The 14th International Workshop on Atmospheric Icing of Structures, Chongqing, China, May 8 - May 13, 2011

WRF ATMOSPHERIC ICING VALIDATION FOR MOUNT ZAO, JAPAN,
DURING 19-29 APRIL 20091

Evgeny A. Podolskiy1*, Bjørn Egil K. Nygaard2
CEMAGREF - Centre de Grenoble, St.-Martin-d'Heres cedex, France;
2
Norwegian Meteorological Institute, Oslo, Norway*Email: evgeniy.podolskiy@gmail.com

Abstract: A mesoscale atmospheric model Weather Research &Forecasting (WRF, ver. 3.2.1) was used to reconstruct mid-spring
episodes of soft rime formation at the Mt. Zao, Japan, during 1929 April 2009. One particularly interesting form of soft rime wasobserved at the Karitadake peak (1758 m). It was presented byfeather-looking opaque aggregates of granular ice 15-35 cm long,called “shrimp’s tale” in Japanese. Based on an analysis of modelgenerated results we could 1) find a good quantitative agreementof modeled and observed wind and temperature time series at
Jizosancho ropeway station; 2) could identify two icing events(lasting for 26-41 hours each, with temperatures between −6.3º
and −0.1ºC, relatively constant westerly strong winds up to 26 m
s-1, and cloud liquid water content up to 0.72-1.05 g m-3). By
considering the general synoptic situation for the simulation
period, we found that both in-cloud icing episodes corresponded
to a passage of low pressure systems; 3) confirm that a highresolution modeling (1.1 km) was much more accurate thansimulations with coarser grids (10 and 3.3 km).

liquid water content were not available).
2. RESULTS AND DISCUSSION
A good agreement could be found between main
meteorological elements measured at Jizosancho ropeway
station and calculated by the model in the finest domain.
Periods corresponding to predicted negative airtemperatures (°C) and cloud liquid water content higher
than zero for Jizosancho and Karitadake were used for
detection of the start and end of icing events (Fig. 1b). Two
prolonged episodes (26-41 hours) were identified on 23-24
and 26-28 of April 2009, produced by passage of two low
pressure systems over the Northern Honshu with predicted
temperatures between −6.3°C and −0.1°C, westerly winds
up to 26 m s-1 (Fig. 2), and cloud water liquid content
reaching as high as 0.72-1.05 g m-3.

1. INTRODUCTION

100

(a)

(b)

-10 ºC

0 ºC

Zao

WRF_Jizosancho
WRF_Karitadake
WRF+Jizosancho

90

80

70

Wind speed, km h-1

Mount Zao, an active volcano at the Northern Honshu(38.14º N, 140.44º E; 1841 m) is very famous for its
impressive and well studied low density ice accretion ontrees, referred in Japan as “ice monsters”, and attracting
thousands of visitors every winter. An interesting form ofsoft rime, called “shrimp’s tale” in Japanese (Fig. 1a), wasobserved at one of the peaks of the mountain (Karitadake,
1758 m) at the end of April 2009. In order to validate the
WRF model, we attempted to reproduce episodes of incloud icing for this particular locality by using 10-days
simulation with a triple nested domain configuration (10,
3.3 and 1.1 km horizontal grid spacing). We employedglobal analysis data from the Global Forecast System,
NCEP (with 1º horizontal grid spacing), derived terrestrial
data from 10′-30″USGS and MODIS datasets, and used
Thompson scheme for cloud microphysics. For groundcontrol, model-generated results were compared tomeasured day-time series from Jizosancho ski lift station,
placed 3.4 km away (measurements of icing and cloud

60

50

40

30

20

10

0
-25

-20

-15

-10

-5

0

5

Air temperature, °C

Figure 2: Joint distribution wind speed (km h-1) - air temperature(ºC) during in-cloud icing events at the Mont Belair test site,
Canada [1] and modeled for Zao (by different colors).

3. CONCLUSION
This paper summarizes results of numerical experimentwith an objective to reconstruct rime ice accretion episodes
at the Mount Zao, Japan, during 19-29 April 2009, by using
the mesoscale atmospheric WRF model. The WRF
calculations for atmospheric icing revealed a good
capability anda potentialformeteorologicalreconstructions with high degree of accuracy.
4. REFERENCES

Figure 1: (a) Photograph of a so called “shrimp’s tail” soft rime
on a wall of a temple, Mount Zao, Japan (29 April 2009);(b) WRF vertical cross section of liquid water content and airtemperature at 20:00 pm 26 April 2009 (JST), Mt. Zao, Japan.

