ice cloud optical depth from modis cirrus reflectance

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IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 4, NO. 3, JULY 2007 471 Ice Cloud Optical Depth From MODIS Cirrus Reflectance Kerry Meyer, Ping Yang, and Bo-Cai Gao Abstract—An algorithm has been developed to infer ice cloud optical depth from the isolated cirrus reflectance derived from Moderate Resolution Imaging Spectroradiometer (MODIS) observations. The present method is a modification of a previous study, which, due to limitations in the assumed ice particle habit distribution, limited ice cloud optical depth retrieval to the tropics. Here, the bulk scattering properties of ice clouds are updated, utilizing the new ice crystal size and habit distributions developed for the latest MODIS Collection 5 operational cloud retrieval algorithm. The cirrus reflectance parameter, which is derived from reflectance measurements in the 0.66- and 1.375-µm spectral bands, is highly sensitive to ice cloud optical depth, allowing for op- tical depth retrieval. In the retrieval, an effective particle diameter of 50 µm is used, representing the peak of the global distribution of the effective particle sizes from the MODIS operational cloud products. The applicability of the algorithm is illustrated using daily cirrus reflectance data from the MODIS level-3 data set. The present method, which derives the contribution to optical depth by ice clouds only, is complementary to the operational MODIS ice cloud retrieval, which provides a total column optical depth. Index Terms—Atmosphere, cirrus clouds, remote sensing. I. I NTRODUCTION I CE CLOUDS (e.g., cirrus and contrails) are observed quite frequently [1]–[3]; yet, they are one of the least understood components of the Earth’s radiative budget. Previous studies have found that such clouds may influence the long-wave radiation budget near the tropical tropopause [4], [5]. It has been suggested that cirrus clouds can have a warming effect on the atmosphere [6] and contrail cirrus can induce tropospheric warming over the U.S. [7]. Due to the radiative importance of ice clouds, several recent studies have been undertaken to investigate their microphysical and radiative properties. Mea- surements from airborne sensors such as the High-resolution Interferometer Sounder (HIS) have been used to retrieve cir- rus properties [8]. Currently, ice cloud properties can be re- trieved from measurements by several space-borne instruments, Manuscript received December 11, 2006; revised March 5, 2007. This work was supported in part by the National Aeronautics and Space Administration (NASA) under Grant NNG04GL24G and Grant NNG05GL78G, by National Science Foundation CAREER Award ATM-0239605, and by the U.S. Office of Naval Research. The work of K. Meyer is supported in part by the NASA Earth System Science Fellowship under Grant NNG04GQ92H. K. Meyer and P. Yang are with the Department of Atmospheric Sci- ences, Texas A&M University, College Station, TX 77843 USA (e-mail: [email protected].). B.-C. Gao is with the Remote Sensing Division, Naval Research Laboratory, Washington, DC 20375 USA. Digital Object Identifier 10.1109/LGRS.2007.897428 notably the Moderate Resolution Imaging Spectroradiometer (MODIS) [9], [10]. Of major importance in radiative studies is the knowledge of cloud microphysical properties such as optical depth. Cir- rus reflectance in the visible is quite sensitive to ice cloud optical depth. Previously, a technique was introduced [11] to derive the daytime tropical ice cloud optical depth from the MODIS cirrus reflectance data product. The cirrus reflectance product [12] is operationally derived from radiances measured with MODIS channels centered near 0.66 and 1.375 µm. The 1.375-µm channel has unique characteristics, namely, incoming solar radiation is reflected and scattered by ice particles and strongly absorbed by water vapor beneath cirrus clouds. As a result, this channel is most sensitive to the upper level cirrus clouds with little sensitivity to the lower level water clouds and surfaces. On the other hand, this channel is slightly affected by absorption due to water vapor above and within cirrus clouds. In order to make quantitative use of the 1.375-µm channel, Gao et al. [12] developed an empirical approach using a combination of the 0.66- and 1.375-µm channels to estimate the upper level water vapor absorption effect and to derive the reflectance in the visible for the upper level cirrus clouds. From the visible cirrus reflectances in the tropical region, the “pure” ice cloud optical depths (typically with negligible contributions from the low level water clouds and clear surfaces) can be derived [11]. It should be pointed out that, under some extreme dry atmospheric conditions such as over the northern African deserts and Tibetan Plateau, the MODIS 1.375-µm channel can be slightly contaminated by the lower level water clouds and surfaces. In this letter, the tropical ice cloud optical depth retrieval technique of Meyer et al. [11] is updated to include the ice cloud bulk scattering properties developed for the latest MODIS cloud products (Collection 5) and is therefore extended for global retrievals. Reflectance data taken from the MODIS Collection 5 data set are used here. The present method is complementary to the operational MODIS cloud optical depth retrieval algorithm [10], [13] that simultaneously retrieves optical depth and cloud particle effective size for both water and ice clouds. II. METHOD The present method follows a simple algorithm to retrieve ice cloud optical depth. Visible cirrus reflectance data, which were taken from the MODIS atmosphere product, are converted to ice cloud optical depth by use of precalculated lookup tables. A library of such lookup tables is constructed from radiative 1545-598X/$25.00 © 2007 IEEE

