The role of low-level terrain-induced jets in rainfall variability in Tigris-Euphrates Headwaters.

Rainfall variability in the Tigris-Euphrates Headwaters is a result of interaction between topography and meteorological features at a range of spatial scales. Here, we have implemented the Weather Research and Forecasting (WRF) model, driven by NCEP/DOE R2, to better understand these interactions. Simulations were performed over a domain covering most of the Middle-East. The extended simulation period (1983-2013) enables us to study seasonality, interannual variability, spatial variability and extreme events of rainfall. Results showed that the annual cycle of precipitation produced by WRF agrees much more closely with observations than does R2. This was particularly evident during the transition months of April and October, which were further examined to study the underlying physical mechanisms. In both months, WRF improves representation of interannual variability relative to R2, with a substantially larger benefit in April. This improvement results primarily from WRF’s ability to resolve two low-level terrain-induced flows in the region that are either absent or weak in NCEP/DOE: one parallel to western edge of the Zagros Mountains, and one along the East Turkish Highlands. The first shows a complete reversal in its direction during wet and dry days: when flowing southeasterly it transports moisture from the Persian Gulf to the region, and when flowing northwesterly it blocks moisture and transports it away from the region. The second is more directly related to synoptic-scale systems and carries moist, warm air from the Mediterranean and Red Seas toward the region. The combined contribution of these flows explains about 50% of interannual variability in both WRF and observations for April and October precipitation.

Fig. 1A: various components contributing to rainfall of the Tigris-Euphrates Headwaters (TEH), which is the region of interest in this study (black solid box). The arrows represent low- level flows affecting the rainfall of the TEH: Zagros Barrier Jet (ZBJ), Shamal winds, and low-level westerly (LLW) flow. The setup of the high (H) and low (L) pressure regions in this schematic resembles an active ZBJ and rainy day in the TEH. The favorable conditions for the Shamals are different (see the text). The green shading shows the Tigris-Euphrates Basin. The dashed black box is the WRF simulation domain. B: the topographic map of the domain. C: long-term mean (1988-2013) annual precipitation (mm/day) using GPCC daily data.