Showing papers by "Kenneth Sassen published in 2009"

Cirrus clouds and deep convection in the tropics: Insights from CALIPSO and CloudSat

Abstract: [1] Using a 2-year data set of combined lidar and cloud radar measurements from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and CloudSat satellites, the occurrence of tropical cirrus and deep convective clouds is studied. The cloud identification algorithm takes advantage of the ability of the radar to probe deep precipitating clouds and of the lidar to sample even subvisual cirrus clouds. Examined are the frequency of occurrence and the geographical distribution of these clouds, and their apparent interconnections. There is a strong apparent diurnal variability in tropical cirrus mainly over land, with significantly more cirrus detected at night compared to day, but no clear diurnal pattern in deep convective activity. CALIPSO daylight signal noise effects do not appear to be not responsible for the diurnal cirrus pattern, because high, thin tropopause transitional layer (TTL) cirrus do not show a clear diurnal effect. Stratifying the global results by estimated visible cloud optical depth τ, we find that most of the planet's subvisual (τ < ∼0.03) cirrus clouds occur in the tropics and are more frequent at night and over ocean; thin (∼0.03 < τ < ∼0.3) cirrus have their highest global frequencies over equatorial landmasses and in the west Pacific region, and are also more frequent at night but occur mainly over land; and opaque (∼0.3 < τ < ∼3.0) cirrus are spread globally and tend to occur during the day over ocean. Although it is unknown which of the several proposed cirrus cloud formation mechanisms are key in the tropics, the close association of cirrus with convective clouds implies that tropical cirrus are linked to deep convective activity, with the likely exception of TTL cirrus clouds.

A global survey of CALIPSO linear depolarization ratios in ice clouds: Initial findings

Abstract: [1] Linear depolarization ratio (δ) data from the summer/winter seasons over the first 2 years of Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite operations in the green (0.532 μm) laser channel are analyzed and interpreted in terms of ice cloud microphysical properties. That is, we use variations in δ as a proxy of cloud formation and environmental conditions that affect ice crystal shape and orientation. The cloud detection algorithm is tuned mainly to cirrus clouds, but also includes polar stratospheric clouds (PSC) and optically thin, low and midlevel ice clouds at high latitudes. As anticipated from ground-based polarization lidar studies, δ increase with increasing height/decreasing temperature, and the effects of horizontally oriented plate crystals in lowing δ are evident by comparing data obtained close to the nadir (0.3°) and off-nadir (3.0°) pointing directions. These differences in δ average 0.01–0.03, although this anisotropic scattering effect is particularly apparent at low altitudes in the mid and high latitudes. Unexpected findings include decreasing δ with increasing latitude, and δ in detected PSC that are usually similar to cirrus clouds. However, δ in PSC are lower in a belt in the lower stratosphere in the Southern Hemisphere and generally lower in the Northern Hemisphere, but higher in lower stratospheric nacreous clouds in both hemispheres. There are also significant differences in the ice cloud δ measured at night and day, but this is assumed to result from factors associated with day/night differences in CALIPSO data collection. Global average δ are 0.34–0.36 for day, and 0.23–0.26 for night.