A24B-01
Stratocumulus Sensitivity to Aerosols and Dynamics: Evaluating Aerosol-Cloud Parameterizations
Global changes in cloud properties have the potential to significantly impact the Earth’s energy balance. Due to the strong cooling effect of stratocumulus clouds, it is of particular importance to quantify their sensitivities to changing aerosol and dynamical forcings. Prior observational studies have shown instantaneous correlations between aerosols and cloud properties, but have generally been unable to test if these correlations reflect a true causal relationship. Mauger and Norris (GRL, 2007) recently presented a new technique for separately quantifying the impacts of aerosol and dynamical forcings on clouds. The method uses HySPLIT back trajectories to control for the influence of meteorological history on cloudiness. By combining MODIS observations with ECMWF operational analyses, Mauger and Norris found that covariation between aerosol optical depth and lower tropospheric stability (LTS) during the previous 48 hours led to an overestimate of the cloud sensitivity to aerosols. Controlling for variations in LTS reduced the estimated sensitivity by 54%. The present work extends the analysis by estimating partial derivatives of cloud properties with respect to aerosols and meteorology, and by applying the technique to model evaluation. Both GFDL and NCAR have recently implemented interactive aerosol-cloud schemes in their GCMs. Prior validation studies have typically focused on comparison of mean fields. By instead examining cloud response on daily time scales, this method provides new diagnostic information on model performance. Specifically, model and observational sensitivities are computed by estimating partial derivatives of cloud properties with respect to aerosol and meteorological forcings. Partial derivatives are estimated by compositing data into high, low, and middle terciles, and considering variations in one variable while holding others constant. The results provide a set of statistically robust estimates of stratocumulus sensitivities, obtained from both observations and model output. Comparison of the two provides unique information for model validation and for diagnosing sources of model error.
A24B-02
The potential influence of Saharan dust on the latitude and intensity of the Atlantic ITCZ
The latitudinal distribution and intensity of cumulus convection in the Tropical Atlantic is investigated in nine years of satellite observations and general circulation model simulations including the climatology and radiative forcing of Saharan dust. We explore the impact of the direct radiative forcing by Saharan dust on the latitude and intensity of the ITCZ in the eastern and central tropical Atlantic Ocean. The northward migration of the ITCZ through the boreal summer season corresponds closely with the seasonal variations in sea surface temperature (SST). Saharan dust is transported over the Atlantic Ocean to the north of the ITCZ in synoptic plumes associated with the passage of African easterly waves. The direct radiative forcing of dust aerosols results in a cooling of SST to the north of the ITCZ, and solar absorption results in mid-tropospheric heating. This heating acts to displace the ITCZ to the north of where it would be in the absence of the aerosols and slightly enhance the intensity of convection in the ITCZ in idealized GCM experiments with and without Saharan dust. The GCM results are explored and compared with satellite data of rainfall, SST, and aerosol optical depth during the TRMM/Terra period in order to estimate the potential variations in ITCZ location and activity attributable to the direct radiative forcing by Saharan dust.
A24B-03
Strengths and Limitations of MISR Aerosol Products for Large-Scale, Long-Term Climate Studies
The NASA Earth Observing System's Multi-angle Imaging SpectroRadiometer (MISR) and MODerate resolution Imaging Spectroradiometer (MODIS) have produced data records nearly eight years long, widely recognized and used due to their frequent, global coverage, radiometric stability, spatial resolution down to 250 m, and broad spectral and angular coverage. Aerosol products generated routinely from these data records include aerosol amount, and type. The MISR stereo-derived height product contains maps of cloud and aerosol source plume heights. Both statistical and case-by-case analyses illustrate the strengths of these products, but also reveal some limitations that must be considered, especially for large-scale, long-term aerosol climate studies. This presentation will review our current understanding of the strengths and limitations, illustrated with examples from a number of recent studies, and will highlight ongoing work aimed at further refining the MISR aerosol data record for climate applications.
A24B-04
Detection of the Arctic Dehydration-Greenhouse Feedback Using Satellite Observations and a Regional Climate Model
Datasets from the CloudSat radar reflectivity and the CALIPSO lidar backscattering measurements have provided a new regard on Arctic winter cloud systems, as well as on the way aerosols determine their formation and evolution. In this presentation, we emphasize the role of sulfates in the cooling and dehydrating air masses from cold low pressure systems entering the Arctic during the cold season. Using combined information from satellite instruments and the Northern Aerosol Regional Climate Model (NARCM), we show that a high sulfate fraction in the aerosol field is much likely to favour the production of bigger ice crystals which, by increased precipitation rates, accelerate both the depletion of atmospheric water content and the reduction of its greenhouse effect. This is linked to the property of sulfuric acid to lower the freezing point of haze droplets, leading to a type of thin ice clouds (TIC-2) extending deeply in the troposphere and increasing the radiative cooling rate with further strenghtening of water deposition and sedimentation of large ice crystals. This enhanced dehydration-IR cooling induced by acidic IFN is a case of dehydration-greenhouse feedback (DGF). In contrast, pristine aerosols favour the formation of large amounts of small crystals and non-precipitating thin ice cloud (TIC-1) systems. Because of their high sensitivity to discriminate crystal sizes, TIC-2 types are easily detected by both instruments, lidar and radar, whereas TIC-1 type is only seen by the lidar due to its capability to detect smaller crystals. Therefore, CloudSat and CALIPSO datasets are used to heuristically discriminate Arctic TIC in two crystal size categories (radar/lidar versus lidar-only). A calculation of the correlation between the occurrence of these cloud types and the aerosol sulfate fraction is obtained by joining satellite information to an aerosol field simulation from NARCM. Humidity field from the Atmospheric InfraRed Sounder (AIRS) is also used to analyse a cold cyclone dehydration case over the Arctic Ocean in January 2007. Results show that the DGF process could be responsible for climatic tropospheric cooling when a cold region is subject to an increase in sulfuric acid concentrations.
