Peter V. Hobbs

The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. XII: A Diagnostic Modeling Study of Precipitation Development in Narrow Cold-Frontal Rainbands

Abstract The highest precipitation rates in midlatitude cyclones are often associated with the narrow cold-frontal rainband. In this paper the formation of precipitation in this type of rainband is analyzed with the aid of a diagnostic cloud model and field measurements. Model results indicate that the high precipitation rates in narrow cold-frontal rainbands are associated with graupel. The graupel forms when ice particles, which originate in the stratiform cloud ahead of the rainband, grow rapidly by riming after entering the strong updraft (and hence a region of high liquid water content) associated with the rainband. When this source of input ice particles is not present the precipitation is somewhat weaker, but still mainly associated with graupel. In this case, the graupel forms as small, frozen drops grow by riming. The drops form in the updraft region due to the autoconversion of cloud water and they are frozen by collisions with small ice crystals. A series of sensitivity studies dealing with var...

Physical, chemical, and optical properties of regional hazes dominated by smoke in Brazil

Gas and particle measurements are described for optically thick regional hazes, dominated by aged smoke from biomass burning, in the cerrado and rain forested regions of Brazil. The hazes tended to be evenly mixed from the surface to the trade wind inversion at 3–4 km in altitude. The properties of aged gases and particles in the regional hazes were significantly different from those of young smoke (<4 min old). As the smoke aged, the total amount of carbon in non-methane hydrocarbon species (C<11) was depleted by about one third due to transformations into CO2, CO, and reactive molecules, and removed by dry deposition and/or by conversion to particulate matter. As the smoke particles aged, their sizes increased significantly due to coagulation and mass growth by secondary species (e.g., ammonium, organic acids and sulfate). During aging, condensation and gas-to-particle conversion of inorganic and organic vapors increased the aerosol mass by ∼20–40%. One third to one half of this mass growth likely occurred in the first few hours of aging due to the condensation of large organic molecules. The remaining mass growth was probably associated with photochemical and cloud-processing mechanisms operating over several days. Changes in particle sizes and compositions during aging had a large impact on the optical properties of the aerosol. Over a 2 to 4 day period, the fine particle mass-scattering efficiency and single-scattering albedo increased by 1 m2 g−1, and ∼0.06, respectively. Conversely, the Angstrom coefficient, backscatter ratio, and mass absorption efficiency decreased significantly with age.

Individual aerosol particles from biomass burning in southern Africa: 1. Compositions and size distributions of carbonaceous particles

[1] Individual aerosol particles in smoke plumes from biomass fires and in regional hazes in southern Africa were studied using analytical transmission electron microscopy (TEM), which allowed detailed characterization of carbonaceous particle types in smoke and determination of changes in particle properties and concentrations during smoke aging. Based on composition, morphology, and microstructure, three distinct types of carbonaceous particles were present in the smoke: organic particles with inorganic (K-salt) inclusions, ‘‘tar ball’’ particles, and soot. The relative number concentrations of organic particles were largest in young smoke, whereas tar balls were dominant in a slightly aged (1 hour) smoke from a smoldering fire. Flaming fires emitted relatively more soot particles than smoldering fires, but soot was a minor constituent of all studied plumes. Further aging caused the accumulation of sulfate on organic and soot particles, as indicated by the large number of internally mixed organic/sulfate and soot/sulfate particles in the regional haze. Externally mixed ammonium sulfate particles dominated in the boundary layer hazes, whereas organic/sulfate particles were the most abundant type in the upper hazes. Apparently, elevated haze layers were more strongly affected by biomass smoke than those within the boundary layer. Based on size distributions and the observed patterns of internal mixing, we hypothesize that organic and soot particles are the cloudnucleating constituents of biomass smoke aerosols. Sea-salt particles dominated in the samples taken in stratus clouds over the Atlantic Ocean, off the coast of Namibia, whereas a distinct haze layer above the clouds consisted of aged biomass smoke particles. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; KEYWORDS: biomass burning, carbonaceous aerosol, individual particles, TEM