[1]

M. Farzaneh and K. Savadjiev, “Statistical Analysis of Icing Event
Data for Transmission Line Design Purposes,” in Atmospheric Icingof Power Networks, M. Farzaneh, Ed. Springer, 2008, pp. 31–81.

WRF atmospheric icing validation for Mount Zao,
Japan, during 19-29 April 2009
Evgeny A. Podolskiy

Bjørn Egil K. Nygaard

CEMAGREF - Centre de Grenoble
St.-Martin-d'Heres cedex, France
evgeniy.podolskiy@gmail.com

Norwegian Meteorological Institute
Oslo, Norway
bjorn.e.nygaard@met.no

Abstract—A mesoscale atmospheric model Weather Research& Forecasting (WRF, ver. 3.2.1) was used to reconstruct midspring episodes of soft rime formation at the Mt. Zao, Japan,
during 19-29/4/2009. Interesting form of soft rime wasobserved at the Karitadake peak (1758 m). It was presented
by feather-looking opaque aggregates of ice 15-35 cm long,called “shrimp’s tale” in Japanese. Based on an analysis ofmodel-generated results we could 1) find a good quantitative
agreement of modeled and observed meteorological time series
at Jizosancho ropeway station; 2) could identify two icing
events (lasting for 26-41 hours each, with temperatures
between −6.3º and −0.1ºC, strong westerly winds up to 26 m s1
, and liquid water content up to 0.72-1.05 g m-3)
corresponding to a passage of low pressure systems; 3)
confirm that a high resolution modeling (1.1 km) was much
more accurate than simulations with coarser grids (10 and 3.3
km).
WRF model; in-cloud icing; rime; "shrimp's tail"; Mt. Zao;

I.

INTRODUCTION

Importance of ice accretion modeling and lack ofmeasured icing data have been discussed in many works([1]-[3]). Probably, the most quoted disaster, exemplifying
significance of atmospheric icing on structures occurred in
January 1998 in Canada, when more than two million
people were left without electricity [1]. Except transmission
lines, other risks are associated with transport and a growing
economy of clean power generation by wind in harshconditions, icing of meteorological instruments [3] or inflight hazards. Scarcity of available data and complexity
with measurements of some critically important parameters
for icing models, like cloud liquid water content (CLWC) or
median volume diameter of the droplet size distribution [3],could be overcome by use of fine-grid weather forecast
models [4]. In the present paper, we would like to perform
validation of the Weather Research & Forecasting (WRF)
atmospheric model and to demonstrate how some particularatmospheric icing events of ground-based structures at highaltitude mountain area could be identified by modelingmeans, despite an extremely small amount of supporting insitu measurements. For examples on comprehensive studies
using WRF for icing simulations the reader may refer to [4][6]. Peculiar ice deposits, discussed in this study, wereobserved on vertical surfaces at the top of Mt. Zao, Japan,
on the 29/4/2009 [7]. These accretion forms shown at thephotographs (Fig. 1) are called “shrimp’s tails” in Japanese,due to their striking resemblance to tails [8]. Such kind oficing is known to be formed from supercooled water cloud

Figure 1. Photo showing build-up of a so called ‘shrimp’s tail’ rime.

droplets colliding with objects. It is related to a so called“dry growth”, and are being called “rime” [1]. In thisparticular case - “soft rime” due to its low density 0.3 g cm-3[9]. Similar feathery structures can be recognized pretty
often at photographs from all around the globe showing
objects covered with agglomerated rime mass. Though inour case, every single formation (15-35 cm) was presented
by individual independent structure, beautifully stemmingout of a single point where it was attached to an ice-freesurface. In this particular study, we attempted to reproduce
meteorological conditions responsible for a formation ofsuch distinct discrete rime structures (when almost nothing
was known on ice accretion evolution in time).
II.

