All cloud fields exhibit variable structures (bulge) and heterogeneities in water distributions. With the development of multidimensional radiative models by the atmospheric community, it is now possible to describe horizontal heterogeneities of the cloud medium, to study these influences on radiative quantities. We have developed a complete radiative cloud scene generator, called MONET (French acronym for: MOdelisation des Nuages En Tridim.) to compute radiative cloud scene from visible to infrared wavelengths for various viewing and solar conditions, different spatial scales, and various locations on the Earth. MONET is composed of two parts: a cloud medium generator (CSSM -- Cloud Scene Simulation Model) developed by the Air Force Research Laboratory, and a multidimensional radiative code (SHDOM -- Spherical Harmonic Discrete Ordinate Method) developed at the University of Colorado by Evans. MONET computes images for several scenario defined by user inputs: date, location, viewing angles, wavelength, spatial resolution, meteorological conditions (atmospheric profiles, cloud types)... For the same cloud scene, we can output different viewing conditions, or/and various wavelengths. Shadowing effects on clouds or grounds are taken into account. This code is useful to study heterogeneity effects on satellite data for various cloud types and spatial resolutions, and to determine specifications of new imaging sensor.

Algorithms developed to estimate cloud optical properties from satellite observations are based on radiative transfer models that assume clouds to be plane-parallel homogeneous entities. In this study, the effect of the plane-parallel assumption on the retrieval of cloud optical thickness and effective particle radius is examined through the use of a three- dimensional (3-D) radiative transfer model. Satellite observed radiances are simulated for 3-D stratus and convective cloud fields. Optical properties are retrieved from these fields and compared to the known inputs. Results show that for the stratus field the plane-parallel assumption is reasonable for most solar and viewing geometries. However, for a combination of a high solar zenith angle and a viewing angle towards the sun the bias becomes significant at high spatial resolutions. In contrast, retrieving cloud properties for the convective cloud at any spatial scale is not possible using the plane- parallel assumption.

It has been shown that simple 1-D radiative transfer models can be used with narrow-band satellite radiance observations to estimate the surface radiation budget with a very good overall accuracy. However, as the temporal and spatial resolution increase, large errors may occur in the presence of non-homogeneous cloud fields as comparisons with surface observations indicate. This suggests that 3-D cloud effects become increasingly important for the radiance field observed by the satellite sensor, and that simple 1-D retrieval schemes are not adequate under general cloudy conditions. We used a 3- D Monte Carlo model with simulated cloud liquid water and ice fields to compute radiances at the top of the atmosphere and surface radiation fluxes. Stratiform an deep convective cloud systems are simulated using the PSU/NCAR Mesoscale Model (MM5). The radiance data base thus simulated is analyzed to determine the impact of horizontal resolution and cloud geometry on the relationship between 3-D cloud properties, top of the atmosphere radiance and surface irradiance. The goal is to determine the 3-D cloud characteristics that are responsible for the deterioration of the 1-D surface radiation retrieval scheme and find possible corrections for the cloud 3-D effects.

Improved techniques for remote sensing of cirrus are needed to obtain global data for assessing the effect of cirrus in climate change models. Model calculations show that the far infrared/sub-millimeter spectral region is well suited for retrieving cirrus Ice Water Path and particle size parameters. Especially useful cirrus information is obtained at frequencies below 60 cm^-1 where single particle scattering dominates over thermal emission for ice particles larger than about 50 micrometer. Earth radiance spectra have been obtained for a range of cloud conditions using an aircraft-based Fourier transform spectrometer. The Far InfraRed Sensor for Cirrus (FIRSC) is a Martin-Puplett interferometer which incorporates a polarizer for the beamsplitter and can be operated in either intensity or linear polarization measurement mode. Two detector channels span 10 to 140 cm^-1 with a spectral resolution of 0.1 cm^-1; achieving a Noise Equivalent Temperature of approximately 1K at 30 cm^-1 in a 4 sec scan. Examples are shown of measured and modeled Earth radiance for a range of cloud conditions from 1998 and 1999 flights.

