NASA Technical Reports Server (NTRS) 19970023383: Comparison of Ice Cloud Particle Sizes Retrieved From Satellite Data Derived From In Situ Measurements

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by  NASA Technical Reports Server (NTRS)

Repr, nted from the preprint volume of the 9th Conference on
NASA-CR-204711 Atmospheric Radiation, 2-7 February 1997 Longt3each,
California, by the AMS, Boston, Massac_husetts
P2.36 COMPARISON OF ICE CLOUD PARTICLE SIZES RETRIEVED FROM
SATELLITE DATA DERIVED FROM IN SITU MEASUREMENTS
,.'i,,,.- ._.:
Qingyuan Hart 1., William B. Rossow 2, Joyce Chou 1and Ronald M. Welch 1
_lnstitute of Atmospheric Sciences
South Dakota School of Mines and Technology
2NASA Goddard Institute for Space Studies
1. INTRODUCTION
be an inappropriate representation of ice crystal
Cloud microphysical parameterizations have scattering. There are measurements from both
attracted a great deal of attention in recent years laboratory and field observations showing that
due to their effect on cloud radiative properties and cirrus cloud scatering phase function is different
cloud-related hydrological processes in large-scale from that of regular hexagonal columns or their
models. The parameterization of cirrus particle size aggregates (e.g., Francis 1995, Gayet et al. 1995,
has been demonstrated as an indispensable Spinhirne et al. 1996). It is possible that using
component in the climate feedback analysis. phase functions based upon hexagonal columns in
Therefore, global-scale, long-term observations of remote sensing may lead to underestimates of
cirrus particle sizes are required both as a basis of crystal sizes if the dominant particles in the cloud
and as a validation of parameteriza-tions for climate are highly irregular crystals (Mishchenko et al.,
models. While there is a global scale, long-term 1996). The problem is that there is no proper way
survey of water cloud droplet sizes (Hart et al. to determine the dominant shapes of ice crystals in
1994), there is no comparable study for cirrus ice a cirrus cloud by current remote sensing
crystals. This study is an effort to supply such a instruments. A final problem is that the definitions
data set. of ice crystal sizes used in remote sensing
applications and in situ measurements are different.
Validation of satellite remote sensing Details of different definitions used in literature are
techniques for the retrieval of cirrus microphysics discussed in the text. The above four problems
has been rare. This is due to difficulties of have to be considered before any meaningful
temporally and spatially collocating satellite passing comparison can be performed.
and in situ aircraft flight. As the pioneer effort of
validation during 1986 FIRE I IFO, Smith et al. Recent aircraft measurements from the Central
(1990) and Heymsfield et al. (1990) found that ice Pacific Experiment (CEPEX) supply a better
particle sizes retrieved by satellite remote sensing c_portunity for intercomparisons between these
and those measured by in situ aircraft are measurements and values retrieved from satellite
considerably different. Particle sizes of about 200 sensors by eliminating the first two problems.
t_m were found from in situ measurements, as First, to overcome the lower limit of 2D-C probe, a
compared to values of "60 _m from the remote video ice particle sampler (VIPS) was included to
sensing results (Wielicki et al. 1990). These obtain images of particles as small as 5 t_m.
differences were attributed both to vertical Second, the aircraft had 20% of its flight time at an
inhomogeneities of cirrus particle sizes in clouds altitude within 500 m of the cloud tops (Heymsfield
and to a limitation in the measurement of small and McFarquhar 1995), making the results
particle sizes by the microphysical probes comparable to those from satellite retrievals.
(Heymsfield et al., 1990; Wielicki et al. 1990).
The validation strategy of the present study,
There are two other possible explanations for while still lacking simultaneous observations from
differences between remotely sensed cirrus particle satellites and aircraft, is to compare the range of
sizes and aircraft measurements. First, the phase ice crystal sizes retrieved by satellite remote
function used in the remote sensing retrievals may sensing with typical values derived from CEPEX in
situ measurements. Definitions of particle size
used in these two techniques are different, but a
relationship between them is derived. We also use
Corresponding author address: Q. Han, Institute of direct calculation of ice crystal effective diameters
Atmospheric Sciences, South Dakota School of Mines D, from CEPEX in situ measurement data.
