[1] The goal of this study is to determine how H 2 O and HDO measurements in water vapor can be used to detect and diagnose biases in the representation of processes controlling tropospheric humidity in atmospheric general circulation models (GCMs). We analyze a large number of isotopic data sets (four satellite, sixteen ground-based remote-sensing, five surface in situ and three aircraft data sets) that are sensitive to different altitudes throughout the free troposphere. Despite significant differences between data sets, we identify some observed HDO/H 2 O characteristics that are robust across data sets and that can be used to evaluate models. We evaluate the isotopic GCM LMDZ, accounting for the effects of spatiotemporal sampling and instrument sensitivity. We find that LMDZ reproduces the spatial patterns in the lower and mid troposphere remarkably well. However, it underestimates the amplitude of seasonal variations in isotopic composition at all levels in the subtropics and in midlatitudes, and this bias is consistent across all data sets. LMDZ also underestimates the observed meridional isotopic gradient and the contrast between dry and convective tropical regions compared to satellite data sets. Comparison with six other isotope-enabled GCMs from the SWING2 project shows that biases exhibited by LMDZ are common to all models. The SWING2 GCMs show a very large spread in isotopic behavior that is not obviously related to that of humidity, suggesting water vapor isotopic measurements could be used to expose model shortcomings. In a companion paper, the isotopic differences between models are interpreted in terms of biases in the representation of processes controlling humidity.Citation: Risi, C., et al. (2012), Process-evaluation of tropospheric humidity simulated by general circulation models using water vapor isotopologues: 1. Comparison between models and observations,

Abstract. Ground-based Fourier transform infrared (FTIR) measurements of solar absorption spectra can provide ozone total columns with a precision of 2 % but also independent partial column amounts in about four vertical layers, one in the troposphere and three in the stratosphere up to about 45 km, with a precision of 5-6 %. We use eight of the Network for the Detection of Atmospheric Composition Change (NDACC) stations having a long-term time series of FTIR ozone measurements to study the total and vertical ozone trends and variability, namely, Ny-Ålesund (79

Abstract. We present lower/middle tropospheric columnaveraged CH 4 mole fraction time series measured by nine globally distributed ground-based FTIR (Fourier transform infrared) remote sensing experiments of the Network for the Detection of Atmospheric Composition Change (NDACC). We show that these data are well representative of the tropospheric regional-scale CH 4 signal, largely independent of the local surface small-scale signals, and only weakly dependent on upper tropospheric/lower stratospheric (UTLS) CH 4 variations. In order to achieve the weak dependency on the UTLS, we use an a posteriori correction method. We estimate a typical precision for daily mean values of about 0.5 % and a systematic error of about 2.5 %. The theoretical assessments are complemented by an extensive empirical study. For this purpose, we use surface in situ CH 4 measurements made within the Global Atmosphere Watch (GAW) network and compare them to the remote sensing data. We briefly discuss different filter methods for removing the local small-scale signals from the surface in situ data sets in order to obtain the in situ regional-scale signals. We find good agreement between the filtered in situ and the remote sensing data. The agreement is consistent for a variety of timescales that are interesting for CH 4 source/sink research: day-to-day, monthly, and inter-annual. The comparison study confirms our theoretical estimations and proves that the NDACC FTIR measurements can provide valuable data for investigating the cycle of CH 4 .Published by Copernicus Publications on behalf of the European Geosciences Union. E. Sepúlveda et al.: NDACC FTIR and GAW surface in situ tropospheric CH 4

[1] This paper presents a comparative study of shortwave downward radiation (SDR) measurements and simulations, obtained with the radiative transfer model LibRadtran, at the Baseline Surface Radiation Network (BSRN) site of Izaña Atmospheric Observatory (IZA, Spain). The analysis is based on cloud-free days between March 2009 and August 2012 (386 days), including aerosol-free and Saharan mostly pure mineral dust conditions and comparing the day-to-day, annual, and interannual variability. The observed agreement between simulations and measurements is excellent: the variance of daily measurements overall agrees within 99% with the variance of daily simulations, and the mean bias (simulations-measurements) is -0.30˙0.24 MJm -2 (-1.1˙0.9%) for global, -0.16˙0.34 MJm -2 (-0.4˙0.9%) for direct, and +0.02˙0.25 MJm -2 (+0.9˙9.2%) for diffuse SDR. Furthermore, the diurnally averaged aerosol radiative forcing (DF) and radiative forcing efficiency (DF eff ) due to Saharan mostly pure mineral dust events has been computed at Izaña Observatory. The mean DF values are -7˙1, -96˙5, and 44˙2 Wm -2 for global, direct, and diffuse BSRN SDR, respectively (mean aerosol optical depth, AOD, at 500 nm of 0.18˙0.01), whereas the mean DF eff values are -59˙6, -495˙11, and 230˙8 Wm -2 per unit of AOD at 500 nm for global, direct, and diffuse BSRN SDR, respectively. These values highlight the importance of scattering processes for mineral dust aerosols: the ratio between DF and the corresponding SDR without aerosols is 2.5% for diffuse SDR versus 0.2% for direct SDR. This illustrates the significant potential of mineral dust particles to cool the Earth-atmosphere system.

Abstract. We present two years of measurements of water vapour (H2O) and its isotopologue ratio (δD, the standardized ratio between H216O and HD16O) made at two remote mountain sites on Tenerife Island in the subtropical North Atlantic. We show that the data – if measured during nighttime – are well representative for the lower/middle free troposphere. We use the measured H2O-δD pairs, together with dust measurements and back-trajectory modelling for analysing the moisture pathways to this region. We can identify four principally different transport pathways. The first two pathways are linked to transport from high altitudes and high latitudes, whereby the respective air can be dry, due to last condensation occurring at low temperatures, as well as humid, due to cross isentropic mixing with lower level and more humid air during transport since last condensation. The third pathway is transport from lower latitudes and lower altitudes, whereby we can identify rain re-evaporation as an occasional source of moisture. The fourth pathway is linked to the African continent, where during summer dry convection processes over the Sahara very effectively inject humidity from the boundary layer to higher altitudes. This so-called Saharan Air Layer (SAL) is then advected westward over the Atlantic and contributes to moisten the free troposphere. We demonstrate that different pathways leave distinct fingerprints on the measured H2O-δD pairs.