Jonathan Petters February 20, 2009 Naval Research Lab Marine Meteorology Division Dynamical impacts of surface and atmospheric radiative heating on cloud systems 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)

Atmospheric Solar Radiative Transfer

Atmospheric Solar Radiative Transfer -Modeled

Each model column is its own plane- parallel atmosphere!

Independent Column Approximation (ICA) – leads to radiative heating errors in the atmosphere and surface hotspots cloudside heating

Errors in surface heating due to ICA - Cumulonimbus? Markowski et al. (1998) Surface cooling of ~3K observed under anvil shadow

Surface solar irradiance – model supercell (ARPS) - ICA Frame, Petters, Markowski and Harrington (2009) - Solar zenith angle of 47° - azimuth just S of W

Surface solar irradiance – same supercell – Monte Carlo Frame, Petters, Markowski and Harrington (2009) - Solar zenith angle of 47° - azimuth just S of W

How might we rectify these surface heating errors? Tilt model columns (titled ICA -> TICA)

Frame, Petters, Markowski and Harrington (2009) Use of TICA lessens error ICA – Monte Carlo TICA – Monte Carlo Surface solar irradiance

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)

Cloud shading in Cb – Dynamical impact? No radiation = no shadow! Little vertical wind shear near surface Frame, PhD Dissertation (2008)

Cloud shading in Cb – Dynamical impact? shadow Added vertical wind shear near surface Cooling under anvil -> stabilize surface layer -> less vertical mixing Frame, PhD Dissertation (2008)

Lemon and Doswell (1979) With anvil shadowing, rear-flank gust front accelerates, can undercut mesocyclone, leading to weakening of storm Anvil

Use of TICA improves atmospheric heating calculations as well Not important here! Where then? cloudside heating

Photo: Alexei Korolev North of Barrow, AK Quite homogeneous cloud field Errors due to use of ICA in such a cloud field not likely to be large Stratocumulus! Radiatively driven

Photo: Amy M Dobrzyn Bloomsburg, PA Inhomogeneous Sc field Errors due to use of ICA in modeling such a cloud field important (?)

What do we know about the impact of solar heating on stratocumulus? Can be thin, broken, light drizzle Can be thick, overcast, heavy drizzle Examine further with ICA treatment of radiation first! Stabilizes cloud layer with respect to subcloud

 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 Experimental Platform Find model Sc cloud fields sensitive to changes in solar heating

No Sun Sun at 45° Overhead Sun No drizzle allowed Solar forcing thins model cloud layer significantly CDNC – cloud droplet number concentration

Drizzle production lessens as solar forcing increases No Sun Sun at 45° Overhead Sun Same as above Drizzle allowed

Increased CDNC -> reduced liquid water path when sun is overhead No drizzle allowed, change CDNC No Sun Sun at 45° Overhead Sun Same as above High CDNC

Difference in integrated radiative heating high CDNC – low CDNC Less heating More heating

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

 Good candidate for study – broken cloud field sensitive to small changes in solar forcing Testing importance of atmospheric radiative heating errors in Sc

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

 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) Testing importance of atmospheric radiative heating errors in Sc

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!

Thank You! Questions/Comments?

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

Supercell Schematic and Pic