Changes In Solar Zenith Angle Research Articles

Influence of cloudy and clear-sky partitions, aerosols, and geometry on the recent variability in surface solar irradiance components in northern France

Abstract. Surface solar irradiance (SSI) is a fundamental parameter whose components (direct and diffuse) and variabilities are highly influenced by changes in atmospheric content and scene parameters. The respective importance of cloudy-sky conditions and atmospheric aerosols on SSI evolutions is region dependent and only partially quantified. Here we provide a comprehensive analysis of SSI variabilities recorded in northern France, a region with extensive variability in sky conditions and aerosol loads. Through the application of automatic filtering methods to 1 min resolution SSI ground-based measurements over Lille, sky conditions are classified as clear-sky, 11 %; clear-sun-with-cloud, 22 %; and cloudy-sun situations, 67 % from 2010 to 2022, for which we analyze the statistics and variabilities in the global horizontal irradiance (GHI), beam horizontal irradiance (BHI), and diffuse horizontal irradiance (DHI). Coincident photometric measurements of aerosol properties and radiative-transfer simulations provide the means to conduct a multivariate analysis of the SSI observed trends and year-to-year evolutions and to estimate aerosol and cloud forcings under clear-sun conditions. The analysis of the record value of all-sky GHI in spring 2020 attributes 89 % of the changes to the exceptional sunlight conditions (57 % of clear-sun situations). It highlights also for that season the importance of solar zenith-angle changes, whose positive effects on clear-sun conditions surpass those due to aerosols. Our results show all-sky GHI and BHI positive trends of around +4.0 and +4.4 Wm-2yr-1, respectively, in both spring and summer, which are explained by more than 60 % by an increase in clear-sun occurrences of +1 % yr−1. Additional significant BHI increases under clear-sun conditions are mainly explained in spring by the negative trend in aerosol optical depth (−0.011 yr−1) and partly by angular effects in summer. Moreover, we find that clear-sun-with-cloud situations are frequently marked by irradiance enhancement due to clouds, with 13 % more GHI on a monthly average and 10 % additional diffuse proportion than in clear-sky situations. Under such conditions, clouds add on average 25 W m−2 of diffuse irradiance that sets the GHI at the remarkable level of pristine (aerosol-and-cloud-free) conditions or even higher, by more than +10 W m−2 in summer and for low aerosol loads. Overall, our results highlight the dominant and complex influence of cloudy conditions on SSI, which precedes or combines with that of aerosols and geometrical effects, and leads to a remarkable global level of SSI in clear-sun-with-cloud situations.

Impacts of clumping effect on canopy reflectance using 3D radiative transfer modeling

The product of leaf area index (LAI) and clumping index (CI) quantifies the effective leaf abundance and distribution across the landscape, and therefore, governs the radiation absorption, evapotranspiration, and carbon assimilation processes in the terrestrial ecosystems. Previous studies were mainly focused on developing inversion methods applicable to large scale for retrieving LAI and CI from multi-angular satellite observations. However, a few studies focused on quantifying the sensitivity of canopy bidirectional reflectance distribution function (BRDF) to changes in CI in a forward manner, hampering an accurate understanding of the relationship between CI and BRDF. In this study, we simulated how BRDF responds to changes in CI in Qinghai spruce (Picea crassifolia) forests based on a 3D radiative transfer model LESS and ground-measured data. We found that the LESS model effectively simulated the hot-spot, roof, and bowl-edge characteristics of the canopy BRDF by changing the sun-sensor geometry. We constructed forest scenes with variable CI (ranging from 0.4 to 0.8) to investigate the clumping effect on BRDF with different solar and observation angles. The red band bidirectional reflectance factor (BRF) showed higher sensitivity to changes in CI than that in the near-infrared (NIR) band. Canopy BRFs in the red band along the principal plane and cross principal planes measured in different seasons showed consistent sensitivity to changes in CI, suggesting that the red band BRF is helpful for CI inversion for forests with different levels of foliage clumping. In the NIR band, canopy BRFs along the principal plane measured in growing seasons [with solar zenith angle (SZA) <40°] and the cross principal plane measured in non-growing seasons (with SZA >40°) were sensitive to changes in CI in highly clumped forests (with CI ranging from 0.4 to 0.6). However, canopy BRF in the NIR band showed low sensitivity to changes in CI in highly clumped forests (CI <0.6), especially along the cross principal plane when SZA was approximately 10°. The simulated BRFs in the red and NIR bands showed relatively low sensitivity to changes in SZAs at a VZA of 40° and 0°, respectively. We highly recommend including the red band BRF for CI retrievals, and using a VZA of 40° in the red band and 0° in the NIR band may help reduce the CI estimation uncertainty caused by changes in SZA. This research provides a new perspective for understanding the sensitivity of multi-angular satellite data to changes in canopy structural characteristics of vegetation in global ecosystem studies and may help improve CI estimations using the multiangular optical remote sensing data.

