Retrieval and Uncertainty Analysis of Land Surface Reflectance Using a Geostationary Ocean Color Imager

Land surface reflectance (LSR) is well known as an essential variable to understand land surface properties. The Geostationary Ocean Color Imager (GOCI) be able to observe not only the ocean but also the land with the high temporal and spatial resolution thanks to its channel specification. In this study, we describe the land atmospheric correction algorithm and present the quality of results through comparison with Moderate Resolution Imaging Spectroradiometer (MODIS) and in-situ data for GOCI-II. The GOCI LSR shows similar spatial distribution and quantity with MODIS LSR for both healthy and unhealthy vegetation cover. Our results agreed well with in-situ-based reference LSR with a high correlation coefficient (>0.9) and low root mean square error (<0.02) in all 8 GOCI channels. In addition, seasonal variation according to the solar zenith angle and phenological dynamics in time-series was well presented in both reference and GOCI LSR. As the results of uncertainty analysis, the estimated uncertainty in GOCI LSR shows a reasonable range (<0.04) even under a high solar zenith angle over 70°. The proposed method in this study can be applied to GOCI-II and can provide continuous satellite-based LSR products having a high temporal and spatial resolution for analyzing land surface properties.

Bestimmung optischer Dichten stratosphärischer Aerosole aus den Farbverhältnissen historischer Gemälde – kann das funktionieren?

&lt;p&gt;Die M&amp;#246;glichkeit, quantitative Information &amp;#252;ber die stratosph&amp;#228;rische Aerosolbefrachtung &amp;#8211; insbesondere in Zeitr&amp;#228;umen vulkanisch verst&amp;#228;rkter Aerosole &amp;#8211; aus den Farbverh&amp;#228;ltnissen historischer Gem&amp;#228;lde abzuleiten, erscheint auf den ersten Blick sehr vielversprechend. Tats&amp;#228;chlich wurde dieser Ansatz in einigen Studien verwendet, um die optische Dichte stratosph&amp;#228;rischer Aerosole nach st&amp;#228;rkeren Vulkanausbr&amp;#252;chen &amp;#252;ber einen Zeitraum von ca. 500 Jahren zu bestimmen. In diesem Vortrag evaluieren wir die Verl&amp;#228;sslichkeit dieses Ansatzes mit Hilfe von Simulationen mit dem Strahlungstransfermodell SCIATRAN und sch&amp;#228;tzen die Fehler der abgeleiteten optischen Dichten basierend auf plausiblen Unsicherheiten relevanter Parameter ab. Wir zeigen, dass die Unsicherheiten in einigen wichtigen Parametern &amp;#8211; die f&amp;#252;r historische Eruptionen typischerweise nur unzureichend bekannt sind &amp;#8211; zu &amp;#228;hnlichen Ver&amp;#228;nderungen in den Rot-Gr&amp;#252;n-Farbverh&amp;#228;ltnissen f&amp;#252;hren k&amp;#246;nnen wie massive Vulkanausbr&amp;#252;che (z.B. des Tambora 1815 oder des Krakatao 1883). Von den untersuchten Effekten hat die angenommene Gr&amp;#246;&amp;#223;enverteilung der stratosph&amp;#228;rischen Aerosole den gr&amp;#246;&amp;#223;ten Einfluss auf die Farbverh&amp;#228;ltnisse und damit die abgeleiteten optischen Dichten. F&amp;#252;r Sonnenzenitwinkel (SZA von engl. Solar Zenith Angle) von mehr als 80 Grad kann auch die angenommene stratosph&amp;#228;rische Ozonmenge die abgeleiteten optischen Dichten signifikant beeinflussen. F&amp;#252;r SZA gr&amp;#246;&amp;#223;er als 90 Grad weisen die horizontnahen Farbverh&amp;#228;ltnisse eine dramatische Abh&amp;#228;ngigkeit vom SZA auf, so dass f&amp;#252;r diese F&amp;#228;lle eine Bestimmung optischer Dichten praktisch unm&amp;#246;glich ist. Abschlie&amp;#223;end gehen wir auf die Frage ein, wie stark die langfristige Ver&amp;#228;nderung der Farben eines Gem&amp;#228;ldes die optischen Dichten beeinflussen kann.&lt;/p&gt;

On the relationship between energy input to the ionosphere and the ion outflow flux under different solar zenith angles

