High spectral resolution Fourier transform spectrometry has been used to derive cirrus cloud optical properties. Data from both ground- and aircraft-based observations, combined with lidar and radiosonde measurements, will be shown from various field campaigns to demonstrate our approach. Ground-based results are from the Atmospheric Emitted Radiance Interferometer, based at the Atmospheric Radiation Measurement program Cloud and Radiation Testbed. Aircraft measurements are from the Scanning High-resolution Interferometer Sounder acquired during the SAFARI-2000 mission in South Africa. It is anticipated that long-term cirrus measurements on a seasonal scale will produce a cloud climatology based on synoptic conditions which can be parameterized on a global scale for use in GCMs.

The lidar has long been proposed as a potential remote sensor of cloud microphysical and optical parameters. The conventional lidar has had only mixed success because retrieval in a medium of such density requires an independently measured boundary value deep in the cloud and a relation between backscatter and extinction. The solution we propose is to make detection at multiple fields of view (MFOV) and exploit the additional information provided by multiple scattering. In this paper, we compare MFOV-based lidar retrievals with in situ measurements of the liquid water content and droplet diameters in liquid-phase stratus clouds. The results show good correlation between the lidar solutions and the in situ data, but a constant bias of 20-30% depending on the parameter. The bias is reduced to 10% if comparisons are restricted to the cloud base region accessible to the lidar. Another significant conclusion is that the lidar solutions are, within 20-30%, statistically representative of the complete layer in spite of the limited penetration range.

Approximate analytical solutions for the degree of polarization of light reflected from water clouds are presented. They are valid in the case of optically thick clouds at visible and infrared channels. Equations obtained are used to develop a new semi-analytical cloud retrieval algorithm.

Measured cloud spectra have been used to develop an initial cloud detection scheme for the assimilation of Infrared Atmospheric Sounding Interferometer (IASI) data. Using realistic data before the instrument is in orbit will help ensure that IASI data can be used at numerical weather prediction (NWP) centers soon after launch. The measured data have been taken by the Airborne Research Interferometer Evaluation System (ARIES) - specifically designed to gather data representative of IASI from an aircraft. The development of the cloud detection scheme and its final form are discussed. The scheme was designed to require no additional background information. Representative spectra are used to create a set of empirical orthogonal functions (EOF). EOF sets were based on 78 diverse modeled clear air spectra, supplemented by selected measured spectra. IASI data could be compressed using such EOF. Tests were carried out on various cloud detection procedures using separate measured spectra. Video data gave independent verification of cloudy and cloud free views. The results were encouraging; clouds were detected in the measured test data with the prospect of extending the results to more general data.

Remote sensing measurements of cirrus clouds are crucial for improving global climate models. Spectral measurements of the far-infrared region provide especially useful information to retrieve cirrus ice water path and particle size properties. Earth radiance spectra of this region have been obtained for a range of cloud conditions using the airborne instrument FIRSC (Far-InfraRed Sensor for Cirrus). The instrument flew on board the high altitude Proteus aircraft in the ARM-FIRE Water Vapor Experiment (AFWEX) during the Nov - Dec 2000 Intensive Operations Period. FIRSC is a Martin-Puplett type Fourier transform spectrometer with two channels covering the ranges 10 - 33 cm^-1 and 80 -140 cm^-1 at a resolution of 0.1 cm^-1. It has achieved a noise equivalent temperature of approximately 1K at 30 cm^-1 using a scan duration of 4 seconds. For the first time this far-infrared data has been compared to data from the thermal IR NAST-I (NPOESS Airborne Sounder Testbed - Interferometer), which was part of the same payload during the AFWEX flights. Retrievals of cirrus ice water path and particle size from the FIRSC data are presented.

The problem of cloud field recognition from the NOAA satellite data is urgent for solving not only meteorological problems but also for resource-ecological monitoring of the Earth's underlying surface associated with the detection of thunderstorm clouds, estimation of the liquid water content of clouds and the moisture of the soil, the degree of fire hazard, etc. To solve these problems, we used the AVHRR/NOAA video data that regularly displayed the situation in the territory. The complexity and extremely nonstationary character of problems to be solved call for the use of information of all spectral channels, mathematical apparatus of testing statistical hypotheses, and methods of pattern recognition and identification of the informative parameters. For a class of detection and pattern recognition problems, the average risk functional is a natural criterion for the quality and the information content of the synthesized decision rules. In this case, to solve efficiently the problem of identifying cloud field types, the informative parameters must be determined by minimization of this functional. Since the conditional probability density functions, representing mathematical models of stochastic patterns, are unknown, the problem of nonparametric reconstruction of distributions from the leaning samples arises. To this end, we used nonparametric estimates of distributions with the modified Epanechnikov kernel. The unknown parameters of these distributions were determined by minimization of the risk functional, which for the learning sample was substituted by the empirical risk. After the conditional probability density functions had been reconstructed for the examined hypotheses, a cloudiness type was identified using the Bayes decision rule.

In this study we compare different approaches to retrieve Cloud Top Height (CTH), Cloud Effective Emissivity (CEE), and the Cloud Particle Size (CPS) from aircraft high-spectral resolution infrared measurements. Two independent methods are used to infer CTH. One approach is based on a high spectral resolution version of the CO₂ Slicing algorithm characterized by a statistically based selection of the optimal channel pairs. Another approach the Minimum Local Emissivity Variance algorithm (MLEV) takes advantage of high-resolution observations in the 8-12 micron region to simultaneously derive CTH and CEE. Once CTH has been retrieved a third method, based on the comparison between simulated and observed radiances, is used to infer CPS and CEE. Simulated radiances are computed for 18 microwindows between 8.5 and 12 microns. The cirrus scattering calculations are based on three-dimensional randomly oriented ice columns assuming six different particle size distributions. Multiple scattering calculations are performed for 26 different cloud optical thicknesses (COT) between 0 and 20. The simulated radiances are then compared to the observed radiances to infer COT and CPS for each spectral measurement. We applied these approaches to High-resolution Interferometer Sounder (HIS), National Polar-Orbiting Operational Environmental Satellite System Airborne Sounder Testbed-Interferometer (NAST-I) and Scanning-HIS (S-HIS) data. The preliminary results, consistent between the different algorithms, suggest that the high spectral resolution measurements improve the accuracy of the cloud property retrievals.

