Observations of Sahara desert dust glaciating clouds at

Observations of Sahara desert dust
glaciating clouds at -20ºC compared to
-35ºC for dust-free similar clouds
Daniel Rosenfeld and Alon Ronen
July 2007
Dust as Ice Nuclei
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Desert dust starts to be active as ice nuclei (IN)
at temperatures colder than -11°C [Schaefer,
1949]. Most of dust minerals are IN at -21°C.
¾
about 70% of the nuclei in the centers of snow
crystals were clay minerals [Kumai, 1976]
¾
Saharan dust can act as both small and giant
CCN, depending on size and composition
[Rosenfeld et al., 2001]
The A-Train
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Satellite
constellation
orbiting earth
at 705 km
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Aqua, CloudSat,
CloudSat Calipso, PARASOL & Aura
Sun-synchronized orbit. equatorial pass at 1:30pm
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http://www-calipso.larc.nasa.gov/about/atrain.php
2007 03 09 12:10 UT
2007 03 09 9-16 UT
2007 03 09 16:00 UT
A
B
2007 03 09 12:10 UT
C
A
C
B
B
2007 03 09 12:10 UT
B
C1
C2
C1
C2
B
C1
Ice virga from water clouds
D
C
A
B
C
D
2007 03 09 12:00 UT
B
A
B
C
D
2007 03 09 12:00 UT
2007 02 22 10-14 UT
2007 02 22 10-14 UT
2007 02 22 10-14 UT
A
B
-30ºC
C
B
-35ºC
C
-21ºC
A
NOAA/AVHRR 2007 02 22 12:36 UT
MSG 2007 02 22 10-14 UT
F
E
F
E
D
D
NOAA/AVHRR 2007 02 22 12:36 UT
Layer clouds that glaciate at -12ºC
The picture is not from this case and is only for illustration
D
D
NOAA/AVHRR 2007 02 22 12:36 UT
E
E
F
F
Layer clouds that glaciate at -20ºC
The picture is not from this case and is only for illustration
E
F
2007
2007 02
02 22
22 12:36
12:36 UT
UT
Conclusion
¾
¾
¾
¾
¾
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Dusty thin layer clouds are composed of small drops.
These layer clouds glaciate at -20 to -21ºC.
Similar dust-free layer clouds glaciate at -35ºC, apparently
by homogeneous freezing.
Thicker dusty layer clouds have small drops, and glaciate
at -10 to -13ºC.
Dusty convective clouds have large drops.
The most vigorous convective elements glaciate at -20ºC.
¾
Desert dust appears to start glaciating clouds at -10ºC,
and induces a complete glaciation below -20ºC.
¾
Desert dust dissipates super-cooled clouds by snow-out.