Changes in the height of the pollution boundary layer and their meteorological effects on the distribution of surface ozone concentrations

Focusing on the key air pollution regions in China by using hourly automatic weather data, ground-based and high-altitude meteorological sounding data, and near-surface O3 monitoring data, here, we try to quantify the relationship between boundary layer meteorological condition and near-surface O3 concentrations. The key meteorological element includes changes in solar zenith angle, cloud height, atmospheric condensation rate, and the associated change in the boundary layer height. We also try to better understand the mechanisms by which meteorological conditions affect near-surface O3 concentrations, and it is found that the exponential increase in near-surface O3 concentrations after sunrise (called the O3 concentration entrainment, EZ) is meaningfully associated with exceeding the threshold of a water vapor condensation rate (fc) that is often closely linked to a significant rise in the pollution boundary layer and that this proves to be diagnostically important for understanding the O3 EZ. Diurnal variations in solar zenith angle and boundary layer height are key meteorological factors influencing the large increase in near-surface O3 concentration entrainment.

Fluorescence Line Height Extraction Algorithm for the Geostationary Ocean Color Imager

Chlorophyll fluorescence is an important indicator of the physiological state of phytoplankton in water bodies. The new generation of ocean color satellite remote sensors usually sets fluorescence bands to detect the phytoplankton fluorescence line height (FLH). Yet, the Geostationary Ocean Color Imager (GOCI) offers no FLH products so far, and the FLH results calculated using the fluorescence band (680 nm) and the two baseline bands (660 and 745 nm) have numerous negative values and are quite different from the FLH products of other satellite ocean color sensors. To address this problem, we established an FLH retrieval algorithm suitable for GOCI. We simulated the spectral datasets of different water types using the radiative transfer model HydroLight and established the band conversion relationship from 680 to 685 nm based on the simulated datasets. The remote sensing reflectance after band conversion was applied to the FLH product inversion, significantly reducing the number of negative FLH values and appreciably improving data availability for GOCI FLH products (from 14.78% to 66.73% on average). The new FLH product has a good correlation with the field-measured data (R2 = 0.73), and the relative error was 6.95%. Moreover, after band conversion, the FLH products retrieved by GOCI are in good agreement with the FLH products of MODIS, and fusion products can be further produced to improve the spatiotemporal resolution of the data. In addition, the radiative transfer simulation datasets also revealed that changes in solar zenith angle have little effect on FLH inversion. The hourly GOCI-derived FLH has good spatiotemporal continuity and can clearly reflect the diurnal variation of FLH. It can provide a stable FLH algorithm for further recovery of time-series GOCI FLH products and research on diurnal changes in FLH.

M3 electron density layer in the dayside ionosphere of Mars: Analysis of MAVEN ROSE observations