The recognition of composition and of microphysical characteristics of aerosol impurities is one of the most urgent tasks in monitoring of atmosphere. Some approaches to its solution are considered in the work. At first stage we were limited to five substances: fine-dispersion water and dust as background aerosol components of atmosphere; coarse- dispersion tributilamin, turbine oil and petroleum as possible impurities. Tributilamin was chosen as spectral analog of V gases. The modeling of input spectra and the recognition were carried out on 12 discrete lines in 2.9 - 4 micrometers spectral rage. As is well known, in this range the considered impurities have the pronounced spectral dependences of aerosol backscattering, the so-called spectral resonances. Spectra of aerosol backscattering for these substances were calculated with Mie theory. We applied evolutionary algorithm (genetic algorithm) and also more traditional optimization methods, namely gradient descent method, for recognition procedure. The comparative analysis of the mentioned methods was done; the concrete results of recognition and the dependence of recognition efficiency on the number of wavelength channels and on the accuracy of spectrum recording are given.

The MSX Satellite observed background terrain, clouds and atmospheric structure simultaneously in spectral bands ranging from the UV (~ 200 to 400 nm) through the visible range (400-900 nm) to the mid-wave infrared (MWIR, ~ 4.5 micrometers ) over a fifteen month operational period. The high spatial and temporal resolution multi-spectral image data sets allow characterization of the background scene structure by statistical and Fourier analysis techniques. In this paper, we use the multi-spectral image observations to compare the Fourier-space (Spatial Power Spectral Density, PSD, and correlation length) descriptors for the various sources of observed structure. Each MSX band views background structure as a unique combination terrain, clouds, and atmospheric temperature and density fluctuation sources having unique statistical and Fourier-space attributes. We selected the MSX image bands to accomplish altitude sounding in that each spectral image represents the background scene structure at an altitude defined by the peak response of the atmospheric weighting function, which is approximately at the altitude of unit optical depth along the sensor LOS. The results of comparing cloud and atmospheric structure properties in the band-selected altitude ranges are twofold. In these scenes, the overall intensity PSD's are characterized by multi-variate distributions in which each component is distinguishable by the PSD slope and the correlation function. For example, the cloud scenes have PSD's and correlation lengths that are distinguishable from atmospheric density and temperature fluctuations. Similarly, the presence of atmospheric gravity wave structure, observed in MWIR scenes, creates identifiable features in the PSD and correlation functions for these bands. The characteristic PSD's most often observed for cloud, and atmospheric structure appear to converge to k^-5/3 behavior.

The key problem in aerosol retrieval over land is to distinguish between surface and atmospheric contribution to the satellite reflectance. In principle a method similar to the classical Dense Dark Vegetation could be used over brighter surfaces if the surface BRDF could be described with sufficient accuracy. Studying a time series of data, taking into account geometrical conditions, and assuming the ground BRDF to be constant over several days, variations of the satellite signal may be mainly attributed to variations of the atmospheric optical properties. By fitting a subset of satellite observations associated with ground photometer data, the best-fit of BRDF parameters could be determined. Using then this surface characterization as an input of the inversion process, the aerosol optical thickness can in principle be retrieved routinely. Such method has been already explored with a time series of VEGETATION data for several Western European sites. The aerosol optical thickness retrieved from the satellite data and that derived from CIMEL measurements were in good agreement, even for cases of high optical depth. The method has been now improved with the sensor SeaWifS. Aerosol properties retrieved from SeaWifS data have been compared with those measured in-situ by sunphotometers.

We present an improved version of GACP (Global Aerosol Climatology Project) algorithm which uses channel 1 and 2 radiances of the Advanced Very High Resolution Radiometer (AVHRR) to retrieve aerosol optical thickness and Angstrom exponent over the ocean. We specifically discuss recent changes in the algorithm as well as the results of a sensitivity study analyzing the effect of several sources of retrieval errors not addressed previously. Uncertainties in the AVHRR radiance calibration (particularly in the deep- space count value) may be among the major factors potentially limiting the retrieval accuracy. On the other hand, the performance of two-channel algorithms weakly depends on a specific choice of the aerosol size distribution function. The updated algorithm is applied to a 10-year period of observations (July 1983 - Aug 1994), which includes data from NOAA-7, NOAA-9 (February 1985 - November 1988),and NOAA-11 satellites. The results are posted on the world wide web at http:gacp.giss.nasa.gov/retrievals. The NOAA-9 record shows no discernable long-term trends in the global and hemisphere averages of the optical thickness and Angstrom exponent. On the other hand, there is a discontinuity in the Angstrom exponent values derived from NOAA-9 and NOAA-11 data and a significant temporal trend in the NOAA-11 record. The latter are unlikely to be related to the Pinatubo eruption and may be indicative of a serious calibration problem.

In the presence of broken clouds over snow or ice covered surfaces it is difficult for UV/VIS spaceborne sensors to distinguish between clouds and high reflecting surfaces. These uncertainties lead to problems in the ozone retrieval. A new approach presented here characterizes the inhomogeneous pixel area (with broken clouds) by deriving an effective albedo and cloud-top-height. These parameters are determined by minimizing the difference between GOME measured sun-normalized radiances and corresponding model spectra in the spectral range of O₄ absorption near 370 nm. Extrapolation of this effective albedo to short wavelengths enables to improve the ozone retrieval. Besides this new retrieval method some new calibration corrections are made for the GOME spectra which take into account instrumental effects from dark current or straylight and the degradation from GOME.

