Ocean models that neglect mass and momentum contributions from precipitation can have a
systematic bias in sea surface height (SSH). Here, a new rainfall scheme is introduced into
the Regional Ocean Modelling System (ROMS) to incorporate the effects of precipitation
mass. When precipitation is added to the sea surface, it spreads out via surface gravity waves
that increase in propagation speed with increasing water depth. Over several days, the SSH
increase due to the precipitation mass added created a geostrophic adjustment, generating
anti-cyclonic geostrophic currents around the SSH increase. The transfer of momentum from
precipitation to the sea surface, or rain stress, can also be important. In the case study of a
real tropical cyclone, Monica passing North Australia, the effect of incorporating
precipitation mass is compared with other processes affecting the storm surge: surface wind,
inverse barometer effect and rain stress. The maximum SSH response is 170.6 cm for the
wind effect, 61.5 cm for the inverse barometer effect, 7.5 cm for the effect of rain stress and
6.4 cm for the effect of rain mass. Each process has been shown to have different spatial
influences. The effect of rain mass has a strong remote influence compared to the inverse
barometer effect and the effect of rain stress. This is particularly seen in semi-enclosed bays.