Retrieving Neptune's aerosol properties from Keck OSIRIS observations. I. Dark regions

We present and analyze three-dimensional data cubes of Neptune from the OSIRIS integral-field spectrograph on the 10-m Keck telescope, from July 2009. These data have a spatial resolution of 0.035"/pixel and spectral resolution of R~3800 in the H and K broad bands. We focus our analysis on regions of Neptune's atmosphere that are near-infrared dark- that is, free of discrete bright cloud features. We use a forward model coupled to a Markov chain Monte Carlo algorithm to retrieve properties of Neptune's aerosol structure and methane profile above ~4 bar in these near-infrared dark regions. Using a set of high signal-to-noise spectra in a cloud-free band from 2-12N, we find that Neptune's cloud opacity is dominated by a compact, optically thick cloud layer with a base near 3 bar and composed of low albedo, forward scattering particles, with an assumed characteristic size of ~1$\mu$m. Above this cloud, we require a vertically extended haze of smaller (~0.1 $\mu$m) particles, which reaches from the upper troposphere (~0.6 bar) into the stratosphere. The particles in this haze are brighter and more isotropically scattering than those in the deep cloud. When we extend our analysis to 18 cloud-free locations from 20N to 87S, we observe that the optical depth in aerosols above 0.5 bar decreases by a factor of 2-3 or more at mid- and high-southern latitudes relative to low latitudes. We also consider Neptune's methane (CH$_4$) profile, and find that our retrievals indicate a strong preference for a low methane relative humidity at pressures where methane is expected to condense. Our preferred solution at most locations is for a methane relative humidity below 10% near the tropopause in addition to methane depletion down to 2.0-2.5 bar. We tentatively identify a trend of lower CH$_4$ columns above 2.5 bar at mid- and high-southern latitudes over low latitudes.Comment: Published in Icarus: 15 September 201

Monitoring Atmospheric Dust Spring Activity at High Southern Latitudes on Mars using OMEGA

This article presents a monitoring of the atmospheric dust in the south polar region during spring of martian year 27. Our goal is to contribute to identifying the regions where the dust concentration in the atmosphere shows specific temporal patterns, for instance high, variable, and on the rise due to lifting or transport mechanisms. This identification is performed in relation with the seasonal ice regression. Based on a phenomenological examination of the previous results, hypothesis regarding the origin of aerosol activity of the southern polar region are proposed. This is of paramount importance since local dust storms generated in this region sometimes grow to global proportions.

Atmospheric and Surface Processes, and Feedback Mechanisms Determining Arctic Amplification: A Review of First Results and Prospects of the (AC)3 Project

Mechanisms behind the phenomenon of Arctic amplification are widely discussed. To contribute to this debate, the (AC)3 project has been established in 2016. It comprises modeling and data analysis efforts as well as observational elements. The project has assembled a wealth of ground-based, airborne, ship-borne, and satellite data of physical, chemical, and meteorological properties of the Arctic atmosphere, cryosphere, and upper ocean that are available for the Arctic climate research community. Short-term changes and indications of long-term trends in Arctic climate parameters have been detected using existing and new data

Data assimilation in atmospheric chemistry models: current status and future prospects for coupled chemistry meteorology models

Abstract. Data assimilation is used in atmospheric chemistry models to improve air quality forecasts, construct re-analyses of three-dimensional chemical (including aerosol) concentrations and perform inverse modeling of input variables or model parameters (e.g., emissions). Coupled chemistry meteorology models (CCMM) are atmospheric chemistry models that simulate meteorological processes and chemical transformations jointly. They offer the possibility to assimilate both meteorological and chemical data; however, because CCMM are fairly recent, data assimilation in CCMM has been limited to date. We review here the current status of data assimilation in atmospheric chemistry models with a particular focus on future prospects for data assimilation in CCMM. We first review the methods available for data assimilation in atmospheric models, including variational methods, ensemble Kalman filters, and hybrid methods. Next, we review past applications that have included chemical data assimilation in chemical transport models (CTM) and in CCMM. Observational data sets available for chemical data assimilation are described, including surface data, surface-based remote sensing, airborne data, and satellite data. Several case studies of chemical data assimilation in CCMM are presented to highlight the benefits obtained by assimilating chemical data in CCMM. A case study of data assimilation to constrain emissions is also presented. There are few examples to date of joint meteorological and chemical data assimilation in CCMM and potential difficulties associated with data assimilation in CCMM are discussed. As the number of variables being assimilated increases, it is essential to characterize correctly the errors; in particular, the specification of error cross-correlations may be problematic. In some cases, offline diagnostics are necessary to ensure that data assimilation can truly improve model performance. However, the main challenge is likely to be the paucity of chemical data available for assimilation in CCMM

