Implementation of water tracers in the Met Office Unified Model

Abstract. There is an increasing need to understand how water is cycled and transported within the atmosphere, to aid water management. Here, atmospheric water tracers are added to the Met Office Unified Model (UM) to allow tracking of water within the model. This requires the implementation of water tracers in the following parts of the model code: large-scale advection, surface evaporation, boundary layer mixing, large-scale precipitation (microphysics), large-scale clouds, stochastic physics, and convection. A single water tracer is found to track all water in the model to a high degree of accuracy during a 35 year simulation; the differences are typically less than 10−16 kg kg−1 at the end of every time step, prior to a very small adjustment to prevent the build up of numerical error. The increase in computing time for each water tracer is between 3.1 % and 3.8 %, depending on the model resolution. The model development is tested by using the water tracers to find the sources of precipitation in a historical UM simulation. As expected, the majority of precipitation is found to be sourced directly from the ocean, with the recycling of water over land becoming increasingly important downwind across continents. The UM results for the mean evaporative source properties of precipitation are comparable to those of the ECHAM6 atmospheric model, with some interesting local differences over Antarctica, Greenland, and the Indian monsoon region. Finally, global model hydrological fluxes are derived from the water tracers to show the amount of precipitation sourced from the land and ocean separately, illustrating the additional information that can be provided from the new development.