Claudia J. Stubenrauch

Assessment of Global Cloud Datasets from Satellites: Project and Database Initiated by the GEWEX Radiation Panel

Clouds cover about 70% of Earths surface and play a dominant role in the energy and water cycle of our planet. Only satellite observations provide a continuous survey of the state of the atmosphere over the entire globe and across the wide range of spatial and temporal scales that compose weather and climate variability. Satellite cloud data records now exceed more than 25 years; however, climate data records must be compiled from different satellite datasets and can exhibit systematic biases. Questions therefore arise as to the accuracy and limitations of the various sensors and retrieval methods. The Global Energy and Water Cycle Experiment (GEWEX) Cloud Assessment, initiated in 2005 by the GEWEX Radiation Panel (GEWEX Data and Assessment Panel since 2011), provides the first coordinated intercomparison of publicly available, standard global cloud products (gridded monthly statistics) retrieved from measurements of multispectral imagers (some with multiangle view and polarization capabilities), IR soun...

Hyperspectral Earth Observation from IASI: Five Years of Accomplishments

The Infrared Atmospheric Sounding Interferometer (IASI) forms the main infrared sounding component of the European Organisation for the Exploitation of Meteorological Satellitess (EUMETSATs) Meteorological Operation (MetOp)-A satellite (Klaes et al. 2007), which was launched in October 2006. This article presents the results of the first 4 yr of the operational IASI mission. The performance of the instrument is shown to be exceptional in terms of calibration and stability. The quality of the data has allowed the rapid use of the observations in operational numerical weather prediction (NWP) and the development of new products for atmospheric chemistry and climate studies, some of which were unexpected before launch. The assimilation of IASI observations in NWP models provides a significant forecast impact; in most cases the impact has been shown to be at least as large as for any previous instrument. In atmospheric chemistry, global distributions of gases, such as ozone and carbon monoxide, can be produ...

Clouds as Seen by Satellite Sounders (3I) and Imagers (ISCCP). Part I: Evaluation of Cloud Parameters

Abstract The improved initialization inversion (3I) algorithms convert TIROS-N Operational Vertical Sounder observations from the National Oceanic and Atmospheric Administration (NOAA) polar-orbiting environmental satellites into atmospheric temperature and water vapor profiles, together with cloud and surface properties. Their relatively good spectral resolution and coverage make IR sounders a very useful tool for the determination of cloud properties both day and night. The iterative process of detailed comparisons between cloud parameters obtained from this global dataset, which is available in the framework of the NOAA–National Aeronautics and Space Administration Pathfinder Program, with time–space-collocated observations of clouds from the recently reprocessed International Satellite Cloud Climatology Project (ISCCP) dataset has led to an improved 3I cloud analysis scheme based on a weighted-χ2 method described in the second article of this series. This process also provides a first evaluation of th...

Characteristics of the TOVS Pathfinder Path-B Dataset

Abstract From 1979 to present, sensors aboard the NOAA series of polar meteorological satellites have provided continuous measurements of the earths surface and atmosphere. One of these sensors, the TIROS-N Operational Vertical Sounder (TOVS), observes earth-emitted radiation in 27 wavelength bands within the infrared and microwave portions of the spectrum, thereby creating a valuable resource for studying the climate of our planet. The NOAA–NASA Pathfinder program was conceived to make these data more readily accessible to the community in the form of processed geophysical variables. The Atmospheric Radiation Analysis group at the Laboratoire de Meteorologie Dynamique of the Centre National de la Recherche Scientifique of France was selected to process TOVS data into climate products (Path-B). The Improved Initialization Inversion (3I) retrieval algorithm is used to compute these products from the satellite-observed radiances. The processing technique ensures internal coherence and minimizes both observ...

