The Tropics: Convective Processes

1. The Tropics: Convective Processes M. D. Eastin

2. Outline • Physical Processes • Radiation • Surface Fluxes • Atmospheric Stability • Organization of Convection • Rainfall – Diurnal Variability • Convective Parameterizations M. D. Eastin

3. The Tropics: Convective Processes M. D. Eastin

4. The Tropics: Convective Processes • Radiational Cooling: • The top of the atmosphere is always • cooling everywhere (~1.5ºC/day) • Energy budget requires heatingto • alsoalways occur everywhere • Atmospheric Heat Sources: • Solar Radiation ~15% • Surface Fluxes** ~50% • Latent Heat Release** ~35% • Adiabatic Sinking** ~35% • ** Only at surface but across 100% of area • ** At all levels but only across 10% of area • ** At all levels but across 90% of area Annual Mean Outgoing Longwave Radiation (W/m2) Zonal Mean Outgoing Longwave Radiation (W/m2) Mean OLR ~250 W/m2 ~1.5 ºC/day OLR ~ σT4 TBB ~ 257 K Z ~ 7.5 km P ~ 400 mb M. D. Eastin

5. The Tropics: Convective Processes • Surface Fluxes: • Energy transfer from the warmer/moister body to the cooler/drier body • For moisture, the transfer is always from the ocean to the atmosphere • For temperature, the transfer is usually from the ocean to the atmosphere • (not the case over cold water ocean currents – along coasts in eastern Pacific) • Both transfers are a function the heat/moisture difference and wind speed • Provides an avenue to transfer oceanic solar heating to the upper atmosphere • for release via radiational cooling Deep Convection Standard Flux Equations Low-level Inflow to Convection (e.g. the ITCZ) M. D. Eastin

6. The Tropics: Convective Processes • Surface Fluxes: • Fluxes are maximum in trade • wind regions with a minimum • near the equator • Latent heat fluxes are maximum • over water and forested regions • (oceanic LHF ~120 W/m2) • Sensible heat fluxes are maximum • over land (deserts in particular) • (oceanic SHF ~10 W/m2) • The majority of energy is tied to • latent heat fluxes (i.e. water • vapor and condensation) Annual Mean Latent Heat Flux (W/m2) Annual Mean Sensible Heat Flux (W/m2) M. D. Eastin

7. The Tropics: Convective Processes • Stability: • The mean tropical atmosphere is • conditionally unstable • If low-level forced ascent (via • convergence) can lift air parcels to • their level of free convection (LFC), • deep convection will occur • Surface fluxes act to decrease stability • (make more unstable) in the lower • atmosphere so less lifting is required • Radiational cooling acts to decrease • stability in upper atmosphere so • convection can reach higher altitudes • Typical CAPE values ~1200-1600 J/kg • (Compare to typical mid-latitude severe • weather CAPE values ~3000 J/kg) Moist Lapse Rate Radiational Cooling Impact Dry Lapse Rate Mean Tropical Lapse Rate (Conditionally Unstable) Altitude Surface Flux Impact Temperature M. D. Eastin

8. The Tropics: Convective Processes • Organization of Convection: • At any given time, deep convective • clouds occupy ~10% of the total area • Active convection (i.e. strong updrafts) • occupies ~1% of the total area • Individual convective clouds only last 1-2 hrs • Convection tends to repeatedly develop in • same area (but gradually propagate) • Advantage: Increased mid-level moisture in • the convective area reduces the negative • impacts of entrainment on future convection • Advantage: Less low-level convergence is • required for future convection • The positive feedbacks “locks” convection into • occurring in certain regions of the Tropics Global IR Composite: 23 October 2006 1200 UTC TRMM Radar Composite: 23 October 2006 1200-1500 UTC M. D. Eastin

9. The Tropics: Convective Processes • Organization of Convection: • Persistent convection associated with • ITCZ, SPCZ, monsoons, squall lines, • tropical cyclones, Rossby waves, and • Kelvin waves • Daily OLR data for a 2-month period • averaged between 0ºN and 20ºN at • each longitude and plotted as a • function of longitude and time • Low OLR = Deep Convection • Note: Convection persists in same • general longitude band • Features appear to propagate • both east and west over the • course of several days Deep Convection (Western Pacific) M. D. Eastin

10. The Tropics: Convective Processes • Rainfall: • At any given 24 hour period, precipitation • falls across ~10% of the Tropics • Heavy precipitation occupies < 1% of area • Tropical rainfall has strongdiurnal signal • over both land and ocean • Ocean • Maximum occurs in early morning (3-6 am) • Lagged (~3 hr) response to maximum in • radiational cooling and destabilization • of the upper atmosphere (permits deeper • convection and more rainfall) • Land • Maximum occurs in late afternoon (3-6 pm) • Lagged response to maximum in surface • heating (fluxes) and destabilization of • the lower atmosphere (increased CAPE, • deeper convection, more rainfall) M. D. Eastin

