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H. G. Leighton

Abstract

The variation in the evolution of a cloud droplet spectrum with collision efficiency is examined for sets of efficiencies based on different fluid mechanical models. The results indicate that the variation of collision efficiencies for droplet radii ≲30 µm are large enough to give appreciable variation in the droplet spectrum development. This is in contrast to the relative insensitivity to variations in the collision efficiencies of larger drops.

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H. G. Leighton

Abstract

A comparison is made between the evolution of initially exponential snowflake size distributions growing by deposition and aggregation as calculated with a numerical model and with the approximate analytical model of Passarelli. The agreement between the two models implies that to a good approximation the assumptions of the analytical model are consistent with the additional assumption that the distribution maintains an exponential shape.

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Zhanqing Li and H. G. Leighton

Abstract

A new technique for estimating broadband reflectance from Advanced Very High-Resolution Radiometer (AVHRR) narrowband reflectances in channel 1 and 2 is developed. The data used are simultaneous and coincident narrowband and broadband measurements made by the AVHRR and Earth Radiation Budget Experiment (ERBE) radiometers aboard NOAA-9 during four days in July 1985 in the region north of 60°N. The limitations and inefficiency of classical regressional methods when applied to datasets with high spatial auto-correlation, which is often the case for remotely sensed data, are discussed. A statistical variable, Moran's I, is introduced, which is specifically designed for testing against a null hypothesis of spatial independence. On the basis of Moran's I and a correlogram analysis of the spatial autocorrelation of measured reflectances, the data are sampled to provide a spatially independent dataset. In addition to sampling, the data are also screened with respect to spatial homogeneity. Both scene-dependent and scene-independent regressional models are developed that are based on these spatially independent datasets. The rms errors of the predicted broadband reflectance are found to be 1.0, 1.8, 2.0, and 3.1 for the ocean, land, ice-snow, and cloud data, respectively. The effects of scene discrimination and solar and viewing geometry on the regressions are investigated, and comparisons are made between two-channel and single-channel models. The use of two solar channels is found to give a significant improvement in the predicted broadband reflectance for datasets in which there is no scene discrimination, a small improvement for measurements over land, and no improvement for the other homogeneous scene types. Geometric factors are found to have no significant effect on the regressions.

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Kazuhiko Masuda, H. G. Leighton, and Zhanqing Li

Abstract

An earlier parameterization that relates the outgoing solar flux at the top of the atmosphere to the flux absorbed at the surface is modified and extended to allow for variations in atmospheric properties that were not considered in the original parameterization. Changes to the parameterization have also been introduced as a result of better treatment of water vapor absorption in the detailed radiative transfer calculations. Corrections are introduced that account for the height of the surface (surface pressure), ozone amount, aerosol type and amount, and cloud height and cloud type, which is characterized by an effective cloud droplet radius. Finally, the results of applying the parameterization to Earth Radiation Budget Satellite measurements are compared with the measurements of the net solar flux measured from two instrumented towers.

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A. S. Koziol and H. G. Leighton

Abstract

The significance of the influence of turbulence on collisions and coalescence of small cloud droplets is still an outstanding problem. In particular, the growth of droplets in the radius range 10 to 15 µm is not well understood. The present research examines whether or not turbulence affects the growth rate of such small drops by simulating trajectories of two hydrodynamically interacting droplets in a turbulent field. The trajectories were calculated with a model based on linear Stokes hydrodynamics. Turbulence was modeled in the form of random Fourier modes with both the space and time spectra prescribed. Both spectra were characterized by Kolmogorov scaling. The space spectrum was modeled in the inertial and dissipation subranges. On the basis of scale analysis, only small-scale time variations were allowed, and the so-called Eulerian–Lagrangian time spectrum was applied. The results show that most collision rates increase moderately in a turbulent flow characterized by rates of energy dissipation of the order of 1, 10, and 100 cm2 s−3.

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H. G. Leighton and R. R. Rogers

Abstract

The growth of cloud droplets by condensation and coalescence in a strong updraft is investigated for different cloud conditions and different initial droplet distributions. Growth by coalescence is determined by solving the stochastic collection equation, and growth by condensation is calculated from the diffusion equation.

The results indicate that under suitable conditions, starting from an initial droplet distribution centered at about 8 µm radius with a dispersion of 0.2, an appreciable number of cloud droplets will grow to precipitation sizes in times of the order of 10–15 min. The results are sensitive to the initial droplet concentration and moisture content of the cloud. The importance of combining coalescence and condensation raises the question of the validity of calculations of rain formation by coalescence only.

