Variations in the spectral scattering coefficient of marine particles [b p ()] were measured at 241 locations in oceanic (case 1) and coastal (case 2) waters around Europe. The scattering coefficient at 555 nm normalized to the dry mass of particles [b (555)] was, on average, 1.0 and 0.5 m 2 g Ϫ1 in case 1 and case 2 waters, respectively. characterized by a high density that counterbalances the effect of a higher refractive index. In the Baltic Sea, b (555) was similar to values found in other coastal waters despite the fact that particles were dominantly organic, m p which may result from higher absorption relative to scattering. A smaller than expected increase of b p () toward short wavelengths is attributed to significant absorption that increases toward the shorter wavelengths and reduces scattering, whether particles are living, detrital, or mineral. Our analyses suggest that the determination of b may m p be significantly sensitive to the porosity of the filter used to assess the dry mass of particles.Light scattering by suspended particles is generally the first-order determinant of reflectance variability in coastal 1 Corresponding author (marcel@obs-vlfr.fr). AcknowledgmentsThis study was mainly funded by the European Commission (Environment and Climate Program, contract ENV4-CT96-0310), and partially by the European Space Agency and by the U.S. Office of Naval Research Environmental Optics Program (grant N00014-98-1-0003). Ship time was provided by the Reedereigemeinschaft Forschungsschiffahrt and Institut National des Sciences de l'Univers. We thank Louis Prieur, who made possible our participation in the Almofront-2 cruise aboard R.V. L'Atalante. We are grateful to the crews of R.V. Victor Hensen and R.V. Tethys 2, and to Commerc'Air SA staff for their support during field experiments. We also thank G. M. Ferrari, G. Obolensky, N. Hoepffner, F. Lahet, K. Oubelkheir, and E. Roussier for their help during measurements,
[1] A method to derive two-layer cloud properties from concurrent visible, near-infrared, and infrared observations is described. It is a modification of a single-layer scheme and is applied to Spinning Enhanced Visible Infrared Imager (SEVIRI) observations and validated against coincident A-Train data, principally to evaluate the accuracy and characterize cloud top pressure (CTP) estimates. CTP values obtained from the single-layer scheme applied to multilayer clouds are significant overestimates of the upper layer value. The effect is usually larger than that on coincident IR-only retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS), and this characteristic can be traced to the use of visible wavelength observations. However, the solution cost from the optimal estimation method is found to be especially high in multilayer situations and is a strong indicator of CTP accuracy. Tighter thresholds on the solution cost select, with increasing stringency, scenes with single-layer or opaque upper layer cloud. High-cost (presumed multilayer) pixels are reprocessed with the scheme adapted to simulate a two-layer cloud and with only infrared measurements. The upper cloud is represented by the parameters of the original formulation; the additional lower cloud layer is gray and has a proxy height given by the surface temperature. Despite the simplicity of the cloud-atmosphere modeling under the upper layer, results obtained from the two-layer scheme are promising. Upper layer CTPs are of comparable accuracy to the single-layer cases, lower-layer CTPs show some useful accuracy, and upper layer optical depths correlate well with radar observations.
Abstract. We use the CloudSat 2006–2016 data record to estimate snowfall over the Greenland Ice Sheet (GrIS). We first evaluate CloudSat snowfall retrievals with respect to remaining ground-clutter issues. Comparing CloudSat observations to the GrIS topography (obtained from airborne altimetry measurements during IceBridge) we find that at the edges of the GrIS spurious high-snowfall retrievals caused by ground clutter occasionally affect the operational snowfall product. After correcting for this effect, the height of the lowest valid CloudSat observation is about 1200 m above the local topography as defined by IceBridge. We then use ground-based millimeter wavelength cloud radar (MMCR) observations obtained from the Integrated Characterization of Energy, Clouds, Atmospheric state, and Precipitation at Summit, Greenland (ICECAPS) experiment to devise a simple, empirical correction to account for precipitation processes occurring between the height of the observed CloudSat reflectivities and the snowfall near the surface. Using the height-corrected, clutter-cleared CloudSat reflectivities we next evaluate various Z–S relationships in terms of snowfall accumulation at Summit through comparison with weekly stake field observations of snow accumulation available since 2007. Using a set of three Z–S relationships that best agree with the observed accumulation at Summit, we then calculate the annual cycle snowfall over the entire GrIS as well as over different drainage areas and compare the derived mean values and annual cycles of snowfall to ERA-Interim reanalysis. We find the annual mean snowfall over the GrIS inferred from CloudSat to be 34±7.5 cm yr−1 liquid equivalent (where the uncertainty is determined by the range in values between the three different Z–S relationships used). In comparison, the ERA-Interim reanalysis product only yields 30 cm yr−1 liquid equivalent snowfall, where the majority of the underestimation in the reanalysis appears to occur in the summer months over the higher GrIS and appears to be related to shallow precipitation events. Comparing all available estimates of snowfall accumulation at Summit Station, we find the annually averaged liquid equivalent snowfall from the stake field to be between 20 and 24 cm yr−1, depending on the assumed snowpack density and from CloudSat 23±4.5 cm yr−1. The annual cycle at Summit is generally similar between all data sources, with the exception of ERA-Interim reanalysis, which shows the aforementioned underestimation during summer months.
A model developed recently by Loisel and Stramski [Appl. Opt. 39, 3001-3011 (2000)] for estimating the spectral absorption a(lambda), scattering b(lambda), and backscattering b(b)(lambda) coefficients in the upper ocean from the irradiance reflectance just beneath the sea surface R(lambda, z = 0(-)) and the diffuse attenuation of downwelling irradiance within the surface layer ?K(d)(lambda)?(1) is compared with measurements. Field data for this comparison were collected in different areas including off-shore and near-shore waters off southern California and around Europe. The a(lambda) and b(b)(lambda) values predicted by the model in the blue-green spectral region show generally good agreement with measurements that covered a broad range of conditions from clear oligotrophic waters to turbid coastal waters affected by river discharge. The agreement is still good if the model estimates of a(lambda) and b(b)(lambda) are based on R(lambda, z = 0(-)) used as the only input to the model available from measurements [as opposed to both R(lambda, z = 0(-)) and ?K(d)(lambda)?(1) being measured]. This particular mode of operation of the model is relevant to ocean-color remote-sensing applications. In contrast to a(lambda) and b(b)(lambda) the comparison between the modeled and the measured b(lambda) shows large discrepancies. These discrepancies are most likely attributable to significant variations in the scattering phase function of suspended particulate matter, which were not included in the development of the model.
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