Solar Spectral Flux Radiometer Research Articles

Above-aircraft cirrus cloud and aerosol optical depth from hyperspectral irradiances measured by a total-diffuse radiometer

Abstract. This study develops the use of spectral total and diffuse irradiance measurements, made from a prototype hyperspectral total-diffuse sunshine pyranometer (SPN-S), to retrieve layer fine-mode aerosol (τaer) and total optical depths from airborne platforms. Additionally, we use spectral analysis in an attempt to partition the total optical depth into its τaer and cirrus cloud optical depth (τcld) components in the absence of coarse-mode aerosols. Two retrieval methods are developed: one leveraging information in the diffuse irradiance and the other using spectral characteristics of the transmitted direct beam, with each approach best suited for specific cloud and aerosol conditions. The SPN-S has advantages over traditional sun photometer systems, including no moving parts and a low cost. However, a significant drawback of the instrument is that it is unable to measure the direct-beam irradiance as accurately as sun photometers. To compensate for the greater measurement uncertainty in the radiometric irradiances, these retrieval techniques employ ratioed inputs or spectral information to reduce output uncertainty. This analysis uses irradiance measurements from the SPN-S and the solar spectral flux radiometer (SSFR) aboard the National Aeronautics and Space Administration's (NASA) P-3 aircraft during the 2018 deployment of the ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) campaign and the 2019 Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex) mission to quantify above-aircraft cirrus τcld and derive vertical profiles of layer τaer. Validation of the τaer retrieval is accomplished by comparison with co-located measurements of direct solar irradiance made by the Sky-Scanning Sun-Tracking Atmospheric Research (4STAR) and in situ measurements of aerosol optical depth. For the aggregated 2018 ORACLES results, regression between the SPN-S-based method and sun photometer τaer values yields a slope of 0.96 with an R2 of 0.96, while the root mean square error (RMSE) is 3.0×10-2. When comparing the retrieved τaer to profiles of integrated in situ measurements of optical extinction, the slope, R2, and RMSE values for ORACLES are 0.90, 0.96, and 3.4×10-2, and for CAMP2Ex they are 0.94, 0.97, and 3.4×10-2, respectively. This paper is a demonstration of methods for deriving cloud and aerosol optical properties in environments where both atmospheric constituents may be present. With improvements to the low-cost SPN-S radiometer instrument, it may be possible to extend these methods to a broader set of sampling applications, such as ground-based settings.

Spatiotemporal Heterogeneity of Aerosol and Cloud Properties Over the Southeast Atlantic: An Observational Analysis

AbstractThe southeast Atlantic has expansive aerosol plumes overlying clouds for a third of each year. Aerosol optical depths (AODs) were measured from the airborne Sun photometer and lidar during the 2016 NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) field campaign. We compare these measurements with one another and with collocated Moderate Resolution Imaging Spectroradiometer (MODIS) observations at native spatial resolutions using <15‐min and 3‐h temporal collocation criteria. We find better statistical relationships for the <15‐min temporal resolution, indicating that AODs in the southeast Atlantic commonly vary below three‐hourly temporal scales over MODIS spatial resolutions. We also use the airborne Solar Spectral Flux Radiometer (SSFR) to conduct the first comprehensive evaluation of attenuation‐corrected below‐aerosol cloud optical depths (CODs) from MODIS and the Spinning Enhanced Visible and Infrared Imager (SEVIRI). MODIS COD retrievals improve their agreement with the SSFR when accounting for overlying aerosol attenuation whereas SEVIRI CODs are mostly underestimated.

The effect of low-level thin arctic clouds on shortwave irradiance: evaluation of estimates from spaceborne passive imagery with aircraft observations