We investigate the characteristics of the topside M3 layer in the Martian dayside electron density profiles obtained from the Radio Occultation Science Experiment (ROSE) onboard the MAVEN spacecraft. M3 layer is a local enhancement in the electron density above the prominent ionospheric M2 peak. ROSE measurements have a wider latitudinal and solar zenith angle (SZA) coverage as compared to the spacecraft observations used by previous studies for characterizing the M3 layer. We analyzed 179 dayside (SZA <85∘) electron density profiles from July 2016 to December 2020. The analysis show that the typical altitude of M3 layer is ∼180 (±10) km, with a density of ∼8 (±3) x 103 cm−3, and occur ∼43 (±8) km above the M2 peak. These values are consistent with those reported by an earlier study using MGS RO data. The density of M3 layer seems to be insensitive to changes in SZA in the range of 55∘ to 85∘. The increasing trend in the M3 peak altitude with increasing SZA might be indicative of M3 forming at constant pressure levels, similar to M2. In the northern hemisphere, both the M3 peak density and altitude seem insensitive to latitudinal changes. The most commonly cited reasons for the production of the M3 layer are the enhancement in the electron temperature and the converging vertical plasma motion caused by the crustal magnetic field. Our simulations using a one-dimensional photochemical model suggest that decreased dissociative recombination rate due to electron temperature enhancements may not contribute to the formation of M3 layer. We conclude that the presence of crustal magnetic field also could not wholly explain the formation of M3 layer.

Impact of scene-specific enhancement spectra on matched filter greenhouse gas retrievals from imaging spectroscopy

Matched filter techniques have been widely used for retrieval of greenhouse gas enhancements from imaging spectroscopy datasets. While multiple algorithmic techniques and refinements have been proposed, the greenhouse gas target spectrum used for concentration enhancement estimation has remained largely unaltered since the introduction of quantitative matched filter retrievals. The magnitude of retrieved methane and carbon dioxide enhancements, and thereby integrated mass enhancements (IME) and estimated flux of point-source emitters, is heavily dependent on this target spectrum. Current standard use of molecular absorption coefficients to create unit enhancement target spectra does not account for absorption by background concentrations of greenhouse gases, solar and sensor geometry, or atmospheric water vapor absorption. We introduce geometric and atmospheric parameters into the generation of scene-specific unit enhancement spectra to provide target spectra that are compatible with all greenhouse gas retrieval matched filter techniques. Specifically, we use radiative transfer modeling to model four parameters that are expected to change between scenes: solar zenith angle, column water vapor, ground elevation, and sensor altitude. These parameter values are well defined, with low variation within a single scene. A benchmark dataset consisting of ten AVIRIS-NG airborne imaging spectrometer scenes was used to compare IME retrieved using a matched filter algorithm. For methane plumes, IME resulting from use of standard, generic enhancement spectra varied from −22 to +28.7% compared to scene-specific enhancement spectra. Due to differences in spectral shape between the generic and scene-specific enhancement spectra, differences in methane plume IME were linked to surface spectral characteristics in addition to geometric and atmospheric parameters. IME differences were much larger for carbon dioxide plumes, with generic enhancement spectra producing integrated mass enhancements −76.1 to −48.1% compared to scene-specific enhancement spectra. Fluxes calculated from these integrated enhancements would vary by the same percentages, assuming equivalent wind conditions. Methane and carbon dioxide IME were most sensitive to changes in solar zenith angle and ground elevation. We introduce an interpolation approach that can efficiently generate scene-specific unit enhancement spectra for given sets of parameters. Scene-specific target spectra can improve confidence in greenhouse gas retrievals and flux estimates across collections of scenes with diverse geometric and atmospheric conditions.

Inherent Optical Properties‐Reflectance Relationships Revisited

AbstractUnderstanding the relationship between remote sensing reflectance, Rrs(λ) and the inherent optical properties (IOPs) of natural waters is potentially a key to improving our ability to determine biogeochemical constituents from radiometric measurements. These relationships are usually described as a function of absorption, a(λ), and backscattering, bb(λ), coefficients, with the literature providing various forms of equation operating on either bb(λ)/a(λ) or bb(λ)/[a(λ)+bb(λ)] to represent the impact of variations in light field geometries and changes in sea‐water composition. The performance of several IOP‐reflectance relationships is assessed using HydroLight radiative transfer simulations covering a broad range of Case 1 and Case 2 water conditions. While early versions of IOP‐reflectance relationships assigned variability to associated proportionality factors (e.g., f/Q) or low‐order polynomial functions, recent studies have demonstrated relationships between Rrs(λ) and bb(λ)/[a(λ)+bb(λ)] are well‐characterized by nonlinear (high‐order polynomial), monotonic functions. This study demonstrates that this approach is also valid for relationships operating on bb(λ)/a(λ) and that there is no intrinsic benefit to functions operating on bb(λ)/[a(λ)+bb(λ)] compared to bb(λ)/a(λ) for Case 2 waters, contrary to recent suggestions in the literature. In all cases it is necessary to carefully consider the performance of best fit relationships across the full range of variability of IOPs and Rrs(λ), with higher order polynomials required to enable equivalent performance across the range of natural variability. The analysis further demonstrates insignificant wavelength sensitivity across the visible region, limited sensitivity to changes in solar zenith angle and extends to relationships for below surface remote sensing reflectance, rrs(λ).

