<p>Precipitation over East Asia (EA) simulated in six experiments using the Met Office Unified Model (MetUM) is compared with observations and ERA-Interim reanalysis. These simulations are performed at resolutions of 200–40 km and with both atmosphere-only and air-sea coupled configurations. EA precipitation in MetUM is systematically overestimated, especially over southeastern EA and the Tibetan Plateau. Moisture sources for simulated and observed regional EA precipitation are traced using the Water Accounting Model-2layers (WAM-2layers) – a moisture tracking model that traces moisture sources using a combination of evaporation, atmospheric moisture and circulation. Biases in moisture sources are linked to biases in precipitation. For southeastern EA, positive precipitation biases are linked to errors in mid-latitude sources rather than tropical ocean sources. Increasing resolution reduces precipitation biases over the Tibetan Plateau. From the perspective of moisture source, this reduction comes from reduced remote moisture source that is blocked by the better representations of topography at higher resolution. Including coupling does not necessarily improve EA precipitation, however, coupling alters moisture sources. Because the effect of coupling on moisture source varies on location and sign, the collective impact on EA precipitation may not be noticeable. By using WAM-2layers, these changes in moisture sources can be attributed to changes in SST, circulation and associated evaporation. WAM-2layers can be a useful tool to identify model biases that cause biases in regional precipitation.</p>