Abstract. Cloud optical properties such as optical thickness along with surface albedo are important inputs for deriving the shortwave radiative effects of clouds from spaceborne remote sensing. Owing to insufficient knowledge about the snow or ice surface in the Arctic, cloud detection and the retrieval products derived from passive remote sensing, such as from the Moderate Resolution Imaging Spectroradiometer (MODIS), are difficult to obtain with adequate accuracy – especially for low-level thin clouds, which are ubiquitous in the Arctic. This study aims at evaluating the spectral and broadband irradiance calculated from MODIS-derived cloud properties in the Arctic using aircraft measurements collected during the Arctic Radiation-IceBridge Sea and Ice Experiment (ARISE), specifically using the upwelling and downwelling shortwave spectral and broadband irradiance measured by the Solar Spectral Flux Radiometer (SSFR) and the BroadBand Radiometer system (BBR). This starts with the derivation of surface albedo from SSFR and BBR, accounting for the heterogeneous surface in the marginal ice zone (MIZ) with aircraft camera imagery, followed by subsequent intercomparisons of irradiance measurements and radiative transfer calculations in the presence of thin clouds. It ends with an attribution of any biases we found to causes, based on the spectral dependence and the variations in the measured and calculated irradiance along the flight track. The spectral surface albedo derived from the airborne radiometers is consistent with prior ground-based and airborne measurements and adequately represents the surface variability for the study region and time period. Somewhat surprisingly, the primary error in MODIS-derived irradiance fields for this study stems from undetected clouds, rather than from the retrieved cloud properties. In our case study, about 27 % of clouds remained undetected, which is attributable to clouds with an optical thickness of less than 0.5. We conclude that passive imagery has the potential to accurately predict shortwave irradiances in the region if the detection of thin clouds is improved. Of at least equal importance, however, is the need for an operational imagery-based surface albedo product for the polar regions that adequately captures its temporal, spatial, and spectral variability to estimate cloud radiative effects from spaceborne remote sensing.

Above-cloud aerosol radiative effects based on ORACLES 2016 and ORACLES 2017 aircraft experiments

Abstract. Determining the direct aerosol radiative effect (DARE) of absorbing aerosols above clouds from satellite observations alone is a challenging task, in part because the radiative signal of the aerosol layer is not easily untangled from that of the clouds below. In this study, we use aircraft measurements from the NASA ObseRvations of CLouds above Aerosols and their intEractionS (ORACLES) project in the southeastern Atlantic to derive it with as few assumptions as possible. This is accomplished by using spectral irradiance measurements (Solar Spectral Flux Radiometer, SSFR) and aerosol optical depth (AOD) retrievals (Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research, 4STAR) during vertical profiles (spirals) that minimize the albedo variability of the underlying cloud field – thus isolating aerosol radiative effects from those of the cloud field below. For two representative cases, we retrieve spectral aerosol single scattering albedo (SSA) and the asymmetry parameter (g) from these profile measurements and calculate DARE given the albedo range measured by SSFR on horizontal legs above clouds. For mid-visible wavelengths, we find SSA values from 0.80 to 0.85 and a significant spectral dependence of g. As the cloud albedo increases, the aerosol increasingly warms the column. The transition from a cooling to a warming top-of-aerosol radiative effect occurs at an albedo value (critical albedo) just above 0.2 in the mid-visible wavelength range. In a companion paper, we use the techniques introduced here to generalize our findings to all 2016 and 2017 measurements and parameterize aerosol radiative effects.

Arctic Radiation-IceBridge Sea and Ice Experiment: The Arctic Radiant Energy System during the Critical Seasonal Ice Transition

Abstract The National Aeronautics and Space Administration (NASA)’s Arctic Radiation-IceBridge Sea and Ice Experiment (ARISE) acquired unique aircraft data on atmospheric radiation and sea ice properties during the critical late summer to autumn sea ice minimum and commencement of refreezing. The C-130 aircraft flew 15 missions over the Beaufort Sea between 4 and 24 September 2014. ARISE deployed a shortwave and longwave broadband radiometer (BBR) system from the Naval Research Laboratory; a Solar Spectral Flux Radiometer (SSFR) from the University of Colorado Boulder; the Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research (4STAR) from the NASA Ames Research Center; cloud microprobes from the NASA Langley Research Center; and the Land, Vegetation and Ice Sensor (LVIS) laser altimeter system from the NASA Goddard Space Flight Center. These instruments sampled the radiant energy exchange between clouds and a variety of sea ice scenarios, including prior to and after refreezing began. The most critical and unique aspect of ARISE mission planning was to coordinate the flight tracks with NASA Cloud and the Earth’s Radiant Energy System (CERES) satellite sensor observations in such a way that satellite sensor angular dependence models and derived top-of-atmosphere fluxes could be validated against the aircraft data over large gridbox domains of order 100–200 km. This was accomplished over open ocean, over the marginal ice zone (MIZ), and over a region of heavy sea ice concentration, in cloudy and clear skies. ARISE data will be valuable to the community for providing better interpretation of satellite energy budget measurements in the Arctic and for process studies involving ice–cloud–atmosphere energy exchange during the sea ice transition period.

