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Abstract
Many theoretical studies have shown that aerosol-induced changes in the earth-atmosphere albedo might be an important climate change mechanism. However, there has been a lack of experimental documentation of albedo changes caused by actual aerosol layers with measured properties. Here we report an incident in which the measured surface-plus-atmosphere albedo was increased by about 0.01 (from 0.11 to 0.12) by a transient aerosol layer. We also report simultaneous measurements of the aerosol by a multi-wavelength sunphotometer, a lidar, a nephelometer and other radiometers, and we use these aerosol measurements to deduce an expected albedo change for comparison to the measurements.
Specifically, we combine the aerosol measurements with several assumed refractive indices to derive a time-dependent aerosol optical model for the day of the incident. We then use this model in a two-stream radiative calculation to compute the expected time-dependent aerosol-layer albedo. Finally, we compute aerosol-plus-surface albedos by modifying a familiar expression to account for changing solar zenith angle and the diffuseness of surface reflectivity. Use of the aerosol model in this expression yields a calculated time-dependent atmosphere-plus-surface albedo that agrees with the measurements, provided an aerosol refractive index of about 1.50−0.01i is assumed. This refractive index value is in accord with the aerosol backscatter-to-extinction ratios measured simultaneously by the lidar and sunphotometer.
To our knowledge, this incident is the first in which an aerosol-induced albedo change and the responsible aerosol have been simultaneously measured to this degree of detail. Although the incident was too brief to be climatically significant, the analysis is significant because it provides a practical methodology for incorporating measured properties of aerosol layers into efficient albedo-change calculations. This methodology, which uses ground-based measurements to characterize elevated aerosol layers, could be applied to more widespread and persistent (hence, climatically significant) aerosol layers. Moreover, the agreement between measured and calculated albedos in this incident provides an initial validation of the methodology for not uncommon surface and aerosol conditions. More general measurements, including better complex refractive index determinations, are required to further validate and apply the methodology.
Abstract
Many theoretical studies have shown that aerosol-induced changes in the earth-atmosphere albedo might be an important climate change mechanism. However, there has been a lack of experimental documentation of albedo changes caused by actual aerosol layers with measured properties. Here we report an incident in which the measured surface-plus-atmosphere albedo was increased by about 0.01 (from 0.11 to 0.12) by a transient aerosol layer. We also report simultaneous measurements of the aerosol by a multi-wavelength sunphotometer, a lidar, a nephelometer and other radiometers, and we use these aerosol measurements to deduce an expected albedo change for comparison to the measurements.
Specifically, we combine the aerosol measurements with several assumed refractive indices to derive a time-dependent aerosol optical model for the day of the incident. We then use this model in a two-stream radiative calculation to compute the expected time-dependent aerosol-layer albedo. Finally, we compute aerosol-plus-surface albedos by modifying a familiar expression to account for changing solar zenith angle and the diffuseness of surface reflectivity. Use of the aerosol model in this expression yields a calculated time-dependent atmosphere-plus-surface albedo that agrees with the measurements, provided an aerosol refractive index of about 1.50−0.01i is assumed. This refractive index value is in accord with the aerosol backscatter-to-extinction ratios measured simultaneously by the lidar and sunphotometer.
To our knowledge, this incident is the first in which an aerosol-induced albedo change and the responsible aerosol have been simultaneously measured to this degree of detail. Although the incident was too brief to be climatically significant, the analysis is significant because it provides a practical methodology for incorporating measured properties of aerosol layers into efficient albedo-change calculations. This methodology, which uses ground-based measurements to characterize elevated aerosol layers, could be applied to more widespread and persistent (hence, climatically significant) aerosol layers. Moreover, the agreement between measured and calculated albedos in this incident provides an initial validation of the methodology for not uncommon surface and aerosol conditions. More general measurements, including better complex refractive index determinations, are required to further validate and apply the methodology.
Abstract
We compare a series of 85 dustsonde measurements and 84 lidar measurements made in midlatitude North America during 1974–80. This period includes two major volcanic increases (Fuego in 1974 and St. Helens in 1980), as well as an unusually clean, or background, period in 1978–79. An optical modeling technique is used to relate the dustsonde-number data to the lidar-backscatter data. The model includes a range of refractive indices and of size distribution functional forms, to show its sensitivity to these factors. Moreover, two parameters of each size distribution function are adjustable, so that each distribution can be matched to any two-channel dustsonde measurement.
We show how the mean particle radius for backscatter, rB , changes in response to size distribution changes revealed by the dustsonde channel ratio, N r>0.15/N r>0.25. (N r>x is the number of particles with radius larger than x microns.) In early 1975, just after the Fuego injection, N r>0.15/N r>0.25 was ∼3, and the corresponding rB , was ∼0.5 μm; by early 1980, when N r>0.15/N r>0.25 had increased to eight or larger, rB had correspondingly decreased to ∼0.25 μm. Throughout the 1975–76 Fuego decay, rB always exceeded 0.3 μm; thus, lidar backscatter was influenced primarily by particles larger than those that contribute most to N r>0.15 and N r>0.25. This is in accord with the shorter lidar background-corrected, 1/e decay time: 7.4 months, versus 10.4 and 7.9 months for N r>0.15 and N r>0.25.
The modeling technique is used to derive a time series of dustsonde-inferred peak backscatter mixing ratio, which agrees very well with the lidar-measured series. The best overall agreement for 1974–80 is achieved with a mixture of refractive indices corresponding to aqueous sulfuric acid at about 210 K with an acid-weight fraction between 0.6 and 0.85.
Abstract
We compare a series of 85 dustsonde measurements and 84 lidar measurements made in midlatitude North America during 1974–80. This period includes two major volcanic increases (Fuego in 1974 and St. Helens in 1980), as well as an unusually clean, or background, period in 1978–79. An optical modeling technique is used to relate the dustsonde-number data to the lidar-backscatter data. The model includes a range of refractive indices and of size distribution functional forms, to show its sensitivity to these factors. Moreover, two parameters of each size distribution function are adjustable, so that each distribution can be matched to any two-channel dustsonde measurement.
We show how the mean particle radius for backscatter, rB , changes in response to size distribution changes revealed by the dustsonde channel ratio, N r>0.15/N r>0.25. (N r>x is the number of particles with radius larger than x microns.) In early 1975, just after the Fuego injection, N r>0.15/N r>0.25 was ∼3, and the corresponding rB , was ∼0.5 μm; by early 1980, when N r>0.15/N r>0.25 had increased to eight or larger, rB had correspondingly decreased to ∼0.25 μm. Throughout the 1975–76 Fuego decay, rB always exceeded 0.3 μm; thus, lidar backscatter was influenced primarily by particles larger than those that contribute most to N r>0.15 and N r>0.25. This is in accord with the shorter lidar background-corrected, 1/e decay time: 7.4 months, versus 10.4 and 7.9 months for N r>0.15 and N r>0.25.
The modeling technique is used to derive a time series of dustsonde-inferred peak backscatter mixing ratio, which agrees very well with the lidar-measured series. The best overall agreement for 1974–80 is achieved with a mixture of refractive indices corresponding to aqueous sulfuric acid at about 210 K with an acid-weight fraction between 0.6 and 0.85.
Abstract
In January–February 2003, the 14-channel NASA Ames airborne tracking sun photometer (AATS) and the NASA Langley/Ames diode laser hygrometer (DLH) were flown on the NASA DC-8 aircraft. The AATS measured column water vapor on the aircraft-to-sun path, while the DLH measured local water vapor in the free stream between the aircraft fuselage and an outboard engine cowling. The AATS and DLH measurements have been compared for two DC-8 vertical profiles by differentiating the AATS column measurement and/or integrating the DLH local measurement over the altitude range of each profile (7.7–10 km and 1.1–12.5 km). These comparisons extend, for the first time, tests of AATS water vapor retrievals to altitudes >∼6 km and column contents <0.1 g cm−2. To the authors’ knowledge, this is the first time suborbital spectroscopic water vapor measurements using the 940-nm band have been tested in conditions so high and dry. Values of layer water vapor (LWV) calculated from the AATS and DLH measurements are highly correlated for each profile. The composite dataset yields r 2 0.998, rms difference 7.7%, and bias (AATS minus DLH) 1.0%. For water vapor densities AATS and DLH had r 2 0.968, rms difference 27.6%, and bias (AATS minus DLH) −4.2%. These results for water vapor density compare favorably with previous comparisons of AATS water vapor to in situ results for altitudes <∼6 km, columns ∼0.1 to 5 g cm−2, and densities ∼0.1 to 17 g m−3.
Abstract
In January–February 2003, the 14-channel NASA Ames airborne tracking sun photometer (AATS) and the NASA Langley/Ames diode laser hygrometer (DLH) were flown on the NASA DC-8 aircraft. The AATS measured column water vapor on the aircraft-to-sun path, while the DLH measured local water vapor in the free stream between the aircraft fuselage and an outboard engine cowling. The AATS and DLH measurements have been compared for two DC-8 vertical profiles by differentiating the AATS column measurement and/or integrating the DLH local measurement over the altitude range of each profile (7.7–10 km and 1.1–12.5 km). These comparisons extend, for the first time, tests of AATS water vapor retrievals to altitudes >∼6 km and column contents <0.1 g cm−2. To the authors’ knowledge, this is the first time suborbital spectroscopic water vapor measurements using the 940-nm band have been tested in conditions so high and dry. Values of layer water vapor (LWV) calculated from the AATS and DLH measurements are highly correlated for each profile. The composite dataset yields r 2 0.998, rms difference 7.7%, and bias (AATS minus DLH) 1.0%. For water vapor densities AATS and DLH had r 2 0.968, rms difference 27.6%, and bias (AATS minus DLH) −4.2%. These results for water vapor density compare favorably with previous comparisons of AATS water vapor to in situ results for altitudes <∼6 km, columns ∼0.1 to 5 g cm−2, and densities ∼0.1 to 17 g m−3.
Abstract
We present a model of stratospheric aerosol optical properties (refractive index and relative size distribution) and their variability. The model's purposes are 1) providing flexible, efficient means for converting between different aerosol macroproperties (e.g., number or mass concentration, extinction or backscatter coefficient), and 2) quantifying the uncertainties in the conversion process. The latter purpose is achieved by including the results of a sensitivity analysis in the model output products.
The model has three layers, the boundaries of which are defined by tropopause height. Each layer includes a set of empirically based refractive indices and relative size distribution types. In contrast to previous models, this model allows for a range of sulfuric acid and ammonium sulfate refractive indices within the “inner stratospheric” layer (∼2 to 20 km above the tropopause, where the major peak in aerosol mixing ratio occurs). We show that nine different analytical types of size distribution previously recommended for this layer can be parameterized in terms of channel ratio—i.e., the relative size distribution indicator that has been extensively measured by dustsondes.
When so parameterized, all nine inner stratospheric function types give very similar results for the several conversion ratios of interest. This parameterization allows considerable saving of computer time while preserving the flexibility to handle certain types of size distribution change. We show that the inner stratospheric parameterization works because all nine inner stratospheric size distribution types are relatively narrow, and their optical integrals of interest are determined primarily by a size range that is well characterized by channel ratio.
Data from previous measurements made near the tropopause are used to demonstrate that, in that region, size distributions are broader than any of the inner stratospheric types, and that their optical integrals are strongly influenced by particles too large to be characterized by channel ratio. Hence, in the layer near the tropopause, conversion ratios can differ significantly from the inner stratospheric values; consequently, parameterization by channel ratios is not successful.
We develop methods for deriving vertical profiles of several conversion ratios and their uncertainties. We also demonstrate an application of the model: deriving profiles of number density and its uncertainty from satellite-measured profiles of extinction and its uncertainty. A companion paper applies the model to the task of validating satellite measurements of stratospheric aerosol extinction.
Abstract
We present a model of stratospheric aerosol optical properties (refractive index and relative size distribution) and their variability. The model's purposes are 1) providing flexible, efficient means for converting between different aerosol macroproperties (e.g., number or mass concentration, extinction or backscatter coefficient), and 2) quantifying the uncertainties in the conversion process. The latter purpose is achieved by including the results of a sensitivity analysis in the model output products.
