Water Vapor

1
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 P. B.
Russell1, P. Pilewskie2, J. Redemann3, J.
Livingston4, B. Schmid3, R. Kahn5, A. Chu6, W.
Gore1, J. Eilers1, J. Pommier4, S. Howard4, C.
McNaughton7, A. Clarke7, S. Howell7 1NASA Ames
Research Center, Moffett Field, CA, 2LASP/PAOS
University of Colorado, Boulder, CO, 3Bay Area
Environmental Research Institute, Sonoma, CA,
4SRI International, Menlo Park, CA, 5Jet
Propulsion Laboratory, California Institute of
Technology, Pasadena, CA, 6NASA Goddard Space
Flight Center, Greenbelt, MD, 7U. Hawaii,
Honolulu, HI
Satellite Validation
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.
Comparison of Coincident AATS-14, MODIS, and MISR
AOD Retrievals for 22 July Terra Overflight
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
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. 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
DFnet/ DAOD 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.
Broadband (350-700nm) aerosol radiative forcing
efficiencies and radiative forcings
(instantaneous and 24h-avg.)
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).
Blue case studies used for average result Red
criterion used to exclude case study
Example
Summary 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.
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.

Water Vapor
Measured water-leaving irradiance in Gulf of NH
shows water to be very "black", i.e., relatively
low levels of chlorophyll-A. Sea surface
spectral albedo/water-leaving irradiance needs to
be adapted for use with MISR data for
constraining low level AOT algorithms convert to
water-leaving radiance via BRDF.
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.
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.