The Impact of Saharan Dust on Atlantic Convective Systems: A Case Study

Tropical weather systems such as the African Easterly Waves (AEWs), the African Easterly Jet (AEJ), and tropical cyclones usually interact with the Saharan Air Layer (SAL) and mineral dust aerosols embedded in the SAL. Previous studies have debates about the strength and even the direction of the impact from the dust and SAL on the development of the tropical systems. In this study, the sole impact from the dust on a hurricane is quantified using carefully designed numerical experiments. Hurricane Earl (2010) was originated from an AEW disturbance over Africa. It was influenced by the dusty SAL especially in its early development stage. We conduct numerical experiments using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) to simulate Hurricane Earl (2010) starting from a weak disturbance. Experiments with 36-km resolution show that without moist convection, dust slightly weakens the low-pressure system in North Africa by cooling the atmosphere. This scenario mainly results from a reduction of the boundary layer heating during the daytime and from the enhancement of the outgoing longwave radiative cooling during the nighttime. The zonal mean of the AEJ is intensified slightly (less than 1m/s) in its southern part and weakened in its northern part when dust is included in the model. Adding dust weakens the AEWs at 850 hPa and 600 hPa levels and the tropical cyclone. High-resolution (4-km) cloud-resolving model experiments show that dust deepens the system slightly but does not affect the track. In the tropical low stage, dust increases the low-level cloud at 1-2 km height. It reduces rainfall in the outer edge of SAL region between 250 km to 350 km radii while it increases rainfall in the inner edge at around 200 km. The associated latent heat release shifts the updraft radially inward. The dynamical consequence is the acceleration of the tangential wind and the mid-level circulation. In the tropical depression stage, adding dust increases the number of cloud droplets in most of the regions and enhances the convection around the center, which generates more ice, snow, and rain. Overall, the impact of dust aerosols on Earl is small, the center minimum sea-level pressures only differ by less than 1 hPa at the end of the simulation. This suggests that pure meteorological model may be sufficiently accurate to forecast hurricanes if the dynamical and thermodynamical features of the SAL are properly described. In order to predict aerosols, one can use a simple chemistry model such as GOCART which only has aerosol direct effects. Using the Ensemble Kalman Filter (EnKF) to assimilate conventional observations and MODIS aerosol optical depth (AOD) data, both forecasts of the track of the storm and the aerosol can be improved

Characterization of Smoke Particles Toward Improved Remote Sensing Retrievals and Chemical Transport Modeling

