Photo courtesy of Paul Lawson/J.H. Bain An Overview of Cirrus
Cloud Thinning and Determining Its Scientific Feasibility David L.
Mitchell Desert Research Institute, Reno, Nevada, USA
Slide 2
Role of Cirrus Clouds in Earths Radiation Budget
Slide 3
Homogeneous Freezing Nucleation: T 145%. Higher nucleation
rates, smaller crystals, lower fall speeds, greater IWP & cloud
coverage. Coldest = strongest greenhouse effect T = -40C Frozen
haze droplet Heterogeneous Nucleation Processes: T > - 40C, 100%
< RH i < 145% Lower nucleation rates, larger ice crystals,
higher fall speeds, lower IWP & cloud coverage, more OLR.
Frozen droplet
Slide 4
KEY POINTS Cirrus CE is not SRM, but can be called RM or TRM
Assumes homogeneous nucleation. Since ice crystals are produced at
a lower rate with heterogeneous nucleation, introducing ice nuclei
will result in larger ice crystals with higher fall speeds. Higher
ice fall speeds reduce cirrus cloud coverage and ice content, which
allows more outgoing longwave radiation (OLR) to escape to space.
These higher ice fall speeds also remove ice from the upper
troposphere (UT) at a higher rate, dehydrating the UT and thus
allowing additional OLR to escape to space. Global climate model
simulations show that this process can cool the planet by about 2 W
m -2 on average, or a mean global surface cooling of 1.4K.
Slide 5
Cirrus CE will only work if homogeneous ice nucleation
dominates ice production in non-anvil cirrus clouds. 1.Evidence
from field campaigns show that either homo- or heterogeneous ice
nucleation may dominate, and this may depend on mineral dust
concentration that depends on latitude and season. Recent findings
suggest ~ 90% of cirrus are formed through heterogeneous
nucleation. But in what locations were the field campaigns?
2.Storelvmo and Herger (JGR, 2014) show that seeding 15% of the
globe having the highest solar noon zenith angles at any given time
yields the same global mean cooling as a seeding strategy that
involves 45% of the globe, cooling the planet by 1.4C on average.
Field campaigns were generally not conducted at these high
latitudes where mineral dust concentrations should be minimal. 3.To
determine the scientific feasibility of cirrus CE, we need to know
whether homogeneous nucleation dominates at high latitudes (e.g.
the Polar Regions), and if so, is there a seasonal dependence? The
most practical way to answer these questions may be to use
satellite remote sensing, but that requires a means of evaluating
the mode of ice nucleation (i.e. homo- or heterogeneous
nucleation).
Slide 6
Evidence of homogeneous freezing nucleation? Results from
SPARTICUS over the central USA. PSD were averaged over 5C
temperature intervals. Homogeneous nucleation is possible when T
< -40C, and is associated with higher concentrations of small
ice crystals.
Slide 7
A Proposed Satellite Retrieval for Ice Nucleation Mode
Assessment The effective absorption optical depth ratio, eff :
Sensitive to small ice crystals eff = abs (12 m)/ abs (11 m) or abs
(12 m)/ abs (10.5 m) adjusted for scattering Channels for CALIPSO
IIR microphysical parameter PSD example from Cotton et al. (2012,
QJRMS) with T = -38C & sampled 2.75 km below cloud top in
synoptic cirrus. PSD is composite from 5 probes, sampled over
northern Scotland. Ice crystal size dependence of the wave
resonance contribution (i.e. tunneling) to absorption efficiency
(which determines eff ). Wavelength dependence of Q abs for 2 PSD
with and without small crystals. The difference in Q abs between
PSD at the indicated IIR channels is due to wave resonance. Note
that eff is also defined as Q abs (12 m)/Q abs (11 m), adjusted for
scattering effects.
Slide 8
For T < -60C, N for continental anvils appears consistent
with homogeneous nucleation, but not N for maritime anvils. Scatter
is reduced by using N/IWC. Near cloud top N/IWC may depend mostly
on the ice nucleation rate, while further below it depends on both
ice nucleation and ice particle mass growth.
Slide 9
Krmer et al. (AMS Cloud Phys. Mtg.) found the ice nucleation
mode can be discriminated based on IWC and RH, with higher IWC at a
given T associated with homogeneous nucleation. Heterogeneous
nucleation? Homogeneous nucleation? Heterogeneous nucleation? A
surprisingly tight relationship was found between eff & N/IWC
that can be used to retrieve N/IWC. A tentative condition for
homogeneous nucleation might be eff > 1.15. Another tentative
condition for homogeneous nucleation might be IWC > ~ 510 -4 g m
-3 for T < -55C, following recent results from Krmer et al.
Slide 10
homogeneous heterogeneous Based on these criteria, synoptic
cirrus sampled during SPARTICUS appear to be formed through
heterogeneous or a mixture of homo- and heterogeneous nucleation
near cloud top. Temperature dependence of the ice particle number
concentration N for synoptic mean PSD (left). PSD have been
averaged over 5C intervals. Right panel shows the same N after
normalizing it by the mean PSD IWC. Vertical bars are standard
deviations; points without bars are based on less than 3 PSD.
Slide 11
Global views of eff (along with cloud emissivity and brightness
temperature) in mid-June from GOES-11; courtesy of Andy Heidinger,
Univ. Wisconsin, USA
Slide 12
A Possible Satellite Retrieval Method 1.Using the Imaging
Infrared Radiometer (IIR) and CALIOP lidar aboard CALIPSO, retrieve
eff and the IWC profile in the upper cloud. 2.Based on eff and the
IWC profile, estimate whether homo- or heterogeneous nucleation is
dominating. 3.Determine whether this discrimination can be
facilitated using the lidar depolarization ratio (an ice particle
shape index). 4.Regardless of how this affects climate engineering,
this knowledge is needed for accurate assessments of the cirrus
cloud climate-radiative feedback.
Slide 13
SUMMARY & END OF TALK
Slide 14
From Storelvmo & Herger, JGR, 2014. MAM JJA SON DJF
HOMHET_5% (5% of dust particles produce IN) HOMHET_50% (50% of dust
particles produce IN) HOMHET_100% (100% of dust particles produce
IN)
Slide 15
MODIS retrievals over ocean near Costra Rica during the TC4
campaign, based on the 11 & 12 m channels. eff calculated from
in situ (ice PSD) measurements in maritime (TC4) and continental
(SPARTICUS) cirrus. Retrieved and in situ TC4 eff values are
consistent for T > -55C. Retrieved eff might not be sensitive to
uppermost cloud if IWC is very low.