Effects of coastal topography on climate: High-resolution simulation with a regional climate model

High-resolution regional climate simulations driven by NCEP-reanalysis for the eastern Mediterranean (EM) region covering 48 yr (1961-2008) are used to examine the coastal effects over the EM region. The present study uses the ICTP-RegCM3 to downscale to a 10 km resolution over the EM with a 50 km driving nest. The high-resolution simulation captures strong temperature gradients as a result of resolving the steep topography over the eastern Black Sea and Mediterranean coasts of Turkey, as well as the Ionian coast of Greece. In terms of the validation based on the coastal meteorological stations, the annual temperature bias in the high-resolution simulation is <0.7°C for the Black Sea and Mediterranean regions over Turkey and the same bias is 1°C in the 50 km simulation. Moreover, the model is able to capture the observed warming trend in summer temperatures during the last 5 decades. In terms of the precipitation simulation, the error in the 10 km simulation (17%) over the Black Sea region is much lower than that in the 50 km simulation (42%) when considering coastal meteorological stations. In addition, existence of many small islands over the EM domain is another unresolved challenge for climate simulations. The small islands tend to be overwhelmed by the influence of sea-surface temperature, and local climate effects related to topography are suppressed in 10 km temperature simulations. However, the precipitation error in the 10 and 50 km simulations based on the island stations (Crete, Rhodes and Cyprus) over the EM domain are 12 and 24%, respectively. In terms of the cross-section analysis comparing model elevation to simulated precipitation, the implementation of highresolution simulation illustrates the importance of modeling the complex terrain over the eastern Black Sea and Mediterranean coasts of Turkey, as the precipitation error was reduced to 7 and 18%, respectively.