New infrared satellite sounders will give greatly increased spectral resolution. The expected improvements, such as increased vertical resolution of temperature and humidity profiles in numerical weather prediction (NWP) models, will rely on accurate cloud detection. Simulation of the expected radiances requires infrared spectra from clouds with known physical properties. ARIES, the Airborne Research Interferometer Evaluation System, is mounted on the U.K. Meteorological Office's research aircraft to gather data in preparation for the Infrared Atmospheric Sounding Interferometer (IASI). ARIES has a 1 cm^-1 wavenumber maximum spectral resolution over the range 600 to 3000 cm^-1 wavenumbers (wavelength, 16(DOT)7 to 3(DOT)3 micrometer). Infrared data over various cloud types have been measured along with in-situ temperature and humidity profiles -- often including microphysical measurements from within the cloud. With spectra from adjacent cloud free air, these provide data to model the cloud signature for NWP assimilation cloud detection. The data readily demonstrate the basic spectral signatures due to cloud:reduction of brightness temperature in the window regions, slope varying with cloud properties, solar reflection in the near-infrared and the opaque cloud top in the CO₂ sounding regions. A cloud detection algorithm is being developed that is intended as the first step in NWP processing. The algorithm takes advantage of the likely method of data compression to be used for IASI spectral data, using Empirical Orthogonal Functions (EOF). The EOFS used here are for cloud-free spectra and the cloud detection algorithm uses the magnitude of a residual -- which will reflect the presence of cloud, amongst other spectral features not represented in the EOFS. If additional EOFS representing cloud where added the cloud detection algorithm could also use these additional values which would be contained in the predictor for the EOFS.

Temporal analysis has been applied to a sequence of cloud top pressure (CTP) images and cloud optical thickness (TAU) images stored in the International Satellite Cloud Climatology Project (ISCCP) D1 database located at the NASA Goddard Institute for Space Studies (GISS). Each pixel in the D1 data set has a resolution of 2.5 degrees or 280 kilometers. These images were collected in consecutive three-hour intervals for the entire month of April 1989. The primary objective of this project was to develop a sequence of storm tracks from the satellite images to follow the formation, progression and dissipation of storm systems over time. Composite images where created by projecting ahead in time and substituting the first available valid pixel for missing data and a variety of CTP and TAU cut-off values were used to identify regions of interest. Region correspondences were determined from one time frame to another yielding the coordinates of storm centers. These tracks were compared to storm tracks computed from sea level pressure data obtain from the National Meteorological Center (NMC) for the same time period. The location of sea level storm center provides insight as to whether storms have occurred anywhere in a region and can be helpful in determining the presence or absence of storms in a general geographic region.

In this paper excellent results on rain pattern tracking are achieved by merging the information extracted from a weather radar data sequence by means of a correlation and a shape analysis approach. The two methods, namely the Tracking Radar Echoes by Correlation corrected by Continuity (COTREC) and the modal matching shape representation are jointly applied to the problem. This choice allows to exploit the advantages of both a global and a more local approach to a motion-based interpretation of rain events. Some results are shown, extracted from data sequences recorded by a C-band weather radar in Northern Italy.

Atmospheric circulation systems have being shown to produce observable signatures on spaceborne synthetic aperture radar (SAR) imagery of the ocean surface. Capillary and small gravity ocean waves of roughly the scale of the SAR electromagnetic wavelength, the so-called Bragg waves, provide the surface roughness that allows for SAR mapping of both ocean and atmospheric mesoscale features. Two RADARSAT SAR images were acquired over Hurricane Bonnie on 25 and 27 August 1998. Simultaneous Doppler weather radar observations were also acquired at next generation radar (NEXRAD) stations on the U.S. East coast. These coincident datasets were coregistered and analyzed. The observed ocean patterns in the C-band HH SAR images correlate well with the cyclonic circulation pattern of the hurricane winds and the presence of convective cells within the system. Significant signal attenuation in regions of heavier rainfall is observed in the SAR images. Understanding rain effects in SAR imagery is of increasing importance as we develop new scatterometer-like algorithms to produce high-resolution winds from spaceborne SAR imagery. Increased knowledge of hurricane SAR signatures will also help us derive reliable information on offshore storms that cannot be easily obtained from other sources.

The large potential opportunities in study of the top and average atmosphere can be realized at remote sensing through a use of the bistatic polarization scanning lidar. In the report, a process of scattering of polarization radiation by crystalline cloud to bistatic technique of sensing is investigated. The translucent plate crystal, as an object of study of scattered radiation on a separate particle is considered. The choice of such model is caused in the report. There are obtained the expressions for scattering cross sections within the physical optics formalism. As a result of the numerical analysis the regular dependences of depolarization ratio and ratios of parameters of Stokes vector on the orientation angle of plate with respect to the sensing direction, the orientation of the polarization plane, the refractive index of plate have been established. It is arrived the obviously expressed dependence of scattering cross section from plate radii.

