Surface radiation measurements and simultaneous ground-based measurements of the atmosphere during the FIRE'91 cirrus field experiment provided an opportunity to identify crucial measurements and parameterization deficiencies in current cloud-radiation models. Comparisons between measured and calculated broadband surface fluxes with only a small data subset already reveal these needs: accurate humidity and aerosol vertical profiles for clear cases, accurate vertical extinction profiles and dimensions for clouds, and understanding of the (solar) scattering properties of cirrus

Ackerman, T.

Bergstrom, R.

Deluisi, J.

Kinne, S.

English

NASA Technical Reports Server

COMPARISONS OF DOWNWELLING RADIATION TO MODEL PREDICTIONS
BASED ON GROUNDBASED MEASUREMENTS DURING FIRE'91
S.Kinne and R.Bergstrom NASA-Ames, Moffett Field, CA 94035
T.Ackerman Pennsylvania State, University Park, PA 16802
J.DeLuisi NOAA, Boulder, CO 80303
ABSTRACT. Surface radiation measurements and simultaneous ground- _
based measurements of the atmosphere during the FIRE'91 cirrus field :
experiment provided an opportunity to identify crucial measurements
and parameterization deficiencies in current cloud-radiation models.
Comparisons between measured and calculated broadband surface _
fluxes with only a small data subset already reveal these needs:
(I) accurate humidity and aerosol vertical profiles for clear cases,
(2) accurate vertical extinction profiles and dimensions for clouds,
(3) understanding of the (solar) scattering properties of cirrus.
1. INTRODUCTION
The recent FIRE'91 cirrus field experiment provided an unique
opportunity to validate atmospheric models. Concentrating on the
radiative transfer modeling aspect - by comparing measured downward
fluxes at the surface to modeled fluxes - we like to address the
questions: 'How well can simple models simulate radiative properties
of the atmosphere?' and 'How well can models translate directly and
indirectly measured atmospheric properties into radiative fluxes?'
The radiative transfer model is described first. The employed
data are introduced next. Finally, comparisons for cloud-free and
cloudy cases, in particular cirrus clouds cases, are discussed.
2. MODEL
Radiative transfer calculations of hemispheric downward
broadband fluxes are based on a four-stream code at eight solar
wavelengths and on a two-stream code at twelve infrared wavelengths,
as the entire spectrum has been subdivided into twenty bands.
Absorption by atmospheric gases in these bands is expressed via
exponential sum-fitting and based on the HITRAN-database. Although
the selection of radiative method, spectral resolution and
absorption approximation can notably affect calculated fluxes, the
chosen model is found to be sufficiently accurate. Deviations of
surface broadband fluxe_ to values based on more accurate models
and/or spectral resolution (less than 4%) are found to be small
compared to the measurement errors. In all these models horizontal
homogeneity has been assumed. This assumption, which is particularly
poor for many cloud conditions, can create significant deviations
between measured and calculated fluxes. To reduce errors due to the
observed inhomogeneity only average flux values for time-periods of
at least five minutes are used.
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3. DATA
In-situ measurements and simultaneous remote sensing methods
during the FIRE'91 field e_xperiment characterized the atmosphere.
Only those measurements are listed that are used in our study:
(i) Vertical profiles of atmospheric variables (e.g. temperature,
humidity) are defined by up to 3 hour frequent NOAA radiosonde
launches. For times between launches interpolations are used.
(2) Cloud properties are based on remote sensing data from the
ground, including the Penn State University 94 Ghz radar for
cloud dimensions and 10-channel sunphotometer for ci_ cloud
optical depths. Ice cloud particles from the FIRE'86 cirrus
experiment are adopted, as FIRE'91 data were not available yet.
(3) Downward hemisDheric broadband sol_and infrar_ fl_es are
provided by Eppiey radiometers of the Penn State University,
and also by NOAA radiometers, employed as part of SPRECTRE.
Measurements (i) and (2) provide the input to the model. Calculated
downward broadband surface fluxes, are compared to measurement (4).
4. COMPARISONS
Comparisons between measured and calculated fluxes carry a
combination of model related errors, which are generally small, and
an uncertain contribution of data related errors. Flux comparisons
for four clear and four cloudy days are given in Figures 1 and 2.
CLEAR DAYS
18. NOV 21. NOV 24. NOV 4. DEC
