An air-sea coupled model is developed by combining the Advanced Regional Prediction System (ARPS) and Princeton Ocean Model (POM). The two models were coupled through exchanges of momentum, heat and salinity fluxes at the air-sea interface. The coupled model is used to simulate squall lines observed during TOGA COARE. The simulations agree well with the observations. The simulated results showed that the precipitation-induced fresh water flux creates a thin low-salinity stable layer near the sea surface. This stable layer shortens the response time of sea surface to atmospheric forcing. Consequently, the coupled solutions in the atmospheric and the ocean appear significantly differently depending on whether precipitation is included in the coupling.