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

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

3. The Tropics: Convective Processes
Tropical
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 heating to
also always 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
Tropical
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)
Tropical
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)
Tropical
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 ~ 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
Tropical
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 UTC
TRMM Radar Composite: 23 October UTC
Tropical
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)
Tropical
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 strong diurnal 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)
Tropical
M. D. Eastin

11. Simple Mass Flux Parameterization (an entraining cumulus cloud)
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 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?
Tropical
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
Tropical
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 do not adequately account for:
Mid-level stratus clouds
Cirrus clouds
Both are very important to the Earth’s radiation balance…why should we care?
Tropical
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?
Tropical
M. D. Eastin

15. References Tropical M. D. Eastin
Betts, A. K., 1997: The parameterization of deep convection. The Physics and Parameterization of Moist Atmospheric
Convection. Ed. Roger K. Smith, Kluwer Academic Publishers,
Climate Diagnostic Center’s (CDCs) Interactive Plotting and Analysis Webage
( )
Gray, W. M., and R. W. Jacobson, 1977: Diurnal variation of deep cumulus convection. Mon. Wea. Rev., 105,
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,
Jorgensen, D. P., and M. A. LeMone, 1989: Vertical velocity characteristics of oceanic convection. J. Atmos. Sci.,
46,
Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-year Reanalysis Project. Bull Amer Met. Soc., 77,
Lucas, C., E. J. Zipser, and M. A. LeMone, 1994: Vertical velocity in oceanic convection off tropical Australia.
J. Atmos. Sci., 51,
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,
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,
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,
Tropical
M. D. Eastin