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Thomas P. Ackerman

Abstract

A one-dimensional, time-dependent model of the boundary layer has been developed to study the effects of pollutants an local meteorological variables. Radiative terms in the model are computed using a fourstream discrete-ordinate method, convective terms are parameterized at the surface using transfer coefficients, and dynamical terms are evaluated from available data. The model is compared initially to the Great Plains data with good results. The results of several model runs with varying types of pollutant concentrations are then discussed. The dominant conclusion that can be drawn from these results is that strong compensation between radiative and convective effects and between low-level heating and inversion rise act to minimize the effect of urban pollutants on surface and atmospheric temperatures. The model results also show that urban pollutants tend to warm the surface slightly and, possibly, to suppress the rate of inversion rise.

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Charles N. Long and Thomas P. Ackerman

Abstract

Pyranometers have been used for many years to measure broadband surface incoming solar irradiance, data that is necessary for surface energy budget, cloud forcing, and satellite validation research. Because such measurements are made at a specific location, it is unclear how representative they may be of a larger area. This study attempts to determine a reasonable spacing between measurement sites for such research by computing the correlation, and standard deviation from perfect correlation, between simultaneous measurements of incoming solar irradiance for a network of surface measurement sites covering a 75 km × 75 km area. Using 1-min data collected from this network of 11 sites during the NASA First ISSCP Radiation Experiment/Surface Radiation Budget Project temporal averages were calculated. The correlation between any two of these sites was determined by comparing simultaneous measurement averages for the 55 possible combinations of site pairs, along with the distances between them. In an attempt to remove the effect of the diurnal cycle, thus leaving clouds as the primary influence on correlation of the radiation field, model results for a clear day were used to normalize measured irradiances and correlations were again calculated.

For individual days, the correlation between sites varied widely, depending primarily on the type of cloud cover the region experienced that day. Removal of the diurnal cycle, as expected, significantly decreased these correlation values. Comparisons using the continuous experiment records from 13 October through 2 November 1986, however, show that a relatively high degree of correlation existed with or without the diurnal cycle removed. Plotting these correlation coefficients versus the distance between sites, the expected trend for a decrease in correlation with increasing distance is observed. Results also confirm that, whether using the complete record for the duration of the experiment or by individual day, the correlation between site station pairs increases with increasing averaging times. Finally, the standard deviation from perfect correlation suggests a predictive relationship within about 6% of clear-sky irradiance for daily averages at a distance of 75 km. Thus, a spacing of 150 km between measurement sites seems reasonable for studies of midlatitude frontal weather regimes using daily averages over periods of weeks or more.

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Gerald G. Mace and Thomas P. Ackerman

Abstract

A topic of current practical interest is the accurate characterization of the synoptic-scale atmospheric state from wind profiler and radiosonde network observations. The authors have examined several related and commonly applied objective analysis techniques for performing this characterization and considered their associated level of uncertainty both from a theoretical and a practical standpoint. A case study is presented where two wind profiler triangles with nearly identical centroids and no common vertices produced strikingly different results during a 43-h period. It is concluded that the uncertainty in objectively analyzed quantities can easily be as large as the expected synoptic-scale signal. In order to quantify the statistical precision of the algorithms, the authors conducted a realistic observing system simulation experiment using output from a mesoscale model. A simple parameterization for estimating the uncertainty in horizontal gradient quantities in terms of known errors in the objectively analyzed wind components and temperature is developed from these results.

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Roger Marchand, Nathaniel Beagley, and Thomas P. Ackerman

Abstract

Vertical profiles of hydrometeor occurrence from the multiscale modeling framework (MMF) climate model are compared with profiles observed by a vertically pointing millimeter wavelength cloud radar (located in the U.S. southern Great Plains) as a function of the large-scale atmospheric state. The atmospheric state is determined by classifying (or clustering) the large-scale (synoptic) fields produced by the MMF and a numerical weather prediction model using a neural network approach. The comparison shows that for cold-frontal and post-cold-frontal conditions the MMF produces profiles of hydrometeor occurrence that compare favorably with radar observations, while for warm-frontal conditions the model tends to produce hydrometeor fractions that are too large with too much cloud (nonprecipitating hydrometeors) above 7 km and too much precipitating hydrometeor coverage below 7 km. It is also found that the MMF has difficulty capturing the formation of low clouds and that, for all atmospheric states that occur during June, July, and August, the MMF produces too much high and thin cloud, especially above 10 km.

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Michael P. Jensen, Thomas P. Ackerman, and Stephen M. Sekelsky

Abstract

In this study the radiative impact of three separate cirrus anvil systems that occurred during the Maritime Continent Thunderstorm Experiment is investigated. Retrievals of microphysical cloud properties and an independent radar measurement are used to develop an appropriate set of radar reflectivity factor (Z)–ice water content (IWC) parameterizations. This set of parameterizations is then applied to the reflectivity field of a scanning 5.2-cm radar. The three-dimensional ice water structure is used as input to a two-stream radiative transfer model using an independent pixel approximation for several different stages in the life cycle of the cloud system. Peak radiative heating/cooling occurs at many different levels from just below the tropopause down to the freezing level. This behavior is attributed to spatial variability of the anvil cloud–top height. There is a distinct difference between the average radiative heating profile in the presence of island-based convection as compared with oceanic convection. The island-based convection results in a heating profile that concentrates cloud-top solar heating and IR cooling higher in the atmosphere and with a greater magnitude in comparison with studies of oceanic convection. Island-based thunderstorms can play a major role in the large-scale radiative energy balance. The net radiative convergence averaged over a simplified diurnal cycle and over a 120 × 120 km2 grid box containing an island-based thunderstorm and its associated anvil cloud is near zero. When considering the energy balance over the tropical western Pacific, it is important to consider the “Maritime Continent” region with all of its small islands separately from the oceanic regime.