Ice Particle Concentrations in Clouds

Abstract Measurements and observations have been made on the development of ice in 90 cumulus (cumulus and cumulonimbus) and 72 stratiform (altocumulus, altostratus, nimbostratus, stratocumulus, and stratus) clouds. Ice particle concentrations significantly in excess of those to be expected from ice nucleus measurements (i.e., ice enhancement) were measured in 42 of the cumuliform and 36 of the stratiform clouds. For the complete data set, and for cloud top temperatures (TT) between −6° and −32°C, the maximum concentrations of ice particles (Imax in L−1) in the clouds were essentially independent of TT(r=0.32). However, Imax was strongly dependent on the broadness of the cloud droplet size distribution near cloud top. If the breadth of the droplet size distribution is measured by DT, such that the cumulative concentration of droplets with diameters ≥DT exceeds a prescribed value, then for −32≤TT≤−6°C:where n=8.4 and DO=18.5 μm for the cumuliform clouds and n=6.6 and DO=19.4 μm for the stratiform clouds. W...

Chemical apportionment of aerosol column optical depth off the mid‐Atlantic coast of the United States

Aerosol column optical depths derived from airborne Sun photometer and in situ measurements of aerosol properties in 14 vertical profiles off the mid-Atlantic coast of the United States in June show excellent agreement. Simultaneous measurements of the chemical compositions of the aerosol allows an assessment of the chemical apportionment of the aerosol column optical depths. The optical depths had essentially three chemical components, which, in order of descending average contributions, were condensed water, carbonaceous species, and sulfate. These results do not support the common assumption that sulfate dominates aerosol optical depths in polluted regions.

Emission factors of hydrocarbons, halocarbons, trace gases and particles from biomass burning in Brazil

Airborne measurements of the emissions of gases and particles from 19 individual forest, cerrado, and pasture fires in Brazil were obtained during the Smoke, Clouds, and Radiation-Brazil (SCAR-B) study in August-September 1995. Emission factors were determined for a number of major and minor gaseous and particulate species, including carbon dioxide, carbon monoxide, sulfur dioxide, nitrogen oxides, methane, nonmethane hydrocarbons, halocarbons, particulate (black and organic) carbon, and particulate ionic species. The magnitude of the emission factors for gaseous species were determined primarily by the relative amounts of flaming and smoldering combustion, rather than differences in vegetation type. Hydrocarbons and halocarbons were well correlated with CO, which is indicative of emissions primarily associated with smoldering combustion. Although there was large variability between fires, higher emission factors for SO2 and NOχ were associated with an increased ratio of flaming to smoldering combustion; this could be due to variations in the amounts of sulfur and nitrogen in the fuels. Emission factors for particles were not so clearly associated with smoldering combustion as those for hydrocarbons. The emission factors measured in this study are similar to those measured previously in Brazil and Africa. However, particle emission factors from fires in Brazil appear to be roughly 20 to 40% lower than those from North American boreal forest fires.

Effects of black carbon content, particle size, and mixing on light absorption by aerosols from biomass burning in Brazil

Black carbon mass absorption efficiencies of smoke particles were measured for various types of biomass fires during the Smoke, Clouds, and Radiation-Brazil (SCAR-B) experiment using thermal evolution measurements for black carbon and optical absorption methods. The obtained results range between 5.2 and 19.3 m2 g−1 with an average value of 12.1±4.0 m2 g−1. Particle size distributions and optical properties were also measured to provide a full set of physical parameters for modeling calculations. Mie theory was used to model the optical properties of the particles assuming both internal and external mixtures coupling the modeling calculations with the experimental results obtained during the campaign. For internal mixing, a particle model with a layered structure consisting of an absorbing black carbon core, surrounded by a nonabsorbing shell, was assumed. Also, for internal mixing, a discrete dipole approximation code was used to simulate packed soot clusters commonly found in electron microscopy photographs of filters collected during the experiment. The modeled results for layered spheres and packed clusters explain black carbon mass absorption coefficients up to values of about 25 m2 g−1, but measurements show even higher values which were correlated with the chemical composition and characteristics of the structure of the particles. Unrealistic high values of black carbon absorption efficiencies were linked to high concentrations of K, which influence the volatilization of black carbon (BC) at lower temperatures than usual, possibly causing artifacts in the determination of BC by thermal technique. The modeling results are compared with nephelometer and light absorption measurements.