Changes in the surface broadband shortwave radiation budget during the 2017 eclipse

Abstract. While solar eclipses are known to greatly diminish the visible radiation reaching the surface of the Earth, less is known about the magnitude of the impact. We explore both the observed and modeled levels of change in surface radiation during the eclipse of 2017. We deployed a pyranometer and Pandora spectrometer instrument to Casper, Wyoming, and Columbia, Missouri, to measure surface broadband shortwave (SW) flux and atmospheric properties during the 21 August 2017 solar eclipse event. We performed detailed radiative transfer simulations to understand the role of clouds in spectral and broadband solar radiation transfer in the Earth's atmosphere for the normal (non-eclipse) spectrum and red-shift solar spectra for eclipse conditions. The theoretical calculations showed that the non-eclipse-to-eclipse surface flux ratio depends strongly on the obscuration of the solar disk and slightly on the cloud optical depth. These findings allowed us to estimate what the surface broadband SW flux would be for hypothetical non-eclipse conditions from observations during the eclipse and further to quantify the impact of the eclipse on the surface broadband SW radiation budget. We found that the eclipse caused local reductions of time-averaged surface flux of about 379 W m−2 (50 %) and 329 W m−2 (46 %) during the ∼3 h course of the eclipse at the Casper and Columbia sites, respectively. We estimated that the Moon's shadow caused a reduction of approximately 7 %–8 % in global average surface broadband SW radiation. The eclipse has a smaller impact on the absolute value of surface flux reduction for cloudy conditions than a clear atmosphere; the impact decreases with the increase in cloud optical depth. However, the relative time-averaged reduction of local surface SW flux during a solar eclipse is approximately 45 %, and it is not sensitive to cloud optical depth. The reduction of global average SW flux relative to climatology is proportional to the non-eclipse and eclipse flux difference in the penumbra area and depends on cloud optical depth in the Moon's shadow and geolocation due to the change in solar zenith angle. We also discuss the influence of cloud inhomogeneity on the observed SW flux. Our results not only quantify the reduction of the surface solar radiation budget, but also advance the understanding of broadband SW radiative transfer under solar eclipse conditions.

Improvements of 6S Look-Up-Table Based Surface Reflectance Employing Minimum Curvature Surface Method

We propose a methodology employing an interpolation technique on the Second Simulation of a Satellite Signal (6S) look-up table (LUT) to improve surface reflectance retrieval using Himawari-8/Advanced Himawari Imager (AHI). A minimum curvature surface (MCS) technique was used to refine the 6S LUT, and the solar zenith angle (SZA) and viewing zenith angle (VZA) increments were narrowed by 0.5°. The interpolation processing time was relatively short, about 3172 s per channel, and the interpolated xa and xb were well represented by the changes in SZA and VZA. An evaluation of the interpolated xa and xb for six cases revealed a relative mean absolute error of less than 5% for all channels and cases; however, a slight difference was evident for higher values of SZA and VZA. To evaluate the surface reflectance, we compared the surface reflectance derived using 6S LUT with that calculated using 6S only. Application of the interpolated 6S LUT showed a lower relative root mean square error (RRMSE) of 0.65% to 9.29% for all channels, than before interpolation. The improvement in surface reflectance measurements increased with the SZA. For a SZA above 75°, the RRMSE improved significantly for all channels (by 11.33–45.1%). In addition, when the MCS method was applied, the surface reflectance measurements improved without spatial discontinuity and showed good agreement with 6S results in a linear profile analyses. Thus, the method proposed can improve LUT based surface reflectance measurements in less time and increase the availability of surface reflectance data based on geostationary satellites.