Observation of the spectrally invariant properties of clouds in cloudy-to-clear transition zones during the MAGIC field campaign

We use the spectrally invariant method to study the variability of cloud optical thickness τ and droplet effective radius reff in transition zones (between the cloudy and clear sky columns) observed from Solar Spectral Flux Radiometer (SSFR) and Shortwave Array Spectroradiometer-Zenith (SASZe) during the Marine ARM GPCI Investigation of Clouds (MAGIC) field campaign. The measurements from the SSFR and the SASZe are different, however inter-instrument differences of self-normalized measurements (divided by their own spectra at a fixed time) are small. The spectrally invariant method approximates the spectra in the cloud transition zone as a linear combination of definitely clear and cloudy spectra, where the coefficients, slope and intercept, characterize the spectrally invariant properties of the transition zone. Simulation results from the SBDART (Santa Barbara DISORT Atmospheric Radiative Transfer) model demonstrate that (1) the slope of the visible band is positively correlated with the cloud optical thickness τ while the intercept of the near-infrared band has high negative correlation with the cloud drop effective radius reff even without the exact knowledge of τ; (2) the above relations hold for all Solar Zenith Angle (SZA) and for cloud-contaminated skies. In observations using redundant measurements from SSFR and SASZe, we find that during cloudy-to-clear transitions, (a) the slopes of the visible band decrease, and (b) the intercepts of the near-infrared band remain almost constant near cloud edges. The findings in simulations and observations suggest that, while the optical thickness decreases during the cloudy-to-clear transition, the cloud drop effective radius does not change when cloud edges are approached. These results support the hypothesis that inhomogeneous mixing dominates near cloud edges in the studied cases.

A spectral method for discriminating thermodynamic phase and retrieving cloud optical thickness and effective radius using transmitted solar radiance spectra

Abstract. A new retrieval scheme for cloud optical thickness, effective radius, and thermodynamic phase was developed for ground-based measurements of cloud shortwave solar spectral transmittance. Fifteen parameters were derived to quantify spectral variations in shortwave transmittance due to absorption and scattering of liquid water and ice clouds, manifested by shifts in spectral slopes, curvatures, maxima, and minima. To retrieve cloud optical thickness and effective particle radius, a weighted least square fit that matched the modeled parameters was applied. The measurements for this analysis were made with the ground-based Solar Spectral Flux Radiometer in Boulder, Colorado, between May 2012 and January 2013. We compared the cloud optical thickness and effective radius from the new retrieval to two other retrieval methods. By using multiple spectral features, we find a closer fit (with a root mean square difference over the entire spectra of 3.1% for a liquid water cloud and 5.9% for an ice cloud) between measured and modeled spectra compared to two other retrieval methods which diverge by a root mean square of up to 6.4% for a liquid water cloud and 22.5% for an ice cloud. The new retrieval introduced here has an average uncertainty in effective radius (± 1.2 μm) smaller by factor of at least 2.5 than two other methods when applied to an ice cloud.

Upper-troposphere and lower-stratosphere water vapor retrievals from the 1400 and 1900 nm water vapor bands

Abstract. Measuring water vapor in the upper troposphere and lower stratosphere is difficult due to the low mixing ratios found there, typically only a few parts per million. Here we examine near-infrared spectra acquired with the Solar Spectral Flux Radiometer (SSFR) during the first science phase of the NASA Airborne Tropical TRopopause EXperiment (ATTREX). From the 1400 and 1900 nm absorption bands we infer water vapor amounts in the tropical tropopause layer and adjacent regions between altitudes of 14 and 18 km. We compare these measurements to solar transmittance spectra produced with the MODerate resolution atmospheric TRANsmission (MODTRAN) radiative transfer model, using in situ water vapor, temperature, and pressure profiles acquired concurrently with the SSFR spectra. Measured and modeled transmittance values agree within 0.002, with some larger differences in the 1900 nm band (up to 0.004). Integrated water vapor amounts along the absorption path lengths of 3 to 6 km varied from 1.26 × 10−4 to 4.59 × 10−4 g cm−2. A 0.002 difference in absorptance at 1367 nm results in a 3.35 × 10−5 g cm−2 change of integrated water vapor amounts; 0.004 absorptance change at 1870 nm results in 5.50 × 10−5 g cm−2 of water vapor. These are 27% (1367 nm) and 44% (1870 nm) differences at the lowest measured value of water vapor (1.26 × 10−4 g cm−2) and 7% (1367 nm) and 12% (1870 nm) differences at the highest measured value of water vapor (4.59 × 10−4 g cm−2). A potential method for extending this type of measurement from aircraft flight altitude to the top of the atmosphere is discussed.

CalNex cloud properties retrieved from a ship‐based spectrometer and comparisons with satellite and aircraft retrieved cloud properties

An algorithm to retrieve cloud optical thickness and effective radius (reff) from spectral transmittance was applied to radiance and irradiance observations of the Solar Spectral Flux Radiometer (SSFR) during the Research at the Nexus of Air Quality and Climate Change Campaign (CalNex). Data from an overcast day, 16 May 2010, was used to validate the algorithm. Retrievals from the SSFR, deployed on the Woods Hole Oceanic Institute R/V Atlantis, were compared to retrievals made from an airborne SSFR, the Geostationary Operations Environmental Satellite (GOES), an Atlantis‐based microwave radiometer, and the Moderate Resolution Imaging Spectroradiometer. In situ observations of reffduring a flight over the Atlantis were compared to the Atlantis SSFR and GOES retrievals. The cloud statistics for the CalNex campaign were compared to previous studies. The agreement between the different retrievals, quantified by determining the number of coincident observations when retrieval uncertainty overlapped, improved as the difference between the field‐of‐views (FOV) of the instruments decreased. It is shown that averaging the 1 Hz SSFR observations to the 15 minute GOES interval cannot fully account for the impact of the different FOVs. The average in situ reff (7.7 μm) fell between the average reffretrieved using the Atlantis‐based SSFR radiance (5.7μm) and irradiance (9.5 μm). The CalNex clouds showed a diurnal pattern observed in previous studies of marine boundary layer clouds in the region. The distribution of cloud optical thickness and liquid water path during CalNex was shown to be a gamma distribution, consistent with previous studies of high cloud fraction marine boundary layer clouds.

The Shannon information content of hyperspectral shortwave cloud albedo measurements: Quantification and practical applications

The Shannon information content provides an objective measure of the information in a data set. In this paper, we quantify the information content of hyperspectral liquid water cloud measurements over a spectral range (300–2500 nm) representing approximately 95% of the total energy in the solar spectrum. We also use the Shannon information content to analyze the cloud retrieval wavelengths and weightings used by the Solar Spectral Flux Radiometer (SSFR) and to determine the cumulative information in the SSFR retrieval. These applications illustrate the utility of the Shannon information content in guiding the effective processing of hyperspectral data. Such efficiency is of growing importance considering the push toward spectrally resolved satellite measurements of reflected solar irradiance used to study climate.

A spectral method for retrieving cloud optical thickness and effective radius from surface-based transmittance measurements

Abstract. We introduce a new spectral method for the retrieval of optical thickness and effective radius from cloud transmittance that relies on the spectral slope of the normalized transmittance between 1565 nm and 1634 nm, and on cloud transmittance at a visible wavelength. The standard dual-wavelength technique, which is traditionally used in reflectance-based retrievals, is ill-suited for transmittance because it lacks sensitivity to effective radius, especially for optically thin clouds. Using the spectral slope rather than the transmittance itself enhances the sensitivity of transmittance observations with respect to the effective radius. This is demonstrated by applying it to the moderate spectral resolution observations from the Solar Spectral Flux Radiometer (SSFR) and Shortwave Spectroradiometer (SWS), and by examining the retrieval uncertainties of the standard and the spectral method for data from the DOE ARM Southern Great Plains (SGP) site and a NOAA ship cruise (ICEALOT). The liquid water path (LWP) is derived from the retrieved optical thickness and effective radius, based on two different assumptions about the cloud vertical profile, and compared to the simultaneous observations from a microwave radiometer. Optical thickness and effective radius is also compared to MODIS retrievals. In general, the effective radius uncertainties were much larger for the standard retrieval than for the spectral retrieval, particularly for thin clouds. When defining 2 μm as upper limit for the tolerable uncertainty of the effective radius, the standard method returned only very few valid retrievals for clouds with an optical thickness below 25. For the analyzed ICEALOT data (mean optical thickness 23), the spectral method provided valid retrievals for 84 % of the data (24 % for the standard method). For the SGP data (mean optical thickness 44), both methods provided a high return of 90 % for the spectral method and 78 % for the standard method.