The ionosphere is one of the important sources for magnetospheric plasma, particularly for heavy ions with low charge states. We investigate the effect of solar illumination on the number flux of ion outflow using data obtained by the Fast Auroral SnapshoT (FAST) satellite at 3000–4150 km altitude from 7 January 1998 to 5 February 1999. We derive empirical formulas between energy inputs and outflowing ion number fluxes for various solar zenith angle ranges. We found that the outflowing ion number flux under sunlit conditions increases more steeply with increasing electron density in the loss cone or with increasing precipitating electron density (> 50 eV), compared to the ion flux under dark conditions. Under ionospheric dark conditions, weak electron precipitation can drive ion outflow with small averaged fluxes (~ 107 cm−2 s−1). The slopes of relations between the Poynting fluxes and outflowing ion number fluxes show no clear dependence on the solar zenith angle. Intense ion outflow events (> 108 cm−2 s−1) occur mostly under sunlit conditions (solar zenith angle < 90°). Thus, it is presumably difficult to drive intense ion outflows under dark conditions, because of a lack of the solar illumination (low ionospheric density and/or small scale height owing to low plasma temperature). Graphical abstract

Top-of-atmosphere albedo bias from neglecting three-dimensional cloud radiative effects

Clouds cover on average nearly 70% of Earth’s surface and regulate the global albedo. The magnitude of the shortwave reflection by clouds depends on their location, optical properties, and three-dimensional (3D) structure. Due to computational limitations, Earth system models are unable to perform 3D radiative transfer calculations. Instead they make assumptions, including the independent column approximation (ICA), that neglect effects of 3D cloud morphology on albedo. We show how the resulting radiative flux bias (ICA-3D) depends on cloud morphology and solar zenith angle. We use high-resolution (20–100 m horizontal resolution) large-eddy simulations to produce realistic 3D cloud fields covering three dominant regimes of low-latitude clouds: shallow cumulus, marine stratocumulus, and deep convective cumulonimbus. A Monte Carlo code is used to run 3D and ICA broadband radiative transfer calculations; we calculate the top-of-atmosphere (TOA) reflected flux and surface irradiance biases as functions of solar zenith angle for these three cloud regimes. Finally, we use satellite observations of cloud water path (CWP) climatology, and the robust correlation between CWP and TOA flux bias in our LES sample, to roughly estimate the impact of neglecting 3D cloud radiative effects on a global scale. We find that the flux bias is largest at small zenith angles and for deeper clouds, while the albedo bias is most prominent for large zenith angles. In the tropics, the annual-mean shortwave radiative flux bias is estimated to be 3.1±1.6 W m−2, reaching as much as 6.5 W m−2 locally.

Plasma and magnetic field conditions during radar blackouts at Mars.

&lt;p&gt;Large scale solar wind disturbances such as Interplanetary Coronal Mass Ejections (ICMEs) have a major impact on planetary systems.&amp;#160; At Mars, for example, Solar Energetic Particles released during the process that creates the ICME cause large scale radar blackouts as a result of enhanced ionisation at lower altitudes than normal.&amp;#160; The increased absorption of the radar signals can last for up to 10 &amp;#8211; 12 days, depending on the operational frequency of the radar.&amp;#160; These events occur at all latitudes and local times but there does appear to be a peak in occurrence at a solar zenith angle of about 160o, i.e. deep in the tail of the Martian plasma system. Using data from MAVEN, Mars Express and Mars Reconnaissance Orbiter we investigate the background plasma&amp;#160; and magnetic field conditions, which occur at the same time as these events to investigate how the SEP impact on the nightside atmosphere.&amp;#160; This will provide crucial evidence for plasma transport in the Martian system, in particular during the passage of ICMEs.&lt;/p&gt;

Cassini CAPS-ELS Observations of Negative Ions in Titan's Ionosphere: Solar Zenith Angle - Density Trends

&lt;p&gt;The discovery of heavy organic anions by in situ measurements using Cassini&amp;#8217;s CAPS Electron Spectrometer (ELS) in Titan&amp;#8217;s ionosphere was an unexpected result of the Cassini mission (Coates et al, 2007, Waite et al, 2007); a complete reconsideration of chemical processes in this enigmatic atmosphere was necessary as a result. These negative ions can be associated with complex hydrocarbon and nitrile processes which are linked to haze formation at lower altitudes. Cassini&amp;#8217;s CAPS ELS observed negative ions during Titan encounters at altitudes below 1400 km. The ions can reach masses over 13,000 amu/q (Coates et al., 2009), while recurring peaks in the mass spectra can be used to identify different mass groups as reported by Coates et al. (2007) and Wellbrock et al. (2013, 2019). Studying density and mass trends of these groups helps to identify controlling factors of the production and destruction mechanisms, and ultimately to improve our understanding of how organic macromolecules can be produced by naturally occurring abiotic processes. In this study we examine the effects different solar zenith angle conditions might have on both the light and heavy negative ion mass groups, and consider the role of processes such as photodetachment and dissociative electron attachment. We also compare the negative ion data with RPWS electron measurements and discuss the possible implications associated with the above processes.&lt;/p&gt;