Comprehensive accuracy assessment of MODIS daily snow cover products and gap filling methods

The accuracy of the standard Moderate Resolution Imaging Spectroradiometer (MODIS) daily snow cover products (Collection 5) and several of the most common and computationally frugal gap-filling methods were validated using 12 years of daily observations from over 800 Snow Telemetry (SNOTEL) stations. While several factors affect snow cover accuracy to some extent, the largest controls are associated with land cover type; accuracy fell from the maximum in croplands (94.9%) to the minimum for observations with a land cover type of water (79.5%). The second largest impact on accuracy is attributed to the changes in solar zenith angle, where accuracy decreased from a maximum of ∼96% at 42° to a minimum of ∼84% at 58°. Based on the results of this work, the highest accuracy binary daily snow cover dataset can be achieved by reclassifying the fractional snow band using a no-snow/snow threshold of 10 from the MODIS Terra sensor. Gap-filling is best accomplished by a temporal filter on the same dataset, and the window length depends on the desired completeness of the final dataset and the amount of missing values in the area of interest. Decreases in accuracy caused by this gap-filling method taper off after 7 days. Therefore, if a larger temporal window is needed to adequately fill in a time series it is best to extend the window to the largest gap present.

Ionospheric Effects of the Sudden Stratospheric Warming in 2009: Results of Simulation with the First Version of the EAGLE Model

In this paper, we discuss perturbations in neutral temperature, total electron content (TEC), and critical frequency of the maximum of the F2 layer (foF2) during the sudden stratospheric warming in January 2009. The calculations were performed using the first version of the EAGLE (Entire Atmosphere Global Model), which is a combination of the models of the low–middle atmosphere (HAMMONIA) and the upper atmosphere (GSM TIP). The EAGLE reproduces observed stratospheric warming and related mesospheric cooling in the northern polar cap in January 2009. At thermospheric altitudes, the neutral temperature perturbations have a quasi-wave character with a wavelength of ∼40 km in the vertical direction. Our results indicate that the HAMMONIA model should be used in the EAGLE instead of the GSM TIP model for the neutral temperature calculations in the altitude region from 80 to 120 km. It is shown that the obtained model foF2 and TEC perturbations are mainly related to seasonal variations. The most-pronounced perturbations in the ionospheric electron density due to stratospheric warming are formed near the equator and are basically negative. Our analysis of the neutral temperature and electron density perturbations made it possible to conclude that the dependence of ionospheric parameters on seasonal changes in solar zenith angle is stronger than for the thermosphere parameters.

Landsat 5 Thematic Mapper reflectance and NDVI 27-year time series inconsistencies due to satellite orbit change

