Experiments Using Atmospheric River Reconnaissance Dropsondes

The US Naval Research Laboratory Marine Meteorology Division has participated in the 2018 and 2019 Atmospheric River (AR) Reconnaissance Program by providing adjoint-based sensitivity information used in the flight-track planning process, as well as forecasting support. Subsequently, NRL is examining the impact of the dropsonde observations in the Navy Global Environmental Model (NAVGEM)-NRL Variational Data Assimilation System Accelerated Representer hybrid 4DVAR system using Forecast Sensitivity Observation Impact (FSOI) and data denial experiments. The goal is to both study the impact of the dropsondes on the forecasts, and to use the observations to identify issues or biases within the current model and DA system. FSOI results indicate that the AR reconnaissance dropsondes have comparable impact to the North American radiosonde network for 24-h global forecast error reduction for the cases when at least two aircraft are flown. Although assimilation of moisture, temperature and wind observations generally have a positive impact, the moisture observations are less impactful than the wind or temperature observations, and this is the case for both the dropsondes and radiosondes. Preliminary analysis to understand why the moisture observations are not more impactful indicates that moisture observations are being rejected at a higher rate than wind or temperature observations. Statistical comparisons indicate that changes to the specified error covariances may enhance the impact of the moisture observations on the analyses and forecasts. Evaluation of the differences between the observations and the model background indicate that the model tends to be too moist in the middle troposphere, and has a low wind bias at high wind speeds, and high wind bias at low wind speeds. Data denial experiments indicate that, as expected, the differences between NAVGEM and European Centre for Medium-Range Weather Forecasts (ECMWF) analyses of lower-tropospheric temperature, wind, and moisture over the eastern North Pacific are reduced by as much as 10% when AR dropsondes are deployed and assimilated. However, the impacts on conventional error statistics over North America are mixed. The next steps are to look specifically at the impact of the AR dropsondes on short-range forecasts of integrated vapor transport and precipitation on the North American west coast.