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Page 1: Ice Cloud Optical Depth From MODIS Cirrus Reflectance

IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 4, NO. 3, JULY 2007 471

Ice Cloud Optical Depth FromMODIS Cirrus Reflectance

Kerry Meyer, Ping Yang, and Bo-Cai Gao

Abstract—An algorithm has been developed to infer icecloud optical depth from the isolated cirrus reflectance derivedfrom Moderate Resolution Imaging Spectroradiometer (MODIS)observations. The present method is a modification of a previousstudy, which, due to limitations in the assumed ice particle habitdistribution, limited ice cloud optical depth retrieval to the tropics.Here, the bulk scattering properties of ice clouds are updated,utilizing the new ice crystal size and habit distributions developedfor the latest MODIS Collection 5 operational cloud retrievalalgorithm. The cirrus reflectance parameter, which is derivedfrom reflectance measurements in the 0.66- and 1.375-µm spectralbands, is highly sensitive to ice cloud optical depth, allowing for op-tical depth retrieval. In the retrieval, an effective particle diameterof 50 µm is used, representing the peak of the global distributionof the effective particle sizes from the MODIS operational cloudproducts. The applicability of the algorithm is illustrated usingdaily cirrus reflectance data from the MODIS level-3 data set. Thepresent method, which derives the contribution to optical depthby ice clouds only, is complementary to the operational MODISice cloud retrieval, which provides a total column optical depth.

Index Terms—Atmosphere, cirrus clouds, remote sensing.

I. INTRODUCTION

ICE CLOUDS (e.g., cirrus and contrails) are observed quitefrequently [1]–[3]; yet, they are one of the least understood

components of the Earth’s radiative budget. Previous studieshave found that such clouds may influence the long-waveradiation budget near the tropical tropopause [4], [5]. It hasbeen suggested that cirrus clouds can have a warming effect onthe atmosphere [6] and contrail cirrus can induce troposphericwarming over the U.S. [7]. Due to the radiative importanceof ice clouds, several recent studies have been undertaken toinvestigate their microphysical and radiative properties. Mea-surements from airborne sensors such as the High-resolutionInterferometer Sounder (HIS) have been used to retrieve cir-rus properties [8]. Currently, ice cloud properties can be re-trieved from measurements by several space-borne instruments,

Manuscript received December 11, 2006; revised March 5, 2007. This workwas supported in part by the National Aeronautics and Space Administration(NASA) under Grant NNG04GL24G and Grant NNG05GL78G, by NationalScience Foundation CAREER Award ATM-0239605, and by the U.S. Office ofNaval Research. The work of K. Meyer is supported in part by the NASA EarthSystem Science Fellowship under Grant NNG04GQ92H.

K. Meyer and P. Yang are with the Department of Atmospheric Sci-ences, Texas A&M University, College Station, TX 77843 USA (e-mail:[email protected].).

B.-C. Gao is with the Remote Sensing Division, Naval Research Laboratory,Washington, DC 20375 USA.