Abstract The Landsat 5 Thematic Mapper (TM) sensor provided the longest single mission terrestrial remote sensing data record but temporally sparse station keeping maneuvers meant that the Landsat 5 orbit changed over the 27 year mission life. Long-term Landsat 5 TM reflectance inconsistencies may be introduced by orbit change induced solar zenith variations combined with surface reflectance anisotropy, commonly described by the Bi-directional Reflectance Distribution Function (BRDF). This study quantifies the local overpass time and observed solar zenith angle changes for all the Landsat 5 TM images available at two latitudinally separated locations along the same north-south Landsat path (27) in Minnesota (row 26) and Texas (row 42). Over the 27 years the Landsat 5 orbit changed by nearly 1 h and resulted in changes in the Landsat 5 observed solar zenith angle of > 10°. The Landsat 5 orbit was relatively stable from 1984 to 1994 and from 2007 to 2011, but changed rapidly from 1995 to 2000, and from 2003 to 2007. Rather than directly examine Landsat 5 TM reflectance time series a modelling approach was used. This was necessary because unambiguous separation of orbit change induced Landsat reflectance variations from other temporal variations is non-trivial. The impact of Landsat 5 orbit induced observed solar zenith angle variations on the red and near-infrared reflectance and derived normalized difference vegetation index (NDVI) values were modelled with respect to different Moderate-Resolution Imaging Spectroradiometer (MODIS) BRDF land cover types. Synthetic nadir BRDF-adjusted reflectance (NBAR) for the Landsat 5 TM observed and a modelled reference year 2011 solar zenith were compared over the 27 years of acquisitions. Ordinary least squares linear regression fits of the NBAR difference values as a function of the acquisition date indicated an increasing trend in red and near-infrared NBAR and a decreasing trend in NDVI NBAR due to orbit changes. The trends are statistically significant but small, no more than 0.0006 NDVI/year. Comparison of certain years of Landsat 5 data may result in significant reflectance and NDVI differences due only to Landsat 5 orbit changes and cause spurious detection of “browning” vegetation events and underestimation of greening trends. The greatest differences will occur when 1995 Landsat 5 TM data are compared with 2007 to 2011 data; NDVI values could be up to 0.11 greater in 1995 than in 2011 for anisotropic land cover types and up to 0.05 greater for average CONUS land cover types. A smaller number of Landsat 5 TM images were also examined and provide support for the modelled based findings. The paper concludes with a discussion of the implications of the research findings for Landsat 5 TM time series analyses.

Characterization of the lower layer in the dayside Venus ionosphere and comparisons with Mars

The influence of solar zenith angle (SZA) and solar irradiance has been well characterized for the V2 layer in the Venus ionosphere, but not the V1 layer, where previous efforts were limited by data scarcity and incomplete SZA coverage. Here we use more than 200 radio occultation profiles from Venus Express with good SZA coverage to characterize how the V1 peak altitude, peak density, and morphology respond to changes in SZA and solar activity. The V1 and V2 peak altitudes vary little with SZA, and both peak electron densities vary with SZA in an approximately Chapman-like manner. These results imply that the thermal structures of the atmosphere and ionosphere between ∼125km and ∼140km vary little with SZA. As solar activity increases, the ratio of the V1 to V2 peak density increases, and the V1 morphology changes more than the V2 morphology. These results are due to the soft X-ray flux increasing relative to the EUV flux as solar activity increases. We compare the behavior of the V1 layer to the analogous M1 layer at Mars, and find that their peak altitudes respond differently to changes in SZA and solar activity. The V1 peak density also increases more with solar activity than the M1 peak density. These distinct behaviors arise from differences in their underlying neutral atmospheres.

An improved method for correction of air temperature measured using different radiation shields

The variation of air temperature measurement errors using two different radiation shields (DTR502B Vaisala, Finland, and HYTFZ01, Huayun Tongda Satcom, China) was studied. Datasets were collected in the field at the Daxing weather station in Beijing from June 2011 to May 2012. Most air temperature values obtained with these two commonly used radiation shields were lower than the reference records obtained with the new Fiber Reinforced Polymers (FRP) Stevenson screen. In most cases, the air temperature errors when using the two devices were smaller on overcast and rainy days than on sunny days; and smaller when using the imported rather than the Chinese shield. The measured errors changed sharply at sunrise and sunset, and reached maxima at noon. Their diurnal variation characteristics were, naturally, related to changes in solar radiation. The relationships between the record errors, global radiation, and wind speed were nonlinear. An improved correction method was proposed based on the approach described by Nakamura and Mahrt (2005) (NM05), in which the impact of the solar zenith angle (SZA) on the temperature error is considered and extreme errors due to changes in SZA can be corrected effectively. Measurement errors were reduced significantly after correction by either method for both shields. The error reduction rate using the improved correction method for the Chinese and imported shields were 3.3% and 40.4% higher than those using the NM05 method, respectively.