Rethinking the correction for absorbing aerosols in the OMI- and TROPOMI-like surface UV algorithms

Satellite estimates of surface UV irradiance have been available since 1978 from the TOMS UV spectrometer and have continued with significantly improved ground resolution using the Ozone Monitoring Instrument (OMI 2004–current) and Sentinel 5 Precursor (S5P 2017–current). The surface UV retrieval algorithm remains essentially the same: it first estimates the clear-sky UV irradiance based on measured ozone and then accounts for the attenuation by clouds and aerosols, applying two consecutive correction factors. When estimating the total aerosol effect in surface UV irradiance, there are two major classes of aerosols to be considered: (1) aerosols that only scatter UV radiation and (2) aerosols that both scatter and absorb UV radiation. The former effect is implicitly included in the measured effective Lambertian-equivalent scene reflectivity (LER), so the scattering aerosol influence is estimated through cloud correction factor. Aerosols that absorb UV radiation attenuate the surface UV radiation more strongly than non-absorbing aerosols of the same extinction optical depth. Moreover, since these aerosols also attenuate the outgoing satellite-measured radiance, the cloud correction factor that treats these aerosols as purely scattering underestimates their aerosol optical depth (AOD), causing underestimation of LER and overestimation of surface UV irradiance. Therefore, for correction of aerosol absorption, additional information is needed, such as a model-based monthly climatology of aerosol absorption optical depth (AAOD). A correction for absorbing aerosols was proposed almost a decade ago and later implemented in the operational OMI and TROPOMI UV algorithms. In this study, however, we show that there is still room for improvement to better account for the solar zenith angle (SZA) dependence and nonlinearity in the absorbing aerosol attenuation, and as a result we propose an improved correction scheme. There are two main differences between the new proposed correction and the one that is currently operational in OMI and TROPOMI UV algorithms. First, the currently operational correction for absorbing aerosols is a function of AAOD only, while the new correction additionally takes the solar zenith angle dependence into account. Second, the second-order polynomial of the new correction takes the nonlinearity in the correction as a function of AAOD better into account, if compared to the currently operational one, and thus better describes the effect by absorbing aerosols over a larger range of AAOD. To illustrate the potential impact of the new correction in the global UV estimates, we applied the current and new proposed correction for global fields of AAOD from the aerosol climatology currently used in OMI UV algorithm, showing a typical differences of ±5 %. This new correction is easy to implement operationally using information of solar zenith angle and existing AAOD climatology.

Comparison of GRUAN RS92 and RS41 Radiosonde Temperature Biases

In this study, we validated the consistency of the GRUAN RS92 and RS41 datasets, versions EDT.1 and GDP.2, in the upper troposphere and lower stratosphere (200–20 hPa), through dual launch campaigns at the GRUAN site and using the radio occultation (RO) product and the ERA5 reanalysis from ECMWF as standards for double difference comparison. Separate comparisons with the references were also performed in order to trace the origin of the bias between the two instruments. Then, the performance of the GRUAN raw temperature correction algorithm was evaluated, from the aspects of day–night, the solar zenith angle, and the pressure level, for GDP.2 version products. The results show that RS92.EDT.1 has a warm bias of 0.355 K, compared to RS41.EDT.1, at 20 hPa, during daytime. This bias was found to mainly originate from RS92.EDT.1, based on the separate comparison with RO or ECMWF ERA5 data. RS92.GDP.2 is consistent with RS41.GDP.2, but a separate comparison indicated that the two original GDP.2 products have a ~1 K warm bias at 20 hPa during daytime, compared with RO or ECMWF ERA5 data. The GRUAN correction method can reduce the warm bias up to 0.5 K at 20 hPa during daytime. As a result, this GRUAN correction method is efficient, and it is dependent on the solar zenith angle and pressure level.

Analyzing of UV Index with the Time Variation for Baghdad

In order to better realize the effects of UV index (UVI) reaching the earth surface, measurements of effective UVI were carried out during the period of one year over Baghdad; (Lat.33.32- Long.44.45), which receives highly amounts of annual solar radiation. A daily data analysis of UVI is found to reach the highest value during summer reaching the value of 11, and a minimum in winter with the value of 1. A relation between UV index and the solar zenith angle was also, it is found that the UVI is highly dependent on the sun elevation where the atmospheric optical path becomes shorter as sun elevation heighten. It can be concluded that Baghdad city exposure to higher amounts of UVI during summer and several hedges must be taken to avoid health harm implications.