Dynamical downscaling precipitation over Southwest Asia: Impacts of radiance data assimilation on the forecasts of the WRF-ARW model Jianjun Xu a, b, ⁎, Alfred M. Powell, Jr. c a Nanjing University of Information Science and Technology, Nanjing, China b College of Science, George Mason University, Fairfax, VA, USA c NOAA/NESDIS/Center for Satellite Applications and Research (STAR), Camp Springs, MD, USA article info abstract Article history: Received 11 July 2011 Received in revised form 15 February 2012 Accepted 6 March 2012 Based on the dynamical downscaling with the Advanced Research Weather (WRF-ARW) me- soscale model, the accuracy of the precipitation forecasts in Southwest Asia has been assessed. Results show that the accuracy of the 24-h and 48-h forecasts for precipitation is closely relat- ed to the complex topography of the mountain areas. To understand the impacts of the initial condition uncertainties on accuracy of the dynamical downscaling, a series of data assimilation experiments has been performed. The Advanced Television and Infrared Observation Satellite Operational Vertical Sounder (ATOVS) radiance observations and a data assimilation system named the Gridpoint Statistical Interpolation (GSI), developed by the National Centers for Environmental Prediction (NCEP), were used in this study. The results show that the satellite data provides beneficial information for improv- ing the initial conditions for the dynamical model system and the “forecast” errors are reduced for most locations within the 24-h hindcasts. © 2012 Elsevier B.V. All rights reserved. Keywords: Southwest Asia Precipitation Data assimilation 1. Introduction Precipitation forecasts in Central and Southwest Asia (SWA) are often very complex because of mesoscale varia- tions induced by the diverse topography (Fig. 1). This is a predominately semi-arid to arid region surrounded by the Black and Caspian Seas in the north, the Mediterranean Sea in the west, the Arabian Sea and Persian Gulf in the south, Himalaya Mountains in the east, and crossed by the impres- sive Tauros and Zagros mountains. The Zagros Plateau in Iran includes broad areas above 1500 m in elevation, and iso- lated peaks as high as 4000 m. The climate of this area ranges from steppe to desert, with large areas of the region receiving little to no precipitation. For example, the precipitation in Iran and Afghanistan primarily falls during winter storms moving eastward from the Mediterranean, with the high mountains of the region intercepting most of the water and the interior high plains left with large stretches of barren de- sert. This wintertime precipitation generally occurs between the months of November and April. Much of the precipitation falls as snow in the higher elevations and the timing and amount of snowmelt are important factors for the irrigated agriculture prevalent in the region. In eastern Pakistan, the primary rainfall season is in the summer, associated with the northernmost advance of the Asian monsoon. The mon- soonal system results in a summertime maximum in precip- itation in the northern mountain regions of Pakistan but generally suppresses rainfall over Iran and Afghanistan. The most striking climatological feature of Southwest Asia is a strong North to South precipitation gradient, stretching from the humid highlands in Turkey (>1 m per year) to de- serts in Syria and Saudi Arabia (b 100 mm per year). The gra- dient is interesting from a physical perspective, as it represents the combined influences of latitude, topography, and local evaporation. Atmospheric Research 111 (2012) 90–103 ⁎ Corresponding author at: College of Science, George Mason University, 4400 University Drive, Fairfax, VA 22030, USA. E-mail address: Jianjun.xu@noaa.gov (J. Xu). 0169-8095/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2012.03.005 Contents lists available at SciVerse ScienceDirect Atmospheric Research journal homepage: www.elsevier.com/locate/atmos