INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 32: 1226–1237 (2012) Published online 17 May 2011 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/joc.2347 Spatial patterns and regimes of daily precipitation in Iran in relation to large-scale atmospheric circulation Tayeb Raziei, a * Abbas Mofidi, b Jo˜ ao A. Santos c and Isabella Bordi d a Soil Conservation and Watershed Management Research Institute (SCWMRI), Tehran, Iran b Department of Geography, Ferdowsi University of Mashhad, Mashhad, Iran c Centre for the Research and Technology of Agro-Environmental and Biological Sciences (CITAB), University of Tr´ as-os-Montes and Alto Douro5001-801 Vila Real, Portugal d Department of Physics, University of Rome “La Sapienza”, Rome, Italy ABSTRACT: 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 KEY WORDS daily precipitation; atmospheric circulation types; precipitation regimes; Iran; APHRODITE Received 6 October 2010; Revised 28 February 2011; Accepted 2 April 2011 1. Introduction Space-time variability of precipitation in a region plays an essential role in water resource management, motivat- ing many research efforts to identify large-scale atmo- spheric circulation types leading to precipitation events and their effects on the frequency and amount of pre- cipitation (Lamb, 1977; Bogardi et al., 1994; Frakes and Yarnal, 1997). Many authors have studied the possible linkages between large-scale atmospheric circulation and precipitation (e.g. Corte-Real et al., 1998; Romero et al., 1999; Wibig, 1999; Xoplaki et al., 2000; Kidson, 2000; Santos et al., 2005). Owing to the complex orography (Figure 1) and wide latitudinal extent – from the Hadley cell descend- ing branch up to mid-latitudes – precipitation in Iran is highly variable both in space and time. Precipita- tion occurrences and amounts vary from the relatively humid western mountainous areas to the arid and semi- arid regions of central and eastern Iran, which are char- acterized by sporadic precipitation events. Additionally, *Correspondence to: Tayeb Raziei, Soil Conservation and Watershed Management Research Institute (SCWMRI), P.O. Box 13445-1136, Tehran, Iran. E-mail: tayebrazi@scwmri.ac.ir the increasing water demand, due to population growth, urbanisation, and industrialisation, has further increased the vulnerability to droughts. As such, a comprehensive analysis of the relationship between precipitation regimes (PR) in Iran and large-scale atmospheric circulation is of great interest. It should be noted that the existence of such relationship is of crucial importance not only for the study area, but also for other geographical contexts, since it enables the prediction of precipitation events at regional scale given the atmospheric pre-conditioning factors. Therefore, the present study illustrates a method- ology that can be easily applied in other regions with reliable precipitation datasets. Domroes et al. (1998), by applying a Principal Com- ponent Analysis (PCA) and Cluster Analysis (CA) to monthly precipitation recorded at 71 stations irregu- larly distributed across Iran, identified 5 sub-regions with different PRs. Dinpashoh et al. (2004), using 77 sta- tions throughout Iran and following a similar approach, isolated 7 climatic sub-regions. Soltani et al. (2007) also analysed PRs using monthly precipitation time series from 28 sites, and 3 main climatic groups were found by applying a hierarchical cluster analysis. Using a dense Copyright  2011 Royal Meteorological Society