Jetstream-31 (J31) in ITCT-INTEX (Intercontinental Transport and Chemical

Jetstream-31 (J31) in ITCT-INTEX (Intercontinental Transport and Chemical

Jetstream-31 (J31) in ITCT-INTEX
(Intercontinental Transport and Chemical TransformationIntercontinental Chemical Transport Experiment)

Aerosol, cloud, water vapor, and sea surface
radiative properties and effects measured by
airborne sunphotometer and solar spectral flux
radiometer off New England in summer 2004

ICARTT: International Consortium for Atmospheric Research on
Transport & Transformation

P. B. Russell1, P. Pilewskie2, J. Redemann3, J. Livingston4, B. Schmid3,
5
6
1
1
4
4
R. Kahn , A. Chu , W. Gore , J. Eilers , J. Pommier , S. Howard ,
7
7
7
C. McNaughton , A. Clarke , S. Howell
NASA Ames Research Center, Moffett Field, CA, 2LASP/PAOS University of Colorado, Boulder, CO,
3
Bay Area Environmental Research Institute, Sonoma, CA, 4SRI International, Menlo Park, CA,
5
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 6NASA Goddard Space Flight
Center, Greenbelt, MD, 7U. Hawaii, Honolulu, HI
1

Satellite Validation
Comparison of Coincident AATS-14, MODIS, and MISR AOD Retrievals for 22 July Terra Overflight

J31

AA
O
:N
T
ITC
INTEX:
NASA

Comparisons like those to the left, supplemented by results from
the U. Hawaii HiGEAR in situ package on the DC-8, are being
used to evaluate MISR research retrievals and the ability to
retrieve aerosol intensive properties.

J31 GOALS in ITCT-INTEX-ICARTT
Assess the radiative impact of the aerosols advecting from North America out
over the Northwestern Atlantic Ocean.
Clear-sky Impact: Aerosol Direct Effect
Impact Via Clouds: Aerosol Indirect Effect
Quantify the relationships between those radiative impacts and aerosol amount
and type.
Contribute water spectral albedo measurements to help improve satellite aerosol
retrievals

AATS-14 measurements were acquired during a near-surface J31 transect coincident in time and space with a Terra overflight on 22 July. AATS-14 AODs represent mean values along the low altitude flight segment, and vertical bars
depict the spread (no horizontal ticks), the standard deviation (wide ticks), and the measurement and retrieval uncertainty (narrow ticks). Corresponding vertical bars on the MODIS and MISR AOD values reflect the expected uncertainties
in those retrievals. MISR Version 15 and Version 16 AOD retrievals are shown. No AOD contributions from the boundary layer below the aircraft altitude (~0.09 km) have been added to the AATS-14 AOD values.

Aerosol Radiative Forcing Efficiency
Objectives of the
Ames Airborne Tracking Sunphotometer
(AATS-14)
and Solar Spectral Flux Radiometer (SSFR) in
ITCT-INTEX/ ICARTT
AATS

Validate Satellites (AOD spectra, H2O columns)
Test Closure (Consistency) among Suborbital Results
Test Chemical-Transport Models Using AOD Profiles
Assess Regional Radiative Forcing by Combining Satellite
and Suborbital Results

Methodology
The combination of coincident and simultaneous AATS and SSFR measurements yields plots of
net spectral irradiance as a function of aerosol optical depth as measured along horizontal flight
legs (gradient plots). From the slope of these plots we determine the change in net radiative flux
per change in aerosol optical depth, dF/dAOD, or aerosol radiative forcing efficiency [W m-2
AOD-1]. This manner of deriving forcing efficiency is called the aerosol gradient method. Unlike
ground-based measurements of direct aerosol radiative forcing which rely upon the advection of
varous air masses over a measurement site during an extended period of time, the airborne
method has the advantage of being quasi instantaneous.

Cloud and Sea Surface Properties
Top panel: Examples of upwelling and
downwelling spectral irradiance from above
cloud during J31 flight in 20 July 2004. Middle
panel: Comparison between measured (SSFR)
and best-fit model spectral albedo (ratio of
upwelling to downwelling irradiance) for this
case. Lower panel: Residual between best
measured and best-fit modeled spectrum. The
well defined and unique minimum determines
the retrieved effective radius/ optical depth pair
(effective radius 10 um, cloud optical depth 10).

Step-by-step:
1. Measure simultaneous change in spectral aerosol optical depth (AATS-14) and spectral
net irradiance (SSFR) across AOD gradient.
2. Slope of the regression of Fnet vs. AOD yields
Fnet/ AOD = aerosol radiative forcing efficiency
3. This constitutes an observationally-based estimate of aerosol radiative effects.
4. Advantage over ground-based methods = quasi-instantaneous, because of short
horizontal distances.
5. Need to consider (and correct for) effects of changing solar zenith angle and changing
column water vapor contents during low-level leg.
*Blue case studies used for average result
** Red criterion used to exclude case study

Example

SSFR

Broadband (350-700nm) aerosol radiative forcing efficiencies and radiative forcings (instantaneous
and 24h-avg.)