AREA DESCRIPTIONS, METHODS AND MATERIALSTUDIED

A. Study site
Mt. Zao presents an active volcano at the Northern
Honshu island, Japan (38.14º N, 140.44º E), located inapproximately 50 km from the Pacific Ocean. It is acomplex exposed ensemble of stratovolcanoes with amaximum elevation of 1841 m. Zao is famous for its
impressive and well studied low density ice accretion ontrees, referred in Japan as “ice monsters”. Mechanisms ofrime formation at Mt. Zao and its chemical-composition
could be found in [9]-[11].
B. Meteorological data
Meteorological data from Jizosancho ropeway station (N38.15°, E 140.43°) we kindly provided by Zao Ropeway
Company. This site is located 1661 m asl and about 3.4 km
northwest of our place of interest at the Karitadake peak
(1758 m asl, N 38.12°, E 140.44°), where photographs weretaken (Fig. 1). The measurements were based on hourly
instantaneous manual records of air temperature (Ta) andwind, conducted only between 7 am and 5 pm (or 6 am and6 pm), and for some days supported by operators notes [12].
The latter were particularly useful, since there were nomeasurements of precipitation, CLWC or in-cloud icing. Air
temperature sensor were installed near to the station at 3 m
above the ground; propeller type wind sensor were placed at
the top of one of ropeway towers below the station, at ~25
m above the ground (location of the tower is ~1600 m asl).
No negative effects of icing on the sensors were reported
during the period of interest.
C. Weather model WRF
The atmospheric mesoscale non-hydrostatic model
Weather Research and Forecasting (WRF, ver. 3.2.1) was
jointly developed by US National Centre for AtmosphericResearch (NCAR) and several other institutions [13]. Being
developed as a community modeling system, WRF slowly
replaces its predecessor, the MM5, and is widely used
nowadays for a wide spectrum of weather prediction andresearch applications [5]. WRF includes advanced
microphysical processes, in particular, the Thompson cloud
microphysics scheme [14], that allows to focus on in-cloud
icing. For the present study, simulation were made with atriple nested domain (10, 3.3 and 1.1 km horizontal grid
spacing) for 10 days period during 19-29/4/2009 (Fig. 2a).
Nested domains, centered over Mt. Zao, were large enoughto avoid any lateral boundary effects. We employed global
reanalysis data from the Global Forecast System, NCEP
(with 1° horizontal grid spacing and 6-hourly products), andderived terrestrial data from USGS and MODIS datasets(with 10 , 2 and 30 horizontal grid corresponding to 1st,
2nd and 3rd domains) (Fig. 2b). Thirty five model levels
were set up for vertical direction of the model, with a top at
50 hPa. Before going into icing questions, we had toperform ground control of the model results extracted at one
hour intervals. Simulated Ta and horizontal windcomponents (U, V) were compared to in-situ measurements
at Jizosancho. For a quantitative comparison, we had tomodify reported wind speed and direction into U and V
components, by assigning azimuth degree values to cardinal
directions and producing vector components.
(a)D. Method for detection of in-cloud icing episodes
Weather elements favoring formation of in-cloud icing
are known: it is Ta < 0°C, and CLWC > 0 (also meaning thatan altitude of an object of interest is above the cloud base)[2]. For ice accretion modeling purposes the most importantfactors are rate of icing and duration of icing event. We do
not attempt to estimate the former, usually obtained from anequation called Makkonen model, employed for ice
accretion on cylinders [1], due to a lack of any ice load ordensity measurements which could be used for references.
Accordingly, we will determine only meteorologicalelements, and would try to identify duration of icing
episodes by means of modeling with WRF. For details about
ice accretion estimates based on modeling results the reader
may refer to [4] and [14].
III.

RESULTS AND DISCUSSION

A. Model output and observations
To compare simulated and observed values we arereferring to the nearest grid point to real location. Validation
tests were made to determine effects produced bydifferences between the model and real elevations (causedby smoothing). The real location of the Jizosancho ropeway
station is about 1625 - 1661 m asl (see above for details),
but the model height is only 1471 m asl. Similar situation
with Karitadake peak, which is reduced by 364 m in themodel. These reductions may affect variable fields, and we
may expect harsher weather at higher altitudes, especiallyfor Karitadake. It has been shown that under suchconditions, CLWC should be extracted from the real height,
rather than the lowest model layer [6]. Some previous
studies on icing assumed equal temperatures for even higher
altitude differences between weather station and object ofinterest (300–600 m), and roughly calculated wind speed bypower-law relation [15]. We could not find any significant
differences between liquid water content extracted from thelowest model levels. The best reproduction of Ta and windobservations could be achieved at the real height modellevel (though improvements were very small comparing tosurface level estimations). Accordingly, we would bereferring to the real height model level for Ta and wind, andto the lowest model level for CLWC.

(b)

Figure 2. (a) Nesting of 3 WRF domains. (b) WRF surface topographyof the finest domain, d3, with the Pacific Ocean on the eastern boundary ofthe domain. Karitadake is indicated with a triangle, Jizosancho - by dot.

