On Trade-Wind Cumulus Cold Pools

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Jody Reed
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1 On Trade-Wind Cumulus Cold Pools Paquita Zuidema & Zhujun Li Reg Hill, Ludovic Bariteau, Bob Rilling, Chris Fairall, Alan Brewer, Bruce Albrecht, Jeff Hare

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

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 combine ship-based data (flux, radar & lidar) (R/V Seward Johnson, Jan. 9-25, 2005) with satellite imagery & S-Pol radar data no a-priori expectations

classical view of shallow cumulus ignores precipitation (Malkus 1958;

2003) cloud streets parallel to the wind or randomly distributed

, 1998) StCu cold pools: moist, little vertical mixing (Stevens et al.

2005; Savic-Jovcic & Stevens, 2008; Wang & Feingold, 2009; Wood et al.

7 classical view of shallow cumulus ignores precipitation (Malkus 1958; Augstein 1973; Nitta &Esbensen, 1974; Albrecht 1993; Siebesma et al. 2003) cloud streets parallel to the wind or randomly distributed clouds (LeMone & Pennel 1976; Nair et al., 1998) StCu cold pools: moist, little vertical mixing (Stevens et al., 2005; vanzanten et al. 2005; Savic-Jovcic & Stevens, 2008; Wang & Feingold, 2009; Wood et al., 2011) precipitation rates ~ mm/day Savic-Jovcic & Stevens 2008 qv ϑ v Xue et al rainrates ~ mm/hr but no wind shear, smaller spatial scale

8 ~ 10 km scale moist cold pool rain ~ 5 mm/day considers wind shear

9 more similar to deep tropical convection? (Barnes and Garstang 1982; Zipser1969, 1977; Young et al. 1995; Houze and Betts 1981; Addis et al. 1984; Kingsmill and Houze 1999; Saxen and Rutledge1998; Betts 1976; Gaynor and Ropelewski 1979) 1) 2) ~ 2mm/hr threshold consistent with RICO data

10 1979, MWR (note cirrus anvil)

11 11 Jan S-Pol ship track

12 11 Jan X-band (ship) S-Pol ship track

13 ship data

15 ship data

$fraction < 0.1 otherwise Nuijens et al.$

16 Jan. 9, 10, 13 disturbed S-Pol echo fraction < 0.1 otherwise Nuijens et al., 2009

17 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

18 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

19 ? q v,θ e, u 0? T decrease pre-rain to rainrate max.

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

22 12 UTC sonde

23 Jan 14

24 Jan. 14: larger cold pool, stronger winds (by ~1.5 m/s), other nearby cold pools. ship in recovering cold pool, then hits convection

25 cold pool recovery zone (grey) surface air T ~ SST qa constant/decreasing RH ~ 73% deepening BL, rising CB&LCL, updraft > 5 m/s (10 micron MOPA lidar)

26 16 UTC sonde after cnvxn cold pool up to 200m, drier air above, wind structure unremarkable

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

27 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?

28 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

29 1) 2) ~ 2mm/hr threshold consistent with RICO data

30 cloud vertical structure: indications of multiple cloud levels 5 4 a) wind profiler humidity gradients b) mixing line

31 c) double-lobed sounding structure d) ceilometer cloud base distribution

32 1987, JAS

33 JAS, 1997

34 recent papers highlight importance of vertical structure of evaporation - and its variation across models

35 our current view

36 Conclusions/Summary: shallow cumuli precipitation within mesoscale arcs downdrafts dry air of lower ϑ e 2. propagates faster downwind than mean winds 3. q v ~ constant within cold pool 4. new (non-precipitating) cloud after ϑ v recovery 5. mesoscale arcs favor higher-wvp environments 6. observations differ from other cold pool obs convective triggering mechanism not yet clearly elucidated clouds appear to generate secondary inversions

37 just accepted into JAS (not yet on early on-line release)

38 further work: is our intuition correct? 1) Jan 19 nested- WRF simulation Ping Zhu, FIU, Miami, FL

39 2) DYNAMO experiment

40 Thank you! Questions/Comments?

42 Jan 19 case (Abel&Shipway, 2007; Snodgrass 2009) differently forced

On Trade-Wind Cumulus Cold Pools

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 key finding from RICO: