Guyancourt, France
Guyancourt, France

Time filter

Source Type

Jouan C.,University of Quebec at Montréal | Jouan C.,Laboratoire Atmosphre | Girard E.,University of Quebec at Montréal | Pelon J.,Laboratoire Atmosphre | And 3 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2012

Extensive measurements from ground-based sites and satellite remote sensing (CloudSat and CALIPSO) reveal the existence of two types of ice clouds (TICs) in the Arctic during the polar night and early spring. The first type (TIC-2A), being topped by a cover of nonprecipitating very small (radar unseen) ice crystals (TIC1), is found more frequently in pristine environment, whereas the second type (TIC-2B), detected by both sensors, is associated preferentially with a high concentration of aerosols. To further investigate the microphysical properties of TIC-1/2A and TIC-2B, airborne in situ and satellite measurements of specific cases observed during Indirect and Semi-Direct Aerosol Campaign (ISDAC) have been analyzed. For the first time, Arctic TIC-1/2A and TIC-2B microstructures are compared using in situ cloud observations. Results show that the differences between them are confined in the upper part of the clouds where ice nucleation occurs. TIC-2B clouds are characterized by fewer (by more than 1 order of magnitude) and larger (by a factor of 2 to 3) ice crystals and a larger ice supersaturation (of 15-20%) compared to TIC-1/2A. Ice crystal growth in TIC-2B clouds seems explosive, whereas it seems more gradual in TIC-1/2A. It is hypothesized that these differences are linked to the number concentration and the chemical composition of aerosols. The ice crystal growth rate in very cold conditions impinges on the precipitation efficiency, dehydration and radiation balance. These results represent an essential and important first step to relate previous modeling, remote sensing and laboratory studies with TICs cloud in situ observations. © 2012. American Geophysical Union. All Rights Reserved.


Zhao C.,Lawrence Livermore National Laboratory | Xie S.,Lawrence Livermore National Laboratory | Klein S.A.,Lawrence Livermore National Laboratory | Protat A.,Laboratoire Atmosphre | And 17 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2012

Accurate observations of cloud microphysical properties are needed for evaluating and improving the representation of cloud processes in climate models and better estimate of the Earth radiative budget. However, large differences are found in current cloud products retrieved from ground-based remote sensing measurements using various retrieval algorithms. Understanding the differences is an important step to address uncertainties in the cloud retrievals. In this study, an in-depth analysis of nine existing ground-based cloud retrievals using ARM remote sensing measurements is carried out. We place emphasis on boundary layer overcast clouds and high level ice clouds, which are the focus of many current retrieval development efforts due to their radiative importance and relatively simple structure. Large systematic discrepancies in cloud microphysical properties are found in these two types of clouds among the nine cloud retrieval products, particularly for the cloud liquid and ice particle effective radius. Note that the differences among some retrieval products are even larger than the prescribed uncertainties reported by the retrieval algorithm developers. It is shown that most of these large differences have their roots in the retrieval theoretical bases, assumptions, as well as input and constraint parameters. This study suggests the need to further validate current retrieval theories and assumptions and even the development of new retrieval algorithms with more observations under different cloud regimes. © Copyright 2012 by the American Geophysical Union.


Yamauchi M.,Swedish Institute of Space Physics | Dandouras I.,CNRS Institute for research in astrophysics and planetology | Rme H.,CNRS Institute for research in astrophysics and planetology | El-Lemdani Mazouz F.,Laboratoire Atmosphre
Geophysical Research Letters | Year: 2012

Near the equatorial plasmapause at around 4-5 Earth radius (R E) geocentric distance, pancake distributed tens of eV ions are sometimes found, as previously reported by Olsen et al. (1987). Cluster CIS data during perigee traversals in 2001-2002 (nearly 200 traversals) revealed new features of these equatorially-trapped warm ions. (1) The characteristic energy of He + is often higher than that of H +. (2) Some events show non-thermal ring distribution for He + rather than superthermal pancake distribution. H + can also have the ring distribution in such events. (3) While majority of the events are dispersion-free, some events show energy-time dispersion, indicating drifts from different local times. (4) The time scale of the development is about an hour, which is much shorter than the drifting time of these ions around the Earth. Cluster statistics also confirmed some results from the previous studies: (5) These ions are confined within a few degrees of latitudinal range near the equator, and have nearly 90 pitch angles. (6) At a geocentric distance of about 4-4.5 R E where Cluster traversed the equator during its perigee, the probability of observing clear events is about 40-45% in the noon and dusk sectors and about 20-25% in the night-to-dawn sector. (7) They are dominated by H + with variable content of He +. The He +/H + ratio is much less than 5% for the majority of the cases. © 2012. American Geophysical Union. All Rights Reserved.

Loading Laboratoire Atmosphre collaborators
Loading Laboratoire Atmosphre collaborators