Using the Hurricane Nature Run to Understand Cold Pools and Their Possible Role in Cyclogenesis

1. Using the Hurricane Nature Run to Understand Cold Pools and Their Possible Role in Cyclogenesis Kieran Bhatia

2. Understanding Cold Pools as Density Currents • Region of negative buoyancy and high density caused by convection overhead • Cold pools are like density currents: flow of relatively dense fluid embedded in a lighter fluid • Hydrostatic pressure differences result from density differences between the two fluids • This horizontal pressure gradient leads to propagation of the cold pool • C=k*

3. Cold Pool Characteristics • Varies based on source of convection: • Preceded by heavy rainfall • Temperature anomaly = 1 K • Water vapor mixing ratio anomaly = 1.5 g/kg • Strong downdrafts • Increased latent and sensible heat flux

4. Cold Pools Triggering Convection • Triggering role of cold pools is largely attributed to the dynamical process of lifting the environmental boundary layer at the cold pool front • The ability of gust front to lift air to the LFC is crucial • Deep convection also propagating through thermodynamic mechanism • High water vapor amounts at the edges of downdrafts • Tompkins’ Mechanism

5. Gravity Waves or Cold Pools: Which One Dominates? • Often there are events close to existing storms but are out in front of cold pool • Internal GWs excited by convective activity (flow, latent heat release, etc.) • They both play a role! Gravity Wave? Cold Pool?

6. Environment’s Effects on Cold Pools • When there is environmental shear, the shear direction is important to storm evolution • Shear vectors point in the same direction as density current motion vector (downshear propagation) then convection is amplified (vice versa true) • Optimal shear strength vs. propagation speed vs. ambient flow speed

8. Simulation 1: Identifying Cold Pools

9. Gust Front

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11. Cross-Section

12. Simulation 2: Shear’s Role?

14. Are Cold Pools Important for Tropical Cyclone Genesis? • Evaporation of mesoscale convective system precipitation can lead to development of moist vortex mid-level air • All the characteristics of cold pools are associated with cyclogenesis: • Increased surface fluxes • Higher vertical velocities • Vorticity generation

16. Tropical Cyclone Cold Pool Genesis Parameter • Identify all cold pools within a certain radius of high relative vorticity • Consistent cold pool selection criteria • Compute parameter and multiple linear regression TCCPGP= (# of CP over area)* Mean* (flux*thetae_pert* streamwise shear* downdraft velocity*(?))

17. References • Tompkins, Adrian M., 2001: Organization of Tropical Convection in Low Vertical Wind Shears: The Role of Cold Pools. J. Atmos. Sci., 58, 1650–1672. • Moncrieff, Mitchell W., Changhai Liu, 1999: Convection Initiation by Density Currents: Role of Convergence, Shear, and Dynamical Organization. Mon. Wea. Rev., 127, 2455–2464. • Bister, M., and K. Emanuel, 1997: The genesis of Hurricane Guillermo: TEXMEX analyses and a modeling study. Mon. Wea. Rev, 125,2662–2682. • Bryan, George H., Richard Rotunno, 2008: Gravity Currents in a Deep Anelastic Atmosphere. J. Atmos. Sci., 65, 536–556. • Mapes, Brian E., 1993: Gregarious Tropical Convection. J. Atmos. Sci., 50, 2026–2037. • Fovell, Robert G., Gretchen L. Mullendore, Seung-Hee Kim, 2006: Discrete Propagation in Numerically Simulated Nocturnal Squall Lines. Mon. Wea. Rev., 134, 3735–3752. • Holloway, Christopher E., J. David Neelin, 2009: Moisture Vertical Structure, Column Water Vapor, and Tropical Deep Convection. J. Atmos. Sci.,66, 1665–1683. • Lima, Maria Andrea, James W. Wilson, 2008: Convective Storm Initiation in a Moist Tropical Environment. Mon. Wea. Rev., 136, 1847–1864. • Zuidema, Paquita, and Coauthors, 2012: On Trade Wind Cumulus Cold Pools. J. Atmos. Sci., 69, 258–280. • Markowski, P., and Y. Richardson, 2010: Mesoscale Meteorology in Midlatitudes. Wiley-Blackwell, 424 pp.