Impacts of Aeolus horizontal Line‐Of‐Sight (HLOS) wind assimilation on the Korean integrated model (KIM) forecast system

Abstract

The Korean Integrated Model (KIM) forecast system, based on a hybrid fourdimensional ensemble-variational method, was extended to assimilate Horizontal Line-Of-Sight (HLOS) wind observations from the Atmospheric Laser Doppler Instrument (ALADIN) on board the Atmospheric Dynamic Mission Aeolus satellite. In a global cycling experiment, assimilation of Aeolus HLOS wind observations led to reductions in the average root-mean-square error of 0.8 and 0.5% for the zonal and meridional wind analyses when compared against European Center for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) analyses. Even though the observed variable is wind, there was also an overall beneficial impact on analyses of the mass variables. In the Southern Hemisphere (SH), the reduced analysis errors led to forecast skill improvements out to 72 h. In contrast, in the Northern Hemisphere (NH) there was relatively little reduction of analysis errors, but wind forecasts were nevertheless improved, and these positive impacts lasted longerout to 120 h rather than 72 h. Experiments suggest that the relatively poor long-range performance in the SH high latitudes was due to problems with the mass increments derived from Aeolus wind increments via the ensemble-based part of the hybrid background error covariance (B), which eventually led to adverse effects on the wind variables as forecasts progressed in the SH. This study shows that it is necessary to estimate the ensemble B in the Antarctic region with its high elevation more accurately in order to effectively use the Aeolus observation information.

Assessment for operational assimilation of horizontal line of sight winds from the European Space Agency's Aeolus at the Met Office

“…This is in contrast to the results of George et al (2021), who mostly saw the smallest impact in the Northern Hemisphere when assimilating from May 20 to June 22, 2020. Studies by Lee et al (2022) and Garrett et al (2022), where Aeolus was assimilated in a period during Northern Hemisphere summer, found the strongest impacts occurred in the Southern Hemisphere and Tropics. Wind observations often have the greatest impact in the winter hemisphere, where the jets are stronger, which may account for this difference.…”

“…Lee et al (2022) also saw improved analysis errors in the tropical upper troposphere mean zonal wind analysis. Garrett et al (2022) observed changes to the zonal mean zonal wind analysis in the Tropics but at higher altitude than Lee et al (2022) did. Garrett et al (2022) used the National Oceanic and Atmospheric Administration (NOAA) FV3GFS model to assimilate Aeolus HLOS winds using four-dimensional ensemble-variational assimilation, from August 2 to September 16, 2019, a very similar period to Laroche and St-James (2022).…”

“…Nevertheless, improvements were still seen in Northern Hemisphere forecasts. Lee et al (2022) also saw improved analysis errors in the tropical upper troposphere mean zonal wind analysis. Garrett et al (2022) observed changes to the zonal mean zonal wind analysis in the Tropics but at higher altitude than Lee et al (2022) did.…”

“…When assimilating reprocessed HLOS winds, which included a bias correction, they found the impact on the forecast accuracy was positive. Lee et al (2022) assimilated Aeolus HLOS winds into the Korean Integrated Model using a hybrid four-dimensional ensemble-variational method from June 16 to July 31, 2020. During this period, they found that the improvements to analysis errors when assimilating Aeolus resulted in improved forecasts in the Southern Hemisphere, but the analysis error reductions were much smaller in the Northern Hemisphere.…”

The Contribution of Aeolus Wind Observations to ECMWF Sea Surface Wind Forecasts

“…Despite the unforeseen signal loss and lower precision, Aeolus winds were assimilated into the ECWMF model through the four‐dimensional variational (4D‐Var) data assimilation technique to improve operational weather forecasts (Rennie et al., 2021). To assess the added value of Aeolus observations to NWP, many institutions have conducted Observing System Experiments (OSEs) with global NWP models, including ECMWF, National Oceanic and Atmospheric Administration (NOAA), Deutscher Wetterdienst (DWD), Météo‐France, Environment and Climate Change Canada, Korean Integrated Model (KIM), etc (Borne et al., 2023; Garrett et al., 2022; Laroche & St‐James, 2022; Lee et al., 2023; Pourret et al., 2022; Rennie et al., 2021). The evaluations demonstrated that with Aeolus data assimilation, the wind vector forecasts are improved by up to 4%, particularly in the upper troposphere and/or lower stratosphere over the tropics and polar regions (Garrett et al., 2022; Laroche & St‐James, 2022; Pourret et al., 2022; Rennie et al., 2021).…”