The optical parameters of the hazy yellow sand dust over the Takula Makan desert in China and the surface reflectance were estimated, using multi-viewing reflectances and polarizations at the wavelength 443nm in several points selected from ADEOS/POLDER data taken on April 10, 1997. In this case, it was assumed that the land surface is the diffuse reflector and the number size distribution of yellow sands is represented by the Junge power-law. The optical parameters of dust clouds and the ground reflectance were determined such that the relative absolute error between the observed reflectance and polarization and those obtained from the simulation of radiative transfer in the atmosphere-ground system is minimum. As a result, it was found that the refractive index of yellow sand dust is 1.36 to 1.45, the optical thickness of dust clouds is 0.44 to 0.54, the index of Junge power-law is -4.2 to -5.0 and the surface reflectance is 0.1 to 0.17.

Remote sensing of atmosphere is changing rapidly thanks to the development of high spectral resolution infrared space- born sensors. The aim is to provide more and more accurate information on the lower atmosphere, as requested by the World Meteorological Organization (WMO), to improve reliability and time span of weather forecasts plus Earth's monitoring. In this paper the performance of the Infrared Atmospheric Sounding Interferometer (IASI) is analyzed looking directly at the products: temperature and water vapor.

The Infrared Atmospheric Sounding Interferometer (IASI), which is due for launch in 2005 aboard the European METOP satellite, will record the infrared radiation of the Earth/atmosphere system in a nadir-viewing geometry. The sensitivity studies carried out on simulated IASI spectra allowed us to highlight its capability to retrieve both total and tropospheric column amounts of ozone. Two algorithms based on neural network techniques have been developed to retrieve these quantities. The averaging kernels characterizing the results show that the information corresponding to the different atmospheric layers can be isolated. However, for the tropospheric ozone retrieval, the lower stratosphere is not totally decoupled, which induces larger uncertainties for the tropospheric columns than for the total columns. Nevertheless, very satisfactory performance has been achieved for both algorithms, with inversion errors smaller than 5 % for the total column retrieval and smaller than 20 % for the tropospheric column retrieval. The Interferometer Monitor for Greenhouse Gases (IMG), which flew aboard the Japanese ADEOS platform was based on the same observation technique as IASI, which enabled us to test our algorithms on real data. Both algorithms have been applied to these data for June 1997. The distributions obtained are in good agreement with the Total Ozone Mapping Spectrometer (TOMS) measurements.

In the laser absorption spectrometry (LAS) system with the open atmosphere, scintillation caused by the atmospheric turbulence is the principal interference for the system accuracy. However, the scintillation could be used for monitoring of the gas flux because it includes the information of the flow of the atmosphere. Therefore we developed a LAS system and attempted to monitor both the water vapor density and the scintillation simultaneously and individually. In the system, the scan time for the absorption lines is set shorter than the principal fluctuation period of the scintillation for reducing the influence of the scintillation. Based on the actual data about temporal sequence of transmittance for the laser beam through the target air, it was found that the scintillation was almost frozen if the time for spectral scanning was done within about 0.1 second. With this scan time, some indoor experiments were carried out and their data were split into the absolute absorption of the water vapor and the scintillation with numerical filters. As the results, actual fluctuation data of the water vapor density buried under the scintillation noise appeared, and the correlation between the fluctuation of the water vapor and the scintillation was able to be estimated.

In order to measure vertical profiles of minor gas concentrations in the stratosphere, Improved Limb Atmosphere Spectrometers (ILAS and ILAS-II) have been developed. ILAS was the first generation sensor and made observations in 1996 and 1997. ILAS-II will measure atmospheric limb transmittance in 66 spectral bands (whereas 44 for ILAS) in the thermal infrared region by observing solar ray passed through the atmosphere. Vertical profiles of minor gases are simultaneously retrieved by a spectral fitting algorithm with an onion-peeling method for vertical profiling. This algorithm adopts a precise radiative transfer calculation and is very accurate, but usually the standard radiative transfer calculation needs huge volume of line-by-line calculations of molecular absorption to simulate theoretical limb transmittance spectra by using the HITRAN database. Methods for accelerating the algorithm have been required. In the ILAS operational program, a table look-up method, which needs an excellent computer system, was used for rapid calculations. We proposed a simplified method, which predicts the gas profiles from the measured limb transmittance spectra and vertical profiles of atmospheric pressure and temperature without iterative calculations by using a multiple regression technique. The Principal Component Expansion (PCE) is used for reducing the scale of the multiple regression model. In the training process, coefficients of the model are estimated from the previously retrieved data sets including measured limb transmittance spectra, vertical profiles of atmospheric pressure and temperature, and retrieved gas profiles. Then, the method predicts gas profiles from the newly measured limb transmittance spectra and pressure and temperature profiles. The validity of the method was confirmed by numerical simulation using the MODTRAN v.3.5 radiative transfer code. The proposed method was also applied to the actual 3474 ILAS observation data sets. The model trained by 3373 data sets well predicted the gas profiles for another 100 data sets which are selected randomly . This proposed method can be used for quick look of ILAS-II measured data and for generating the initial profiles for the operational spectral fitting algorithm.

Raytheon and the U.S. Army have been developing laser remote chemical sensors for the last decade. This has included advanced transmitter and sensor development, field testing, and concepts for spacecraft, aircraft, unmanned aerial vehicles, ground-mobile transports, and fixed sites. The WILDCAT sensor utilizes a wavelength agile CO₂ laser with output energy of 1 J/pulse at a repetition rate of 100 Hz for ranges of 40 km. The receiver is composed of a 60 cm dia. telescope and HgCdTe detector, integrated into a gimbal system with full hemispherical scan. Algorithms allow for real-time data processing and concentration map display. An all solid state sensor breadboard has also been developed that is capable of 300 (mu) J output at 8-12 micrometers and 300 Hz repetition rate. The system is based on a Nd:YAG pump slab laser and two-stage, angle-tuned, optical parametric oscillator wavelength shifter. The system provides for power efficiency, compactness, and light weight that are consistent with manportability. Anticipated horizontal range is 3 km on the ground and 5 km vertically. Analysis of a space-based, low earth orbit system shows that chemical and biological species detection can be performed effectively by sensors derived from the laser components developed under these programs.

