Abstract
The forecast motion of tropical cyclones (TCs) critically depends on the evolution of the layer-averaged steering flow, which is associated with features proximate and remote to the TC. Given this, it is of interest to objectively identify the locations and aspects of the steering flow that will have the biggest impact on subsequent TC track forecasts, which in turn could be used to identify where to take supplemental observations, such as from aircraft, or extra rawinsondes. This paper describes the application of the ensemble-based sensitivity method to evaluate the sensitivity of TC track forecasts, which was used for synoptic surveillance flight planning for 55 potential missions during the 2019–21 seasons. TC track sensitivity can be calculated from either operational ECMWF or GEFS ensemble output (following the GEFS upgrade to version 12). Several automated methods are developed and described that provide sensitivity guidance that is useful and can be quickly interpreted, including a time-integrated track metric, and defining the steering wind within a coordinate framework along the axis of greatest position variability or vorticity. For the majority of cases, the sensitivity to the steering wind is maximized within 500 km of the TC center, particularly in the vicinity of nearby weaknesses in the subtropical ridge, with comparatively less sensitivity to upstream midlatitude features.
Significance Statement
Tropical cyclone motion is primarily influenced by the steering flow, which is determined by the evolution of various atmospheric features, and can be characterized by large uncertainties due to the lack of observations and/or low predictability. This study describes an ensemble-based method of predicting where uncertainties in the steering flow have the largest impact on subsequent tropical cyclone forecasts, which can be used to identify locations where additional aircraft observations may benefit the forecast. Over a 3-yr period, tropical cyclone track forecasts were found to be most sensitive to the steering flow within 500 km of the center of the tropical cyclone, with some additional sensitivity to weaknesses in the subtropical ridge, which allows the TC to move poleward. Future work will expand this methodology toward computing sensitivity for tropical cyclone hazard forecasts.
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