A global coupled ensemble data assimilation system using the Community Earth System Model and the Data Assimilation Research Testbed

Abstract

This paper presents a description of the CESM/DART ensemble coupled data assimilation (DA) system based on the Community Earth System Model (CESM) and the Data Assimilation Research Testbed (DART) assimilation software. The CESM/DART should be viewed as a flexible system to support the DA needs of the CESM research community and not as a static reanalysis product. In this implementation of the CESM/DART, conventional insitu observations of the ocean and atmosphere are assimilated into the respective component models of the CESM using a 30‐member ensemble adjustment Kalman filter (EAKF). CESM/DART is run in a “weakly coupled” configuration wherein observations native to each climate system component only directly impact the state vector for that component. Information is passed between components indirectly through the short‐term coupled model forecasts that provide the EAKF background ensemble. This system leverages previous ensemble DA development for the Community Atmosphere Model and Parallel Ocean Program models using the DART EAKF. The CESM/DART project is a step towards providing increasingly useful DA capabilities for the CESM research community. Results are presented for our prototype 12‐year reanalysis, run from 1970 to mid 1982. Multiple lines of evidence demonstrate that the system is capable of constraining the CESM coupled model to simulate the historical variability of the climate system in the well‐observed Northern Hemisphere. A collection of monthly average variables, climate mode indices, observation diagnostics and snapshots of synoptic weather in the ocean and atmosphere are compared to established datasets, showing especially good agreement in the Northern Hemisphere. A discussion of the CESM/DART as a modular, community facility and the benefits and challenges associated with this vision is also included.

A Reliability Budget Analysis of CESM‐DART

“…The DART system is an open-source community developed software used to implement the EAKF data assimilation technique into different models (Anderson et al, 2009). The application of DART to the CESM (Karspeck et al, 2018) provides a novel depiction of climate data for the 12-year period that it was run and made available to us (1971)(1972)(1973)(1974)(1975)(1976)(1977)(1978)(1979)(1980)(1981)(1982). CESM-DART uses CESM1.1.1, which is a coupled model, which combines the Community Atmospheric Model Version 4 (Gent et al, 2011), with the Community Land Model (CLM4), the Community Ice Code (CICE4), CESM River Transport Model, and the LANL Parallel Ocean Program (POP2) for a coupled earth climate model (Hurrell et al, 2013).…”

“…The components are coupled together every 24 hr. ACARS and BUFR data (see Karspeck et al, 2018, for the data sources) is used to adjust CAM and POP state vectors. Aircraft, radiosonde, and a few satellite measurements of wind and temperature are used for the atmosphere model.…”

Tropical climate variability in the Community Earth System Model: Data Assimilation Research Testbed

“…The DART system is an open-source community developed software used to implement the ensemble adjustment Kalman filter (EAKF) data assimilation technique into different models (Anderson et al 2009). The application of DART to the CESM (Karspeck et al 2018) provides a novel depiction of climate data for the period that it was run (1970)(1971)(1972)(1973)(1974)(1975)(1976)(1977)(1978)(1979). CESM-DART is run with a 30-member ensemble, with data assimilated every 6 h for atmospheric fields and 24 h for oceanic components.…”

“…The components are coupled together every 24 h. ACARS data is used to adjust CAM state vectors and BUFR data is used for POP state vectors. Localization and inflation methods are explained by Karspeck et al (2018).…”

“…Penny and Hamill 2017). Recently, a weakly-coupled data assimilation system (Karspeck et al 2018) was developed at the National Center for Atmospheric Research (NCAR) using the Community Earth System Model (CESM) in the Data Assimilation Research Testbed (DART), which is an Ensemble Adjustment Kalman Filter-based (EAKF) scheme (Anderson et al 2009). Invoking such a procedure in an Earth System Model could provide new insights into the role of ocean-atmosphere coupling on the climate processes that have occurred over the historical record.…”