Assessing a New Coupled Data Assimilation System Based on the Met Office Coupled Atmosphere–Land–Ocean–Sea Ice Model

Abstract

A new coupled data assimilation (DA) system developed with the aim of improving the initialization of coupled forecasts for various time ranges from short range out to seasonal is introduced. The implementation here is based on a “weakly” coupled data assimilation approach whereby the coupled model is used to provide background information for separate ocean–sea ice and atmosphere–land analyses. The increments generated from these separate analyses are then added back into the coupled model. This is different from the existing Met Office system for initializing coupled forecasts, which uses ocean and atmosphere analyses that have been generated independently using the FOAM ocean data assimilation system and NWP atmosphere assimilation systems, respectively. A set of trials has been run to investigate the impact of the weakly coupled data assimilation on the analysis, and on the coupled forecast skill out to 5–10 days. The analyses and forecasts have been assessed by comparing them to observations and by examining differences in the model fields. Encouragingly for this new system, both ocean and atmospheric assessments show the analyses and coupled forecasts produced using coupled DA to be very similar to those produced using separate ocean–atmosphere data assimilation. This work has the benefit of highlighting some aspects on which to focus to improve the coupled DA results. In particular, improving the modeling and data assimilation of the diurnal SST variation and the river runoff should be examined.

Coupling of surface air and sea surface temperatures in the CERA‐20C reanalysis

“…NWP centres have recently started to develop coupled DA systems to provide more consistent AO analysed states for forecast applications, with the most recent examples being the GFDL's ECDA (Zhang et al, ), NCEP's CFSR (Saha et al, ), Canadian CanSIPS (Merryfield et al, ), UK Met Office's coupled DA system (Lea et al, ) and ECMWF's CERA (Laloyaux et al, ) . Such systems use a common assimilation window for both ocean and atmosphere, but still with separated analysis of increments, and are usually called ‘weakly’ coupled DA as no cross‐medium background‐error covariances are used (Lu et al, ; Sluka et al, ).…”

Improved SST‐Precipitation Intraseasonal Relationships in the ECMWF Coupled Climate Reanalysis

“…One major limitation in uncoupled reanalyses is the lack of atmospheric feedbacks onto the ocean boundary conditions. For producing more consistent ocean‐atmosphere reanalyses, coupled general circulation models have recently started to be used in DA, with the most recent examples being the NCEP's Climate Forecast System Reanalysis (CFSR; Saha et al, , ), UK Met Office's coupled DA system (Lea et al, ) and the Coupled European Centre for Medium‐range Weather Forecasts (ECMWF) ReAnalysis system (CERA; Laloyaux et al, ). Such systems are based on coupled models with DA implemented individually for the atmosphere and ocean components and are usually named as “weakly” coupled DA systems as no cross‐medium background error covariances are directly used (Lu et al, ; Sluka et al, ).…”

GEOS‐S2S Version 2: The GMAO High‐Resolution Coupled Model and Assimilation System for Seasonal Prediction

“…All components are coupled together using the Earth System Modeling Framework (Hill et al, 2004) and the Modeling Analysis and Prediction Layer interface layer (Suarez et al, 2007). Various operational centers are developing Coupled AODAS systems (Brassington et al, 2015;Dee et al, 2014), in which different components (e.g., atmosphere and ocean) of the Earth system are analyzed separately (Laloyaux et al, 2016;Lea et al, 2015) or simultaneously (Sluka et al, 2016;Wada & Kunii, 2017). The GEOS-S2S Coupled AODAS relies on an precomputed near-real-time atmospheric analysis and performs an analysis of the ocean state.…”