B. Ground control
We will discuss here only results produced by the finest
domain (1.1 km, d3), and just note, that the latter had themost realistic estimations. Coarser domains (10 and 3.3 km),for example, have produced similar amplitudes and shapesof the curves for Ta , but approximately 5°C higher (d1),thus showing almost no temperatures what would be below
0°C. In general, a good agreement could be found for main
meteorological elements measured at the Jizosancho andpredicted by the model, as shown on Fig. 3. However, a few
exceptions may be noticed, namely underestimated Ta aswell as overestimated on 20 and 24/4/2009, and on
25/4/2009, respectively (Fig. 3a), and wind velocities
overestimated by the model on the 26/4/2009 (Fig. 3d).
Though, we see that the model has produced realistic

estimations, including wind directions (Fig. 3b&c).
Simulated
continuoustime-series
andday-time
measurements complement one another, allowing us toconsider a continuous variation of parameters important for
icing, and to fulfill lacking night time measurements. Basing
on realistic WRF representation of meteorological elements
for Jizosancho, we also include modeled curves for
Karitadake peak (Fig. 3), and show simulated CLWC
variation (Fig. 3e) for the both sites in attempt to identify
potential periods of icing.
C. In-cloud icing
Periods corresponding to predicted negative Ta and
CLWC > 0 for Jizosancho and Karitadake are shown on Fig.
4. Two prolonged episodes are clearly seen between 23 and24/4 and between 26 and 28/4/2009. Few shorter events
could be also identified (1-3 hours for Jizosancho, and 2-10
hours at Karitadake) (Fig. 4). Duration of the two longest
episodes (for convenience called here ‘the first’ and ‘thesecond’), for Jizosancho was 36 and 41 hours, and for
Karitadake 26 and 32 hours, respectively (Fig. 4). Bothepisodes corresponded to a passage of low pressure systems(996 and 986 hPa) over the Northern Honshu area (on 22
and 26/4/2009) [16]. In general, conditions were harsher at
Karitadake, except CLWC, showing maximum values for
Jizosancho (Fig. 3e). The prevailing wind direction modeled
during icing events is from the West or WNW, withmaximum wind speeds up to 26.0 m s-1 (Fig. 3d). Fig. 5
shows vertical cross-sections of simulated Ta and CLWC
exemplifying the 1st and the 2nd icing episodes. It could bedemonstrated that Mt. Zao is above the cloud base for both
events, with maximum CLWC above the top, illustrating
orographic lifting of air (Fig. 5). Icing period at Karitadakeis slightly shorter due to not-continuous or disrupted
conditions (Fig. 4), caused by variation of CLWC (Fig. 3e).
Behavior of the latter may be associated with shading
effects of Zao peak on clouds, which is located between twosites. Both icing episodes, detected by WRF, agree well with
operators’ reports [12], noting presence of shrimp’s tails 1520 cm long on trees in mornings of 24, 27 and 28/4/2009
and recognizable on 29/4/2009 at Jizosancho (Fig. 4).
According to operators’ reports, rime, formed during the 1st
episode, fully dropped off before morning of 26/4; total
shedding of an accretion formed during the 2nd episode,happened until afternoon 30/4/2009 (Fig. 4). Shedding can
be a result not only of Ta above melting, but also of rain,wind or gravity, and present not a trivial issue for
cumulative ice load studies. Some studies use the Ta above
zero for 3 hours or more as a simple unloading criterion[15]. If we refer to predicted Ta alone (Fig. 3a), we may
suggest that shedding should have started the following day(4 hours of Ta >0), and intensified during a long warming
episode (up to +3.7°C) between 25 and 26/4. Though, as it
was already mentioned before, modeling results did not
reproduce perfectly daytime Ta measurements on 25/4 (Fig.
3a), so our conclusion has obvious limitations. Second
shedding should have started during 8 hours of positive Ta
on the next day (29/4/2009) when pictures were taken, andsimulation time was over (Fig. 1 & 3a). Ropeway operators’
report [12] noted a full drop off on the next day, 30/4/2009

(a)

(b)

(c)

(d)

(e)

(f)
Figure 3. Meteorological elements modeled for Karitadake and Jizosancho,
and measured at the latter site, during 19-29/4/2009. (a) Ta; (b) U and (c) V
components of wind (+/− correspond to W/E, or S/N directions); (d) Wind
speed; (e) modeled CLWC; (f) modeled accumulated total grid scale snow
and ice, and observed amount of new snow, reported in the morning [12].

Figure 4. Timeline with periods there modeled parameters are satisfying tometeorological conditions usually associated with in-cloud atmosphericicing. Filled triangles indicate operators’ reports about rime presence [12],empty triangles – complete shedding; arrow - time when Fig. 1 was taken.

