Retrieval of foreign-broadened water vapor continuum coefficients from emitted spectral radiance in the H2O rotational band from 240 to 590 cm(-1)

Characterization of Aerosol Size and Microphysical Properties from Multi-Wavelength Raman Lidar Measurements: Inter-Comparison with in Situ Sensors Onboard the ATR 42 in the Framework of HyMEX-SOP1

This extended abstract reports measurements that were carried out by the Raman lidar system BASIL in the frame of the Hydrological Cycle in the Mediterranean Experiment – Special Observation Period 1 (HyMeX-SOP1). A specific case study was selected revealing the presence of variable aerosol properties at different altitudes. Specifically, Raman lidar measurements on 02 October 2012 reveal the presence of two distinct aerosol layers, a lower one extending up to ~3 km and an upper one extending from 3.5 km to 4.7 km. Aerosol and size microphysical properties are determined from multi-wavelength measurements of particle backscattering and extinction profiles based on the application of a retrieval scheme which employs Tikhonov’s inversion with regularization. Inversion results suggest a size distribution with the presence, in both the lower and upper aerosol layer, of two particle modes (a fine mode, with a radius of ~0.2 μm, and a coarse mode, with radii in the range 2-4 μm), volume concentration values of 2-4 mm3cm-3 and effective radii in the range 0.2-0.6 μm.

This effort benefited from the dedicated flights of the French research aircraft ATR42, equipped with a variety of in situ sensors for measuring aerosol/cloud size and microphysical properties. Aerosol size and microphysical properties retrieved from multi-wavelength Raman lidar measurements were compared with simultaneous and co-located in-situ measurements.

CO2 Profiling by Space-Borne Raman Lidar

As clearly reported in the IPCC fifth Assessment Report, CO2 emissions are already producing destructive effects to the plant ecosystem through the alteration of soil-atmosphere interaction mechanisms.

Although the space and ground network for CO2 monitoring has regularly expanded over the past 50 years, it does not guarantee the necessary spatial and temporal resolution needed for a quantitative analysis of sources and sinks. For the purpose of estimating forests’ carbon capturing capabilities, accurate measurements of CO2 gradients between the forest floor and the top of the canopy, which ultimately translates into the capability to measure CO2 concentration profiles. Space sensors provide CO2 measurements above forest canopies, which do not allow to properly estimate Gross Primary Production (GPP).

These observational gaps could be addressed with an active remote sensing system in space based on the vibrational Raman lidar technique. CO2 profile measurements are possible, together with simultaneous measurements of the temperature and water vapour mixing ratio profile and a variety of additional variables (aerosol backscatter profile, aerosol extinction profile, PBL depth, cloud top and base heights, cloud optical depth). An assessment of the expected performance of the system has been performed based on the application of an analytical simulation model developed at University of Basilicata.

Water Vapour and Temperature Measurements by Raman Lidar in the Frame of the NDACC

In November 2012, the University of BASILicata Raman Lidar system (BASIL) was approved to enter the International Network for the Detection of Atmospheric Composition Change (NDACC). Since then measurements were routinely carried out on a once per week basis. This paper illustrates specific measurement examples from this effort, with a dedicated focus on temperature and water vapour measurements, with the ultimate goal to provide a characterization of the system performance. Case studies illustrated in this paper demonstrate the ability of BASIL to perform measurements of the temperature profile up to 50 km and of the water vapour mixing ratio profile up to 15 km, based on an integration time of 2 hours and a vertical resolution of 150 m, with measurement bias not exceeding 0.1 K and 0.1 g kg−1, respectively. Raman lidar measurements are compared with measurements from additional instruments, such as radiosondings and satellite sensors (IASI and AIRS), and with model re-analyses data (ECMWF and ECMWF-ERA). Comparisons in this paper cover the altitude interval up to 15 km for water vapour mixing ratio and up to 50 km for the temperature. Comparisons between BASIL and the different sensor/model data in terms of water vapour mixing ratio indicate a mean absolute/relative bias of -0.024 g kg−1(or -3.9 %), 0.342 g kg−1(or 36.8 %), 0.346 g kg−1 (or 37.5 %), -0.297 g kg−1 (or -25 %), -0.381 g kg−1 (or -31 %), when compared with radisondings, AIRS, IASI, ECMWF, ECMWF-ERA, respectively. For what concerns the comparisons in terms of temperature measurements, these indicate a mean absolute bias between BASIL and the radisondings, AIRS, IASI, ECMWF, ECMWF-ERA of -0.04, 1.99, 0.48, 0.14, 0.62 K, respectively. Based on the available dataset and benefiting from the circumstance that the Raman lidar BASIL could be compared with all other sensor/model data, it has been possible to estimate the absolute bias of all sensors/datasets, this being 0.004 g kg−1/0.30 K, 0.021 g kg−1/-0.34 K, -0.35 g kg−1/0.18 K, -0.346 g kg−1/-1.63 K, 0.293 g kg−1/-0.16 K and 0.377 g kg−1/0.32 K in terms of water vapour mixing ratio/temperature for BASIL, the radisondings, IASI, AIRS, ECMWF, ECMWF-ERA, respectively.

Validation of H2O continuum absorption models in the wave number range 180-600 cm(-1) with atmospheric emitted spectral radiance measured at the Antarctica Dome-C site

This work presents the results concerning the analysis of a set of atmospheric emitted (down welling) spectral radiance observations in the spectral range 180 to 1100 cm(-1) acquired at the Dome-C site in Antarctica during an extensive field campaign in 2011-2012. The work has been mainly focused on retrieving and validating the coefficients of the foreign contribution to the water vapour continuum absorption, within a spectral range overlapping the water vapour rotational band. Retrievals have been performed by using a simultaneous physical retrieval procedure for atmospheric and spectroscopic parameters. Both day (summer) and night (winter) spectra have been used in our analysis. This new set of observations in the far infrared range has allowed us to extend validation and verification of state-of-art water vapour continuum absorption models down to 180 cm(-1). Results show that discrepancies between measurements and models are less than 10% in the interval 350-590 cm(-1), while they are slightly larger at wave numbers below 350 cm(-1). On overall, our study shows a good consistency between observations and state-of-art models and provides evidence toward needing to adjust absorptive line strengths. Finally, it has been found that there is a good agreement between the coefficients retrieved using either summer or winter spectra, which are acquired in far different meteorological conditions.

Recent advances in measurement of the water vapour continuum in the far-infrared spectral region

We present a new derivation of the foreign-broadened water vapour continuum in the far-infrared (far-IR) pure rotation band between 24 μm and 120 μm (85-420 cm(-1)) from field data collected in flight campaigns of the Continuum Absorption by Visible and IR radiation and Atmospheric Relevance (CAVIAR) project with Imperial College's Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) far-IR spectro-radiometer instrument onboard the Facility for Airborne Atmospheric Measurement (FAAM) BAe-146 research aircraft; and compare this new derivation with those recently published in the literature in this spectral band. This new dataset validates the current Mlawer-Tobin-Clough-Kneizys-Davies (MT-CKD) 2.5 model parametrization above 300 cm(-1), but indicates the need to strengthen the parametrization below 300 cm(-1), by up to 50 per cent at 100 cm(-1). Data recorded at a number of flight altitudes have allowed measurements within a wide range of column water vapour environments, greatly increasing the sensitivity of this analysis to the continuum strength.