Universal Instrument Simulation Wrapper (UISW): A Lightweight and Flexible Observation Simulation Library in Support of Cal/Val, Data Assimilation and Digital Twin Projects

Projects involving Cal/Val, data assimilation, and digital twin all require the generation of synthetic observations. Those observations are generated either using direct model outputs or reanalysis products. Simulating satellite retrievals at Levels-2 to -4 requires interpolation of model values to the observation locations, while simulation of Level-1 satellite observations such as radiance requires application of additional observation operators (e.g., radiative transfer models) that transform the variables from the model space (e.g., profiles of temperature, humidity, and hydrometeors) to the observation space (e.g., radiance and brightness temperature). Such calculations usually pose challenges to researchers with limited experience of running complicated observation operators such as the radiative transfer model, and the calculations can become more complicated when more than one type of background or observation operator is involved in the study, a common scenario when attributing the simulation error to different error sources.

To address these challenges, we have developed a Lightweight, flexible, modular Fortran library, the Universal Instrument Simulation Wrapper (UISW), that automates synthetic observation generation, aiding Cal/Val, data assimilation, and digital twin projects. UISW includes high-level I/O modules to facilitate easy access to Levels-1 to -4 satellite data in the NetCDF, HDF, and HDF5 formats using Fortran. For Level-1 radiance simulation, UISW interfaces two most state-of-the-art fast radiative transfer models, Community Radiative Transfer Model (CRTM) developed at the JCSDA and the Radiative Transfer for TOVS (RTTOV) adopted by the ECMWF, which are widely used for atmospheric remote sensing and data assimilation. UISW supports radiance simulations using various backgrounds, including several reanalyses developed at ECMWF, NOAA, and NASA, and model outputs from the regional models ARW, NU-WRF, and SCALE. Notably, UISW's strength lies in its flexibility to switch backgrounds or radiative transfer models within a single program, simplifying the error attribution process. In this presentation, we will show several applications using the UISW, including simulation of Level-2 GPS-RO wet profiles and GMI-based integrated hydrometeor retrievals, bias characterization for Level-1 CrIS clear-sky radiance spectrum using multiple backgrounds and radiative transfer models, all-sky GOES ABI simulations for the Canadian Wildfire, and the ocean-surface sensitive clear-sky radiance simulations using coupled atmosphere-ocean model states from a coupled general circulation model. Through UISW, the complexities of observation generation are streamlined, accelerating satellite-based studies.