Early Results from the MODIS Cloud Algorithms

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cloud detection optical, microphysical, and cloud top properties. Early Results from the MODIS Cloud Algorithms. S. Platnick 5,2 , S. A. Ackerman 1 , M. D. King 2 , W. P. Menzel 3,1 , B. A. Baum 4,1 , et al. - PowerPoint PPT Presentation


  • Early Results from theMODIS Cloud Algorithms cloud detectionoptical, microphysical, and cloud top propertiesS. Platnick5,2, S. A. Ackerman1, M. D. King2, W. P. Menzel3,1, B. A. Baum4,1, et al.AGU Fall Meeting San Francisco, CA December 19, 20001 U. Wisconsin/CIMSS, 2 NASA GSFC, 3 NOAA NESDIS, 4 NASA LaRC, 5 UMBC/JCET

  • OutlineMODIS - a quick introductionMODIS cloud productsAlgorithm descriptions and example retrievalsData statusS. Platnick, AGU, 19 Dec. 2000MODIS (MODerate resolution Imaging Spectroradiomter)

  • Filter radiometer, 4 detector arrays, 36 spectral bands; 250 m, 500 m, 1 km spatial resolutionOnboard calibration via Solar Diffuser/Stability Monitor, Spectral Radiometric Calibration Assembly instrumentsFirst light on 24 February 2000 (Terra launch 18 Dec 1999)Science teams organized into atmosphere, land, and ocean discipline groupsMODIS instrument highlightsS. Platnick, AGU, 19 Dec. 2000

  • MODIS Atmosphere Global 19 April 2000L1B True color RGB (0.65, 0.56, 0.47 m bands)S. Platnick, AGU, 19 Dec. 2000example data granule coverage (5 min)

  • Pixel level products (Level 2) overview Cloud mask for determining clear-sky Cloud top properties Cloud optical, microphysical properties (optical thickness, effective particle size, water path, thermodynamic phase, cirrus reflectance)Unique aspects New algorithms from greater spectral coverage Heritage algorithms at higher spatial resolution Includes QA (processing and assessment info)MODIS cloud products, granule levelS. Platnick, AGU, 19 Dec. 2000

  • Gridded time-average products (Level 3) Daily, 8 day, monthly composites containing all atmosphere products 1 x 1 equal angle grid mean, standard deviation, marginal and joint probability distributions quick look available on the web

    details at the MODIS atmosphere web pagehttp://modis-atmos.gsfc.nasa.govMODIS cloud products, global compositesS. Platnick, AGU, 19 Dec. 2000

  • Cloud mask(S. Ackerman, R. Frey, K. Strabala U. Wisconsin/CIMSS)Bottom of the algorithmic food chain, input to all MODIS products. 1 km nadir spatial resolution day & night, (250 m day) 17 spectral bands (0.55-13.93 m, incl. 1.38 m) 11 spectral tests (function of 5 ecosystems)temporal consistency test over ocean, desert (nighttime) spatial variability test over ocean48 bits per pixel including individual test results and processing path; generation of clear sky mapsBits 1,2 give combined test results as: confident clear, probably clear, uncertain, obstructed/cloudy (clear sky conservative)S. Platnick, AGU, 19 Dec. 2000

  • Cloud mask, cont. Spectral tests use fuzzy thresholds, examples include low cloud tests: 3.9 m - 11 m BT high cloud tests: 13.9 m (CO2), 1.38 m (H2O), 11-12 m BT 1.6 m snow/ice test NIR/VIS reflectance test; IR tests (dependent on sfc emissivity, PW, aerosols); et al.S. Platnick, AGU, 19 Dec. 2000Ackerman, S. A. et al. 1998: JGR, 103, 32141-32157.

  • Cloud mask, validation activities Mask consistent with radar/lidar cloud boundary measurements at Oklahoma ARM CART site and ER-2 observations during spring 2000 campaign (including correct snow identification).Improvements being made for sun glint, warm cloud in arid ecosystems, Antarctica, nighttime low cloud over land, nighttime snow/ice surfacesRegional/global validation is ongoing. Gobal clear sky composite images being used to identify problem regions.S. Platnick, AGU, 19 Dec. 2000

  • aaMODIS cloud mask example(confident clear is green, probably clear is blue, uncertain is red, cloud is white) 1.6 m image0.86 m image11 m image3.9 m imagecloud maskSnow test(impacts choice of tests/thresholds)VIS test(over non-snowcovered areas)3.9 - 11 BT test for low clouds 11 - 12 BT test (primarily for high cloud) 13.9 m high cloud test(sensitive in cold regions)

  • MODIS 5-8 September 2000Band 31 (11.0 m) Daytime Clear sky Brightness Temperature

  • MODIS 5-8 September 2000Band 1, 4, 3 (R/G/B) Daytime Clear sky Reflectance Composite

