Diurnal variations of cloud and relative humidity profiles across the tropics.

Even though the diurnal cycle of solar forcing on the climate system is well defined, the diurnal evolutions of water vapor and clouds induced by the solar forcing are not yet established across the tropics. Here we combine recent satellite observations of clouds profiles and relative humidity profiles to document the diurnal variations of the water vapor and clouds vertical distributions over all the tropics in June-July-August. While the daily mean water vapor and cloud profiles are different between land and ocean, their diurnal variations with respect to their daily means exhibit similar features. Relative humidity profiles and optically thin cloud fraction profiles vary together which maximize during night-time in the entire troposphere and a minimize in day-time. The fraction of optically opaque clouds peak in the free troposphere in the early afternoon, transforms into a high altitude positive anomaly of optically thin clouds from nightfall to sunrise. In addition, land regions exhibit a daily low thin cloud positive anomaly, while oceanic regions exposed to subsidence air motions exhibit positive anomalies of opaque clouds in the lower atmosphere during the second half of the night, which grow until sunrise.

Computation of a single-scattering matrix for nonspherical particles randomly or horizontally oriented in space.

The computation of the scattering properties of cirrus cloud ice crystals by the ray-tracing approach is described. A light beam is represented by its Stokes quadrivector I, which describes intensity as well as polarization, and the scattering properties of particles (molecules, droplets, or ice crystals) are introduced by means of a 4 x 4 transformation matrix M known as the Mueller matrix, or M matrix. Obtaining such a matrix for each kind of particle gives access to a complete description of the scattering medium. Most computations of the M matrices of cirrus ice crystals have introduced several simplifying hypotheses, by using basic shapes, by assuming a random orientation of the particles, or both. The present study focuses on the calculation of the complete M matrix for a specific shape of particles (i.e., hexagonally based crystals) either with optional oscillation about the horizontal plane or with random orientation. The validity of the computation code is checked against specific well-known cases for randomly oriented particles. For horizontally oscillating particles the computation of this matrix is a new result. Sensitivity of the M matrix to the following variables is studied: refractive index, amplitude of oscillation, particle shape and size, and angle of incidence.