i ...... caLl--atOm
2c0_
_ ,, - oF-
Ol S_ , 0
I 5 r "'tld "I ' *
0 _t L''" " - *
io 12 14 lo 12 _4 io 12 14 _C 12 I_
._,( {Iocol. n} nixie (_e,. _) _lu(: (i_..l. n) nl._E (Ioc_. ,)
Figure i. Comparison of measured and modeled hemispheric broadband
downward solar and infrared fluxes on four cloud-free days. Errors
(in %) for solar and infrared fluxes are given in the lower panels.
118
Clear sky flux comparisons are based on ten minute averages.
For the solar spectral region, deviations of less than 5% are even
smaller than expected from instrumental error analysis. For the
infrared spectral region clear-sky deviations are larger, in part
due to inaccurate humidity data, as radiosonde launches are sparse.
Cloudy sky flux comparisons are based on five minute averages.
Average values reduce effects from often observed cloud horizontal
inhomogeneities and from the lack of instrumental co-location. For
the solar spectral region large deviations are found for situations
involving optically thick clouds, as optical depths are uncertain.
Under cirrus conditions (e.g. Dec.5), systematically higher modelled
transmissions indicate solar asymmetry-factors smaller than 0.75.
For the infrared region the good agreement of less than i0 W/m2
mainly reflects an accurate height positioning for the clouds base.
5. CONCLUSION
Simple radiation models can reasonably well reproduce surface
fluxes under clear-sky conditions. However, the schemes frequently
fail to reproduce surface fluxes for cloudy conditions, mostly due
to insufficient information about cloud microphysics and structure.
CLOUDY DAYS
2S. NOV 26. NOV $. DEC S. DEC
a,_. _ZTtUl aO el _Ir_tt_ _ deaz am. dear al_. QZT'._ al_4 "l_tlLt'_
pro.. de_" ; _n'_ pro. _l.osu'_ras pro. _ pro. de._r /
_oo. , :_or "- Jt
¢ .. . ... "., J
!
O; " OI
,ok, 1/'@.Lu, ,of ,o '
-_,0 )- . , , - 1
JO 12 )_ _0 t_ _ 10 12 1_ _0 12 _
T;_( (kxm, n) TIU( (_:o,. n) Tt_( (kxe. n) _'IU( (kxo_. _)
Figure 2. Comparison of measured and modeled hemispheric broadband
downward solar and infrared fluxes on four (partly-) cloudy days.
Corresponding flux errors and general cloud conditions are shown.
119
SMALL COLUMNS (1:1) [tau=l] 0.55um
1.0
70
0.8:
<z 0.6;
E
0.4
0.2
ONe ;
o
...... •_q....,,_i_-c9_ ._;'_
qe_ k
c] - 0.9. 0.7, 0.5 " /_-"
20 40 60 80
SOLAR ZENITH ANGLE (deg)
LARGE COLUMNS (1:3) [tau=l] 0.55urn
1.0
Z
_o
)%.
O9
0.6
II
1--
_ 0.4
I--
0
ul
,,-,0.2
W
CE
0.0
0
, , , . ,
...... g_.%2
20 40 60 80
SOLAR ZENITH ANGLE (deg)
Figure 3. Comparison of reflection and transmission from different
assumptions to single scattering properties of ice-crystal columns.
Particular disturbing is the current inability to model solar
scattering of ice crystals. The results suggest asymmetry-factors of
0.70-0.75, which is lower than theoretical calculations on columns
suggest (Takano et al., 1989). However, ice crystals are typically
more complex, and often contain cavities (e.g., Milosevitch et al.,
1992). New theoretical studies (e.g., Macke 1992) are encouraging.
The use of asymmetry-factor in place of accurate phasefunctions in
the model seems to lead to too much solar transmission, especially
for smaller particles (Figure 3, also Fu et al., 1993) and may have
contributed to the discrepancy in the flux comparison. Consequently,
nonspherical single scattering corrections should be reevaluated.
This study reflects only an initial comparison with limited
data-sets. The incorporation of more data, which were unavailable at
this time for model calculations is in progress. This includes the
use of in-situ cloud microphysical measurements (e.g. NCAR aircraft
and balloon sondes) and additional ground based remote sensing data
(e.g., NASA-Ames solar direct/diffuse and infrared radiometers, AFGL
full sky camera, NOAA microwave radiometer and radar data).
REFERENCES
Fu Q. and Y. Takano
On the limitation of using the asymmetry factor for radiative
transfer involving cirrus clouds. Athos. Res., in press, 1993.
Macke, A.
Scattering of light by arbitrary shaped ice particles. ADD1.
Q_ in print 1992.
Miloshevich, L.M., A.J. Heymsfield, P.M. Norris
Microphysical measurements in cirrus clouds from ice crystal
replicator sondes launches during Fire II. llth Intern. Conf.
on Clouds and PreciD. Proceed. 1 525-528, 1992.
Takano, Y. and K.-N. Liou
Solar radiative transfer of cirrus clouds. Part i: Single
scattering and optical properties of hexagonal ice crystals.
J. Atmos. Sci. 46, 3-19, 1989.
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Comparisons of downwelling radiation to model predictions based on groundbased measurements during FIRE 1991

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