Satellite observations of atmospheric CO₂ are the key to answering important questions regarding spatial and temporal variabilities of carbon sources and sinks. Global measurements sampling the air above land and oceans allow oceanic flux to be distinguished from terrestrial flux. Continuous sampling on frequent basis allows seasonal variations to be distinguished. This study quantifies the potential value of satellite-based measurements of column- integrated CO₂ concentrations in terms of the carbon source/sink information that can be derived from these concentrations via inverse modeling. We discuss the utility of the carbon flux inversions in terms of both spatial and temporal resolution, compare capabilities of active and passive approaches to the measurements, and demonstrate the feasibility of high precision CO₂ column concentration retrievals.

Tankers are a wide spread and important emission source of VOC's. Up to now emission data are available from measurements inside the storage tanks to investigate explosion protection only. Different emission reduction systems are under discussion such as vapor recovery units. The environmental impact of these alternatives shall be investigated. The VOC emissions of tankers were investigated in a river harbor. Fence-line monitoring by Differential Optical Absorption Spectroscopy (DOAS) was performed to investigate different emission sources during activities of tankers in harbors. Benzene, toluene and p-xylene concentrations were measured by a mono-static DOAS using 3 retro-reflectors. The inverse method was applied on the basis of these non-intrusive measurements of effluent concentrations in the exhaust plume and meteorological measurements to determine the emission source strengths. Inversion of effluent's dispersion was performed by a Lagrangian model driven by wind fields of the Eulerian model MISKAM considering influences from buildings and the orography upon the streaming fields. The emission source strengths were determined from unloading and loading of gasoline, from ventilation of storage tanks of tankers into ambient air which is of common practice if the tankers are operated without load or if the tankers are to be refueled with a different product than before and from using the land-site gasoline vapor recovery unit in a tank farm for ventilation.

A sampling strategy and a signal processing technique are proposed to overcome Non Uniform Beam Filling (NUBF) errors on mean Doppler velocity measurements made by spaceborne weather radars. Effects of non uniformity of rainfall within the main antenna lobe in terms on the accuracy of standard estimators are first briefly shown, so as to point out that the bias introduced by NUBF on mean Doppler velocity estimates can be greater than the standard deviation of the estimated velocity, and that it depends on the along-track distribution of reflectivity. Then the sampling strategy is described, based on an oversampling of the integrated data in the along-track direction in order to retrieve information about the reflectivity pattern at the sub-beam scale. The proposed processing technique, named Combined Frequency-Time (CFT) technique, exploits the time series of spectra at fixed range to resolve the NUBF induced bias. The results and the evaluation of performances achievable by means of CFT, were obtained by applying a 3D spaceborne Doppler radar simulator to a 3D dataset of reflectivity and mean Doppler velocity measured through the NASA/JPL airborne Doppler radar ARMAR. The radar system considered here is a nadir-looking, Ku band radar with a sufficiently wide antenna. It is shown how the error on mean Doppler velocity estimates can be reduced by means of CFT to the level predicted for such a radar system in the case of uniformly filled resolution volume (UBF).

Suspended aerosol particles play a significant role in the Global Change issue. Every year huge quantities of Saharan dust are transported to the European continent and North Atlantic Sea. Since May 1, 2000, an elastic backscatter lidar station is in operation in Athens, Greece, in the frame of the European LIDAR Network (EARLINET), aiming at the monitoring of extreme aerosol events over the Eastern Mediterranean Sea. During the first year of operation 20 cases of Saharan dust transport to the East Mediterranean Sea (EMS), were successfully observed. The lidar data are correlated with meteorological and satellite observations, including the aerosol index (AI data from the Total Ozone Mapping Spectrometer (TOMS) and the SeaWiFS satellite images. The analysis of the data collected so far, made possible a first statistical approach of the vertical and horizontal extent and of the seasonal variation of free tropospheric Saharan dust layers over the EMS area. The main results of this work are: (1) multiple dust layers of variable thickness (0.5 - 4 km) are systematically observed in the altitude region 2 - 6 km, (2) Saharan dust outbreaks take place all year round, with more pronounced frequencies during the summer period and can persist for a few days (1- 5), (3) 24 - 48 hours are usually needed for the Saharan air masses to reach the EMS area, and finally (4) the TOMS AI values can reach 1.5 - 3.5 over Greece. These lidar data gave the aerosol vertical structure over the Eastern Mediterranean Sea during Saharan dust outbreaks and can be a direct input for global radiative transfer models.

The principles of design, problems of engineering realization, some results of first applications of two- frequency scanning Linear Frequency Modulation (LFM) lidars are discussed in the present paper. Basic method of transformation of one-frequency radiation into two-frequency one is described. The requirements to parameters of probing radiation, dept of modulation, band of scanning and modulating frequencies, laws of scanning and modulation are defined, spectral and temporal resolution are calculated. The different variants of two-frequency scanning LFM lidars with direct and heterodyne detection are examined, in particular: with probing frequencies being located inside absorption line; with the first probing frequency inside the absorption line and the second one outside it. The block- scheme of a two-frequency scanning LFM lidar is described, and parameter requirements of two-frequency radiators and lidar photoreceivers are discussed.