Digital Object Identifier 10.1109/LGRS.2007.897428

notably the Moderate Resolution Imaging Spectroradiometer(MODIS) [9], [10].

Of major importance in radiative studies is the knowledgeof cloud microphysical properties such as optical depth. Cir-rus reflectance in the visible is quite sensitive to ice cloudoptical depth. Previously, a technique was introduced [11] toderive the daytime tropical ice cloud optical depth from theMODIS cirrus reflectance data product. The cirrus reflectanceproduct [12] is operationally derived from radiances measuredwith MODIS channels centered near 0.66 and 1.375 µm. The1.375-µm channel has unique characteristics, namely, incomingsolar radiation is reflected and scattered by ice particles andstrongly absorbed by water vapor beneath cirrus clouds. As aresult, this channel is most sensitive to the upper level cirrusclouds with little sensitivity to the lower level water clouds andsurfaces. On the other hand, this channel is slightly affectedby absorption due to water vapor above and within cirrusclouds. In order to make quantitative use of the 1.375-µmchannel, Gao et al. [12] developed an empirical approach usinga combination of the 0.66- and 1.375-µm channels to estimatethe upper level water vapor absorption effect and to derive thereflectance in the visible for the upper level cirrus clouds. Fromthe visible cirrus reflectances in the tropical region, the “pure”ice cloud optical depths (typically with negligible contributionsfrom the low level water clouds and clear surfaces) can bederived [11]. It should be pointed out that, under some extremedry atmospheric conditions such as over the northern Africandeserts and Tibetan Plateau, the MODIS 1.375-µm channelcan be slightly contaminated by the lower level water cloudsand surfaces.

In this letter, the tropical ice cloud optical depth retrievaltechnique of Meyer et al. [11] is updated to include the ice cloudbulk scattering properties developed for the latest MODIS cloudproducts (Collection 5) and is therefore extended for globalretrievals. Reflectance data taken from the MODIS Collection 5data set are used here. The present method is complementary tothe operational MODIS cloud optical depth retrieval algorithm[10], [13] that simultaneously retrieves optical depth and cloudparticle effective size for both water and ice clouds.

II. METHOD

The present method follows a simple algorithm to retrieve icecloud optical depth. Visible cirrus reflectance data, which weretaken from the MODIS atmosphere product, are converted toice cloud optical depth by use of precalculated lookup tables.A library of such lookup tables is constructed from radiative

1545-598X/$25.00 © 2007 IEEE

Page 2: Ice Cloud Optical Depth From MODIS Cirrus Reflectance

472 IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 4, NO. 3, JULY 2007

Fig. 1. Normalized scattering phase functions for the 18 effective diameters(from 10 to 180 µm) used in this study.

transfer calculations using the discrete ordinates radiative trans-fer (DISORT) method [14].

A. Bulk Scattering Properties

Constructing the lookup library necessary for ice cloudoptical depth retrieval requires realistic bulk scattering prop-erties of ice clouds for the radiative transfer calculations.To remain consistent with the operational MODIS products,the bulk scattering properties [15], [16] developed for theMODIS Collection 5 algorithms are used here, representingthe primary difference between the current method and that ofMeyer et al. [11]. These scattering properties were developedassuming a habit distribution or a mixture of idealized icecrystal shapes, consisting of bullet rosettes, solid and hollowcolumns, plates, aggregates, and droxtals.

The bulk scattering properties are computed by averagingthe single-scattering properties of the individual ice crystalhabits over the prescribed habit distribution and ice particle sizedistributions obtained from in situ observations. Included areparameters such as the mean effective diameter, extinction effi-ciency, single-scattering albedo, asymmetry factor, and fractionof delta transmission, as well as the normalized phase functioncomputed at 498 scattering angles (from 0◦ to 180◦). The bulkscattering properties used here are computed for 18 effectivediameters, ranging from 10 to 180 µm [15], [16].

