A coupled atmosphere and multi-layer land surface model for improving heavy rainfall simulation

Abstract

A multi-layer land surface model (SOLVEG) is dynamically coupled to the non-hydrostatic atmospheric model (MM5) in order to better represent spatial variations and changes in land surface characteristics compared with the land surface parameterization schemes included in the MM5. In this coupling, calculations of the atmosphere and the land surface models are carried out as independent tasks of different processors; a model coupler controls these calculations and data exchanges among models using the Message Passing Interface (MPI) libraries. This coupled model is applied to the record-breaking heavy rain events occurred in Kyushu Island, the southernmost of Japan's main islands, from July 20 to July 25 in 2006. The test computations are conducted by using both the coupled model and the original land surface parameterization of MM5. The results of these computations show that SOLVEG reproduce higher ground temperatures than land surface parameterization schemes in the MM5. The results indicate mat feedbacks of the land surface processes between MM5 and SOLVEG play an important role in the computations. The most pronounced difference is in the rainfall simulation that shows the importance of coupling SOLVEG and MM5. The coupled model accurately reproduced the heavy rainfall events observed in Kyushu Island compared to the original MM5 from both the spatial and the temporal point of view. This paper shows that realistic simulation of rainfall events strongly depends on land-surface processes interacting with cloud development that depends on surface heat and moisture fluxes, which in turn are mainly determined by land surface vegetation and soil moisture storage. The soil temperature/moisture changes significantly affect the localized precipitation and modest improvement in the land surface representation can enhance the heavy rain simulation MM5-SOLVEG coupling showed a clear image of the land surface-atmosphere interactions and the dynamic feedback on convection initiation, storm propagation and precipitation.