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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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Randolph D. Borys, Kapin Tan, and William Cotton

Until recently, investigations of the temperature structure of the planetary boundary layer have been confined to the use of balloon soundings (tethered balloon sondes, rawinsondes), disposable dropsondes, or high performance instrumented aircraft. These methods can be quite restrictive in their ability to obtain detailed temporal and spatial resolutions, especially in areas of limited accessibility. The operating cost of an instrumented aircraft also may be prohibitive. From this perspective, the use of an ultralight sounder—a meteorological sensor mounted on a motorized glider—is described, and its versatility is discussed. This system was employed in measuring the vertical temperature structure in mountainous terrain during the winter months of 1981–82. The system's capability to obtain detailed vertical temperature structure, as attested by the data gathered, renders it invaluable in the study of the planetary boundary layer in complex mountainous terrain.

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Randolph D. Borys and Melanie A. Wetzel

The Storm Peak Laboratory (SPL), operated by the Atmospheric Sciences Center of the Desert Research Institute, is now located in a newly constructed permanent building at elevation 3210 m (10 530 ft) above mean sea level in the northwestern Colorado Rocky Mountains. The laboratory provides a site for the conduct of basic and applied research in the atmospheric sciences, hands-on instruction in meteorology for students ranging from middle school through graduate school, and high-elevation atmospheric measurement programs for various scientific groups, agencies, and private companies. This article provides a background of the history of SPL, its past and current activities, and a description of the facilities and opportunities available at the laboratory.

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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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Brian J. Billings, Vanda Grubišić, and Randolph D. Borys

Abstract

A persistent cold-air pool in the Yampa Valley of northwestern Colorado was simulated with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). The observed cold-air pool, which was identified by temperature measurements along a line of surface stations ascending the eastern side of the valley, remained in place throughout the day of 10 January 2004. The baseline simulation with horizontal resolution of 1 km, which is close to the resolution of operational regional mesoscale model forecasts, neither matched the strength of the observed cold-air pool nor retained the cold pool throughout the day. Varying the PBL parameterization, increasing the vertical resolution, and increasing the model spinup time did not significantly improve the results. However, the inclusion of snow cover, increased horizontal resolution, and an improved treatment of horizontal diffusion did have a sizable effect on the forecast quality. The snow cover in the baseline simulation was essential for preventing the diurnal heating from eroding the cold pool, but was only sufficient to produce a nearly isothermal temperature structure within the valley, largely because of an increased reflection of solar radiation. The increase of horizontal resolution to 333 and 111 m resulted in a stronger cold-air pool and its retention throughout the day. In addition to improving the resolution of flow features in steep terrain, resulting in, for example, less drainage out of the valley, the increase in horizontal resolution led to a better forecast because of a reduced magnitude of horizontal diffusion calculated along the terrain-following model surfaces. Calculating horizontal diffusion along the constant height levels had a beneficial impact on the quality of the simulations, producing effects similar to those achieved by increasing the horizontal resolution, but at a fraction of the computational cost.

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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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Edward E. Hindman, Elizabeth J. Carter, Randolph D. Borys, and David L. Mitchell

Abstract

Supercooled cloud droplets were inertially impacted onto “cloud-sieves” at a mountaintop location. The large cross-sectional areas of the sieve meshes permitted grams of cloud water to be passively collected in minutes. Each sieve was constructed from specific diameter cylindrical strands and collected all cloud droplets larger than a critical size. Procedures are developed to produce liquid water content (LWC) and chemical composition values as a function of droplet-size interval.

The sieve LWC measurements were compared with simultaneous LWC measurements obtained from a standard cloud droplet spectrometer. The sieve and spectrometer values were consistent for droplets between approximately 4 and 13 µm in diameter. The sieves overestimated the water contents of larger and smaller droplets in low LWC clouds (<0.1 gm−3). In high LWC clouds, the sieve LWC values for all droplet sizes closely approximated the spectrometer values.

Sources of error were investigated. Rime “feathers” and frost grew on the larger sieves in low-LWC clouds, capturing droplets smaller than the sieves critical size. Frost growth on the smallest sieve overestimated LWC values of the smallest droplets. Procedures are suggested to overcome these limitations.

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