Fig. 1 shows the normalized scattering phase functions of all18 effective diameters for the band centered at 0.66 µm. Graylines indicate effective diameters less than or equal to 60 µm.Note the presence of the 22◦ and 46◦ halos (peaks in the phasefunction)—phenomena resulting from the hexagonal shapes ofthe ice particles. The strong forward peaks exhibited here resultfrom the relatively large size of ice crystals compared to thewavelengths. For the DISORT calculations used to compute thelookup library, these forward peaks are truncated, and the phasefunctions are expanded in terms of Legendre polynomials usingthe δ-fit method [17] with 32 streams. These bulk scattering

Fig. 2. Histogram of retrieved ice particle effective diameter from fouryears of Aqua MODIS observations (September 2002, through August 2006).Effective diameter data are taken from the operational MODIS atmosphereproduct.

properties offer a significant improvement over previous ver-sions [15], [16].

B. Lookup Library

The retrieval lookup library consists of 4864 precalculatedlookup tables, each categorized by the geometrical configura-tion of the Sun and the satellite (i.e., the solar/satellite zenithangles and relative azimuth). Each lookup table consists of vis-ible ice cloud reflectance values calculated for 23 optical depthsranging from 0.002 to 100.0. To effectively simulate visiblecirrus reflectance, the lookup tables, which were constructedfrom DISORT calculations, are generated under the assumptionof a single-layer ice cloud in a transparent atmosphere withno surface reflectance; i.e., the reflection and scattering in thissystem are due only to the ice cloud layer.

Under the above habit distribution, DISORT calculationsreveal that cirrus reflectance values are sensitive to ice particleeffective size. This sensitivity results in a shift in the lookup ta-ble when moving from one effective size to the next. To accountfor this, an effective particle size is assumed for all ice clouds.Previously, it was found that the peak in the global distributionof ice cloud particle effective diameter (Deff), which wasobtained from the MODIS Collection 004 data set, is around50 µm [18]. Fig. 2, which depicts a histogram of effectivediameters obtained from four years of the Aqua MODIS level-3Collection 5 atmosphere product, reveals similar results. Forthis letter, the scattering properties associated with an ef-fective diameter of 50 µm are used to compute the lookuptables.

Fig. 3 shows a sample lookup table (the solid curve) gen-erated for an effective particle diameter of 50 µm. The solarand satellite zenith angles are 30◦ and 0◦, respectively, andthe relative azimuth angle is 60◦. Uncertainty estimates, whichare expressed in terms of the standard deviation of ice particleeffective size (σdeff = 17.0 µm, calculated from the data inFig. 2), are shown as horizontal lines. The solid horizontal linescorrespond to uncertainties due to errors in effective diameterof ±σdeff , the dashed lines to errors of ±2σdeff . When cirrusreflectance is small, the absolute uncertainties in optical depth

Page 3: Ice Cloud Optical Depth From MODIS Cirrus Reflectance

MEYER et al.: ICE CLOUD OPTICAL DEPTH FROM MODIS CIRRUS REFLECTANCE 473

Fig. 3. Sample lookup table for solar and satellite zenith angles of 30◦ and0◦, respectively and a relative azimuth angle of 60◦, assuming an ice particleeffective diameter of 50 µm. Solid (dashed) horizontal lines denote opticaldepth uncertainties due to ±1 standard deviation (±2 standard deviation) errorsin effective diameter.

are significantly less than when reflectance is large. Relativeuncertainties corresponding to ±σdeff errors range fromroughly −14% to 9%.

III. RESULTS

The applicability of the present method is demonstratedusing cirrus reflectance data from the MODIS on Aqua. Level-3, or global, retrievals are shown here, though retrievals usingthe more localized level-2 (1-km resolution) data set are possi-ble. The level-3 data set has 1◦ spatial resolution and consistsof parameters derived from the level-2 retrievals.

The retrieval itself is straightforward. Because measuredcirrus reflectance is dependent on scattering angle, theSun/satellite view geometry is used to select the appropriatelookup table for each data point. The visible cirrus reflectanceis then matched with the corresponding ice cloud optical depthalong the curve in the lookup table. For example, using thelookup table shown in Fig. 3, a cirrus reflectance of 0.25roughly corresponds to an ice cloud optical depth of around 5.