For good visibility lidar return signals may be analyzed using the classical lidar equation which describes the single scattering contribution only. Here the range, from which a contribution to the return signal comes, is proportional to the time difference between emission and reception. For dense cloud sensing with a ground-based lidar or for a spaceborne lidar system, the return signal contains also essential contributions from higher orders of multiple scattering. In this case the physical range or the distance along the emitted beam, from which the contribution comes, is no longer proportional to the time elapsed since emission. The elapsed time is only proportional to the photon path-length. Thus making the analysis of the return signal much more difficult. Which part of the return signal comes from what range and, hence, from which type of scatterers? The diffusion process of multiple scattering of light in the atmosphere is non-isotropic and extremely complicated. The key to the solution of the problem is the simulation of multiple scattering lidar returns where the separate orders of scattering are tracked. Such information about the diffusion of the laser beam is needed to give a better understanding of the extend of contribution from the type of scatterers to the return signal. In this paper, we offer such a non-trivial analysis of the diffusion of the laser beam in the cloud modeled by two kinds of atmospheric particles, i.e., aerosols and ice crystals, by using a multi-dimensional contribution distribution for different orders of scattering. This is done by conditioning the probability of return e.g. by the time elapsed, the order of scattering, the distance from the axis of the direction of emission, and the distance of the projection of the point of contribution on this axis to the emitter. This gives a fairly complete information about the diffusion process as it is seen from the receiver.

Introduction of satellite services using higher frequency bands such as Ku- and Ka-band requires the characterization of tropospheric propagation factors that are normally considered negligible at lower frequency bands. Clouds scattering and attenuation are considered one such factor. Clouds are present during a large fraction of an average year and cloud scattering and attenuation, together with gaseous absorption, will determine the system performance under non-rainy conditions. In this paper, macro- and micro-physical properties of clouds over Xi'an region are discussed based on meteorologic observation data. The theory and mechanism of backscattering for clouds are analyzed. According to statistics parameters of the clouds over Xi'an region, the quantity calculations of the backscattering cross sections for various reflection mechanism of cloud are dealt with. The results calculated show that the major contributions for the backscattering cross section come possibly from clear-air and humidity turbulence and also from distributions of cloud particles, especially, at higher frequencies. These quantification calculation results on the basis of Xi'an typical clouds properties confirm previous results and it is significance for satellite communication systems, particularly, low availability satellite links and active and passive remote sensing, etc.

The Vaisala ceilometer LD-40 'Tropopauser' is a compact eye-safe lidar measuring continuously under all possible climatic conditions and scanning the atmosphere up to a height of 13000 m. It uses laser diodes with 855 nm wavelength that are pulsed at an average frequency of 4000 Hz. The distance of the system's range bins is 7.5 m. Its main purpose is reporting cloud base heights and vertical visibility for aviation safety purposes. Ceilometers are capable of retrieving additional atmospheric parameters. These parameters include a vertical profile of planetary boundary layer aerosol backscatter, and an estimation of the cloud coverage for all cloud layers detected. The main focus of this paper however is directed on the investigation of backscatter data from falling precipitation. The LD-40 ceilometer is able to detect virga and precipitation reaching the ground, and it can distinguish between frozen and non-frozen precipitation by examining backscatter data profiles. This ability is caused by the biaxial lidar optics in combination with the different scattering phase functions of droplets, soft hail, and snow. Results of LD-40 measurements gathered on a regular basis are presented together with comparison data obtained from precipitation and visibility meters.

The DIAL system and the HYPACT model are two useful means to study plume evolution and dispersion. We developed a mobile DIAL system, mounted onto a truck. The DIAL is based on a single TEA CO₂ laser source, able to switch between the 'on' and 'off' lines. This set-up allows rapid tuning of the two lines and keeps the misalignment within a range of 0.1 mrad, moderately below the beam divergence. The receiving system is a Newtonian telescope. All the instrumentation and the telescope are located inside the van. A large size scanning mirror, installed on the roof, sends the laser radiation into the atmosphere and, at the same time, it collects the backscattered light. To test all instrumentation, preliminary measurements have been completed to monitor a water vapor plume of a cement factory. The measurements are qualitatively compared against a dispersion plume model run initially from climatological fields.

A mathematical description of atmospheric gases remote monitoring based on continuous-wave laser diode ladar (CW-LD ladar) is worked out. Dependences of echo-signal amplitudes and frequencies versus gas concentration at definite path range are obtained. Analysis of both probing signal modulation parameters and absorption line parameters on each-signal characteristics is carried out. Results of CW- LD-ladar numerical simulation are presented.

Detection and identification of organic gases and the most toxic chemical agents in the atmosphere by diode lidars in the band of the overtones and combined tones of the C-H vibration are proposed. This approach based on the spectroscopic data of the main toxic substances, which were measured in our laboratory and also on the field experiment using diode lidar. Our experience involved both compact 1 microJoule pulsed diode backscattering lidar (905 nm) with single photon detector and 3 mW continuous wave (CW) differential absorption (1500 - 1650 nm) lidar (DIAL). An example of the clouds and aerosol plume (located at 6 km far from lidar) monitoring by backscattering lidar are presented.

This article presents the results of laboratory tests and full-scale investigations of the capabilities of UV laser- based Raman-luminescent lidars when they are applied in the remote sensing of atomically, biologically, and chemically (ABC) dangerous objects. It also demonstrates the capabilities of applying image-processing television systems with space-time resolution operating both in the visible and ultraviolet regions to monitor ABC dangerous objects and ionization sources.

