As part of our efforts to characterize the cloud properties at the North Slope of Alaska (NSA) Cl... more As part of our efforts to characterize the cloud properties at the North Slope of Alaska (NSA) Cloud and Radiation Testbed (CART), we have used microwave radiometers (MWRs) and multi-filter rotating shadowband radiometers (MFRSRs) at Barrow (a coastal site) and Atqasuk (an inland site) to measure
Boundary-layer processes responsible for the formation of anomalous ozone profiles over the North Atlantic
In the summer of 1992, a series of ozone profiles were obtained with an airplane flying over the ... more In the summer of 1992, a series of ozone profiles were obtained with an airplane flying over the western North Atlantic Ocean as part of the North Atlantic Regional Experiment (NARE). Over the course of the experiment, ozone profiles with widely divergent characteristics were obtained, and concurrent measurements and analyses of meteorological data and other chemistry data suggested a number of mechanisms that could contribute to the observed behavior. In one intriguing category of ozone profiles observed on several days, the ozone values were generally low (<25--30 ppb) in the first 1--1.5 km above the surface but jumped rather sharply above that height to concentration values two or more times greater. These higher ozone values remained nearly constant to a height of approximately 2.5 km, the normal maximum sampling altitude for the airplane on these flights. The jump in ozone concentrations coincided closely with a sudden increase of potential temperature {theta}; below the le...
The results of some recently analyzed diffusion experiments are examined to study the effects of ... more The results of some recently analyzed diffusion experiments are examined to study the effects of source height on lateral and vertical dispersion characteristics. 0s at elevated release heights is shown to be a poor predictor of the lateral plume spread near the surface; q, near ground level is more useful at all downwind distances. For relatively short downwind distances, vertical dispersion for elevated releases is shown to be significantly larger than for surface releases.
Modifications to the properties of the boundary layer arising from surface flux variations over a... more Modifications to the properties of the boundary layer arising from surface flux variations over an area of 10 5 km 2 are simulated with a mesoscale model using realistic meteorology and distributions of surface fluxes. The surface fluxes are calculated with the SiB2 land parameterization scheme, which is driven by vegetation, soil, and meteorological data from the U.S. Department of Energy's Cloud and Radiation Testbed in Oklahoma and Kansas. Simulations were carried out for 15 case study days in July of 1995, during which synoptic conditions were relatively settled and significant flux contrasts existed over the site. Despite the prevalence of conditions apparently favorable to the development of thermally induced secondary circulations, the domain-averaged profiles of potential temperature and mixing ratios and the mean mixed-layer heights showed little differences when the spatially varying fluxes were replaced with uniform fluxes equal to the averages of the varying ones. The effects on simulated cloud fractions were also small except on one day. Sensitivity tests were made with a checkerboard distribution of contrasting surface fluxes and very light ambient wind speeds. These tests show that the use of such idealized conditions in numerical experiments can lead to serious overestimates of the likely strength of secondary circulations, the magnitudes of vertical velocities generated by convergence and divergence, and the importance of mesoscale fluxes of heat, moisture, and momentum.
The behaviour of intermittent turbulent temperature fluxes is characterized in terms of a flux in... more The behaviour of intermittent turbulent temperature fluxes is characterized in terms of a flux interval, defined as the time interval over which flux values are computed, and a flux threshold value. Fluxes whose magnitudes exceed the threshold value are defined as flux events while the times when the threshold is not exceeded are defined as gaps. Turbulent temperature flux data from five sites in Kansas, Utah, and Washington State, U.S.A. are examined within this descriptive framework. The turbulent event fraction f turb , the ratio of the time occupied by turbulent events to the total sampling time, is found to depend linearly on the average flux for the sampling period over a range of flux intervals, threshold values, and sampling times. As the average flux for a sampling period decreases toward zero, the median magnitude of the fluxes during the gaps also decreases but the median fluxes during the turbulent events become nearly independent of the sampling period average. A wide range of values of f turb is found for gradient Richardson numbers less than 0.3, indicating the possibility of considerable intermittency under weakly to moderately stable conditions
Th is report was prepared as an acco un t o f w or k spon so red by an agency o f the U nited Sta... more Th is report was prepared as an acco un t o f w or k spon so red by an agency o f the U nited States Govern me nt. Nei t her the U nited Stat es Go vernment nor any agency thereof, nor any of their employees, m akes an y warranty, express or impl ied, or ass umes any legal lia bility or respo n sibility for the accuracy, co mp leteness, or usefuln ess of any in for mati on , appara tus, p roduct , or process disclosed, or represe nts that its use w o ul d not i nfri nge p riva tely owned rights. Re ference herein to an y sp ecif ic co m m erci al pro duct, pro cess, or service by trade n ame, trademark , man ufactu re r, o r o therwi se, does not ne cessarily con stitute or i m pl y its endorse m ent, reco m men d ation , or favor in g by the U ni ted Sta tes Go ve rn me n t o r any age n cy t hereo f. Th e views and opi nio n s of authors exp ress ed herei n do not necess arily state o r re flect those of the Un ited State s Gove rn me nt o r any agen cy t hereof.
