MICROPHYSICAL CHARACTERIZATION OF THE ISRAELI CLOUDS
FROM AIRCRAFT AND SATELLITES
Ronen Lahav and Daniel Rosenfeld
Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel
1. INTRODUCTION
Preliminary results of extensive cloud physics
measurements in Israel are reported. These
measurements were done during the 1998-1999
winter as part of the research program of the
Israeli rain enhancement project.
In this study we tried to document the cloud
evolution in a way that would be easy for
comparison with NOAA/AVHRR data. The
AVHRR microphysical inferences were validated
by the in-situ measurements. Based on 10
flights, we were able to identify four major factors
determining the cloud microstructure in Israel,
which are used for a microphysical classification
of the clouds.
2. THE AIRCRAFT MEASUREMENTS
Data was obtained from the Israeli King air
C90 cloud physics aircraft, in which the authors
were the flight scientists.
The main instruments used in this study were
SPP-20, 2DC, King hot wire cloud liquid water
contents, temperature, dew point, pressure, GPS
and ball variometer. The data system was the
SEA-200.
c. After reaching the target area we penetrated
young growing convective elements in vertical
steps of 1000 feet from cloud base to the altitude
were the SLWC (supercooled liquid water
content) completely depleted or up to the cloud
top if water persisted there. The parameters that
were analyzed are cloud liquid water content
(CLW); maximum up and down drafts; 2DC
particle types and their maximum sizes; drop
size distribution and their number concentrations;
temperature and dew point.
d. Monitoring clouds separately over sea and
inland, for documenting potential differences
between the two areas.
The AVHRR and TRMM overpasses within 2
hours of the flight were analyzed. The analyses
were done using the methods of Rosenfeld and
Lensky (1998), and Rosenfeld (1999; 2000)
3. RESULTS
2.1 Data Processing
All the research flights were conducted in the
same way so we could compare them to each
other.
a. The takeoff was from Sde-Dov, which is
located just 200 meters from the Tel Aviv coast
line, in a parallel orientation. Therefore sea spray
was readily detectable by the SPP-20, starting
from the takeoff run.
b. We flew to the target area passing and
penetrating the cloud bases.
_________________________________________
Corresponding author’s address: Ronen Lahav,
Inst. of Earth Sciences, The Hebrew University of
Jerusalem, Jerusalem 91904, Israel;
E-Mail: ronenl@vms.huji.ac.il.
FIGURE 1. Cloud droplet concentration [cm-3] as
function of depth above cloud base, as determined by
the temperature. Cloud base is at 70 C over the sea
and 40 C over land.
Figures 1 and 2 show the large difference
between the land and the sea as measured at
15.1.99. The air in this day was hazy, probably
from local air pollution. No sea spray was seen
or detectable. Fig 1 shows that inland the drop
concentration is higher than over the sea, with
local concentration above 1000 drops cm -3 near
cloud base decreasing with height. Over the sea
the concentration varies between 200-350 drops
cm-3. Fig 2 shows the difference in the effective
radius of the same case. Over the land the
effective radius is smaller by about 2-3 μm for
the same depth, as obtained from the
temperature relative to cloud base temperature.
Fig 3 shows that the satellite retrieved r e
(effective radius) decreases when moving from
sea inland, following the aircraft measurements.
The clouds over the sea exceeded the 14-μm
precipitation threshold at -20C isotherm while
inland it barely reached it at -110C isotherm. The
differences between the land and the sea are
probably due to the local air pollution above the
land.
Figures 4-8 show a case of clouds forming in
air mass containing desert dust limiting the
visibility to 5 km. The flow was from the sea
inland. The dust was originated over North
Africa,
and
moved
through
the
east
Mediterranean to Israel. The strong surface wind
(SW, 20 knot) caused a stormy sea with much
sea spray.
FIGURE 2. Same as Fig 1, but for the SPP-20
measured cloud droplets effective radius [μm].
990115
-30
FIGURE 4. Cloud Droplet concentration [cm-3] as
measured by the SPP-20 over the Sea and Land on
21.3.99.
Sea
Land
o
T [ C]
-20
-10
0
10
0
5
10
15
r
eff
20
25
30
35
[m m]
FIGURE 3. The 30th percentile effective radius [μm]
of cloud droplets as measured by the NOAA-14
Satellite over Sea and Galilee (inland) on the 15.1.99.
The vertical line is the 14-μm precipitation threshold.
Fig 4 shows that the concentration is changing
in the range 300-600 droplets cm -3 and there is
almost no differences between sea and land.
However according to Fig 5 the re over land is
smaller by 2 μm than over the sea.
Figures 6 and 7 show the drop size distribution
at different depths above cloud base, as
indicated by the temperature relative to cloud
base temperature. The distribution over the sea
is wider. We suggest that the sea spray is
responsible for that. Because the desert dust
prevailed equally over sea and land, it is unlikely
that the desert dust was responsible for widening
the distribution over sea. When the clouds are
moving from sea inland they lose their large CCN
(sea spray), but remain with the desert dust, and
so become more continental.