Wildfires increase in extent, intensity, and frequency across the globe over the recent decades. The uncontrolled fires trigger cascading effects on local ecosystems and the fire emissions pose a higher risk to air quality and climate. Wildfire emissions contain a variety of trace gases and particulate matters. The particle-phase emissions, especially those light-absorbing species including black carbon (BC) and brown carbon (BrC), significantly affect the regional and global climate by modulating the radiative transfer phenomena in the atmosphere. A great discrepancy still exists between model- and observation-based estimates of aerosol-radiation interactions (ARI). The discrepancy is partially attributed to the mischaracterizations of aerosol microphysical properties in current chemical transport models and the misinterpretation of satellite observational data. Motivated these challenges, this dissertation tends to advance the knowledge in wildfire studies from two aspects: (1) assessing the radiative effects of fire-emitted particles by incorporating their morphological and optical properties into a radiative transfer algorithm, and (2) developing an improved algorithm to retrieve the subpixel fire properties.Objective 1: Nascent BC particles exhibit an aggregated appearance. We applied electron tomography (ET) coupled with a slice-by-slice voxel filling algorithm to reconstruct the 3D morphology of BC aggregates. The morphological and optical properties of the BC aggregates are respectively studied with the Q-space analysis and discrete dipole approximation approach. Our study indicates that the ET reconstructed aggregates are different in morphological and optical characteristics than those resolved from the traditional 2D microscopic analysis or modeling aggregation processes. Additionally, BC aerosols undergo an aging process as they are emitted into the atmosphere. The particle-scale characterization was further extended to the aged BC particles by adding different levels of coating onto the nascent BC. In this part of work, we numerically investigated the variation of fractal characteristics as BC is coated. The morphologies of coated BC particle fit well with the ideal fractal law when its radius of gyration is identical to that of the bare BC core. However, using the same law is difficult to fit the structures of heavily or unevenly coated BC aggregates. Our findings suggest a more realistic parameterization of both nascent and aged BC needs to be incorporated in climate models. The microphysical characteristics of fire-emitted particles were then incorporated into radiative transfer models to evaluate their radiative effects. We integrated the Mie code with the successive order of scattering (SOS) algorithm to simulate the polarimetric signals at the top of the atmosphere. The modeled polarization quantities have exhibited potential to distinguish particles with distinct light-absorbing properties. Moreover, we integrated the above-mentioned fractal particle model and the associated optical properties of aggregated particles into an optical computation module, Flexible Aerosol Optical Depth (FlexAOD), as well as an offline radiative transfer algorithm based on DIScrete Ordinates Radiative Transfer (DISORT) principle, to re-evaluate the ARI of BC in the wildfire regions in the northwest US. Our results suggest that BC morphologies have noticeable impacts on aerosol optical depth (AOD), and the resulting radiative forcings. Objective 2: The sporadic occurrence and the dynamically evolving nature of wildfires requires measurement techniques with broad spatiotemporal coverages and high resolution. Satellite-based products thus have been widely used in estimating the emission rates of atmospheric pollutants. Additionally, many atmospheric and meteorological applications require the fraction of fire area at the subpixel scale and fire temperature to estimate the plume injection height and understand mechanisms of the following pyroconvection processes. A thermodynamically-constrained algorithm was developed which utilizes the radiance at middle infrared (MIR) and thermal infrared (TIR) wavelengths to retrieve the subpixel fire characteristics. This algorithm considers the heat transfer phenomena beyond solely the fire area to include the adjacent heated land. By doing this, we resolved a continuously changed temperature profile outside the fire area. Furthermore, the comparisons of the retrieved fire temperature and area fraction between the improved and the traditional bi-spectral algorithms via a Williams Flats fire test case during the 2019 FIREX-AQ campaign show the improved algorithm outputs a lower fire temperature but significantly larger fire area fraction than the traditional method. It implies that this new algorithm can further reconcile the significant underestimation of fire emissions estimated by burned-area based approach

An assessment of aerosol optical properties from remote-sensing observations and regional chemistry–climate coupled models over Europe

Atmospheric aerosols modify the radiative budget of the Earth due to their optical, microphysical and chemical properties, and are considered one of the most uncertain climate forcing agents. In order to characterise the uncertainties associated with satellite and modelling approaches to represent aerosol optical properties, mainly aerosol optical depth (AOD) and Ångström exponent (AE), their representation by different remote-sensing sensors and regional online coupled chemistry–climate models over Europe are evaluated. This work also characterises whether the inclusion of aerosol–radiation (ARI) or/and aerosol–cloud interactions (ACI) help improve the skills of modelling outputs.Two case studies were selected within the EuMetChem COST Action ES1004 framework when important aerosol episodes in 2010 all over Europe took place: a Russian wildfire episode and a Saharan desert dust outbreak that covered most of the Mediterranean Sea. The model data came from different regional air-quality–climate simulations performed by working group 2 of EuMetChem, which differed according to whether ARI or ACI was included or not. The remote-sensing data came from three different sensors: MODIS, OMI and SeaWIFS. The evaluation used classical statistical metrics to first compare satellite data versus the ground-based instrument network (AERONET) and then to evaluate model versus the observational data (both satellite and ground-based data).Regarding the uncertainty in the satellite representation of AOD, MODIS presented the best agreement with the AERONET observations compared to other satellite AOD observations. The differences found between remote-sensing sensors highlighted the uncertainty in the observations, which have to be taken into account when evaluating models. When modelling results were considered, a common trend for underestimating high AOD levels was observed. For the AE, models tended to underestimate its variability, except when considering a sectional approach in the aerosol representation. The modelling results showed better skills when ARI+ACI interactions were included; hence this improvement in the representation of AOD (above 30 % in the model error) and AE (between 20 and 75 %) is important to provide a better description of aerosol–radiation–cloud interactions in regional climate models

Tropospheric BrO plumes in Arctic spring – A comparison of TROPOMI satellite observations and model results

