A novel satellite mission concept for upper air water vapour, aerosol and cloud observations using integrated path differential absorption LiDAR limb sounding

We propose a new satellite mission to deliver high quality measurements of upper air water vapour. The concept centres around a LiDAR in limb sounding by occultation geometry, designed to operate as a very long path system for differential absorption measurements. We present a preliminary performance analysis with a system sized to send 75 mJ pulses at 25 Hz at four wavelengths close to 935 nm, to up to 5 microsatellites in a counter-rotating orbit, carrying retroreflectors characterized by a reflected beam divergence of roughly twice the emitted laser beam divergence of 15 µrad. This provides water vapour profiles with a vertical sampling of 110 m; preliminary calculations suggest that the system could detect concentrations of less than 5 ppm. A secondary payload of a fairly conventional medium resolution multispectral radiometer allows wide-swath cloud and aerosol imaging. The total weight and power of the system are estimated at 3 tons and 2,700 W respectively. This novel concept presents significant challenges, including the performance of the lasers in space, the tracking between the main spacecraft and the retroreflectors, the refractive effects of turbulence, and the design of the telescopes to achieve a high signal-to-noise ratio for the high precision measurements. The mission concept was conceived at the Alpbach Summer School 2010

Critical Evaluations Of Modis And Misr Satellite Aerosol Products For Aerosol Modeling Applications

The study of uncertainties in satellite aerosol products is essential to aerosol data assimilation and modeling efforts. In this study, with the assistance of ground- based observations, uncertainties in Moderate Resolution Imaging Spectroradiometer (MODIS) collection 5 Deep Blue (DB), Multi-Angle Imaging Spectroradiometer (MISR) version 22 aerosol products, and the newly released collection 6 Dark Target over-ocean and DB products were evaluated. For each product, systematic biases were analyzed against observing conditions. Empirical correction procedures and data filtering steps were generated to develop noise and bias reduced DA-quality aerosol products for modeling related applications. Special attention was also directed at the potential low bias in satellite aerosol optical depth (AOD) climatology due to misclassification of aerosols as clouds over Asia. A heavy aerosol identifying system (HAIS) was developed through the combined use of the Ozone Monitoring Instrument (OMI) and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) products for detecting heavy smoke aerosol plumes. Upon extensive evaluation, HAIS was applied to one year of collocated OMI, CALIOP, and MODIS data to study the misclassifications related low bias. This study suggests that the misclassification of heavy smoke aerosol plumes by MODIS is rather infrequent and thus introduces an insignificant low bias to its AOD climatology. Still, this study confirms that misclassification happens in both active- and passive- based satellite aerosol products and needs to be studied for forecasting these events

Evaluation of satellite-based aerosol datasets and the CAMS reanalysis over the ocean utilizing shipborne reference observations

Reliable reference measurements over the ocean are essential for the evaluation and improvement of satelliteand model-based aerosol datasets. Within the framework of the Maritime Aerosol Network, shipborne reference datasets have been collected over the Atlantic Ocean since 2004 with Microtops Sun photometers. These were recently complemented by measurements with the multi-spectral GUVis- 3511 shadowband radiometer during five cruises with the research vessel Polarstern. The aerosol optical depth (AOD) uncertainty estimate of both shipborne instruments of ±0:02 can be confirmed if the GUVis instrument is cross calibrated to the Microtops instrument to account for differences in calibration, and if an empirical correction to account for the broad shadowband as well as the effects of forward scattering is introduced. Based on these two datasets, a comprehensive evaluation of aerosol products from the Moderate Resolution Imaging Spectroradiometer (MODIS) flown on NASA's Earth Observing System satellites, the Spinning Enhanced Visible and Infrared Imager (SEVIRI) aboard the geostationary Meteosat satellite, and the Copernicus Atmosphere Monitoring Service reanalysis (CAMS RA) is presented. For this purpose, focus is given to the accuracy of the AOD at 630 nm in combination with the Ångström exponent (AE), discussed in the context of the ambient aerosol type. In general, the evaluation of MODIS AOD from the official level-2 aerosol products of C6.1 against the Microtops AOD product confirms that 76% of data points fall into the expected error limits given by previous validation studies. The SEVIRI-based AOD product exhibits a 25% larger scatter than the MODIS AOD products at the instrument's native spectral channels. Further, the comparison of CAMS RA and MODIS AOD versus the shipborne reference shows similar performance for both datasets, with some differences arising from the assimilation and model assumptions. When considering aerosol conditions, an overestimation of AE is found for scenes dominated by desert dust for MODIS and SEVIRI products versus the shipborne reference dataset. As the composition of the mixture of aerosol in satellite products is constrained by model assumptions, this highlights the importance of considering the aerosol type in evaluation studies for identifying problematic aspects. © Author(s) 2020