A new technique for measuring cloud liquid water, mean droplet radius and droplet number density has been presented elsewhere and is reviewed here. A new extension of the theory is then presented which allows multiple scattering to be quantified. The technique is based on simultaneously measuring Raman and Mie scattering from cloud liquid droplets using a Raman lidar. The intensity of Raman scattering is known to be proportional to the amount of liquid present in cloud droplets. This fact is used as a constraint on calculated Mie intensity to calculate droplet radius and number density. The technique is tested using Raman lidar data. The general relationship of retrieved average radius and number density is consistent with traditional cloud physics models.

The Clouds and Earth's Radiant Energy System (CERES) experiment, the first satellite project devoted to monitoring cloud macrophysical and microphysical properties simultaneously with the broadband radiation field, is designed to dramatically improve our understanding of the relationship between clouds and the Earth's radiation budget. The first CERES instruments flew on the Tropical Rainfall Measuring Mission (TRMM) satellite between 35 degrees N and 35 degrees S with the Visible Infrared Scanner (VIRS), a 2-km resolution imager with five channels: 0.65, 1.6, 3.75, 10.8, and 12 micrometer beginning in January 1998. Cloud amount, height, temperature, phase, effective particle size, and water path are derived from the VIRS radiances and validated using surface radar and lidar data. Droplet radii are largest over ocean and smallest over land. Mean droplet radius is larger than that from earlier studies. The mean ice diameter is 61 micrometer. Variations of cloud parameters with temperature and viewing and solar zenith angle are given. Surface observations of liquid water path and droplet size agree well with the VIRS retrievals. This is the first analysis of cloud microphysical properties covering all times of day using all available pixels and viewing angles for half of the globe. Seasonal and diurnal variations of the cloud properties are presented.

A method for cloud top thermodynamical phase determination has been developed, using polarimetric data from the spaceborne instrument POLDER. Eight months of data, corresponding to the POLDER/ADEOS operational period, have now been processed. In our present studies, two complementary approaches were developed to evaluate the accuracy of the cloud phase retrieval algorithm. At the global scale, the POLDER phase information is compared to ground based synoptic weather reports from the Extended Edited Synoptic Cloud Report. This database allows one to select observations of various cloudiness situations (cloud amount and types, multilayer clouds, thin cirrus), which are then compared with matching POLDER observations. The second approach consists in analyzing data from the Millimeter Wave Cloud Radar (Souther Great Plain -- ARM site) which are available for nearly the entire operational POLDER period. The sampling capabilities of the radar (data are acquired every 10 seconds) allows one to compare POLDER cloud top phase with cloud top pressure and temperature, derived from the MMCR data combined with radiosounding measurements. Both approaches show the consistency of the POLDER phase product but also lead to interesting results concerning the cloud phase problem.

This paper describes the Receiver of the Microwave Humidity Sounder being developed by MMS under contract to Eumetsat. The MHS instrument is the new generation of instrument that will follow the AMSU-B one. After a detailed description, the performance of the Receiver is given.

The National Polar-orbiting Operational Satellite System (NPOESS) Aircraft Sounder Testbed-Interferometer (NAST-I) is one of two airborne infrared sounder systems currently being used to evaluate future spaceborne advanced sounder designs. The NAST-I instrument is a cross-track scanning Fourier Transform Spectrometer (FTS) that measures the upwelling radiation in the infrared spectrum between 645 - 2700 cm^-1 (15.5 - 3.7 micrometer) at a high-spectral resolution of 0.25 cm^-1. Each observation has a spatial resolution of 2.6 km from NASA's ER-2 high-altitude aircraft, which operates 20 km above the surface. Measurements from this instrument in its first year of operation have not only contributed to risk reduction studies for future IR sounders but have also provided valuable datasets from three different climate regimes. The spatial coverage, 40 km swath width, has facilitated evaluation of non-linear retrieval algorithms using high-spectral resolution information content and provided a means for further validation of infrared radiative transfer models. The capabilities of the NAST-I instrument have already been tested under varied field conditions such as tropical, mid-latitude summer, and mid- latitude winter regimes. Preliminary results from these field experiments have demonstrated favorable sounding capability under such conditions, including intensive tropical cyclone environments (Hurricane Bonnie, August 1998 and Hurricane Georges, September 1998). In addition, repeated observations over the same geographic location near Andros Island in the Bahamas have provided additional information on the temporal change and spatial distribution of water vapor responding to complex mesoscale and large-scale dynamic processes. The framework for future spaceborne IR sounders will be well established by current and future observations made by the NAST-I instrument with its capability to remotely sense atmospheric state variables and cloud radiative properties.