and Technology, 501 E. St Joseph St., Rapid City, Differences from this comparison then can be used
SD 57701-3995
to estimate the possible bias caused by the
368 AMERICANMETEOROLOGICALSOCIETY
assumptioonfinappropriaptehase
functions. The
results reveal a close agreement between results 3.1 Different D¢finiti0ns of Particle Sizes
from the CEPEX measurements and from satellite
retrievals suggesting that the use of the hexagonal There are many different definitions of cloud
column phase functions in remote sensing of particle sizes, each used for different purpose (e.g.,
particles size may be justified. Foot, 1988; Wielicki et al., 1990; Ebert and Curry,
1992; Heymsfield and McFarquhar, 1996; Fu and
2. METHODOLOGY Liou, 1993). For water clouds, the best parameter
describing the scattering light is the effective radius
The data used to retrieve ice crystal sizes is ro (Hansen and Travis 1974). ro has been widely
the ISCCP CX data derived from the NOAA-9 used inremote sensing of water cloud microphysics
AVHRR from January, April, July, and October (Foot 1988, Nakajima and King 1990, Han et al.
1987 and 1988. The detailed description of CX 1994). For ice crystals, however, the non-spherical
data can be found in Schiffer and Rossow (1985), shape leads to many different ways to describe
Rossow et al. (1991) and Han et al. (1994). The particle sizes. While many of the definitions are
radiative transfer model used to simulate the very useful for characterizing particle sizes for in
AVHRR radiances is described in a previous paper situ measurements, they are not appropriate in
(Han et al. 1994). Shapes and orientations of ice remote sensing techniques because the information
crystals are assumed to be hexagonal columns and about the areas of irregular shapes is not available.
plates randomly orientated in the atmosphere. Ray For example, Foot (1988) defined a generalized ro
tracing techniques are used to calculate phase ina way analogous to that of water droplets which
functions for different size distributions. Five can be used for any particle shape. But the Dj in
different size distributions from observations are the numerator of his expression cannot be derived
used, i.e., cold cirrus (D,=23.9 t_m), warm cirrus from scattering properties. It is related to cross
(De=47.6 p.m), -40°C cirrus (De=64.1 I_m), Nov. 1 sectional area A/ in different ways depending on
cirrus (D, =75.1 _m) and Cirrus uncinus (D, = 123.6 particle shape, which need to be obtained from in
i_m). Phase functions of these five size situ measurements. For remote sensing purposes,
distributions for channels 1 and 3 are applied in the Fu and Liou (1993) defined D° as the ratio between
model for calculations of multiple scattering. particle volume and cross-sectional area. These
two quantities are related to the absorption
The particle size retrieval is initiated with a coefficient and scattering coefficient, respectively
cloudy pixel which is determined by the ISCCP (Pollack and Cuzzi 1980).
cloud detection procedure. The retrieval procedure
is limited to latitude +50 ° due to difficulties with The relationships between different size
proper cloud detection and retrieval of optical definitions are by no means straightforward. For
thicknesses over highly reflective surfaces at example, Do cannot be regarded as about twice as
extreme solar zenith angles. To alleviate possible large as re, as is the case for water droplets. Using
3-D effects, this analysis is limited to image pixels measured data, Ou et al. (1995) correlated the
viewed near nadir for which the reflected radiation values of D, of Fu and Liou (1993) and ro used by
is less affected by cloud geometry. Ice clouds in Wielicki et al. (1990) using measured data and
this study are determined to be those clouds developed a fourth-degree polynomial.
defined by Tc < 240K. The detailed retrieval
scheme, sensitivity tests for effect of partial cloud We examine the relationship between two
cover and supercooled water contamination are definitions of ice crystal size: 1) Do used for our
decribed in Han et al. (1996). retrieval and 2) re used in CEPEX by Heymsfield and
McFarquhar (1996). This relationship is based on
3. COMPARISON WITH IN SlTU the fact that for a certain ice crystal size
MEASUREMENTS distribution in a specific cirrus cloud, while the
definition of effective particle size can be different,
With a better in situ measurement dataset and the ice water content and the extinction coefficient
using the same definition of particle size, it is still P,xt should be the same. For D°, we have the
uncertain how different the particle size results may relation boxt= IWC(-6.656x 10.3+3.686/Do) (Fu and
be between those retrieved by satellite remote Liou, 1993), where IWC is the ice water content.