Halogen radicals can drastically alter the polar atmospheric chemistry. This is made evident by a recurrent destruction of boundary layer ozone during polar springs called ozone depletion events (ODEs). ODEs are caused by enhanced concentrations of reactive halogens, in particular bromine monoxide (BrO) radicals. Measurements suggest that there are two distinct sets of environmental conditions favoring the emissions of bromine to the atmosphere, namely cold and stable meteorological conditions on one side and less stable conditions associated with low-pressure systems on the other. This thesis investigates the importance of these differing environmental conditions on a pan-Arctic scale by comparing TROPOMI satellite observations of BrO with the results of an meterology model coupled with atmospheric chemistry (WRF-CHEM) for the Arctic spring of 2019. For the retrieval of tropospheric BrO from satellite measurements an algorithm is developed which allows to assess the tropospheric partial column without reliance on external input. Compared to other retrieval algorithms, it enables the full utilization of TROPOMI’s high spatial resolution (7 × 3.5 km²) while also avoiding biases from the use of model data. Satellite observations are used to validate model assumptions. It was demonstrated that a bromine release mechanism from the snow-pack employed in numerous models is unfit to predict ODEs in early February at high solar zenith angles. Case studies demonstrated that the observed spatial patterns and large BrO columns observed during late polar spring in association with polar cyclones can be explained by the intrusion of bromine into the free troposphere. It was shown that the magnitude of bromine emissions from blowing snow in polar cyclones is likely overestimated. A seasonal dependence in the environmental conditions favoring bromine release was established. Calm meteorological conditions favor the occurrence of ODEs during early polar spring. During late March and April, ozone was identified as limiting factor for BrO formation and results indicated high wind speeds as favorable meteorological parameter for bromine release. A statistical analysis of spatial extent and shape of ODEs was conducted, showing a scale of 40 km to 1000 km for bromine enhanced air masses

Analysis of the aerosol-radiation-cloud interactions through the use of regional climate/chemistry coupled models