On the spatio-temporal representativeness of observations

The discontinuous spatio-temporal sampling of observations has an impact when using them to construct climatologies or evaluate models. Here we provide estimates of this so-called representation error for a range of timescales and length scales (semi-annually down to sub-daily, 300 to 50 km) and show that even after substantial averaging of data significant representation errors may remain, larger than typical measurement errors. Our study considers a variety of observations: ground-site or in situ remote sensing (PM2.5, black carbon mass or number concentrations), satellite remote sensing with imagers or lidar (extinction). We show that observational coverage (a measure of how dense the spatiotemporal sampling of the observations is) is not an effective metric to limit representation errors. Different strategies to construct monthly gridded satellite L3 data are assessed and temporal averaging of spatially aggregated observations (super-observations) is found to be the best, although it still allows for significant representation errors. However, temporal collocation of data (possible when observations are compared to model data or other observations), combined with temporal averaging, can be very effective at reducing representation errors. We also show that ground-based and wideswath imager satellite remote sensing data give rise to similar representation errors, although their observational sampling is different. Finally, emission sources and orography can lead to representation errors that are very hard to reduce, even with substantial temporal averaging

An AeroCom–AeroSat study: intercomparison of satellite AOD datasets for aerosol model evaluation

To better understand and characterize current uncertainties in the important observational constraint of climate models of aerosol optical depth (AOD), we evaluate and intercompare 14 satellite products, representing nine different retrieval algorithm families using observations from five different sensors on six different platforms. The satellite products (super-observations consisting of 1 degrees x 1 degrees daily aggregated retrievals drawn from the years 2006, 2008 and 2010) are evaluated with AErosol RObotic NETwork (AERONET) and Maritime Aerosol Network (MAN) data. Results show that different products exhibit different regionally varying biases (both under- and overestimates) that may reach +/- 50 %, although a typical bias would be 15 %-25 % (depending on the product). In addition to these biases, the products exhibit random errors that can be 1.6 to 3 times as large. Most products show similar performance, although there are a few exceptions with either larger biases or larger random errors. The intercomparison of satellite products extends this analysis and provides spatial context to it. In particular, we show that aggregated satellite AOD agrees much better than the spatial coverage (often driven by cloud masks) within the 1 degrees x 1 degrees grid cells. Up to similar to 50 % of the difference between satellite AOD is attributed to cloud contamination. The diversity in AOD products shows clear spatial patterns and varies from 10 % (parts of the ocean) to 100 % (central Asia and Australia). More importantly, we show that the diversity may be used as an indication of AOD uncertainty, at least for the better performing products. This provides modellers with a global map of expected AOD uncertainty in satellite products, allows assessment of products away from AERONET sites, can provide guidance for future AERONET locations and offers suggestions for product improvements. We account for statistical and sampling noise in our analyses. Sampling noise, variations due to the evaluation of different subsets of the data, causes important changes in error metrics. The consequences of this noise term for product evaluation are discussed

Interaction between aerosols and the mesoscale convective systems over the tropical continents

