Transcript Satellite Remote-Sensing of Small Ice Crystal Concentrations in

Satellite Remote Sensing of Small Ice
Crystal Concentrations in Cirrus Clouds
David L. Mitchell
Desert Research Institute, Reno, Nevada
Robert P. d’Entremont
Satellite Meteorology Group
Atmospheric and Environmental Research
Lexington, Massachusetts
R. Paul Lawson
SPEC, Bouider, Colorado
Further details of this work may be found at
http://www.dri.edu/Projects/Mitchell/
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Problem 1: Instruments aboard aircraft that measure
the ice particle size distribution (PSD) cannot
accurately measure the concentrations of small ice
crystals. Larger ice particles may shatter on the
probe inlet, producing tiny artifact ice crystals.
Problem 2: Global climate models are very sensitive
to the concentrations of small ice crystals in cirrus.
Possible solution: Improved instruments + new
remote sensing methods that estimate small ice
crystal concentrations.
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How Photon Tunneling Can be Used to Remotely
Detect Small Ice Crystals in Cirrus
Photon tunneling is the process by which radiation
beyond the physical cross-section of a particle is
absorbed. Tunneling is strongest when:
1) Effective size and wavelength are comparable
1) Particle shape is spherical or quasi-spherical
1) The real refractive index is relatively large
Therefore tunneling contributions at terrestrial
wavelengths are greatest for smallest (D < 60 μm)
ice crystals. To detect the tunneling signal is to
detect small ice crystals.
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Size dependence of tunneling for single ice crystals
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Ice particle size distribution (PSD) scheme
for evaluating potential tunneling contributions
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Decomposition of the two PSD modes
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MADA calculation using climatological ice crystal shapes
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Method for Estimating Small Crystal Concentrations
- using effective emissivities -
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TC4 Case Study - Sunday 22 July 2007
18 UTC Sun 22 Jul 07
200-mb NWP Winds
Cloudsat (radar)
overpass
Aircraft flight
tracks
Flights between
1400 and 2000 UTC
0 2
4
6
8
10
6-hr Precipitation (mm)
12
14
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2D-S Probe for measuring ice particle size distributions
From Lawson et al. 2006, JTech.
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Blue = cirrus; white = CBs; yellow = low cloud
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MODIS Retrieval Results
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MODIS Retrieval Results
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2D-S measurements for 22 July appear consistent
with retrieved estimates of small-mode contributions
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MODIS Retrieval Results
homogeneous
freezing nucleation
begins?
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Summary
1.
The absorption of radiation by photon tunneling affects the small mode
of the PSD but not the large mode, making thermal emission due to
tunneling an ideal signal for detecting small ice crystals.
2.
The tunneling signal is the emissivity difference between 12 and 11 μm
wavelength. This was not previously understood.
3.
A satellite retrieval methodology has been developed whereby it is
possible to retrieve the small-to-large mode ice crystal concentration
ratio, the ice crystal concentration for a given IWC, the IWP and De. The
PSD is also estimated, even when it is bimodal.
4.
Retrievals using a new satellite remote sensing methodology suggest a
pronounced PSD small mode is often a measurement artifact.
5.
The retrieved dependence of De and N on cloud temperature appear to
support the homogeneous freezing nucleation mechanism. The
absorption optical depth ratio (12 to 11 µm) of 1.08 could be used as a
metric for the new MG microphysics.
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22 July 2007 Case Study: MODIS Retrieval Results
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Remote Sensing of Small Ice Crystals (D < 60 μm) and the
Process of Photon Tunneling
Incident Ray
Tunneling depends
on real refractive
index, which
changes abruptly
between 11 and 12
μm wavelength.
Tunneling can
account for up to
45% of absorption
in ice clouds for
terrestrial
radiation.
Tunneling
Surface Wave
Critical Angle
Diffracted Ray
Surface
Wave
Large-angle Diffraction
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MODIS Retrieval Results
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Cloud Optical Properties Depend on the Ice Crystal
Mass- and Projected Area-Dimension Relationships
That Characterize the PSD
β
•
Mass = m = α D
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Projected area = P = γ D
σ
Constants giving P & m = function of temperature for
mid-latitude cirrus. Based on 22 cirrus flight missions,
104 horizontal legs and 15,000 km of in-cloud sampling,
and Heymsfield et al. (2007).
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Method for Estimating Small Crystal Amounts
• Begin with satellite retrievals of cloud temperature and emissivity (ε)
at 11 and 12 μm wavelength channels.
• Use the retrieved cloud temperature to estimate the PSD mean sizeD
and dispersion (ν) for large and small mode. Difference between the solid
and dashed curves results primarily from differences in the contribution of
the PSD small mode to the IWC. This also determines effective diameter,
De. Note that the large modeD and ν have little effect on above curves.
• Locate the retreived Δε and the ε(11 μm) by (1) incrementing the
modeled IWP to increase ε(11 μm) and (2) incrementing the small mode
IWC, which elevates the curve.
4. If all IWC is in the small mode and retrieved Δε and ε(11 μm) is still
not located, then decrease small modeD to locate them.
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Method for Estimating Small Crystal Amounts
- continued 1. If the retrieved point lies below the “large mode only” curve (e.g. a
dashed curve), then systematically decreaseD for large mode until a
match is obtained. Negative Δε values correspond to maximum allowed
large modeD values.
• This methodology retrieves IWP, De, the small-to-large mode ice
crystal concentration ratio, and the ice particle concentration for a
given IWC. It also estimates the complete PSD (even when it is bimodal).
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Tunneling Efficiencies for Different Crystal Shapes
(from Mitchell et al. 2006, JAS)
CRYSTAL
TUNNELING EFFICIENCIES
SHAPE
Small Mode of SD
Large Mode Mean of SD .
D = 14 to 20 μm D = 53 μm D = 170 μm
Aggregate
0.35
Bullet Rosette
Hex. Column
0.30
0.70
0.90
Hollow Column
0.15
0.40
0.65
0.70
0.15
0.40
0.50
0.15
Hex. Plate
0.60
0.15
0.00
Droxtal
1.00
0.80
0.40
Red circles indicate typical crystal shapes found in the small or large mode.
Efficiencies are one reason tunneling is- mostly
manifested in the small mode.
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MODIS Retrieval Results
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