A72B-0168 Simulation of the Climate of South-West Asia with a Regional Model J.P. Evans R. Smith Yale University, P.O. Box , New Haven, CT United States R. Oglesby NASA/MSFC/NSSTC, 320 Sparkman Dr., Huntsville, AL United States The ability of the regional model RegCM2 to simulate the climate of South-West Asia is examined. The climate of the region displays high spatial, seasonal and interannual variability, providing a strong test of climate model capabilities. RegCM2 captures the spatial variability of temperature despite cold biases being present in the model. RegCM2 does not capture the annual cycle of precipitation on the Black and Caspian Sea coasts where very steep topography exists, nor on the eastern Mediterranean coast where the coastal mountains are not resolvable. RegCM2 does capture the seasonal cycle in the Fertile Crescent and Zagros mountains, where it is strongly influenced by a plateau circulation above the Iranian plateau. It is shown that accurate simulation of precipitation in these regions, including the inter-annual variability, requires the correct simulation of both storm tracks and topographic interactions. Through the use of regional climate models the concentration and transport pathways of water vapor through the Middle East can be explored. The results demonstrate that short lived 'events' have little impact on monthly mean atmospheric fields yet provide a significant amount of the precipitated water which flows in the Tigris river. 1. Introduction South-West Asia (Figure 1) is interesting for several reasons. Over the last 8 thousand years, the landscape has been massively altered by developing human activity, including forest removal, rangeland degradation by plowing, grazing and trampling, and water course damming and diversion. Rapid population growth, political conflict and water scarcity are common throughout the area, rendering it sensitive to changes in climate. Indeed climate changes may have already had significant impact on the history of the region. Our study region includes the archeologically important Fertile Crescent, the birthplace of agriculture and civilization. The region is characterized by steep climatological gradients between the central desert regions and the surrounding water bodies and sizeable Tauros and Zagros mountain ranges. 2. Model, domain and experiment Saudi Arabia Syria Iraq Turkey Iran Mediterranean Persian Gulf Figure 1: RegCM2 domain RegCM2 Description Dynamical component based on MM4 hydrostatic compressible primitive equation terrain following  vertical coordinate modifications include detailed representation of radiative transfer surface physics-soil hydrology (BATS) planetary boundary layer convective precipitation scheme RegCM2 model run Centered at 45N 35E ~8,000,000km 2 total domain Topography and landuse are interpolated from a global 10 min dataset Initial and boundary conditions are extracted from the ECMWF TOGA analysis Begins 1 st january 1990 (1 st month of run is discarded) Covers 5 years grid spacing = 25 km time step = 90 s Black Sea Caspian Sea 3. Results 3.1 Surface temperature and precipitation While RegCM2 is able to capture the spatial distribution of temperature quite well, Figure 2 clearly shows a significant cold bias is present. Figure 3 shows that the model is able to capture only some aspects of the precipitation field well. The annual cycle of precipitation is modeled well in the Fertile Crescent and Zagros Mountains subregions however problems exist in each of the other subregions. Figure 5: Monthly average precipitation for each subregion. Figure 4: domain showing the focus subregions. 1. South-East Black Sea coast, 2. South-West Caspian Sea coast, 3. Eastern Mediterranean coast, 4. Fertile Crescent (headwaters of Tigris river), 5. Zagros Mountains and 6. Saudi desert. Figure 3: Observed and modeled mean annual precipitation Figure 2: Observed mean annual temperature and model bias 3.3 Water Vapor transport The fifteen largest precipitation events in the Fertile Crescent region were studied to establish where the water vapor leading to each event came from. Presented are results from a typical storm which occurred on the 16 th February Figure 9 shows snapshots of the storm, which clearly shows the easterly movement through the domain. Figure 9: Precipitation and winds. a) 24hrs before event; b) 12hrs before event; c) the event a bc abc Figure 10: Water vapor mixing ratio at 800hPa. a) 24hrs before event; b) 12hrs before event; c) the event. As well as the moisture coming from the west, an intense ‘river’ of moisture also enters from the south. Figure 12: Vertical cross sections showing the water vapor transport into the box defined by the white square, 6 hours before the precipitation maximum. From the west a broad region of moist air approaches while from the south a more confined ‘river’ of very moist air is evident. Figure 11: Mass of water entering the area of interest. Clearly large contributions come from both the west and the south. While water vapor transport into the Fertile Crescent region from the west is important, water vapor approaching from the south was at least as important in most of the precipitation events. Thus the Persian Gulf/Arabian Sea are important water sources for the Fertile Crescent. 3.2 Influence of terrain The terrain exerts a significant influence over the precipitation falling in the Fertile Crescent and Zagros Mountains regions. Figure 6 shows the Tigris-Euphrates valley being dominated by rising motion during the winter months as the westerly winds are forced to rise over the mountains. During summer the winds are turned down the valley and a large zone of descending air is present. Figure 7 shows this change in vertical motion is accompanied by the appearance of a summer temperature anomaly over the Iranian plateau. Figure 6: Omega and horizontal 700hPa Figure 7: Temperature and horizontal 700hPa Figure 8: Vertical cross-section of temperature and 34N This summer temperature anomaly over the Iranian plateau is associated with a break in the dominance of westerly winds. It induces a kind of plateau circulation effect which can be seen in Figure 8. Over the peaks of the mountain range easterly winds from the Iranian plateau meet westerly winds from the Mediterranean. This convergence of air masses above the 800 hPa level strongly restrains air from below this level from moving up into the convergence zone. Instead the low level air mass is more easily turned to flow along the mountain range towards the south-east.