The Ontar Corporation (www.Ontar.com) has developed several products for atmospheric remote sensing to calculate radiative transport, atmospheric transmission, and sensor performance in both the normal atmosphere and the atmosphere disturbed by battlefield conditions of smoke, dust, explosives and turbulence. These products include: PcModWin: Uses the USAF standard MODTRAN model to compute the atmospheric transmission and radiance at medium spectral resolution (2 cm-1) from the ultraviolet/visible into the infrared and microwave regions of the spectrum. It can be used for any geometry and atmospheric conditions such as aerosols, clouds and rain. PcLnWin: Uses the USAF standard FASCOD model to compute atmospheric transmission and emission at high (line-by-line) spectral resolution using the HITRAN 2000 database. It can be used over the same spectrum from the UV/visible into the infrared and microwave regions of the spectrum. HitranPC: Computes the absolute high (line-by-line) spectral resolution transmission spectrum of the atmosphere for different temperatures and pressures. HitranPC is a user-friendly program developed by the University of South Florida (USF) and uses the international standard molecular spectroscopic database, HITRAN. LidarPC: A computer program to calculate the Laser Radar/L&n Equation for hard targets and atmospheric backscatter using manual input atmospheric parameters or HitranPC and BETASPEC - transmission and backscatter calculations of the atmosphere. Also developed by the University of South Florida (USF). PcEosael: is a library of programs that mathematically describe aspects of electromagnetic propagation in battlefield environments. 25 modules are connected but can be exercised individually. Covers eight general categories of atmospheric effects, including gases, aerosols and laser propagation. Based on codes developed by the Army Research Lab. NVTherm: NVTherm models parallel scan, serial scan, and staring thermal imagers that operate in the mid and far infrared spectral bands (3 to 12 micrometers wavelength). It predicts the Minimum Resolvable Temperature Difference (MRTD) or just MRT) that can be discriminated by a human when using a thermal imager. NVTherm also predicts the target acquisition range performance likely to be achieved using the sensor.

We present results from a preliminary analysis of the multi-instrument MFRSR dataset from ARM CART (SGP) site Extended Facilities (Northern Oklahoma and Southern Kansas). Single-instrument retrievals are performed using our previously reported direct-diffuse algorithm. Retrieval products include daily time series of the aerosol optical depth, column mean aerosol particle size, NO₂ and ozone column amounts. The distribution of MFRSR sites at the SGP (central and extended facilities) allows us to combine the retrievals from the individual instruments into series of 2D images. These 2D images facilitate the investigation of spatial and temporal variability of aerosols and trace gases. These images can be also used to evaluate satellite aerosol data products (e.g. MODIS, TOMS, EO-1).

We analyze integrated water vapor (IWV) estimated by Global Positioning System (GPS) measurements at the Milano-Linate airport, Italy, throughout October 1999, during the Mesoscale Alpine Programme (MAP) Special Observing Period (SOP). IWV was obtained from zenith path delay parameters, estimated every two hours, during GPS data analysis. Different strategies for GPS data analysis (network configuration, parameter weighting, path delay models) were tested in order to achieve the best accuracy in IWV estimates. The results are compared to balloon sounding data and are analyzed with the help of other data, such as from numerical weather prediction models, that were available during the MAP campaign. We find mean and RMS errors of 0.5 kg/m² and 1.4 kg/m², respectively, when compared tsounding data. The highest accuracy was found for GPS observations using data down to 5 - 10 degrees elevation angles in a network including far stations (more than 1000 km away). This analysis will be pursued for the study of episodes of strong convection leading to heavy precipitation encountered during the MAP SOP. Therefore, the GPS-derived IWV covering the MAP investigation area will be used.

Light scattering characteristics by small ice particles modeled by polydisperse, randomly oriented, finite circular cylinders at both the visible, and near-infrared 1.38 micrometers wavelengths are investigated. The effects caused by an increase in aspect ratio and by changes in effective variance of particle size distributions are addressed. Comparison of our results with those calculated by Zakharova and Mishchenko (2000) shows that light scattering properties of small ice particles modeled by finite circular cylinders are different from those modeled by spheroids having the same aspect ratio. The results are significant in optical modeling and applications in remote sensing.

Scattering properties, i.e., the integral photometric characteristics, of polydisperse, randomly oriented small ice crystals modeled by finite circular cylinders at some infrared (IR) wavelengths are calculated by using the exact T-matrix approach. The effects caused by various aspect ratios and effective variances of size distributions are investigated. The results further show that scattering properties of small ice cylinders in the IR region depend strongly on wavelengths, particle size distributions with different x and v_e and aspect ratios. Both the 25micrometers and 3.979micrometers together have some potential applications for remote sensing of cirrus and other ice clouds.

The Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) to be launched on ESA's environmental monitoring satellite ENVISAT-1 in November 2001, will measure the backscattered radiances in the UV, VIS and NIR of the solar spectrum. Apart from its nadir viewing mode, the instrument can also observe in limb viewing geometry. Such measurements will provide information about atmospheric trace gas distribution with high vertical resolution. A new generation of radiative transfer models is required in order to enable precise retrievals from UV and VIS limb observations. These models must (1) take scattering of light in a spherical shell atmosphere into account, and (2) be linearized with respect to the vertical trace gas distribution to be retrieved. As a first approach we present a linearized radiative transfer model in pseudo-spherical approximation, which employs the forward-adjoint perturbation theory. This method is based on two separate radiative transfer calculations: the usual forward and the adjoint formulation. The derivative of the radiance with respect to the trace gas concentration in a homogeneous model layer can easily be calculated, using the internal radiance field of both calculations. Thus, the proposed model carries out both the linearization with respect to changes in the vertical trace gas distribution and the modeling of backscattered radiances with reasonable numerical effort. Backscattered radiances are simulated with an accuracy better than 0.002 %. For the linearization the model achieves an accuracy better than 0.04 % in the derivatives.

Understanding the effect of aerosol on clouds and cloud systems is one of the major challenges in climate research. Local studies suggest a multitude of aerosol influences on cloud properties, some increasing cloud albedo and some decreasing it. The net effect remains uncertain. NASA's TRIANA mission, which would see the whole sunlit face of the Earth continually, would provide simultaneous data on aerosol properties and cloud reflectivity from its EPIC multi-spectral imager. With TRIANA's unique position at the L-1 Lagrange point, these data would be available not only globally but also over the entire daytime, well suited to accommodate the often short lifetimes of aerosol and investigations around diurnal cycles. This pilot study explores the ability to detect relationships between aerosol properties and cloud-reflectivity with advanced statistical methods and applies to any L-1 imager with similar capabilities. Prototype results using data from the EOS Terra platform are presented.

