Cirrus Clouds and the Large-Scale Atmospheric State: Relationships Revealed by Six Years of Ground-Based Data

Abstract

The properties of cirrus clouds observed at the Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) in Oklahoma are documented from a nearly continuous 6-yr record of 35-GHz cloud radar data. Cirrus frequency over the ACRF is 23% and 28% of the time in the warm (May–September) and cold seasons (November–March), respectively, with maxima and minima during the period studied of 30% and 16% in the warm season and 34% and 24% in the cold seasons. Cirrus, as defined here, reveal a seasonal oscillation in their macroscale properties that can be traced to the seasonal deepening of the troposphere in the Southern Plains region. While the average bulk microphysical properties do not change significantly from season to season, the variability of certain parameters demonstrates seasonal change. It is shown that the properties of cirrus clouds vary perceptively with the large-scale vertical motion. Using NCEP–NCAR reanalysis data to define the large-scale meteorological state when cirrus are observed at the ACRF, the authors find that cirrus tend to exist within a maximum in upper-tropospheric humidity and downstream of the peak upper-tropospheric vertical motion. Cirrus that exist in large-scale ascent upstream of the synoptic-scale middle-tropospheric ridge axis are shown to have higher water contents than cirrus that exist in large-scale subsidence downstream of the ridge axis, although the overall nature of the statistical distributions of water contents do not change greatly, suggesting that it may be difficult to parameterize the properties of cirrus based solely on large-scale vertical motion. The layer-mean particle size, on the other hand, shows no such sensitivity to the large-scale vertical motion.

Modelling of cirrus clouds – Part 2: Competition of different nucleation mechanisms

“…This depth is on the lower end of typical vertical extensions for cirrus clouds (see e.g. Mace et al, 2006) but is still in the typical range for an extratropical cirrus cloud and an ice supersaturated layer (Spichtinger et al, 2003). For the reference cases the layer is located between 9.5 km and 10.5 km, while for sensitivity studies we also consider a higher (10.5≤z ISSR ≤11.5 km), hence colder, and a lower, hence warmer, layer (8.5≤z ISSR ≤9.5 km).…”

Impact of deep convection in the tropical tropopause layer in West Africa: in-situ observations and mesoscale modelling

“…Enhanced values of BSR indicate the presence of aerosols and values of δ a above 10% indicate the presence of ice crystals . The mechanism of cirrus formation in the uppermost troposphere is still matter of debate, in particular whether ice particles formation can be directly linked to deep convective systems (see for instance Pfister et al (2001) and Mace et al (2006)). The use of ice particles as tracer for convection may be ambiguous.…”

Cirrus cloud macrophysical and optical properties over North China from CALIOP measurements

“…But, they differ from the Platt et al (1998) equatorial study. This is attributed to the relatively higher occurrence frequency of the physically thick cirrus in the equatorial region, which is often associated with deep convective activities in the Intertropical Convergence Zone (ITCZ) (Mace et al, 2006). Figure 4b exhibits a PDF histogram of 0.1 intervals of cirrus visible optical depth.…”