Aerosol properties and radiative effects in the United States East Coast haze plume: An overview of the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX)

Aerosol effects on atmospheric radiation are a leading source of uncertainty in predicting climate change. The Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) was designed to reduce this uncertainty by measuring and analyzing aerosol properties and effects on the United States eastern seaboard, where one of the worlds major plumes of urban/industrial haze moves from the continent over the Atlantic Ocean. The TARFOX intensive field campaign was conducted July 10–31, 1996. It included coordinated measurements from four satellites (GOES-8, NOAA-14, ERS-2, Landsat), four aircraft (ER-2, C-130, C-131A, and a modified Cessna), land sites, and ships. A variety of aerosol conditions was sampled, ranging from relatively clean, behind frontal passages, to moderately polluted, with aerosol optical depths exceeding 0.5 at midvisible wavelengths. Gradients of aerosol optical thickness were sampled to aid in separating aerosol effects from other radiative effects and to more tightly constrain closure tests, including those of satellite retrievals. Early results from TARFOX include demonstration of the unexpected importance of carbonaceous compounds and water condensed on aerosol in the United States East Coast haze plume, chemical apportionment of the aerosol optical depth, measurements of aerosol-induced changes in upwelling and downwelling shortwave radiative fluxes, and generally good agreement between measured flux changes and those calculated from measured aerosol properties. This overview presents the TARFOX objectives, rationale, overall experimental approach, and key initial findings as a guide to the more complete results reported in this special section and elsewhere.

Physical and optical properties of young smoke from individual biomass fires in Brazil

Physical and optical characteristics of particles in smoke from 19 fires were measured in Brazil during the 1995 burning season as part of the Smoke, Clouds, and Radiation-Brazil (SCAR-B) project. The University of Washington C-131A measured particle sizes and absorption and scattering properties in very young smoke (<4 min old). These properties are related to fuel type, fire intensity, combustion efficiency, and particle composition. The count median diameter (CMD) of particles from tropical forest fires were strongly and positively correlated with the combustion efficiency. The particle volume median diameter (VMD) of the particles from forest fires did not correlate well with combustion efficiency, but it was highly correlated with the emission factors of particles and unsaturated hydrocarbons. The median diameter and standard deviation of the particle size spectra for smoke from grass and cerrado fires did not correlate with either the combustion efficiency or any emission factor. The measured particle radiative properties correlated well with the measured particle sizes and compositions, and the relationships between these parameters are described fairly well by Mie theory. The optical properties of smoke from individual biomass fires in Brazil differ significantly from those of smoke from biomass burning in North America. In particular, the total light-scattering coefficient for smoke particles in Brazil is, on average, 15% less than for smoke particles in North America. Also, the average values of the single-scattering albedos of smoke particles in Brazil are 0.05 to 0.1 less than those in North America.

Use of the Ångstrom exponent to estimate the variability of optical and physical properties of aging smoke particles in Brazil

In situ airborne measurements from the Smoke, Clouds and Radiation-Brazil (SCAR-B) study show that during aging over 1–4 days the physical and optical properties of smoke particles are correlated. Consequently, if one optical or physical property of the smoke particles is determined, other properties can be derived. This methodology is validated using multiwavelength Angstrom exponents determined from the ground-based Sun photometer measurements in SCAR-B. It is shown that the Angstrom exponent determined from Sun photometers for the wavelength intervals 339–437 nm and 437–669 nm are well correlated with particle size, single-scattering albedo, and the backscatter ratio (r2>0.8). Therefore, when almucantar sky radiance data are not available and for remote sensing applications (such as MODIS), some of the uncertainties in the properties of smoke particles can be reduced by applying these relationships. Using this methodology, major oscillations were observed in smoke particle properties in Brazil on timescales of ∼5–15 days, resulting in variations of the volume median diameter and single-scattering albedo of ±0.04 μm and ±0.05, respectively. In comparison, the mean value of the dry smoke particle volume median diameter and single-scattering albedo over all of Brazil was 0.27 μm and 0.86, respectively. A daily cycle in smoke particle properties was also observed. The weekly and seasonal variability in the single-scattering albedo is shown to have significant consequences for retrieving aerosol optical depths from satellite measurements.