NASA's Earth Observing System (EOS) is part of an international program for studying the Earth from space using a multiple-instrument, multiple-satellite approach. The Clouds and the Earth's Radiant Energy System (CERES) experiment is designed to monitor changes in the Earth's radiant energy system and cloud systems and to provide these data with sufficient simultaneity and accuracy to examine critical cloud/climate feedback mechanisms which may play a major role in determining future changes in the climate system. The first EOS satellite (Terra), scheduled for launch this year, and the EOS-PM satellite, to be launched in late 2000, will each carry two CERES instruments. The first CERES instrument was launched in 1997 on the Tropical Rainfall Measuring Mission (TRMM) satellite. The CERES TRMM data show excellent instrument stability and a factor of 2 to 3 less error than previous Earth radiation budget missions. The first CERES data products have been validated and archived. The data consist of instantaneous longwave and shortwave broadband radiances, top- of-atmosphere fluxes, scene types, and time and space averaged fluxes and albedo. A later data product will combine CERES radiances and high-resolution image data to produce cloud properties and fluxes throughout the atmosphere and at the surface.

Eight continuous months of earth-nadir-viewing radiance measurements from the 3-channel Tropical Rainfall Measuring Mission (TRMM), Clouds and the Earth's Radiant Energy System (CERES) scanning radiometric measurement instrument, have been analyzed. While previous remote sensing satellites, such as the Earth Radiation Budget Experiment (ERBE) covered all subsets of the broadband radiance spectrum (total, longwave and shortwave). CERES has two subset channels (window and shortwave) which do not give continuous frequency coverage over the total band. Previous experience with ERBE indicated the need for us to model the 'equivalent' daytime longwave radiance using a window channel regression, which will allow us to validate the performance of the instrument using a three-channel inter-comparison. Limiting our consideration to the fixed azimuth plane, cross-track, scanning mode (FAPS), each nadir-viewing measurement was averaged into three subjective categories called daytime, nighttime, and twilight. Daytime was defined as any measurement taken when the solar zenith angle (SZA) was less than 90 degrees; nighttime was taken to be any measurement where the SZA was greater than 117 degrees; and twilight was everything else. Our analysis indicates that there are only two distinct categories of nadir-view data; daytime, and non-daytime (i.e., the union of the nighttime and twilight sets); and that the CERES longwave radiance is predictable to an accuracy of 1%, based on the SZA, and window channel measurements.

Mean fluxes of solar radiation at different atmospheric levels are calculated by two different methods: (1) the approximate method (under the assumption of random cloud overlap) and (2) the method of closed equations based on the Monte Carlo solution of equations for mean intensity in two-layer broken clouds. Calculations are made for characteristic parameters of typical cloud systems (St)-(Ci), (St)-(As), (Cu)-(Ci), (Cu)- (As) at midlatitudes of the Northern Hemisphere. It is shown that, depending on the geometrical and optical cloud parameters, the difference in upward (at the upper boundary of the atmosphere) and downward scattered (at the level of the underlying surface) radiative fluxes between different calculation techniques varies from 5 - 10% in (St)-(Ci), (St)- (As) clouds to tens of percent at large solar zenith angles in (Cu)-(Ci), (Cu)-(As) clouds.

Transmission spectra recorded using a Fourier Transform Spectrometer at the Atmospheric Radiation Measurements (ARM) Southern Great Plains (SGP) site are analyzed in order to derive atmospheric temperature profiles. The analysis is carried out using the Sequential Evaluation Algorithm for Simultaneous and Concurrent Retrieval of Atmospheric Parameter Estimates (SEASCRAPE) code, which uses a priori information to arrive at a statistically meaningful retrieval. Two different a priori profiles are used, the first is the SEASCRAPE retrieved temperature and water vapor profiles derived from ARM SGP emission spectra recorded near in time to the transmission spectra, the second is a sonde average temperature profile with a reasonable local water profile also used for the initial emission retrievals. We find that the best results are arrived at by using the emission retrieval as the a priori profiles as the initial starting point. Using the sonde a priori results in temperature and water vapor profiles that do not replicate the spectrum well.

In this paper a new regularization technique is introduced and applied to the problem of retrieval of vertical temperature profiles in the atmosphere from remote sensing data. This is a key issue in Meteorology since it provides an important input for weather forecasting models, mainly in the Southern Hemisphere, where there are large areas uncovered by data collecting ground stations. The new regularization technique is derived from the well known Maximum Entropy method, and is based on the maximization of the entropy of the vector of second-differences of the unknown parameters. Simulations using real satellite data achieved a good agreement with radiosonde measurements. Numerical simulations have also shown that the temperature profiles retrieved with the new technique are relatively independent on the choice of the initial guess.