A new method for deriving aerosol solar radiative forcing and its first application within MILAGRO/INTEX-B

Abstract. We introduce a method for deriving aerosol spectral radiative forcing along with single scattering albedo, asymmetry parameter, and surface albedo from airborne vertical profile measurements of shortwave spectral irradiance and spectral aerosol optical thickness. The new method complements the traditional, direct measurement of aerosol radiative forcing efficiency from horizontal flight legs below gradients of aerosol optical thickness, and is particularly useful over heterogeneous land surfaces and for homogeneous aerosol layers where the horizontal gradient method is impractical. Using data collected by the Solar Spectral Flux Radiometer (SSFR) and the Ames Airborne Tracking Sunphotometer (AATS-14) during the MILAGRO (Megacity Initiative: Local and Global Research Observations) experiment, we validate an over-ocean spectral aerosol forcing efficiency from the new method by comparing with the traditional method. Retrieved over-land aerosol optical properties are compared with in-situ measurements and AERONET retrievals. The spectral forcing efficiencies over ocean and land are remarkably similar and agree with results from other field experiments.

A new method to retrieve the aerosol layer absorption coefficient from airborne flux density and actinic radiation measurements

A new method is presented to derive the mean value of the spectral absorption coefficient of an aerosol layer from combined airborne measurements of spectral net irradiance and actinic flux density. While the method is based on a theoretical relationship of radiative transfer theory, it is applied to atmospheric radiation measurements for the first time. The data have been collected with the Spectral Modular Airborne Radiation Measurement System (SMART‐Albedometer), the Solar Spectral Flux Radiometer (SSFR), and the Actinic Flux Spectroradiometer (AFSR) during four field campaigns between 2002 and 2008 (the Saharan Mineral Dust Experiment (SAMUM), the Influence of Clouds on the Spectral Actinic Flux in the Lower Troposphere (INSPECTRO) project, and the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites and Aerosol, Radiation, and Cloud Processes Affecting Arctic Climate (ARCTAS/ARCPAC) projects). The retrieval algorithm is tested in a series of radiative transfer model runs and then applied to measurement cases with different aerosol species and loading. The method is shown to be a feasible approach to obtain the mean aerosol absorption coefficient across a given accessible altitude range. The results indicate that the method is viable whenever the difference of the net irradiance at the top and bottom of a layer is equal to or higher than the measurement uncertainty for net irradiance. This can be achieved by a high optical depth or a low single‐scattering albedo within the layer.

Examining the impact of overlying aerosols on the retrieval of cloud optical properties from passive remote sensing

Haywood et al. (2004) show that an aerosol layer above a cloud can cause a bias in the retrieved cloud optical thickness and effective radius. Monitoring for this potential bias is difficult because space‐based passive remote sensing cannot unambiguously detect or characterize aerosol above cloud. We show that cloud retrievals from aircraft measurements above cloud and below an overlying aerosol layer are a means to test this bias. The data were collected during the Intercontinental Chemical Transport Experiment (INTEX‐A) study based out of Portsmouth, New Hampshire, United States, above extensive, marine stratus cloud banks affected by industrial outflow. Solar Spectral Flux Radiometer (SSFR) irradiance measurements taken along a lower level flight leg above cloud and below aerosol were unaffected by the overlying aerosol. Along upper level flight legs, the irradiance reflected from cloud top was transmitted through an aerosol layer. We compare SSFR cloud retrievals from below‐aerosol legs to satellite retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) in order to detect an aerosol‐induced bias. In regions of small variation in cloud properties, we find that SSFR and MODIS‐retrieved cloud optical thickness compares within the uncertainty range for each instrument while SSFR effective radius tend to be smaller than MODIS values (by 1–2 μm) and at the low end of MODIS uncertainty estimates. In regions of large variation in cloud properties, differences in SSFR and MODIS‐retrieved cloud optical thickness and effective radius can reach values of 10 and 10 μm, respectively. We include aerosols in forward modeling to test the sensitivity of SSFR cloud retrievals to overlying aerosol layers. We find an overlying absorbing aerosol layer biases SSFR cloud retrievals to smaller effective radii and optical thickness while nonabsorbing aerosols had no impact.