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Zhanqing Li, H. G. Leighton, Kazuhiko Masuda, and Tsutomu Takashima

Abstract

Measurements of radiation budgets, both at the top of the atmosphere (TOA) and at the surface, are essential to understanding the earth's climate. The TOA budgets can, in principle, be measured directly from satellites, while on a global scale surface budgets need to be deduced from TOA measurements. Most methods of inferring surface solar-radiation budgets from satellite measurements are applicable to particular scene types or geographic locations, and none is valid over highly reflective surfaces such as ice or snow. In addition, the majority of models require inputs such as cloud-optical thickness that are usually not known.

Extensive radiative transfer modeling for different surface, atmospheric, and cloud conditions suggests a linear relationship between the TOA-reflected flux and the flux absorbed at the surface for a fixed solar zenith angle (SZA). The linear relationship is independent of cloud-optical thickness and surface albedo. Sensitivity tests show that the relationship depends strongly on SZA and moderately on precipitable water and cloud type. The linear relationship provides a simple parameterization to estimate surface-absorbed flux from satellite-measured reflected flux at the TOA. Unlike other models, the present model makes explicit use of the SZA. Precipitable water is included as a secondary parameter. Surface-absorbed fluxes deduced from this simple parameterized model generally agree to within 10 W m−2 with the absorbed fluxes determined from detailed radiative transfer calculations, without including information on the presence or absence of cloud, cloud type, optical thickness, or surface type.

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Chong Wei, H. G. Leighton, and R. R. Rogers

Abstract

Using radiometer data collected during the Canadian Atlantic Storms Program, we have investigated five different methods of estimating the path-integrated, or columnar, cloud liquid water. The methods consist of one- and two-channel physical retrievals, the standard method of linear statistical inversion using two channels, and two statistical methods that proceed from an initial determination of several empirical regressions between measured and computed quantities. Though differing in details and complexity, the methods gave estimates of cloud liquid that did not deviate greatly from one another. We assessed the accuracy of the methods by simulation. Using hypothetical profiles of cloud liquid in archival soundings, we calculated the atmospheric emission and thus the brightness temperatures that would be measured in the two channels of the radiometer. These values were taken as data for the five methods, and the amount of liquid was calculated. Results showed that the three statistical methods were more accurate than the physical methods, but no one of the three was significantly better than the others. In the four methods requiring measurements in two channels, the columnar water vapor is computed as part of the retrieval procedure. A comparison of the computed with the actual vapor amounts showed that one of the statistical methods employing empirical regressions was the most accurate for vapor retrieval. For this optimum method, the rms deviation of the measured columnar liquid from its actual value was 0.159 mm and the rms deviation of the columnar vapor was 0.867 mm. As fractions of the overall average liquid and vapor in the simulations, these deviations amount to 37% and 8.7% respectively. If cases are excluded in which the liquid amount is small or nonexistent, the fractional deviation of the liquid estimates decreases and that of the vapor increases.

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R. E. Stewart, H. G. Leighton, P. Marsh, G. W. K. Moore, H. Ritchie, W. R. Rouse, E. D. Soulis, G. S. Strong, R. W. Crawford, and B. Kochtubajda

The Mackenzie River is the largest North American source of freshwater for the Arctic Ocean. This basin is subjected to wide fluctuations in its climate and it is currently experiencing a pronounced warming trend. As a major Canadian contribution to the Global Energy and Water Cycle Experiment (GEWEX), the Mackenzie GEWEX Study (MAGS) is focusing on understanding and modeling the fluxes and reservoirs governing the flow of water and energy into and through the climate system of the Mackenzie River Basin. MAGS necessarily involves research into many atmospheric, land surface, and hydrological issues associated with cold climate systems. The overall objectives and scope of MAGS will be presented in this article.

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Robin W. Pascal, Margaret J. Yelland, Meric A. Srokosz, Bengamin I. Moat,, Edward M. Waugh, Daniel H. Comben, Alex G. Cansdale, Mark C. Hartman, David G. H. Coles, Ping Chang Hsueh, and Timothy G. Leighton

Abstract

Waves and wave breaking play a significant role in the air–sea exchanges of momentum, sea spray aerosols, and trace gases such as CO2, but few direct measurements of wave breaking have been obtained in the open ocean (far from the coast). This paper describes the development and initial deployments on two research cruises of an autonomous spar buoy that was designed to obtain such open-ocean measurements. The buoy was equipped with capacitance wave wires and accelerometers to measure surface elevation and wave breaking, downward-looking still and video digital cameras to obtain images of the sea surface, and subsurface acoustic and optical sensors to detect bubble clouds from breaking waves. The buoy was free drifting and was designed to collect data autonomously for days at a time before being recovered. Therefore, on the two cruises during which the buoy was deployed, this allowed a variety of sea states to be sampled in mean wind speeds, which ranged from 5 to 18 m s−1.

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