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Abstract
A model of a cumulus cloud field, parameterized as right circular cylinders, has been used to estimate the uncertainties in longwave radiation calculations of irradiances and heating rates caused by neglecting the dimensions of the clouds. The results show that the irradiances and heating rates are nonlinear functions of the absolute cloud amount, height to radius ratio, and the cloud base and top altitudes. Cylindrically shaped clouds result in more downward irradiance at the surface (1–4%) and less escaping the atmosphere (up to 8%) than from flat plate clouds. The subcloud layer experiences as much as 20% more beating from cylindrical clouds than from flat plates, whereas the tropospheric column may experience up to 10% greater radiative cooling. Although the cloud parameters result in nonlinear effects, these effects may he taken into account with the use of easily calculable functions.
Abstract
A model of a cumulus cloud field, parameterized as right circular cylinders, has been used to estimate the uncertainties in longwave radiation calculations of irradiances and heating rates caused by neglecting the dimensions of the clouds. The results show that the irradiances and heating rates are nonlinear functions of the absolute cloud amount, height to radius ratio, and the cloud base and top altitudes. Cylindrically shaped clouds result in more downward irradiance at the surface (1–4%) and less escaping the atmosphere (up to 8%) than from flat plate clouds. The subcloud layer experiences as much as 20% more beating from cylindrical clouds than from flat plates, whereas the tropospheric column may experience up to 10% greater radiative cooling. Although the cloud parameters result in nonlinear effects, these effects may he taken into account with the use of easily calculable functions.
An analysis of the infrared irradiance data obtained from the upward- and downward-looking pyrgeometers on the NCAR Electra indicates that there are nonsystematic errors in the Monsoon Experiment (MONEX) data archived at the World Data Centers. The errors often exceed 8% of the downward flux and may be eliminated only by reanalyzing the 1 Hz data archived at NCAR. In addition, uncertainties in the measurement of the temperatures of the pyrgeometer surfaces lead to uncertainties in the corrected data, which may be important to particular applications.
An analysis of the infrared irradiance data obtained from the upward- and downward-looking pyrgeometers on the NCAR Electra indicates that there are nonsystematic errors in the Monsoon Experiment (MONEX) data archived at the World Data Centers. The errors often exceed 8% of the downward flux and may be eliminated only by reanalyzing the 1 Hz data archived at NCAR. In addition, uncertainties in the measurement of the temperatures of the pyrgeometer surfaces lead to uncertainties in the corrected data, which may be important to particular applications.
Abstract
Comparisons of zonally averaged outgoing longwave fluxes estimated from 10 μm radiance observations on NOAA polar orbiting satellites with flat plate observations on the Nimbus 6 ERB experiment have shown the NOAA estimates to be systematically larger than the ERB data. The NOAA flux estimation technique has been examined with the use of a new radiative transfer model and with several new assumptions concerning the properties of clouds in the field of view of the satellite to determine if there are situations which will lead to such differences. The analysis indicates that the operational estimation technique overestimates fluxes for middle and high cloud conditions. The use of new assumptions concerning the cloud cover and the use of a radiation model different from that used in the initial study will reduce the NOAA-ERB flux differences by approximately 30%.
Abstract
Comparisons of zonally averaged outgoing longwave fluxes estimated from 10 μm radiance observations on NOAA polar orbiting satellites with flat plate observations on the Nimbus 6 ERB experiment have shown the NOAA estimates to be systematically larger than the ERB data. The NOAA flux estimation technique has been examined with the use of a new radiative transfer model and with several new assumptions concerning the properties of clouds in the field of view of the satellite to determine if there are situations which will lead to such differences. The analysis indicates that the operational estimation technique overestimates fluxes for middle and high cloud conditions. The use of new assumptions concerning the cloud cover and the use of a radiation model different from that used in the initial study will reduce the NOAA-ERB flux differences by approximately 30%.
Abstract
Cumulus cloud bulk geometry, size, and spatial distributions have long been recognized as important factors for longwave radiative transfer under broken cloud conditions. Most current climate models, however, still ignore these factors and estimate the effects of broken cumulus clouds as the cloud amount–weighted average of clear and black-cloud overcast conditions, that is, the black plate approximation. Although several groups have adopted the simplicity of the black plate approximation and extended it to include the effects of cloud geometry, cloud size, and spatial distributions by defining an effective cloud fraction, the validity of these parameterizations has long been assumed because of inadequate measurements of the instantaneous atmospheric radiative properties. Now ground-based measurements at the Atmospheric Radiation Measurement Program southern Great Plains Cloud and Radiation Test Bed site allow the derivation of the effective cloud fraction, absolute cloud fraction, cloud aspect ratio, and many other variables characterizing cumulus clouds. Using an empirically determined sampling period of 10 min, several different parameterizations for effective cumulus cloud fraction were tested by comparing effective amounts derived from hemispheric flux observations with values predicted by the parameterizations. Within the range of data and among the models tested, the better results were obtained with the cuboidal model with exponential cloud size and spatial distributions, the random cylinder model, the regular cuboidal model, and the shifted-periodic array cuboidal model. However, there are few cases in the range of greatest sensitivity where model comparisons demonstrate larger disparity.
