Horizontal Distribution of Ice and Water

1
Horizontal Distribution of Ice and Water in
Arctic Stratus Clouds During MPACE
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. 1ice,
2mixed, 3water
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 98915 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,
24342455. 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, doi10.1029/2007JD0086
46.
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.