The response of the climate systems to aerosols and their effect on the radiative budget of the Earth is the most uncertain climate feedback and one of the key topics in climate change mitigation. Air quality-climate studies (AQCI) are a key, but uncertain contributor to the anthropogenic forcing that remains poorly understood. To build confidence in the AQCI studies, regional-scale integrated meteorology-atmospheric chemistry models are in demand. The main objective of the present Thesis is the characterization of the uncertainties in the climate-chemistry-aerosol-cloud-radiation system associated to the aerosol direct and indirect radiative effects caused by aerosols over Europe, employing an ensemble of fully-coupled climate and chemistry model simulations. The first topic covered deals with the microphysics parameterization configuration of an online-coupled model. The differences when using two microphysics schemes within the Weather Research and Forecasting coupled with Chemistry (WRF-Chem) model are analyzed. The evaluated simulations come from the Air quality Model Evaluation International Initiative (AQMEII) Phase 2. The impact on several variables is estimated when selecting Morrison vs. Lin microphysics. The results showed smaller and more numerous cloud droplets simulated with the Morrison and therefore this scheme is more effective in scattering shortwave radiation. Also, the impact of biomass burning (BB) aerosols on surface winds during the Russian heat wave and wildfires episode is studied. The methodology consists of three WRF-Chem simulations over Europe, run under the context of EuMetChem COST Action ES1004, differing in the inclusion (or not) of aerosol-radiation (ARI) and aerosol-cloud interactions (ACI). These aerosols can affect surface winds where emission sources are located and further from the release areas. Local winds decrease due to a reduction of shortwave radiation at the ground, which leads to decreases in 2-m temperature. Atmospheric stability increases when considering aerosol feedbacks, inducing a lower planetary boundary layer height. This Dissertation also investigates the ability of an ensemble of simulations to elucidate the aerosol-radiation-cloud interactions. An assessment of whether the inclusion of atmospheric aerosol radiative feedbacks during two aerosol case studies of an ensemble of on-line coupled models improves the simulation results for maximum, mean and minimum 2-m temperature is done. The simulations (COST Action ES1004) are evaluated against observational data from E-OBS database. In both episodes, a general underestimation of the studied variables is found, being most noticeable in maximum temperature. The biases are improved when including ARI or ARI+ACI in the dust case. Although the ensemble does not outperform the individual models (in general), its improvements when including ARI+ARI are more remarkable. Last, an improvement of the spatio-temporal variability and correlation coefficients when aerosol radiative effects are included is found. Finally, the representation of the ACI in regional-scale integrated models when simulating the climate-chemistry-cloud-radiation system is analyzed. It complements the temperature analyses. The evaluated simulations are run in the context of AQMEII Phase 2 and include the ARI+ACI interactions. Simulations are evaluated against the (ESA) Cloud_cci data. Results show an underestimation(overestimation) of cloud fraction (CFR) over land(ocean) areas, which could be related to satellite retrieval missing thin clouds. Lower bias and mean absolute error (MAE) are found in the ensemble Cloud optical depth (COD) and cloud liquid ice path (CIP) are generally underestimated. The differences are related to microphysics. The development of this Thesis has contributed to the state of the art in AQCI studies. Although including aerosol feedbacks does not modify the bias, the spatio-temporal variability and correlation coefficients are improved.Los procesos climáticos que representan mayor incertidumbre respecto a la modificación del balance radiativo terrestre son los relacionados con los aerosoles atmosféricos. Por tanto, representa uno de los temas clave a la hora de establecer políticas de mitigación del cambio climático. Los estudios de las interacciones calidad del aire/clima (AQCI) son por tanto de especial relevancia, contribuyendo a su vez a la comprensión de las incertidumbres asociadas a los forzamientos antropogénicos. Con el fin de crear confianza en los estudios de AQCI, un campo científico de especial interés, el uso de modelos meteorológicos y químicos de escala regional acoplados está en alza. En este contexto, el principal objetivo de esta Tesis es la caracterización de las incertidumbres del sistema climaquímica-aerosol-nubes-radiación, asociadas a los efectos radiativos directo e indirecto causados por los aerosoles sobre Europa. El primer aspecto tratado es la configuración del modelo acoplado, relacionado con la parametrización de la microfísica. Se han estudiado y analizado las diferencias al usar dos esquemas de microfísica diferentes con el modelo meteorológico acoplado con la química (WRF-Chem). Las simulaciones estudiadas han sido realizadas bajo la segunda fase de la iniciativa internacional sobre evaluación de la modelización de la calidad del aire (AQMEII). Se ha estimado el impacto de varias variables bajo los siguientes esquemas de microfísica: Morrison versus Lin, sobre periodos de tres meses durante el 2010 en Europa. Los resultados obtenidos muestran que la parametrización Morrison simula gotas de nube más pequeñas y más numerosas, siendo por tanto más efectivo a la hora de dispersar la radiación de onda corta. Así mismo, se han estudiado los efectos de los aerosoles procedentes de la quema de biomasa (BB) sobre los vientos en superficie durante la ola de calor y fuegos de Rusia. La metodología consiste en tres simulaciones con el modelo WRF-Chem sobre Europa, realizadas bajo la iniciativa EuMetChem COST Action ES1004. Éstas difieren en la inclusión (o no) de las interacciones aerosol-radiación y aerosol-nubes (ARI y ACI). Los resultados muestran que estos aerosoles pueden afectar los vientos en superficie no solo sobre la fuente de emisión sino también alejados de ella. Los vientos locales disminuyen debido a que la radiación de onda corta que llega a la superficie se reduce, lo que supone un descenso en la temperatura a dos metros. Además, la estabilidad atmosférica aumenta cuando se tienen en cuenta las realimentaciones producidas por los aerosoles sobre la meteorología, provocando una menor altura de la capa límite planetaria. Finalmente, esta Tesis evalua la representación de las interacciones ACI en modelos de escala regional acoplados complementando el análisis colectivo de temperatura. Las simulaciones analizadas se llevaron a cabo bajo la segunda fase de AQMEII (Europa,2010) e incluyen las interacciones ARI+ACI. Las simulaciones son evaluadas frente datos de la Agencia Espacial Europea (ESA) del proyecto Cloud de la iniciativa sobre cambio climático (CCI). La variable fracción de nubes (CFR) se subestima(sobreestima) sobre tierra(océano), lo que puede ser debido a que los satélite infraestiman las nubes finas sobre el océano. El bias y el error absoluto medio (MAE) son menores al considerar el promedio del conjunto de simulaciones. En general, la profundidad óptica de la nube (COD), así como el camino de hielo líquido en la nube (CIP) son subestimadas sobre todo el dominio. Las diferencias encontradas entre los modelos se deben a los diferentes esquemas de microfísica empleados. El desarrollo de esta Tesis ha contribuído al estado del arte de los estudios sobre AQCI. A pesar que incluir las realimentaciones de los aerosoles no contribuye a la mejora del sesgo de los modelos, hay una mejora en la variabilidad espacio-temporal y los coeficientes de correlación. Son necesarios más estudios para mejorar su representación