Numerical experiments on cloud streets in the lee of island arcs

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Numerical experiments on cloud streets in the lee of island arcs
Numerical experiments on cloud streets in the lee of island arcs during cold-air outbreaks
Hiroaki Kawase
Graduate School of Life and Environmental Sciences,
University of Tsukuba, Tsukuba, Japan
Tomonori Sato1
Japan Science and Technology Agency, Kawaguchi, Japan
Fujio Kimura2
Frontier Research Center for Global Change, Yokohama, Japan
______
1
Present Affiliation: Life and Environmental Sciences, University of Tsukuba, Tsukuba,
Japan.
2
Additional affiliation: Life and Environmental Sciences, University of Tsukuba, Tsukuba,
Japan
Abstract
Numerous clouds streets often appear in the lee of island arcs during cold-air outbreak.
These clouds are quite similar each other in shape and appearance. We found that two
different kinds of cloud streets are coexistent in them, one is formed by a mountain and
another is formed by the temperature contrast between islands and the sea. Since difference
in the characteristics between them is small, it is difficult to simply discriminate by satellite
images. However, two kinds of the cloud streets can be observationally classified by the
statistics of the satellite images with the help of numerical experiments.
1. Introduction
During winter monsoons, many cloud streets are frequently observed around the islands
of Japan. Similar cloud streets appear off the eastern coast of the United States [Grossman,
1990] and over the Bering Sea [Walter, 1980] during cold-air outbreaks. Figure 1 shows the
infrared image obtained by Geostational Meteorological Satellite (GMS) at 06 JST, 18
February 2001. Thick cloud streets (hereafter referred to as cloud streets) appear over
southern ocean of Japan, for example, in the lee of Mt. Tsurugi and in the lee of the Kii
Channel.
The frequency of cloud streets depends on the ambient wind speed and direction at 850
hPa [Kawase and Kimura, 2005; Nishina, 1984; Kurosaka, 1982]. Using numerical simula-
tions, Kang and Kimura [1997] investigated the generation mechanism of cloud streets in the
lee of an isolated mountain. They reported that two factors, static instability and a
topographically induced mechanical disturbance, are required to generate cloud streets. They
also stated that heat flux from the sea surface maintains the cloud streets in the lee of
mountains. When the ambient wind goes over a mountain, part of the airflow sometimes goes
around the summit and generates vortices in the lee of the mountain. The lee vortices cause
the development of clouds. Wind speed, static stability, and mountain height dominate the
criteria for whether wind goes around a mountain. Triggering of cloud streets are controlled
by the mountain wave, which has been discussed by many scientists. Smith [1980] obtained
flow patterns passing over an isolated obstacle with a linear theory. Smolarkiewicz and
Rotunno [1989] explained the generation mechanism of lee vortices using numerical
simulations and field observations.
This paper states that two different kinds of cloud streets are coexistent in the cloud
streets in the lee of island arcs, one is formed by a mountain and another is formed by the
temperature contrast between islands and the sea.
2. Numerical Experiments
The numerical model applied to this study is a modified Regional Atmospheric
Modeling System (TERC-RAMS), presented by Yoshikane et al. [2001] and Sato and Kimura
[2004]. The original RAMS was developed at Colorado State University [Pielke et al., 1992].
Figure 2 shows the simulating domains of the two-way nested model: (a) a coarse grid system
with 12 km grid intervals and (b) a fine grid system with 3 km grid intervals. The height of
the model atmosphere, which is divided into 30 layers, is 19 km above sea level. The initial
condition for the coarse grid system is interpolated from the global reanalysis data provided
by the National Center for Environmental Prediction (NCEP reanalysis data) at 2100 JST, 17
Feb 2001. The lateral boundary conditions of the coarse grid system are also interpolated from
NCEP reanalysis data given at six-hour intervals. The sea-surface temperature (SST), which
was assumed to be constant during the integration period, is specified using the monthly mean
Reynolds SST data for Feb 2001.
Four numerical experiments were conducted in this study. Table 1 shows the
specifications of the numerical experiments. A CTRL run was carried out to reproduce the
cloud streets. Numerical experiments without mountains (F run and F-WGST run) were
carried out to clarify the importance of mountains. To investigate the importance of the
thermal contrast between land and sea, two more numerical experiments, M-WGST and
F-WGST, were conducted with a constant warm ground surface temperature (GST) which
roughly corresponded to the SST around the land.
