Observational Undersampling in Tropical Cyclones and Implications for Estimated Intensity

The maximum surface wind speed is an important parameter for tropical cyclone operational analysis and forecasting, since it defines the intensity of a cyclone. Operational forecast centers typically refer the wind speed to a maximum 1- or 10-min averaged value. Aircraft reconnaissance provides measurements of surface winds; however, because of the large variation of winds in the eyewall, it remains unclear to what extent observing the maximum wind is limited by the sampling pattern. Estimating storm intensity as simply the maximum of the observed winds is generally assumed by forecasters to underestimate the true storm intensity. The work presented herein attempts to quantify this difference by applying a methodology borrowed from the observing system simulation experiment concept, in which simulated “observations” are drawn from a numerical model. These “observations” may then be compared to the actual peak wind speed of the simulation. By sampling a high-resolution numerical simulation of Hurricane Isabel (2003) with a virtual aircraft equipped with a stepped-frequency microwave radiometer flying a standard “figure-four” pattern, the authors find that the highest wind observed over a flight typically underestimates the 1-min averaged model wind speed by 8.5% ± 1.5%. In contrast, due to its corresponding larger spatial scale, the 10-min averaged maximum wind speed is far less underestimated (1.5% ± 1.7%) using the same sampling method. These results support the National Hurricane Center’s practice, which typically assumes that the peak 1-min wind is somewhat greater than the highest observed wind speed over a single reconnaissance aircraft mission.