A numerical monochromatic polarized plane-parallel radiative transfer code over sea has been developed for a non-scattering atmosphere in local thermodynamical equilibrium. In present work a non-linear inversion technique has been applied, based on the Levenberg-Marquardt Method, to a TMI late summer scene over the South of Mediterranean Sea. Temperature, water vapor and cloud liquid water atmospheric profiles have been retrieved over a wind-roughened sea surface. Eventually, integrated water vapor and cloud liquid water content have been estimated. A quasi-contemporary SSM/I acquisition over the same region has been utilized to derive integrated water vapor and cloud liquid water burden, using a statistical well- known algorithm. The TMI and SSM/I derived estimates are then compared for the selected scene.

Satellite measurements at high spectral resolution and span that avoid gas absorption bands and determine aerosol spectral optical properties are necessary for obtaining aerosol optical thickness values at the reference wavelength of 550 nm (hereinafter AOT). GOME (Global Ozone Monitoring Experiment on board the ERS-2 spacecraft) measurements fit such requirements, with a suitable spectral resolution over the region between UV and near IR while presenting a low spatial (320 X 40 Km²) and temporal resolution. The present new method overcomes these limitations by combining aerosol optical characteristics retrieved from GOME with METEOSAT visible data; the latter allow for monitoring aerosol events with adequate temporal resolution over wide cloud-free oceanic areas. The AOT results from fitting the measured broad-band visible METEOSAT radiance with the simulated radiance from radiative transfer calculations; aerosol properties estimated from GOME data are the essential input parameters. Several parameterizations of aerosol microphysical quantities have been tested to improve the AOT retrievals.

A UV/Vis DOAS spectrometer (GASCOD, Gas Analyzer Spectrometer Correlating Optical Differences) was installed at Monte Cimone station in 1993 and since then it has been measuring zenith scattered solar radiation at sunset and sunrise. During 1995 it was possible to investigate two spectral regions, about 50 nm width, centered at 365 nm and 436 nm while later we only have measurements at 436 nm available. The spectra obtained during the 1995 - 96 period have been processed with DOAS technique to obtain column amounts of NO₂ and O₃. The seasonal and diurnal variation of the NO₂ column amounts is shown with a summer maximum (about 1.2 X 10¹⁷ mol(DOT)cm^-2 for p.m. value and 6 X 10¹⁶ mol(DOT)cm^-2 for a.m.) and winter minimum (about 2 X 10¹⁶ mol(DOT)cm^-2 for a.m. and 5 X 10¹⁶ mol(DOT)cm^-2 for p.m.). An anomalous spring increase in p.m. NO₂ value during 1995 is investigated through a vertical distribution analysis. The gas profile is retrieved through a Chahine inversion algorithm applied to the slant columns measured at different solar zenith angle. In fact the air mass factor variation with solar zenith angle can be used to extract information about the gas concentration at each atmospheric layers. A consistent and frequent tropospheric increase in NO₂ a.m. concentration is evident. The method and the results obtained are discussed.

Polar Stratospheric Clouds (PSC) appear in the polar zones of the Earth in the winter. These clouds are known to cause enhanced chemical ozone destruction. Methods for optical remote-sensing of PSC in use or under development at the Swedish Institute of Space Physics are discussed with respect to their advantages and limitations. Especially multistatic imaging may become a valuable additional tool for PSC studies.

A new pseudo-spherical 3-D radiative transfer model has been written to calculate radiances and weighting functions required by inversion algorithms to retrieve minor species profile and column amount information from limb measurements. Limb radiances obtained from the ODIN/OSIRIS (Optical Spectrograph and InfraRed Imager System) satellite instrument (to be launched Spring 2000) will be used to study depletion processes and transport dynamics in the Earth's atmosphere through the study of trace species. Model simulations have been carried out for a suite of tangent heights, wavelengths, tangent solar zenith and azimuthal angles and lambertian surface albedos.The effects of Rayleigh and aerosol scattering as well as ozone absorption have been modeled. Comparisons with a 3-D Monte-Carlo model show that there is generally less than 1 - 2% difference for single-scattering and less than 10% for multiple scattering radiances at a fraction of the computational cost.