Observations and modeling of ice cloud shortwave spectral albedo during the Tropical Composition, Cloud and Climate Coupling Experiment (TC4)

Ice cloud optical thickness and effective radius have been retrieved from hyperspectral irradiance and discrete spectral radiance measurements for four ice cloud cases during the Tropical Composition, Cloud and Climate Coupling Experiment (TC4) over a range of solar zenith angle (23°–53°) and high (46–90) and low (5–15) optical thicknesses. The retrieved optical thickness and effective radius using measurements at only two wavelengths from the Solar Spectral Flux Radiometer (SSFR) irradiance and the Moderate Resolution Imaging Spectroradiometer Airborne Simulator (MAS) were input to a radiative transfer model using two libraries of ice crystal single‐scattering optical properties to reproduce spectral albedo over the spectral range from 400 to 2130 nm. The two commonly used ice single‐scattering models were evaluated by examining the residuals between observed spectral and predicted spectral albedo. The SSFR and MAS retrieved optical thickness and effective radius were found to be in close agreement for the low to moderately optically thick clouds with a mean difference of 3.42 in optical thickness (SSFR lower relative to MAS) and 3.79 μm in effective radius (MAS smaller relative to SSFR). The higher optical thickness case exhibited a larger difference in optical thickness (40.5) but nearly identical results for effective radius. The single‐scattering libraries were capable of reproducing the spectral albedo in most cases examined to better than 0.05 for all wavelengths. Systematic differences between the model and measurements increased with increasing optical thickness and approached 0.10 between 400 and 600 nm and selected wavelengths between 1200 and 1300 nm. Differences between radiance and irradiance based retrievals of optical thickness and effective radius error sources in the modeling of ice single‐scattering properties are examined.

Apparent absorption of solar spectral irradiance in heterogeneous ice clouds

Coordinated flight legs of two aircraft above and below extended ice clouds played an important role in the Tropical Composition, Cloud and Climate Coupling Experiment (Costa Rica, 2007). The Solar Spectral Flux Radiometer measured up‐ and downward irradiance on the high‐altitude (ER‐2) and the low‐altitude (DC‐8) aircraft, which allowed deriving apparent absorption on a point‐by‐point basis along the flight track. Apparent absorption is the vertical divergence of irradiance, calculated from the difference of net flux at the top and bottom of a cloud. While this is the only practical method of deriving absorption from aircraft radiation measurements, it differs from true absorption when horizontal flux divergence is nonzero. Differences between true and apparent absorption are inevitable in any inhomogeneous atmosphere, especially clouds. We show, for the first time, the spectral shape of measured apparent absorption and compare with results from a three‐dimensional radiative transfer model. The model cloud field is created from optical thickness and effective radius retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator and from reflectivity profiles from the Cloud Radar System, both on board the ER‐2. Although the spectral shape is reproduced by the model calculations, the measured apparent absorption in the visible spectral range is higher than the model results along extended parts of the flight leg. This is possibly due to a net loss of photons into neighboring cirrus‐free areas that are not contained within the model domain.

Aircraft measurements of spectral surface albedo and its consistency with ground‐based and space‐borne observations

Spectral surface albedo, a boundary condition which needs to be accurately known for aerosol remote sensing, surface aerosol forcing, and radiative transfer calculations, also strongly affects Earth's radiation balance. The difficulty in deriving surface albedo from space and aircraft observations lies mainly in the atmospheric correction, especially in aerosol‐burdened regions. Because of the different scales, comparing satellite retrievals with airborne or ground‐based observations is not straightforward. We use Solar Spectral Flux Radiometer (SSFR) measurements of upward and downward irradiance from aircraft altitude during Megacity Initiative: Local and Global Research Observations (MILAGRO) to determine spectral surface albedo at ground stations and along the flight track (over the wavelength range 350 to 2100 nm), thereby linking flight‐level retrieved measurements to larger‐scale satellite observations in the polluted Mexico City environment. Our approach involves iteratively adjusting the surface albedo input of a SSFR specific radiative transfer model until the modeled upward irradiance matches the SSFR measurements at flight level. A sensitivity analysis of surface albedo to aerosol optical properties provides a retrieval uncertainty, which can outweigh the SSFR instrument uncertainty under highly variable conditions (or uncertain measurements) of aerosol optical depth and asymmetry parameter. Comparisons between spectral surface albedo derived from the SSFR, Multi‐Filter Rotating Shadowband Radiometer, and the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument onboard the NASA‐EOS Terra and Aqua satellites are shown with differences of 6–10% and 0.025–0.05 units, respectively. Along‐track comparisons between the SSFR and MODIS show that two instruments (aircraft and satellite) can capture inhomogeneous surface albedo scene changes.

