Convective cloud system size and structure: thermodynamic forcing
and modification by aerosols
Eric Wilcox, Desert Research Institute, Reno NV [email protected]
Derek Posselt, JPL, Pasadena, CA
Tianle Yuan, NASA GSFC, Greenbelt MD
Rick Thomas, University of Birmingham
P. S. Praveen, International Centre for Integrated Mountain Development, Kathmandu
Kristina Pistone, NASA Ames Research Center
Frida Bender, Stockholm University
V. Ramanathan, Scripps/UCSD
Wintertime Northern Indian Ocean
Meteosat 8, Mar. 2 2017
Jan-Mar 1999 aerosol radiative
forcing at the surface
Ramanathan et al. JGR (2001)
Convective invigoration hypothesis
• Clouds impacted by more CCN glaciate at a colder temperature and exhibit a thicker mixedphase layer
Rosenfeld and Woodley (2003)
Convective invigoration hypothesis
• Clouds impacted by more CCN glaciate at a colder temperature and exhibit a thicker mixedphase layer.
• Polluted cloud penetrate deeper and exhibit a broader anvil.
Rosenfeld and Woodley (2003)
Rosenfeld et al. (2008)
Cloud detection
MODIS
AMSR-E
• Detect and spread algorithm (Boer and Ramanathan 1997) – Detect core
of deep convection and spread out to attach anvil cloud based on MODIS
IR brightness temperatures (Roca and Ramanathan 2000).
• Evaluate scales of clouds
• Evaluate effects of thermodynamic environment on scales and scaledependent properties.
Cloud detection
MODIS
AMSR-E
• Detect and spread algorithm (Boer and Ramanathan 1997) – Detect core
of deep convection and spread out to attach anvil cloud based on MODIS
IR brightness temperatures (Roca and Ramanathan 2000).
• Evaluate scales of clouds
• Co-locate with MERRA reanalysis to evaluate effects of thermodynamic
environment on scales and scale-dependent properties.
• Then do the same with simulations with the Goddard Cumulus
Ensemble Model.
• Simulate brightness temperatures with the Goddard satellite data
simulator unit (SDSU).
• Apply identical cloud detection algorithm.
Relationship between cloud size and CAPE/shear
winter
ocean
JFM 2009-2012
>167000 clouds
• Cloud horizontal scale increases systematically with increasing CAPE and vertical shear core of horizontal
wind at MERRA resolution in the core of the cloud.
Relationship between cloud size and CAPE/shear
winter
ocean
JFM 2009-2012
>167000 clouds
summer
ocean
JJA 2004-2007
>339000 clouds
• Cloud horizontal scale increases systematically with increasing CAPE and vertical shear of horizontal wind.
• Summer environment exhibits smaller clouds for same values of CAPE and shear.
Average
CAPE and shear = 2x
Sensitivity of cloud structure to aerosols and environment
wintertime oceanic clouds
• Cold cloud fraction is fraction of cloud area colder than 220 K.
• Warmer 89 GHz brightness temperature with increasing AOD.
• Relationship between cold cloud fraction and cloud size does not
depend on aerosol amount.
• Can get a larger cloud for same cold cloud area if the shear is greater.
Sensitivity of cloud structure to aerosols and environment
wintertime oceanic clouds
• Relationship between the brightness temperature at the coldest part of the cloud and cloud scale
does not change with AOD.
• For high shear environments clouds of equivalent size are produced with lower core cloud tops
compared to low shear environments.
Sensitivity of cloud structure to aerosols and environment
•
MODIS JFM
Polluted clouds larger than clean clouds for low values
of CAPE and shear.
Sensitivity of cloud structure to aerosols and environment
MODIS JFM
Polluted clouds larger than clean clouds for low values of
CAPE and shear.
summertime oceanic clouds
Controlling for aerosol sampling errors
1. Sorting clouds according to CCN concentration from aerosol model coupled to
meteorological reanalysis (Jeff Pierce).
Controlling for aerosol sampling errors
1. Sorting clouds according to CCN concentration from aerosol model coupled to
meteorological reanalysis (Jeff Pierce).
2. Sensitivity studies with the Goddard Cumulus Ensemble Model.
• 2048 x 2048 km grid for 60 day DYNAMO period.
• Vary CCN concentrations: 400, 1600 cm-3.
• CSU-RAMS two-moment cloud microphysics.
Dynamo simulations
GCE - DYNAMO
MODIS JFM
Simulated clouds are smaller, but do increase with CAPE and shear in a
similar fashion as the observations.
Sensitivity of cloud structure to aerosols and environment
MODIS JFM
Polluted clouds larger than clean clouds for low values of
CAPE and shear.
Higher values of CAPE and shear in the model may have
been cut due to low numbers of samples.
GCE - DYNAMO
CARDEX Experiment site: Hanimaadhoo Island, Maldives
February-March 2012
Black carbon suppresses turbulence in the boundary layer
As the aerosol number concentration in the boundary layer increases:
• turbulent kinetic energy is reduced,
• boundary layer top is lower,
• latent heat flux from surface reduced from 99 to 61 W m-2.
Wilcox et al., PNAS (2016)
Black carbon suppresses turbulence in the boundary layer
Profiles with UAV aircraft show that more polluted boundary layer is:
• warmer (+1 K),
• more humid (+8% RH),
• has a thicker saturated cloud layer, and
• has cloud tops that penetrate deeper into the free troposphere.
Summary
• Properties of convective clouds depend systematically on the size of the cloud.
• This provides a convenient means of assessing the relationships between cloud structure, the thermodynamic properties of the
environment, and the cloud scales.
• Cloud size increases systematically with CAPE and shear of the environment around the core of the cloud – with significant variability
and strong regional differences.
• While IR/MW brightness temperatures indicate the microphysical modification of polluted clouds, the cold cloud fraction is far more
sensitive to changes in shear than variations in AOD.
• Controlling for CAPE/shear suggests the signature of invigoration for clouds in the low CAPE/shear environment. Clouds in the high
CAPE/shear environment exhibit opposite behavior.
• Model so far confirms similar for the low CAPE/shear environment.
• During winter monsoon turbulent fluxes north of the ITCZ are suppressed by aerosols.
Supported by the Science of Terra/Aqua program
CARDEX funded by the National Science Foundation