cloud fraction: can it be defined and measured? …cloud fraction, anyway? clouds have a strong...

CLOUD FRACTION: CAN IT BE DEFINEDAND MEASURED? AND IF WE KNEW IT

WOULD IT BE OF ANY USE TO US?Stephen E. Schwartz

Upton NY USA

www.bnl.gov/envsci/schwartz

Cloud Properties, Observations, and their Uncertainties

CLOUD FRACTION: CAN IT BE DEFINED AND MEASURED? AND IF WE KNEW IT

WOULD IT BE OF ANY USE TO US?

CONCLUSIONSNo.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.

Warren, Hahn, London, Chervin, Jenne

Domain ObservationsMillions

Cloud cover%

Land 116 52.4Ocean 43.3 64.8Global 159 61.2

CLOUD FRACTION BY MULTIPLE METHODS 2 Surface, 3 satellite methods at U.S. Southern Great Plains; 10 years data

0.30.40.50.60.7

1.0˚ PATMOS!X0 6 12 18 24

ARSCL (Lidar, Radar)TSI (Total Sky Imager)

0.5˚ GOES1.0˚ PATMOS!X

2.5˚ ISCCP D1

2 4 6 8 10 12

98 00 02 04 06 08 10Year

All!S

ky C

loud F

racti

on

Local Hour Month

0.30.40.50.60.7

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

TSI (FOV 100°)

Ground

Satellite

Modified from Wu, Liu, Jensen, Toto, Foster & Long, JGR, in review

Domain 55-140 kmon a side

Total Sky Imager

Although different approaches yield different instantaneous, local CF, they would be expected to yield the same average CF.

MULTIPLE APPROACHES TO DETERMINING CLOUD FRACTION

TSI (FOV 100°)

Cloud Radars (FOV 0.2°; 3.5 mrad)

Ground

Satellite

Modified from Wu, Liu, Jensen, Toto, Foster & Long, JGR, in review

Cloud Lidars (FOV 80 µrad)

Domain 55-140 kmon a side

ARSCLActive Remote Sensing of CLouds

Total Sky Imager

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

Spatial resolution. 6% 25% 100%

Threshold.

1

0Clou

d Fr

actio

n

0.450.400.350.300.25Threshold

COMPARISON OF CLOUD FRACTION BY DIFFERENT METHODS

Hourly cloud fraction at SGP by multiple methods, May, 2009 ARSCL 1.0

0.8

0.6

0.4

0.2

0.01.00.80.60.40.20.0

1.0

0.8

0.6

0.4

0.2

0.01.00.80.60.40.20.0

TSI

GOES

ActiveRemoteSensing

by Radar,Lidar

TotalSky

Imager

Satellite, 0.5˚

1:1 line

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

1.0

0.8

0.6

0.4

0.2

0.01.00.80.60.40.20.0

CF GOES

1.0

0.8

0.6

0.4

0.2

0.01.00.80.60.40.20.0

CF GOES

All points, May, 2009 Points within 5% of 0 or 1 in both data sets excluded

r2 = 0.78 r2 = 0.44

CF

ARSC

L

CF

ARSC

LLeast squares fit

Excluding all-cloud and no-cloud scenes reduces variance accounted for by the regression from 78% to 44%.

n = 1346 n = 620

0

Cou

nts

10 CF GOES

0

Cou

nts

10ARSCL CF

TIME SERIES OF CLOUD FRACTION BY MULTIPLE METHODS

ARM SGP site (north central OK) May 13, 2009

Date, May, 2009, UTC

1

0.5

0

Clo

ud F

ract

ion

12005/13/09

00005/14/09

ARSCL

TSI GOES

Substantial variation among methods.

TIME SERIES OF CLOUD FRACTION BY MULTIPLE METHODS

ARM SGP site (north central OK) May 13, 2009

Date, May, 2009, UTC

1

0.5

0

Clo

ud F

ract

ion

12005/13/09

00005/14/09

ARSCL

TSI GOES

TSI 30 s

141600 141630 141700

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, 1416-1417141600 141630 141700

141600 141630 141700

TSI threshold misses thin visible cloudsSubstantial changes at 30-s intervals as clouds are blown by wind.

