing the winter precipitation fields is provided. [ABSTRACT FROM AUTHOR]

The relationships between large-scale atmospheric circulation types and seasonal regimes of daily precipitation over Iran are assessed using daily precipitation from a high-resolution gridded dataset provided by the Asian Precipitation-Highly Resolved Observational Data Integration Towards the Evaluation of Water Resources (APHRODITE) Project. Regional spatial modes of daily precipitation variability were identified by S-mode Principal Component Analysis (PCA) with Varimax rotation, applied to the subset of days when at least 10% of all grid-points over Iran received precipitation ≥ 5 mm. The study refers to the period 1961-2004 and is carried out for each season (excluding summer) separately. To characterize the dynamical features associated with each regional precipitation regime (PR), composites of daily atmospheric fields are computed by only averaging days with rotated PCA scores ≥ 1.5 (strong positive phase). In autumn and winter, Iran is divided into five PRs, while four PRs are identified in spring. Results suggest that the spatial distribution of precipitation over Iran is largely governed by the geographical position of both the mid-tropospheric trough over the Middle East and the Arabian anticyclone. In fact, in almost all PRs, the trough, as a pre-conditioning factor, leads to regional-scale ascending motions, whereas the Arabian anticyclone induces low-tropospheric moisture transports from southern water bodies into the cyclonic systems near Iran, triggering rain-generating conditions. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

Aiming at better understanding the causes and processes leading to meteorological dry and wet episodes in western Iran, the present study investigates the space and time variability of winter dry/wet events and their associated large-scale atmospheric driving circulations. Using monthly precipitation data of 140 weather stations for 1965-2000, meteorological dry/wet spells are assessed through the 1-month Standardized Precipitation Index (SPI). By applying the Principal Component Analysis (PCA) with Varimax rotation to the SPI field, three sub-regions of independent climatic variability are identified. Large-scale daily atmospheric circulation patterns are then classified into twelve circulation types (CTs) by applying PCA to the 500 hPa geopotential height fields coupled with the non-iterative K-means clustering technique, in which extreme rotated principal component scores, in their positive and negative phases, are considered as potential groups and their centroids are computed by averaging all days that fulfil the extreme score criterion. The linkage between daily CTs and winter dry/wet spells in the region is investigated by correlating the frequencies of occurrence of each CT with the time series of dry/wet events, derived from both the rotated principal component scores of SPI and the SPI series at three representative stations of the identified sub-regions. Severe/extreme events have also been considered. A performance index is applied to test the relationship between CTs and dry/wet events. Results show statistically significant correlations between the frequencies of occurrence of the identified CTs and of dry/wet spells at the three representative stations, suggesting that particular CTs are potential predictors for the winter dry/wet events in western Iran. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]