GACP Projects
Theoretical studies of microphysical processes in marine boundary-layer clouds
Form A: GACP ACCOMPLISHMENT REPORT
Name:
Andrew Ackerman, Co-I Brian Toon
Institution: University of Colorado
TITLE: Theoretical studies of microphysical processes in marine boundary-layer clouds
ABSTRACT:
Marine boundary layer clouds cover over a third of the ocean surface and reflect much more sunlight than the underlying ocean surface, thereby playing an important role in the global shortwave radiation budget. Cloud-climate feedbacks are poorly understood and represent a major uncertainty in predicting global climate change: satellite observations indicate that warm, low-lying, marine clouds decrease in optical depth as cloud-top temperatures decrease, yet the mechanisms responsible for this negative feedback have not been adequately addressed.
Although general circulation models (GCMs) are the ultimate tool for predicting global climate change, computational limits impose coarse spatial resolutions on them, which require highly-parameterized representations of clouds. For these global models to stand a chance of offering reliable, long-term predictions, a much greater understanding of regional-scale cloud processes is required. Measurements in ship tracks indicate that variations in microscale conditions can affect cloud properties and boundary-layer dynamics. For example, increased aerosol concentrations have been observed to result in increased cloud coverge in ship tracks, which has been attributed to a decrease in precipitation with increasing droplet concentrations. However, during the Indian Ocean Experiment (INDOEX), the areal coverage of boundary layer clouds was very low, and the boundary layer over much of the Indian Ocean was thick with strongly absorbing haze, suggesting a very different scenario, in which solar heating by the aerosols leads to decreased cloud coverage, due to evaporation of inactive clouds. Cloud-resolving models represent a invaluable tool for investigating such feedbacks, yet the parameterizations of important processes such as the production of precipitation in stratiform clouds in these models tend to be either overly simplified or dependent upon unknown parameters.
To study these issues, we have merged our detailed models of aerosol and cloud microphysics and radiative transfer with a thee-dimensional large-eddy simulation code. We have used this coupled model to study the dependence of boundary-layer and cloud dynamics on aerosol concentrations under clean conditions. We find that when aerosol concentrations are significantly depleted, not only can the boundary layer be compressed, but stratiform cloud layers break up into shallow cumulus. We are also using model simulations to investigate the response of tradewind cumulus to solar absorption by strongly absorbing haze, such as found over the Indian Ocean during INDOEX. We plan to also use the model to further study the effects of cloud microphysical variations on cloud properties and dynamics, using measurements of ship tracks obtained during the Monterey Area Ship Tracks (MAST) experiment. We also will use these simulations to evaluate and suggest improvements for parameterizations of precipitation in cloud-resolving models. Additionally, we propose to use our models to investigate the mechanisms behind feedbacks between cloud optical depths and cloud-top temperatures, and to evaluate parameterizations of cloud-topped boundary layers used in GCMs, such as the NASA GISS model and the
GOALS:
Our overriding goal is to improve parameterizations of cloud-topped boundary layers used in large-scale models. Of particular interest and importance are the feedbacks between cloud dynamics and populations of boundary layer aerosols, and the factors controlling cloud coverage.
OBJECTIVES:
Investigate the dependence of the diurnal variations in tradewind cumulus in the presence of strongly absorbing aerosols, such as observed over the Indian Ocean during INDOEX.
Explore the dependence of cloud properties (such as cloud liquid water content, physical thickness, and areal coverage) on droplet concentrations, using model simulations and measurements of ship tracks from the MAST experiment. We will also use these simulations to evaluate precipitation parameterizations used in cloud-resolving models.
Use our numerical models to explore the dependence of cloud optical depths on cloud-top (and sea surface) temperatures, as obtained from satellite observations.
Evaluate parameterizations of boundary-layer clouds used in GCMs.
APPROACH:
We use numerical model simulations to investigate relationships found in observations.
