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Report on the Cirrus Symposium

De Bilt, The Netherlands, 2 February, 2003

Organizers: B. Bregman (bregman@knmi.nl) and P. Stammes (stammes@knmi.nl)

Ice clouds play an important role in the radiation budget, chemical processing and the ozone budget. They are, however, poorly represented in large-scale Chemistry-Transport Models (CTMs) and Chemistry-Climate Models (CCMs), since the formation and physical properties are not well understood. This caveat formed the reason for the symposium at KNMI, De Bilt, Netherlands, February, 2003.

B. Kärcher outlined a parameterization scheme for homogeneous freezing of ice clouds. In many cases the time scale of depositional growth of nucleated ice crystals is fast compared to that of the freezing event; the number of crystals formed is rather insensitive to details of the freezing aerosol size distribution and number, but increases rapidly with updraft speed and decreases with temperature. Subvisible cirrus clouds (SVCs) preferentially form at low temperatures (<215 K) and small updraft velocities (< few cm s^-1). A limited number (<0.1 cm^-3) of effective heterogeneous Ice Nuclei (IN) can control the formation and properties of SVCs.

The data from the INterhemispheric differences in Cirrus properties from Anthropogenic emissions (INCA) campaign provide compelling evidence for both homogeneous and heterogeneous freezing and for prevalence of meso-scale variability in vertical velocities driven by ubiquitous gravity waves (GW). These findings render the parameterization of cirrus in large-scale models more difficult (Figure 1), because global information about small-scale temperature fluctuations in GW and the distribution and properties of IN is not yet available.

(a)	(b)
Figure 1. Temporal evolution of ice crystal size distributions in a synoptic wave with a peak amplitude of 5 cm s-1 (left) and with superimposed small-scale temperature fluctuations consistent with INCA observations (right). The aerosol consists of 400 cm -3 liquid supercooled droplets and 0.01 cm-3 IN, the former (latter) freezing at ~150% (130%) relative humidity over ice. The marked differences between the size spectra are brought about by the combined action of buoyancy waves and the presence of few efficient IN. [Figure provided by B. Kärcher].

Recently the ECWMF and ECHAM model vertical winds were compared to the INCA observations. The models significantly underestimate the mean velocities and variability. As a result, the ice cloud number densities were underestimated. In the ECHAM model, the results are improved by superimposing a ‘turbulent kinetic energy’ (TKE) parameter on the vertical velocities, but still remain unsatisfactory.

P. Siebesma discussed the representation of ice clouds in the ECWMF model. The occurrence frequency of high thin ice clouds was compared with observations from the NOAA satellite [Jakob, 2003]. The comparison reveals an underestimation by the ECMWF model in middle and high latitudes and an overestimation in the ITCZ. The reason is in the relatively crude treatment of microphysical processes of cirrus. In addition, the major source and sink terms for liquid and ice water are an order of magnitude larger than the mean state. One can argue whether process-oriented or statistical approaches should be used in large-scale models.

D. Donovan presented ice particle size retrievals from radar and lidar observations of cirrus at the ARM SGP site. The effective radius depends on the physical shapes of particles. Different shapes were considered and complex poly-crystals seemed to give the best agreement [Donovan et al., 2002]. Generally the size distribution was bi-modal, with a peak at around 10 micron and a broad tail with radii of 100-200 microns. The size spectrum strongly depends on both temperature and ice water content (IWC), and the bi-modal distribution appeared at and above IWC of 0.011 g m^-3.

W. Knap discussed a comparison between global multi-angle (polarized) radiance measurements made by the POLDER satellite instrument and model calculations of the angular-dependent radiation field over ice clouds [Labonnote et al., 2001; Knap et al., 2003], see Figure 2. For this comparison, model clouds consisting of different ice particles are used: hexagonal crystals with smooth/rough surfaces and with/without air bubble inclusions, and the ISCPP polycrystal. It is found that the POLDER measurements are adequately simulated using smooth hexagons with air bubble inclusions or rough hexagons without inclusions. Slightly less favourable results are obtained for the polycrystal. Clear disagreement between model and measurements is obtained for the pristine hexagon.

Figure 2. Relative spherical albedo difference for cirrus clouds. Polarized radiances are measured with POLDER and model calculations are performed for a cloud consisting of rough hexagons with an aspect ratio of 2.5. [From Knap et al., 2003].

In conclusion, cirrus observations show that both homogeneous and heterogeneous freezing are important, a bi-modal size distribution explains the ARM radar/LIDAR observations, and an optical ice crystal model with imperfect or air-bubble hexagons seems to give best agreement with POLDER satellite observations.

Current representation of cirrus and SVCs in assimilation and climate models shows significant discrepancies with observations, due to crude assumptions in the physical formation and loss processes of ice clouds. More work on meso-scale dynamical variability and heterogeneous freezing is required.

Different parameterization approaches may be needed via physical process modelling, statistical model PDFs and detailed cloud-resolving models.

References

Donovan, D.P., and A.C.A.P. van Lammeren, First ice cloud effective particle size parameterization based on combined lidar and radar data, GRL, 29, 10.1029/2001GL013731, 2002.

Jakob, C., PhD Thesis, 2003.

Kärcher, B. and Lohmann, U., A parameterization of cirrus cloud formation: Homogeneous freezing including effects of aerosol size, J. Geophys. Res., 107, 4698, 2002.

Kärcher, B., and Ström, J., The roles of dynamical variability and aerosols in cirrus cloud formation, Atmos. Chem. Phys., 3, 823-838, 2003.

Knap, W. H., et al., 2003. Modelling light scattering in ice clouds using various ice crystal models: validation with POLDER and ATSR-2 measurements (in preparation).

Labonnote, C. et al., Polarized light scattering by inhomogeneous hexagonal monocrystals. Validation with ADEOS-POLDER measurements, J. Geophys. Res., 106, 12139-12153, 2001.