A24B-05
Concentrations of Aerosol related Particulate Matter and its darkening of Arctic Glacier Ice Surfaces
The glacier surface albedo is important for ice melt. Studies have revealed that even small changes in the surface albedo often have significant impact on the resulting ice melt. For instance a 15% reduction in surface reflectance on the Greenland ice sheet is sufficient to cause over one meter annual excess ice melt, which is the magnitude of present thinning may places on the Greenland ice sheet margin. Except for minor areas around Nunataks as well as moraine material the surface the particulate matter (PM) constitutes of aerosols accumulated on the glacier surface for hundreds or thousands of years. In PM on the glacier surface a significant biological activity exists due to abundance of algae, bacteria and other micro- organisms. The biological activity is able to effectively bind particles to the biomass in so-called "cryoconite". The result is a favourable substrate for bacterial activity, which "flocculate" the micro particles and thus affects the glacier surface albedo. Besides biological activity formation of "cryoconite holes" (depressions/holes in the ice filled with concentrated matter) also affects the resulting albedo. The combined effects and interaction between biological activity, material matter and formation of cryoconite holes makes the resulting albedo a complex parameter in the surface energy budget. However, one inherent factor affecting it all is the ice surface mass concentration of PM predominately from aerosols. We here present measured concentrations of PM from three different glacier types and locations in the Arctic - namely the north- eastern Greenland ice sheet margin, the valley glacier Midre Loweenbreen and the Vestfonna ice cap, both on Svalbard.
A24B-06
Might anthropogenic aerosol be stimulating Arctic sea ice melting through their effects on clouds?
It has recently been shown that aerosol have the capacity to increase low cloud longwave emissivity in the Arctic. Aerosols are most abundant in the Arctic during winter and spring, when any increased thermal emission by clouds is likely to augment the rate of seasonal sea-ice melt.Aerosol also indirectly cools the Arctic surface in summer, provided it is black, and the sea-ice or snow have melted. However, averaged over the year, the net indirect forcing of the surface is strongly positive. Measurements at Barrow indicate that polluted clouds emit 10 to 15 W/m2 more longwave radiation than clean clouds. Notably, the magnitude of the effect appears to be influenced not only by increased density of droplet absorption cross-sections, but also by aerosol-cloud-radiation interactions. We demonstrate numerical simulations that illustrate how pollution amplifies cloud-top radiative cooling, and that this translates to more substantial, and more highly radiative, Arctic stratus.
A24B-07
Radiative Impact of Boreal Smoke in the Arctic: Observed Versus Modeled
The Arctic climate is modulated, in part, by the presence of aerosols that affect the horizontal and vertical distribution of radiant energy passing through the atmosphere, directly through interactions with solar and terrestrial radiation and indirectly through interactions with cloud particles. During summer 2004 forest fires destroyed vast areas of boreal forest in Alaska and western Canada, releasing smoke into the atmosphere that was dispersed widely. Smoke passing over instrumented field sites near Barrow, Alaska was monitored to determine its physical and optical properties and its impact on the surface radiation balance. Empirical determinations of the direct radiative forcing by the smoke were used to corroborate simulations made using the Moderate Resolution Transmittance radiative transfer code, MODTRAN®5. Radiative forcing varying with solar angle and surface type was evaluated at the surface, at the top of the atmosphere (TOA) and within the intervening atmosphere. The TOA results are used to corroborate retrievals from polar orbiting satellites. Smoke cools the surface while warming those layers in which it resides, increasing atmospheric stability and possibly suppressing cloud formation. TOA forcing is especially sensitive to surface albedo, evidenced in both the model results and satellite retrievals. Cooling to space occurs over dark ocean areas while warming occurs over bright snow and ice covered regions. Surface cooling and corresponding layer heating are the dominant radiative effects of boreal smoke at high northern latitudes. Should the frequency and intensity of boreal fires increase in the future due to climate change, the more persistent presence of smoke in the atmosphere may be manifest as a negative feedback at the surface with variable impact on cloud distributions depending on complicated, competing greenhouse and albedo effects of clouds.
A24B-08
Experimental validation of light scattering and absorption theories of fractal-like carbonaceous aerosol agglomerates
The optical coefficients of size-selected carbonaceous aerosol agglomerates measured at a wavelength of 870 nm are compared with those predicted by three theories, namely Rayleigh-Debye-Gans (RDG) approximation, volume-equivalent Mie theory, and integral equation formulation for scattering (IEFS). Carbonaceous agglomerates, produced via flame synthesis, were size-selected using two differential mobility analyzers (DMAs) in series, and their scattering and absorption coefficients were measured with nephelometry and photoacoustic spectroscopy. Scanning electron microscopy, along with image processing techniques, were used for the parameterization of the structural properties of the fractal-like agglomerates. The agglomerate structural parameters were used to evaluate the predictions of the optical coefficients based on the three light scattering and absorption theories. The results indicate that the RDG approximation agrees within 10% of the experimental results and the exact electromagnetic calculations of the IEFS theory. The experimental scattering coefficient is over predicted by the volume-equivalent Mie theory by a factor of ~3.2. Also, the RDG approximation-predicted optical coefficients showed pronounced sensitivity to changes in monomer mean diameter, the count median diameter of the agglomerates, and the geometric standard deviation of the agglomerate number size distribution.