A technique to map monthly average global illuminance from satellite data in the tropics using a simple semi-empirical model

This paper presents a technique to map monthly average hourly global illuminance from satellite data. A semi-empirical model relating monthly average global illuminance to cloud index, precipitable water, total ozone column (TOC), aerosol optical depth (AOD) and air mass was developed. Data for the cloud index, AOD and TOC were obtained from the visible imagery data of MTSAT-1R, MODIS/Terra and OMI/Aura satellites respectively, while precipitable water was extracted from NCEP/NCAR reanalysis database. The model was formulated using global illuminance measured at four stations in Thailand for a four-year period and validated with an independent one-year data set. Values of monthly average hourly global illuminance calculated from the model and those obtained from the measurements were in good agreement, with a root mean square difference (RMSD) and mean bias difference (MBD) of 8.1% and −0.8%, respectively. The model was used to calculate monthly average hourly global illuminance over Thailand and the results were displayed as illuminance maps. The maps reveal diurnal and seasonal effects mainly in response to solar zenith angle changes and cloud cover related to the southwest and northeast monsoons.

Assessment of the impact of the orbital drift of SPOT-VGT1 by comparison with SPOT-VGT2 data

The Système Pour l’Observation de la Terre 5 – VEGETATION 2 (SPOT 5 – VGT2) instrument started to drift in orbit from early 2011, but currently the overpass time is still within mission requirements (less than 20 min deviation from 10:30 local equator crossing time). To determine the operational lifetime of the VGT2 instrument, it was necessary to investigate the impact of orbital drift beyond these mission requirements such that the VGT time series remains consistent over time. To this purpose, the impact of orbital drift on reflectance values and on the normalized difference vegetation index (NDVI) from the VGT1 instrument (onboard SPOT 4) in the period May 2009 to April 2012 was investigated using paired observations from VGT1 and VGT2. The comparison is thus a relative one, which has the advantage that land-cover change or trends in land-cover change are not interfering with the analysis. VGT2 acted as the reference against which the VGT1 data were compared. First, the magnitude of the solar zenith angle (SZA) change in the overlapping time period was investigated. This SZA change is largest near the equator and decreases considerably with increasing latitude. The overpass difference of VGT1 and VGT2 of 37 min resulted in maximum SZA changes of 9°, which is still well within the SZA variability of standard VGT products (i.e. 20°). Second, for a number of selected sites of 1° × 1° size with one dominant land cover, we investigated in detail the time evolution of some measures of agreement and disagreement. In the third analysis, systematic subsamples of the global images were used. From these images, the relative difference between VGT1 and VGT2 – with VGT2 as reference – was assessed as a function of the SZA of VGT2 and for different ranges of SZA difference. An increase of 10–20% in the relative difference between VGT1 and VGT2 was observed for the four spectral bands. The impact on the NDVI was negligible. Based on the obtained results, an alternative compositing approach is formulated in order to maintain the quality of the VGT standard products with a similar orbital drift as investigated in this study.

Within-day variability of particulate organic carbon and remote-sensing reflectance during a bloom of Phaeocystis antarctica in the Ross Sea, Antarctica

We examined the within-day variability in seawater optical properties and biogeochemical constituents for a high-latitude location in the Ross Sea, Antarctica, during development of the annual spring phytoplankton bloom. Measurements of particulate organic carbon concentration (POC), chlorophyll-a concentration (Chl), and particle size distribution were conducted at 4–6 hour intervals in parallel with determinations of the spectral absorption and attenuation coefficients of particles, and the spectral remote-sensing reflectance of the surface ocean (Rrs). Surface POC and Chl exhibited more than a twofold variation throughout the day in the continuous presence of natural light. A minimum occurred near local noon coinciding with peak solar irradiance, a maximum in the evening, and a subsequent decrease throughout the night-time hours. These patterns were accompanied by large changes in the magnitude and spectral shape of Rrs, including the blue-to-green spectral band ratios used in ocean colour algorithms for estimating POC and Chl. The variability in Rrs could not be explained by changes in solar zenith angle, but was consistent with observations of within-day variations in spectral absorption and scattering by particles which were influenced by changes in the particle concentration and size distribution. The accuracy of an empirical ocean colour algorithm for estimating POC from Rrs was unaffected by within-day variability, implying that short-term variations in surface POC can be potentially monitored by multiple within-day measurements of Rrs, through means of in situ and remote sensing observations if available. Our findings also suggest that within-day changes in POC can be significant compared with the variability observed on meso-scale spatial scales, potentially confounding the interpretation of remote-sensing data obtained from temporal and spatial compositing of images measured at different times within a single day.

