1. Introduction
Ž .
The appearance of freezing rainrdrizzle supercooled rainrdrizzle is not a rare phenomenon, especially when a rain cloud or melting layer exists above a cold layer
Ž .
near the ground surface e.g., Stewart, 1992; Stewart and Crawford, 1995; Zerr, 1997 . On the other hand, it is worthy of consideration that meteorological conditions exist
where supercooled rainrdrizzle could form under low temperatures in the mid-winter season of the Arctic regions.
Ž .
Ohtake 1963 reported that in cold regions, supercooled rain sometimes fell even when the temperature was below 08C, at and above the ground. As to the problem of
Ž .
Ž .
formation of supercooled rain, Bocchieri 1980 and Huffman and Norman 1988 reported that about 44 and 30, respectively, of the freezing rainrdrizzle examined
by radiosonde observations had no warm layer with air temperature higher than 08C. Ž
. Ž
. Recently, Strapp et al.
1996 and Stuart and Isaac
1999 compiled the detailed
climatology of freezing rainrdrizzle for North America. Moreover, in-situ aircraft observations have revealed the presence of supercooled drizzle drops in stratiform
Ž clouds with temperatures colder than 08C e.g., Cober et al., 1995; Cober et al., 1996;
. Isaac et al., 1996 .
A few researchers have observed supercooled raindropsrdrizzle drops in the mid- Ž
. winter season of the Arctic region Magono and Kikuchi, 1980; Kajikawa et al., 1988 .
However, in those observations, the formation mechanism has not been proved clearly. This paper describes some considerations regarding the meteorological conditions and
Ž
X
the characteristics of supercooled drizzle observed at Inuvik, N.W.T., Canada 68822 N,
X
. 133842 W in the mid-winter season during the experiment ‘‘Studies on Water vapor,
Ž .
Aerosols and Nuclei Transportation, and Snow Crystals in the Arctic WANTS-Arctic ’’ Ž
. Kikuchi and Asuma, 1999 .
2. Observations
Supercooled drizzle drops were photographed by a camera installed on a polarizing microscope. Traces of those particles spread on a glass plate showed nearly circular
shapes, as seen in Fig. 1. The size of drizzle drops was derived from the calibration curve obtained by a spraying method in a cold room. In addition to the glass plate
method, a catching method in a laboratory dish poured silicon oil and replica method Ž
. Kajikawa et al., 1988 were used for the evaluation of precipitation intensity and size
distribution.
3. Results and consideration
Supercooled drizzle was observed on December 20, 21 and 27, 1995. Among the three cases, the maximum precipitation intensity recorded 0.0046 mmrh at 09:05
Ž .
Ž .
L.S.T., December 27. Mean volume diameter size and maximum size D were 144
ma x
and 233 mm, 148 and 203 mm, 164 and 418 mm in the order, December 20, 21, 27.
Ž . Ž .
Fig. 1. Examples of the supercooled drizzle drops A spread on a glass plate and ice pellets B observed on Ž .
December 20, 1995, and frozen drops on snow crystal C observed on December 27, 1995.
Fig. 2 shows the diurnal variation of surface air temperature and the type of precipitation particles on December 20. In the upper part of this figure, the snow crystals
obtained by our observation are shown using the graphic symbols by Magono and Lee Ž
. Ž
. 1966 and Kikuchi and Asuma 1999 , including the supercooled drizzle drops and ice
pellets. Solid lines in this figure indicate the appearance time of the particles. The short vertical lines attached to the solid lines denote the change of riming state of crystals. The
small and large solid circles beneath the solid lines indicate the rimed crystals and the Ž
. crystals with frozen small raindrops diameter 100 mm , respectively. In the polar
Ž regions, such snow crystals with several small raindrops were observed previously e.g.,
. Kikuchi, 1972; Kikuchi and Uyeda, 1979 .
Fig. 2. Diurnal variation of the surface air temperature and precipitation particles at Inuvik on December 20, Ž
. 1995. Graphic symbols of snow crystals follow the manner of Magono and Lee 1966 and Kikuchi and
Ž .
Asuma 1999 . Solid lines and short vertical lines attached to them denote appearance of particles and change of riming state of crystals, respectively. Small and large solid circles beneath the solid lines indicate the rimed
Ž .
crystals and crystals with frozen small raindrops diameter 100 mm , respectively.
It is evident from Fig. 2 that the supercooled drizzle drops always fell with ice pellets, and snow crystals with small raindrops, even when surface air temperature was
about y238C. This situation was similar to the cases of December 21, 27. Since the formation mechanism of supercooled drizzle drops in a polar region is a
very interesting subject, for a start, it is necessary to examine the temperature and Ž
humidity profiles of the 3 days at Inuvik Upper Air Station the distance from our .
observation site, about 10 km . The vertical profiles of temperature and humidity with respect to water are shown in Fig. 3, at the nearest time that the supercooled drizzle
appeared. Ž
It can be seen from Fig. 3 that there are warm and saturated layers regarded as the .
layer of relative humidity 0 98 accompanying a temperature inversion at about 900 ; 800 hPa level in the three cases. The depth of these layers is about 500 ; 1500 m
Fig. 3. Vertical profile of air temperature and relative humidity with respect to water at Inuvik.
M. Kajikawa
et al.
r Atmospheric
Research 52
2000 293
– 301
298
Table 1 Relationship between the maximum size of supercooled drizzle drops and the features of cloud layer
Date Maximum size of
Maximum value of Maximum value of
Depth of cloud layer supercooled drizzle
liquid water pass air temperature of the layer
saturated with respect to water saturated with respect to water
Ž .
Ž .
Ž .
Ž .
D LWP
T D Z
ma x max
December 20, 1995 233 mm
0.003 cm y11.58C
690 m December 21, 1995
203 0.008
y9.9 530
December 27, 1995 418
0.010 y7.3
1460
and the temperature range is about y7 ; y178C. The wind direction of the three cases was roughly from south and east around 800 and 1000 hPa level, respectively. Thus, it is
suggested that the advection of warm air mass from the Pacific Ocean. Since a layer of air temperature higher than 08C was not detected in this analysis, the melting of snow
crystals is impossible. It is considered, therefore, that the supercooled drizzle drops were formed in the cloud layer forming in a maritime air mass over Inuvik by condensation–
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coalescence process, as indicated by Strapp et al. 1996 . Table 1 shows the relationship between the maximum size of supercooled drizzle
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Ž .
drops D and the features of cloud layer with the data of liquid water pass LWP
max
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measured by a passive microwave radiometer Ishida et al., 1998 at the ground level. Although the other factors do not indicate a clear correlation, the maximum size
increases as the depth of saturated layer increases. The relationship between the depth of Ž
. Ž
. the saturated layer D Z and the maximum size D
of supercooled drizzle drops, ice
max
pellets and frozen drops on snow crystals is shown in Fig. 4. In this figure, other data Ž
. Magono and Kikuchi, 1980; Sakurai and Ohtake, 1981 obtained in the arctic region are
Ž also shown. In these three kinds of particles, only frozen drops on snow crystals snow
. crystals with frozen drops fell independently. Except for one case, the size of the drops
is smaller than the supercooled drizzle drops and ice pellets in the same D Z.
Ž .
Ž .
Fig. 4. The relationship between the depth D Z of the water saturated layer and the maximum size D of
ma x
supercooled drizzle drops.
It is apparent from Fig. 4 that the maximum size of supercooled drizzle drops increases as the depth of water saturated layer increases. This result suggests again the
condensation–coalescence process in a supercooled cloud, as the formation mechanism of supercooled drizzle drops.
4. Conclusion
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