Abstract
Cumulus cloud bulk geometry, size, and spatial distributions have long been recognized as important factors for longwave radiative transfer under broken cloud conditions. Most current climate models, however, still ignore these factors and estimate the effects of broken cumulus clouds as the cloud amount–weighted average of clear and black-cloud overcast conditions, that is, the black plate approximation. Although several groups have adopted the simplicity of the black plate approximation and extended it to include the effects of cloud geometry, cloud size, and spatial distributions by defining an effective cloud fraction, the validity of these parameterizations has long been assumed because of inadequate measurements of the instantaneous atmospheric radiative properties. Now ground-based measurements at the Atmospheric Radiation Measurement Program southern Great Plains Cloud and Radiation Test Bed site allow the derivation of the effective cloud fraction, absolute cloud fraction, cloud aspect ratio, and many other variables characterizing cumulus clouds. Using an empirically determined sampling period of 10 min, several different parameterizations for effective cumulus cloud fraction were tested by comparing effective amounts derived from hemispheric flux observations with values predicted by the parameterizations. Within the range of data and among the models tested, the better results were obtained with the cuboidal model with exponential cloud size and spatial distributions, the random cylinder model, the regular cuboidal model, and the shifted-periodic array cuboidal model. However, there are few cases in the range of greatest sensitivity where model comparisons demonstrate larger disparity.
Abstract
A new model has been developed for calculating vertical profiles of longwave irradiance and heating rates. The infrared active bands taken into account include the pure rotational water vapor, the 6.3 µm water vapor, the 15 µm carbon dioxide, the 14 and 9.6 µm ozone, the 7.66 µm methane and the 7.78 µm nitrous oxide band systems. Nimbus 3 IRIS radiance spectra obtained on clear days near the BOMEX array were compared with theoretically calculated spectra to test the spectral and frequency integrated quality of the calculations for the 400 to 1400 cm−1 region. These comparisons combined with pessimistic estimates of errors in spectral regions not observed indicate that the clear-sky upward flux at the top of the atmosphere may be calculated to within 3%.
Abstract
A new model has been developed for calculating vertical profiles of longwave irradiance and heating rates. The infrared active bands taken into account include the pure rotational water vapor, the 6.3 µm water vapor, the 15 µm carbon dioxide, the 14 and 9.6 µm ozone, the 7.66 µm methane and the 7.78 µm nitrous oxide band systems. Nimbus 3 IRIS radiance spectra obtained on clear days near the BOMEX array were compared with theoretically calculated spectra to test the spectral and frequency integrated quality of the calculations for the 400 to 1400 cm−1 region. These comparisons combined with pessimistic estimates of errors in spectral regions not observed indicate that the clear-sky upward flux at the top of the atmosphere may be calculated to within 3%.
Abstract
In the longwave part of the spectrum, clouds are generally modeled in GCMs as flat black plates. The true effective cloud cover for transmittance of infrared radiation may be larger or smaller than the fractional cloud cover normal to the surface because of emittance of radiation from the sides of clouds into the clear sky and because the sides may have a finite cross section normal to the view and a vertical thermal gradient. The authors have derived the effective cloud cover as a function of zenith angle in terms of the cloud cover normal to the surface for several models of cumulus clouds with measured spatial and size distributions as a function of aspect ratio (height to radius or half-width) and shape. The effective cloud cover is shown as a function of cloud shape and aspect ratio as well as spatial distribution. The effective cloud cover is also sensitive to the thermal gradient between the cloud top and its base.
Abstract
In the longwave part of the spectrum, clouds are generally modeled in GCMs as flat black plates. The true effective cloud cover for transmittance of infrared radiation may be larger or smaller than the fractional cloud cover normal to the surface because of emittance of radiation from the sides of clouds into the clear sky and because the sides may have a finite cross section normal to the view and a vertical thermal gradient. The authors have derived the effective cloud cover as a function of zenith angle in terms of the cloud cover normal to the surface for several models of cumulus clouds with measured spatial and size distributions as a function of aspect ratio (height to radius or half-width) and shape. The effective cloud cover is shown as a function of cloud shape and aspect ratio as well as spatial distribution. The effective cloud cover is also sensitive to the thermal gradient between the cloud top and its base.