Presence of aerosols in the upper troposphere can have significant impacts on the Earth’s radiative energy budget. However, the aerosol–cloud relationship represents the largest uncertainty in the radiative energy budget. Relationships between aerosols and the mesoscale convective systems (MCSs) are complicated and difficult to ascertain, due in large part to inadequate availability of satellite datasets until recent years. Variation of aerosol impacts with meteorological parameters and the relative influence of these parameters on the convective strength of the MSCs can also be attributed to limited detectability of aerosol invigoration effects. To address the interaction between aerosol and the MCSs, I first address the influence of MCS on the distribution of the aerosols, which is poorly known on a global scale. Then, I investigate the influence of aerosol on MCSs. This dissertation addresses these problems by collocating a suite of geostationary and polar orbital satellites at three different phases of their convective lifecycle. First, I estimate the extent of upper tropospheric aerosol layers (UT ALs) surrounding the MCSs and explore the relationships between UT AL extent and the morphology, location, and developmental stages of collocated MCSs in the tropics over equatorial Africa, South Asia, and the Amazon basin between June 2006 and June 2008. I identify that the most extensive UT ALs over equatorial Africa are associated with the mature MCSs, while the most extensive UT ALs over South Asia and the Amazon basin are associated with the growing MCSs. Convective aerosol transport over Amazonia is weaker than that observed over the other two regions despite similar transport frequencies, likely due to smaller sizes and shorter mean lifetimes of Amazonian MCSs. Variations in UT ALs in the vicinity of tropical MCSs are primarily explained by variations in the horizontal sizes of the associated MCSs and are not related to aerosol loading in the lower troposphere. Relationships between convective properties and aerosol transport are relatively weak during the decaying stage of convective development. Then I estimate the relative influence of aerosols and other meteorological parameters on MCS strength and longevity using collocated samples of MCSs from January 2003 to June 2008. The results show that relative humidity (RH) and convective available potential energy (CAPE) have the strongest impacts on MCS lifetime and enhance the lifetime of the MCSs by 6-36 hours when other parameters such as vertical wind shear (VWS) and aerosols are kept constant. Aerosols also enhance the convective lifetime of MCSs, however at a much weaker rate (6-24h) and only when RH and VWS are high. Moreover, aerosol influence on convective lifetime is detected during the mature and decaying phases only. At the continental scale, aerosols explain 20-27% of the total variance of MCSs’ lifetime over equatorial South America, but explain only 8% of the same over equatorial Africa. South Asian MCSs are more strongly influenced by meteorological parameters and MCS-associated aerosols when they are over the ocean than when over the land since most MCSs form and develop over the oceans. After that, I estimate the influence of aerosols and other meteorological parameters on MCSs’ rain rate (RR). Results show that an increase in aerosol concentration enhance IWC and suppress RR and LH during all three phases of convective lifetime. Increasing aerosol concentrations suppress RR at the rate of -0.38 mm/h and -0.47 mm/h during the growing, decaying phases when VWS is high and at a rate of -0.30 mm/h during the mature phase when RH is low. Meteorological parameters such as VWS and RH have significant effects on these aerosol influences. The suppression of RR is also associated with a decrease in latent heat released by large hydrometeors. Aerosols explain 16%, 23%, and 29% of RR’s variance during the growing, mature and decaying phases, respectively, as estimated by a multiple linear regression method. Consequently, aerosols enhance IWC of the MCSs inside the anvil up to 0.72, 1.41, 0.82 mg/m3 and enhance the total integrated reflectivity of the larger-sized ice particles up to 8, 11, and 18 dBZ in the convective core regions during the growing, mature and decaying phases, respectively. In contrast, changes (one standard deviation) in CAPE and RH enhance the RR up to 0.35 mm/h. This dissertation study provides the first satellite based global tropical assessment of the relative influences of aerosols and meteorological conditions on MCSs’ lifetime, rain rate, and IWC and the mutual dependence of these influences. It also shows how aerosols influence the rain rate, cloud ice and lifetime of the MCSs, varying within their lifecycle and between different tropical continents ranging from humid equatorial South America during wet season and big monsoonal systems over South Asia to relatively dry equatorial Africa with high aerosol loading. In doing so, this work has also advanced our capability to evaluate whether or not aerosols could increase convective lifetime by suppressing rain rate and invigorating the MCSs on climate scale and what are the favorable meteorological conditions for aerosol to affect the lifetime of the MCSs. Our results also provide an interpretive framework for devising and evaluating numerical model experiments that can examine relationships between convective properties and ALs transported in the upper troposphere. In the future, we would like to investigate the influence of different meteorological parameters and aerosols on extra tropical MCSs and on self-aggregation of convection.Geological Science

