Simulated convective invigoration processes at trade-wind cumulus cold pool

boundaries

Zhujun Li and Paquita Zuidema

University of Miami

Ping Zhu

Florida International University

AMS Cloud Conference, Boston, 2014

Precipitation from shallow convection (cloud tops below 0°C level )

Photos taken during BACEX (Barbados 2010)

Photo taken during RICO (Rauber et al. 2007)

CoolMoist

θe

Drizzle from shallow rain•No change in θe •Cool + Moist

Rain from more intense shallow convection•Downdraft•Lower θe Cold + dry•Denser cold pool air•Outflow boundary

Rain evaporation and changes in sub-cloud layer

Zuidema et al. 2012

The invigoration and suppression of convection due to shallow cumulus cold pools

Moisture convergence and Mechanical lifting

Denser Cold pool air

qv

Environmental air

0 buoyancy lineBuoyancy <0Buoyancy 0≧

z

(Thermodynamic) (Dynamic)

Simulation setup• Weather Research and Forecasting Model

(WRF) simulation on RICO case• Five 2-way nested domain centered at 61.7°

W 18° N• Microphysics scheme:

Thompson scheme (double-moment)

•Simulated period: 0000 UTC January 19, 2005 to 0600 UTC January 20, 2005 (post cold front influence, observed precipitation only from shallow convection)only use last 24 hours for analysesoutput every 1 minute•Boundary conditions: NCEP reanalysis 1° resolution•Soundings from ship are assimilated and nudged in the coarser domains•Largest domain size: 972 × 972 km•Innermost domain size: 24 × 24 km; surface to 10 hPa•Grid spacing (innermost domain): Δx=Δy=100m •nz= 77 levels Δz=6m~200m (below 4km)

Four nested domains on goes-12 vis satellite imagery

Used to initiate previous RICO LES comparison study, no cold pools discussed (vanZanten et al. 2011)

Domain averaged profiles compared with soundings on ship and land

Averaged vertical profiles from simulation:•Well represent the average soundings of the day•Capture the wind shift inversion at 3 km•Moister than the land sounding above 2 km

Jan. 19northeasterly

Simulated surface cold pool properties compared to observations

The changes of surface air properties within cold pools are similar to observed changes within cold pool during RICO

Simulated cold pools

January 19 observation

Observation from other days during RICO

Identify cold pool downwind boundary for the research interest

€

b < 0

€

Δθ v < 0

t=t0

t=t0+ t

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Δ

The cold pool downwind boundary at each vertical level

Averaged RR > 2 mm hr-1 over 6 x 6 km

Cold pool at each level

•Negatively buoyant:

•Associated with precipitation:

Updraft: w >0.5 m s-1

updrafts inside the CPAR

updrafts outside the region (not related to cold pool effect)

1 km

The cold pool ambient region

Cold pool ambient region (CPAR)

CPAR updraft vs Non-CPAR updraft (80-m level)

CPAR updrafts are moister, with higher θe

Difference within same output minute

Expansion speed C* = Ucp - Umean

C*

Mean wind sp

eed

U mean

Averaged speed of cold pool downwind boundary Ucp

Moisture advection due to cold pool downwind boundary expansion

Greater expansion speed correspond to greater ambient moisture anomaly

A measure of the cold pool strength

Mechanical lifting due to cold pool expansion compared to buoyancy (80-m level)

The convergence due to cold pool expansion is more relevant to the enhancement of updraft speed compared to the buoyancy

Cloud base level updrafts and cloud water path

CPAR updrafts are able to produce more CWP due to the enhanced updraft speed

Cloud

Rain

Cold pool air

Conclusions

• The low level updrafts within the “cold pool ambient region” are moister than other updrafts

• Cold pool boundary propagation causes moisture convergence, increasing the moisture anomaly of updrafts by lifting more air parcel with higher θe

• The updraft speed in the “cold pool ambient region” is more affected by the lifting due to cold pool boundary expansion than the buoyancy, and is correlated with the cloud water overhead.

Li et al., JAS, 2014

References Li Z., P. Zuidema, and P. Zhu, 2014: Simulated convective invigoration processes at trade-wind cumulus cold pool boundaries. J. Atmos. Sci., in press, doi: http://dx.doi.org/10.1175/JAS-D-13-0184.1Zuidema, P., Z. Li, et al., 2012: On trade wind cumulus cold pools. J. Atmos. Sci., 69, 258–280.Barnes, G. M., and M. Garstang, 1982: Subcloud layer energetics of precipitating convection. Mon. Wea. Rev., 110, 102–117Rauber, R. M., and Coauthors, 2007: Rain in shallow cumulus over the ocean: The RICO campaign. Bull. Amer. Meteor. Soc., 88, 1912–1928Thompson, G., P. R. Field, R. M. Rasmussen, and W. D. Hall, 2008: Explicit forecasts of winter precipitation using an improved bulk microphysics scheme. part ii: Implementation of a new snow parameterization. Mon. Weather Rev., 136, 5095–5115.vanZanten, M. C., and Coauthors, 2011: Controls on precipitation and cloudiness in simulations of trade-wind cumulus as observed during RICO. J. Adv. Model. Earth Syst., 3, M06001Zhu, P., B. A. Albrecht, V. P. Ghate, and Z. Zhu, 2010: Multipole-scale simulations of stratocumulus clouds. J. Geophys. Res., 115, D23 201.