TASKS COMPLETED:
1) Completed the merging of our microphysics and radiative transfer models with a large-eddy simulation code.
2) Participated in an international cloud model intercomparison study (GCSS) focused on tradewind cumulus, which tested our large-eddy simulation code.
3) Participated in the INDOEX field experiment. Primary responsibility was releasing GLASS sondes from the Kaashidoo Climate Observatory, and analyzing the data.
4) Investigating the conditions under which visible ship tracks form, we found through simulations with the coupled model that when aerosol concentrations are significantly depleted, the stratocumulus- topped marine boundary layer breaks up into shallow cumulus.
FUTURE PLANS:
1) Use large-eddy simulations with parameterized microphysics to investigate the response of tradewind cumulus to solar absorption by soot. This investigation was prompted by the observation during INDOEX of extensive boundary layers filled with haze particles having single scattering albedos between 0.8 and 0.9.
2) Use large-eddy simulations with detailed microphysics to investigate the response of the cloud-topped boundary layer to injections of aerosols by modeling ship tracks measured during the MAST experiment. As a by-product we will assess precipitation parameterizations used in cloud-resolving models.
3) Use large-eddy simulations with parameterized microphysics to investigate the processes underlying the observed dependence of cloud optical depths on sea surface temperatures.
4) Use large-eddy simulations with detailed microphysics to evaluate parameterizations of boundary-layer clouds used in GCMs.
RESULTS:
We simulated tradewind cumulus, based on ATEX observations, using large-eddy simulations. As part of our submission to the GEWEX Cloud System Studies (GCSS) Boundary Layer Cloud working group, we found the surprising result that the simulated cloud coverage depends strongly on the sub-grid scale mixing length. When the mixing length is assumed to be constant, the model produces an average cloud coverage of 1/2, as was observed. However, with a more realistic treatment of the sub-grid scale mixing length, in which the length scale is reduced in stable regions (Deardorff, 1980), the model produces overcast conditions, in which a stratiform cloud layer forms at the trade inversion. In this case, decreased mixing between cloudy updrafts and the slowly subsiding environmental air pumps more water up to the base of the trade inversion, where a thin cloud layer maintains itself through radiatively-driven cloud-top cooling. Although not part of the intercomparison (in which sedimentation of condensed water was ignored), in subsequent simulations we found that allowing precipitation to operate reduced the cloud coverage back to the observed range of values.
We used the coupled model (large-eddy simulations with detailed microphysics and radiative transfer) to investigate the response of the stratocumulus-topped marine boundary layer to depletion of aerosol concentrations due to droplet coalescence. We found that below an average droplet concentration of about 10 cm-3, the boundary layer compresses during a 24 hour simulation (see PS Figure) faster than the imposed large-scale subsidence, and that the stratiform cloud layer evolves into a broken cumulus field.
We have run preliminary large-eddy simulations with parameterized microphysics and detailed radiative transfer, and find that maximum heating rates of only 1 K/d (due to solar absorption by strongly absorbing haze) results in a significant reduction in cloud coverage during the afternoon.
Form B: GACP SIGNIFICANT HIGHLIGHTS
Name: Andrew Ackerman, Co-I Brian Toon
Institution: University of Colorado
SIGNIFICANT HIGHLIGHTS:
We have used our coupled model (large-eddy simulations with detailed microphysics and radiative transfer) to investigate the response of the stratocumulus-topped marine boundary layer to depletion of aerosol concentrations due to droplet coalescence. As with our previous 1-D results (Ackerman et al., 1993), we found that below an average droplet concentrations of about 10 cm^-3, the boundary layer is compressed significantly during a 24 hour simulation. However, the compression is slower in the large-eddy simulations, because it is resisted by vigorous convection that is not treated adequately in the 1-D model. Another, more significant effect not represented in the 1-D model is that the stratiform cloud layer breaks up into shallow cumulus clouds. Before the cloud layer breaks up, mixing is driven by narrow, negatively buoyant downdrafts, which result from cloud-top radiative cooling; after the break up, mixing is driven by narrow, positively buoyant updrafts. These are the first model simulations to show a change in cloud coverage due to changes in aerosol concentrations, and add to the weight of previous studies showing that boundary layer mixing and cloud dynamics are affected by changes in aerosol concentrations.
