In-up-and-out — William M. Gray’s Contribution and Legacy to Tropical Cyclone Inner-Core Characteristics

This presentation covers Dr. Bill Gray’s contributions towards understanding tropical cyclone (TC) inner-core structure characteristics and variability, and how those insights are utilized today in operations. Gray and Shea (1973a, b) utilized 533 radial flight legs executed by the National Hurricane Research Laboratory over a 13-year period (1957–69) to develop dynamic and thermodynamic inner-core composites. These were further stratified by intensity change, intensity, translation speed, and other parameters to identify useful features. The Gray and Shea work provided insight on PBL inflow, supergradient imbalances, and eye/eyewall thermodynamic patterns. We will briefly discuss modern TC reconnaissance datasets and composite research which branched from this seminal work.

A second example highlights the Gray philosophy of compartmentalizing complex structures – such as the TC radial wind field – into conceptual models highlighting distinct features. A Gray student, Merrill (1984), postulated TC wind structure with three characteristics: intensity (maximum sustained 10-meter wind), size (extent of the TC vortex from RMW to radius of gale-force wind), and strength (average wind speed of the vortex), motivated by the poor correlation of TC gale-force wind radii to intensity. From this work, he proposed a metric for size called the Radius of Outer Closed Isobar (ROCI). Weatherford and Gray (1998a, b) further defined the mean tangential wind velocity within a 1°–2.5°-latitude radius from the TC center as the outer-core wind strength (OCS), and noted only weak correlations with central pressure. However, the relationship between OCS and central pressure improved when eye size was considered, and a high correlation was also found between OCS and radius of gale-force winds. Late in his career, Gray also postulated pressure-RMW relationships in an unpublished book.

The above research provided motivation that gale-force wind predictions may have skill, and spurred further research into multivariate relationships for wind and pressure. In his unpublished WMO monograph (1995) he wrote “…it might be possible to use satellite or coast dual-Doppler radar information to infer improved relationships concerning the relationship between the radial distribution of tangential winds and the magnitude and character of the inner-core convection.” Members of the Gray research project and others, no doubt inspired by such ideas, followed with an operational-oriented focus on empirical gale-force prediction/analysis schemes (e.g., Cocks and Gray 2002, Knaff et al. 2007, 2011, 2017, Sampson and Knaff 2015, and Sampson et al. 2017, Chan and Chan 2012, 2015); analyses from infrared satellite data (e.g., Mueller et al. 2006, Kossin et al. 2007, Knaff et al. 2014, 2015, 2016), and microwave sounders (The wind-pressure relationship is used operationally by TC Warning Centers, and many methods to assess and predict wind radii are either transitioning or have transitioned to operations. Studies of TC structure climatology have also been conducted by Gray project members (Chan and Chan 2012) and by others in the meteorology community (Kimball and Mulekar 2004; Jinnan et al. 2007). From this research has evolved a TC structure dataset known as the “extended best track”, available at http://rammb.cira.colostate.edu/research/tropical_cyclones/tc_extended_best_track_dataset/ . Beginning in 2004, storm size information has also been incorporated into the National Hurricane Center's post-analysis HURican DATabase known as HURDAT2 (Jarvinen et al. 1984; Landsea and Franklin 2013). HURDAT2 is available at: http://www.aoml.noaa.gov/hrd/hurdat/Data_Storm.html .

One final example, and perhaps a less well-known facet of Gray’s legacy, is the first detailed tornado composite study of Novlan and Gray (1974) in landfalling TCs. This research identified the importance of the land interface in generating tornadoes due to increased low-level vertical shear and maximized low-level convergence. As originally postulated by Smith’s (1965) limited dataset, Novlan and Gray also confirmed tornadoes concentrated in the right front quadrant from 100-400 nm radially from the storm center, establishing an additional hazard of concern during landfalling events. The primary findings of Novlan and Gray (1974) have changed little with time even with larger and more modern datasets (e.g., McCaul 1991). However, nuances in tornado patterns have emerged, summarized by Edwards (2012), which will be outlined in the presentation. Today, the NOAA recognizes that special forecast attention is necessary for EF0, EF1, and occasionally EF2 tornadoes during landfalling events. In the United States, TC tornado prediction today is a coordinated effort between the Storm Prediction Center, the National Hurricane Center, and the local National Weather Service office staged as outlooks, watches, and warnings.

In conclusion, we leave you with one of Gray’s strongest research ideals as stated in his invited lecture to the 12th World Meteorological Organization Congress: “There is much to be learned about tropical cyclones from the wide variety of observational data sets that are now available. Although each type of data is inadequate in itself, an improved underlying synthesis of the cyclone’s internal physics and its environment becomes possible if one is able to piece together the interlocking association between the man different data sources. We need to better integrate the satellite, radar, aircraft, surface buoys, surface ships, rawindsondes and related data sets.” This is still true today.

References are available by request, and most are cited within:

Klotzbach, P. J., J. C. L. Chan, P. J. Fitzpatrick, W. M. Frank, C. W. Landsea, and J. L. McBride, 2017: The science of William M. Gray – his contributions to the advancement of tropical meteorology and tropical cyclones. Bull. Amer. Meteor. Soc., in press.