Retrieval methods for limb-scanning measurements usually adopt the assumption of horizontal homogeneity for the parcel of atmosphere sounded by the analyzed observations. For along-track observations from an orbiting platform, the dimension of the probed parcel of atmosphere may approach 2000 km. Horizontal homogeneity assumption in the inversion algorithms induces an error on the retrieved atmospheric parameters. Two questions arise from these considerations: 1) how to characterize these errors, 2) how to avoid this assumption if the size of the induced errors is not acceptable. In order to answer these questions, an innovative forward and retrieval model (geo-fit) was developed which does not use horizontal homogeneity assumption. In this approach the radiative transfer is made through a two-dimensional inhomogeneous atmospheric field. The retrieval algorithm is based on the simultaneous analysis of all the limb-scanning measurements relating to a given orbit. This feature allows to gather information from several contiguous limb-scanning sequences on a target atmospheric parameter at a given location, and therefore to improve the trade-off between accuracy and horizontal resolution. The obtained results show that the horizontal homogeneity assumption induces a significant systematic error on the retrieved atmospheric parameters, especially in the presence of strong horizontal gradients.

Light scattering property of environment is very important in theoretical study and application of the remote sensing. What's more, it is valuable for infrared radiation, imaging, and the detection of target and tracking. In this paper, solar and atmospheric background radiation, and their scattering property from target are discussed. According to electromagnetic radiant and scattering theories, BRDF (Bidirectional Reflectance Distribution Function) is introduced. BRDF is a very important quantity that shows the radiation and reflection feature of target. Lowtran model is an effective method of calculating the spectral distribution of solar and atmospheric radiation. Here it is applied to compute solar and atmospheric background radiation, which is incident to target. The intensity of incident light and the scattering radiance are linked by BRDF. The relative equations are deduced. Thus, the light spatial and spectral scattering distribution is given. As a special example, for the Lambert's surface, the equations are simplified. As a result, the spatial and spectral distribution scattering curves of solar and atmospheric background radiation incident to target are present. They show the different spatial and spectral scattering features of target, at the same time, the coherent and incoherent scattering quantities are discussed.

By Monte Carlo simulation we investigated specific features of the radiation regime in the atmosphere caused by multiple light reflection between clouds and surface for solar radiation transfer and for light propagation from earth- based sources. In particular, the following effects were observed for earth-based sources of light: the increase of the distance of light propagation for a point source and the possible intensification of luminous flux near the surface. Concerning solar radiation transfer in the atmosphere- surface system in the presence of optically thin clouds and high probability of light-reflection on the surface we studied the following two effects: the increase of the radiation intensity near the Earth's surface and the decrease of the integral albedo of the system.

A comparison between simulated data and measurements performed by means of a spectroradiometer has been done. We searched a correlation between Aerosol Optical Depth, measured the over a wide spectral range, and TOMS Aerosol Index, which is satellite retrieved. This comparison has been done for both desertic aerosol (measurements taken in Namibia, 1998) and rural aerosol (measurements realized in Southern Italy, 2000 and 2001). At the same time, the simulation code MODTRAN has been used, in order to obtain both synthetic Aerosol Optical Depth and Aerosol Index. Finally, we compared measured and simulated correlation, finding, a good agreement for desertic aerosol.

We present a new software toolbox based on LabVIEW, that simulates the return signal of a backscatter lidar. The beam propagates through two different media: a layer, which is enclosed by two parallel planes and a surrounding medium. A large variety of environmental and instrumental conditions can be chosen for the calculation. Multiple scattering can be taken into account. This new version M contains a variety of new modules including a hard target which can be tilted to simulate the effect of pulse extension.

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) is a high-resolution limb sounder on board the European polar platform ENVISAT, scheduled for launch this summer. Three main characteristics converge in MIPAS which make it a very useful instrument for non-LTE studies: its wide spectral coverage (4.15-14.6 micrometers or 680-2275 cm^-1); high spectral resolution (0.03 cm^-1), and high sensitivity; all of this in addition to its global spatial coverage. In this paper we present an overview of the non-LTE studies that have been carried in preparation for the analysis of MIPAS data, including the evaluation of non-LTE effects in the operational processing, focused in the stratosphere, and the retrieval of species that normally emit under non-LTE conditions. The current mission plan for measuring the non-LTE upper atmosphere is described, as well as the general purpose non-LTE retrieval scheme developed for analyzing those measurements.

Atmospheric emissions at 5.3 micrometers will be measured by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), a high-resolution limb sounder on board the European polar platform ENVISAT, scheduled to be launched in 2001. Measured spectra at 5.3 micrometers contain information on important atmospheric quantities such as NO volume mixing ratio, thermospheric temperature, and chemical NO production rates. However, the scientific analysis of this spectral region has to deal with complex non-local thermodynamic equilibrium (non-LTE) effects. A conventional non-LTE retrieval approach using ab initio vibrational temperatures cannot be applied due to rotational and spin-orbit non-LTE of NO in the thermosphere, and the dependence of NO state populations on the NO abundance itself caused by chemical excitations. An innovative non-LTE retrieval method enabling the treatment of vibrational, rotational, and spin non-LTE as well as a dependence of the non-LTE state distribution on the retrieval target quantities has thus been developed for the MIPAS data analysis. The ability of the developed non-LTE inversion tool to retrieve NO abundance profiles, thermospheric temperature profiles, and NO mean production rates by NO₂ photolysis in the stratosphere and N+O₂ combination in the thermosphere is demonstrated by means of a feasibility study.

During its two missions of about one week duration each in November 1994 and in August 1997 the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment observed spectrally resolved mid- and far- infrared emissions from the earth limb in the altitude regime from the troposphere to the lower thermosphere. The measurements concentrated on high spatial resolution in all three dimensions in the lower part of the height range. At upper mesosphere/lower thermosphere altitudes global ozone concentrations during day and night, daytime carbon-dioxide densities, and atmospheric temperatures were derived using a comprehensive non-LTE model. The derived parameters compare well with other experimental data. Pronounced horizontal structures in latitude as well as in longitude are found in all parameters showing that dynamical influences are of importance also in the upper mesosphere/lower thermosphere. Carbon dioxide mixing ratios start to depart from their tropospheric values at altitudes as low as 70 - 80 km. Atmospheric temperatures of 140 K and below were retrieved near the mesopause at high northern latitudes in August. These data are supported by the simultaneous observation of polar mesospheric clouds.

