objective theory some initial results - asr...norm. zen.rad. (red) cod(red) cod(red) clou dop tica...
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Stephen E. Schwartz and Dong HuangBrookhaven National Laboratory, Upton NY
[email protected] [email protected]://www.ecd.bnl.gov/steve
Daniela Viviana VladutescuNew York City College of Technology, CUNY
Short-Range Structure of Cloud Optical Depth in North Central Oklahoma Determined by High Resolution Photography From the Surface
Atmospheric Systems Research Science Team Meeting, May 2-5, 2016
RADIATION TRANSFER CALCULATIONS WITH DISORT
NORMALIZED ZENITH RADIANCE: Zenith radiance per incoming direct normal solar irradiance, function of cloud optical depth (COD) and solar zenith angle (SZA)
W m-2 nm-1 sr-1 per W m-2 nm-1, or sr-1
Rayleigh sky is dark. Clouds initially increase zenith radiance.
46
0.012
46
0.12
46
1rs ,ecnaidar dezilamro
N-1
0.001 0.01 0.1 1 10 100Cloud optical depth
cos (SZA) 1.00 0.95 0.90 0.85 0.80 0.70 0.60 0.50
Solid, 460 nmDashed, 640 nm
0.25
0.20
0.15
0.10
0.05
0.00Nor
m. r
adia
nce,
sr-1
1086420Cloud optical depth
Red Blue
CLOUD ALBEDO calculated from COD
Cloud albedo is nearly independent of color.Cloud albedo varies linearly with COD for COD 3.<~
2
4
68
0.1
2
4
68
1
Albe
do
0.01 0.1 1 10 100Cloud optical depth
cos(SZA) 1.00 0.95 0.90 0.85 0.80 0.70 0.60 0.50
BlueRed
10-5
10-4
10-3
10-2
10-1
100
Clo
ud a
lbed
o
0.01 0.1 1 10 100Cloud optical depth
cos(SZA) 1.00 0.95 0.90 0.85 0.80 0.70 0.60 0.50
BlueRed
Slope = 1
Red/(Red + Blue) a commonly used cloud discriminant:
Define RADIATIVE CLOUD FRACTION, a variable that ranges from 0 to 1 as COD increases:
Radiative cloud fraction can be determined on pixel-by-pixel basis.
COLOR: Sky is blue, clouds are white
rRRB(μ0, )= RRnz(μ0, )
RRnz(μ0, )+ RB
nz(μ0, )
0.60.50.40.30.20.10.0R
ed/(R
ed +
Blu
e)
1086420Cloud optical depth
cos(SZA) 1.00 0.95 0.90 0.85 0.80 0.70 0.60 0.50 0.30
Fr (μ0, ) rRRB(μ0, ) rRRB(μ0,0)3
1.21.00.80.60.40.20.0
Rad
iativ
e cl
oud
fract
ion
1086420Cloud optical depth
cos(SZA) 1.00 0.95 0.90 0.85 0.80 0.70 0.60 0.50 0.30
For Red = 640 nm and Blue = 460 nm, Red/(Red + Blue)ranges from about 0.22 to 0.52 with increasing COD.
CLOUD OPTICAL DEPTH by inverting radiance
0.2
0.1
0.00.050.00
6543210C
loud
opt
ical
dep
th
0.250.200.150.100.050.00Normalized zenith radiance, sr-1
640 nm, Red 460 nm, Blue
Cos (SZA) = 0.850.25
0.20
0.15
0.10
0.05
0.00Nor
mal
ized
zen
ith ra
dian
ce, b
lue,
sr-1
0.250.200.150.100.050.00
Normalized zenith radiance, red, sr-1
Descending,High COD
Ascending,Low COD
1:1 line
COD is not monotonic; inversion is limited to thin clouds, COD <~ 3.
APPROACHCharacterize cloud radiative properties at high resolution
from the surface.
Digital photography with long-focal-length lens.Camera facing upwards takes photos at regular intervals.Use radiative transfer model to relate counts in camera to
radiance and then to cloud optical depth, cloud albedo,radiative cloud fraction.
COMMERCIALLY AVAILABLEHIGH-RESOLUTION CAMERA
1200 mm equivalent 35 mm focal length; f / 5.6
vs.
