Abstract. Marine stratocumulus cloud properties, and the free-tropospheric environment above them, are examined in NASA A-Train satellite data for cases where smoke from seasonal burning of the West African savannah overlay the persistent southeast Atlantic stratocumulus cloud deck. CALIPSO space-borne lidar observations show that features identified as layers of aerosol occur predominantly between 2 km and 4 km. Layers identified as cloud features occur predominantly below 1.5 km altitude and beneath the layer of elevated smoke aerosol. The diurnal mean shortwave heating rates attributable to the absorption of solar energy in the aerosol layer is nearly 1.5 K d −1 for an aerosol optical thickness value of 1, and increases to 1.8 K d −1 when the smoke resides above clouds owing to the additional component of upward solar radiation reflected by the cloud. As a consequence of this heating, the 700 hPa air temperature above the cloud deck is warmer by approximately 1 K on average for cases where smoke is present above the cloud compared to cases without smoke above cloud. The warmer conditions in the free-troposphere above the cloud during smoke events coincide with cloud liquid water path values that are greater by 20 g m −2 and cloud tops that are lower for overcast conditions compared to periods with low amounts of smoke. The observed thickening and subsidence of the cloud layer are consistent with published results of large-eddy simulations showing that solar absorption by smoke above stratocumulus clouds increases the buoyancy of free-tropospheric air above the temperature inversion capping the boundary layer. Increased buoyancy inhibits the entrainment of dry air through the cloud-top, thereby helping to preserve humidity and cloud cover in the boundary layer. The direct radiative effect of absorbing aerosols residing over a bright cloud deck is a positive radiative forcing (warming) at the top of the atCorrespondence to: E. M. Wilcox (eric.wilcox@dri.edu) mosphere. However, the greater liquid water path for cases of smoke overlaying cloud contributes an additional negative semi-direct radiative forcing (cooling) of climate in locations such as the southeast Atlantic Ocean owing to the enhanced albedo of the thicker cloud.

Abstract.Observations from Earth observing satellites indicate that dark carbonaceous aerosols that absorb solar radiation are widespread in the tropics and subtropics. When these aerosols mix with clouds, there is generally a reduction of cloudiness owing to absorption of solar energy in the aerosol layer. Over the subtropical South Atlantic Ocean, where smoke from savannah burning in southern Africa resides above a persistent deck of marine stratocumulus clouds, radiative heating of the smoke layer leads to a thickening of the cloud layer. Here, satellite observations of the albedo of overcast scenes of 25 km 2 size or larger are combined with additional satellite observations of clouds and aerosols to estimate the top-of-atmosphere direct radiative forcing attributable to presence of dark aerosol above bright cloud, and the negative semi-direct forcing attributable to the thickening of the cloud layer. The average positive direct radiative forcing by smoke over an overcast scene is 9.2±6.6 W m −2 for cases with an unambiguous signal of absorbing aerosol over cloud in passive ultraviolet remote sensing observations. However, cloud liquid water path is enhanced by 16.3±7.7 g m −2 across the range of values for sea surface temperature for cases of smoke over cloud. The negative radiative forcing associated with this semi-direct effect of smoke over clouds is estimated to be −5.9±3.5 W m −2 . Therefore, the cooling associated with the semi-direct cloud thickening effect compensates for greater than 60 % of the direct radiative effect. Accounting for the frequency of occurrence of significant absorbing aerosol above overcast scenes leads to an estimate of the average direct forcing of 1.0±0.7 W m −2 contributed by these scenes averaged over the subtropical southeast Atlantic Ocean during austral winter. The regional average of the negative semi-direct forcing is −0.7±0.4 W m −2 . Therefore, smoke aerosols overlaying the decks of overcast marine stratocumulus clouds considered here yield a small net positive radiative forcing, which results from the difference of two larger effects.