Figure 3 shows the horizontal distribution of vertically integrated cloud water at 0600
JST, 18 Feb 2000, obtained by a CTRL run. Cloud streets in the lee of the Kii Channel and in
the lee of Mt. Tsurugi can be simulated by the model. These correspond to two cloud streets
observed in the satellite image shown in Fig. 1. The model results indicate another cloud
street in the lee of Mt. Ishizuchi similar to that in the satellite image. Figure 4a indicates the
distribution of convergence on the lowest layer simulated in the CTRL run at 0600 JST, 18
Feb 2000. The convergence lines can be seen in the lee of the Kii Channel and also in the lee
of Mt. Tsurugi; these lines correspond well to the simulated cloud streets shown in Fig. 3.
Figure 4b indicates the distribution of convergence calculated by the F run, which was
performed to investigate the mechanical effects of mountains. No convergence line appears in
the lee of Mt. Tsurugi, while a convergence line appears in the lee of the Kii Channel as well
as in the CTRL run.
The convergence line in the lee of Mt. Tsurugi appears in the CTRL run but not in the F
run. This fact means that the mountain effects, which seem to be the mechanical effects of
mountains, generate this convergence line. Since the cloud street appears in the lee of the Kii
Channel even in the F run, the mechanical effects of mountains do not play an important role
in generating this cloud street. The contrast between land and sea, including the differences in
temperature and roughness, seems to be more important than the mountain’s effects on cloud
streets in the lee of the Kii Channel.
The difference between GST and SST was estimated to be about 17 K in the F run at 06
JST, 18 Feb 2000, when the cloud street clearly appeared. In order to investigate the effects of
the thermal contrast, another flat experiment, an F-WGST run, was conducted assuming a
warmer GST, which resulted in a negligible thermal contrast.
Figure 4c shows the horizontal
distribution of convergence simulated by the F-WGST run. The convergence line in the lee of
the Kii Channel had disappeared. No cloud street was generated in the experiment assuming
neither mountains nor a thermal contrast between land and sea.
The M-WGST run was also conducted with realistic topography but without the
thermal contrast between land and sea. In this run, the convergence line clearly appeared in
the lee of Mt. Tsurugi, but it did not appear in the lee of the Kii Channel (figure not shown).
Similar numerical experiments were conducted focusing on the cloud streets in the lee
of Ise Bay. Some numerical experiments indicated that the cloud streets simulated in the lee of
Ise Bay had similar characteristics to those which formed in the lee of the Kii Channel (figure
not shown).
3. Discussion
The numerical experiments described above suggest that the thermal contrast is the
main trigger of cloud streets in the lee of channels and bays, while mechanical force is the
main trigger of cloud streets in the lee of mountains. The contrast in surface roughness
between land and sea does not seem to significantly affect the formation of cloud streets.
Several papers have investigated the effect of thermal contrast on cloud streets. Sikora
et al. [2001] examined anomalous cloud lines (ACLs) appearing in the Chesapeake and
Delaware Bays in the USA. They presented the hypothesis that ACLs are formed by the same
mechanism of the cloud lines in the Great Lakes [Hjelmfelt, 1990]. When northerly cold air
flows over an inland sea (about 20 km wide), land breezes formed on both sides of the inland
sea converge there, and then a convection system develops along the convergence line over
the inland sea. However, they did not substantially validate their hypothesis.
Nagata et al. [1986] investigated the formation mechanism of a convergence cloud band
appearing east of the Korea Peninsula in the Sea of Japan. They conducted numerical
experiments and indicated that the land-sea contrast in thermal properties between the
peninsula and the Sea of Japan plays the leading role in the formation of the convergent cloud
band. The horizontal scale of this convergence cloud line was so large that it was difficult to
discuss the similarity to the cloud streets in the lee of channels and bays.
Kawase and Kimura [2005] state that the cloud streets that form in the lee of the Kii
Channel and Ise Bay tend to appear more frequently when the ambient wind is weaker, while
the cloud streets in the lees of the Shikoku and Kyushu Mountains do not show such a
tendency. Their results also suggest that the generation mechanism of these cloud streets
seems to be different in the lee of mountains than in the lee of channels or bays.
The formation of cloud streets should be affected by the diurnal variation of the land
surface temperature if the thermal contrast generates cloud streets. By conducting an
additional numerical experiment, we found that the convergence line becomes clearer at night
than during the day in the lee of the Kii Channel (figure not shown) as the thermal contrast
during the night is more remarkable than that during the day. This experiment not only
supports the hypothesis that thermal contrast is a main trigger of a cloud street in the lee of a
channel and the diurnal variation of cloud streets, but it also gives an observational method
for validating the mechanism.