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Jeffrey N. Cuzzi, Thomas P. Ackerman, and Leland C. Helmle

Abstract

We present an approximate technique for solving the radiative transfer problem for fluxes reflected, transmitted, and absorbed by a homogeneous scattering layer. The technique is a straightforward “delta-function” modification of a solution to the transfer equation in closed form via the discrete-ordinates method with four streams. By the use of four streams, significant improvement in accuracy is obtained for anisotropic phase functions over that obtained by similarly employed two-stream approximations with delta-functions. However, the computational effort is increased only slightly. We present equations for the general case of a layer underlaid by a reflecting Lambert surface. Graphical comparisons are given of fractional error resulting from the use or this method with that resulting from the use of typical four-stream and delta-two-stream (or delta-Eddington) techniques.

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Thomas P. Ackerman, Kuo-Nan Liou, and Conway B. Leovy

Abstract

A four-stream, multi-layered radiative transfer model has been developed to treat the problem of the transfer of infrared radiation in an atmosphere containing both scatterers and absorbers. Each atmospheric layer is isothermal and contains a uniform concentration of scatterers and absorbers. To facilitate the computations, the infrared spectrum was divided into four bands. In each band empirical transmission functions were fitted by a series of exponential functions. Test calculations of the infrared cooling rate were made using the empirical transmission functions and the fitted transmission functions. The resulting cooling rate profiles exhibit good agreement with each other. A model of a typical urban aerosol was developed using recent experimental results on the spectral dependence of the complex refractive index and the size distributions of aerosols. Atmospheric cooling rates as a function of height were computed on a band by band basis, with and without aerosols, in order to compare the effect of aerosols to the effect of H2O and CO2 in cooling the boundary layer. The presence of large, but realistic, concentrations of aerosol can substantially increase the infrared cooling of the aerosol-containing layer.

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Lyle D. Wilson, Judith A. Curry, and Thomas P. Ackerman

Abstract

This paper investigates the satellite detection of low-level ice crystal clouds in the Arctic using a radiative transfer model and aircraft observations for six cases. Brightness temperatures at the top of the atmosphere are calculated for the six cases and for the corresponding clear-sky cases. Sensitivity studies are conducted to determine the sensitivity of brightness temperature at the top of the atmosphere to ice crystal layer optical depth, clear-sky brightness temperature, and height of the top of the ice crystal layer. These results are interpreted in the context of the ISCCP cloud retrieval algorithm.

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Gerald G. Mace, Eugene E. Clothiaux, and Thomas P. Ackerman

Abstract

The properties of midlatitude cirrus clouds are examined using one year of continuous vertically pointing millimeter-wave cloud radar data collected at the Atmospheric Radiation Measurement Program Southern Great Plains site in Oklahoma. The goal of this analysis is to present the cloud characteristics in a manner that will aid in the evaluation and improvement of cirrus parameterizations in large-scale models. Using a temperature- and radar reflectivity–based definition of cirrus, the occurrence frequency of cirrus, the vertical location and thickness of cirrus layers, and other fundamental statistics are examined. Also the bulk microphysical properties of optically thin cirrus layers that occur in isolation from other cloud layers are examined. During 1997, it is found that cirrus were present 22% of the time, had a mean layer thickness of 2.0 km, and were most likely to occur in the 8.5–10-km height range. On average, the cirrus clouds tended to be found in layers in which the synoptic-scale vertical velocity was weakly ascending. The mean synoptic-scale vertical motion in the upper troposphere as derived from Rapid Update Cycle model output was +0.2 cm s−1. However, a significant fraction of the layers (33%) were found where the upper-tropospheric large-scale vertical velocity was clearly descending (w < −1.5 cm s−1). Microphysical properties were computed for that subset of cirrus events that were optically thin (infrared emissivity < 0.85) and occurred with no lower cloud layers. This subset of cirrus had mean values of ice water path, effective radius, and ice crystal concentration of 8 g m−2, 35 μm, and 100 L−1, respectively. Although all the cloud properties demonstrated a high degree of variability during the period considered, the statistics of these properties were fairly steady throughout the annual cycle. Consistent with previous studies, it is found that the cloud microphysical properties appear to be strongly correlated to the cloud layer thickness and mean temperature. Use of these results for parameterization of cirrus properties in large-scale models is discussed.

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Kuo-Nan Liou, Qiang Fu, and Thomas P. Ackerman

Abstract

We present a systematic development of the delta-four-stream approximation for calculations of radiative fluxes in planetary atmospheres. We illustrate that an analytic solution for this approximation can be derived explicitly with minimum computational effort for flux calculations. Relative accuracy checks for reflection, transmission, and absorption for numerous asymmetry factors, single-scattering albedos, optical depths, and solar zenith angles have been performed with respect to the “exact” results computed from the adding method for radiative transfer. Overall, results from the delta-four-stream approximation yield relative accuracies within about 5%. This approximation is well suited to radiative transfer parameterizations involving flux and heating calculations in aerosol and cloudy atmospheres.

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