Orbit Characteristics and View Angle Effects on the Global Cloud Field

Brent C. Maddux1,2

Steven A. Ackerman1

Steve Platnick3

Paul Hubanks4

1Cooperative Institute for Meteorological Satellite Studies 2Department of Atmospheric and Oceanic Sciences-U of Wisconsin3NASA Goddard Space Flight Center, Greenbelt, Maryland4Wyle Information Systems, McLean, VA 22102

Motivation

• Comparison to other cloud datasets (apples-to-apples)

• Place ‘error bars’ on global mean statistics

• Quantify cloud variability globally

Global Means

0 .25 .5 .75 1.0

• Daytime Cloud Fraction

• Coast lines evident

• Differences where expected: maritime stratocumulus decks, SH land, etc.

Terra 9 Year Mean

Aqua 7 Year Mean

MODIS Cloud Fraction Terra (Red) and Aqua (Black)

Global Cloud Amount AnomalyCA

Ano

mal

y (%

)

1970 1975 1980 1985 1990 1995 2000 2005 2010

3

2

1

0

-1

-2

-3

Cloud Fraction vs Viewing Angle

• 7 years of Aqua and Terra

• 16% increase from near nadir to edge of scan

• View angle effect not constant for all cloud types

Cloud Fraction vs Sensor Zenith Angle

Sensor Zenith Angle

Single Day in the MODIS Orbit

0 .25 0.5 .75 1.0

Terra Daytime Cloud Fraction• Geostationary-like view

• 16 day orbit procession

• U-Shape at edges of convection and cloudy regions

• Increase in pixel size with view angle

Daytime Cloud Fraction Mean

Nadir to 10°

60° to edge of scan°

Nadir to 10° minus 60 and greater

• Differences not uniform

• Largest differences not where thin high clouds exist

Nadir vs Edge of ScanNear Nadir Near Edge of Scan

0 0.2 0.4 0.6 0.8 1

15 20 25 30 35

200 375 550 775 900

5 10 15 20 25

Cloud Fraction (%)

Cloud Top Pressure (hPa)

Cloud Effective Radius Ice (μm)

Cloud Effective Radius Liquid (μm)

15 20 25 30 35

5 10 15 20 25

Cloud Optical Depth Ice

Cloud Optical Depth Liquid

• Changes are not uniform

•Largest changes in CF aren’t the same as largest changes in optical properties

•Near Nadir ≤10°•Near Edge of Scan ≥50°

L3 Statistical Uncertainty Study: Zonal Retrieval Statistics vs. VZAEffective Radius, water clouds

fwd. nadir binforward oblique bin (53° to 65° VZA)backward oblique bin (-53° to -65° VZA)

0 ≥2412

operational (all bins)

L3 Statistical Uncertainty Study: Global Retrieval Statistics vs. Zenith AngleEffective Radius, water clouds, MODIS Aqua, April 2005

Mean Cloud Fraction Difference (MOD35-MOD06)

0 10 20 30 40 (%)

MOD06 cloud fraction is a quality assured subset of MOD35 to retrieve better cloud optical properties

Mean Cloud Fraction Difference in Percent (MOD35-MOD06)/MOD35

0 30 60 (%)

Differences are due to the QA stuff(clear sky restoral and cloud edges, thin clouds, and surfaces influences).

CTP histogram from CTP (red) and CTPvsOD (blue) histogram

Cloud Fraction

Difference in high and low cloud fraction

-30 0 30

Cloud Fraction Day Difference for 7 yrs (Aqua minus Terra)

OCT

JUL

APR

JAN

NOV

AUG

MAY

FEB

DEC

SEP

JUN

MAR

Grid Cell Size and Swath Overlap

Latitudes Included

EQ 15 30 45 60 75 90

50

40

30

20

10

Cloud Top Pressure vs Latitude

Swath overlap causes more cloud to be averaged into the middle cloud bin

Grid Cell Size and Swath Overlap Cloud Top Pressure Histogram

High Cloud <400 hPaMid Cloud >400 and <700 hPa Low Cloud >700 hPa

Summary

• Cloud fractions need to characterize the global cloud trend within ±1%

• View angle dependencies are large across swath

– 16% cloud fraction (>60 locally)– 30hPa for cloud top pressure (>200 hPa locally)– 2μm for effective radii (10μm locally)– 2 for optical depth (20 locally)