Simulation of the spatial distribution of mineral dust and its direct radiative forcing over Australia

Direct radiative forcing by mineral dust is important as it significantly affects the climate system by scattering and absorbing short-wave and long-wave radiation. The multi-angle imaging spectro radiometer (MISR) and cloud–aerosol lidar with orthogonal polarisation (CALIOP) aerosol data are used to observe mineral dust distribution over Australia. In addition, the weather research and forecasting with chemistry (WRF/Chem) model is used to estimate direct radiative forcing by dust. At the surface, the model domain clear-sky short-wave and long-wave direct radiative forcing by dust averaged for a 6-month period (austral spring and summer) was estimated to be −0.67 W m−2 and 0.13 W m−2, respectively. The long-wave warming effect of dust therefore offsets 19.4% of its short-wave cooling effect. However, over Lake Eyre Basin where coarse particles are more abundant, the long-wave warming effect of dust offsets 60.9% of the short-wave cooling effect. At the top of the atmosphere (TOA), clear-sky short-wave and long-wave direct radiative forcing was estimated to be −0.26 W m−2 and −0.01 W m−2, respectively. This leads to a net negative direct radiative forcing of dust at the TOA, indicating cooling of the atmosphere by an increase in outgoing radiation. Short-wave and long-wave direct radiative forcing by dust is shown to have a diurnal variation due to changes in solar zenith angle and in the intensity of infrared radiation. Atmospheric heating due to absorption of short-wave radiation was simulated, while the interaction of dust with long-wave radiation was associated with atmospheric cooling. The net effect was cooling of the atmosphere near the surface (below 0.2 km), with warming of the atmosphere at higher altitudes.

Hydroxyl in the stratosphere and mesosphere – Part 1: Diurnal variability

Abstract. Diurnal variations in hydroxyl (OH) in the stratosphere and mesosphere are analyzed using measurements from the Aura Microwave Limb Sounder (MLS). The primary driver for OH diurnal variations is the ultraviolet actinic flux that initiates the photochemical production of reactive hydrogen species. The magnitude of this flux is governed largely by changes in solar zenith angle (SZA) throughout the day, and OH diurnal variations are well approximated by an exponential function of the secant of SZA. Measured OH concentrations are fit to a function of the form exp[−βsec(SZA)], where the parameter β is a function of altitude. We examine the magnitude of β and show that it is related to the optical depths of ultraviolet absorption by ozone and molecular oxygen. Values of β from SLIMCAT model simulations show the same vertical structure as those from MLS and the average level of agreement between model and measurements is 6%. The vertical profile of β from MLS can be represented by a simple analytic formulation involving the ozone and water vapor photodissociation rates. This formulation is used to infer the altitude dependence of the primary production mechanisms for OH: the reaction of excited-state atomic oxygen with water vapor versus the direct photodissociation of water vapor.

Long-Term Dependencies of Annual and Semiannual Components of NmF2 Over Concepción

In this work an attempt is made to determine whether the annual and semiannual components of the NmF2 observed over Concepcion (36.8°S; 73.0°W), Chile, exhibit long-term trends. Monthly median hourly values of NmF2 determined from ionosonde observations are analyzed. Although the assessed trends for NmF2 and for the semiannual and annual components of NmF2, are hardly significant by any statistical test, they exhibit a systematic diurnal evolution. NmF2 trends are almost null during night time and negative from around noon to the early evening. The same can be said for trends of the amplitudes of the semiannual component. On the contrary, trends for the annual component are always positive and exhibit a rather irregular diurnal pattern. It seems reasonable then to particularly look for an explanation of the semiannual component long-term changes when concerned with long-term changes of NmF2. The semiannual variation is linked to composition and thermospheric wind effects, and solar zenith angle changes which may be connected to changes of the strength of the global circulation. But, more study is needed to envisage the changes on the semiannual component of NmF2 which may arise from eventual long-term changes of the global circulation.