The overview of different approaches to the problem of remote sensing of the non-LTE atmosphere by satellite instruments working in the infrared region is presented. Special emphasis is made on the approach, which does not require modeling of the processes driving the nonequilibrium populations of the molecular vibrational states. The method of simultaneous retrieval of kinetic temperature, pressure, concentration of atmospheric gases and nonequilibrium populations of the vibrational states of molecules of atmospheric gases (vibrational temperatures) from the high altitude limb infrared radiance measurements in 15 micrometers region is described. The results of the application of this method to the interpretation of the limb radiance spectra measured by the CRISTA instrument are presented. The kinetic temperature profiles derived in the altitude range 40 - 120 km are shown and the features of the thermal structure of this atmospheric region are compared with climatological data. The magnitude of the non-LTE effect for the CO₂ vibrational states which give origin to 15 micrometers transitions is analyzed on the basis of retrieved CO₂ vibrational temperatures. The vertical distributions of the CO₂ volume mixing ratio in the mesosphere and lower thermosphere derived from the CRISTA measurements are presented and compared with model data. The problem of the influence of the amount of a priori information on the retrieval results is discussed on the basis of calculations of the information content by the Shannon approach.

An extensive database on spatial structure in the infrared radiance of the middle and upper atmosphere has been collected by the Mid-Course Space Experiment (MSX). The observed radiance contains spatial structure down to the scale of hundreds of meters. This spatial structure results from local fluctuations in the temperature and densities of the radiating states of the emitting molecular species as well as fluctuations in radiation transport from the emitting regions to the observer. A portion of this database has been analyzed to obtain statistical parameters characterizing stochastic spatial structure in the observed radiance. Using simple models, the observed statistics have been shown to agree with prior observations and theoretical models of stochastic spatial structure generated by gravity waves for special viewing geometries. The SHARC model has been extended to predict the statistics of stochastic fluctuations in infrared radiance from the statistics characterizing temperature fluctuations in the middle and upper atmosphere for arbitrary viewing geometries. SHARC model predictions have been compared with MSX data and shown to be in generally good agreement. Additional work is in progress to account for the statistics characterizing small spatial scale fluctuations.

The MSX SPIRIT III infrared radiometer and interferometer/spectrometer completed 233 episodic data collection events of remotely sensed atmospheric limb and terrestrial scenes from April 1996 to February 1997. The below the horizon (BTH) or terrestrial scenes were recorded in two mid-wavelength infrared (MWIR) radiometer bands centered near 4.3 microns. The above the horizon (ATH) measurements were measured simultaneously in the MWIR bands and four long wavelength infrared (LWIR) bands extending over the wavelength range from 6.8 to 25.1 microns. MSX results included the initial space based MWIR observations of gravity waves in the stratosphere. This source of atmospheric structure was detected in approximately 30 percent of the BTH and low limb MWIR measurements. As anticipated the MSX MWIR BTH results also included structure produced by clouds with large contrast ratios produced by high altitude clouds in the equatorial region. The MSX MWIR and LWIR measurements include observations of the radiance and structure associated with a range of atmospheric phenomena including clouds, gravity waves, airglow, day- night terminator transitions and aurora. Representative samples of these and other significant finding in the MSX measurements of infrared terrestrial and earth limb backgrounds are presented.

The nonequilibrium middle and upper atmosphere are very dynamic regions that are structured vertically and horizontally by the presence of persistent temperature inversion layers and by the passage of both atmospheric gravity waves and transient frontal disturbances or bores. The infrared emissions from this part of the atmosphere are already typically not in local thermodynamic equilibrium (LTE) and are further perturbed by the presence of this pervasive atmospheric structure. The inevitable result is highly structured atmospheric emissions that reflect the structure of the atmosphere. Understanding the structure of the atmosphere is essential to understanding the structure of the radiation that it emits. At the same time understanding how atmospheric structure perturbs atmospheric radiation provides a means to sense the perturbing atmospheric processes remotely. We examine methods to calculate the LTE/non-LTE radiative response to temporal and spatial variations of the atmosphere and give examples of applications. We also compare our results with existing field data. Finally, we discuss a proposed new NASA optical/infrared experiment (Waves Explorer) to sense atmospheric gravity waves remotely from earth orbit on a global basis and characterize their sources.

The Waves middle class Explorer mission (WE) is proposed to observe and quantify the effects of small-scale internal Gravity Waves (GW) in the Earth's atmosphere from source regions in the troposphere and lower stratosphere to the mesosphere, lower thermosphere, and ionosphere (MLTI) where the GW have their most dramatic effects. These are now understood to be a key element in defining large-scale circulation, thermal and constituent structures, and variability of the stratosphere and MLTI. The WE instrumentation consists of 5 nadir and limb viewing sensors of the wave perturbed emission structure due to GW throughout the source and affected regions. The WE PI is Prof. G.R. Swenson. This paper addresses the measurement strategy and implementation for two of these instruments, the Source Wave And Propagation Imager (SWAPI), and the Hydroxyl Airglow Wave Imager (HAWI). The SWAPI uses multi-spectral sublimb imaging measurements in the CO₂ (nu) ₃ band near 4210 nm to identify GW sources, and their propagation through the stratosphere. Its measurement strategy is driven by data, particularly sublimb images in the CO₂ (nu) ₃ band that were obtained by instrumentation deployed on the Midcourse Space Experiment (MSX) satellite, and by the WE team member's data analysis and models. Similarly team member's ground based observational experience and data analysis drives the HAWI measurement strategy.