The model has three layers, the boundaries of which are defined by tropopause height. Each layer includes a set of empirically based refractive indices and relative size distribution types. In contrast to previous models, this model allows for a range of sulfuric acid and ammonium sulfate refractive indices within the “inner stratospheric” layer (∼2 to 20 km above the tropopause, where the major peak in aerosol mixing ratio occurs). We show that nine different analytical types of size distribution previously recommended for this layer can be parameterized in terms of channel ratio—i.e., the relative size distribution indicator that has been extensively measured by dustsondes.
When so parameterized, all nine inner stratospheric function types give very similar results for the several conversion ratios of interest. This parameterization allows considerable saving of computer time while preserving the flexibility to handle certain types of size distribution change. We show that the inner stratospheric parameterization works because all nine inner stratospheric size distribution types are relatively narrow, and their optical integrals of interest are determined primarily by a size range that is well characterized by channel ratio.
Data from previous measurements made near the tropopause are used to demonstrate that, in that region, size distributions are broader than any of the inner stratospheric types, and that their optical integrals are strongly influenced by particles too large to be characterized by channel ratio. Hence, in the layer near the tropopause, conversion ratios can differ significantly from the inner stratospheric values; consequently, parameterization by channel ratios is not successful.
We develop methods for deriving vertical profiles of several conversion ratios and their uncertainties. We also demonstrate an application of the model: deriving profiles of number density and its uncertainty from satellite-measured profiles of extinction and its uncertainty. A companion paper applies the model to the task of validating satellite measurements of stratospheric aerosol extinction.
This paper describes the life cycle of the background (nonvolcanic) stratospheric sulfate aerosol. The authors assume the particles are formed by homogeneous nucleation near the tropical tropopause and are carried aloft into the stratosphere. The particles remain in the Tropics for most of their life, and during this period of time a size distribution is developed by a combination of coagulation, growth by heteromolecular condensation, and mixing with air parcels containing preexisting sulfate particles. The aerosol eventually migrates to higher latitudes and descends across isentropic surfaces to the lower stratosphere. The aerosol is removed from the stratosphere primarily at mid- and high latitudes through various processes, mainly by isentropic transport across the tropopause from the stratosphere into the troposphere.
This paper describes the life cycle of the background (nonvolcanic) stratospheric sulfate aerosol. The authors assume the particles are formed by homogeneous nucleation near the tropical tropopause and are carried aloft into the stratosphere. The particles remain in the Tropics for most of their life, and during this period of time a size distribution is developed by a combination of coagulation, growth by heteromolecular condensation, and mixing with air parcels containing preexisting sulfate particles. The aerosol eventually migrates to higher latitudes and descends across isentropic surfaces to the lower stratosphere. The aerosol is removed from the stratosphere primarily at mid- and high latitudes through various processes, mainly by isentropic transport across the tropopause from the stratosphere into the troposphere.
Abstract
Hemispherical backscattering cross sections σb of spherical particles are calculated using a recently derived analytic expression. Results are compared with σb values obtained by numerical integration. It is found that the analytic formula gives exact values of the hemispherical backscattering cross sections and also saves computer time. The behavior of σb in the limits of very small and very large spheres is discussed. As an aid in utilizing simple models of climate change due to aerosols, the percentage of incident solar radiation scattered into the backward hemisphere is calculated for a range of particle sizes and complex refractive indices. Similar results are also presented for the ratio of absorption to hemispheric backscattering, a critical parameter in many aerosol climate models.
Abstract
Hemispherical backscattering cross sections σb of spherical particles are calculated using a recently derived analytic expression. Results are compared with σb values obtained by numerical integration. It is found that the analytic formula gives exact values of the hemispherical backscattering cross sections and also saves computer time. The behavior of σb in the limits of very small and very large spheres is discussed. As an aid in utilizing simple models of climate change due to aerosols, the percentage of incident solar radiation scattered into the backward hemisphere is calculated for a range of particle sizes and complex refractive indices. Similar results are also presented for the ratio of absorption to hemispheric backscattering, a critical parameter in many aerosol climate models.
Abstract
The angular variation of the intensity of light scattered from a collimated beam by airborne soil particles and the size distribution of the particles were measured simultaneously 1.5 m above the ground. These measurements gave an estimate of the complex index of refraction m=n RE−n IM i of airborne soil particles, where n RE is the real part and n IM the imaginary part of the refractive index.
