Can we detect cirrus changes
with remote sensing methods ?
Bernd Kärcher
DLR Institut für Physik der Atmosphäre
with contributions from W. Haag, J. Ovarlez, J Ström
CIRAMOSA and beyond
Final project meeting
CNES HQ, Paris, November 20/21, 2003
mesoscale gravity waves
T(t)
dehydration and
denitrification
HNO3
H2O
liquid particles
and ice nuclei
cirrus
Measurement strategy during INCA
formation
layer
developed
cloud, virga
• When do cirrus clouds form ?
• What are the freezing mechanisms ?
Analysis of
data taken
inside and
outside of
clouds
Freezing mechanisms in cirrus conditions
A strong indirect effect of aerosols
on cirrus clouds is possible if
• liquid particles and IN are, at least in part,
externally mixed
• liquid aerosols and ice nuclei (IN) have distinct
freezing thresholds
(IN < 130% vs > 145%)
Indirect aerosol effect on cirrus clouds
Indirect aerosol effect on cirrus clouds
The presence of IN may lead to
• a suppression of ice crystal concentrations in
cirrus clouds
• a bimodal ice crystal size distribution, even at a
constant cooling rate
• differences in the vertical cloud structure and
cloud cover
• enhanced sedimentation fluxes through a small
number of large crystals originating from IN
The magnitude of the indirect
aerosol effect depends on
• spectrum (pdf) of updraft speeds or cooling rates
• number densities and freezing relative humidities
of ice nuclei
• degree of external vs internal mixing of IN with
liquid aerosol particles
Ubiquitous mesoscale temperature fluctuations
minimize the impact of heterogeneous IN.
Distributions of RHI
outside of cloud
inside cloud
Distributions of RHI
outside of cloud
inside cloud
Lessons from RHI observations
• homogeneous freezing controls cirrus formation
in clean and polluted air masses
• direct evidence for heterogeneous ice formation
in cirrus clouds
• number of IN (< 0.01 cm-3) too small to strongly
alter cirrus properties (weak neg. Twomey
effect) at the prevailing rapid cooling rates
The combined effect of IN and small-scale
fluctuations of temperature compared to synoptic
cooling may lead to
• high ice crystal concentrations even in the
presence of IN
• complex evolution of ice crystal size distributions
Synoptic versus mesoscale waves
Bimodality induced by IN and
small-scale waves
synoptic wave
only
(5 cm/s peak)
Atmospheric modeling
• Microphysical domain-filling trajectory
simulations based on ECMWF wind fields
with superimposed mesoscale temperature
oscillations
• Domain 25o-75oN, tropopause region (+1 km to
–2 km around local TP), 3 month-averages
Sep/Oct/Nov 2000
• H2O mixing ratio reinitialized every 6 hours at
RHI < 95% and constant above to generate
realistic supersaturation spectra
(W. Haag, Ph.D. thesis)
Cloud occurrence frequency / Cloud cover
Cloud cover changes: add IN
Cloud cover changes: alter wave prop‘s
Do aerosol particles control the
properties of cirrus clouds ?
• Small-scale temperature oscillations and
homogeneous freezing seem to be
universal features of cirrus clouds
• Regarding their impact on cloud, changes in
aerosol may easily be masked by dynamical
changes
• IN may impact dehydration and seed lower level
clouds, but would not alter overall cloud
prop‘s at the observed concentration levels
of O(0.01 cm-3)
Cirrus changes quantified (prelimin. list)
Changes in cloud cover caused by
• vertical air motion variability: ± 5%
• adding IN freezing @ 130%: ± 3%
Changes in median cloud parameters caused by
• vertical air motion variability: up to factor 3 (ni),
± 10-20% (ri and IWC), ± 15-50% (E), ± 10% (fsvc)
• adding < 0.01 cm-3 IN freezing @ 130%: < factor 3
reduction (ni), < 15%increase (ri), < 25% reduction
(IWC), < factor 2 reduction (E), < 60%increase (fsvc)
Can we detect cirrus changes with
remote sensing methods ?
• Changes appear to be more pronounced for mean cloud
parameters
• Likely strong contrast induced by weak wave activity
over the oceans vs intense orographic forcing
• Aerosol-induced changes likely to be more pronounced
for more abundant and/or more potent IN
This is a discussion topic .....
A careful evaluation of
dynamical changes in cloud formation
is required
• to assess cirrus cloud properties and trends in
a future climate
• before conclusions regarding the role of
aerosol composition or other anthropogenic
factors are made