This study shows that the temperature information in the upper stratosphere can be derived from the SAGE II 385-nm observations. The preliminary results indicate that the zonal mean temperature increases with altitude below 50 km and decreases above 50 km. At 50 km, a regional maximum of 263 K is located in the tropics, and a minimum of 261 K occurs in the subtropics in both hemispheres. The derived long-term temperature changes from 1985 to 1997 reveal a statistically significant negative trend of -2 to -2.5 K/decade in the tropical upper stratosphere and about -2 K/decade in the subtropics near the stratopause. At latitudes poleward of 50 degrees, the results show a statistically significant positive trend of about 1 K/decade in the upper stratosphere. The preliminary results also show large annual temperature oscillations in the extratropics with a maximum amplitude of approximately 8 K located at about 44 km near 50 degrees in both hemispheres during local summer. In addition, the semiannual oscillation is found to be a maximum in the tropics with a peak amplitude of approximately 3.3 K located at about 42 km during the equinox.

Lidar returns from clouds include successive scattering order contributions. Under several assumptions, they can be evaluated by a simple analytical model, where only forward and backward scattering events are considered. Such a model has been developed, which moreover accounts for realistic receiver/emitter characteristics. Two experimental settings, enabling multiple scattering contribution identification, are considered. They are referred as <<2-FOV>> and <<off-axis>> methods. The latter requires no special additional feature to common lidar devices. Both are examined through a simple simulated case, and two measurements performed on cirrus clouds layers. 2-FOV method leads to an accurate estimate of ice crystals diffraction peak width. Off-axis approach, although theoretically efficient and experimentally less requiring, proved to be rather critical, and has to be properly improved.

The aim of this poster is to determine the droplet size distribution in low water clouds from measured optical parameters. Those measurements are obtained by a Lidar system. It consists of two reception telescopes: the first is near the beam send to the atmosphere, the second is at 7.7 m-distance from the first. The backscattering signal collected by the first telescope gives after an Klett's inversion, the volume extinction coefficient profile in the cloud. The double- scattering signal collected by the second telescope associated with the volumique extinction coefficient profile, gives by an inversion method, the double-scattering phase function which is correlated to a log-normal size distribution. The collected signals are detected simultaneously. A verification of the water phase is made by the depolarization ratio. Because of the situation of Lannion city, near the sea, swept by predominant winds from West, frequent fronts are present. So, a large diversity of clouds exists, which is a good experimental ground for testing and validating the theory presented here. Results from ENSSAT Lidar measurements will be presented.

The Lidar technique is an efficient tool for continuous monitoring of clouds and aerosols, with high temporal and spatial resolution. Lidar systems can provide long-term accurate information on cloud top and base heights and their optical depth. The spatial distribution of clouds and the diurnal variation of cloud properties are important parameters in many operational and research applications (i.e. radiative transfer modeling, energy balance, meteorology, etc.). In this paper, we present the application of a compact mobile lidar system in monitoring of cloud properties, over Athens, Greece. Preliminary lidar measurements were performed during summer- autumn 1994, focusing on the study of short-time variability of spectral cloud properties at two wavelengths (355 nm and 532 nm), using a compact Nd:YAG laser. The Lidar dataset acquired is analyzed during selected cases and conclusions are drawn.

The Global Ozone Monitoring Experiment (GOME) is an atmospheric chemistry instrument on-board the ERS-2 satellite which is able to measure a range of important atmospheric trace constituents on a global scale. Atmospheric UV/visible backscatter spectra obtained by the GOME spectrometer were used to retrieve column amounts of key trace species associated with biomass burning events and ozone hole chemistry. In particular, the column distributions of ozone (O₃), nitrogen dioxide (NO₂), formaldehyde (CH₂O), and bromine-monoxide (BrO) were retrieved on an operational basis. The differential optical absorption spectroscopy technique (DOAS) is applied to backscatter spectra and yields slant column distributions of the aforementioned species. Additionally, the vertical columns of O₃ and NO₂ are provided. A strong enhancement of both the NO₂ and CH₂O contents were detected during the severe biomass burning event in September 1997 in SE Asia. A higher NO₂ content is apparent over a large area within the smoke clouds, where formaldehyde is detected only in areas closest to combustion sources. BrO has been detected on a global scale and under Antarctic winter (ozone hole) conditions. The knowledge about the spatial distribution and the amount of BrO is of high relevance because BrO is a key species for the depletion of stratospheric ozone.