Abstract
The accuracy of radiation models is a critical issue in climate studies. However, calculations from different radiation models used in climate calculations disagree with one another, and with more detailed models, at levels significant to many climate problems. With several new advances in the field of radiation modeling, it is possible to develop more accurate band models and validate them against radiation observations of known high accuracy. In this paper, a new accurate narrowband longwave radiative transfer model for clear-sky conditions is developed. In the first part of this study, only water vapor effects are included, and the model results are tested against line-by-line radiative transfer model (LBLRTM) calculations.
In the model development, it is first shown that traditional techniques for estimating Malkmus statistical model parameters from the line compilation and line-by-line models cannot be trusted to give accurate transmittance function. A new technique is then described that calculates water vapor line transmittances with good agreement with LBLRTM calculations (i.e., with rms errors less than 0.01 for more than 97% of the intervals). The water vapor continuum is included in a manner consistent with the water vapor line absorption. Fluxes calculated with the model agree with LBLRTM to about 1 W m−2 for the entire vertical range of the atmosphere for several test cases. The heating rate errors are reduced by as much as 0.25°C day−1 below the tropopause for the test cases compared with the original narrowband model.
Abstract
The accuracy of radiation models is a critical issue in climate studies. However, calculations from different radiation models used in climate calculations disagree with one another, and with more detailed models, at levels significant to many climate problems. With several new advances in the field of radiation modeling, it is possible to develop more accurate band models and validate them against radiation observations of known high accuracy. In this paper, a new accurate narrowband longwave radiative transfer model for clear-sky conditions is developed. In the first part of this study, only water vapor effects are included, and the model results are tested against line-by-line radiative transfer model (LBLRTM) calculations.
In the model development, it is first shown that traditional techniques for estimating Malkmus statistical model parameters from the line compilation and line-by-line models cannot be trusted to give accurate transmittance function. A new technique is then described that calculates water vapor line transmittances with good agreement with LBLRTM calculations (i.e., with rms errors less than 0.01 for more than 97% of the intervals). The water vapor continuum is included in a manner consistent with the water vapor line absorption. Fluxes calculated with the model agree with LBLRTM to about 1 W m−2 for the entire vertical range of the atmosphere for several test cases. The heating rate errors are reduced by as much as 0.25°C day−1 below the tropopause for the test cases compared with the original narrowband model.
Abstract
The plane-parallel horizontal (PPH) hypothesis used to approximate clouds in GCMs neglects three-dimensional cloud effects. Such effects can amount to as much as 20 W m−2 in longwave radiation. Several investigators have proposed accounting for longwave three-dimensional cloud effects by using information on the probability of clear line of sight (PCLoS) to modify the PPH approximation. This study investigates the PCLoS at the Atmosphere Radiation Measurement (ARM) Program’s Tropical Western Pacific (TWP) site. PCLoS is estimated for single-layer tropical marine cumulus cases for 2-h intervals using the Whole Sky Imager (WSI) observations at the Manus and Nauru sites. PCLoS estimates are compared to calculations from a set of simple PCLoS models using measured cloud field statistics as input. A summary of the PCLoS at the TWP site is presented in addition to a statistical summary of retrieved cloud field characteristics. The results are used to investigate the spatial variability of the PCLoS and to test the usefulness of the parameterization of effective cloud fraction.
Abstract
The plane-parallel horizontal (PPH) hypothesis used to approximate clouds in GCMs neglects three-dimensional cloud effects. Such effects can amount to as much as 20 W m−2 in longwave radiation. Several investigators have proposed accounting for longwave three-dimensional cloud effects by using information on the probability of clear line of sight (PCLoS) to modify the PPH approximation. This study investigates the PCLoS at the Atmosphere Radiation Measurement (ARM) Program’s Tropical Western Pacific (TWP) site. PCLoS is estimated for single-layer tropical marine cumulus cases for 2-h intervals using the Whole Sky Imager (WSI) observations at the Manus and Nauru sites. PCLoS estimates are compared to calculations from a set of simple PCLoS models using measured cloud field statistics as input. A summary of the PCLoS at the TWP site is presented in addition to a statistical summary of retrieved cloud field characteristics. The results are used to investigate the spatial variability of the PCLoS and to test the usefulness of the parameterization of effective cloud fraction.
