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The instrument set-up designed by the PreViBOSS project for the ParisFog field campaign is suitable to sound microphysical properties of droplets and interstitial aerosols during developed fog in a semi-urban environment. Developed fog is defined as LWC -3 and the temperature vertical gradient over 30 m, ΔT, smaller than 0.04 K/m. Visibility averaged over November 2011 is 385±340 m (with rare values larger than 1000 m), and month average of LWC is 60±60 mg m -3 . The droplet effective radius decreases from 14 to 4 μm when the number concentration increases from less than 10 to 220 cm -3 . Particle extinction coefficient is computed by Mie theory applied on size distribution observed during developed fog in ambient conditions by both PALAS WELAS and DMT FM100. Comparison with particle extinction coefficient directly measured by the Degreanne DF20 visibilimeter demonstrates satisfying agreement, within combined uncertainties. Ratio of computed over measured particle extinction coefficient is 1.15±0.35. Visibility smaller than 1000 m at 3 m above ground level is observed not only during developed fog but also during shallow fog, which presents a significant vertical gradient, as ΔT > 0.4 K/m. In this case, LWC is highly variable and may be observed below 7 mg m -3 . The consequent month average of LWC is 30±80 mg m -3 . The optical counters miss large droplets significantly contributing to extinction in shallow fogs. Consequently, it is not possible to reproduce with satisfaction the particle extinction coefficient in shallow fog. Fog type may be distinguished by association of groundbased visibilimeter and MSG/SEVIRI. When clear-sky is given by EUMETSAT/NWCSAF cloud type product while visibility is observed smaller than 1000 m at SIRTA, in 75% cases a shallow fog occurs, and in other cases, horizontal heterogeneity characterises the developed fog within the SIRTA pixel, as during the dissipation phase. Moreover, consistently, low and very low clouds are mostly detected by the satellite product when developed fog is observed by ground-based instrumentation.

The SIRTA (Site instrumental de Recherche par Teledetection Atmospherique) is a ground-based platform located 25km south of Paris in France. The SIRTA observatory was created in 1999 by the French research institute IPSL (Institut Pierre Simon Laplace) to conduct research programs in order to improve the knowledge of radiative and dynamic processes in the atmosphere as well as complex interactions between clouds and aerosols. The objective is to better comprehend climate changes and evolution of environment using a suite a state-of-art active and passive remote sensing instruments. Two ground platforms, a wooden tower, a roof platform and a building (where the lidar operates) are the main facilities of SIRTA. The project team is composed of six persons to ensure the station operations from instrument deployment, maintenance, data transfer and preliminary data analysis. The SIRTA infrastructure enables to conduct many research activities that involve the cloud and aerosol lidar. Some of them will be discussed: the development of the STRAT (Structure of the Atmosphere) algorithm dedicated to automatically discriminate atmospheric layers and retrieve geophysical parameters from lidar profiles, and the CALIPSO validation using the dual-channel backscatter lidar deployed at SIRTA.

NASA's clouds and the Earth's radiant energy system (CERES) program is a key component of the Earth observing system (EOS). The CERES proto-flight model (PFM) instrument is to be launched on NASA's tropical rainfall measuring mission (TRMM) platform on 1 November 1997. Each CERES instrument contains three scanning thermistor bolometer radiometers to monitor the longwave and visible components of the Earth's radiative energy budget. An integral part of analyzing these measurements will be the use of high-resolution cloud imager data in conjunction with data from the CERES instruments. The use of high-resolution cloud imager data requires that the point spread function (PSF), or the dynamic response of the radiometric channels as they scan across a far-field point source, be well characterized. The PSF is determined by the field-of-view of the radiometric channel, its optical geometry, and the time response of the thermistor bolometer and its associated signal conditioning electronics. The PSF of the CERES instruments is measured in the laboratory using a state of the art radiometric calibration facility (RCF) developed by TRW. Intrinsic difficulties in making this measurement suggest that a better understanding of the data could be obtained by the use of an independent instrument model. High-level first-principle dynamic electrothermal models of the CERES radiometric channels have been completed under NASA sponsorship. These first-principle models consist of optical, thermal and electrical modules. Accurate optical characterization of the channels is assured by Monte-Carlo- based ray-traces in which tens of millions of rays are traced. Accurate thermal and electrical characterization is assured by transient finite-difference formulations involving thousands of nodes to describe thermal and electrical diffusion within the thermistor bolometer sensing elements and the instrument mechanical structure. The signal conditioning electronics are also included in the models. Numerical simulations of the PSF's of the CERES proto-flight model (PFM) radiometric channels have been completed. This paper presents a comparison between the measured PSF and the independent numerically predicted PSF for the CERES proto-flight model total channel. Agreement between the measured and predicted PSF's is excellent. The result of this agreement is a high confidence in the model to predict other aspects of instrument performance. For example, the model may now be used to predict channel PSF's for elevation scan rates different from the nominal Earth scan rate.

