Deng and Mace (2006) JAMC

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22 Φεβ 2014 (πριν από 3 χρόνια και 8 μήνες)

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Deng and Mace (2006)
JAMC


In addition to the microphysical properties of cirrus clouds, knowledge of cirrus
radiative and dynamical processes is important for understanding the maintenance

of cirrus layers within the atmosphere, and this

understanding
is critical to develop
realistic representations

of cirrus in large
-
scale models. Few studies have

evaluated
the representation of cirrus in numerical

models mainly because of the paucity of
reliable observations

that are collected on space and time scales

relevant

to models.
Such an approach was pioneered by

Heymsfield

(1977), who found that cirrus
p
roperties

depend strongly on the ambient vertical velocity and

temperature.
Heymsfield and Donner (1990) proposed

a simple scheme for parameterizing ice
water
content

(IWC) in general circulation models from the large
-
scale

vertical
velocity and temperature based on balancing

the water vapor deposition and the ice
particle

sedimentation. This parameterization is meaningful for

cirrus clouds
generated in situ by
synoptic
-
scale lifting

of a moist layer (e.g., Boehm et al. 1999).
However,

Mace et al. (1997, 2006) show that cirrus are found in

environments in
which the large
-
scale atmosphere is

weakly ascending on average, but nearly half the
cirrus

are observed in w
eak large
-
scale subsidence. To unveil

the relationship
between the cirrus properties and dynamical

and radiative processes, it is important to
know

t
he vertical velocity on different scales (Gultepe and

Starr 1995), but
particularly on the cloud scale
where

saturation ratios and subsequent particle
formation are

determined by the magnitude of the ascent.



Vertical velocity in ice clouds
-

Status report


Pavlos Kollias and Jennifer Comstock


Vertical air motions ac
ting on a wide range of spatial
(1
-
100
km) and temporal scales
(seconds to hours) have

been suggested as the primary mechanism for reaching

ice
-
supersaturation and subsequently cirrus cloud

formation (Kärcher and Ström, 2003).


Facilitate:




microphysical retrievals (actually coupled) and thus

help to assess ice clouds
radiative properties



particle sedimentation rates in cirrus clouds and sensitivity of GCM future climate
simulations

(Mitchell et al., 2008)


Vertical velocities in cirrus


Measured extensively using
aircraft observations


though

data is not well utilized

E
xamine data from past campaigns
(FIRE II, ARM Cloud IOP 2000,

SPartICus, etc.)


Example: Gultepe et al. (1995)


cirrus measured during FIRE II


Retrievals of vertical velocities in Cirrus


Radar Methods (examples)


Averaging Doppler Velocity over long time periods i.e. Orr and Kropfli (1999)

• Solve explicitly within the sample volume using 3 Doppler moments (Deng and
Mace, 2006)


Limitations: Sensitive to larger particles that are typically falling, missing nucleati
on
zones where crystals form


Coherent Doppler lidar


U
nder
-
explored for cirrus studies


Example from Grund et al. (2001) using High
-
Resolution Doppler Lidar


Poten
tial to study vertical velocity
in cirrus nucleating zones and

crystal fall speeds
using z
enith lidar

or nadir on
-
board aircraft.



Limitations


limited to optically

thin clouds



Challenges


The majority of the available (published) ice
cloud remote sensing techniques
have
focused on ice microphysical retrievals (e.g., IWC and effective
radius)


Aircraft
-
based VV measurement
s

are a great source of in
-
sit
u data for vertical air
motion and microphysics (SPARTICUS) and future retrieval development efforts

should take advantage of such observations.


Straw man suggestions:


Start by analyzing

the long record of Doppler m
easurements at the ARM sites to
derive
a
cirrus
cloud

“Doppler velocity” climatology (e.g., variance,

identification of
turbulent/gravity scales of motion)


Revisit the Z
-
V and Doppler spectra based t
echniques that could accomplish
separation of ice clouds microphysics and dynamics




Midlatitude Cirrus

http://www.ssec.wisc.edu/~baum/Cirrus/MidlatitudeCirrus.html


Midlatitude
cirrus often display 3 distinct layers:



S
mall particles in “generating region” near cloud top



G
rowth region containing pristine ice crystals in middle region



Su
blimation layer near cloud base, with largest particles

Size sorting occurs because of the relatively low vertical velocities; the situation may
be completely different for cirrus generated near centers of strong convection such as
anvils.


ASR


How does
vertical velocity

vary in height, space, time, and stren
gth? How does
this variability impact precipitation formation, cirrus production, and convective
system lifetime?


How do cloud properties (water contents, sizes, number concentrations, cloud
thickness,
vertical velocities
) relate to the near
-
surface
properties (surface
fluxes, temperature, moisture, vertical velocity, and aerosol) and their
variability?


What role does mesoscale inhomogeneity of liquid water or
vertical velocity

on
scales of tens of km play in determining the radiative properties and
drizzle rates
in low clouds?


What processes (concentrations,
vertical velocity
, entrainment, riming, etc.)
determine precipitation efficiency in cold clouds? What are the relative
influences of persistent slow precipitation vs. periodic strong precipitati
on
associated with storms on the Arctic precipitation budget?


What relationships among various microphysical and dynamical factors (i.e.
particle sizes, cloud thickness,
vertical velocity
, turbulence, ambient moisture,
and aerosol) determine low cloud pre
cipitation onset, rate, and efficiency?