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

We use a moist adjoint model to compute the initial-condition perturbations that minimize the significant 48-72-h synoptic-scale forecast errors associated with the 15 November 2018 cyclone over the East Coast of the United States. These adjoint-optimal perturbations, which have a maximum amplitude of about 2 K in potential temperature, are widespread, extending throughout the troposphere and across much of North America. We investigate the most impactful subsets of the perturbations by truncating them in physical and spectral space and rescaling them to be equal in an energy norm to the full, unmodified perturbations. When the perturbations are confined to localized target regions, including those with the highest adjoint sensitivity or the strongest convective instability, they have weaker impacts on the forecast than when the perturbations within the target regions 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 greater impacts on the forecast than when the perturbations are filtered to retain only wavelengths shorter than 1000 km. These results suggest that the 15 November 2018 forecast bust was strongly sensitive to widespread, large-scale uncertainties in the initial conditions, rather than those in localized, small-scale regions that could more feasibly be reduced by targeted observations.

Left: 600-hPa potential-temperature adjoint perturbations (color fill every 0.5 K) for the (b) box, (d) full, and (e) hole experiments. GFS analysis for 600-hPa geopotential height (black contours, contoured every 15 dam). Green box in (b), (e) bounds the box and hole regions.

Right: 72-h SLP errors (color fill, contoured at 3.5 hPa) for (a) control (unperturbed) simulation and for the adjoint perturbations (b) box, (d) full, and (e) hole. SLP contours from the GFS analysis (black) and the forecast (magenta) contoured every 6 hPa.