sensing and those measured by in situ For to, a regression from Figure 6 of Heymsfield and
measurements. The differences can be used to McFarquhar (1996) gives pox,=lWC(3.3459x10
estimate the potential effect of assuming phase 3+3.0981/ro). The resulting relationship between
functions based upon simple hexagonal crystals Do and re is D°=3.686rol(1.0x102ro+3.098).
instead of fractals. Figure 1 shows this relationship which reveals that
9TH ATMOSPHERIC RADIATION 369
1) D. is slightly larger than the r. values when
particle size is small (r. < 60 I_m), and 2) smaller A near global survey of ice crystal effective
than the r. values when particle size is large sizes D. has been conducted for Jan, Apr, Jul, and
(r.>100 I_m) The typical range of r. of about 50 Oct of 1987 (Han et al, 1996) The results show
I_m to 100 l_m in the CEPEX measurement that there is no distinct contrast between conti
(Heymsfield, personal communication) is similar to nental and maritime clouds, as found for water
values of D. of 50 I_m to 90 I_m cloud droplets (Han et al 1994) It appears that
the microphysics of low-level liquid water clouds
Also shown in Figure 1 are relationships are affected by CCNs near the ground whereas the
between D_.and three other r. definitions reported microphysics of cirrus clouds are influenced by
in the literature They are r. definitions used by upper air aerosols Observations of CCN vertical
Wielicki et al (1990), Ebert and Curry (1992) and profiles from five different geographical locations
Foot (1988). The fourth-degree polynomial (Hoppel et al 1973) found that at higher altitudes
reported by Ou et al. (1995) is used for relationship (around 35 kin), there are no systematic
between D. and the rodefinition used by Wielicki et differences between oceanic and continental
al (1990} For other re definitions, the relationship environments.
between D. and ro (Ebert and Curry 1992) is
Do=3686r.l(1010x102r.+2431) The relation- For comparison with CEPEX results, over the
ship between D= and r. (Foot 1988) is region 165°E to 170°W and 2°N and 18°S (region
Do=3686r.l(6656x103ro+ 1500), which has of CEPEX measurements), the average value of
been derived by Moss et al (1995) Do= 532 _m
3.3 Results of CEPEX and Kwalalein Measurements
i.=0
The CEPEX observations were conducted in
March and April 1993 in the central Pacific area.
I II
The microphysical measurements were made by a
II ..'"" Lear Jet, which can reach altitudes up to 13.5 kin,
100 flying in anvils within the area bounded by 165°E
to 170°W and 2°N to 20°S. The Lear Jet was
E equipped with a PMS-2DC, a PMS-2DP, a FSSP-
300, oil coated slides and a video ice particle
v
sampler (VIPS) (Heymsfield and McFarquhar 1996,
McFarhuhar and Heymsfield 1996).
5O
Figure 2 is an example of size distributions
I
derived from VIPS images (thin lines) and 2D-C
/ / ,, measurements (thick lines) during CEPEX
at -39.8°C, 10.9 km, April 4, 1993 (after
Heymsfield and McFarquhar 1996). The
=
measurements were made in anvils far from their
O _.0 ISC !50
convection cores. Using the aspect ratio suggested
by Ebert and Curry (1992}, D, can be calculated
accordingly. It is 79.4 mm for using 2D-C probe
Figure 1 Comparison of different definitions of ice
only and 40.4 l_m for 2D-C plus VlPS. This result
crystal size
shows the importance of including VlPS in the in
situ measurements.
3.2 Results of Satellite Retrievals
The calculation of D, is also conducted for the
A validation against FIRE I cirrus IFO data has
in situ measurement data in the vicinity of Kwaja-
been conducted using AVHRR LAC data. The land
surface reflectances for channels 1 and 3 over the lein, Marshal Islands (8°N, 168°W), in 1973. In
the Kwajalein experiment, the particle sizes were
Wausau region (Wisconsin) were determined as
measured using a PMS 1D-C probe (range from 20
0.167 and 0.038, respectively. For this case, the
to 3000 I_min 20 IJm intervals), a PMS 1D-P probe
average value of D'59.2 I_m, which is consistent
with the results of Ou et al. (1995) and Wielicki et (range from 140 to 2100 I_m in 140 mm intervals)
and a PMS axial scat[er;ng spectrometer probe
al. (1990). Note that Wielicki et al. were using
(range from 2 to 30 I_m in 2 l_m intervals)
LANDSAT data and using a different definition of re
(Heymsfield and McFarquhar 1996). The measure-
and different phase functions.
