On Trade-Wind Cumulus Cold Pools

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Harold Bell
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1 On Trade-Wind Cumulus Cold Pools Paquita Zuidema & Zhujun Li U of Miami Reg Hill, Ludovic Bariteau, Bob Rilling, Chris Fairall, Alan Brewer, Bruce Albrecht, Jeff Hare AGU Dec 2011

reminiscent of cold pool outflow boundaries Snodgrass

2 key finding from RICO: nearly all cloud producing > 1 mm/hr rainrates associated with arc-shaped formations reminiscent of cold pool outflow boundaries Snodgrass et al., 2009; Rauber et al., 2007 ~100 km Rauber et al. 2007

3 visually determined, unanticipated

4 open questions: how does shallow cumulus precipitation relate to the subcloud layer energetics & thermodynamic structure? can we learn anything about the convective triggering mechanism? what are the characteristics of the cold pools?

5 combine ship-based data (flux, radar & lidar; R/V Seward Johnson, Jan. 9-25, 2005) with satellite imagery & S-Pol radar data

6 shallow cloud precipitation: no a-priori expectations moistens sub-cloud layer, little change to ϑ e Malkus 1958; Nitta and Esbensen, 1974; Augstein et al., 1973; Albrecht 1993; Siebesma et al but mesoscale org. of shallow cloud streets or popcorn cumuli; LeMone and Pennell 1976; Nair et al ϑ increase e Stevens et al., 2005; van Zanten et al. 2005; Savic-Jovcic and Stevens, 2008; Wang and Feingold 2009; Wood et al moist outflow capable of forming new convection Xue et al but precipitation ~ mm/day not mm/hr and no wind shear or slight surface stratocumulus squall-line Jensen et al tropical deep convection: can draft down air drier than environment Zipser 1969, 1977; Betts 1976; Gaynor and Ropelewski, 1979; Houze and Betts 1981; Barnes and Garstang, 1982; Addis et al. 1984; Young et al. 1995; Saxon and Rutledge 1998; Kingsmill and Houze, 1999 threshold at ~2 mm/hr? Barnes and Garstang 1982 mesoscale arcs documented Warner et al but deep convection assumes ice microphysics, and recovery process can be influenced by mesoscale subsidence

7 11 Jan S-Pol ship track

8 11 Jan X-band (ship) S-Pol ship track

9 during 9-24 January: 37 recorded surface rain events 17 on undisturbed days (Nuijens et al., 2009) 15 clearly associated with mesoscale arcs 10 through convection center & > 2 mm/hr max. rainrate > 2km cloud depth (X-band) pre-rain to rain maximum

10 cold pool : a region or pool of relatively colder air surrounded by warmer air; any large-scale mass of colder air (AMS glossary) sought an explicit connection to precip: beginning: > 0 mm/hr end: Tair ~ SST or max Tair

11 undisturbed disturbed sensible/latent pre-rain to maximum rainrate

12 gust winds faster than mean

convection propagation speed estimates (both cases): ship surface winds increased by ~ 2-4 m/s visible imagery tracking ~2m/s increase Spol radar tracking: ~3-4 m/s increase

13 convection propagation speed estimates (both cases): ship surface winds increased by ~ 2-4 m/s visible imagery tracking ~2m/s increase Spol radar tracking: ~3-4 m/s increase Jan 11 sonde ~3 m/s increase ship X-band consistent with new (struggling) convection to west of main line density current dynamically (attempting to) trigger new convection upwind?

14 cloud radar reflectivities, composited by WVP, show almost all precipitation in highest-tercile WVP one interpretation: convxn more likely to flourish when moving into more moist air

15 trade wind cumulus precipitation: - capable of drying sub-cloud layer (similar to deep convection) -also appears capable of upper-level downdrafts (not shown) - gust winds but recovery zone winds unchanged (unlike deep convection) -convective triggering process still unclear to us but seems both dynamical (wind shear; outflow convergence) & thermodynamical (properties of air convxn is moving into) continuing work with simulations (Ping Zhu) & new observations (eq. Indian Ocean) JAS, early on-line release

16 Thank you for your time Questions/Comments?

17 our current view ~ km across

18 JAS (early on-line release)

On Trade-Wind Cumulus Cold Pools

On Trade-Wind Cumulus Cold Pools Paquita Zuidema & Zhujun Li Reg Hill, Ludovic Bariteau, Bob Rilling, Chris Fairall, Alan Brewer, Bruce Albrecht, Jeff Hare key finding from RICO: nearly all cloud producing