A consistent coupling of two‐moment microphysics and bulk ice optical properties, and its impact on radiation in a regional weather model

We present a consistent coupling between two‐moment microphysics and bulk ice optics in the Met Office's 1.5‐km resolution regional weather model and study its impact on top‐of‐atmosphere (TOA) short‐ and long‐wave irradiances. The coupling links the prognostic moments (total mass and number) to bulk ice optical properties through the mass‐equivalent spherical radius using Padé approximants. Model runs were evaluated for Darwin, Australia (January–March 2017) and the UK (December 2017–March 2018). Using this consistent coupled parametrisation, we demonstrate improved simulation of TOA short‐wave irradiances over both regions compared to the non‐consistent ice optical parametrisation when validated against satellite observations. Similar improvements were found for TOA long‐wave irradiances over Darwin, though the consistent parametrisation was slightly too transmissive over the UK. Overall, our more consistent two‐moment coupling between microphysics and ice optics leads to generally better prediction of radiation fields than single‐moment parametrisations.