Cloud Properties and Their Seasonal and Diurnal Variability from TOVS Path-B

Eight years of cloud properties retrieved from Television Infrared Observation Satellite-N (TIROS-N) Observational Vertical Sounder (TOVS) observations aboard the NOAA polar orbiting satellites are presented. The relatively high spectral resolution of these instruments in the infrared allows especially reliable cirrus identification day and night. This dataset therefore provides complementary information to the International Satellite Cloud Climatology Project (ISCCP). According to this dataset, cirrus clouds cover about 27% of the earth and 45% of the Tropics, whereas ISCCP reports 19% and 25%, respectively. Both global datasets agree within 5% on the amount of single-layer low clouds, at 30%. From 1987 to 1995, global cloud amounts remained stable to within 2%. The seasonal cycle of cloud amount is in general stronger than its diurnal cycle and it is stronger than the one of effective cloud amount, the latter the relevant variable for radiative transfer. Maximum effective low cloud amount over ocean occurs in winter in SH subtropics in the early morning hours and in NH midlatitudes without diurnal cycle. Over land in winter the maximum is in the early afternoon, accompanied in the midlatitudes by thin cirrus. Over tropical land and in the other regions in summer, the maximum of mesoscale high opaque clouds occurs in the evening. Cirrus also increases during the afternoon and persists during night and early morning. The maximum of thin cirrus is in the early afternoon, then decreases slowly while cirrus and high opaque clouds increase. TOVS extends information of ISCCP during night, indicating that high cloudiness, increasing during the afternoon, persists longer during night in the Tropics and subtropics than in midlatitudes. A comparison of seasonal and diurnal cycle of high cloud amount between South America, Africa, and Indonesia during boreal winter has shown strong similarities between the two land regions, whereas the Indonesian islands show a seasonal and diurnal behavior strongly influenced by the surrounding ocean. Deeper precipitation systems over Africa than over South America do not seem to be directly reflected in the horizontal coverage and mesoscale effective emissivity of high clouds.

Clouds as Seen by Satellite Sounders (3I) and Imagers (ISCCP). Part II: A New Approach for Cloud Parameter Determination in the 3I Algorithms

Abstract First comparisons of improved initialization inversion (3I) cloud parameters determined from TIROS-N Operational Vertical Sounder observations with time–space-collocated clouds from the recently reprocessed International Satellite Cloud Climatology Project (ISCCP) dataset have shown a reasonable agreement between all cloud types, with exception of the stratocumulus regions off the western coasts. Here, 3I clouds were found systematically thinner and higher than ISCCP clouds. These results have initiated a careful investigation of the methods used to convert measurements from IR sounders into cloud parameters. All existing methods get very sensitive to the chosen temperature profile toward lower cloud heights, due to a denominator approaching zero. This leads to a bias like the one seen in the comparison with ISCCP. Therefore, a new 3I cloud scheme has been developed, based on a weighted-χ2 method, which calculates the effective cloud amount from the CO2-band radiances, but weighted differently ac...

Did the Eruption of the Mt. Pinatubo Volcano Affect Cirrus Properties

Some observations suggest that the volcanic aerosols produced by the Mt. Pinatubo eruption may have altered cirrus. The authors look for evidence that such modification of cirrus is extensive enough to be climatically significant by comparing three satellite-based cirrus datasets produced by the International Satellite Cloud Climatology Project (ISCCP) analysis, the split-window analysis, and the Improved Initialization Inversion (3I) analysis. Since the former two have not been compared in detail before, such a comparison was conducted here. When applied to the Advanced Very High Resolution Radiometer (AVHRR) data, both the ISCCP and splitwindow analyses identify about 0.2‐0.3 cirrus cloud amounts in tropical latitudes; however, there are detailed differences of classification for about half of these clouds. The discrepancies are attributed to the simplified assumptions made by both methods. The latter two datasets are derived from infrared radiances, so they are less sensitive to volcanic aerosols than the ISCCP analysis. After the Mt. Pinatubo eruption, the ISCCP results indicate a notable decrease of thin cirrus over ocean, accompanied by a comparable increase of altocumulus and cumulus clouds; over land, there are no significant changes. In contrast, results from the split-window and 3I analyses show little change in thin cirrus amount over either ocean or land that is associated with the volcanic eruption. The ISCCP results can, therefore, be understood as a misclassification of thin cirrus because additional reflected sunlight by the volcanic aerosol makes the cirrus clouds appear to be optically thicker. Examinations of the split-window signature show no significant change in infrared emissivity (or optical thickness). These results indicate that the Mt. Pinatubo volcanic aerosol did not have a significant systematic effect on tropical cirrus properties (such as cloud amount and optical thickness), but do not exclude the occurrence of temporary, local effects. Hence, these results suggest that there is no significant climate feedback produced by aerosol‐ cirrus‐radiative interactions.

