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The third version of the Regional Atmosphere and Land (RAL3) science configuration is documented. Developed through international partnership, RAL configurations define settings for the Unified Model atmosphere and Joint UK Land Environment Simulator when applied across timescales with kilometre and sub-km scale model grids. The RAL3 configuration represents a major advance compared to previous versions by delivering a common science definition suitable for application to tropical and mid-latitude regions. Developments within RAL3 include the introduction of a double-moment microphysics scheme and a bi-modal cloud scheme, replacing use of a single-moment scheme and different cloud schemes for mid-latitudes and tropics in previous versions. Updates have been implemented to the boundary layer scheme and a consolidation of land model settings to be more consistent with Global Atmosphere and Land (GAL) science configurations. Physics developments aimed to address priorities for model performance improvement identified by users. This paper documents the RAL3 science configuration, including a series of iterative revisions delivered since its first release, and their characteristics. Evidence is provided from the variety of assessments of RAL3, relative to the previous version (RAL2). Collaborative development and evaluation across organisations has enabled evaluation across a range of domains, grid-spacing and timescales. The analysis indicates more realistic precipitation distributions; improved representation of clouds and of visibility; a continued trend to more realistic representation of convection; and reduced near-surface wind speeds, but a persistent cold temperature bias. Overall the convective-scale verification scores and climatological model distributions relative to observations improve for the majority of variables. Ensemble results show improvements to the spread-error relationship. User feedback from subjective assessment activities has also been positive. Differences between RAL3 revisions and RAL2 are further illustrated through process-based analysis of a convective system over the UK. The latest RAL3 configuration (RAL3.3) is therefore recommended for research, operational numerical weather prediction and climate production at km and sub-km scales. [ABSTRACT FROM AUTHOR]

The Met Office limited‐area model (LAM) data assimilation (DA) system is based on an hourly 4D‐Var method with lateral boundary conditions (LBCs) provided by the Met Office global model (GM). This set‐up was introduced operationally in summer 2017. The work presented here improved the representation of large‐scale dynamics in the LAM DA, which is now integrated on an extended domain and therefore also needs to represent larger scales than were present in 2017. To improve the representation of the large‐scale motions in the convective‐scale system, we choose to take advantage of the better estimation of these scales from the host model. The method presented here is called large‐scale blending (LSB), a spectral nudging solution which is applied to the Met Office LAM (UKV). Spectral nudging allows the LAM evolution to be constrained by a host model with a better representation of the larger scales while preserving the smaller scales predicted by the LAM. In this article, we investigate different options for the implementation of LSB, such as cut‐off wavelength and update frequency. The LAM forecast is typically improved for various variables over a forecast range from about T+10$$ T+10 $$ to T+20$$ T+20 $$ by the introduction of LSB. We also see a reduction in the internal gravity‐wave activity generated when new lateral boundary conditions are introduced to the LAM from the latest GM forecast. This research proves the benefits of a better representation of large‐scale motions for LAM forecasts. [ABSTRACT FROM AUTHOR]