Edward Zipser Department of Atmospheric Sciences University of Utah, Salt Lake City, UT, USA

1. Necessary conditions for intensification of tropical cyclones: The role of mesoscale systems and convective intensity Edward Zipser Department of Atmospheric Sciences University of Utah, Salt Lake City, UT, USA ed.zipser@utah.edu

2. Hot Towers: Of vital importance for the Hadley Cell and for hurricanes, but how, exactly, are they defined? • “We postulated that all, or nearly all the mass reaching great heights in the core ascends rapidly in a few nearly undilute convective ‘hot’ towers, rather than by a uniform and gradual “mass circulation”…..”The Daisy data have in fact confirmed the importance of the hot tower mechanism and the concentration of the storm’s heat release into 100-200 individual buoyant elements.” (Riehl and Malkus, 1961) • In Simpson et al. (1998), “On the role of ‘hot’ towers in tropical cyclone formation”, strong, deep convective towers were documented during the formation of Cyclone Oliver during TOGA COARE, which were extremely deep (17-18 km) and electrified, but only a few of them produced lightning. (EZ: Indicating “moderate” updrafts) • It has become part of the lexicon that “hot towers” are common in tropical cyclones, and it is often assumed that they are necessary for genesis and for intensification. However, if the term “hot tower” is defined at all in the literature, that definition varies a great deal from paper to paper. More clarity is needed!

3. “Hot Tower”: The time has come to distinguish between CBs with different specific properties • Hennon et al. (2013) show that developing TCCs have coldest IR Tb that are about 2-3°C colder than nondevelopingTCCs. • Kelley et al. (2004, 2005) show that TCs that intensify have higher IR and 20 dBZ radar tops than other TCs. • Jiang (2012) shows that intensifying TCs have higher, colder IR tops than other TCs, in the inner core, but these differences are not substantial, indicating that “hot towers” are neither necessary nor sufficient for intensification (rapid or slow) • “Hot towers refer to horizontally small, deep tropical CBs with intense rapidly rising cores that reach or penetrate the tropopause” (Tao and Jiang 2012) • KEY Q: What is the measure of tropopause penetration, and especially, what is the measure of “intense rapidly rising cores”? Specifically, if we knew W(z), what would we decide is “intense enough” and in what range of altitudes??? We argue here that high IR tops and intense W are not the same thing.

4. Higher echo tops are correlated with intensification in some datasets Kelley et al. (GRL 2004)

5. But CBs with 17 km echo tops over oceans, and in TCs, are much less intense than CBs with 17 dBZ tops over continents \ Kelley et al. MWR 2010

6. After G.Heymsfield et al., JAS 2010; Scatter plot of maximum height of 30 and 40 dBZ echo top vs. maximum updraft estimated in each CB from 39 overpasses of deep CBs by the ER-2. INFERENCE: Fairly good relationship, may justify using dBZ profile as a proxy for max W QUESTION: Is the scatter characteristic, or is the true relationship tighter than it seems here. REASON: These are instantaneous measurements that may well miss either the maximum updraft or the maximum reflectivity

7. After G.Heymsfield et al., JAS 2010; relationship between updrafts and dBZ (z) profiles

8. It is true that colder IR tops, higher radar echoes, and more ice scattering all are more favorable for RI, but…. Jiang, MWR2012

9. BUT USING THIS TRUTH AS A PREDICTOR IS NOT THE ANSWER. Jiang, MWR2012

10. Less than 5% of hurricanes that eventually reached Category 4 or 5 had a radar echo of 40 dBZ reaching as high as 7 km in the eyewall (inner core) region: Hence_Houze_JAS2012

11. NEXT, let’s look at a series of slides showing statistics from the TRMM Precipitation Radar (PR) over 818 TC inner cores, all having “favorable” conditions for RI, showing distribution with respect to the 850 – 200 hPa shear vector, of • “Very intense” convection (20 dBZ top > 14 km) • “Moderately intense” convection (20 dBZ top 10 – 14 km) • “Moderate” convection (20 dBZ top 4 – 10 km)

12. NEXT, let’s look at a series of slides showing statistics from the TRMM Precipitation Radar (PR) over 818 TC inner cores, all having “favorable” conditions for RI, showing distribution with respect to the 850 – 200 hPa shear vector, of • “Very intense” convection (20 dBZ top > 14 km) • “Moderately intense” convection (20 dBZ top 10 – 14 km) • “Moderate” convection (20 dBZ top 4 – 10 km) sorted by whether the TC was • Weakening (W) • Neutral (N; little change) • Slowly intensifying (SI), or • Rapidly intensifying (RI), with these split into • Early stages of RI (RI – initial), or • Later stages of RI (RI – continuing) SLIDES COURTESY OF HAIYAN JIANG, FIU, PAPER IN REVIEW

13. Occurrence of VERY INTENSE convection

14. Occurrence of MODERATELY INTENSE convection

15. Occurrence of MODERATELY INTENSE convection

16. Occurrence of MODERATE convection

17. Occurrence of MODERATE convection

19. Hot Towers as RI predictors: Conclusions • RI is more likely than not if extremely tall convection is in the eyewall or inside the radius of max. winds ………but……… • It is far from necessary or sufficient, and it is not at all clear that the convection need be intense or deep. • For a dataset of 818 TRMM PR overpasses of TCs with RI “plausible” (103/818 with RI), a large, quasi-symmetric coverage of moderate convection seems to be the most reliable indicator of RI. (Tao/Jiang, personal communication)

20. Time to revisit the role of “hot towers”? • What if Kerry Emanuel was correct? • Many years ago, he (+ Schubert, Hack, Willoughby, Shapiro) constructed axisymmetric models that always intensified. • In 2003, K.E. wrote “…numerically simulated storms invariably spin up to their maximum potential intensity (MPI), while few real storms achieve this limit.” Ann. Rev. Earth Planet. Sci.

21. Time to revisit the role of “hot towers”? • What if Kerry Emanuel was correct? • Many years ago, he (+ Schubert, Hack, Willoughby, Shapiro) constructed axisymmetric models that always intensified. • In 2003, K.E. wrote “…numerically simulated storms invariably spin up to their maximum potential intensity (MPI), while few real storms achieve this limit.” Ann. Rev. Earth Planet. Sci. Modest Proposal: GIVEN a Tropical Cyclone that has attained near-symmetric surface winds, strong enough for substantial sea-air enthalpy flux, near-symmetric rainfall distribution and latent heat release is sufficient for intensification. THEN, in the absence of disruptive factors, which may include not only unfavorable large scale environments, but also asymmetrical intense convection, the storm will proceed toward its maximum potential intensity (MPI).

22. Time to revisit the role of “hot towers”? Modest Proposal: GIVEN a Tropical Cyclone that has attained near-symmetric surface winds, strong enough for substantial sea-air enthalpy flux, near-symmetric rainfall distribution and latent heat release is sufficient for intensification. THEN, in the absence of disruptive factors, which may include not only unfavorable large scale environments, but also asymmetrical intense convection, the storm will proceed toward its maximum potential intensity (MPI). IN OTHER WORDS, instead of seeking factors for storm strengthening, seek factors that would interfere with strengthening.

23. Thank you!Questions, and contrary opinions, are always welcome in science,so let’s hear from you!