Nitric acid is a key gas for understanding the processes leading to ozone depletion in Antarctica. The Antarctica ozone hole is a cyclic phenomenon that begins in early September when the region exits the polar night and fully develops in October-November. Nitric acid is the primary constituent of stratospheric aerosol. In the Antarctic region, when the temperature gets below 195K, it condenses from the gas phase to NAT or nitric acid trihydrate (HNO3·3H2O) in the form of ice crystals. Recently, we have developed a new forward/inverse model capable of computing the top-ofatmosphere infrared spectral radiance in all-sky conditions (clear/cloudy, day/night) and surface type (land or ocean). The new forward/inverse system has been applied to IASI (infrared Atmospheric Sounder interferometer) to retrieve O3 and HNO3 simultaneously. We have analyzed data for September 2021 and 2023, and HNO3 column retrievals have been compared to those observed with the MLS (Microwave Limb Sounder) instrument to check the capability of IASI to estimate HNO3. The paper also addresses the relationship between HNO3 and O3, especially at the onset of the ozone hole in the early spring. It will be shown that ozone depletion is paralleled by a consistent diminishing of HNO3 from the gas phase.
A Cloud Identification and Classification algorithm named CIC is illustrated. CIC is a machine learning method used for the classification of far and mid infrared radiances which allows to classify spectral observations by relying on small size training sets. The code is flexible meaning that can be easily set up and can be applied to diverse infrared spectral sensors on multiple platforms. Since its definition in 2019, the CIC has been applied to many observational geometries (airborne, satellite and ground-based) and is currently adopted as the scene classificator of the end-2-end simulator of the next ESA 9th Earth Explorer, the Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) which will spectrally observe the far infrared part of the spectrum with unprecedent accuracy. The algorithm has been recently improved to enhance its sensitivity to thin clouds (and also to surface features) and to increase the cloud hit rates in challenging conditions such as those characterizing the polar regions. The newly introduced metric is presented in details and the set-up procedures are discussed since they are critical for a correct application of the code. We illustrate the definition of the metric, the calibration process and the code optimization. The issues related to the definition of the reference training sets and to the classification of multiple classes are also presented.
The new σ-IASI/F2N radiative transfer model is an advancement of the σ-IASI model, introduced in 2002. It enables rapid simulations of Earth-emitted radiance and Jacobians under various sky conditions and geometries, covering the spectral range of 3-100μm. Successfully utilized in δ-IASI, the advanced Optimal Estimation tool tailored for the IASI MetOp interferometer, its extension to the Far Infrared (FIR) holds significance for the ESA Earth Explorer FORUM mission, necessitating precise cloud radiative effect treatment, crucial in regions with dense clouds and temperature gradients. The model's update, incorporating the "linear-in-T" correction, addresses these challenges, complementing the "linear-in-tau" approach. Demonstrations highlight its effectiveness in simulating cloud complexities, with the integration of the "linear-in-T" and Tang correction for the computation of cloud radiative effects. The results presented will show that the updated σ-IASI/F2N can treat the overall complexity of clouds effectively and completely, at the same time minimizing biases.
We have developed a new forward model for all sky radiative transfer calculations in the spectral range 10 to 2760 cm−1 . This new code, which we call σ-IASI/F2N, allows us to calculate in all-sky based on an original parametrization of the optical depth of atmospheric gases and clouds. Clouds are represented through the atmospheric profiles of Liquid Water Content (LWC), Ice Water Content (IWC), and effective radii of both water droplets (re) and ice crystals (De). The cloud parametrization relies on suitable scaling laws, which make the radiative transfer equations for a cloudy atmosphere identical to those for a clear atmosphere. Therefore, the difficulties in applying a multiple-scattering algorithm to a partly cloudy atmosphere are avoided, and the computational efficiency is practically the same as that for a clear atmosphere. The new radiative transfer code has been coupled with an inverse scheme based on the Optimal Estimation methodology. The problem of dimensionality of the data and parameter space is handled by considering suitable transforms, which allow the representation of the radiances (data space) and the atmospheric state vector (parameter space) through a set of reduced components. The dimensionality is diminished through the random Projections transform for the radiance space, whereas we use the usual Principal Component Analysis for the parameter space. The scheme’s performance has been evaluated using the Infrared Atmospheric Sounder Interferometer (IASI) spectral radiances. The soundings are collocated with analyses from the European Centre for Medium-Range Forecasts (ECMWF) model. The ECMWF analyses provide the basic information, i.e., the first guess state vector to initialize the inverse scheme. The forward/inverse technique uses the whole IASI spectral coverage (645 to 2760 cm−1 ). As such, it is the first scheme for all sky using the full IASI spectrum to retrieve clouds and atmospheric parameters simultaneously. This new forward/inverse model is exemplified through the analysis of a set of IASI soundings over the Antarctica continent on 9 September 2021 at the onset of the ozone hole. We will show that infrared retrievals add information to assess ozone’s spatial extent and depletion.
The international experiment EAQUATE (European AQUA Thermodynamic Experiment) was held in September 2004 in Italy and in the United Kingdom. The Italian phase, performed in the period 6-10 September 2004, was mainly devoted to assessment and validation of performances of new IR hyperspectral sensors and benefits from data and results of measurements of AQUA and in particular of AIRS. It is also connected with the preparatory actions of MetOp mission with particular attention to calibration and validation of IASI products (as water vapour and temperature profiles), characterization of semitransparent clouds and study of radiative balance, demonstrating the role of ground-based and airborne systems in validation operations.
The Italian phase of the campaign was carried out within a cooperation between NASA Langley Research Center, University of Wisconsin, the Istituto di Metodologie per l'Analisi Ambientale (CNR-IMAA), the Mediterranean Agency for Remote Sensing (MARS) and the Universities of Basilicata, Bologna and Napoli. It involved the participation of the Scaled Composites Proteus aircraft (with NAST thermal infrared interferometer and microwave radiometer, the Scanning HIS infrared interferometer, the FIRSC far-IR interferometer), an Earth Observing System-Direct Readout Station and several ground based instruments: four lidar systems, a microwave radiometer, two infrared spectrometers, and a ceilometer. Radiosonde launches for measurements of PTU and wind velocity and direction were also performed as ancillary observations. Four flights were successfully completed with two different AQUA overpasses. The aircraft flew over the Napoli, Potenza and Tito Scalo ground stations several times allowing the collection of coincident aircraft and in- situ observations.
The clear sky radiative energy balance in the infrared spectrum is investigated with particular attention to the role of water vapor pure rotational band in the spectral range 10-600 cm-1. Main results for Tropical and Sub Arctic Winter atmospheres are in good agreement with those previously reported by other authors. Multiple scattering layers, simulating the presence of cirrus clouds, are used in Tropical atmosphere. The cloudy tropospheric heat balance is studied by the introduction of spectral power densities instead of spectral heating rates. Distribution of radiant energy between air molecules and cloud ice crystals is taken into account and the effects of cirrus absorption and scattering are distinguished. Finally a comparison between Mie and Fu parameterization is performed to understand the effects of more realistically shaped cirrus particles.
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