In order to investigate the diurnal variation of the cloud streets, we analyzed GMS
infrared images obtained during January, February, and December 1997 to 2001. The GMS
images were classified by the ambient wind direction and velocity. The ambient wind was
estimated by the area-averaged wind (30N-35N, 132.5E-137.5E) at 850 hPa, which was
obtained from NCEP reanalysis data. The estimated six-hour ambient wind was interpolated
to hourly data. The diurnal variation was investigated in only one specific category, in which
the wind speed was between 10 m/s and 20 m/s and the wind direction was between NW and
WNW, because Kawase and Kimura [2005] previously showed that cloud streets most
frequently appear in this category. When no cloud street could be observed in the analysis
domain (Figure 2b), the images were rejected from this analysis. As a result, 242 images were
selected during the daytime (from 1100 to 1600 JST), and 306 cases were selected during the
nighttime (from 0100 to 0600 JST).
The threshold of the equivalent black body temperature (TBB) was assumed to be 265
K to detect clouds, which is sufficiently lower than the lowest SST. Cloud detection occurred
only over the sea since the land surface temperature was sometimes lower than the threshold,
making the detection of low-level clouds difficult.
Figure 5 shows the difference in the frequency of clouds between the daytime and
nighttime. The lighter shades indicate higher cloud frequencies during the night than during
the day. Clouds appear more frequently at night in the Kii Channel and the Bungo Channel.
On the other hand, the difference in cloud frequency between day and night is small in the lee
of Mt. Tsurugi as long as the downwind distance is short.
Over the ocean and far from land, clouds appear more frequently at night than during
the day. The difference in the frequency of clouds between day and night is caused by the
static stability of the lower level atmosphere above the sea, which has a weak diurnal
variation. At night, the low-level temperature tends to be lower than that during the day, while
the sea-surface temperature remains nearly constant. As a result, at night, the atmosphere
becomes slightly less stable, convection develops more easily, and the cloud frequency
becomes higher.
The diurnal variation of cloud streets obtained by satellite data analysis agrees well
with the results of the numerical experiments. The thermal contrast between a channel and
surrounding land generates land breeze circulations. Since land breeze circulation is stronger
at night, convergence appears more clearly in a channel because the thermal contrast between
land and sea is larger at night. This fact means that the thermal contrast between a channel and
the surrounding land plays a critical role in cloud street formation in the lee of a channel. This
mechanism gives a trigger effect similar to the mechanical disturbance in the lee of a
mountain, which generates cloud streets there. As a strong sensible heat flux from the sea
surface maintains the convection [Kang and Kimura, 1997], cloud streets extend a long
distance from a channel or bay.
4. Conclusion
We found that two different kinds of cloud streets are coexistent in the cloud streets in
the lee of the island of Japan, one is formed by a mountain and another is formed by the
temperature contrast between islands and the sea. Since the difference in the characteristics
between them is small, it was difficult to simply discriminate by satellite images without
assistance from numerical experiments. The numerical experiments suggested that there
should be a small difference in the diurnal variation of frequency of the cloud formation
between them. Actually, the statistical analysis of satellite data indicates that cloud streets
formed by the thermal contrast appear slightly more frequently at night than during the day.
Acknowledgments
We thank Drs. Y. Hayashi, H. L. Tanaka, H. Ueda, and other members of the University of
Tsukuba for their helpful suggestions.
Reference list
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Figure captions
Fig. 1
GMS infrared image at 06 JST 18 in Feb 2000.
Fig. 2
Model domains for (a) a coarse grid system (grid interval, 15 km) and (b) a fine grid
system (grid interval, 3 km). The contour interval is 300 m.
Fig. 3
The distribution of integrated cloud water simulated by CTRL run at the same time
as Fig.1, 06 JST on 18 Feb 2000.
Fig. 4
The distribution of convergence at the near surface simulated by three different
models at 06 JST, 18 Feb 2000. a) CTRL run, b) F run, and c) F-WGST run. The convergence
zone is shaded.
Fig.5
The difference in the mean frequency of clouds during the day and at night. The
lighter shade shows a higher cloud frequency at night than during the day.
1
Table 1. Specification of numerical experiments
Run name
CTRL run
F run
M-WGST run
F-WGST run
Ground Surface Temperature
predict
predict
constant warm temperature
constant warm temperature
Orography
with mountain
flat
with mountain
flat

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