Arctic MixedPhase Cloud Properties Derived

1
Arctic Mixed-Phase Cloud Properties Derived from
Surface-Based Sensors
Matthew D. Shupea,b, Sergey Y.
Matrosova,b, Taneil Uttalb aCooperative
Institute for Research in Environmental Sciences
University of Colorado and
bNOAA - Environmental Technology Laboratory,
Boulder, CO
Note All box-and-whisker plots indicate the
median (middle line), 25th and 75th percentiles
(edges of box), 5th and 95th percentiles (end of
whiskers) and mean (symbol) of the data.
Identifying and Characterizing Mixed-Phase Clouds
Cloud Presence and Morphology

• Many Arctic mixed-phase cloud properties
  are derived from measurements made during the
  Surface Heat Budget of the Arctic Ocean (SHEBA)
  Program in the Beaufort/Chukchi Seas in 1997-1998
  and at the ARM North Slope of Alaska (NSA) site
  since March 1998. Single-phase ice and liquid
  cloud properties are also derived however, these
  are only used here for comparison with some
  mixed-phase properties.
• Measurements/instruments
• Profiles of radar reflectivity, mean Doppler
  velocity, and spectrum width from the 35-GHz
  millimeter cloud radar (MMCR),
• Column integrated liquid water path (LWP) and
  precipitable water vapor from microwave
  radiometer (MWR) measurements,
• Lidar depolarization ratio (DR) measurements
  which provide information on cloud phase,
• Radiosonde profiles of temperature and humidity.
• Classification Methods
• Mixed-phase clouds are defined here as distinct,
  cloud layers containing both liquid and ice
  water, although not necessarily in all volumes at
  the same time. These clouds are identified using
  the measurements listed above and a combination
  of the following list of criteria (which is not
  exhaustive) 1) Below-freezing temperatures 2)
  a positive LWP 3) depolarization ratios both
  below 0.11 (signifying liquid water) and above
  0.11 (signifying ice) 4) radar moments that are
  consistent with the presence of multiple phases
  (for an example, see the spectrum width
  discussion below).
• Retrieval Methods
• The ice properties of mixed-phase clouds are
  derived from radar reflectivity measurements
  under the assumption that the relatively larger
  ice particles in these clouds dominate the radar
  signal over the relatively smaller liquid
  droplets. Vertically-resolved ice water content
  (IWC) and particle mean size (Dmean) are
  retrieved from the reflectivity (Z) using
  standard power law relations of the form IWC
  a Zb, where the b exponent is fixed and the a
  coefficient is prescribed to vary with season
  based on retrievals of this coefficient from an
  IR-based ice retrieval method over the course of
  the SHEBA annual cycle. Profiles of cloud liquid
  properties are not discussed here however, the
  vertically integrated LWP from single-layered
  mixed-phase clouds is derived from MWR
  measurements.