Retrieve cloud droplet radius, optical depth,
and liquid water path

ICARTT Forcing Efficiencies per unit 499nm AOD, all 14 cases

Compare/validate with P-3 MIDAS, P-3 microphysics, satellite
retrievals (MODIS), microwave/radar retrievals from the Ron
Brown.
Relate these cloud properties to near-cloud aerosol properties
(from other investigators and platforms)

Provide water spectral albedo measurements
to help improve satellite aerosol retrievals
Joint AATS-SSFR
Study effect of over-cloud AOD on cloud property retrievals
by SSFR and satellites
Derive Spectra of Aerosol Absorbing Fraction (1-SSA) from
Spectra of Radiative Flux and AOD.

Time-series of spectral AOD and down-welling broadband solar flux across AOD gradient
encountered on July 21, 2004.

J31 flight path (cyan) during the minimum altitude leg that acquired
the AATS and SSFR data shown at left and below.

Derive Aerosol Radiative Forcing from
Simultaneously Measured Radiative Flux and
AOD Gradients

This poster reports progress on objectives in
gold font.
font For additional examples, see the
presentations posted at http://geo.arc.nasa.gov/
sgg/INTEX/presentations/presentations.html

Summary

Changes in SSFR band-integrated downwelling flux as a function of AATS-14 derived
aerosol optical depth for the J31 flight of 21 July 2004; slope yields the direct aerosol
forcing efficiency (defined per unit AOD). Note that approximately 60% of the change in
irradiance over the entire band comes from the visible portion of the spectrum where water
vapor absorption is negligible. [/Redemann et. al.,/ 2005]

Radiative fluxes (narrowband and broadband, net and downwelling) plotted vs. midvisible
AOD, all as measured when underflying an AOD gradient on 21 July 2004.

In INTEX/ITCT, we observed a total of 16 horizontal AOD gradients, with 10 gradients
well suited for our analysis because of the small changes in solar zenith angle during
the gradient measurements. More than half of the AOD gradients (at a wavelength of
499 nm) were greater than 0.1 and extended over distances less than 40 km. Within
the 10 case studies we found a high variability in the derived instantaneous aerosol
forcing efficiencies (forcing per unit optical depth) for the visible wavelength range
(350-700nm), with a mean of -79.6Wm-2 and a standard deviation of 21.8Wm-2(27%).
The mean instantaneous forcing efficiency for the visible plus near-IR wavelength
range (350-1670nm, not shown here) was derived to be 135.3Wm-2 with a standard
deviation of 36.0Wm-2(27%). An analytical conversion of the instantaneous forcing
efficiencies to 24h-average values yielded -45.813.1Wm-2 (meanstd) for the visible
and -82.923.1Wm-2 (meanstd) for the visible plus near-IR wavelength range,
respectively.

RV Ron Brown as seen from J31 flyby

Conversion to 24-hr averages
350-700nm net flux change for 10 ICARTT cases, assuming ssa=0.9-1, g=0.65-0.7

Water Vapor
NASA Ames Sunphotometer ICARTT
6

5

at 200 ft altitude, 17 July 2004

7/26/2004 19.21-19.514 UT ascent
6

5

Altitude [km]

AATS-14
4

4

3

3
In situ

2

2

1

1

0
0
1
2
Columnar Water Vapor [g/cm2]

0

0
5
10
3
Water Vapor Density [g/m ]

Left frame: AATS-retrieved column water vapor in profile on J31 Flight 15, 26
July 2004 in ITCT/INTEX-A. Right frame: AATS water vapor density profile
obtained by differentiating profile in left frame, compared to density from J31 in
situ sensor.

* Measured water-leaving irradiance in
Gulf of NH shows water to be very
"black", i.e., relatively low levels of
chlorophyll-A.

Absolute (left panel) and relative (right panel) spectral aerosol radiative forcing efficiency for the 21 July 2004
case (see above). Relative forcing efficiency is derived by normalizing by the incident solar irradiance and
serves to remove the influence of the distribution solar radiation. Note that outside of the gas (primarily water)
absorbing bands the relative forcing efficiency is a smooth and monotonic function of wavelength as expected
from aerosol extinction.

Upper panel: up- (green spectrum) and down-welling spectral irradiance over the
Gulf of Maine from the J31 ICART flight on 17 July 2004. Lower panel: sea surface
spectral albedo.

* Sea surface spectral albedo/waterleaving irradiance needs to be adapted
for use with MISR data for constraining
low level AOT algorithms; convert to
water-leaving radiance via BRDF.

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