Jonathan PettersFebruary 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)

Cloud

Shadow

Cloud

Shadow

From Sun

Atmospheric Solar Radiative Transfer

Cloud

To Sun

Atmospheric Solar Radiative Transfer -Modeled

Cloud

To Sun

Diff use

Direct

I nfinitely Long Layers

Each model column is its own plane-parallel atmosphere!

Cloud

To Sun

Cloud

Shadow Shadow

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

hotspotscloudside 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 - Solar zenith angle of 47°angle of 47°- azimuth just - azimuth just S of WS of W

Surface solar irradiance – same supercell – Monte Carlo

Frame, Petters, Markowski and Harrington (2009)

- Solar zenith - Solar zenith angle of 47°angle of 47°- azimuth just - azimuth just S of WS of W

How might we rectify these surface heating errors?

Tilt model columns (titled ICA -> TICA)

To Sun

Anvil

CbCloud Shadow

Frame, Petters, Markowski and Harrington (2009)

Use of TICA lessens error

ICA – Monte CarloICA – Monte Carlo TICA – Monte CarloTICA – 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)

Anvil

CbCloud

Cloud shading in Cb – Dynamical impact?

No radiation = no shadow!

Little vertical wind shear near surface

Frame, PhD Dissertation (2008)

Anvil

CbCloud

Cloud shading in Cb – Dynamical impact?

shadow Added vertical wind shear near surface

Cooling under anvil -> stabilize surface layer -> less vertical mixingFrame, PhD Dissertation

(2008)

Lemon and Doswell (1979)

With anvil shadowing, rear-flank gust front accelerates, can undercut mesocyclone, leading to weakening of storm

Anvil

To Sun

Anvil

CbCloud

Shadow

Use of TICA improves atmospheric heating calculations as well

Not important here! Where then?

cloudside heating

Photo: Alexei KorolevNorth 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 DobrzynBloomsburg, PA

Inhomogeneous Sc field

Errors due to use of ICA in modeling such a cloud field important (?)

DayNight

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 timestepno 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 aboveHigh CDNC

Difference in integrated radiative heatinghigh CDNC – low CDNC

Less heating

More heating

No Sun

Sun at 45°

Overhead Sun

Same as aboveHigh 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 fieldWith ICA treatment of radiationWithout 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