In this paper we describe a field measurement program for the evaluation of plume depletion model... more In this paper we describe a field measurement program for the evaluation of plume depletion models using simultaneously released, depositing and nondepositing tracers. The testing of such models requires at least three kinds of information: a description of the meteorology that governs atmospheric transport and diffusion, an estimate of how rapidly the material of interest deposits to the surface, and a measurement of the airborne concentration distributions that result from the atmospheric diffusion and deposition of this material. The second requirement is particularly troublesome, since estimates of deposition rates vary over several orders of magnitude. Data were obtained from six dual-tracer releases during moderately stable to near-neutral conditions. The behavior of the nondepositing tracer was used to deduce the diffusion meteorology, while both tracers were used to determine C/sub d//C/sub o/, the ratio of the crosswind-integrated, depositing and nondepositing tracer concentrations. The deposition velocity of the depositing tracer was also obtained from the dual-tracer concentrations, using a novel method based on the surface depletion model of Horst (1977). The measured values of C/sub d//C/sub o/, were then compared to the predicted values for each of four Gaussian plume-depletion models.
Turbulent Structure of Stably Stratified Nocturnal Slope Flows
Abstract : Turbulence characteristics of wind and temperature fields in drainage flows over a sim... more Abstract : Turbulence characteristics of wind and temperature fields in drainage flows over a simple slope were studied. Data was collected on a simple, nearly two-dimensional mountain ridge with little vegetation. Anemometers, thermistors, and thermometers were used to collect mean and turbulent wind and temperature data 150 m below the ridgeline. Evidence for katabatically driven winds was found for a wide range of ambient conditions whenever a surface inversion was formed. When a velocity jet characteristic of good slope flows was present, the vertical profiles of turbulence were found to differ significantly from those over flat terrain, with the turbulent kinetic energy no longer a monotonically decreasing function of height. Richardson number profiles revealed a value well in excess of critical values in the region of the jet. The existence of an internal mixing layer, found by other investigators in deeper drainage flows, was not confirmed. Calculations of turbulent kinetic energy budgets showed a close balance between shear production and viscous dissipation, with buoyancy forces playing a relatively minor role. Above the jet, the contribution from the vertical heat flux was only a few percent of the shear production term and resulted from a near-calculation of the contributions from the heat flux normal to the sloping surface and flux parallel to the surface. In this region, the heat flux parallel to the slope was upslope, resulting in an energy production term, while below the jet it was downslope, resulting in an energy loss. Keywords: Complex terrain.
The treatment of subgrid-scale variations of surface properties and the resultant spatial variati... more The treatment of subgrid-scale variations of surface properties and the resultant spatial variations of sensible and latent heat fluxes has received increasing attention in recent years. Mesoscale numerical simulations of highly idealized conditions, in which strong flux contrasts exist between adjacent surfaces, have shown that under some circumstances the secondary circulations induced by land-use differences can significantly affect the properties of the planetary boundary layer (PBL) and the region of the atmosphere above the PBL. At the Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) site, the fluxes from different land-surface types are not expected to differ as dramatically as those found in idealized simulations. Although the corresponding effects on the atmosphere should thus be less dramatic, they are still potentially important. From an ARM perspective, in tests of single column models (SCM's) it would be useful to understand the effects of the lower bo...
In this paper we discuss the use of rotating shadowband radiometers and microwave radiometers to ... more In this paper we discuss the use of rotating shadowband radiometers and microwave radiometers to find shortwave cloud optical depth and cloud effective radius at two Arctic sites. These sites are the SHEBA ice camp site (a field study undertaken in 1997 and 1998) and the ARM Barrow (AK) site. Special measures are necessary to process the data from the SHEBA site to account for the harsh environment in which the instruments reside. The analysis shows that, over the summer of 1998, the median cloud optical depth at the SHEBA site is greater than the median cloud optical depth at the Barrow site. The cloud droplet effective radius is less at the SHEBA site than the Barrow site.
