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CLOUD FRACTION: CAN IT BE DEFINED and MEASURED? and IF WE KNEW IT WOULD IT BE of ANY USE to US? Stephen E

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CLOUD FRACTION: CAN IT BE DEFINED and MEASURED? and IF WE KNEW IT WOULD IT BE of ANY USE to US? Stephen E CLOUD FRACTION: CAN IT BE DEFINED AND MEASURED? AND IF WE KNEW IT WOULD IT BE OF ANY USE TO US? Stephen E. Schwartz Upton NY USA Cloud Properties, Observations, and their Uncertainties www.bnl.gov/envsci/schwartz CLOUD FRACTION: CAN IT BE DEFINED AND MEASURED? AND IF WE KNEW IT WOULD IT BE OF ANY USE TO US? CONCLUSIONS No. No. No. I come to bury cloud fraction, not to praise it. - Shakespeare, 1599 WHAT IS A CLOUD? AMS Glossary of Meteorology (2000) A visible aggregate of minute water droplets and/or ice particles in the atmosphere above the earth’s surface. Total cloud cover: Fraction of the sky hidden by all visible clouds. Clothiaux, Barker, & Korolev (2005) Surprisingly, and in spite of the fact that we deal with clouds on a daily basis, to date there is no universal definition of a cloud. Ultimately, the definition of a cloud depends on the threshold sensitivity of the instruments used. Ramanathan, JGR (ERBE, 1988) Cloud cover is a loosely defined term. Potter Stewart (U.S. Supreme Court, 1964) I shall not today attempt further to define it, but I know it when I see it. WHY DO WE WANT TO KNOW CLOUD FRACTION, ANYWAY? Clouds have a strong impact on Earth’s radiation budget: -45 W m-2 shortwave; +30 W m-2 longwave. Slight change in cloud fraction could augment or offset greenhouse gas induced warming – cloud feedbacks. Getting cloud fraction “right” is an evaluation criterion for global climate models. Domain Observations Cloud cover Millions % Land 116 52.4 Ocean 43.3 64.8 Global 159 61.2 Warren, Hahn, London, Chervin, Jenne CLOUD FRACTION BY MULTIPLE METHODS 2 Surface, 3 satellite methods at U.S. Southern Great Plains; 10 years data 0.7 0.6 0.5 0.4 PATMOS−X 1.0˚ 0.3 0612182424681012 Local Hour Month 0.7 ARSCL (Lidar, Radar) 0.6 Sky Cloud Fraction TSI (Total Sky Imager) − 0.5 ISCCP D1 2.5˚ All 0.4 GOES 0.5˚ 0.3 PATMOS−X 1.0˚ 98 00 02 04 06 08 10 Year Wu, Liu, Jensen, Toto, Foster & Long, JGR, in review Different methods yield substantial systematic differences in the mean. Error of 0.1 in cloud fraction is ~ 9 W m-2 in shortwave, 6 W m-2 in longwave. MULTIPLE APPROACHES TO DETERMINING CLOUD FRACTION Satellite TSI (FOV 100°) Total Sky Imager Domain 55-140 km on a side Ground Modified from Wu, Liu, Jensen, Toto, Foster & Long, JGR, in review Although different approaches yield different instantaneous, local CF, they would be expected to yield the same average CF. MULTIPLE APPROACHES TO DETERMINING CLOUD FRACTION Active Remote Satellite ARSCL Sensing of CLouds Cloud Lidars (FOV 80 µrad) Cloud Radars (FOV 0.2°; 3.5 mrad) TSI (FOV 100°) Total Sky Imager Domain 55-140 km on a side Ground Modified from Wu, Liu, Jensen, Toto, Foster & Long, JGR, in review Although different approaches yield different instantaneous, local CF, they would be expected to yield the same average CF. REASONS FOR DIFFERENCES IN MEASURED CLOUD FRACTION Trivial Mismatch of spatial and/or temporal domain. View angle – sidewall effect – cloud aspect ratio. Intrinsic 6% 25% 100% Spatial resolution. 1 Threshold. Cloud Fraction 0 0.25 0.30 0.35 0.40 0.45 Threshold COMPARISON OF CLOUD FRACTION BY DIFFERENT METHODS Hourly cloud fraction at SGP by multiple methods, May, 2009 1.0 ARSCL Active 0.8 Remote 0.6 Sensing 0.4 by Radar, 0.2 1:1 line Lidar 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.0 TSI 0.8 Total 0.6 Sky 0.4 Imager 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 GOES Satellite, 0.5˚ Comparison plots show some skill but substantial differences. CORRELATION IS DOMINATED BY ONES AND ZEROES Hourly cloud fraction at SGP by ARSCL AND GOES, May, 2009 Points within 5% of 0 or 1 All points, May, 2009 in both data sets excluded 1.0 1.0 0.8 0.8 Counts 0.6 0.6 0 0 1 CF GOES Least squares fit 0.4 0.4 CF ARSCL n = 1346 CF ARSCL n = 620 0.2 0.2 Counts 0 2 2 0 1 r = 0.78 r = 0.44 ARSCL CF 0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 CF GOES CF GOES Excluding all-cloud and no-cloud scenes reduces variance accounted