Can Observation Targeting Be a Wild Goose Chase? An Adjoint-Sensitivity Study of a U.S. East Coast Cyclone Forecast Bust

Abstract Efforts to improve midlatitude-cyclone forecasts by deploying supplemental observations in localized target regions often fall short of expectations. We consider a potential contributing factor to these underwhelming results by investigating the initial-condition sensitivity of the 15 November 2018 East Coast cyclone forecast bust. We use a moist adjoint model to compute the initial-condition perturbations that minimize the large 48–72-h synoptic-scale forecast errors associated with this storm. The adjoint-optimal perturbations, which have maximum amplitudes of about 2 K in temperature and 1 m s −1 in horizontal wind speed, are widespread, extending throughout the troposphere and along a ridge–trough pattern covering much of North America. We investigate the most impactful components of the perturbations by truncating them in physical and spectral space and rescaling them to be equal in a domain-integrated energy norm to the full, unmodified perturbations. When the perturbations are confined to a localized target region of strongest sensitivity, they have weaker impacts on the forecast than when the perturbations within the target region are removed and the rest of the perturbations are retained. Additionally, when the perturbations are filtered to retain only wavelengths longer than 1000 km, they have stronger impacts on the forecast than when the perturbations are filtered to retain only wavelengths shorter than 1000 km. These results suggest that midlatitude-cyclone forecast improvements from targeted observations can be overwhelmed by smaller-amplitude but widespread and large-scale initial-condition sensitivities outside of the target region. Significance Statement Poor forecasts of midlatitude cyclones can cause tremendous socioeconomic disruption via unexpected heavy precipitation and damaging winds. One approach to improving these forecasts involves targeting observations in localized regions where initial-condition errors are expected to be most harmful to forecast accuracy. These efforts are expensive, yet they typically produce only minor forecast improvements. By examining a recent poorly forecast midlatitude cyclone, we find that a potential contributing factor to these underwhelming results is that small, but widespread changes to the initial state can be more impactful than the big, but localized changes that targeting is designed to make. This suggests that efforts to reduce initial-condition errors over broad areas can be more economical for improving midlatitude-cyclone forecasts than targeted observations.