The Potential for Improved Short-term Atlantic Hurricane Intensity Forecasts Using Recon-based Core Measurements Andrew Murray, Robert Hart, Florida State University Department of Meteorology [Department of EOAS in May] [ in late 2010] 64 th IHC, Savannah, GA, 3 March 2010 NASA NNX09AC43G [GRIP]

Desire to better use core data for short-term intensity forecasting First note that SHIPS (DeMaria and Kaplan) remains the standard to beat for intensity change in the short-term Potential use of core data: – New insight into evolution of the core – Sawyer-Eliassen non-linear balance theory (Shapiro and Willoughby 1982) argues for use of the TC core as both as potential predictors and predictands – Results of Kossin and Sitkowski (2008) argue that inner core structural changes have predictability Limitations of core data: – Very small subset of global TCs observed by recon – Irregular sampling

Dataset Data used in Atlantic basin VDM Climatology Dataset Period Data SourceNHC ATCF archives Flight Level700 hPa Total Vortex Data Messages (VDMs) 1929 Number of TCs Included83

Piech (2007)

Gulf of Mexico Composite mean VDM evolution using first closed eye report as Time 0

Graphical Display of Prior Composite

Example case: Wilma (2005)

Frequency Distribution Comparison Vortex Message Minimum MSLP (hPa) Maximum Advisory Sustained Surface Wind (kt) Eye Diameter (nm) Minimum MSLP (mb) Max. Sustained Wind (kt)

Mean future 12-hr Intensity Change 12-hour mean wind rate (hr -1 )12-hour SE of the mean Max. Sustained Wind (kt) Eye Diameter (nm)

Need for a multi-parameter prediction system Two-predictor system leads to forecasts of strengthening for TCs 90 kt Prediction based solely on eye diameter and maximum wind speed is insufficient to accurately predict TC intensity changes What predictors should be useful?

Predictor Examples Example raw VDM predictors – Minimum MSLP – Eye temperature and dewpoint Example derived predictors – Temperature gradient across eyewall – Area of eye – Equivalent potential temperature – Inertial stability of the eye – Dewpoint depression of eye * Area of eye

Examples of Transformed Predictors UntransformedTransformed Area*Ti Ln(V 2 /R)V 2 /R (Area*Ti) 0.15

Regression sets First attempt (called NSNC-full): – Does not do development binned by initial intensity. – One equation is derived to be used at all initial intensities. – Seven equations total, one for each forecast time Second attempt (called NSNC-binned): – Uses running bins of 20kt initial intensity for development – Leads to 133 equations (19 binsx7 forecast lengths)

Performance of NSNC-Full Equations Out of sample performance shown Verification performed via removal of one VDM at a time Superior to SHIPS at all forecast lead times with largest improvement at 12 hr 21% improvement of NSNC relative to SHIPS Impact of serial correlation still an issue since only one VDM is removed

Independent Verification Multi-year independent dataset eliminates serial correlation and increases confidence in future performance 11% improvement over SHIPS at 12 hr for both periods  21% improvement when using leave-one- VDM-out method While serial correlation contributed to apparent skill, there remains statistically significant improvement over SHIPS

Performance of Binned Regression Large gap in performance at Category 4/5 Performance at 12 hr indicates that inclusion of storm-scale predictors may be essential to improving short-term forecasts Decreased skill from SHIPS for Cat 5 at 24 hr  surprising, given expectation of weakening for storms of that intensity Distinct relative minima from kt, kt, and kt  Related to ERC predictability (Kossin and Sitkowski 2009)?

Important Caveats The apparent improvement over SHIPS is not an apples to apples comparison SHIPS was developed using TCs over the entire basin, while this study (necessarily) only used TCs flown by recon Further, SHIPS did not have individual equations as function of initial intensity. Further, SHIPS has evolved over time as the science and observations have evolved A more apples to apples study would recalculate SHIPS style using the subset of storms used here, in full and binned, and then intercompare

Conclusions and Future Work Nonetheless, independent testing showed that new technique is comparable to or surpasses the skill of SHIPS for short-term forecasts for the subset of storms flown by recon. Utilization of running bins of 20 kt size provides additional benefit. Predictability of future TC intensity is strongly a function of initial intensity and is not linear. Future: Examine satellite proxies for VDMs to determine if the approach and skill can be extended to non-recon basins [GRIP] If not, potentially argues for more frequent Hurricane Hunter recon missions in the Atlantic basin and expanding recon flights to other basins.