The influence of resolved convective motions on scalar dispersion in hectometric scale numerical weather prediction models

The UK Met Office has a 300 m grid length numerical weather prediction (NWP) model running routinely over London and in research mode city-scale hectometric grid length NWP has become commonplace. It is important to understand how moving from kilometre to hectometre scale grid length NWP influences boundary layer vertical mixing. For a clear-sky convective boundary layer (CBL) case study, using 55 m and 100 m grid length NWP, we demonstrate that CBL vertical mixing of passive scalar is almost fully resolved. Passive scalar converges near the surface after emission from an idealised pollution ground source representing city-scale emissions, and is transported in updrafts preferentially into the upper boundary layer. Approximately 8 km downstream of the source edge this causes 34% lower near-surface concentrations compared to 1.5 km grid length NWP, where vertical mixing is fully parameterised. This demonstrates that resolving ballistic type dispersion, which is not typically represented in NWP vertical mixing parameterisations, can have a leading order influence on city-scale near-surface pollution concentration. We present a simple analytical model that is able to capture diffusive and ballistic dispersion behaviour in terms of effective timescales. The timescale controlling how long it takes passive scalar to become well-mixed in the CBL is ≈ times longer for the 1.5 km compared to the 100 m and 55 m grid length NWP.