370 AMERICANMETEOROLOGICALSOCIETY
ments were collected with a WB57F aircraft, which the relatively simple assumption of the dominant
can reach altitude up to 20 kin. Using the data shapes of ice particles for a particular cirrus cloud,
presented in Heymsfield and McFarquhar (1996), the possible bias caused by this assumption is not
values of Do were calculated for Dec. 18 and 19, significant. This is because the complexity of
1973. These are vertical profiles of ice crystal different shapes and orientations in real clouds.
sizes. For Dec. 18, 1973, the altitudes are 16.4, Note that sensitivity tests conducted between
12.8, 11.9 and 11.7 kin, respectively. The cor- hexagonal columns and aggregates or fractals
responding Do values are: 60.6, 40.9, 43.8, and (Macke 1993, Mishchenko et al. 1996) are based
50.1 I_m, respectively. For Dec. 19 1973, the alti- on monodispersed ice crystal shape alone, i.e., they
tudes are 14, 13.3, 12.7, 12.2, and 11.7 km re- consist of one particle shape only. In any real
spectively. The corresponding Do values are: 30.4, cirrus cloud, many different particle shapes coexist,
32.8, 50.8, 57.2, and 44.6 p.m. and their radiative effect may cancel each other.
For example, some of the aggregated crystals may
cause an overestimate of particle size, but quasi-
De = 40.4/7g .4 spherical shape particles may cause underestimates
I of particle size. The total scattering properties, and
_oSr .... , ! thus the retrieved values of D,, depend not only on
¢- 23:41:00 size distributions but also on "shape distributions".
I
4. SUMMARY AND CONCLUSION
c-
O
A near-global survey of cirrus ice crystal sizes
© is conducted using ISCCP satellite data analysis.
The retrieval scheme uses phase functions based
c
_D upon hexagonal crystals calculated by a ray tracing
u
technique. The results show that global mean
c-
o values of Do are about 60 _m. This study also
investigates the possible reasons for the significant
difference between satellite retrieved effective radii
iOo
('60 p.m) and aircraft measured particle sizes
('200 p.m) during the FIRE I IFO experiment. They
are 1) vertical inhomogeneity of cirrus particle
Z
sizes; 2) lower limit of the instrument used in
10
aircraft measurements; 3) different definitions of
effective particle sizes; and 4} possible inappro-
Figure 2. Example of ice particle size measured dur-
priate phase functions used in satellite retrieval.
ing CEPEX on April 4, 1993.
Acknowledgments. We thank Drs. Takano and Ou
for generously supplying the phase functions of
3.4 Discussion
hexagonal columns. This research was supported
by NASA contract No. NAS1-19077, NAGW-3922,
The above results show that for the same
and NAGW-3788; was partially funded by the US.
region, at the same season, the result from satellite
Department of Energy's (DOE) National Institute for
remote sensing, D°=53.2 p.m, is very close to that
Global Environmental Change (NIGEC} through the
obtained from CEPEX in situ measurements,
NIGEC Great Plains Regional Center at the
D,=57.2 p.m. Although this is not a direct
University of Nebraska-Lincoln (DOE Cooperative
comparison between coincident observations, there
Agreement No. DEFC03-90ER61010). Financial
are no significant differences, such as found in the
support does not constitute an endorsement by
FIRE I IFO experiment (Wielicki et al. 1990). This
DOE of the views expressed in this paper. This
improvement comes from 1) the inclusion of VIPS;
research was also supported by the NASA Contract
2) the near cloud top flights in taking the in situ
NAGW-4791, under the Climate Program managed
measurements; 3) the hexagonal phase function
by Dr. Bob Curran; the ISCCP international manager
assumed for the remote sensing results; and, more
is Dr. Robert A. Schiffer. The ISCCP is part of the
importantly, 4) the unified particle size definition
World Climate Research Program supported by the
used for different techniques.
efforts of several nations.
The close agreement also suggests that
although we are uncertain about the adequacy of
9TH ATMOSPHERIC RADIATION 371
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