An important constraint on tropical cloud-climate feedback
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An important constraint on tropical cloud-climate feedback. Dennis L. Hartmann and Kristin Larson Geophysical Res. Lett ., 2002. Main r esult:. T he emission temperature of tropical anvil clouds doesn’t depend on SST, thus it will remain constant during climate change. Positive feedback.

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An important constraint on tropical cloud-climate feedback

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An important constraint on tropical cloud climate feedback

An important constraint on tropical cloud-climate feedback

Dennis L. Hartmann and Kristin Larson

Geophysical Res. Lett., 2002


An important constraint on tropical cloud climate feedback

Main result:

The emission temperature of tropical anvil clouds doesn’t depend on SST, thus it will remain constant during climate change.

Positive feedback

This is relevant for climate change, because LW radiative effects of tropical convection are dominated by convective anvil clouds.


An important constraint on tropical cloud climate feedback

  • In the tropics: the heat balance is between convective heating and radiative cooling

  • The most active convection is constrained to the altitude where radiative cooling is efficient

  • Observations show a peak of probability of optically thick cloud tops (i.e. anvils) at 200 hPa and that clear-sky cooling rate decreases rapidly above 200 hPa

  • The radiative cooling is balanced by adiabatic heating (subsidence). The rapid decrease in clear-sky cooling is accompanied by a strong convergence of mass at 200 hPa, which is balanced by a strong divergence of mass from the convective regions


An important constraint on tropical cloud climate feedback

Hypothesis:

Even if SST increases, the emission temperature of anvil clouds remains the same.


An important constraint on tropical cloud climate feedback

Physical explanation:

  • The temperature at which tropical convection detrains and convective anvil tops occur is constrained by the Clausius-Clapeyron relationship.

  • This is because anvil tops occurs where cooling rate declines rapidly, and this happens when water vapor emission becomes inefficient.

  • Low water vapor emissivity is due to low saturation vapor pressure, which is related principally to the airtemperaturethrough C-C, rather than to pressure.


An important constraint on tropical cloud climate feedback

Test of hypothesis with model:

  • 3D Radiative-Convective model in which they specify SST and solve for equilibrium climate of the troposphere as a function of SST

  • Convective (non-convective) regions of the models are defined to be those with ice visible optical depths > (<)0.1


An important constraint on tropical cloud climate feedback

Results (I):

  • Clear-sky cooling decreases rapidly with height way before tropopause. As T drops < 200 K, emission from water vapor becomes inefficient.

  • As SST goes up, the pressure at which rapid cooling occurs goes down (height goes up). This is because the troposphere warms up more than surface, so that the pressure (height) where T drops < 200 K gets lower (higher).


An important constraint on tropical cloud climate feedback

T@ max/min vertical vel.

T@ cooling rate drops off

T@ const 200 hPa

T@ high clouds anvils


An important constraint on tropical cloud climate feedback

Results (II):

  • The T where cooling rate drops is about const. compared to the T at fixed p = 200 hPa.


An important constraint on tropical cloud climate feedback

  • Ideas for CRG -> Fall 2012:

  • ENSO (mechanisms, teleconnections, prediction, asymmetry, climate change…)

  • IPCC/Anthropogenic climate change

  • Aerosols radiative impacts on climate

  • TCs and climate variability

  • Models uncertainty

  • Decadal variability/predictability

  • Stochastic climate models/Noise

  • ITCZs, single or not

  • Teleconnections

  • Stratocumulus clouds

  • Subtropical highs (already done?)

  • Simple climate models


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