Evaluating The Impact Of Above-Cloud Aerosols On Cloud Optical Depth Retrievals From Modis

Using two different operational Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) cloud optical depth (COD) retrievals (visible and shortwave infrared), the impacts of above-cloud absorbing aerosols on the standard COD retrievals are evaluated. For fine-mode aerosol particles, aerosol optical depth (AOD) values diminish sharply from the visible to the shortwave infrared channels. Thus, a suppressed above-cloud particle radiance aliasing effect occurs for COD retrievals using shortwave infrared channels. Aerosol Index (AI) from the spatially and temporally collocated Ozone Monitoring Instrument (OMI) are used to identify above-cloud aerosol particle loading over the southern Atlantic Ocean, including both smoke and dust from the African sub-continent. MODIS and OMI Collocated Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data are used to constrain cloud phase and provide contextual above-cloud AOD values. The frequency of occurrence of above-cloud aerosols is depicted on a global scale for the spring and summer seasons from OMI and CALIOP, thus indicating the significance of the problem. Seasonal frequencies for smoke-over-cloud off the southwestern Africa coastline reach 20-50% in boreal summer. We find a corresponding low COD bias of 10–20% for standard MODIS COD retrievals when averaged OMI AI are larger than 1.0. No such bias is found over the Saharan dust outflow region off northern Africa, since both MODIS visible and shortwave in channels are vulnerable to dust particle aliasing, and thus a COD impact cannot be isolated with this method. A similar result is found for a smaller domain, in the Gulf of Tonkin region, from smoke advection over marine stratocumulus clouds and outflow into the northern South China Sea in spring. This study shows the necessity of accounting for the above–cloud aerosol events for future studies using standard MODIS cloud products in biomass burning outflow regions, through the use of collocated OMI AI and supplementary MODIS shortwave infrared COD products