Fig. 4(a) shows the derived MODIS cirrus reflectance ob-served by Aqua on April 23, 2006. The image is scaled asshown by the color bar. Black regions denote missing data,whereas violet regions have cirrus reflectance at or near zero.Red regions correspond to cirrus reflectance values of 0.2 orgreater. Note the absence of data in the southern polar regiondue to the daytime-only aspect of this retrieval. Furthermore,orbital tracks are clearly evident in the tropics.

Fig. 4(b) shows the retrieved ice cloud optical depth corre-sponding to the reflectance data in Fig. 4(a). Similar to Fig. 4(a),black regions denote missing data, violet regions denote opticaldepths at or near zero, and red regions denote optical depths of5 or greater. In this image, the tropical region appears to haveless missing data than in that of Fig. 4(a). This is becausedata points with cirrus reflectance equal to zero are reportedas missing data in the operational data set. Here, sensor pixelcounts are used to identify such data points, which are then setto zero.

Fig. 4(c) shows the operationally retrieved MODIS ice cloudoptical depth for April 23, 2006. Note that, especially over the

Fig. 4. (a) Derived cirrus reflectance taken from Aqua on April 23, 2006.Regions of black denote missing data. (b) Retrieved optical depth correspond-ing to the cirrus reflectance image shown in (a). (c) MODIS operational icecloud optical depth for the same day.

tropics, the observable patterns in Fig. 4(b) and (c) are similar.Note also that this image has larger values of optical depththan those in Fig. 4(b). This is because the MODIS operationalretrieval is a total column optical depth, whereas the presentmethod is limited exclusively to the contribution by ice clouds.

Fig. 5(a) shows the derived MODIS cirrus reflectance ob-served by Aqua on October 23, 2006. This image is scaledsimilar to that in Fig. 4(a). Note here, in contrast to Fig. 4(a), themissing data over the northern polar region due to the seasonalsolar patterns. Orbital tracks are clearly evident as well.

Fig. 5(b) shows the retrieved ice cloud optical depth cor-responding to the reflectance data in Fig. 5(a). This image isscaled similar to that in Fig. 4(b). As in Fig. 4(b), there is lessmissing data over the tropics than is illustrated by the cirrusreflectance image.

Fig. 5(c) shows the MODIS operational ice cloud opti-cal depth for October 23, 2006. Much like the images inFig. 4, this image has larger values of optical depth than thoseshown in Fig. 5(b). Again, the location and patterns of iceclouds, especially across the tropics, compare well betweenFig. 5(b) and (c).

IV. SUMMARY

A method has been developed, following a simple lookuptable approach, to globally retrieve ice cloud optical depth from

Page 4: Ice Cloud Optical Depth From MODIS Cirrus Reflectance

474 IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 4, NO. 3, JULY 2007

Fig. 5. (a) Derived cirrus reflectance taken from Aqua on October 23, 2006.Regions of black denote missing data. (b) Retrieved optical depth correspond-ing to the cirrus reflectance image shown in (a). (c) MODIS operational icecloud optical depth for the same day.

the derived visible cirrus reflectance in the MODIS atmosphereproduct. This method is an update of a previous techniquethat limited optical depth retrieval to the tropics. For consis-tency with the MODIS operational retrieval algorithms, thenew bulk scattering properties of ice clouds developed for theCollection 5 data set are used here. Radiative transfer cal-culations revealed that visible cirrus reflectance is dependentnot only on optical depth, but also on ice particle effectivesize as well. Because of this, an effective diameter of 50 µm,corresponding to the peak in the histogram of MODIS globallyderived effective particle diameters, is assumed for lookuptable calculations. The applicability of the algorithm is thenillustrated with two sample cases from the Aqua satellite. Themain advantage of the present method is that the retrievedice cloud optical depths are mostly attributable to upper levelcirrus clouds, whereas both upper level cirrus clouds and lowerlevel water clouds can sometimes contribute to the ice cloudoptical depths derived from the present MODIS operationalcloud algorithms.

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