11. The Tropics: Convective Processes • Convective Parameterizations: • Nearly all global numerical models must • parameterize the effects of clouds and their • associated latent heat release • Deep convection occurs on scales of 2-10 km • Model resolution on scales of 20-100 km • Therefore, convection is not resolved but its • impacts must be accounted for in the model • Most parameterizations are based on either: • Radiative-Convective equilibrium • Moisture flux convergence • Mass flux convergence • All parameterization schemes are very loosely • based on limited observations Simple Mass Flux Parameterization (an entraining cumulus cloud) Big Questions for each Scheme: What “triggers” the convection? How is the heating distributed? M. D. Eastin

12. The Tropics: Convective Processes • Convective Parameterizations: • Radiative-Convective Equilibrium Schemes • Cooling via radiation or advection destabilizes the lapse rate • Convection is triggered to adjust lapse rate to an “equilibrium state” • Adjustment conserves total energy whereby total column heating via condensation is • directly proportional to total column drying (e.g. moisture loss via precipitation) • Latent heating profile defined by differences between the initial and adjusted sounding • No mass fluxes, no entrainment, no downdrafts • Moisture Convergence Schemes • Low-level moisture convergence triggers convection if forced ascent produces an • unstable parcel at the PBL top (i.e. low-level RH builds until sounding becomes unstable) • Cloud depth and latent heating profile are a function of CAPE in large-scale sounding • Cloud quickly dissipates and “detrained” moisture is “added” to the large scale sounding • No mass fluxes, no downdrafts • Mass Convergence Schemes • Convection triggered if low-level mass convergence produces unstable parcel at PBL top • Incorporates downdrafts in the cloud and the environment • Cloud depth, entrainment, and the net heating profile are functions of both upward and • downward fluxes of mass and moisture • Most complex, most realistic, and thus most often employed in models M. D. Eastin

13. The Tropics: Convective Processes • Convective Parameterizations: • Many convective parameterization schemes also account for: • Shallow non-precipitating convection • Precipitating and non-precipitating low-level stratocumulus • Current convective parameterization schemes donotadequately account for: • Mid-level stratus clouds • Cirrus clouds • Both are very important to the Earth’s radiation balance…why should we care? M. D. Eastin

14. The Tropics: Convective Processes • Summary: • Convection is a means to global energy balance • Radiational cooling is always occurring everywhere • at the top of the atmosphere • Atmosphere must gain heat to offset cooling • Surface fluxes • Latent heat release (convection) • Adiabatic heating (sinking in clear regions) • Variations in stability and convection • Distribution / Organization of convection • Convective Parameterizations • Why do we need them? • How do they work? M. D. Eastin

15. References Betts, A. K., 1997: The parameterization of deep convection. The Physics and Parameterization of Moist Atmospheric Convection. Ed. Roger K. Smith, Kluwer Academic Publishers, 255-280. Climate Diagnostic Center’s (CDCs) Interactive Plotting and Analysis Webage ( http://www.cdc.noaa.gov/cgi-bin/PublicData/getpage.pl ) Gray, W. M., and R. W. Jacobson, 1977: Diurnal variation of deep cumulus convection. Mon. Wea. Rev., 105, 1171-1188. Gregory, D., 1997: The mass flux approach to the parameterization of deep convection. The Physics and Parameterization of Moist Atmospheric Convection. Ed. Roger K. Smith, Kluwer Academic Publishers, 297-320. Jorgensen, D. P., and M. A. LeMone, 1989: Vertical velocity characteristics of oceanic convection. J. Atmos. Sci., 46, 621-640. Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-year Reanalysis Project. Bull Amer Met. Soc., 77, 437-471. Lucas, C., E. J. Zipser, and M. A. LeMone, 1994: Vertical velocity in oceanic convection off tropical Australia. J. Atmos. Sci., 51, 3183-3193. Mapes, B. E., 1997: Equilibrium vs. Activation control of large-scale variations of tropical deep convection. The Physics and Parameterization of Moist Atmospheric Convection. Ed. Roger K. Smith, Kluwer academic Publishers, 321-358. Nesbitt, S. W., and E. J. Zipser, 2003: The diurnal cycle of rainfall and convective intensity according to three years of TRMM measurements, J. Climate, 16, 1456-1475. Randall, D. A., P. Ding, and D.-M. Pan, 1997: The Arakawa-Schubert parameterization. The Physics and Parameterization of Moist Atmospheric Convection. Ed. Roger K. Smith, Kluwer Academic Publishers, 281-296. M. D. Eastin