Clouds as Seen by Satellite Sounders (3I) and Imagers (ISCCP). Part III: Spatial Heterogeneity and Radiative Effects

Abstract Their relatively good spectral resolution makes infrared sounders very useful for the determination of cloud properties (day and night), and their coarse spatial resolution has less effect on clouds with large spatial extents like cirrus clouds. The Improved Initialization Inversion (3I) algorithms convert TIROS-N Operational Vertical Sounder observations from the NOAA Polar Orbiting Environmental Satellites into atmospheric temperature and humidity profiles and into cloud and surface properties. On the other hand, the relatively high spatial resolution of the imagers from the geostationary and polar orbiting satellites used in the International Satellite Cloud Climatology Project (ISCCP) is important for the determination of properties of clouds with smaller spatial extents like boundary layer clouds. By combining these quite different datasets some insight into the behavior of retrieved cloud properties with spatial heterogeneity is gained. The effective cloud amount as determined by 3I and ISC...

Retrieval of cirrus ice crystal sizes from 8.3 and 11.1 μm emissivities determined by the improved initialization inversion of TIROS-N Operational Vertical Sounder observations

The improved initialization inversion (31) algorithms convert TIROS-N Operational Vertical Sounder (TOVS) observations from the NOAA polar orbiting environmental satellites into atmospheric temperature and water vapor profiles as well as cloud and surface properties. Because of their relatively high spectral resolution, infrared vertical sounders are especially useful for the identification of cirrus clouds. Differences in cirrus emissivity between the wavelengths 8.3 and 11.1 μm are used to retrieve ice crystal size; the radiative transfer model is based on the anomalous diffraction approximation applied to different crystal morphologies. We present sensitivity studies of ice crystal sizes estimated on a global scale to uncertainty factors in the retrieval as well as to assumptions in the model. On average, cirrus ice crystal mean maximum dimensions lie between 80 and 150 μm or effective ice crystal sizes between 35 and 45 μm. Correlations between estimated cirrus ice crystal mean maximum dimensions and cloud-top temperature seem to be positive in the tropics and midlatitude winter but depend on assumed temperature-dependent ice crystal morphology and size distribution function. These estimates can be helpful for the evaluation of general circulation models. With satellite measurements, one estimates mean ice crystal sizes mostly on the top of the cirrus clouds. However, when the clouds are thinner, the IR sounder can reach deeper into the cloud. Yet a quantitative relation between cloud thickness (effective cloud emissivity) and retrieval height inside the cloud has still to be investigated.

Upper tropospheric humidity and cirrus geometrical and optical thickness: Relationships inferred from 1 year of collocated AIRS and CALIPSO data

Profiles of relative humidity with respect to ice (RHice) determined from spaceborne passive remote sensing suffer a lack of vertical and spatial resolutions. RHice distributions show dry biases compared to in situ observations because geometrically thin moist layers are integrated within coarser vertical resolutions, a direct effect being the underestimation of ice supersaturation (RHice > 100%) occurrence. Collocated data from the Atmospheric Infrared Sounder (AIRS) and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) provide the opportunity to investigate relationships between RHice and geometrical thickness and optical depth of high clouds near the tropopause. “Apparent” optical depths are derived for single-layer high clouds from CALIPSO. By comparing these “apparent” optical depths to cloud infrared emissivities derived from AIRS the multiple scattering contribution is estimated and the optical depths are corrected. Mean RHice increases with cloud geometrical thickness but remains low compared to 100% except for very vertically extended clouds. Optically thicker clouds show on average larger geometrical thickness and larger relative humidity than optically thinner clouds. However, for a comparable geometrical thickness, optically thinner clouds are on average slightly more humid. This study concludes that cloud geometrical thickness has a greater influence than cloud optical depth on RHice integrated within a coarse vertical resolution. Limitations of AIRS humidity observations regarding the detection of ice supersaturation are discussed.