P98  Model Spin-Up Versus Initial Conditions – Does Cold-Start Time Matter That Much?

Zhang, Yongxin Yubao Liu, National Center for Atmospheric Research, Dorita Rostkier-Edelstein, Israel Institute for Biological Research, Israel, William Y. Y. Cheng, Linlin Pan, Yuewei Liu, NCAR, Adam Piterkovski, Government of Israel, Israel

Conventional wisdom holds that the closer the model simulations to the cold-start time in cycling runs the better the model simulations would become because of better initial conditions. This may be true in places where moisture is not a major problem. However, in climatologically dry places such as the Eastern Mediterranean where the only source of moisture is evaporation over the Mediterranean Sea, the spin-up issue becomes important due to the time it takes for the model to initiate, accumulate and transport enough moisture inland.

NCAR has applied the WRF-based real-time four dimensional data assimilation and forecasting system (WRF-RTFDDA) over the Eastern Mediterranean at 30-, 10-, and 3.3-km grid spacing for providing high-resolution model guidance. The uniqueness of WRF-RTFDDA is its capability of continuous data assimilation and forecast cycles that are able to provide dynamically consistent and Òspun-upÓ initial conditions that also include moisture. Using a heavy rainfall case as an example, we will show in this work that the WRF-RTFDDA runs with an earlier cold-start time do a much better job in resolving the observed precipitation over Israel than the WRF-RTFDDA runs with a much later cold-start time. Besides differences in the initial conditions due to different cold-start times, the limiting factor in this case is the spin-up of the moisture field.

Evaporation over the sea is dictated by SST and low-level winds, which in tern determines the moisture build-up and transport. We will show in this work that changes in SST over the Mediterranean Sea impact the spin-up of model moisture, which in turn affect the simulated intensity of precipitation over the Eastern Mediterranean.