A glutamic acid at residue 69(Glu69) in the HLA-DPB1 gene (Glu69) is associated with chronic beryllium disease (CBD) and possibly beryllium sensitization (BeS). This study tested the hypothesis that MHC class II polymorphisms are important in susceptibility to BeS and CBD and that the Glu69 variant is related to markers of disease severity. Genomic DNA was obtained from BeS (n = 50), CBD (n = 104), and beryllium-exposed nondiseased (Be-nondiseased) (n = 125) subjects. HLA-DPB1, -DRB1, and -DQB1 genotypes were determined by (sequence-specific primers) PCR. Disease severity was assessed by pulmonary function and exercise testing. A higher frequency of the DPB1 Glu69 gene was found in CBD and BeS compared with the Be-nondiseased subjects, with odds ratios of 10.1 for CBD vs Be-nondiseased and 9.5 for BeS vs Be-nondiseased. The majority of BeS and CBD subjects displayed non-0201 Glu69 alleles. Glu69 homozygosity was higher in the CBD subjects, while BeS subjects were intermediate and Be-nondiseased lowest. DRB1*01 and DQB1*05 phenotypes were reduced in CBD vs Be-nondiseased subjects, while DRB1*13 and DQB1*06 were associated with CBD in the absence of Glu69. Markers of disease severity, including a lower forced vital capacity, diffusion capacity for carbon monoxide, PaO2 at rest, maximum workload on exercise testing, and a higher arterial-alveolar gradient at rest, were associated with Glu69 homozygosity. We conclude that DPB1 Glu69 is a marker of sensitization and not specific for disease. Glu69 homozygosity acts as a functional marker associated with markers of CBD severity.

The frequency distributions of surface rain rate are evaluated in the Tropical Rainfall Measuring Mission (TRMM) and Special Sensor Microwave/Imager (SSM/I) satellite observations and the NOAA/GFDL global atmosphere model version 2 (AM2). Instantaneous satellite rain-rate observations averaged over the 2.5° latitude × 2° longitude model grid are shown to be representative of the half-hour rain rate from single time steps simulated by the model. Rain-rate events exceeding 10 mm h−1 are observed by satellites in most regions, with 1 mm h−1 events occurring more than two orders of magnitude more frequently than 10 mm h−1 events. A model simulation using the relaxed Arakawa–Schubert (RAS) formulation of cumulus convection exhibits a strong bias toward many more light rain events compared to the observations and far too few heavy rain events. A simulation using an alternative convection scheme, which includes an explicit representation of mesoscale circulations and an alternative formulation of the closure, exhibits, among other differences, an order of magnitude more tropical rain events above the 5 mm h−1 rate compared to the RAS simulation. This simulation demonstrates that global atmospheric models can be made to produce heavy rain events, in some cases even exceeding the observed frequency of such events. Additional simulations reveal that the frequency distribution of the surface rain rate in the GCM is shaped by a variety of components within the convection parameterization, including the closure, convective triggers, the spectrum of convective and mesoscale clouds, and other parameters whose physical basis is currently only understood to a limited extent. Furthermore, these components interact nonlinearly such that the sensitivity of the rain-rate distribution to the formulation of one component may depend on the formulation of the others. Two simulations using different convection parameterizations are performed using perturbed sea surface temperatures as a surrogate for greenhouse gas–forced climate warming. Changes in the frequency of rain events greater than 2 mm h−1 associated with changing the convection scheme in the model are greater than the changes in the frequency of heavy rain events associated with a 2-K warming using either model. Thus, uncertainty persists with respect to simulating intensity distributions for precipitation and projecting their future changes. Improving the representation of the frequency distribution of rain rates will rely on refinements in the formulation of cumulus closure and the other components of convection schemes, and greater certainty in predictions of future changes in both total rainfall and in rain-rate distributions will require additional refinements in those parameterizations that determine the cloud and water vapor feedbacks.

