On Trade-Wind Cumulus Cold Pools

Size: px

Start display at page:

Download "On Trade-Wind Cumulus Cold Pools"

Harvey Sparks
5 years ago
Views:

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

2 if you can t explain something simply, you don t understand it (Feynman?) recently-submitted work, first presentation - here

3 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

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?

5 current (the primary authors ) view

6 combine a ship dataset of surface, radar & lidar measurements (R/V Seward Johnson, Jan. 9-25, 2005) with satellite imagery & S-Pol radar data

$S-Pol echo fraction < 0.1 otherwise Nuijens et al.$

8 Jan. 9, 10, 13 de-emphasized (organized synoptics) S-Pol echo fraction < 0.1 otherwise Nuijens et al., 2009

$cloud top from soundings, scanning Kband radar, LCL ceilometer cloud fraction (18% up to 1.$

9 cloud top from soundings, scanning Kband radar, LCL ceilometer cloud fraction (18% up to 1.5 km); 500 hpa dp/dt surface air T & SST ship & Spol rain rates radiosonde & radiometer WVPs, LWPs latent & sensible fluxes U & V

11 cold pools 37 recorded surface rainrate events in 14 days 17 events on undisturbed days 15/17 arc clouds (visible imagery) all > 2km in height (ship radar) 10/17 ship went through convxn center: > 2 mm/hr max rain all minutes duration half between sunrise&noon

12 surface air dries black: undisturbed days theta_e decreases wind speed often but not always increases latent, sensible fluxes increase Bowen ratio increases

13 3 cases: Jan 11: cold pool diameter ~40 km sounding launched during rain event on west edge Jan 14: cold pool diameter ~ 60 km ship traversed eastern, mostly-recovered cold pool edge Jan. 19: atypical (synoptic forcing) but in literature

14 11:45 as ship moves west to east, samples air increasingly recovered from convxn

15 initial drops in Ta, qa, thetae, short-lived wind speed increase Ta recovers monotonically qa initial quick recovery, then plateaus/decreases when Ta=SST, new cloud development (LWP) not visible to xband radar

16 12 utc sonde went through leading convection: qa increase up to 4 km theta_e km cold pool up to 500 m, moisture minimum at 200 m theta_e profile suggests air origin at 600 m. wind increase up to ~ 3km, sonde surface wind increase from10 to ~13 m/s ship surface wind increase similiar from visible imagery,70km/90min=~12.7 m/s tracking S-Pol blob ~ 14 m/s all speed estimates suggest convective line moves faster than prevailing wind

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

MOPA lidar) brought home (to me) that the convection doesn t

18 cold pool recovery zone (grey) surface air T ~ SST qa constant/decreasing RH ~ 73% deepening BL, rising CB&LCL (10 micron MOPA lidar) brought home (to me) that the convection doesn t necessarily know the environment it s moving into 16utc: strong, narrow updraft & rain

19 jan utc convection propagation speed estimates: ship surface winds increased by ~ 2 m/s visible imagery: ~47km/60min = 13 m/s; ~2m/s increase Spol radar tracking: 15.5 m/s average over 10 minutes ship X-band consistent with new (struggling) convection to west of main line data consistent with density current dynamically (attempting to) trigger new convection to the west

20 sounding released w/ more convxn encroaching cold pool up to 200m, drier air above, wind structure unremarkable

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

detrainment) - helps explain multiple inversions - broad jan 11 thetae

22 Cloud lines propagating faster than mean wind has implications for cloud vertical structure: - westerly shear encourages evaporation aloft (tilting & detrainment) - helps explain multiple inversions - broad jan 11 thetae minimum consistent with upper-level downdraft Jan MHz wind profiler reflectivity

23 1987, JAS

24 current (the primary authors ) view density current depth ~ 200 m, buoyancy depletion ~ 1K/299K has speed of ~ 3 m/s

2008 squall-line behaviour at advance of moist cold pools

25 other work on convective triggering: convergence of moist outflows Xue, Feingold, Stevens (2008) Savic-Jovcic & Stevens 2008 squall-line behaviour at advance of moist cold pools Jensen et al thermodynamic-only generation Tompkins 2001 Tompkins 2001

26 other work on convective triggering: convergence of moist outflows Xue, Feingold, Stevens (2008) squall-line behaviour at advance of moist cold pools Jensen et al thermodynamic-only generation Tompkins 2001 we weren t happy with any of these depictions

27 perhaps more similar to deep tropical convection? 1) 2) ~ 2mm/hr threshold consistent with RICO data

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 further work - assess how general this may be (do heaviest shallow convection always look like little squall lines? is our intuition correct? (e.g., RKWtheory)?) - how well can this be modeled? current collaboration w/ Ping Zhu, but just on Jan 19 case - need to assess model sensitivities

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

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