New CERES Data Examined for Evidence of Tropical Iris Feedback

4 Pages·2002·0.32 MB·English

by  Chambers, Lin H.

2.12 NEW CERES DATA EXAMINED FOR EVIDENCE OF TROPICAL IRIS FEEDBACK
Lin H. Chambers*, Bing Lin and David EYoung
NASA Langley Research Center, Hampton, VA
1. INTRODUCTION measures thermal radiation between 8.10 and
11.79 #m, and the total (TOT) channel measures
New data products from the Clouds and the
the total energy leaving the Earth at all wave-
Earth's Radiant Energy System (CERES; Wielicki
lengths (0.3 - >100 #m, Priestley et al., 2000). The
et al., 1996) instrument were recently released.
Iongwave (LW) radiation is obtained by subtrac-
The Single Scanner Footprint (SSF) data product tion: LW=TOT-SW.
combines radiation budget data from CERES with CERES is primarily a climate instrument, so
cloud property retrievals from an imager on the
great attention has been paid to its calibration. As
same platform to provide a vastly improved charac- a result, the radiative fluxes measured by CERES
terization of the state of the atmosphere. In addi- have been both stable and accurate over the life-
tion, the SSF incorporates new CERES angular time of the instrument. Uncertainties in measured
distribution models (ADM; Loeb et al., 2002) based
radiances are generally below the 0.5% level
on improved scene identification to obtain more
(Priestley et al., 2000). The errors in the instanta-
accurate top-of-atmosphere fluxes from the satel- neous estimated fluxes of SW (13 W/m 2) and LW
lite-measured radiances. Together these (4.3 W/m 2)radiation are mainly due to errors in the
advances allow the study of radiative fluxes for
application of the angular distribution models (Loeb
specific cloud types with unprecedented accu-
et al., 2002) which result from errors in the scene
racy. SSF data products are now available using identification.
the Visible and InfraRed Sensor (VIRS) for the
Tropical Rainfall Measuring Mission (TRMM). 3. DATA ANALYSIS
These data are used here to explore the Iris
hypothesis recently proposed by Lindzen et al., A CERES SSF contains about 130 parameters
2001 (hereafter LCH). describing each CERES footprint. These include
time, position, and viewing angles, surface infor-
2. THE CERES INSTRUMENT mation, filtered and unfiltered radiances, radiative
fluxes at the surface and top of atmosphere, and a
The CERES instrument is a bi-axial scanning
variety of parameters describing the clear and
radiometer (Wielicki et al., 1996). On the TRMM
cloudy portions of the footprint. The latter are
satellite, CERES operated in a quasi-periodic cycle
obtained from imager (VIRS) information and from
of 2 days in Fixed Azimuth Plane Scan (FAPS)
ancillary inputs such as numerical weather predic-
mode and 1 day in Rotating Azimuth Plane Scan tion models (http://asd-www.larc.nasa.gov/ATBD/
(RAPS) mode, with an occasional day of along-
ATBD.html). The nominal 2-km VIRS-pixel derived
track scanning. FAPS mode is basically a cross-
properties are then convolved using the CERES
track scanning mode and obtains maximum geo-
scanner point spread function to obtain cloud prop-
graphic coverage over the orbital swath. RAPS
erties for up to two cloud layers within the -10 km
mode scans in azimuth in order to obtain informa-
CERES footprint. Currently these layers are dis-
tion on the anisotropy of radiation in the full hemi- tinct, with no overlap properties identified. For this
sphere. TRMM is a precessing spacecraft with an
analysis, since radiative fluxes are only available at
orbital inclination of about 35°. It samples all local
the CERES footprint scale, the properties of the
times over a 46 day period.
two layers are area-weighted to obtain footprint-
CERES measures radiative energy in 3 broad mean cloud properties.
channels: the shortwave (SW) channel measures
A set of screening criteria are applied to the
reflected solar radiation, the window (WN) channel data to ensure that footprints with problems are not
used. This leaves more than half a million region-
* Corresponding author address: Lin H. Chambers, ally distributed footprints per day to be analyzed.