Abstract
Longwave radiative transfer under broken cloud conditions is often treated as a problem in cloud bulk geometry, especially for cumulus clouds, because individual clouds are nearly black. However, climate models ignore cloud geometry and estimate the effects of broken cumulus clouds as the cloud amount weighted average of clear and black cloud overcast conditions (i.e., the black plate approximation). To overcome the simplicity of the black plate approximation, the authors developed a more generalized form of cloud geometrical effects on the effective cloud fraction. Following previous work, this form includes parameters that allow a more precise specification of cloud size and spatial distributions. The sensitivity of the generalized form to the variation in cloud size and spatial distributions is discussed in relation to others. Model calculations show that cloud bulk geometrical shapes, aspect ratio, size distribution, and side inclination angle are the primary factors significantly affecting the effective cloud fraction. These parameters are important at all cloud amounts with greatest sensitivity when the cloud amount is between 0.2 and 0.8. On the other hand, cloud spatial distributions do not significantly influence the effective cloud fraction when absolute cloud amount is less than 0.2 and/or when the cloud aspect ratio is less than 0.5. However, in the range of greatest sensitivity with large aspect ratio and absolute amount, model comparisons show large intermodel differences. The model discussed herein is cloud size dependent and applies most directly to small cumulus clouds (i.e., clouds small compared to the area under consideration).
Abstract
Longwave radiative transfer under broken cloud conditions is often treated as a problem in cloud bulk geometry, especially for cumulus clouds, because individual clouds are nearly black. However, climate models ignore cloud geometry and estimate the effects of broken cumulus clouds as the cloud amount weighted average of clear and black cloud overcast conditions (i.e., the black plate approximation). To overcome the simplicity of the black plate approximation, the authors developed a more generalized form of cloud geometrical effects on the effective cloud fraction. Following previous work, this form includes parameters that allow a more precise specification of cloud size and spatial distributions. The sensitivity of the generalized form to the variation in cloud size and spatial distributions is discussed in relation to others. Model calculations show that cloud bulk geometrical shapes, aspect ratio, size distribution, and side inclination angle are the primary factors significantly affecting the effective cloud fraction. These parameters are important at all cloud amounts with greatest sensitivity when the cloud amount is between 0.2 and 0.8. On the other hand, cloud spatial distributions do not significantly influence the effective cloud fraction when absolute cloud amount is less than 0.2 and/or when the cloud aspect ratio is less than 0.5. However, in the range of greatest sensitivity with large aspect ratio and absolute amount, model comparisons show large intermodel differences. The model discussed herein is cloud size dependent and applies most directly to small cumulus clouds (i.e., clouds small compared to the area under consideration).
Abstract
This paper develops a nonlinear statistical method that uses satellite radiance observations directly to estimate the downward longwave radiation (DLR) at the earth's surface, a necessary component of the surface energy budget. The proposed technique has rms regression errors of about 9 W m–2 for clear-sky conditions, and about 4 to 8 W m–2 for overcast conditions, depending on the cloud levels. It is shown that this technique can produce unbiased estimates over a large range of meteorological conditions, which is crucial for climate studies.
Sensitivity studies show that the DLR is most sensitive to errors in the cloud amount on average. Overall, the combined errors for an instantaneous DLR estimate, excluding the effects of the surface pressure errors, range from about 7 to 12 W m–2 when there is a ±10% uncertainty in cloud amount and a ±100 hPa uncertainty in cloud-base pressure. When the cloud amount uncertainty rises to 30%, the combined DLR error ranges from about 10 to 25 W m–2.
This clear-sky DLR estimation technique was validated preliminarily by using simulated radiation data. The DLR differences between estimated and calculated values have a standard deviation of about 9 W m–2 and are unbiased in most conditions.
The validity of the DLR estimation technique has been demonstrated; however, validation for cloudy conditions, comparison with surface observations, and improvements related to surface pressure dependence and skin temperature discontinuity are left for future study.
Abstract
This paper develops a nonlinear statistical method that uses satellite radiance observations directly to estimate the downward longwave radiation (DLR) at the earth's surface, a necessary component of the surface energy budget. The proposed technique has rms regression errors of about 9 W m–2 for clear-sky conditions, and about 4 to 8 W m–2 for overcast conditions, depending on the cloud levels. It is shown that this technique can produce unbiased estimates over a large range of meteorological conditions, which is crucial for climate studies.
Sensitivity studies show that the DLR is most sensitive to errors in the cloud amount on average. Overall, the combined errors for an instantaneous DLR estimate, excluding the effects of the surface pressure errors, range from about 7 to 12 W m–2 when there is a ±10% uncertainty in cloud amount and a ±100 hPa uncertainty in cloud-base pressure. When the cloud amount uncertainty rises to 30%, the combined DLR error ranges from about 10 to 25 W m–2.
This clear-sky DLR estimation technique was validated preliminarily by using simulated radiation data. The DLR differences between estimated and calculated values have a standard deviation of about 9 W m–2 and are unbiased in most conditions.
The validity of the DLR estimation technique has been demonstrated; however, validation for cloudy conditions, comparison with surface observations, and improvements related to surface pressure dependence and skin temperature discontinuity are left for future study.
