Bibliography - J-C Golaz

1. Ackerman, A S., and J-C Golaz, et al., March 2009: Large-eddy simulations of a drizzling, stratocumulus-topped marine boundary layer. Monthly Weather Review, 137(3), doi:10.1175/2008MWR2582.1.
   [ Abstract ]

   Cloud water sedimentation and drizzle in a stratocumulus-topped boundary layer are the focus of an intercomparison of large-eddy simulations. The context is an idealized case study of nocturnal stratocumulus under a dry inversion, with embedded pockets of heavily drizzling open cellular convection. Results from 11 groups are used. Two models resolve the size distributions of cloud particles, and the others parameterize cloud water sedimentation and drizzle. For the ensemble of simulations with drizzle and cloud water sedimentation, the mean liquid water path (LWP) is remarkably steady and consistent with the measurements, the mean entrainment rate is at the low end of the measured range, and the ensemble-average maximum vertical wind variance is roughly half that measured. On average, precipitation at the surface and at cloud base is smaller, and the rate of precipitation evaporation greater, than measured. Including drizzle in the simulations reduces convective intensity, increases boundary layer stratification, and decreases LWP for nearly all models. Including cloud water sedimentation substantially decreases entrainment, decreases convective intensity, and increases LWP for most models. In nearly all cases, LWP responds more strongly to cloud water sedimentation than to drizzle. The omission of cloud water sedimentation in simulations is strongly discouraged, regardless of whether or not precipitation is present below cloud base.

2. Golaz, J-C, J D Doyle, and S Wang, in press: One-way nested large-eddy simulation over the Askervein Hill. Journal of Advances in Modeling Earth Systems. 2/09.
   [ Abstract ]

   Large-eddy simulation (LES) models have been used extensively to study atmospheric boundary layer turbulence over flat surfaces; however, LES applications over topography are less common. We evaluate the ability of an existing model -- COAMPS(R) -- to simulate flow over terrain using data from the Askervein Hill Project. A new approach is suggested for the treatment of the lateral boundaries using one-way grid nesting. LES wind profile and speed-up are compared with observations at various locations around the hill. The COAMPS-LES model performs generally well. This case could serve as a useful benchmark for evaluating LES models for applications over topography.

3. Haywood, J M., Leo J Donner, A Jones, and J-C Golaz, March 2009: Global indirect radiative forcing caused by aerosols: IPCC (2007) and beyond In Clouds in the Perturbed Climate System, Jost Heintzenberg and Robert Charlson, eds., MIT Press, 451-467.
   [ Abstract ]

   Anthropogenic aerosols are thought to exert a significant indirect radiative forcing because they act as cloud condensation nuclei in warm cloud processes and ice nuclei in cold cloud processes. While IPCC (2007) discuss many of the processes associated with the perturbation of cloud microphysics by anthropogenic aerosols, they only provide full quantification of the radiative forcing due to the first indirect effect (referred to by IPCC (2007) as the cloud albedo effect). Here we explain that this approach is necessary if one is to compare the radiative forcing from the indirect effect of aerosols with those from other radiative forcing components such as that from changes in well-mixed greenhouse gases. We also highlight the problems in assessing the effect of anthropogenic aerosols upon clouds under the strict definitions of radiative forcing of IPCC (2007). Although results from GCMs at their current state of development suggest analyzing indirect aerosol effects in terms of forcing and feedback is possible, a key rationale for IPCC’s definition of radiative forcing, a straightforward scaling between an agent’s forcing and the temperature change it induces, is significantly compromised. Feedbacks from other radiative forcings are responses to radiative perturbations, while feedbacks from indirect aerosol effects are responses to both radiative and cloud microphysical perturbations. This inherent difference in forcing mechanism breaks down the consistency between forcing and temperature response. It is likely that additional characterization, such as climate efficacy, will be required when comparing indirect aerosol effects with other radiative forcings. We suggest using the radiative flux perturbation associated with a change from pre-industrial to present-day composition, calculated in a GCM with fixed sea-surface temperature and sea ice, as a supplement to IPCC forcing.

4. Klein, Stephen A., and J-C Golaz, et al., in press: Intercomparisons of model simulations of mixed-phase clouds observed during the ARM Mixed-Phase Arctic Cloud Experiment. Part I: Single layer cloud. Quarterly Journal of the Royal Meteorological Society. 3/08.
5. Ming, Yi, Paul Ginoux, Leo J Donner, Stuart Freidenreich, Larry Horowitz, Ming Zhao, J-C Golaz, and Shian-Jiann Lin, in press: Transport of European Air Pollution influences Arctic climate. Science. 8/08.
   [ Abstract ]

   Arctic climate is changing at a pace faster than the global average in the recent decades (1, 2). Arctic haze (3) - an accumulation of long-range transported aerosols - enhances longwave emissivity of liquid water clouds both by reducing droplet size (4–6) and by increasing liquid condensate, thus exerting substantial surface warming in winter. The formation of Arctic haze and its influence on local climate are poorly understood, and constitutes an important missing piece of the Arctic climate puzzle. Here we find, with the help of a state-of-the-art global climate model with explicit treatment of pollutant transport and aerosol-cloud interactions, that the poleward transport of European air pollution is controlled strongly by the fluctuation in the second climate mode of the North Atlantic - European region. Though accounting for a smaller fraction of the region’s overall climate variability than the first mode (namely the North Atlantic Oscillation), the second mode has its impacts on Arctic climate amplified through modulating the amount of aerosols reaching the Arctic. This is supported by the fact that the surface aerosol concentrations and longwave downward radiative flux measured at locations lying in the model-projected transport pathway show strong correlation with the second mode. A shift of the mode from negative to positive phases doubles the abundance of Arctic haze, and the resulting increase in cloud liquid condensate alone is estimated to warm the surface by 1.8 K or to reduce the wintertime sea ice by 0.16 m. This finding is essential for understanding Arctic climate variability and change.