The Multi-Filter Rotating Shadow-band Radiometer (MFRSR) makes precise simultaneous measurements of the direct solar beam extinction, and horizontal diffuse flux, at six wavelengths (nominally 415, 500, 615, 670, 870, and 940 nm) at one minute intervals throughout the day. MFRSR data obtained at tens of sites throughout the U.S. has been available for several years. This is a potentially very important but as yet underutilized data set. We describe and validate a retrieval algorithm for processing of MFRSR data from clear and partially cloudy days. This method uses consistency between direct normal and diffuse horizontal measurements together with a special regression technique to retrieve daily time series of column mean aerosol particle size, aerosol optical depth, NO₂ and ozone amounts together with the instrument's calibration constants. Our validation studies demonstrate two advantages of our approach compared to the traditional Langley calibration method: less calibration variability and less sensitivity of retrievals to calibration accuracy. This method is currently used for processing data from a growing number of MFRSRs spread throughout the U.S. determining both time and geographic variability of aerosol properties and gaseous column amounts. This method makes the relatively inexpensive and automatic MFRSR an important tool in climatological research.

For atmospheric correction of satellite signals in optical channels, aerosol spectral optical depth and phase functions have to be known. These aerosol parameters have been retrieved from ground measurements of direct and diffuse solar radiation in South-Eastern Kazakhstan. Field experiments were conducted in 1996 - 1998 years during July - September. Only cloud-free days with the stability of atmospheric optical transparency during 2 and more hours of measurements were taken for data processing and analysis. The algorithm for retrieval of aerosol phase function is based on numerical calculations of multiple scattering in the plane-parallel atmosphere with the Lambertian surface by modified method of spherical harmonics. It provides angular values of aerosol phase function within 10 - 35% of error, depending on the wavelength and scattering angle. It is shown that aerosol optical parameters in Kazakhstan are significantly differ from other global and regional aerosol models by both spectral dependence and range of absolute values. The anomal spectral behavior of aerosol optical depth (with the maximum around 0.45 micrometer) was registered in about 50% of cases for clean atmosphere.

New high-spectral resolution satellite sounders will use channels located between CO2 lines for temperature retrievals. Transmittances for these channels are dominated by spectral line wings that are strongly influenced by line-mixing and duration-of-collision effects. Previous studies demonstrated the importance of Q-branch line mixing for atmospheric sounding in the 15 micrometer region. This work presents an improved model of P/R-branch line mixing and duration-of- collision effects on CO2 transmittances in the 4.3 micrometer and 15 micrometer regions, based on laboratory and spectroscopy data. Most line-by-line codes model non- Lorentzian behavior by using the Cousin chi-function. This empirical function incorporates both P/R line-mixing and duration-of-collision effects by using many parameters. It is common to use the Cousin model parameters obtained from the 4 micrometer band in the 15 micrometer region, overestimating the amount of line-mixing. Comparisons to radiance data taken with high resolution interferometers that fly on NASA's ER-2 partially validates our model. The biggest improvements are at 4.3 micrometer where the differences are reduced by more than 2K, compared to using the Cousin model.

We describe a radiative transfer model (SBDART-MOD) that can be used to analyze observations from new moderate resolution sensors such as MODIS. The gaseous absorption and thermal emission produced by this model rely on a pre-calculated correlated k-distribution database. Predictions from a line- by-line radiative transfer model is used to compare and evaluate the accuracy of the correlated-k method in both clear and cloudy conditions.

The Stratospheric Aerosol and Gas Experiment (SAGE) III will make multi-spectral measurements of the oxygen A-band absorption feature near 762 nm that can be used to retrieve profiles of temperature and pressure. The retrieval algorithm is based on a global fitting technique that uses a non-linear least squares procedure to simultaneously fit measured absorptivities from all spectral channels and slant paths. The feasibility of this approach is demonstrated through a series of simulated retrievals using synthetic measurements with realistic noise. An assessment of the expected uncertainties associated with the retrieved temperature and pressure data products is also provided.

Knowledge of water cycle in the atmosphere: that is the amount of water in its three phases is very useful for several applications, such as: weather forecasting, latent heat flux estimation, sea-atmosphere interaction. The present work showed a comparison of total precipitable water and cloud liquid water content retrievals from the well-known SSM/I and the new TMI radiometers, utilizing a physical-based inversion scheme -- developed at the PIN of the University of Florence. The estimations obtained through the physical-based inversion scheme are compared against those worked out thorough well- accepted empirical scheme based on SSM/I data. Hence, a validation of the physical-based inversion scheme, was achieved. For this purpose, a specific data-set was collected and analyzed, obtaining near-coincident crossing of the two radiometers over the Mediterranean sea. This is a contribution to understand the applicability of algorithms developed for the SSM/I towards the TMI, and to assess the improvement due to this new instrument (i.e. TMI). The statistical results show the goodness at utilizing the physical inversion model to estimate useful parameters such as atmosphere total precipitable water distribution over the Mediterranean area, from both the SSM/I and the TMI radiometers, and demonstrate a good retrieval accuracy. That is important especially for data-assimilation applications.

