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.