$350Todd Vorenkamp, B&H Photo, NYC
$180,000Nominal resolution 6 µrad (6 mm at 1 km)
a
b
c
d
e
f
1680
1660
1640
1620
1600
1580
1560
2280224022002160
1900
1800
1700
1600
1500
1400
1300
2500230021001900
3000
2500
2000
1500
1000
500
0
40003000200010000
2 cm
4 cm
8 cm
1 cm
1625
1620
1615
1610
1605
1600
225022402230222022102200
65x103
60
55
50Cou
nts
225022402230222022102200
RESOLVING POWER TEST AT 1 km
Actual resolution 20 µrad (20 mm at 1 km)
17 34 69 86
0.3
0.6
0.9
1.2
1.5
1.8
103
radians
East West
North
South
12:00
16:00
14:00
18:00 20:0022:00
24:00
degrees52
Sun, angular diameter 0.535˚ = 9.3 mrad
Narrow FOV Camera, 22 x 29 mrad = 2 x 3 sun diameters
Both drawn 10 times actual angular dimension
Wide FOV Camera, 120 x 160 mrad,Actual angular dimension
Moonshotfor comparison
Time is UTC; local sun time = UTC - 6.5 h.
OBSERVATION GEOMETRY
10x103
8
6
4
2
0Sens
itivi
ty, c
ount
s J-1
, rel
ativ
e
900800700600500400300Wavelength, nm
WAVELENGTHCALIBRATIONIntegrating sphere,Hg-Xe lamp,Calibrated photodiodePeaks: 460, 640 nm
Identify darkest and brightest regions of images to determine Cmin and Cmax. Scaled normalized radiance Rs is evaluated from measured counts C as
Rs = Rmin +C Cmin
Cmax Cmin(Rmax Rmin )
Rmin and Rmin are from radiation transfer calculations.Agreement of scaled radiances with theory lends confidence to the approach.
SCALING OF RADIANCES from images
ZOOMING IN, IN SPACE AND TIMEFIVE MINUTES IN OKLAHOMA
07-31-2015; UTC time; local sun time = UTC - 6.5 h; 16:34 = 10:04 sun time.Wide field of view 240 × 320 m
Narrow field of view 43 × 57 m
CLOUD OPTICAL DEPTH by inverting radianceRGB Red Red
COD(Red) COD(Blue) COD(Blue) vs COD(Red)
COD(Blue) vs COD(Red)
Image1028
Image1028
Image1041
COD(Red) COD(Blue)
RGB Red Red
Image1041
AcknowledgmentsSupported by the U.S. Department of Energy (DOE) Atmospheric System Research Program (Office of Science, OBER) under Contract No. DE-AC02-98CH10886. We thank the staff at the ARM Central Great Plains site in Lamont, OK, particularly Matt Gibson and James Martin, for assistance in the deployment. Pilot studies atBrookhaven National Laboratory were carried out with Antonio Aguirre (New York City College of Technology, Brooklyn), with support from the National Science Foundation, Louis Stokes Alliance for Minority Participation (LSAMP), and Clement Li (City College of the City University of New York) with support from the DOE Office of Science, Office of Workforce Development for Teachers and Scientists under the Visiting Faculty Program.
CONCLUSIONSPhotography of clouds from the surface provides a novel way of looking at clouds and their radiative effects at much higher resolution than other cloud imaging techniques.
•
Readily available commercial cameras provide a resolution of about 20 µrad (corresponding to 4 cm for cloud base at 2 km), 3 orders of magnitude higher than typical satellite products.
•
Cloud properties are highly variable in space (a few meters or less) and time (a few seconds or less).•
Manuscript about to be submitted! •
Cloud optical depth can be accurately retrieved at native resolution of the camera for optically thin clouds, optical depth < 3.~
•
10 Million determinations of COD, cloud albedo, radiativecloud fraction, etc.,for each image.
•Images and analyses of these should be of value to •cloud modelers.
Doppler Lidar: Thin wimpy clouds
Image1028
Image1041
Cloud albedo (Red) Cloud albedo (Blue) Radiative Cloud FractionCLOUD ALBEDO and CLOUD RADIATIVE FRACTION
0.120.080.040.00Cloud albedo
Image1028 Image1041
1.00.80.60.4Radiative cloud fraction
Image1028 Image1041
RGB Image 1041 Norm. Zen.Rad. (Red) COD(Red)
Norm. Zen.Rad. (Red) COD(Red)COD(Red)
Cloud optical depth structure at CENTIMETER SCALES
THEORYOBJECTIVE SOME INITIAL RESULTS