Standard microscopic analysis procedures were employed to determine n RE. Although a wide range of values was observed, the value 1.525 was taken as representative. By applying Mie scattering theory to each of the observed distributions of particle size, the expected angular variation of the intensity of the scattered light was calculated for a fixed value of n RE and a wide range of values of n IM. For each set of simultaneous measurements, the value of n IM was taken to be that value which provided the best fit to the experimental data. The upper limit of the value of n IM for the airborne particles studied in the experiment was determined to be 0.005 with an uncertainty factor of about 2. The estimate of n IM was found to be fairly insensitive to the assumed value of n RE.
Abstract
The angular variation of the intensity of light scattered from a collimated beam by airborne soil particles and the size distribution of the particles were measured simultaneously 1.5 m above the ground. These measurements gave an estimate of the complex index of refraction m=n RE−n IM i of airborne soil particles, where n RE is the real part and n IM the imaginary part of the refractive index.
Standard microscopic analysis procedures were employed to determine n RE. Although a wide range of values was observed, the value 1.525 was taken as representative. By applying Mie scattering theory to each of the observed distributions of particle size, the expected angular variation of the intensity of the scattered light was calculated for a fixed value of n RE and a wide range of values of n IM. For each set of simultaneous measurements, the value of n IM was taken to be that value which provided the best fit to the experimental data. The upper limit of the value of n IM for the airborne particles studied in the experiment was determined to be 0.005 with an uncertainty factor of about 2. The estimate of n IM was found to be fairly insensitive to the assumed value of n RE.
Abstract
As part of the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) experiment, 10 July–2 August 2001, off the central East Coast of the United States, the 14-channel NASA Ames Airborne Tracking Sunphotometer (AATS-14) was operated aboard the University of Washington’s Convair 580 (CV-580) research aircraft during 10 flights (∼45 flight hours). One of the main research goals in CLAMS was the validation of satellite-based retrievals of aerosol properties. The goal of this study in particular was to perform true over-ocean validations (rather than over-ocean validation with ground-based, coastal sites) at finer spatial scales and extending to longer wavelengths than those considered in previous studies. Comparisons of aerosol optical depth (AOD) between the Aerosol Robotic Network (AERONET) Cimel instrument at the Chesapeake Lighthouse and airborne measurements by AATS-14 in its vicinity showed good agreement with the largest r-square correlation coefficients at wavelengths of 0.38 and 0.5 μm (>0.99). Coordinated low-level flight tracks of the CV-580 during Terra overpass times permitted validation of over-ocean Moderate Resolution Imaging Spectroradiometer (MODIS) level 2 (MOD04_L2) multiwavelength AOD data (10 km × 10 km, nadir) in 16 cases on three separate days. While the correlation between AATS-14- and MODIS-derived AOD was weak with an r square of 0.55, almost 75% of all MODIS AOD measurements fell within the prelaunch estimated uncertainty range Δτ = ±0.03 ± 0.05τ. This weak correlation may be due to the small AODs (generally less than 0.1 at 0.5 μm) encountered in these comparison cases. An analogous coordination exercise resulted in seven coincident over-ocean matchups between AATS-14 and Multiangle Imaging Spectroradiometer (MISR) measurements. The comparison between AATS-14 and the MISR standard algorithm regional mean AODs showed a stronger correlation with an r square of 0.94. However, MISR AODs were systematically larger than the corresponding AATS values, with an rms difference of ∼0.06. AATS data collected during nine extended low-level CV-580 flight tracks were used to assess the spatial variability in AOD at horizontal scales up to 100 km. At UV and midvisible wavelengths, the largest absolute gradients in AOD were 0.1–0.2 per 50-km horizontal distance. In the near-IR, analogous gradients rarely reached 0.05. On any given day, the relative gradients in AOD were remarkably similar for all wavelengths, with maximum values of 70% (50 km)−1 and more typical values of 25% (50 km)−1. The implications of these unique measurements of AOD spatial variability for common validation practices of satellite data products and for comparisons to large-scale aerosol models are discussed.