Atmospheric Sciences, Mail Stop 420, NASA Langley CERES TRMM SSF data are available for the
Research Center, Hampton, Virginia 23681-2199; e-mail period January 1 to August 31, 1998. This
<l.h.chambers@larc. nasa.gov>. includes the peak and decay of the strong 1997-98
ElNifio.Shortperiodsofdataareavailableafter
albedo for the cloudy moist portion of the model
this,butarenotincludedinthisstudybecauseof
are too low, while the corresponding properties of
theprecessinogrbit.Along-tracdkaysarealsonot
the dry portion are too high. Hartmann and Mich-
includedd,uetopoorspatiaslamplingT.oprevent
elsen (2002) have called into question the interpre-
temporaslamplingbiases,the TRMMdataare tation of the observed cloud cover versus SST
analyzedin46-dayintervalstogetradiativeprop- relation.
ertiesoverthefullrangeoflocatlimes.Thisisnec-
From the SSF, radiative properties are
essaryto computea correctinsolation-weighted
obtained for the dry region, which LCH assume
albedo.Detailsoftheaveraginmgethodaregiven
covers half the Tropics, from those footprints hav-
inChamberestal.,2002a.Theradiativeproper-
ing the highest 50% of outgoing LW flux. This cor-
tiesofaparticulaMr-typefootprinatrefoundtobe
responds to emission from the lowest levels of the
verystablefromoneprecessiocnycletothenext,
atmosphere, as occurs with very low upper tropo-
suggestintghattheresultsareinfactrepresenta-
sphere humidity. Properties for the moist region
tive of that cloud/atmosphertyepe;and not
are obtained from the remaining pixels. These are
respondintgoaseasonaolrElNifiosignal. compared to the 3.5 box properties of LCH in
ThreeTropicaOl ceanregionsbetween30° S-
Table 1. The final row combines the LCH Cloudy
30° Nlatitudeareconsideredt:heTropicaWl est
and Clear Moist values for comparison. Note the
Pacific(TWP,130°to190°EastLongitudec,orre-
near-constant net flux measured by CERES, as
spondingtotheregionstudiedbyLCH);theEast
opposed to the large range of net flux in the LCH
Pacific(200° to280° EastLongitude)a;ndthe values.
entireTropicaOl cean.Whencalculatintgheradia-
tivepropertiees,achregionwasbrokenupinto10° Table 1: Radiative Properties for the Tropics
latitudezones.Sincesimilarresultsareobtained
forallthreeregionso,nlythosefortheentireTropi- Area SW OLR, Net Flux,
calOceanaregiveninthispaper.Fortheland Region Coverage Albedo W/m2 W/m2
mask,SSFfootprintsarescreenedby surface
scenetypeforcompletceoveragebywater. CERES Fluxes for Highest and Lowest 50% of LW
4. RADIATIVE PROPERTIES Dry 50% 0.14 292 51.4
Moist 50% 0.32 231 42.4
4.1 The original 3.5 box Iris model
LCH Values
The Iris hypothesis of LCH relied on examina-
tion of the variation of cirrus cloud coverage with Dry 50% 0.21 303 12.8
cloud-weighted Sea Surface Temperature (SST)
over a large area. This was done using the 11 and Cloudy 22% 0.35 138 123.
12 pm split window channels on Japan's Geosta- Moist
tionary Meteorological Satellite, GMS-5. LCH used
Clear 28% 0.21 263 52.8
these observations to develop a 3.5 box climate
Moist
model consisting of the extratropics and the dry
and moist Tropics, the latter further divided into a Moist 50% 0.27 208 84.3
cloudy-moist region containing upper-level cirrus (Cldm+
cloud and a clear-moist region without this high cir-
Clrm)
rus. The GMS instrument provides only limited
information on the cloud properties. Thus, LCH
4.2 Improved Cloudy Moist Definitions
used the brightness temperature at 11 pm, Tll <
260 K, as an indicator of the cloudy moist region. The use of a brightness temperature threshold
Further, LCH used a fairly subjective approach to to identify upper level cirrus cloud is problematic.