These model results are consistent with observations of ship tracks that form under very clean conditions (Hindman et al., 1994; Taylor and Ackerman, 1999). The ambient clouds in those simulations were shallow cumulus reaching the surface. Injections of cloud nuclei from ship exhaust was observed to result in a rapid deepening of the boundary layer and a filling in of the clouds to form a stratocumulus deck.
Form C: FUTURE PLANS
Name: Andrew Ackerman, Co-I Brian Toon
Institution: University of Colorado
Briefly describe your research plans for the second year of GACP.
1) Complete our assessment on tradewind cumulus of the impact of solar heating by strongly absorbing aerosols.
2) Use large-eddy simulations with detailed microphysics to investigate the response of the cloud-topped boundary layer to injections of aerosols by modeling ship tracks measured during the MAST experiment. As a by-product we will assess precipitation parameterizations used in cloud-resolving models.
3) Use large-eddy simulations to investigate the processes underlying the observed dependence of cloud optical depths on sea surface temperatures.
Form D: GACP BIBLIOGRAPHY
Name: Andrew Ackerman
Institution: University of Colorado
BIBLIOGRAPHY:
Papers, reports, and presentations refer to those published during GACP by the principal investigator, co-investigators, and other researchers supported by your agency for aerosol research. Include those in progress or planned.
a. List of publications (including books, book chapters, and refereed papers), using AMS bibliographic citation form.
- Jensen, E. J., A. S. Ackerman, D. E. Stevens, O. B. Toon, and P. Minnis, Spreading and growth of contrails in a sheared environment, J. Geophys. Res., 103, 31557-31567, 1998.
- Ackerman, A. S., O. B. Toon, J. P. Taylor, D. W. Johnson, P. V. Hobbs, and R. J. Ferek, Effects of aerosols on cloud albedo: Evaluation of Twomey's parameterization of cloud sucsceptibility using measurements of ship tracks, in press at J. Atmos. Sci. (as part of a special issue -- accepted in 1997!).
- Ferek, R. J., T. Garrett, P. V. Hobbs, S. Strader, K. Nielsen, D. Johnson, J. P. Taylor, A. S. Ackerman, Y. Kogan, Q. Liu, B. A. Albrecht, and D. Babb, Drizzle suppression in ship tracks, in press at J. Atmos. Sci. (as part of a special issue -- accepted in 1997!).
- Engel, S., D. M. Hunten, A. S. Ackerman, and O. B. Toon: Dissipation of jovian water clouds due to subsidence, provisionally accepted for publication, Icarus.
- Taylor, J. P., and A. S. Ackerman: A case study of pronounced perturbations to cloud properties and boundary layer dynamics due to aerosol emissions, in press at Q. J. R. Meteorol. Soc.
- Ackerman, A. S., O. B. Toon, and D. E. Stevens: Aerosol-driven transitions of the cloud-topped marine boundary layer. In preparation.
- Ackerman, A. S., O. B. Toon, and D. E. Stevens: Impact of solar heating by aerosols on tradewind cumulus. In preparation.
b. List of printed technical reports and non-refereed papers.
c. List of oral presentations or posters at professional society meetings and conferences.
- Ackerman, A. S., O. B. Toon, and D. E. Stevens, Response of the cloud-topped marine boundary layer to depletion of aerosol concentrations, Conference on Cloud Physics (Everett, Washington), pp. 322-324, 1998.
- Ackerman, A.S., Cloud microphysics on Earth and elsewhere (invited talk), From Giant Planets to Cool Stars (Flagstaff, Arizona), 1999.
