TropicalM. D. Eastin The Tropics: Convective Processes

Tropical M. D. Eastin

The Tropics: Convective Processes


Outline

Physical Processes

• Radiation• Surface Fluxes• Atmospheric Stability• Organization of Convection• Rainfall – Diurnal Variability• Convective Parameterizations





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 KZ ~ 7.5 kmP ~ 400 mb



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

Low-level Inflow to Convection (e.g. the ITCZ)

Standard Flux Equations

vqqCLLH airSSTLv )(

vTTCcSH airSSTSp )(



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)



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) Temperature

Alt

itu

de

Moist Lapse Rate

Dry Lapse Rate

Surface Flux Impact

Mean TropicalLapse Rate

(ConditionallyUnstable)

RadiationalCooling Impact


The Tropics: Convective ProcessesOrganization 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



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: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)



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)



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?


The Tropics: Convective ProcessesConvective 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


The Tropics: Convective ProcessesConvective 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?



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?


ReferencesBetts, 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 Physicsand 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 ofTRMM measurements, J. Climate, 16, 1456-1475.

Randall, D. A., P. Ding, and D.-M. Pan, 1997: The Arakawa-Schubert parameterization. The Physics and Parameterizationof Moist Atmospheric Convection. Ed. Roger K. Smith, Kluwer Academic Publishers, 281-296.

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TropicalM. D. Eastin The Tropics: Convective Processes