  • Cloud top properties(P. Menzel, R. Frey, K. Strabala, L. Gumley, et al. NOAA NESDIS, U. Wisc./CIMSS)Cloud top pressure, temperature, effective emissivity Retrieved for every 5x5 box of 1 km FOVs, when at least 5 FOVs are cloudy, day & nightCO2 Slicing technique (5 bands, 12.0-14.2 m) ratio of cloud forcing in 2 nearby bandsretrieve pc; Tc from temperature profilemost accurate for high and mid-level cloudsPreviously applied to HIRS (NOAA POES, 20 km). MODIS 1st satellite sensor capable of CO2 slicing at high spatial resolution.S. Platnick, AGU, 19 Dec. 2000

  • Cloud top properties, validation activities Activities proceeding via early ER-2 effort (March 2000 with lidar and HIS IR interferometer), and NOAA HIRS intercomparisonsCloud top pressure compares well with HIRS and aircraft validation,better than 50 mb.S. Platnick, AGU, 19 Dec. 2000Frey, R. A. et al, 1999: JGR , 104, 24547-24555.

  • CO2 slicingTechnique:- ratio of cloud forcing at two near-by wavelengths- effective emissivity includes cloud fraction in 5x5 box- actual cloud emissivity assumed same for each band- radiance gradient used when clear sky not availableThe more absorbing bands are more sensitive to high clouds, weighting functions S. Platnick, AGU, 19 Dec. 2000Frey, R. A. et al, 1999: A comparison of cloud top heights computed from airborne lidar and MAS radiance data. J. Geophys. Res., 104, 24547-24555.

  • Cloud Mask MODIS 12 March 2000, 1730 UTCsnowclear=green cloud=white uncertain=redMODIS band 3111 mCloud MaskARM CART site

  • Cloud Top Pressure MODIS 12 March 2000, 1730 UTCMODIS band 3111 mCloud top pressure900-1000 mb=purple 500-600 mb=blue 300-400 mb=redARM CART site

  • Comparison of ER2 lidar (nadir view), HIRS (3 hrs later), RAOB, & MODIS Cloud Propertiesover ARM CART site, Oklahomalidareffective emissivity vs. HIRSCTP vs. HIRS

  • Cloud top PressureCloud top TemperatureCloud FractionCloud Effective EmissivityMODIS 5-8 September 2000

  • IR thermodynamic phase algorithm (B. Baum, S. Ackerman, K. Strabala NASA LaRC, U.W. CIMSS)Tri-spectral method, 5 km resolution

    NIR, MWIR reflectance technique being developedS. Platnick, AGU, 19 Dec. 2000ice cloudApril 1996Successwater cloudJan 1993TOGA/COAREStrabala, Menzel, and Ackerman, 1994, JAM, 2, 212-229.Baum et al, 2000, JGR, 105, 11781-11792.

  • IceWaterMixed PhaseUncertainMODIS cloud thermodynamic phase - IR algorithmClouds over Southern India19 April 2000VISIR windowcloud phase

  • MODIS 5-8 September 2000IR retrieval - percent liquid water

  • MODIS 5-8 September 2000IR retrieval - percent ice water

  • MODIS IR phase retrieval vs. Cloud Top Temperature frequency of ice phase & Tc < 253 Kstatistics from 5 Sept day and night, 60N-60S, water surface onlyfrequency (%)

  • frequency (%)MODIS IR phase retrieval vs. Cloud Top Temperature frequency of ice phase & 253< Tc< 273 Kstatistics from 5 Sept day and night, 60N-60S, water surface only

  • frequency (%)MODIS IR phase retrieval vs. Cloud Top Temperature frequency of ice phase & Tc> 273 Kstatistics from 5 Sept day and night, 60N-60S, water surface only

  • Cloud optical, microphysical properties(M. D. King, S. Platnick, M. Gray, E. Moody, J. Li, S.-C. Tsay, et al. NASA GSFC, UMBC)Optical thickness, particle size (effective radius), water path1 km spatial resolution, daytime only, liquid water and ice cloudsLand, ocean, snow/sea ice surfacesSolar reflectance technique, VIS through MWIR (0.65, 0.86, 1.2, 1.6, 2.1, 3.7 m)MODIS 1st satellite sensor with all usefulSWIR, MWIR bandsS. Platnick, AGU, 19 Dec. 2000

  • Cloud optical, microphysical properties, cont.Required input: cloud mask (tuned for cloudy not clear using individual cloud mask tests) cloud top temperature for 3.7 m retrieval cloud top pressure for atmospheric correction (being implemented) cloud phase (currently derived from individual cloud mask tests, not IR or solar tests) surface albedo (currently assigned from IGBPecosystem map & NISE snow/ice data set)Early validation effort as part of SAFARI 2000S. Platnick, AGU, 19 Dec. 2000

