Convective cloud system size and structure: thermodynamic forcing and modification by aerosols Eric Wilcox, Desert Research Institute, Reno NV eric.wilcox@dri.edu 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

Jan-Mar 1999 aerosol radiative forcing at the surface Wintertime Northern Indian Ocean Jan-Mar 1999 aerosol radiative forcing at the surface Meteosat 8, Mar. 2 2017 Ramanathan et al. JGR (2001)

Convective invigoration hypothesis Clouds impacted by more CCN glaciate at a colder temperature and exhibit a thicker mixed- phase layer Rosenfeld and Woodley (2003)

Convective invigoration hypothesis Clouds impacted by more CCN glaciate at a colder temperature and exhibit a thicker mixed- phase 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 scale- dependent 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 Average CAPE and shear = 2x 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.

wintertime oceanic clouds 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. It is also instructive to look at the dependence of some cloud properties as a function of their size. Here the clouds have been sorted according to AOD on the top, where the AOD for each cloud is actually a very large-scale average (5 deg. x 5 deg.) representing the grossest sort of discrimination between clean and polluted clouds. In spite of the very large scale averaging and the availability of AOD measurements only in the clear sky regions between the clouds, we still see a systematic difference in the average of the 89 GHz brightness temperature (averaged over the area of the cloud system) between the clean clouds and the polluted clouds. Warmer 89 GHz brightness temperature for the polluted clouds implies less scattering of upwelling microwave radiation by precipitation-sized ice hydrometeors in the anvil cloud. This is consistent with prior studies showing smaller ice particle effective radius at the tops of polluted deep convective clouds, which might be expected from a decrease in the size of cloud drops in the convective core and reduce collision/coalescence. However, in spite of evidence of a microphysical modification of the polluted clouds, there is no signature of pollution invigorating the clouds (i.e. making them taller – this is not shown in the figure) and no indication of a significant change in the ratio of cold cloud (<220K) area to total cloud area (i.e. no change in the size of the anvil relative to the size of the core as has been hypothesized to result from invigoration). When you sort by shear, however, you see no indication of a change in microphysics (at least not one that would result in a change in the scattering properties of precipitation-size ice hydrometeors), but you do see a systematic difference in the core-to-anvil relationship. The cores of the clouds in the high shear environment are lower (i.e. warmer – not shown) but you can generate a cloud system of a given size with a smaller area of convective core cloud in the high shear environment. Key point: Variations in shear (and probably CAPE too) have a much larger effect on the ratio of the size of anvil to the size of convective core area than do aerosols. Therefore, when looking for signatures of aerosol effects on the coverage of convective clouds it is important to control for variations in CAPE and shear as we have in the methodology underlying this study.

wintertime oceanic clouds Sensitivity of cloud structure to aerosols and environment wintertime oceanic clouds This slide now shows the things labeled “not shown” in the notes for the previous slide. 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 Polluted clouds larger than clean clouds for low values of CAPE and shear. MODIS JFM

summertime oceanic clouds Sensitivity of cloud structure to aerosols and environment Polluted clouds larger than clean clouds for low values of CAPE and shear. MODIS JFM summertime oceanic clouds

Controlling for aerosol sampling errors Sorting clouds according to CCN concentration from aerosol model coupled to meteorological reanalysis (Jeff Pierce).

Controlling for aerosol sampling errors Sorting clouds according to CCN concentration from aerosol model coupled to meteorological reanalysis (Jeff Pierce). 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 Polluted clouds larger than clean clouds for low values of CAPE and shear. MODIS JFM 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