Cirrus Cloud Occurrence as Function of Ambient Relative Humidity: A Comparison of Observations Obtained during the INCA Experiment

ATMOSPHERIC CHEMISTRY AND PHYSICS (Impact Factor: 5.51). 01/2003; 3(2003):1807-1816.
Source: DLR

ABSTRACT Based on in-situ observations performed during
the Interhemispheric differences in cirrus properties from anthropogenic
emissions (INCA) experiment, we introduce and
discuss the cloud presence fraction (CPF) defined as the ratio
between the number of data points determined to represent
cloud at a given ambient relative humidity over ice
(RHI) divided by the total number of data points at that value
of RHI. The CPFs are measured with four different cloud
probes. Within similar ranges of detected particle sizes and
concentrations, it is shown that different cloud probes yield
results that are in good agreement with each other. The CPFs
taken at Southern Hemisphere (SH) and Northern Hemisphere
(NH) midlatitudes differ from each other. Above ice
saturation, clouds occurred more frequently during the NH
campaign. Local minima in the CPF as a function of RHI
are interpreted as a systematic underestimation of cloud presence
when cloud particles become invisible to cloud probes.
Based on this interpretation, we find that clouds during the
SH campaign formed preferentially at RHIs between 140
and 155%, whereas clouds in the NH campaign formed at
RHIs somewhat below 130%. The data show that interstitial
aerosol and ice particles coexist down to RHIs of 70–90%,
demonstrating that the ability to distinguish between different
particle types in cirrus conditions depends on the sensors
used to probe the aerosol/cirrus system. Observed distributions
of cloud water content differ only slightly between the
NH and SH campaigns and seem to be only weakly, if at all,
affected by the freezing aerosols.

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