The radiometer CLIMAT is a highly sensitive field instrument designed for multispectral thermal infrared measurements. Ground-based measurements can be performed. but the instrument has capabilities for operating from aircraft or balloon. The optics consist of an objective lens and a condenser mounted according to the Koehler principle to provide uniform irradiation over the detector surface. The radiometric signal is treated by a fast thermopile detector characterized by a low noise and a very weak temperature dependence of its responsivity. The managing system allows either manual or automated measurements. The energy consumption of the instrument is optimized for a maximum autonomy. The optical and electrical units of the instrument are described. Different experimental studies for measuring the sensitivity accuracy, spectral characteristics, thermal behavior and, field of view of the instrument are described. The instrument is dedicated to ground and vegetation on the one hand. and on the other hand, clouds and atmospheric soundings. The radiometer is also designed for calibrations or analyses of satellite radiometry data. Some atmospheric measurements obtained with a prototype are presented. Prospects are the development and the qualification of a narrow field-of-view instrument adapted to inhomogeneous targets such as cirrus clouds. A 3.7-tim channel and an internal blackbody are under study.

The Earth radiation budget experiment (ERBE) was developed to provide a complete temporal and spatial coverage of the solar reflected and Earth emitted radiation. ERB measurements were resumed in 1994 by the Scanner for Radiation Budget (ScaRaB) mission on a single sateffite. Due to sparse temporal sampling, diurnal variations must be accounted for in order to establish accurate unbiased daily and monthly mean radiant exitance. When the ERBE diurnal interpolation algorithm is used alone, large discrepancies are shown between monthly mean radiative flux of single and multi-sateffite measurements. We extend the algorithm by accounting for diurnally varying cloud cover and thickness using ISCCP data. Significant improvements are found in regions where clouds have a pronounced diurnal cycle.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Current efforts are directed at creating a high-level end-to-end numerical model of scanning thermistor bolometer radiometers of the type used in the Earth Radiation Budget Experiment (ERBE) and planned for the clouds and the earth's radiative energy system (CERES) platforms. The first-principle model accurately represents the physical processes relating the electrical signal output to the radiative flux incident to the instrument aperture as well as to the instrument thermal environment. Such models are useful for the optimal design of calibration procedures, data reduction strategies, and the instruments themselves. The modeled thermistor bolometer detectors are approximately 40 micrometers thick and consist of an absorber layer, the thermistor layer, and a thermal impedance layer bonded to a thick aluminum substrate which acts as a heat sink. Thermal and electrical diffusion in the thermistor bolometer detectors is represented by a several-hundred-node- finite-difference formulation, and the temperature field within the aluminum substrate is computed using the finite-element method. The detectors are electrically connected in adjacent arms of a two-active-arm bridge circuit so that the effects of common mode thermal noise are minimized. However, because of a combination of thermistor self heating, loading of the bridge by the bridge amplifier, and the nonlinear thermistor resistance-temperature relationship, bridge deflections can still be provoked by substrate temperature changes, even when the change is uniform across the substrate. Of course, transient temperature gradients which may occur in the substrate between the two detectors will be falsely interpreted as a radiation input. The paper represents the results of an investigation to define the degree of vulnerability of thermistor bolometer radiometers to false signals provoked by uncontrolled temperature fluctuations in the substrate.

The Earth Radiation Budget Experiment consists ofan array ofradiometric instniments placed in Earth orbit by the National Aeronauticsand Space Administration to monitor the longwave and shortwave components ofthe Earth's radiative budget. Presented is a high-leveldyn amic electrothermal model ofthe nonscanrnng Earth-viewing active cavity radiometers used to measure the Earth's total radiativeexitance. High accuracy is obtained by integrating an optical and thermal radiative model with both a transient conduction model ofthe Suprasil® filter dome and a dynamic electrothermal model of the active cavity. The instrument optical and thermal radiativeperfonnance is characterized by a Monte-Carlo ray-trace representation. Thermal diffusion in the Suprasil® filter dome is modeled bya three-dimensional transient finite difference analysis. Dynamic electrothermal behavior ofthe active cavity is described by a two-dimensional transient finite element model. The paper emphasizes the capabilities ofthe current models, including the ability to performsimulated calibration runs and to simulate the so-called solar blip" phenomenon.1. INTRODUCTION