The relative importance of deep/very deep vs. shallow/moderate convection to rapid intensification: Results from 14 years of TRMM

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1 The relative importance of deep/very deep vs. shallow/moderate convection to rapid intensification: Results from 14 years of TRMM Haiyan Jiang Florida International University Former & Current Graduate Students: Cheng Tao, Joe Zagrodnik (at UW now), Margie Kieper Acknowledgements: Ed Zipser, Dan Cecil, Rob Rogers, Fuqing Zhang, and two anonymous reviewers HFIP RI workshop, UM RSMAS, Nov. 18,

2 Proposed Mechanisms for TC Intensification Symmetric Mechanism: symmetric heating is more important (Ooyama 1969, Shapiro and Willoughby 1982, Nolan et al. 2007). Asymmetric Mechanism: Asymmetric heating by deep convection is the preferred spin-up mechanism vortical hot towers (VHTs) (Hendricks et al. 2004, Montgomery et al. 2006, Nguyen et al. 2008, Montgomery and Smith 2011). For Rapid Intensification (RI) particularly, observational evidence of asymmetric deep convection in TCs undergoing RI is reported in several case studies (Reasor et al. 2009, Guimond et al. 2010; Molinari and Vollaro 2012; Nguyen and Molinari 2012). However, satellite-based statistical studies have indicated that the symmetry of widespread precipitation is important in predicting/initiating RI (Jiang 2012, Kieper and Jiang 2012, Jiang and Ramirez 2013, Zagrodnik and Jiang 2014). 2

3 Definition of Rapid Intensification (RI) General definition: 24 hour Intensity Increase >= 30 kt (Kaplan and DeMaria 2003). This only defines the intensification rate, not the duration, although forecasters usually consider each 24-h period. The concept of RI event: A RI event can continue for hours and contain multiple, continuous, and overlapping 24-h periods in which the intensity increased in each period by 30 kt or more (Kieper and Jiang 2012). Each RI event includes the onset of RI, in the middle of RI, and the ending period of RI. To forecast RI, at least 24-h lead time is generally required. A careful examination found that most above-mentioned observational case studies on deep convection focused on the ending period of RI: Reasor et al (2009) and Reasor and Eastin (2012): the asymmetric deep convection in Hurriane Guillermo (1997) was observed around 18 hours before RI ended and 32 hours after the onset of RI. Guimond et al. (2010): Hot towers in Hurricane Dennis (2005) were observed 22 hours before RI ended. Molinari and Vollaro (2010): supercell/strong lightning was observed only 7 hours before RI ended. Nguyen and Molinari (2012): Deep convection/hot tower in Hurricane Irene (1999) was observed only 5 hours before the RI event ended. 3

4 Review of Satellite-based Statistical Studies All studies reviewed below focused on the precipitation and convective properties in the inner core for RI Initial and/or RI continuing categories (not the RI ending period). So their results have implications on both RI prediction and physical understanding of how RI is initiated and maintained. Jiang (2012) tested the hot tower hypothesis on RI using 11-yr TRMM radar data. It was found that the probability of future 24-h RI does not increase significantly when one or more hot towers (defined as 20 dbz echo top >= 14.5 km) exists in the inner core, and more than 50% of storms underwent RI without hot towers in the inner core. Jiang and Ramirez (2013) found that storms that will undergo RI in the next 24 hours do not necessarily have the most intense convection in the inner core, but they always have the largest raining area & volumetric rain in the inner core than non-ri storms. Zagrondik and Jiang (2014) found that the rainfall frequency is better correlated to TC future intensity change than convective intensity parameters. 4

5 Review of Satellite-based Statistical Studies Kieper and Jiang (2012) found that a symmetric ring ( Margie s ring ) on NRL 37 GHz color images is a very good predictor of RI in the next 24-h. The ring is closed by bright cyan color with some pink regions scattered mostly on the outer edge of the ring asymmetrically. [Lee et al. (2002): Cyan: low-level clouds or warm rain; Pink: deep convection] The bright cyan+pink color ring appeared during RI initial and RI continuing period of 23 out of 28 (82%)RI events in the Atlantic during By using the SHIPS RI Index to select only favorable environmental conditions, the realtime test conducted by M. Kieper predicted all Atlantic RI event in 2008 correctly. 5

