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  • Author or Editor: Randolph D. Borys x
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Randolph D. Borys and Robert A. Duce

Abstract

Particulate lead, aluminum, sodium, vanadium, manganese and iodine, vapor phase iodine and ice nuclei were measured simultaneously in Providence, Rhode Island, over a period of one year. Interrelationships found were governed primarily by the physical properties of the aerosol. Weak positive correlations were observed between lead and ice nuclei. An argument is given using lead as an indicator of the aerosol surface area maximum, or that fraction of the aerosol which contains the greatest number of potential ice nucleating sites. Ice nucleus concentrations appeared to be controlled by large-scale air mass advection to the site studied.

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Melanie A. Wetzel, Randolph D. Borys, and Ling E. Xu

Abstract

Digital data from the National Oceanic and Atmospheric Administration Advanced very High Resolution Radiometer (AVHRR) satellite instrument provides multispectral images in visible near-infrared and thermal infrared wave bands, which have been utilized to develop retrieval techniques for estimating the droplet effective radius and optical depth of land-based fog. The retrieval methods are based on multiple scattering calculations that simulate the increased near-infrared absorption by fog layers with increasing droplet size and liquid water path. The AVHRR thermal window channels are utilized to remove the effects of thermal emission in the near-infrared band.

New instrumentation and field sampling methods have been developed for obtaining detailed vertical profiles of fog droplet size distributions and thermodynamic conditions in fog decks. The in situ measurements derived from the field observations were employed to test the satellite retrieval techniques. Intercomparison shows a close correspondence between field observations and retrieved values of the fog droplet effective radius as well as fog optical depth. Simulated 4-km near-infrared and visible pixel data are also used to test retrievals from GOES-8. The AVHRR and GOES-8 retrievals provide a mapped database of the fog microphysical and depth parameters over the entire region of fog, which may be applied to numerical simulation of fog evolution and pollutant deposition, newcasting of fog visibility hazards, and global monitoring of fog influences on the atmosphere-surface radiation budget.

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Edward E. Hindman, Mechel A. Campbell, and Randolph D. Borys

Abstract

Cloud-droplet spectra and coincident cloud water pH measurements have been made for a portion of ten consecutive winters (1983/84–1992/93) from clouds that enveloped Storm Peak Laboratory in northwestern Colorado; cloud water ion measurements were made for eight of the winters. To determine if the physical and chemical properties are related, the data were stratified into three populations: pH ≤3.6, 3.6 < pH < 4.6, pH ≥4.6. It was found that clouds with the smallest pH values (3.4) had the largest droplet concentrations (N 329 cm−3), smallest mean droplet diameters (D bar = 6.4 µm), and largest ion concentrations (e.g., SO4 4 = 5.7 mg L−1), while clouds with the largest pH values (5.1) had the smallest N values (189 cm−3), largest D bar values (8.0 µm), and smallest ion concentrations (SO4 4 = 3.9 mg L−1). Nevertheless, all three populations had similar liquid water contents (LWC ≅ 0.070 g m−3). The equation LWC = π/66Dbar 3 Nρ where ρ is the density of water, closely describes the relationship between LWC, D bar and N. The range in pH values could not be completely explained by entrainment, or variations in cloud-base height or in LWC; differences in cloud condensation nucleus composition appear to be a major factor. No significant trends in average winter N, D bar and pH values were found in the ten-winter record.

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Stephen M. Saleeby, William R. Cotton, Douglas Lowenthal, Randolph D. Borys, and Melanie A. Wetzel

Abstract

Pollution aerosols acting as cloud condensation nuclei (CCN) have the potential to alter warm rain clouds via the aerosol first and second indirect effects in which they modify the cloud droplet population, cloud lifetime and size, rainfall efficiency, and radiation balance from increased albedo. For constant liquid water content, an increase in CCN concentration (N CCN) tends to produce an increased concentration of droplets with smaller diameters. This reduces the collision and coalescence rate, and thus there is a local reduction in rainfall. While this process applies to warm clouds, it does not identically carry over to mixed-phase clouds in which crystal nucleation, crystal riming, crystal versus droplet fall speed, and collection efficiency play active roles in determining precipitation amount. Sulfate-based aerosols serve as very efficient cloud nuclei but are not effective as ice-forming nuclei. In clouds where precipitation formation is dominated by the ice phase, N CCN influences precipitation growth by altering the efficiency of droplet collection by ice crystals and the fall trajectories of both droplet and crystal hydrometeors. The temporal and spatial variation in both crystal and droplet populations determines the resultant snowfall efficiency and distribution. Results of numerical simulations in this study suggest that CCN can play a significant role in snowfall production by winter, mixed-phase, cloud systems when liquid and ice hydrometeors coexist. In subfreezing conditions, a precipitating ice cloud overlaying a supercooled liquid water cloud allows growth of precipitation particles via the seeder–feeder process, in which nucleated ice crystals fall through the supercooled liquid water cloud and collect droplets. Enhanced N CCN from sulfate pollution by fossil fuel emissions modifies the droplet distribution and reduces crystal riming efficiency. Reduced riming efficiency inhibits the rate of snow growth, producing lightly rimed snow crystals that fall slowly and advect farther downstream prior to surface deposition. Simulations indicate that increasing N CCN along the orographic barrier of the Park Range in north-central Colorado results in a modification of the orographic cloud such that the surface snow water equivalent amounts are reduced on the windward slopes and enhanced on the leeward slopes. The inhibition of snowfall by pollution aerosols (ISPA) effect has significant implications for water resource distribution in mountainous terrain.

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