Aerosol – remote sensing, characterization and aerosol-radiation interaction

Die Wechselwirkung von Aerosol und Strahlung in der Atmosphäre beeinflusst stark die Energiebilanz der Erde. Durch die großräumige Erfassung der horizontalen und vertikalen Verteilung von Aerosoleigenschaften in der Atmosphäre liefern Fernerkundungstechniken einen wichtigen Beitrag zu unserem Verständnis des Klimasystems. Genaue Beobachtungen durch langfristige operationelle Satellitenmissionen und zuverlässige Referenzmessungen vom Boden aus sind auch für die Ableitung und Verbesserung satelliten- und modellgestützter Aerosoldatensätze unerlässlich. Dies gilt insbesondere über dem Ozean. Mittels Fernerkundungsmethoden werden in dieser Dissertation bestimmte optische Eigenschaften von Aerosol und dessen Strahlungseffekt untersucht. Ein Teil der Datengrundlage hierfür wurde auf fünf Fahrten mit dem Forschungsschiff Polarstern mittels eines multispektralen Schattenbandradiometers erhoben. Anhand dieser Daten werden die aus theoretischen Betrachtungen abgeleitete Unsicherheit der Irradianzmessung von etwa 2 % anhand eines Vergleichs mit Sonnenphotometerbeobachtungen an Land und auf dem Schiff bestätigt. Unter Verwendung Schiffs-gestützter Referenzdaten werden im Rahmen dieser Dissertation mehrere weitere Aerosoldatensätze evaluiert. Für zwei satellitengestützte Datensätze können die erwarteten Fehlergrenzen bestätigt und die vom Aerosoltyp abhängigen Einschränkungen aufgrund von Modellannahmen diskutiert werden. Darüber hinaus werden die optischen Eigenschaften von Aerosol in der CAMS-Reanalyse betrachtet. Dabei findet sich die größte Diskrepanz in der Aerosolabsorption, die von der CAMS-Reanalyse um etwa 30 % überschätzt wird. Schließlich wird der Strahlungseffekt von Aerosol für die Region Deutschland und das Jahr 2015 unter unbewölkten Bedingungen mit zwei komplementären Ansätzen untersucht. Hierbei werden Messungen der solaren Einstrahlung an 25 Stationen des Beobachtungsnetzes des Deutschen Wetterdienstes als Datengrundlage verwendet. Einerseits wird ein Ensemble von empirischen Modellen verwendet, um die direkte Strahlungswirkung von Aerosol am Boden mithilfe einer Fehlerminimierung zu bestimmen. Die zugrundeliegenden Annahmen über Aerosol- und atmosphärische Eigenschaften in diesen Modellen werden kritisch analysiert und diskutiert. Im zweiten Ansatz werden explizite Strahlungstransfersimulationen des Strahlungseffekts unter Verwendung der CAMS-Reanalyse genutzt. Weiterhin wird die Sensitivität der Simulationen auf Unsicherheiten in den Eingangsgrößen untersucht, und damit die resultierende Unsicherheit im Strahlungseffekt abgeschätzt. Nach Korrektur von systematischen Abweichungen in der CAMS-Reanalyse hat Aerosol im Jahre 2015 einen mittleren abkühlenden Strahlungseffekt von -10.6 Wm-2 am Boden in Deutschland.The interaction of aerosol and radiation in the atmosphere exerts a strong influence on the Earth's energy balance. Remote sensing techniques provide an important contribution to our understanding of the climate system, by observing the horizontal and vertical distribution of aerosol properties in the atmosphere on a large scale. Accurate observations from long-term operational satellite missions and reliable ground-based reference measurements are essential for deriving and improving satellite- and model-based aerosol data sets. This is especially true over the ocean. In this dissertation, certain optical properties of aerosol particles and their radiation effect are investigated using remote sensing methods. Parts of the considered data basis were collected on five cruises with the research vessel Polarstern using a multispectral shadow-band radiometer. This unique data set contributes to the global available reference observations over the ocean by partially filling known gaps. On this database, an algorithm to evaluate shadow-band radiometer observations for the determination of spectral irradiance and optical properties of aerosol has been advanced. The basis algorithm was developed by the author as part of his master's thesis. The uncertainty of the irradiance measurement of about 2 % derived from theoretical considerations is validated by comparison with sun photometer observations on land and on ship. Using ship-borne reference data, several aerosol products are evaluated as part of this dissertation. For two satellite-based datasets, the expected error bounds has been confirmed and the aerosol-type dependent limitations due to model assumptions in the satellite retrievals are discussed. Furthermore, the optical properties of aerosol considered in the CAMS reanalysis are evaluated. The largest discrepancy is found in the aerosol absorption, which is overestimated by the CAMS reanalysis by about 30 %. Finally, the radiative effect of aerosol is investigated for the region of Germany and the year 2015 under cloud-free conditions using two complementary approaches. Here, measurements of solar irradiance at 25 stations of the observation network of the German Weather Service are used as a data basis. In the first approach, an ensemble of empirical models is used to determine the direct radiative effect of aerosols on the ground using error minimization. The underlying assumptions about aerosol and atmospheric properties in these models are critically analysed and discussed. The second approach quantifies the radiative effect by applying explicit radiative transfer simulations using CAMS reanalysis. The uncertainty in the radiative effect is estimated by studying the sensitivity of the simulations to uncertainties in the input variables. After correcting for systematic deviations in the CAMS reanalysis, aerosol has a cooling radiative effect of -10.6 Wm-2 on the ground in Germany in the annual mean of 2015