Spatial distribution of intermittent turbulence during CASES99
A suite of three sonic anemometers, two minisodars, and a laser scintillometer were used at four ... more A suite of three sonic anemometers, two minisodars, and a laser scintillometer were used at four locations in the Atmospheric Boundary Layer Experiments (ABLE) facility during the Cooperative Atmosphere Surface Exchange Study 1999 (CASES99) to study the spatial and temporal evolution of intermittent turbulence during very stable conditions. Two locations were within the main CASES99 site at Leon, KS, separated by roughly 1 km; the remaining sites were about 15 km east (Beaumont) and 20 km south (Smileyburg). Sudden increases from an otherwise quiescent signal of downward heat flux (as measured by sonic anemometers and lasers scintillometer) or temperature structure parameter correlation with vertical velocity (from minisodars) were used to define intermittent turbulence. It was generally found that there was little or no one-to-one correlation among turbulent episodes at Leon, Beaumont, and Smileyburg. Within the Beaumont and Leon sites, there were multiple instances of near-simultaneous bursts as observed with sonic - minisodar and scintillometer - minisodar comparisons separated by 20 - 100 m, perhaps indicating that the spatial extent of nocturnal bursts of turbulence varies from a few meters to greater than 1 km but rarely extends beyond 10 km. There were several well defined events, such as microfronts and/ormore » density currents, that were observed at all four sites, generally with a time delay. During the 30 day experimental period, there were only 6 nights with largely quiescent conditions punctuated by periods of intermittent turbulence; the remaining nights were characterized by nearly continuous significant downward heat fluxes, forced by strong winds aloft. The six nights were characterized by relatively light winds and predominantly northerly wind directions. The overall character of the turbulence in terms of duration, frequency of occurrence, and range of turbulence intensity was similar at all sites. However, there were significant differences between overall turbulence intensities between sites on individual nights.« less
A field study of the effects of inhomogeneities of surface sensible and latent heat fluxes
In recent years, the problem of characterizing turbulent fluxes of heat, momentum, and moisture o... more In recent years, the problem of characterizing turbulent fluxes of heat, momentum, and moisture over inhomogeneous surfaces has received increasing attention. This issue is relevant to the performance of general circulation models (GCMs), in which a single grid element can encompass a variety of surface and topographical features. Although considerable progress has been made in describing the energy balance at a surface partially covered by vegetation, less is known about how to treat adjacent regions of sharply contrasting surface characteristics. One difficulty is the scarcity of suitable data sets with which to study the problem, particularly on scales of tens to hundreds of kilometers.
This report was prepared as an account of work sponsored by the Uni ted St ates Governmen t. N ei... more This report was prepared as an account of work sponsored by the Uni ted St ates Governmen t. N either the United States nor the Department of Energy, nor an y of thei r employees. nor any o f their contractors. su bcontractors. or their emp lo yees. makes any w arrant y. express or imp lied. or assumes an y legal liabil ity o r respons ibility for the accuracy. co mpleteness or use fu lness of any in formation , apparatu s, prod u ct or process disclosed. or represents that its use wou ld not in fringe pri vately owned rig hts. The views. opinions and conclusions contained in t his report are those of the contractor and do not necessar ily represent those of the United States Government or t he United States Depart ment of Energy.
Cloud-Surface Interaction on the North Slope of Alaska
The radiation balance in the Arctic is widely understood to be a major control for global climate... more The radiation balance in the Arctic is widely understood to be a major control for global climate, and because of the routine occurrence of clouds, cloudradiative interactions are a prominent part of that balance. Curry et al. cited studies that indicated the cloud fraction in the Arctic is a minimum in winter (4070%) and reaches a maximum of approximately 90% in summer. Parameterizations for the interaction between clouds and radiation are generally given in terms of liquid water path (LWP) and droplet or ice crystal effective radius. We have analyzed three years of radiation and radiatively important cloud properties, such as liquid water path and cloud optical thickness, from Alaskas North Slope to see what role surface interactions may play in establishing cloud radiative characteristics. We have focused on summer (warm) clouds, since at that time liquid water clouds provide the strongest radiative forcing and surface interactions are likely to be most important. In this paper, ...
The magnetization of Ag-Mn alloys containing 100 ppm-4 ppm of Mn has been measured from 0.5 K dow... more The magnetization of Ag-Mn alloys containing 100 ppm-4 ppm of Mn has been measured from 0.5 K down to 10 mK. For the high concentrations, ordering sets in at a temperature proportional to impurity concentration; there is no evidence for a spin-compensated Kondo state.
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Papers by JC Doran