Comparing irradiance fields derived from Moderate Resolution Imaging Spectroradiometer airborne simulator cirrus cloud retrievals with solar spectral flux radiometer measurements

During the Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment, the Moderate Resolution Imaging Spectroradiometer (MODIS) airborne simulator (MAS) and the solar spectral flux radiometer (SSFR) operated on the same aircraft, the NASA ER‐2. While MAS provided two‐dimensional horizontal fields of cloud optical thickness and effective ice particle radius, the SSFR measured spectral irradiance in the visible to near‐infrared wavelength range (0.3–1.7 μm). The MAS retrievals, along with vertical profiles from a combined radar/lidar system on board the same aircraft were used to construct three‐dimensional cloud fields, which were input into Monte Carlo radiative transfer models. The simulated field of spectral albedo (ratio of reflected upwelling to incident downwelling irradiance) was compared with the SSFR measurements. For two cases, the relative importance of spatial cloud heterogeneities, various approximations of the single scattering parameters, vertical structure, cirrus optical thickness, and ice crystal effective radius was studied.

Spectral observations and modeling of the Arctic surface radiation environment

Measurements from three spectral instruments, the Solar Spectral Flux Radiometer (SSFR), the Rotating Shadowband Spectrometer (RSS), and the Solar Ultraviolet Spectroradiometer (SUV), are compared to results from the Santa Barbara Moderate Resolution Radiative Transfer Model (SBMOD). SSFR data are from the SHEBA experiment of May–June 1998. RSS and SUV data are from Barrow, Alaska, April 1999. SBMOD is a recently developed model utilizing the correlated‐k method of computing gaseous absorption and convolves the solar source function and the instrument's filter function during computations. Comparisons indicate that absorption of ozone, water vapor, oxygen, and the O2‐O2 absorption complexes are well characterized by the model. There does not appear to be any significant unexplained absorption between 0.4 and 1.0 microns, as evidenced by comparisons between SBMOD and both SSFR and RSS data. However, a more absorptive aerosol was required to reach agreement between SBMOD and RSS diffuse fields than we anticipated. Data from the RSS and SUV possess similar structure over their entire shared spectral range from 400 to 600 nm, and the RSS and SSFR possess similar structure between 500 and 1000 nm. However, below 500 nanometers, measurements from all three instruments diverge noticeably from model results. The residual between the SUV and RSS measurements and SBMOD are similar to those presented in prior studies, suggesting that there may be errors in several widely used representations of the solar spectrum between 400 and 500 nm. The shape of the SSFR's data in this region has yet to be fully explained; aspects of its calibration and operation may be the main source of uncertainty.

Impact of cirrus crystal shape on solar spectral irradiance: A case study for subtropical cirrus

Profiles of in situ measurements of ice crystal size distribution of subtropical cirrus were used to calculate solar spectral irradiances above and below the clouds. Spheres and nonspherical ice crystal habits (columns, hollows, plates, bullets, and aggregates) were assumed in the calculations. The simulation results were compared to irradiance measurements from the NASA Solar Spectral Flux Radiometer. The microphysical and radiation data were collected by three aircraft during CRYSTAL‐FACE. Two cirrus cases (optical thickness of about 1 and 7) from two mission dates (26 and 23 July 2002) were investigated in detail. The measured downwelling and upwelling irradiance spectra above the cirrus could mostly be reproduced by the radiation model to within ±5–10% for most ice crystal habits. Below the cirrus the simulations disagreed with the measured irradiances due to surface albedo variability along the flight track, and nonoptimal colocation between the microphysical and irradiance measurements. The impact of shape characteristics of the crystals was important for the reflected irradiances above the optically thin cirrus, especially for small solar zenith angles, because in this case single‐scattering dominated the solar radiation field. For the cirrus of moderate optical thickness the enhanced multiple scattering tended to diminish particular shape features caused by nonspherical single‐scattering. Within the ice absorption bands the shape‐related differences in the absorption characteristics of the individual nonspherical ice crystals were amplified if multiple scattering prevailed. Furthermore, it was found that below the cloud the shape sensitivity of the downwelling irradiance spectra is larger compared to the nonsphericity effects on reflected irradiances above the cirrus. Finally, it was shown that the calculated cirrus solar radiative forcing could vary by as much as 26% depending on the ice crystal habit.