CLOUD RADIATIVE EFFECTDependence on shortwave optical depth and cloud-top temperature

24-Hour average CRE, north central Oklahoma, at equinox

-200

-100

0

100

0.01 0.1 1 10 100Cloud shortwave optical depth

Shortwave

-200

-100

0

100

200

0 02 04 06 08 001-10

-5

0

5

10

m W ,A

OT ta tceffE evitaidaR duol

C-2

20.0 40.0 60.0 80.0 01.00-60

-40

-20

0

20

40

60

0.0 2.0 4.0 6.0 8.0 0.1-200

-100

0

100

200

0 2 4 6 8 01

Cloud shortwave optical depthCRE is initially linear in optical depth, saturating at high optical depth.



-200

-100

0

100


Shortwave

-200

-100

0

100

200

100806040200-10

-5

0

5

10

m W ,A


C-2

0.100.080.060.040.020-60

-40

-20

0

20

40

60

1.00.80.60.40.20.0-200

-100

0

100

200

1086420

Cloud shortwave optical depth

Disk of sun no longer visible100 g m ⇔-3 100 μm LWP ⇔ 5 12-μm radius drops

Thin clouds

Thin clouds

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

Clou

d Fr

actio

n, %

Martins Noel & Chepfer, JGR, 2011



-200

-100

0

100


Shortwave Tct 280 260 240 220 K Longwave

-200

-100

0

100

200

0 02 04 06 08 001-10

-5

0

5

10

m W ,A


C-2

20.0 40.0 60.0 80.0 01.00-60

-40

-20

0

20

40

60

0.0 2.0 4.0 6.0 8.0 0.1-200

-100

0

100

200

0 2 4 6 8 01

Cloud shortwave optical depthLongwave CRE also initially linear; saturates; depends on cloud-top temp.



-200

-100

0

100


Shortwave Tct 280 260 240 220 K Longwave Net

-200

-100

0

100

200

0 02 04 06 08 001-10

-5

0

5

10

m W ,A


C-2

20.0 40.0 60.0 80.0 01.00-60

-40

-20

0

20

40

60

0.0 2.0 4.0 6.0 8.0 0.1-200

-100

0

100

200

0 2 4 6 8 01

Cloud shortwave optical depthNet CRE depends on optical depth and cloud-top temperature even in sign. Knowledge of cloud fraction tells you nothing about the cloud radiative effect.

Shortwave Tct 280 260 240 220 K Longwave Net

A FOOL’S ERRANDThreshold photometric determination of cloud fraction

Natural color photo

What is the cloud fraction?

A FOOL’S ERRANDThreshold photometric determination of cloud fractionNatural color photo Red/(Red + Blue)

Examine ratio Red/(Red + Blue), common cloud discrimination technique.


0.30, 86%Threshold, Cloud Fraction

Natural color photo Red/(Red + Blue)

Apply cloud mask with threshold R/(R+B) 0.30. Cloud fraction 86%. Threshold is too low.


0.40, 35%0.30, 86%Threshold, Cloud Fraction


Try threshold 0.40. Cloud fraction is 35%. That threshold is too high.


0.35, 63% 0.40, 35%0.30, 86%Threshold, Cloud Fraction


Try threshold 0.35. Cloud fraction is 63%.There is no “just right”. False positives and false negatives.


40x103

30

20

10

0

slexiP

1.0

0.8

0.6

0.4

0.2

0.0

noitcarF duolC

0.450.400.350.300.25

0.35, 63% 0.40, 35%0.30, 86%Threshold, Cloud Fraction


Examination of cloud fraction as function of Red/(Red + Blue) threshold. There is no unique threshold.

R/(R+B)

CLOUD FRACTION: CAN IT BE DEFINEDAND MEASURED? AND IF WE KNEW IT

WOULD IT BE OF ANY USE TO US?

CONCLUSIONSNo!No!No!

Cloud fraction is dead! Do not resuscitate.

cloud fraction: can it be defined and measured? …cloud fraction, anyway? clouds have a strong...

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