Abstract
As part of the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) experiment, 10 July–2 August 2001, off the central East Coast of the United States, the 14-channel NASA Ames Airborne Tracking Sunphotometer (AATS-14) was operated aboard the University of Washington’s Convair 580 (CV-580) research aircraft during 10 flights (∼45 flight hours). One of the main research goals in CLAMS was the validation of satellite-based retrievals of aerosol properties. The goal of this study in particular was to perform true over-ocean validations (rather than over-ocean validation with ground-based, coastal sites) at finer spatial scales and extending to longer wavelengths than those considered in previous studies. Comparisons of aerosol optical depth (AOD) between the Aerosol Robotic Network (AERONET) Cimel instrument at the Chesapeake Lighthouse and airborne measurements by AATS-14 in its vicinity showed good agreement with the largest r-square correlation coefficients at wavelengths of 0.38 and 0.5 μm (>0.99). Coordinated low-level flight tracks of the CV-580 during Terra overpass times permitted validation of over-ocean Moderate Resolution Imaging Spectroradiometer (MODIS) level 2 (MOD04_L2) multiwavelength AOD data (10 km × 10 km, nadir) in 16 cases on three separate days. While the correlation between AATS-14- and MODIS-derived AOD was weak with an r square of 0.55, almost 75% of all MODIS AOD measurements fell within the prelaunch estimated uncertainty range Δτ = ±0.03 ± 0.05τ. This weak correlation may be due to the small AODs (generally less than 0.1 at 0.5 μm) encountered in these comparison cases. An analogous coordination exercise resulted in seven coincident over-ocean matchups between AATS-14 and Multiangle Imaging Spectroradiometer (MISR) measurements. The comparison between AATS-14 and the MISR standard algorithm regional mean AODs showed a stronger correlation with an r square of 0.94. However, MISR AODs were systematically larger than the corresponding AATS values, with an rms difference of ∼0.06. AATS data collected during nine extended low-level CV-580 flight tracks were used to assess the spatial variability in AOD at horizontal scales up to 100 km. At UV and midvisible wavelengths, the largest absolute gradients in AOD were 0.1–0.2 per 50-km horizontal distance. In the near-IR, analogous gradients rarely reached 0.05. On any given day, the relative gradients in AOD were remarkably similar for all wavelengths, with maximum values of 70% (50 km)−1 and more typical values of 25% (50 km)−1. The implications of these unique measurements of AOD spatial variability for common validation practices of satellite data products and for comparisons to large-scale aerosol models are discussed.
Abstract
A large satellite validation experiment was conducted at Poker Flat, Alaska, 16–19 July 1979. Instruments included the SAM II and SAGE satellite sensors, dustsondes impactors, a fitter collector and an airborne lidar. We show that the extinction profiles that were measured independently by SAM II and SAGE agree with each other. We then use a generalized optical model (which agrees with the Poker Flat optical absorption and relative size distribution measurements) to derive extinction profiles from the other measurements. Extinction profiles thus derived from the dustsonde, fitter and lidar measurements agree with the satellite-measured extinction profiles to within the combined uncertainties. (Individual 1 σ uncertainties are, at most heights, roughly 7 to 20% each for the satellite, dustsonde and filter measurements, 30 to 60% for the lidar measurements, and 10 to 20% for the process of converting one measured parameter to another using the optical model.)
The wire impactor-derived results are also consistent with the other results, but the comparison is coarse because of the relatively large uncertainties (±35% to a factor of 4) in impactor-derived mass, extinction, N 0.15 and N 0.25 (Nx is the number of particles per unit volume with radius greater than x μm.) These uncertainties apply to background stratospheric aerosol size distributions, and result primarily from relatively small uncertainties (±8 to ±20% for confidence limits of 95%) in radii assigned to impacted particles, combined with the steepness of background size distributions in the radius range that contributes most to mass, extinction, N 0.15 and N 0.25. Polar nephelometer-measured asymmetry parameters (0.4 to 0.6) agree with a previous balloon photometer inference, but are significantly less than the value (∼0.7) obtained from Mie scattering calculations assuming either model or measured size distributions.