We report on water vapor and Ozone measurements made by a Raman LIDAR system recently built up in Lecce, Italy (40 degrees 20'6' N, 18 degrees 6'41' E). The system uses an excimer laser at 248 nm and detects the Raman backscattered radiation from O₂, N₂ and H₂O and can operate in daytime. Atmospheric transmission function, which is mainly due to Ozone absorption, is found not to vary significantly at the operative LIDAR range (200 - 1000 m). We report two cases of daily evolution of water vapor.

We discuss the intercomparison between the Karlsruhe Optimized and Precise Radiative transfer Algorithm (KO-PRA) and the Reference Forward Model (RFM) codes, which have been designed for analysis of MIPAS-ENVISAT data. The purpose of this intercomparison is to validate the KOPRA algorithm, i.e. to identify and to remove possible errors in the KOPRA (or RFM) code and to quantify the reason of remaining differences. Similar comparisons between the MIPAS Optimized Forward Model (OFM) and the RFM as well as between KOPRA and the RFM have already been performed. To be able to relate on these results, this validation is similarly organized: we perform subsequently more complex tests of ray-tracing, integrated column amounts, homogeneous and limb path calculations of unapodised, apodised and field-of-view (FOV) convolved spectra, using the same isolated CO₂ line as well as the same six MIPAS microwindows. Additionally we compare modeling of CO₂ line-mixing, non-local thermodynamic equilibrium (NLTE), trace gas continua and cross-section spectra. The KOPRA-RFM residuals are below a quarter of the noise- equivalent spectral radiance (NESR) for the isolated CO₂ line as well as for the MIPAS microwindows, i.e. KOPRA fulfills the acceptance criteria requested for the OFM. In most cases the deviations are even clearly below 1 nW/(cm² sr cm^-1), that is more than one order of magnitude below the acceptance threshold. This is valid for unconvolved as well as for ALS (apodised line shape) and FOV convolved spectra. There is also good agreement in modeling of the H₂O-, O₂- and N₂-continua and of CO₂ line-mixing. Larger deviations of up to several nW/(cm² sr cm^-1) occurred for NLTE calculations on the basis of 'default' atmospheric profiles with vertical resolution of 1 or 2.5 km. These differences were found to be due to different layer-averaging of the vibrational temperatures and could be considerably reduced by calculations with a higher vertical resolution of 250 m. Cross-section spectra agree well, if the tabulated data are given independent of pressure, e.g. for ClONO₂ and N₂O₅, and cover the atmospheric temperatures. Due to different temperature extrapolation the deviations increase up to 10 nW/(cm² sr cm^-1) for atmospheric temperatures outside the measuring range. The RFM is not yet adjusted to cross-sections tabulated for non- equidistant temperatures and for atmospheric pressures, like CFC-data in the HITRAN96 database. If these data are used, larger differences arise, e.g. up to 30 nW/(cm² sr cm^-1) between CFC-12 spectra. Avoidance of interpolation by performing homogeneous path calculations for p,T of one of the tabulated cross-section datasets reduces the deviations to below 0.5 nW/(cm² sr cm^-1).

An ESA supported study was carried out for the development of an optimized code for near real time retrieval of altitude profiles of pressure, temperature (p, T) and volume mixing ratio (VMR) of five key species (O₃, H₂O, HNO₃, CH₄ and N₂O) from infrared limb sounding spectra recorded by MIPAS (Michelson Interferometer for Passive Atmospheric Sounding), which will be operated on board ENVISAT-1 satellite. The implemented model is based on the Global Fit approach, i.e. all the limb-scanning spectra are simultaneously fitted, and on the analysis in narrow spectral intervals (microwindows). The trade-off between run time and accuracy of the retrieval was optimized from both the physical and mathematical point of view, with improvements in the program structure, in the radiative transfer model and in the computation of the retrieval Jacobian. The attained performances of the retrieval code are as follows: noise error on temperature less than 2 K at all the altitudes covered by the standard MIPAS scan (8 - 53 km), noise error on tangent pressure less than 3%, noise error on VMR of the target species less than 5% at most of the altitudes of scientific interest covered by the standard MIPAS scan, with a total run time of less than 6 minutes on a SUN SPARC station 20.