Mixed-phase clouds are prevalent in the Arctic.
They occurred 41 of the total time, and 55 of
the time clouds were present, during the yearlong
SHEBA program. Over the course of 6 years at the
NSA site, these clouds occurred 46 of the time,
and 59 of the time that clouds were present.
The annual evolution of monthly mixed-phase cloud
fraction indicates that these clouds occur most
frequently in the spring and fall transition
seasons, with the SHEBA annual cycle shifted 1-2
months earlier than the average NSA cycle. The
transition seasons also exhibit the lowest
monthly-averaged mixed-phase cloud base heights
derived from radar measurements, because in these
seasons low-level stratiform mixed-phase clouds
with ice crystals extending down to, or near, the
surface are the predominant mixed-phase cloud
structure. Mixed-phase clouds are mildly thinner
in May and thicker in mid-summer than in other
times of the year. The temperature of
mixed-phase clouds (Tcld, derived from the
ambient radiosonde temperatures throughout the
depth of the mixed-phase clouds) vary with season
from a monthly average below -20 C in winter to a
maximum warmer than -10 C in June. There is a
high degree of similarity between the mixed-phase
cloud morphology at SHEBA and the NSA.
Figure 2 Annual cycles of monthly mixed-phase
cloud fraction at SHEBA and NSA.
Figure 1 (a) Lidar depolarization ratio, (b)
radar Doppler spectrum width, (c) MWR-derived
LWP, and (d) a temperature sounding during a
mixed-phase cloud.
The Doppler Spectrum Width and Cloud
Classification Figure 1 suggests that, in some
circumstances, a wide radar Doppler spectrum
width can be used to identify mixed-phase cloud
conditions. The top panel shows lidar DR
measurements, where values less than 0.11
(blues) clearly indicate liquid water and higher
ratios indicate ice. In the middle of this case,
both DR measurements and the MWR LWP indicate
that the liquid water disappears (likely through
scavenging by falling ice crystals from above)
for over 8 hours. Radar spectrum width
measurements are wider than 0.4 m s-1 during
the times that the lidar and MWR observations
suggest both liquid and ice water are present,
but are narrower during the times when the other
instruments suggest that no liquid is present.
Figure 3 Monthly and annual mixed-phase cloud
statistics of (a) occurrence fraction (Ac), (b)
cloud base height, (c) cloud thickness, and (d)
cloud temperature for one year at SHEBA (stars)
and six years at NSA (diamonds).
s
Summary
Figure 5 Monthly and annual statistics of cloud
(a) LWP and (b) hours of occurrence for
single-layer mixed-phase (stars) and all-liquid
(diamonds) clouds.
Phase Partitioning with Temperature
Mean Mixed-Phase Properties Retrieved
mixed-phase cloud ice particle sizes (Figure 4)
were, on average, smallest in the winter and
largest in the summer with an annual mean of 93
mm. The retrievals of IWC and IWP do not show
clear annual trends, with annual averages of
0.027 g m-3 and 42 g m-2, respectively.
Mixed-phase cloud LWP is highest in the summer
(Figure 5), with an annual average of 61 g m-2.
To examine the relative vertical distribution
of mixed-phase cloud ice properties, retrieved
profiles were normalized in vertical extent and
in the magnitude of microphysical properties.
Average, normalized profiles show broad IWC
(Figure 6) and Dmean maxima at about two-thirds
of the cloud depth from the base.
Microphysical Properties
SHEBA only
The amount of liquid relative to ice in
mixed-phase clouds broadly increases with
temperature. Figure 7 summarizes the spread in
cloud top temperatures for different bins of the
liquid fraction, defined as LWP/(LWPIWP), for
SHEBA and 6 years at the NSA. On average, the
liquid fraction shows a relatively steep increase
as the temperature increases from -24 to -13 C.
At any given liquid fraction, the temperature
varies over 20-25 C. This average relationship
for both SHEBA and the NSA is similar to the
temperature-phase relationships employed by many
climate models however, the spread in these
observations suggests that parameterizations
based on temperature alone may fail to capture
the natural variability of mixed-phase cloud
phase partitioning.
NSA
SHEBA
Differences Between Mixed- and Single-Phase
Clouds Mixed-phase clouds at SHEBA contained
both more ice and more liquid than single-phase
ice or liquid clouds (i.e., Figs. 4 and 5). The
annual mean mixed-phase cloud Dmean, IWC, IWP,
and LWP were 28, 93, 40 and 30 larger than
their single phase counterparts, respectively.
The profile statistics in Figure 6 also show that
the bulk of the ice mass was somewhat higher in
mixed-phase clouds than in the all-ice clouds
observed at SHEBA. Together, these comparisons
suggest that there are marked differences between
the ice particle formation and growth mechanisms
in action in mixed-phase and all-ice clouds.
These differences are likely tied to the presence
of supercooled liquid water (typically found at
the cloud top) which promotes additional ice
growth over that which occurs in single-phase ice
clouds. In terms of liquid water, the cloud
layer thickness data (derived from both radar and
lidar) suggest that the larger LWPs observed in
mixed-phase clouds are predominantly due to
deeper liquid water layers in those clouds than
in the all-liquid clouds observed at SHEBA.
Figure 6 Annual mean, normalized profiles of
retrieved IWC (bold) and profiles of the standard
deviation (thin) for mixed-phase (solid) and
all-ice (dashed) clouds.
Figure 7 Box-and-whisker plots showing the
liquid fraction LWP/(LWPIWP) versus cloud top
temperature for mixed-phase clouds at (a) SHEBA
for one year and (b) NSA for six years.
A summary of the mean mixed-phase cloud
properties and their general ranges (5th to 95th
percentiles) derived from 6 years at the NSA and
1 year at SHEBA are given above. These
properties represent a small step towards
understanding Arctic mixed-phase clouds.
Substantial work is still needed to further
understand the annual variation of cloud
properties, to determine why mixed-phase clouds
contain more liquid and ice than single-phase
liquid and ice clouds, and to investigate which
other properties (i.e., ice nucleus
concentrations, vertical motions, etc.) may be
used to further constrain the partitioning of
phase in these clouds.
Figure 4 Monthly and annual statistics of cloud
(a) Dmean, (b) IWC, (c) IWP, and (d) hours of
occurrence for mixed-phase (stars) and all-ice
(diamonds) clouds.
Acknowledgements DOE ARM, NSF
SHEBA, NOAA SEARCH NASA
FIRE-ACE and EOS Validation