Abstract
A large satellite validation experiment was conducted at Poker Flat, Alaska, 16–19 July 1979. Instruments included the SAM II and SAGE satellite sensors, dustsondes impactors, a fitter collector and an airborne lidar. We show that the extinction profiles that were measured independently by SAM II and SAGE agree with each other. We then use a generalized optical model (which agrees with the Poker Flat optical absorption and relative size distribution measurements) to derive extinction profiles from the other measurements. Extinction profiles thus derived from the dustsonde, fitter and lidar measurements agree with the satellite-measured extinction profiles to within the combined uncertainties. (Individual 1 σ uncertainties are, at most heights, roughly 7 to 20% each for the satellite, dustsonde and filter measurements, 30 to 60% for the lidar measurements, and 10 to 20% for the process of converting one measured parameter to another using the optical model.)
The wire impactor-derived results are also consistent with the other results, but the comparison is coarse because of the relatively large uncertainties (±35% to a factor of 4) in impactor-derived mass, extinction, N 0.15 and N 0.25 (Nx is the number of particles per unit volume with radius greater than x μm.) These uncertainties apply to background stratospheric aerosol size distributions, and result primarily from relatively small uncertainties (±8 to ±20% for confidence limits of 95%) in radii assigned to impacted particles, combined with the steepness of background size distributions in the radius range that contributes most to mass, extinction, N 0.15 and N 0.25. Polar nephelometer-measured asymmetry parameters (0.4 to 0.6) agree with a previous balloon photometer inference, but are significantly less than the value (∼0.7) obtained from Mie scattering calculations assuming either model or measured size distributions.
Abstract
The air–sea exchange of heat and carbon in the Southern Ocean (SO) plays an important role in mediating the climate state. The dominant role the SO plays in storing anthropogenic heat and carbon is a direct consequence of the unique and complex ocean circulation that exists there. Previous generations of climate models have struggled to accurately represent key SO properties and processes that influence the large-scale ocean circulation. This has resulted in low confidence ascribed to twenty-first-century projections of the state of the SO from previous generations of models. This analysis provides a detailed assessment of the ability of models contributed to the sixth phase of the Coupled Model Intercomparison Project (CMIP6) to represent important observationally based SO properties. Additionally, a comprehensive overview of CMIP6 performance relative to CMIP3 and CMIP5 is presented. CMIP6 models show improved performance in the surface wind stress forcing, simulating stronger and less equatorward-biased wind fields, translating into an improved representation of the Ekman upwelling over the Drake Passage latitudes. An increased number of models simulate an Antarctic Circumpolar Current (ACC) transport within observational uncertainty relative to previous generations; however, several models exhibit extremely weak transports. Generally, the upper SO remains biased warm and fresh relative to observations, and Antarctic sea ice extent remains poorly represented. While generational improvement is found in many metrics, persistent systematic biases are highlighted that should be a priority during model development. These biases need to be considered when interpreting projected trends or biogeochemical properties in this region.
Abstract
The air–sea exchange of heat and carbon in the Southern Ocean (SO) plays an important role in mediating the climate state. The dominant role the SO plays in storing anthropogenic heat and carbon is a direct consequence of the unique and complex ocean circulation that exists there. Previous generations of climate models have struggled to accurately represent key SO properties and processes that influence the large-scale ocean circulation. This has resulted in low confidence ascribed to twenty-first-century projections of the state of the SO from previous generations of models. This analysis provides a detailed assessment of the ability of models contributed to the sixth phase of the Coupled Model Intercomparison Project (CMIP6) to represent important observationally based SO properties. Additionally, a comprehensive overview of CMIP6 performance relative to CMIP3 and CMIP5 is presented. CMIP6 models show improved performance in the surface wind stress forcing, simulating stronger and less equatorward-biased wind fields, translating into an improved representation of the Ekman upwelling over the Drake Passage latitudes. An increased number of models simulate an Antarctic Circumpolar Current (ACC) transport within observational uncertainty relative to previous generations; however, several models exhibit extremely weak transports. Generally, the upper SO remains biased warm and fresh relative to observations, and Antarctic sea ice extent remains poorly represented. While generational improvement is found in many metrics, persistent systematic biases are highlighted that should be a priority during model development. These biases need to be considered when interpreting projected trends or biogeochemical properties in this region.
