3 years ago

Impact of Two‐way Coupling and Sea‐surface Temperature on Precipitation Forecast in Regional Atmosphere and Ocean Model

Benedikt Strajnar, Jure Cedilnik, Anja Fettich, Matjaž Ličer, Neva Pristov, Peter Smerkol, Jure Jerman

Abstract

Six coupled atmosphere‐ocean setups of ALADIN and POM limited‐area models are used to simulate five heavy precipitation events over the Adriatic Sea. We evaluate several approaches to applying the one‐ or two‐way atmosphere‐ocean coupling (during the forecast of the warm‐up assimilation cycle, during the main forecast, or both) and using static SST information in various resolutions: from POM, from MFS (a regional ocean model) or from OSTIA analysis (used also by the global model ECMWF/IFS). The setups are designed in a way that allows for independent evaluation of various SST sources in ALADIN and the strategy of two‐way coupling. SST quality in this setups is verified against satellite observations. Impact on precipitation forecast is quantified by verification on over 900 stations. Results depend on weather situation, frequency of the update of SST and application of two‐way coupling prior and during the events. When SST is used statically in ALADIN, operational‐like forecasts using daily SST analysis from OSTIA are more accurate than those using MFS and especially POM; this illustrates the importance of using fresh information from observations. The two‐way coupling outperformed the one‐way coupling in the cases with heavy and localised convection as the dominating process. The skill of the two‐way coupled experiments was similar in situations with large‐scale synoptic forcing. It was found that including the two‐way coupling in the assimilation cycle few days before the studied weather events impacts the sea temperature forecast but has neutral impact on precipitation scores. Results suggest that ocean data assimilation is necessary in the two‐way coupled system in order to realistically update SST in the system with fresh observations.

Six setups using various SST information, air‐sea coupling and data assimilation are designed. Five heavy precipitation events over the Adriatic Sea were simulated and verified. Two‐way coupling is superior to one‐way coupling, SST update frequency plays a major role.

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