for by the regression from 78% to 44%. TIME SERIES OF CLOUD FRACTION BY MULTIPLE METHODS ARM SGP site (north central OK) May 13, 2009 1 ARSCL GOES TSI 0.5 Cloud Fraction 0 5/13/09 Date, May, 2009, UTC 5/14/09 1200 0000 Substantial variation among methods. TIME SERIES OF CLOUD FRACTION BY MULTIPLE METHODS ARM SGP site (north central OK) May 13, 2009 1 ARSCL GOES TSI TSI 30 s 0.5 Cloud Fraction 0 5/13/09 Date, May, 2009, UTC 5/14/09 1200 141600 141630 141700 0000 Substantial variation among methods. Substantial fluctuation in TSI images taken at 30-second intervals. TOTAL SKY IMAGES AND CLOUD MASKS FROM TSI ALGORITHM ARM SGP site (north central OK) May 13, 2009, 1416-1417 141600 141630 141700 141600 141630 141700 TSI threshold misses thin visible clouds Substantial changes at 30-s intervals as clouds are blown by wind. CLOUD RADIATIVE EFFECT Dependence on shortwave optical depth and cloud-top temperature 24-Hour average CRE, north central Oklahoma, at equinox 100 -2 0 Cloud R adiative Effect at T O A, W m -100 Shortwave -200 0.01 0.1 1 10 100 Cloud shortwave optical depth 10 200 60 200 40 5 100 100 20 0 0 0 0 -20 -100 -100 -5 -40 -200 -60 -200 -10 0 0.02 0.04 0.06 0.08 0.10 0.0 2.0 4.0 6.0 8.0 0.1 0 2 4 6 8 10 0 20 40 60 80 100 Cloud shortwave optical depth CRE is initially linear in optical depth, saturating at high optical depth. CLOUD RADIATIVE EFFECT Dependence on shortwave optical depth and cloud-top temperature 24-Hour average CRE, north central Oklahoma, at equinox 100 Thin clouds -2 0 Cloud R adiative Effect at T O A, W m -100 Shortwave Disk of sun no longer visible -200 100 g m-3⇔100 μm LWP ⇔ 5 12-μm radius drops 0.01 0.1 1 10 100 Cloud shortwave optical depth 10 200 60 200 40 Thin clouds 5 100 100 20 0 0 0 0 -20 -100 -100 -5 -40 -200 -60 -200 -10 0 0.02 0.04 0.06 0.08 0.10 0.0 0.2 0.4 0.6 0.8 1.0 0 2 4 6 8 10 0 20 40 60 80 100 Cloud shortwave optical depth Even clouds of very low optical depth exert substantial radiative effect. PERSISTENT VERY THIN CIRRUS AT MIDLATITUDE SITE Kienast-Sjögren et al., 9th Int. Symp. on Tropospheric Profiling, 2012 Optical depth of cirrus layer estimated from lidar return as 0.003 to 0.004. OPTICALLY THIN CLOUDS CAN BE PREVALENT IN TROPICS Subvisible cirrus detected by lidar from space, DJF 0.01 ≤ τ ≤ 0.03 Cloud Fraction, % Martins Noel & Chepfer, JGR, 2011 CLOUD RADIATIVE EFFECT Dependence on shortwave optical depth and cloud-top temperature 24-Hour average CRE, north central Oklahoma, at equinox 100 -2 0 Cloud R adiative Effect at T O A, W m -100 Shortwave Tct 280 260 240 220 K Longwave -200 0.01 0.1 1 10 100 Cloud shortwave optical depth 10 200 60 200 40 5 100 100 20 0 0 0 0 -20 -100 -100 -5 -40 -200 -60 -200 -10 0 0.02 0.04 0.06 0.08 0.10 0.0 2.0 4.0 6.0 8.0 0.1 0 2 4 6 8 10 0 20 40 60 80 100 Cloud shortwave optical depth Longwave CRE also initially linear; saturates; depends on cloud-top temp. CLOUD RADIATIVE EFFECT Dependence on shortwave optical depth and cloud-top temperature 24-Hour average CRE, north central Oklahoma, at equinox 100 -2 0 Cloud R adiative Effect at T O A, W m -100 Shortwave Tct 280 260 240 220 K Longwave -200 Net 0.01 0.1 1 10 100 Cloud shortwave optical depth 10 200 60 200 40 5 100 100 20 0 0 0 0 -20 -100 -100 -5 -40 -200 -60 -200 -10 0 0.02 0.04 0.06 0.08 0.10 0.0 2.0 4.0 6.0 8.0 0.1 0 2 4 6 8 10 0 20 40 60 80 100 Cloud shortwave optical depth Net CRE depends on optical depth and cloud-top temperature even in sign. Knowledge of cloud fraction tells you nothing about the cloud radiative effect. A FOOL’S ERRAND Threshold photometric determination of cloud fraction Natural color photo What is the cloud fraction? A FOOL’S ERRAND Threshold photometric determination of cloud fraction Natural color photo Red/(Red + Blue) Examine ratio Red/(Red + Blue), common cloud discrimination technique. A FOOL’S ERRAND Threshold photometric determination of cloud fraction Natural color photo Red/(Red + Blue) Threshold, Cloud Fraction 0.30, 86% Apply cloud mask with threshold R/(R+B) 0.30. Cloud fraction 86%. Threshold is too low. A FOOL’S ERRAND Threshold photometric determination of cloud fraction Natural color photo Red/(Red + Blue) Threshold, Cloud Fraction 0.30, 86% 0.40, 35% Try threshold 0.40.
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