Simulated convective invigoration processes at trade-wind cumulus cold pool boundaries

1. Simulated convective invigoration processes at trade-wind cumulus cold pool boundaries Zhujun Li and PaquitaZuidema University of Miami Ping Zhu Florida International University AMS Cloud Conference, Boston, 2014

2. Precipitation from shallow convection (cloud tops below 0°C level ) Photo taken during RICO (Rauber et al. 2007) Photos taken during BACEX (Barbados 2010)

3. Rain evaporation and changes in sub-cloud layer • Rain from more intense shallow convection • Downdraft • LowerθeCold + dry • Denser cold pool air • Outflow boundary • Drizzle from shallow • rain • No change inθe • Cool + Moist θe Cool Moist

4. The invigoration and suppression of convection due to shallow cumulus cold pools Zuidema et al. 2012

5. Moisture convergence and Mechanical lifting (Dynamic) (Thermodynamic) 0 buoyancy line Buoyancy ≧0 Buoyancy <0 z Environmental air Denser Cold pool air qv

6. 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) Four nested domains on goes-12 vis satellite imagery • 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 analyses • output 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)

7. Domain averaged profiles compared with soundings on ship and land Jan. 19 northeasterly • 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 Used to initiate previous RICO LES comparison study, no cold pools discussed (vanZanten et al. 2011)

8. Simulated surface cold pool properties compared to observations Simulated cold pools January 19 observation Observation from other days during RICO The changes of surface air properties within cold pools are similar to observed changes within cold pool during RICO

9. Identify cold pool downwind boundary for the research interest • Negatively buoyant: • Associated with precipitation: Averaged RR > 2 mm hr-1 over 6 x 6 km t=t0 Cold pool at each level t=t0+ t The cold pool downwind boundary at each vertical level

10. Cold pool ambient region (CPAR) 1 km The cold pool ambient region Updraft: w >0.5 m s-1 updrafts inside the CPAR updrafts outside the region (not related to cold pool effect)

11. CPAR updraft vs Non-CPAR updraft (80-m level) Difference within same output minute CPAR updrafts are moister, with higher θe

12. Moisture advection due to cold pool downwind boundary expansion Mean wind speed Umean Expansion speed C* = Ucp - Umean C* A measure of the cold pool strength Averaged speed of cold pool downwind boundary Ucp Greater expansion speed correspond to greater ambient moisture anomaly

13. 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

14. Cloud base level updrafts and cloud water path CPAR updrafts are able to produce more CWP due to the enhanced updraft speed

15. Cloud Rain Cold pool air

17. Conclusions Li et al., JAS, 2014 • 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.

18. 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-13-0184.1 Zuidema, 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, 102–117 Rauber, R. M., and Coauthors, 2007: Rain in shallow cumulus over the ocean: The RICO campaign. Bull. Amer. Meteor. Soc., 88, 1912–1928 Thompson, 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, M06001 Zhu, P., B. A. Albrecht, V. P. Ghate, and Z. Zhu, 2010: Multipole-scale simulations of stratocumulus clouds. J. Geophys. Res., 115, D23 201.