D. Barahona

Direct estimation of the global distribution of vertical velocity within cirrus clouds

Cirrus clouds determine the radiative balance of the upper troposphere and the transport of water vapor across the tropopause. The representation of vertical wind velocity, W, in atmospheric models constitutes the largest source of uncertainty in the calculation of the cirrus formation rate. Using global atmospheric simulations with a spatial resolution of 7 km we obtain for the first time a direct estimate of the distribution of W at the scale relevant for cirrus formation, validated against long-term observations at two different ground sites. The standard deviation in W, σw, varies widely over the globe with the highest values resulting from orographic uplift and convection, and the lowest occurring in the Arctic. Globally about 90% of the simulated σw values are below 0.1 m s−1 and about one in 104 cloud formation events occur in environments with σw > 0.8 m s−1. Combining our estimate with reanalysis products and an advanced cloud formation scheme results in lower homogeneous ice nucleation frequency than previously reported, and a decreasing average ice crystal concentration with decreasing temperature. These features are in agreement with observations and suggest that the correct parameterization of σw is critical to simulate realistic cirrus properties.

Quantifying sensitivities of ice crystal number and sources of ice crystal number variability in CAM 5.1 using the adjoint of a physically based cirrus formation parameterization

We present the adjoint of a cirrus formation parameterization that computes the sensitivity of ice crystal number concentration to updraft velocity, aerosol, and ice deposition coefficient. The adjoint is driven by simulations from the National Center for Atmospheric Research Community Atmosphere Model version 5.1 CAM 5.1 to understand the sensitivity of formed ice crystal number concentration to 13 variables and quantify which contribute to its variability. Sensitivities of formed ice crystal number concentration to updraft velocity, sulfate number, and is sufficient but sulfate number concentration is low, indicating a sulfate-limited regime. Outside of the tropics, competition between homogeneous and heterogeneous nucleation may shift annually averaged sensitivities to higher magnitudes, when infrequent strong updrafts shift crystal production away from purely heterogeneous nucleation. Outside the tropics, updraft velocity is responsible for approximately 52.70% of the ice crystal number variability. In the tropics, sulfate number concentration and updraft jointly control variability in formed crystal number concentration. Insoluble aerosol species play a secondary, but still important, role in influencing the variability in crystal concentrations, with coarse-mode dust being the largest contributor at nearly 50% in certain regions. On a global scale, more than 95% of the temporal variability in crystal number concentration can be described by temperature, updraft velocity, sulfate number, and coarse-mode dust number concentration.

Assessing the Impact of Mineral Dust and Adsorption Activation on Cloud Droplet Formation

Most aerosol-cloud-climate assessment studies assume that aerosol with a substantial fraction of soluble material are the sole source of Cloud Condensation Nuclei (CCN). However, insoluble species can also act as good CCN, even if they lack appreciable amounts of soluble material. The source of hygroscopicity in these particles is the adsorption of water vapor onto the surface of the particle. Moreover, during atmospheric transport, fresh dust undergoes aging which results in a coating of soluble material on its surface that augments its CCN activity. Given that dust may affect precipitation in climate-sensitive areas, the ability to capture the complex impact of mineral dust on cloud droplet formation is an important issue for global and regional models. The “unified dust activation framework” of Kumar et al. (2011) can be used to calculate the CCN activity of both fresh and aged dust. In this study, simulations of droplet number are carried out with the GMI chemical transport model. GMI simulates global atmospheric composition which is used to drive the droplet number calculations of Kumar et al. (2011) parameterization. This new framework is a comprehensive treatment of the inherent hydrophilicity from adsorption and acquired hygroscopicity from soluble salts in dust particles and is used to assess the impact of dust and adsorption activation on the predicted global droplet number concentration.

Assessing aerosol indirect effect through ice clouds in CAM5

Ice clouds play an important role in regulating the Earth’s radiative budget and influencing the hydrological cycle. Aerosols can act as solution droplets or ice nuclei for ice crystal formation, thus affecting the physical properties of ice clouds. Because the related dynamical and microphysical processes happen at very small spatial and temporal scales, it is a great challenge to accurately represent them in global climate models. Consequently, the aerosol indirect effect through ice clouds (ice AIE) estimated by global climate models is associated with large uncertainties. In order to better understand these processes and improve ice cloud parameterization in the Community Atmospheric Model, version 5 (CAM5), we analyze in-situ measurements from various research campaigns, and use the derived statistical information to evaluate and constrain the model [1]. We also make use of new model capabilities (prescribed aerosols and nudging) to estimate the aerosol indirect effect through ice clouds, and quantif...