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Jean-Jacques Morcrette
and
Yves Fouquart

Abstract

Using the shortwave radiation scheme of Fouquart and Bonnel that accounts for scattering and absorption by gases and cloud particles, we study the effect of varying the assumption for the overlap of partially cloudy layers, and the resultant impact upon the heating rate profile, planetary albedo, net flux at the surface, and atmospheric net absorption. In this study, we consider the maximum, minimum, and random overlap assumptions and a radically simple scheme to approximate the radiative effects of a random overlapping of clouds. This simple scheme involves linear combinations of clear and cloudy reflectivities and transmissivities within a layer, and gives, respectively, fluxes and heating rates with maximum differences of 5% and 0.1 K day−1 compared to similar quantities obtained from a full calculation assuming a random overlapping of cloud layers. This former approach, however, is much more time efficient (five times faster for a 3-cloud atmosphere, three times faster in a full-size GCM).

Compared to the random assumption, the maximum overlap assumption gives smaller planetary albedo and larger net flux at the ground, whereas larger planetary albedo and smaller net flux at the ground result from the minimum overlap assumption. These differences tend to smooth out for larger values of the surface reflectivity. Systematic difference in the radiative forcings of a GCM due to these different cloud overlap assumptions largely vary with the cloud generation scheme.

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Jean-Louis Dufresne
,
Catherine Gautier
,
Paul Ricchiazzi
, and
Yves Fouquart

Abstract

Scattering in the longwave domain has been neglected in the first generation of radiative codes and is still neglected in most current GCMs. Scattering in the longwave domain does not play any significant role for clear-sky conditions but recent works have shown that it is not negligible for cloudy conditions. This paper highlights the importance of scattering by mineral aerosols in the longwave domain for a wide range of conditions commonly encountered during dust events. The authors show that neglecting scattering may lead to an underestimate of longwave aerosol forcing. This underestimate may reach 50% of the longwave forcing at the top of atmosphere and 15% at the surface for aerosol effective radius greater than a few tenths of a micron. For an aerosol optical thickness of one and for typical atmospheric conditions, the longwave forcing at the top of the atmosphere increases to 8 W m−2 when scattering effects are included. In contrast, the heating rate inside the atmosphere is only slightly affected by aerosol scattering: neglecting it leads to an underestimate by no more than 10% of the cooling caused by aerosols.

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Jean-Louis Brenguier
,
Hanna Pawlowska
,
Lothar Schüller
,
Rene Preusker
,
Jürgen Fischer
, and
Yves Fouquart

Abstract

The plane-parallel model for the parameterization of clouds in global climate models is examined in order to estimate the effects of the vertical profile of the microphysical parameters on radiative transfer calculations for extended boundary layer clouds. The vertically uniform model is thus compared to the adiabatic stratified one. The validation of the adiabatic model is based on simultaneous measurements of cloud microphysical parameters in situ and cloud radiative properties from above the cloud layer with a multispectral radiometer. In particular, the observations demonstrate that the dependency of cloud optical thickness on cloud geometrical thickness is larger than predicted with the vertically uniform model and that it is in agreement with the prediction of the adiabatic one. Numerical simulations of the radiative transfer have been performed to establish the equivalence between the two models in terms of the effective radius. They show that the equivalent effective radius of a vertically uniform model is between 80% and 100% of the effective radius at the top of an adiabatic stratified model. The relationship depends, in fact, upon the cloud geometrical thickness and droplet concentration. Remote sensing measurements of cloud radiances in the visible and near infrared are then examined at the scale of a cloud system for a marine case and the most polluted case sampled during the second Aerosol Characterization Experiment. The distributions of the measured values are significantly different between the two cases. This constitutes observational evidence of the aerosol indirect effect at the scale of a cloud system. Finally, the adiabatic stratified model is used to develop a procedure for the retrieval of cloud geometrical thickness and cloud droplet number concentration from the measurements of cloud radiances. It is applied to the marine and to the polluted cases. The retrieved values of droplet concentration are significantly underestimated with respect to the values measured in situ. Despite this discrepancy the procedure is efficient at distinguishing the difference between the two cases.

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Frederick M. Luther
,
Robert G. Ellingson
,
Yves Fouquart
,
Stephen Fels
,
Noelle A. Scott
, and
Warren J. Wiscombe

An international program of intercomparison of radiation models has been initiated because of the central role of radiative processes in many proposed climate change mechanisms. Models ranging from the most detailed (line-by-line) to the most-highly parameterized have been compared with each other and with selected aircraft observations. Although line-by-line-model fluxes tend to agree with each other to within one percent (if the water-vapor–continuum absorption is ignored), the less-detailed models show a spread of 10–20 percent. The spread is even larger (30–40 percent) for the sensitivities of the models to changes in important radiation variables, such as carbon dioxide amounts and water-vapor amounts. These spreads are disturbingly large.

Lacking highly accurate flux observations from within the atmosphere, it has been customary to regard line-by-line–model results as “the truth.” However, uncertainties in the physics of line wings and in the proper treatment of the water-vapor continuum make it impossible for the line-by-line models to provide an absolute reference for evaluating less-detailed models. Therefore, a dedicated surface-based field measurement program is recommended in order to properly evaluate model performance; the goal would be to use sophisticated spectrometers to measure accurately spectral radiances rather than integrated fluxes.

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