Further Studies of Observational Undersampling in Flight-level and SFMR Observations

Obtaining the best estimate of tropical cyclone (TC) intensity is vital for operational forecasting centers to produce accurate forecasts and to issue appropriate warnings. Aircraft data are traditionally provide the most reliable information about the TC inner core and surrounding environment, but sampling strategies and observing platforms associated with these TC-penetrating aircraft have inherent deficiencies that contribute to the uncertainty of the intensity estimate. One such instrument, the stepped frequency microwave radiometer (SFMR) on the NOAA WP-3D aircraft, provides surface wind speeds along the aircraft flight track, usually sampling several times near the maximum surface wind speed. Because the design of the flight tracks substantially limits the azimuthal coverage of wind speed observations, it is difficult to truly observe the maximum wind speed during a given flight. Using a high-resolution Weather Research and Forecast (WRF) simulation of Hurricane Isabel (2003), a previous study produced SFMR-like wind speeds along simulated flight tracks. Generally, the 1-minute mean (maximum) surface winds underestimate a 6-hour running mean maximum wind (i.e. best track) by ~7.5-10%.

In this JHT-funded work, a newly developed, high-resolution hurricane nature run (HNR) is utilized in the place of the Isabel simulation in order to assess the underestimation of the maximum wind throughout the TC lifecycle. This particular HNR contains a genesis period, rapid intensification, a period of slight weakening due to eyewall replacement, and finally becomes fairly steady-state as a mature hurricane. Preliminary results find that the mean underestimate of the best-track estimate is on the order of 12-15%, which is considerably higher than determined from the previous Isabel simulation. There are several factors that may have contributed to this increased underestimation, which include increased variance in the HNR maximum wind speed, advancements and improvements to the model configuration, and the stages of TC development previously mentioned. After recalibrating the simulated SFMR method, our results reveal that these increased underestimates are correct and indicate that the current method for sampling TCs may not be optimal for providing the most accurate estimate of the maximum surface wind speed in all stages of the TC lifecycle. Results will be extended to include additional storms with stages and structures not captured by the first HNR.