Jonathan Petters

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Dynamical impacts of surface and atmospheric
radiative heating on cloud systems
Jonathan Petters February 20, 2009 Naval Research
Lab Marine Meteorology Division
Howard W. Barker, Eugene E. Clothiaux, Jason N.S.
Cole, Jeffrey W. Frame, Jerry Y. Harrington, Paul
M. Markowski This work funded by the Department
of Energy Atmospheric Radiation Measurement
Program (DOE ARM)
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Atmospheric Solar Radiative Transfer
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Atmospheric Solar Radiative Transfer
-Modeled
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Each model column is its own plane-parallel
atmosphere!
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Independent Column Approximation (ICA) leads to
radiative heating errors in the atmosphere and
surface
cloudside heating
hotspots
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Errors in surface heating due to ICA -
Cumulonimbus?
Markowski et al. (1998) Surface cooling of 3K
observed under anvil shadow
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Surface solar irradiance model supercell
(ARPS) - ICA
- Solar zenith angle of 47 - azimuth just S of W
Frame, Petters, Markowski and Harrington (2009)
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Surface solar irradiance same supercell
Monte Carlo
- Solar zenith angle of 47 - azimuth just S of W
Frame, Petters, Markowski and Harrington (2009)
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How might we rectify these surface heating errors?
Tilt model columns (titled ICA -gt TICA)
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Use of TICA lessens error
Surface solar irradiance
ICA Monte Carlo
TICA Monte Carlo
Frame, Petters, Markowski and Harrington (2009)
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Use of TICA in Supercell Dynamical impact?
For stationary storms and storms moving slowly in
the direction of anvil shadow, cooling of surface
under anvil shadow can lead to weakening.
Frame, PhD Dissertation (2008)
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Cloud shading in Cb Dynamical impact?
No radiation no shadow!
Little vertical wind shear near surface
Frame, PhD Dissertation (2008)
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Cloud shading in Cb Dynamical impact?
shadow
Added vertical wind shear near surface
Cooling under anvil -gt stabilize surface layer -gt
less vertical mixing
Frame, PhD Dissertation (2008)
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Anvil
With anvil shadowing, rear-flank gust front
accelerates, can undercut mesocyclone, leading to
weakening of storm
Lemon and Doswell (1979)
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Use of TICA improves atmospheric heating
calculations as well
cloudside heating
Not important here! Where then?
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Stratocumulus! Radiatively driven
Quite homogeneous cloud field Errors due to use
of ICA in such a cloud field not likely to be
large
Photo Alexei Korolev North of Barrow, AK
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Inhomogeneous Sc field Errors due to use of ICA
in modeling such a cloud field important (?)
Photo Amy M Dobrzyn Bloomsburg, PA
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What do we know about the impact of solar heating
on stratocumulus?
Can be thick, overcast, heavy drizzle
Can be thin, broken, light drizzle
Stabilizes cloud layer with respect to subcloud
Examine further with ICA treatment of radiation
first!
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Experimental Platform

• Regional Atmospheric Modeling System (RAMS)
• Eddy-resolving mode (2-D)
• Input sounding
• ASTEX (Jiang et al. 2002)
• 30 m vertical resolution, 50 m horizontal
  resolution (64X70X70)
• 2 second model and radiative timestep
• no surface fluxes

Find model Sc cloud fields sensitive to changes
in solar heating
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No Sun Sun at 45 Overhead Sun No drizzle
allowed
Solar forcing thins model cloud layer
significantly
CDNC cloud droplet number concentration
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No Sun Sun at 45 Overhead Sun
Same as above Drizzle allowed
Drizzle production lessens as solar forcing
increases
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No Sun Sun at 45 Overhead Sun
Same as above High CDNC
No drizzle allowed, change CDNC
Increased CDNC -gt reduced liquid water path when
sun is overhead
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Less heating
More heating
Difference in integrated radiative heating high
CDNC low CDNC
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No Sun Sun at 45 Overhead Sun
Same as above High CDNC
Sensitivity to small changes in solar forcing
when sun is overhead broken Sc commonly
observed when sun is overhead too
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Testing importance of atmospheric radiative
heating errors in Sc

• Good candidate for study broken cloud field
  sensitive to small changes in solar forcing

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Finding a candidate model Sc field
CDNC 50/cc Overhead Sun Drizzle
Calculate radiative fluxes through cloud without
ICA offline, note changes in integrated shortwave
heating
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Testing importance of atmospheric radiative
heating errors in Sc

• Monte Carlo radiative transfer model coupled with
  RAMS
• Accurately represents horizontal transport of
  radiation through model domain
• Simulate broken Sc cloud field
• With ICA treatment of radiation
• Without ICA treatment of radiation
• Observe/analyze dynamical impact (if any)

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Summary

• Modeling of radiative transfer leads to errors in
  computed radiative heating in numerical
  atmospheric models
• Errors in surface heating can lead to changes in
  model supercell evolution
• Errors in atmospheric heating might impact
  stratocumulus evolution analysis continues!

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Thank You!Questions/Comments?
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Finding a candidate model Sc field
CDNC 50/cc Sun at 45 Drizzle
Calculate radiative fluxes through cloud without
ICA offline, note changes in integrated shortwave
heating
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Supercell Schematic and Pic
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