select the radiative properties of each box in the When the brightness temperature threshold is set
climate model, subject to the constraint of match- very low, to remove all water clouds, it will also
ing Earth Radiation Budget Experiment (ERBE, miss thinner cirrus clouds. When the threshold is
Barkstrom et al., 1989) global and Tropical mean too high, water clouds will be selected along with
values. Previous studies (Fu et al., 2002 and Lin et the cirrus clouds. Chou et al. (2002) indicated that
al., 2002) suggest that the LCH LW flux and SW T11 < 260K was merely an index for the variation of
thecloudymoistregiona,ndLCHactuallyusedan
of ways of defining the cloudy moist region. These
areafractionabouttwicethatoftheT11<260K
include cloud temperature thresholds, Tc < -15 C
regionforthecloudymoistbox.TheCERESSSF and Tc < -30 C and phase thresholds of Phase >
containsvastlyimprovedinformationon cloud
1.5 (more than half of cloud in footprint is ice) and
propertiesin,cludingcloudtemperaturaendcloud
Phase > 1.9 (cloud in footprint is predominantly
particlecondensatiopnhase(i.e.,wateror ice),
ice). Since cirrus over cumulus is expected to
usingassociateVdIRSimagerdata.Asa result,
occur often in the Tropics, a second set of tests is
retrievedcloudpropertiesthemselvescan be
also performed using only the properties of the
examinetdoobtaina betteridentificatioonfcirrus
upper cloud layer, if that layer covers more than
cloud.Twobasicapproacheasreconsiderehdere:
half the CERES footprint. This captures any
1) TOtestsfind CERESfootprintswherethe
occurrence of extensive and identifiable high cirrus
retrievedcloudtemperatur(eaccountinfgorthe
cloud in the same footprint with identifiable low
emissivityof thinclouds)is suggestiveof ice
cloud. The net flux is quite insensitive to the defini-
clouds2;)cloudwater/icephasetestsfindCERES
tion used, including restriction of the results by
footprintisnwhichthecloudismostlyorpredomi-
footprint cloud fraction. About 85% of the cloudy
nantlyice.
moist pixels are 99% or more cloud covered, con-
InitialvalidationstudiesoftheCERESSSF
firming that cirrus cloud tends to be spatially exten-
cloudpropertiessuggestthatthe TOretrieval sive.
placessinglelayercirruscloudswithin1kmoftheir
actuahl eightw; hilethephaseretrievaisl excellent 5. CLOUD FEEDBACK CALCULATIONS
forsinglelayerclouds(Minnis,2002,personal
communication).Comparisonto groundtruth The radiative properties and area fractions
resultsshowsnoevidencethattheCERESalgo- from the CERES SSF analysis can be inserted in
rithmismissinganythingmorethanisolatedthin the simple climate model used by LCH. As in their
cirrus(0-5%cloudfraction;Chamberset al., study, the tropical cloudy moist area is varied +/-
2002b). Forcirrusoverlowwatercloud,the 30%, as Acldm=Acldm0(l+B), where p=-0.3 to 0.3.
retrievailsnotsowellunderstoodS.omehighcir- The clear moist area, Acm=Acm0(l+T# ), may vary
rusoverlowcloudmaybemissedormis-classifiedthrough a range of dynamics, from following the
inthisanalysis.Howevegr,iventhelackofsensi- change in the cloudy moist area (7=1) to remaining
tivityintheresultsbelow,itdoesnotappearthat constant (7=0).
thiscouldbeamajoirssue.
29O
140
-- 7=1
.... 7=0.5
....... 7=0
Tc<-15 C
120 _ _ Tc<-30C 289
" _ Ph> 1.5
_ Ph> 1.9 t S:
_100 _ LCH QE.. t S"
--¢ -- -D-- - Tc<-15C
1-288
iT 80 _ -- -A-- - T¢<-30C
-- -D- - Ph>l.5 o
:_ [ -- _ - Ph>l.9 ,s 4
287
LCH Vahles{ _
40
20
286 i i i i i i i i i i i
0.1 0.2 0.3
Tropical Cloudy MoistFraction
Threshold CloudFraction Fig. 2 Surface temperature response in 3.5 box model
using various means of defining radiative properties.
Fig. 1 Netflux computed foravariety ofdefinitions of the
cloudy moist region. Solid symbols for results from foot- The global surface temperature response from
print-mean cloud properties. Open symbols for results a change inthe cloudy moist area fraction is shown
from upper layer properties. in Fig. 2. The strong negative feedback found by
LCH is reduced to a much weaker feedback. This
Figure 1shows net radiative fluxes for avariety
feedbackmaybeeitherslightlypositiveorslightly
negativedependinognwhichdefinitioinsusedfor 7. REFERENCES
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