(when Ta was +7.6°C at Jizosancho). We should notice, that
“3 hours of +Ta” approach [15] has its own limitations andseems to be too simplified, since it does not correspond torime shedding during periods than observed Ta were positive
for about 7 hours (24/4/2009) or 8 hours (29/4/2009) as it
was shown above (Fig. 3a). In order to see any differences
between published joint distributions of wind and Ta (usedfor recognition of type of icing on transmission line
conductors by field data [3],[8],[17]) and results of thisstudy, we plot the latter on Fig. 6. It should be noted, thatapproximate limits for the type of accreted ice is known todepend on the shape of icing object, and shift to the left ofthe plot (Fig. 6a) for higher values of CLWC [3]. Theselimits seem to be applicable to our estimations only partly
for the Jizosancho (Fig. 6a). It appears that parameters
obtained for Karitadake are falling mostly into ‘hard rime’or ‘glaze’ type of icing for our case (Fig. 6a). If this is right,
we have an indication that the low density ‘shrimp’s tails’
observed on buildings and other large objects are developed
under meteorological conditions that would cause hard rime
or even glaze on small cylindrical objects such as power line
conductors. An attempt to extend our dataset and to include
synthetic values (i.e., selection of data where measured wind
and Ta values corresponded to times with modeled CLWC

>0), also shows a distribution more typical for hard rime oreven glaze (Fig. 6a).
(a)

(b)

-10 ºC-10 ºC

0 ºC

0 ºC
Zao

the Northern Honshu with temperatures between −6.3° and−0.1°C, westerly winds up to 26 m s-1, and CLWC reachingas high as 0.72-1.05 g m-3. Except demonstration of a great
WRF potential, the obtained meteorological constrains may
be useful for scientific communities involved into stochastic
or physical modeling of complex rapidly evolving rime
icing objects.

Zao

ACKNOWLEDGMENT

Liquid water content, g m-3

Figure 5. WRF vertical cross sections (along latitude N 38.14°) of
CLWC and Ta on (a) 09:00 am 23/4/2009 (JST), corresponding to the first
icing episode (wind: West), and on (b) 20:00 pm 26/4/2009 (JST),
corresponding to the beginning of the second icing episode (wind: WNW).

It is not very clear how these limits (Fig. 6a) could be
employed – for example, could these be used to help us withidentification of correct icing period (e.g., group of dots
100

(b)

30

Wind speed, m s-1

Glaze

90

Hard rime

20

80

10

Wind speed, km h-1

70Soft rime
0

60

-15

-10

-5

Air temperature, ˚C

0

(a)

50
WRF_Jizosancho
WRF_Karitadake
WRF+Jizosancho

40
30
20
10
0
-25

-20

-15

-10

-5

0

5

Air temperature, ˚CFigure 6. (a) Type of icing as a function of wind speed (m s-1) and Ta(dashed lines - after [3]; solid blue lines – after [8]); blue dots - Jizosancho
synthetic selection (see text). (b) Joint distribution “wind speed (km h-1) Ta” during in-cloud icing events at the Mont Belair [17] and Zao.

with average wind speed about 5 m s-1 and Ta −5°C for
Jizosancho; Fig. 6), or to show that observed ice accretioncannot be easily classified as a soft rime by using known
criterions. To explore the icing classification issue in moredetails, we consider obtained distributions in comparison to
known data from the Mont Belair test site in Canada [17].
Fig. 6b shows that the form of these distributions represents
values typical for in-cloud icing with severe wind speeds.
Finally, it is necessary to note that conditions described here
for the Mt. Zao could be also found at other high mountains
located within the boundaries of the finest domain (d3, Fig.
2b) (suggesting that similar rime could be formed at Azuma,
Gassan, Ooasahi-dake, Iide, and Iimori Mts.).
IV.

CONCLUDING REMARKS

We conducted a numerical experiment with an objective
to reconstruct ice accretion episodes at the Mt. Zao, Japan,
during 19-29/4/2009, by using WRF model. Obtained
results illustrated that WRF had performed well, andprovided new data to existing icing measurements (e.g., [3]).
The model helped to detect two icing events 26-41 hours
long, produced by passage of two low pressure systems over

The authors wish to acknowledge O. Abe, K. Okita, H.
Ohba, K. Nishimura, A. Sakai, and H. Nagai for help. The
WRF input data for this study are from the Research Data
Archive, NCAR (http://dss.ucar.edu, ds083.2).
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