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- Author or Editor: Thomas P. Ackerman x
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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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Abstract
A 4-yr climatology (1997–2000) of warm boundary layer cloud properties is developed for the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) site. Parameters in the climatology include cloud liquid water path, cloud-base height, and surface solar flux. These parameters are retrieved from measurements produced by a dual-channel microwave radiometer, a millimeter-wave cloud radar, a micropulse lidar, a Belfort ceilometer, shortwave radiometers, and atmospheric temperature profiles amalgamated from multiple sources, including radiosondes. While no significant interannual differences are observed in the datasets, there are diurnal variations with nighttime liquid water paths consistently higher than daytime values. The summer months of June, July, and August have the lowest liquid water paths and the highest cloud-base heights. Model outputs of cloud liquid water paths from the European Centre for Medium-Range Weather Forecasts (ECMWF) model and the Eta Model for 104 model output location time series (MOLTS) stations in the environs of the SGP central facility are compared to observations. The ECMWF and MOLTS median liquid water paths are greater than 3 times the observed values. The MOLTS data show lower liquid water paths in summer, which is consistent with observations, while the ECMWF data exhibit the opposite tendency. A parameterization of normalized cloud forcing that requires only cloud liquid water path and solar zenith angle is developed from the observations. The parameterization, which has a correlation coefficient of 0.81 with the observations, provides estimates of surface solar flux that are comparable to values obtained from explicit radiative transfer calculations based on plane-parallel theory. This parameterization is used to estimate the impact on the surface solar flux of differences in the liquid water paths between models and observations. Overall, there is a low bias of 50% in modeled normalized cloud forcing resulting from the excess liquid water paths in the two models. Splitting the liquid water path into two components, cloud thickness and liquid water content, shows that the higher liquid water paths in the model outputs are primarily a result of higher liquid water contents, although cloud thickness may a play a role, especially for the ECMWF model results.
Abstract
A 4-yr climatology (1997–2000) of warm boundary layer cloud properties is developed for the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) site. Parameters in the climatology include cloud liquid water path, cloud-base height, and surface solar flux. These parameters are retrieved from measurements produced by a dual-channel microwave radiometer, a millimeter-wave cloud radar, a micropulse lidar, a Belfort ceilometer, shortwave radiometers, and atmospheric temperature profiles amalgamated from multiple sources, including radiosondes. While no significant interannual differences are observed in the datasets, there are diurnal variations with nighttime liquid water paths consistently higher than daytime values. The summer months of June, July, and August have the lowest liquid water paths and the highest cloud-base heights. Model outputs of cloud liquid water paths from the European Centre for Medium-Range Weather Forecasts (ECMWF) model and the Eta Model for 104 model output location time series (MOLTS) stations in the environs of the SGP central facility are compared to observations. The ECMWF and MOLTS median liquid water paths are greater than 3 times the observed values. The MOLTS data show lower liquid water paths in summer, which is consistent with observations, while the ECMWF data exhibit the opposite tendency. A parameterization of normalized cloud forcing that requires only cloud liquid water path and solar zenith angle is developed from the observations. The parameterization, which has a correlation coefficient of 0.81 with the observations, provides estimates of surface solar flux that are comparable to values obtained from explicit radiative transfer calculations based on plane-parallel theory. This parameterization is used to estimate the impact on the surface solar flux of differences in the liquid water paths between models and observations. Overall, there is a low bias of 50% in modeled normalized cloud forcing resulting from the excess liquid water paths in the two models. Splitting the liquid water path into two components, cloud thickness and liquid water content, shows that the higher liquid water paths in the model outputs are primarily a result of higher liquid water contents, although cloud thickness may a play a role, especially for the ECMWF model results.
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.
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.
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.
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.
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.
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.
