Cloud-Climate Feedbacks as a Result of Solar Cloud Absorptionin the SKYHI General Circulation Model

Carynelisa Erlick, Atmospheric Sciences, Hebrew University

V. Ramaswamy, NOAA/GFDL

Lynn Russell, Scripps Institute of Oceanography

Previous Studies

Anomalous Absorption [Kiehl et al. 1995] decrease in shortwave flux to the SFC increase in temperature of upper troposphere decrease in surface wind speed

decrease in Hadley circulation global decrease in precipitation and latent heat release

Stratocumulus Dissipation [Ackerman and Toon 1996; Johnson et al. 2004] decrease in radiative cooling near cloud top decrease in boundary layer mixing

decrease in water vapor supply decrease in liquid water path Forcings and Response: Semi-Direct Effect [Hansen et al. 1997]

decrease in low level cloud cover increase in shortwave flux to the SFC increase in global mean surface temperature

Indo-Asian Haze [Ackerman et al. 2000; Ramanathan et al. 2001; Chung et al. 2002] decrease in net shortwave flux at the SFC decrease in surface temperature increase in boundary layer stability increase in

low level convergence increase in convective rainfall and latent heat release Semi-Direct Effect [Cook and Highwood 2004]

increase in global mean surface temperature and atmospheric temperature increase in atmospheric stability decrease convective cloud amount

Goals of This Work

Simulate effect of moderate continental aerosol absorption (not as strong as anomalous absorption)

Simulate non-uniform forcing (matching pattern of cloudiness) Look at regional responses to global perturbation

Marine vs. Continentally Influenced CloudMonterey Area Ship Track Experiment (MAST), June 1994

clean marine stratocumulus (JDT180, Star Livorno, June 29)

continentally influenced stratocumulus (JDT178, Tai He, June 27)

Radiative Properties of a Cloud Drop Containing Absorbing Aerosols

Calculate the effect refractive index: linear mixing rule for non-absorbing

species* weighted by volume

Maxwell-Garnett theory for absorbing species* assumes a random distribution of

absorbing inclusions in an otherwise homogeneous matrix

Calculate the drop single scattering parameters Mie scattering subroutine [Bohren and Huffman, 1983]:

input: drop radius, concentration, effect refractive index

output: extinction coefficient, single scattering albedo, and asymmetry factor

integrate over all drops in distribution

non-absorbing mixture(sulfates, nitrates, sea salt)

absorbing mixture(soot, dust, organics)

Change in Visible Cloud Single Scattering Albedo

99.0 :cloud influencedlly continenta

999998.0 :cloud marine

abssca

sca

kk

k

General Circulation Model (SKYHI) [Hamilton et al., 1995]

40-level finite difference grid 3.0° x 3.6° latitude-longitude resolution predicted clouds [Wetherald and Manabe, 1988]

a layer is fully cloud covered when RH > 100% low clouds: 6801000 mb, ext ~ 12, reff = 10 m

middle clouds: 440680 mb, ext ~ 3, reff = 10 m

high clouds: 10440 mb, ext ~ 1, reff = 10 m Slingo parameterization for cloud radiative properties [Slingo, 1989] fixed SST’s shortwave radiation [Freidenreich and Ramaswamy, 1999]

exponential sum-fit technique for water vapor transmission delta-Eddington method for reflectance and transmittance of scattering layers

[Joseph et al., 1976] adding method to combine layers [Ramaswamy and Bowen, 1994] wavelengths of perturbation: 0.21.2 m

longwave radiation [Schwarzkopf and Ramaswamy, 1999] simplified exchange approximation method for IR radiative transfer

[Schwarzkopf and Fels, 1991] gaseous absorption approximated over 8 spectral bands

Shortwave Forcing = Instantaneous

less upward flux at TOA less downward flux at SFC

contcontpertpert FFFF

Change in Low Cloud Amount and Surface Temperature

decrease in low cloud amount: pattern of response does not match forcing pattern increase in land surface temperature: heating + dissipation

Change in Equilibrium Shortwave Flux

existing low clouds absorb + less low clouds: even less upward flux at TOA, more downward flux at SFC (change in sign!)

SW flux TOA increases more than SW flux SFC net input to the system the system warms

contcontpertpert FFFF

Change in Global Mean Cloud Amount Profile

controlperturbation

decrease in low cloud amount and total cloud amount

Regional Differences in JJA Response:United States and Europe/E. Asia

controlperturbation

decrease in low cloud amount and total cloud amount increase in shortwave flux to surface increase in stability

* unlike in the RCM the diabatic heating does not translate into an increase in convection but into an increase in horizontal heat advection

decrease in precipitation, soil moisture

Regional Differences in JJA Response:N. Africa

controlperturbation

decrease in low cloud amount, increase in middle and high cloud amounts (overall increase)

decrease in shortwave flux to surface decrease in stability near surface

* like in the RCM the diabatic heating does translate into increase in convection increase in precipitation, soil moisture, evaporation and sublimation, latent heat

release from surface

Equilibrium Change in Precipitation

negative area average in United States, Europe/E. Asia positive area average in N. Africa band of strong increases and decreases around equator

Summary

Globally, absorption of solar radiation by clouds causes a warming of the surface, stabilization of the lower troposphere, and a decrease in precipitation.

Regionally, results may vary. In the United States and Europe/E. Asia horizontal heat flux is more efficient, while in N. Africa there is a distinct local (vertical) balance [Chen and Ramaswamy, 1995].