  • MODIS SAFARI granule RGB composite13 September 2000, 0925 UTCNamibiaEtosha PanAngolamarine stratocumulusER-2 ground trackNamibiaAngolaBotswanaSouth AfricaZambia

  • (>99%)(>95%)(>66%)

  • Data summaryAtmosphere L2 products processed on P.I. system at GSFC (except cloud mask)Cloud products archived at GSFC DAAC series starts at 8/20/00 for L2, 10/31/00 for L3(1d)Current archived products are beta release early data product, useful for familiarity with data formats/parameters, minimal validation, temporaryConsistent processing time series (instrument bias settings, L1B algorithm) underwayOrder through EOS Data Gateway - details at http://modis.gsfc.nasa.gov/DATA/S. Platnick, AGU, 19 Dec. 2000

  • Data summary, cont.Data file infoMOD35 cloud mask ~ 48 MB/granule daytimeMOD06 cloud product ~ 65 MB/granule daytime, 16 MB nighttime, 12 GB/day- L3 atmospheres ~ 400-800 MB/day- L1B ~ 340 MB/granule, 70 GB/dayL2 production system limits currently running at approximately 1x, not sufficient for reprocessing needs S. Platnick, AGU, 19 Dec. 2000

  • minimummaximumstandard deviationmeanL3 optical thickness (liquid water) statistics, 10/2/00, from atmo web page S. Platnick, AGU, 19 Dec. 2000

  • Algorithms summaryMODIS provides an unprecedented opportunity for cloud and other atmospheric studies 36 spectral channels, high spatial resolutionComprehensive set of cloud algorithmsArchive of pixel level retrievals, global statisticsProduct intercomparison for small number of selected day(s) proven useful Validation activities ongoing (gnd. based, in situ, aircraft, satellite intercomparisons, etc.); detailed plans on atmosphere web siteS. Platnick, AGU, 19 Dec. 2000

    Following example shows some of these tests applied to the upper midwest this past March, part of a validation effort with the ER2 and ARM CART ground site in Oklahoma.

    Mask is consistent with the ARM site and ER-2observations, including correct snow identification.Global validation is making use of clear sky composites to identify problem regions. Will eventually be used to tweak thresholds. Improvements being made for the expected problem cases.

    These particular 4 days were chosen as a set of standard days to test ongoing improvements to the algorithms.

    Regions where no clear sky observations were made during the 4 days are shaded gray.

    Note the high temperatures off south-western South America - this image was used to indicate a problem in L1B (personal conversation w/ Ack).

    Hard to see but color scale showing cloud top pressure in 100 mb layers, from the sfc to 300 mb,

    With the lowest cloud height purple, and the highest being red; green is the 400-500 mb layer.

    CHAPS = Collocated HIRS/2 and AVHRR Products

    Global composite from those same 4 days.Example from MAS where water and ice clouds are separated according to both the absolute BT differences as well as the slope.

    Solar refl. technique more problematic because of a greater dependence on particle shape, as well as solar geometry

    Fraction of ice clouds within a 25 km grid.

    With 4 day average, spatial structure of the storm tracks are smoothed out.

    Validation is of course extremely difficult, but one interesting sanity check is to compare phase results with cloud top temperature retrievals.

    Heres the probability of ice clouds when Tc is less then 253 K (plot from Bryan Baum, derived from a single day, Sept 5th, excluding the poles, and only over water).

    The preponderance of reds implies that perhaps 80% of clouds are ice below these temperatures.Probabilities are mostly quite low.

    Required input includes all the information just discussed.

    Uses the cloud mask, but tuned from individual tests - i.e., if the mask is thought of as a clear sky mask, its not obvious that a non-clear pixel is useful for a cloud retrieval.

    Cloud phase is currently determined from individual cloud mask tests while the IR tri-spectral algorithm is being evaluated (and the solar reflectance test is being developed).

    Hope to incorporate MODIS surface albedo product in the near future, meanwhile use the IGBP ecosystem database as a surrogate for surface albedo and for determining the combination of bands to use.

    (Ecosystem map = 10 resolution, will upgrade to 1, derived from 1km IGBP map)

    SAFARI Granule from Sept 13.

    Sc along the Namibian border associated with upwelling along the Benguela current - similar to California stratus. A very interesting area outflow of industrial pollution and fires from the continent is sometimes extreme and a rich phytoplankton source of sulfur in the waters.

    Corresponding ER-2 track for this day shown, note a couple of flight legs along the coast.

    While theres a limited amount of cloud over the land, this ecosystem map is used as a surrogate for the surface albedo, barren/desert along the coast, ...As a reminder, this is derived from individual cloud mask tests. Therefore, in the following retrievals, some cloud regions to the far north and south are processed as if theyre ice clouds.Looks like fairly good continuity over the coast.A subset of L3 statistics are available directly from the web, optical thickness example shown here.


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