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Large negative magnetoresistance and anomalous magnetic properties are found in amorphous Si doped with magnetic rare earth ions near the metal-insulator transition. The resistivity below 50 K rises orders of magnitude above that of comparable composition nonmagnetic alloys and is strongly reduced by a magnetic field. Magnetization measurements show noninteracting moments at high temperature which develop antiferromagnetic interactions below 50 K. We suggest that these results are due to formation below 50 K of a dense concentration of magnetic polarons which localize conduction electrons. [S0031-9007(96)01722-X]

The introduction of cloud condensation nuclei and radiative heating by sunlight-absorbing aerosols can modify the thickness and coverage of low clouds, yielding significant radiative forcing of climate. The magnitude and sign of changes in cloud coverage and depth in response to changing aerosols are impacted by turbulent dynamics of the cloudy atmosphere, but integrated measurements of aerosol solar absorption and turbulent fluxes have not been reported thus far. Here we report such integrated measurements made from unmanned aerial vehicles (UAVs) during the CARDEX (Cloud Aerosol Radiative Forcing and Dynamics Experiment) investigation conducted over the northern Indian Ocean. The UAV and surface data reveal a reduction in turbulent kinetic energy in the surface mixed layer at the base of the atmosphere concurrent with an increase in absorbing black carbon aerosols. Polluted conditions coincide with a warmer and shallower surface mixed layer because of aerosol radiative heating and reduced turbulence. The polluted surface mixed layer was also observed to be more humid with higher relative humidity. Greater humidity enhances cloud development, as evidenced by polluted clouds that penetrate higher above the top of the surface mixed layer. Reduced entrainment of dry air into the surface layer from above the inversion capping the surface mixed layer, due to weaker turbulence, may contribute to higher relative humidity in the surface layer during polluted conditions. Measurements of turbulence are important for studies of aerosol effects on clouds. Moreover, reduced turbulence can exacerbate both the human health impacts of high concentrations of fine particles and conditions favorable for low-visibility fog events.

[1] Two independent satellite retrievals of cloud liquid water path (LWP) from the NASA Aqua satellite are used to diagnose the impact of absorbing biomass burning aerosol overlaying boundary-layer marine water clouds on the Moderate Resolution Imaging Spectrometer (MODIS) retrievals of cloud optical thickness (t) and cloud droplet effective radius (r e ). In the MODIS retrieval over oceans, cloud reflectance in the 0.86-mm and 2.13-mm bands is used to simultaneously retrieve t and r e . A low bias in the MODIS t retrieval may result from reductions in the 0.86-mm reflectance, which is only very weakly absorbed by clouds, owing to absorption by aerosols in cases where biomass burning aerosols occur above water clouds. MODIS LWP, derived from the product of the retrieved t and r e , is compared with LWP ocean retrievals from the Advanced Microwave Scanning Radiometer-EOS (AMSR-E), determined from cloud microwave emission that is transparent to aerosols. For the coastal Atlantic southern African region investigated in this study, a systematic difference between AMSR-E and MODIS LWP retrievals is found for stratocumulus clouds over three biomass burning months in 2005 and 2006 that is consistent with above-cloud absorbing aerosols. Biomass burning aerosol is detected using the ultraviolet aerosol index from the Ozone Monitoring Instrument (OMI) on the Aura satellite. The LWP difference (AMSR-E minus MODIS) increases both with increasing t and increasing OMI aerosol index. During the biomass burning season the mean LWP difference is 14 g m À2 , which is within the 15-20 g m À2range of estimated uncertainties in instantaneous LWP retrievals. For samples with only low amounts of overlaying smoke (OMI AI 1) the difference is 9.4, suggesting that the impact of smoke aerosols on the mean MODIS LWP is 5.6 g m À2 . Only for scenes with OMI aerosol index greater than 2 does the average LWP difference and the estimated bias in MODIS cloud optical thickness attributable to the impact of overlaying biomass burning aerosol exceed the instantaneous uncertainty in the retrievals.Citation: Wilcox, E. M., Harshvardhan, and S. Platnick (2009), Estimate of the impact of absorbing aerosol over cloud on the MODIS retrievals of cloud optical thickness and effective radius using two independent retrievals of liquid water path, J. Geophys.