6. Teixeira, J, B Stevens, C S Bretherton, R T Cederwall, J D Doyle, J-C Golaz, A A M Holtslag, Stephen A Klein, J K Lundquist, D A Randall, A P Siebesma, and P M M Soares, April 2008: Parameterization of the atmospheric boundary layer: A view from just above the inversion. Bulletin of the American Meteorological Society, 89(4), doi:10.1175/BAMS-89-4-453.
7. Wang, S, J-C Golaz, and Q Wang, 2008: Effect of intense wind shear across the inversion on stratocumulus clouds. Geophysical Research Letters, 35, L15814, doi:10.1029/2008GL033865.
   [ Abstract ]

   A large-eddy simulation model is used to examine the impact of the intense cross-inversion wind shear on the stratocumulus cloud structure. The wind shear enhanced entrainment mixing effectively reduces the cloud water and thickens the inversion layer. It leads to a reduction of the turbulence kinetic energy (TKE) production in the cloud layer due to the weakened cloud-top radiative cooling and the formation of a turbulent and cloud free sublayer within the inversion. The thickness of the sublayer increases with the enhanced wind shear intensity. Under the condition of a weaker inversion, the enhanced shear mixing within the inversion layer even lowers the cloud-top height and reduces the entrainment velocity. Finally, increasing wind shear or reducing inversion strength tends to create an inversion layer with a constant bulk Richardson number (∼0.3), suggesting that an equilibrium value of the Richardson number is reached.

8. Wang, S, J Schmidt, and J-C Golaz, in press: Effect of down-gradient turbulent liquid water flux parameterization on condensation in mesoscale models. Journal of Applied Meteorology and Climatology. 11/08.
9. Golaz, J-C, V E Larson, J A Hansen, D P Schanen, and B M Griffin, 2007: Elucidating Model Inadequacies in a Cloud Parameterization by Use of an Ensemble-Based Calibration Framework. Monthly Weather Review, 135(12), doi:10.1175/2007MWR2008.1.
   [ Abstract ]

   Every cloud parameterization contains structural model errors. The source of these errors is difficult to pinpoint because cloud parameterizations contain nonlinearities and feedbacks. To elucidate these model inadequacies, this paper uses a general-purpose ensemble parameter estimation technique. In principle, the technique is applicable to any parameterization that contains a number of adjustable coefficients. It optimizes or calibrates parameter values by attempting to match predicted fields to reference datasets. Rather than striving to find the single best set of parameter values, the output is instead an ensemble of parameter sets. This ensemble provides a wealth of information. In particular, it can help uncover model deficiencies and structural errors that might not otherwise be easily revealed. The calibration technique is applied to an existing single-column model (SCM) that parameterizes boundary layer clouds. The SCM is a higher-order turbulence closure model. It is closed using a multivariate probability density function (PDF) that represents subgrid-scale variability. Reference datasets are provided by large-eddy simulations (LES) of a variety of cloudy boundary layers. The calibration technique locates some model errors in the SCM. As a result, empirical modifications are suggested. These modifications are evaluated with independent datasets and found to lead to an overall improvement in the SCM’s performance.

10. Golaz, J-C, J D Doyle, and S Wang, in press: Large-eddy simulation of flow over the Askervein Hill using COAMPS. Quarterly Journal of the Royal Meteorological Society. 2/07.
11. Larson, V E., A J Smith, M J Falk, K E Kotenberg, and J-C Golaz, 2006: What determines altocumulus dissipation time? Journal of Geophysical Research, 111, D19207, doi:10.1029/2005JD007002.
    [ Abstract ]

    This paper asks what factors influence the dissipation time of altocumulus clouds. The question is addressed using three-dimensional, large-eddy simulations of a thin, midlevel cloud that was observed by aircraft. The cloud might be aptly described as “altostratocumulus” because it was overcast and contained radiatively driven turbulence. The simulations are used to construct a budget equation of cloud water. This equation allows one to directly compare the four processes that diminish liquid: diffusional growth of ice crystals, large-scale subsidence, radiative heating, and turbulent mixing of dry air into the cloud. Various sensitivity studies are used to find the “equivalent sensitivity” of cloud decay time to changes in various parameters. A change from no sunlight to direct overhead sunlight decreases the lifetime of our simulated cloud as much as increasing subsidence by 1.2 cm s−1, increasing ice number concentration by 780 m−3, or decreasing above-cloud total water mixing ratio by 0.60 g kg−1. Finally, interactions among the terms in the cloud water budget are summarized in a “budget term feedback matrix.” It is able to diagnose, for instance, that in our particular simulations, the diffusional growth of ice is a negative feedback.

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