Quantifying aerosol indirect and semi-direct effects on trade wind cumuli
using cloud process models and high resolution satellite observations:
Work plan as part of the CMAI Working Group
Greg McFarquhar and Larry Di Girolamo
Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign
Increases in aerosol concentrations lead to smaller cloud droplets that increase cloud albedo and suppress drizzle (first and second indirect effects). Where significant absorption of solar radiation by carbonaceous aerosols exists, a warming of the cloud layer may lead to a reduction in cloud cover (semi-direct effect). Through these effects, anthropogenic aerosols can modify the surface radiation budget, the vertical profile of atmospheric heating and cooling, evaporation off Earth's surface, detrainment from clouds and fluxes of sensible and latent heat, that in turn impact cloud cover, feedback on the energy budget and hydrological cycle and induce cloud dynamical responses. We propose to use a cloud-resolving model in coordination with satellite observations to quantify aerosol impacts on trade cumuli microphysical, macrophysical and radiative properties over the Indian Ocean and to better understand the processes through which these impacts occur. We focus on trade cumuli because they are nearly ubiquitous over tropical oceans, impact global water and energy budgets, and their properties are highly susceptible to aerosol indirect and semi-direct effects--yet our understanding of their properties and their treatment in climate models is extremely poor. We will focus our modeling and satellite studies on the Indian Ocean region where trade cumuli in pristine and polluted air occur in close proximity to each other during the winter monsoon, and where in-situ data (e.g., aerosol properties, thermodynamic profiles) are available from the Indian Ocean Experiment (INDOEX) to guide and initialize modeling studies.
We will use the 3-d version of the NCAR Eulerian/semi-Lagrangian (EULAG) model, which will have bin-resolved microphysics, to quantify and better understand processes by which aerosols impact cloud cover, microphysical processes and properties, precipitation, vertical velocities (w), detrainment, life cycles, profiles of radiative heating and surface fluxes of sensible and latent heat. We will examine the impacts of aerosols on these quantities over the diurnal cycle, in particular, investigating:
1. impacts of the vertical distribution of aerosols on simulated trade cumuli properties and their feedbacks on heating rates, surface fluxes, w and cloud properties;
2. how different mixing scenarios for black carbon (i.e., internally or externally mixed, embedded as core within haze) influence the absorption of radiation and nucleation of cloud droplets, and hence the resultant feedbacks between simulated cloud properties, heating rates, surface fluxes and w;
3. whether improved representations of drizzle development with bin-resolved microphysics can help better understand the second indirect effect in polluted trade cumuli and the associated feedbacks to surface fluxes, w, heating rates and cloud properties; and
4. the magnitude of indirect and semi-direct forcing on regional scales, which can be compared to similar estimates from large-scale models to assess their representations of aerosol forcing.
A critical element of our research is to evaluate whether model results are consistent with satellite remote sensing observations of aerosol effects on cloud properties. This will involve comparing statistics (e.g., means, spreads) of how modeled and observed cloud properties change depending on aerosol optical depth, single-scattering albedo, aerosol vertical distribution and meteorological forcing. Hence, we will:
1. develop and compare climatologies of trade wind cumuli (fractional coverage, size distribution, height distribution, albedo, optical depth, effective particle size) from EOS-Terra and A-Train satellite data moving away from a “pixel” summary approach to a “cloud object” approach to provide properties of individual clouds rather than of individual pixels; and
2. use these climatologies to examine impacts of aerosols and meteorology on trade cumuli statistics, focusing on the Indian Ocean, where we have large samples of cumuli in both pristine and polluted air. We will assess whether cloud properties change in a consistent manner with varying aerosol properties (e.g., optical depth) or meteorological forcing (e.g., humidity in mixed layer) for models and observations, revisiting representations of model processes when necessary.
The ultimate goals are (1) to gain a process-oriented understanding of how aerosols impact trade cumuli microphysical and dynamical properties, (2) to isolate processes that have the largest impact on energy and water budgets on a regional scale, and (3) to assess whether our regional estimates of semi-direct and indirect effects are consistent with estimates obtained from large-scale models. Knowledge gained from the first two goals will be shared with the CMAI working group to discuss ways of transferring this knowledge to practical parameterizations on cloud-aerosol interactions for climate models. The third goal requires the participation of NASA support personnel under the CMAI framework.