6 What s in the 37 GHz ring? Is it convective? Precipitative? Or Just an indication of increased surface winds? Harnos and Nesbitt (2011) claimed a moderately intense convective ring forming 6-h before RI begins and intensifying over the following 24-h period: 50% occurrence of ice scattering signature (85 GHz PCT<250 K, equivalent to 17 dbz radar echo reaching 10 km, Cecil and Zipser 2002). However, they did not mirror SH microwave overpasses relative to shear before compositing with NH overpasses. If they did, the % occurrence would be much lower. Zagrodnik and Jiang (2014): a ring of only 5% occurrence of TRMM precipitation radar (PR) reflectivity > 20 dbz reaching 10 km for RI continuing storms; but a ring of 70% occurrence of near-surface reflectivity > 20 dbz. This indicates that Margie s ring is more likely to be precipitative than moderately intense convective. From Fig. 3 of Harnos and Nesbitt (2011)) % occurrence of moderately intense convection (reflectivity >20 dbz reaching 10 km) % occurrence of rainfall (nearsfc reflectivity >20dBZ), Zagrodnik and Jiang (2014) 6

7 Questions to answer However, the rain ring found by Zagrodnik and Jiang (2014) is only at 70% occurrence. Could that be due to the smaller footprint size of PR than 37 GHz? Although observational case studies emphasized the role of asymmetric deep convection in RI, satellite-based composite studies suggested that the symmetry of precipitation containing not much asymmetric deep convection is important in predicting and initiating RI. An important question raised from here is, What is the relative importance of convection in different depth/convective intensity to RI? 7

8 A recent work by Tao and Jiang (2014, JCLI manuscript under revision): NEXT, a series of slides will show the statistics/composites from the 14-yr TRMM Precipitation Radar (PR) over 1139 TC inner cores (yes, we flipped SH overpasses relative to shear), all having moderately favorable conditions for RI, showing the frequency distribution with respect to the hpa shear vector, of Very deep convection (20 dbz top > 14 km) Moderately deep convection (20 dbz top km) Moderate convection (20 dbz top 6 10 km) Shallow convection (20 dbz top below 6 km) sorted by the 24-h future TC intensity change of Weakening (W) Neutral (N; little change) Slowly intensifying (SI), or Rapidly intensifying (RI), with these split into Near the onset of RI (RI initial), or In the middle of RI (RI continuing)

9 Percent Occurrence of VERY DEEP Convection (20 dbz top > 14 km; shear direction pointing upward) - In general, very deep convection is very rare in all TCs (<4% occurrence). - More very deep convection for SI storms and the peak is closer to the TC center than weakening and neutral storms (Consistent with Rogers et al. 2013) - The least amount of very deep convection is found in storms near the onset of RI. So it s very unlikely to be a trigger of RI. - As RI continues, very deep convection increases a bit and becomes upshear-left dominant.

10 Percent Occurrence of MODERATELY DEEP Convection (20 dbz top km) - Moderately deep convection is downshear-left dominant for W, N, SI storms, but upshear-left dominant for RI storms. - Significantly increase of moderately deep convection occur in RI continuing storms only, suggesting that RI is likely triggered by other mechanisms and the appearance of more deep convection in the middle of RI is more likely a response or positive feedback to changes in the vortex that occur earlier in the SI to beginning of RI period.

11 Percent Occurrence of MODERATE Convection (20 dbz top 6 10 km) - Moderate convection is downshear-left dominant for all TCs (Corbosiero and Molinari 2002, Chen et al. 2006). - Moderate convection becomes more widespread (max. frequency >= 50%) and more symmetric at the onset of RI, and further more as RI continues.

12 Percent Occurrence of SHALLOW Convection (20 dbz top below 6 km) - Similar as moderate convection, shallow convection also becomes more widespread (max. frequency >= 35%) and more symmetric, and the peak is closer to the storm center for storms near the onset of RI. - As RI continues, the shallow convection does not increase much, just becomes more symmetric. - Shallow convection is downshear left dominant for non-ri storms, but downshear-right dominant for both RI initial and RI continuing storms.

13 Percent Occurrence of ALL Convection (Downgraded to 37 GHz footprint size) - Downshear-left dominant for total convection in all TCs (Corbosiero and Molinari 2002, Chen et al. 2006) - A ring of ~90% occurrence of all convection is found for RI continuing storms (storms that have undergone RI for at least 12 hours and will undergo RI in the next 24 hours). - Finally this is almost exactly Margie s ring (recall that only 82% of RI storms had the 37 GHz ring, as in Kieper and Jiang 2012). - Margie s ring is mostly contributed by shallow-to-moderate convection. Therefore, we argue that it s reasonable to call it a precipitative ring.

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15 Conclusions The contribution to total volumetric rain and total latent heating in the inner core from deep-to-very deep convection is less in RI storms than in non-ri storms, suggesting that deep convection is more likely a symptom rather than a trigger of RI. More widespread and more symmetric shallow-tomoderate convection seems to be the most reliable indicator/predictor of RI. Margie s 37 GHz ring is mainly precipitative. Thanks for you attention! 15