Feingold, G., 2003:
Modeling of the first indirect effect: Analysis of measurement requirements. Geophys. Res. Lett., 30, No. 19, 1997, doi:10.1029/2003GL017967.
[1] We investigate the extent to which aerosol extinction is a
suitable proxy for the aerosol affecting drop formation. First
we use multiple realizations of a cloud model to investigate
the sensitivity of cloud drop effective radius re to aerosol
parameters (size distribution and composition) and
dynamical parameters (updraft and liquid water content). In
general, re is most sensitive to cloud liquid water, a parameter
often ignored in indirect effect analyses. The relative
importance of the other parameters varies for different
conditions but aerosol concentration Na is consistently
important. Updraft plays an increasingly important role
under high aerosol loadings. A breakdown of the individual
aerosol terms contributing to drop size change shows that use
of aerosol extinction as a proxy for size distribution and
composition tends to underestimate the magnitude of the first
indirect effect. This may influence interpretation of current
satellite and surface remote measurements of the indirect
effect. INDEX TERMS: 0305 Atmospheric Composition and
Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric
Composition and Structure: Cloud physics and chemistry; 1610
Global Change: Atmosphere (0315, 0325); 1640 Global Change:
Remote sensing. Citation: Feingold, G., Modeling of the first
indirect effect: Analysis of measurement requirements, Geophys.
Res. Lett., 30(19), 1997, doi:10.1029/2003GL017967, 2003.
[3] In situ measurements, both surface-based and airborne,
address the fundamental microphysical processes by
measuring size distribution and composition. However, for
climate monitoring purposes it is not feasible to perform
global measurements of these parameters with the detail
applied to in situ measurements. Satellites use passive
remote sensors to measure aerosol optical depth ta which
is used as an indicator of aerosol effects on cloud drop
size. Satellites provide global coverage of aerosols and
clouds but do not measure details of aerosol size and
composition. In addition, aerosol measurements are pathintegrated
and not collocated with clouds. Surface based
remote sensors [e.g., Feingold et al., 2003] measure
aerosol extinction a beneath clouds, cloud droplet size,
and liquid water path (LWP) in a single column of air at
scales pertinent to cloud drop formation (100 m). Again,
a (local ta) is used as a proxy for size distribution and
composition.
[4] The question explored in this paper is whether
proxies such as a are adequate, or alternatively, what set
of measurements needs to be made to provide useful
assessments of the first indirect effect. One can view this
as an endeavour to guide measurement strategies in the
coming years.
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