J31 GOALS in ITCT-INTEX-ICARTT

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 Jetstream-31 (J31) in ITCT-INTEX(Intercontinental Transport and Chemical Transformation-Intercontinental Chemical Transport Experiment) ICARTT:International Consortium for Atmospheric Research on Transport & Transformation J31 ITCT: NOAA INTEX: NASA • 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 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 SSFR ICARTT Forcing Efficiencies per unit 499nm AOD, all 14 cases • Retrieve cloud droplet radius, optical depth, and liquid water path • 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 satelliteaerosol retrievals 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. 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. • Derive Aerosol Radiative Forcing from Simultaneously Measured Radiative Flux and AOD Gradients This poster reports progress on objectives in gold font. For additional examples, see the presentations posted athttp://geo.arc.nasa.gov/ sgg/INTEX/presentations/presentations.html Conversion to 24-hr averages 350-700nm net flux change for 10 ICARTT cases, assuming ssa=0.9-1, g=0.65-0.7 RV Ron Brown as seen from J31 flyby at 200 ft altitude, 17 July 2004 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. 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 -45.8±13.1Wm-2 (mean±std) for the visible and -82.9±23.1Wm-2 (mean±std) for the visible plus near-IR wavelength range, respectively. Radiative fluxes (narrowband and broadband, net and downwelling) plotted vs. midvisible AOD, all as measured when underflying an AOD gradient on 21 July 2004. 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] 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.