Horizontal Distribution of Ice and Water in Arctic Stratus Clouds During MPACE

1. Horizontal Distribution of Ice and Water in Arctic Stratus Clouds During MPACE Figure 1. Phase partitioning Michael Poellot, David Brown – University of North Dakota Greg McFarquhar, Gong Zhang – University of Illinois Urbana-Champaign Introduction Radiative properties of clouds are strongly tied to optical depth and phase. Studies have shown that the cloud phase regions are not uniformly distributed (Lawson et al., 2001) and that using a model parameterization with an average phase fraction can lead to significant errors in predicted radiative budgets (Cahalan et al., 1994). Therefore, sub-grid scale variability must be accurately parameterized to get the radiative budget correct and so knowledge of the distribution of ice and water phases is essential. Summary Clouds during the MPACE period were dominated by mixed phase. There were substantial differences in distribution of phase between single and multi-layer cloud cases, which appears to be related to the large scale forcing and airmass trajectory. Multi-layer systems were quite heterogeneous with significant regions of ice phase and relatively low liquid water paths. The lack of ice-only phase in single layer clouds indicates that use of the plane-parallel assumption may be appropriate in this case. Technique In situ measurements of cloud microphysical properties were made using the University of North Dakota Citation aircraft during the Mixed-Phase Arctic Cloud Experiment (MPACE) project. This data set has been processed by the University of Illinois to produce time series of 10-second averages of microphysical parameters, including cloud phase and condensate amount (McFarquhar et al., 2007). MPACE missions where the Citation performed extended horizontal sampling of stratiform cloud conditions were selected for this study. Clustering of cloud phase was determined by binning contiguous occurrences of like phase during horizontal sampling legs. Samples in precipitation below the lowest layer were not included. Assuming a constant sampling speed, the phase cluster time periods can be converted into distance, e.g., 3 samples x 10 sec x 90 m s-1 = 2.7 km. Discussion Multi-layer clouds were sampled on Oct. 5, 6 and 8 and single-layer on Oct. 8 and 10. Fig. 1 shows phase partitioning by mission, and phase distribution for Oct. 6 and Oct. 9 is shown in Fig. 2. Back trajectories for these two flights are shown in Figs. 3. The ice phase dominated 2 of 3 multi-layer cases, occurring throughout the depth of the cloud, and was absent in the single-layer case. Liquid water paths ranged from 70-170 g m-2 on Oct. 9 and only 6-60 g m-2 on Oct. 6. Phase clusters tended to be smaller for the multi-layer cases (Fig 4.), although there was one large region of ice. The single layer clouds were nearly homogeneous in phase (Fig. 5), with cluster size limited by sample segment length. Figure 4. Phase cluster size (left) and length of sampling segments for multi-layer cases. Figure 2. Phase occurrence by height. 1=ice, 2=mixed, 3=water Figure 5. Same as Fig. 4, for single-layer cases. References Lawson, R., B. A. Baker, C. G. Schmitt, and T. L. Jensen, 2001: An overview of microphysical properties of Arctic clouds observed in May and July 1998 during FIRE ACE. J. Geophys. Res., 106, 14 989–15 014. Cahalan, R. F., W. Ridgeway, W. J. Wiscombe, T. L. Bell, and J. B. Snider, 1994: The albedo of fractal stratocumulus clouds. J. Atmos. Sci., 51, 2434–2455. McFarquhar, G.M., G. Zhang, M.R. Poellot, G.L. Kok, R. McCoy, T. Tooman, and A.J. Heymsfield, 2007: Ice properties of single layer stratocumulus during the Mixed-Phase Arctic Cloud Experiment (MPACE). Part I: Observations. J. Geophys. Res.,112, D24202, doi:10.1029/2007JD008646. Figure 3. Backwards trajectories of cloudy air masses originating at Barrow, Alaska for Oct. 6 (left) and Oct. 9 (right). The red, blue, and green lines on Oct. 6 represent the first cloud layer, second cloud layer, and above the second cloud layer, respectively. For Oct. 9 they represent below, in, and above the single cloud layer.