Tropical Cyclone Formation: Results

Transcription

1 Tropical Cyclone Formation: Results from PREDICT (PRE Depression Investigation of Cloud systems in the Tropics) collaborator on this presentation: Dave Ahijevych (NCAR) Chris Davis National Center for Atmospheric Research Boulder, Colorado, USA acknowledgements: NCAR Earth Observing Laboratory for support of PREDICT Mike Montgomery, PREDICT PI Tim Dunkerton Dave Raymond Kerry Emanuel NASA GRIP and NOAA IFEX and to all the tropical meteorology pioneers who have thought about this problem for decades (Riehl, Simpson, Gray, Zipser to name a few)

2 Conceptual Stages of TC Formation Preconditioning Producing a vortex ready for intensification Rotating, very moist and marginally convectively unstable Vortex alignment Transition to bottom heavy mass flux profile Sensitive to vertical shear and relative flow Intensification The (rapid) spinup of vorticity in the lower troposphere bottom heavy mass flux profile Vortical hot towers key references: Bister and Emanuel, 1997 Simpson et al Ritchie and Holland 1997 Montgomery et al Nolan, 2007 Dunkerton et al Tory and Frank, 2010 Montgomery et al Raymond, Sessions and Lopez Carillo, 2011 Davis and Ahijevych, 2012, 2013 Komaromi, 2013 Zawislak and Ziper, 2014

3 When the Atlantic was Active Courtesy CIMSS

4 The Formation of Hurricane Karl 2010

5 A Conundrum TC formation usually accompanied by intense convection A mid tropospheric vortex precedes surface spinup: thermal wind implies greater stability e Elevated mid level, lower sfc. Reduced (or eliminated) CAPE How are these consistent? e Potential Temperature Anomaly MODIS image, Courtesy Naval Research Laboratory Zawislak and Zipser, 2014: MWR

6 Deep or Shallow Convection? Coldest cloud tops occur near time of TC formation Lightning relatively absent Modeling suggests importance of cumulus congestus Wang, 2014: JAS within 100 km of center pre genesis PV change due to congestus; moisture as well within 500 km of center Davis et al. 2014, JAS Lightning flash counts pre TD Ritchie et al., 2014: AMS Conf. see also Leppert et al. 2013, MWR

7 Hypothesis The answer is that the character of convection varies spatially and temporally. Both deep and shallow convection occur Shallow convection may be more important pre genesis near the center of rotation preferentially increases vorticity at low levels detrains water vapor into lower to mid troposphere

8 Dropsonde Composite Profiles from PREDICT (comparison of inner (< 200 km) and outer (> 200 km) radii) Nondeveloping Composite Less Stable Inner Region Pressure (hpa) Developing Composite More Stable Inner Region Pressure (hpa) < 2 days before genesis 95% confidence intervals Temperature Anomaly from Large scale Mean Statistically significant difference: inner vs. outer

9 Stability and Humidity Each circle is one dropsonde L2: composite < 2 days before genesis High RH and greater stability within 200 km of the developing center The inner pouch

10 Aside: What is Special About 200 km? Rota on Period (days) mb tangential wind Radius (degrees) Sept. 10 Sept. 13 Sept. 14 Advective time scale comparable to convection life cycle, roughly h T=2 r/v r V

11 Marked Dependence of Rain Rate on Humidity Holloway and Neelin 2009, JAS Developing Composite Moist Tropical Sounding Composite Dunion, 2011: J. Climate

12 Where does the Moisture Come From? Small cumulonumbi Large areas of anvil cloud Beneath anvils From the NSF/NCAR GV Folkins and Martin, 2005: JAS Shallow Convection

13 Further Examination of Karl day/hour (UTC) Circles denote uncertainty in center location

14 12 UTC 13 September

15 IR 1345 UTC 13 September N E S W N km Dropsonde 900 mb center 700 mb center 500 mb center Easterly Shear Motion of 700 mb center m/s

16 Buoyancy entrainment = 10%/km 1 Height (m) unstable ahead of system Buoyancy All Fall Buoyancy (C) Height (m) marginal near center of vortex Buoyancy All Fall Buoyancy (C)

17 Overall Buoyancy Result Boundary layer parcels from inner core of pre genesis disturbances have less buoyancy than parcels farther out. Shown are profiles NOT through anvils. Effect is more pronounced in anvils.

18 13 September

19 DC 8 Soundings Late on 13 September: 18 h Prior to TC Formation ice on sonde 2244 UTC Height (m) UTC 2244 UTC All Fall Buoyancy (C) UTC Height (m) UTC 2252 UTC All Fall Buoyancy (C) convective inhibition

20 P 3 Flight: Sept 900 hpa center at 0200 UTC 700 hpa center at 0200 UTC

21 0445 UTC IR Image 0500 UTC estimated center at 900 hpa and 700 hpa

22 0645 UTC 14 Sept 700 mb center For reference, here is strong convection 2145 UTC 15 Sept 0620 UTC TRMM PR Convection is widespread but not particularly intense

23 Transition to Deep Convection 19 UTC 14 Sept. 06 UTC 14 Sept. 19 UTC 13 Sept. Courtesy NRL

24 2 Buoy Obs Sept UTC 14th 1 21 UTC 13th h recovery time is equivalent to nearly 200 km of translation: no longer in the moist inner core when PBL recovers Time (h) 12 UTC/13th 00 UTC/14th

25 Air sea Disequilibrium Dev Non Dev Lagrangian recovery time scale: T=H/C E V, h BE 97 Wind speeds NOT significantly different in two samples: 7 m/s

26 T rec = H/C E V s But what is V s? Storm relative vs. Ground Relative Moistening above PBL depends on storm relative flow: less is better (vortex alignment) Surface fluxes depend on earth relative flow + mean flow = Pouch center Vertical Shear? See Rappin and Nolan (2012) Eulerian surface center is place of weakest fluxes, longest recovery, greatest inhibition

27 The Resolution of Our Puzzle The issue is when and where Deep, intense convection, with attendant stratiform rain nearly always present Shallow (congestus) or low top deep convection also important Rather arbitrary time designation of genesis time