-1
Concentration [cm micron ]
990321dsd_sea
DSD +5.0
100
DSD +1.7
DSD -2.6
-3
10
1
0.1
0.01
0.001
0
10
20
30
40
50
Drop diameter [micron]
FIGURE 7. The Drop Size Distribution as measured by
the SPP-20 over the sea on the 21.3.99.
990321
-30
FIGURE 5. The effective radius [μm] of cloud droplets
as measured by the SPP-20 over the Sea and Land
on 21.3.99.
1. Sea
2. Israel
3. Jordan
T [ C]
-20
o
-10
DSD +2.5
100
DSD -2.0
0
DSD -5.0
10
-3
-1
Concentration [cm micron ]
990321dsd_land
1
10
0
5
10
15
r
0.1
0.01
0.001
0
10
20
30
40
50
Drop diameter [micron]
FIGURE 6. The Drop Size Distribution as measured by
the SPP-20 at a distance of about 40-km inland on the
21.3.99
eff
20
25
30
35
[m m]
FIGURE 8. The 30th percentile effective radius [μm] of
cloud droplets as measured by the NOAA-14 Satellite
over Sea, land (west of the Jordan River) and Trance
Jordan on 21.3.99. The vertical line is the 14-μm
precipitation threshold.
4. DISCUSSION AND SUMMARY
Based on analyses of all 10 flights, we were
able to identify four major factors determining the
cloud microstructure in Israel:
According to the satellite analysis presented in
Desert dust.
Fig 8, the re decreased moving from sea inland1st.
Continental aerosols that are not desert
and farther east over Jordan. The clouds at the -2nd.
dust (air pollution).
100C isotherm exceeded the 14-μm precipitation
Sea spray.
threshold (Rosenfeld and Gutman, 1994) over3rd.
4th.
CCN-lean Clean Maritime air.
sea, barely reached it over Israel, and were
mostly below it over Jordan.
Based on those factors we classified the
Israeli clouds into the following types:
1. Maritime clouds in clean marine air.
2. Continental clouds in locally polluted air
without desert dust.
3. Continental clouds in CCN rich Mediterranean
air.
4. Clouds with warm rain in air with non-desertdust-haze.
5. Clouds in air with desert dust, without warm
rain.
6. Clouds in air with desert dust, with warm rain.
We characterized the microphysical properties
of the clouds in each flight. According to that we
found that Maritime clouds in clean marine air
with low concentration of CCN (cloud type 1)
changes to Continental clouds in the center of
Israel (cloud type 2) 10-20 km after crossing the
coast line. Because instability over sea is at least
as strong as over land, the likely explanation of
the differences is the effects from the locally
polluted air. Occasionally the air arrives from the
sea already rich with CCN, forming cloud type 3.
In days with warm rain and poor visibility due
to non-desert-dust-haze (cloud type 4), the cloud
base drop size distribution (DSD) was wide, yet
containing large concentrations of droplets. The
DSD narrowed gradually as the clouds moved
farther inland until most of the warm rain
processes ceased at the eastern border of Israel.
The reason for that may be the wash out of the
large and giant CCN originated as sea spray,
and the added continental aerosols from local
sources.
Convective clouds in heavy dust storm,
limiting the visibility to less than 2 km, had no
warm rain, and up to 3 g m-3 of liquid water were
observed up to -200C. At colder temperatures
that water froze quickly. These clouds are
defined as cloud type 5.
In days where desert dust was observed with
warm rain (cloud type 6), the warm rain
decreased gradually inland, transforming the
clouds into cloud type 5. The warm rain was not
likely caused by desert dust, but rather from the
sea spray that was depleted inland. We suggest
that clouds in hazy (cloud type 4) and dusty air
(cloud type 6) became continental inland slower
than clouds in maritime clean air (cloud type 1)
because the slow deposition of the large CCN as
compared to the fast inclusion of the continental
small CCN.
Those observations help us to understand
better the causes for the large variability of Israeli
clouds. This understanding will lead eventually to
the possibility to assess the suitability of the
clouds to the various possible treatments for rain
enhancement: hygroscopic seeding, glaciogenic
seeding, or no seeding at all.
5. ACKNOWLEDGEMENTS
This study is supported by the Israeli Water
Commission, and by Electrical Mechanical
Services, a subsidiary of MEKOROTH, the Israeli
National Water Company.
6. REFERENCES
1. Rosenfeld, D, and I. M. Lensky,
1998: Satellite based insights into
precipitation formation processes in
continental and maritime convective
clouds. Bull. Amer. Meteor. Soc.,
79, 2457-2476.
2. Rosenfeld, D., and G. Gutman,
1994:
Retrieving
microphysical
properties near the tops of potential
rain clouds by multispectral analysis
of AVHRR data. J. Atmos. Res.,
34,259-283.
3. Rosenfeld,
D.,
1999:
TRMM
observed first direct evidence of
smoke from forest fires inhibiting
rainfall. Geophys. Res. Lett. 26,
3105-3108.
4. Rosenfeld, D., 2000: Suppression of
rain and snow by urban and
industrial air pollution. Science. 287,
1793-1796.