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Atmospheric Research 98 (2010) 426–437
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Atmospheric Research
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a t m o s
Comparison of advection and steam fogs: From direct observation over
the sea
Ki-Young Heo a, Kyung-Ja Ha a,⁎, Larry Mahrt b, Jae-Seol Shim c
a
b
c
Division of Earth Environmental System, College of Natural Science, Pusan National University, Busan, South Korea
College of Oceanic and Atmospheric Science, Oregon State University Corvallis, Oregon, USA
Coastal Disaster Prevention Research Division, Korea Ocean Research & Development Institute, Ansan, South Korea
a r t i c l e
i n f o
Article history:
Received 26 April 2010
Received in revised form 4 August 2010
Accepted 4 August 2010
Keywords:
Sea fog
Advection fog
Steam fog
a b s t r a c t
Sea fog occurs frequently over the Yellow Sea in spring and summer, which causes costly or
even catastrophic events including property damage, marine accidents, public health and
financial losses. Case studies of advection and steam fogs using direct observation over the sea
are constructed to better understand their formation, evolution and dissipation. A southerly
wind supplies moisture to initiate advection fog events (AFs). Approximately −100 to −200 W
m−2 of latent heat flux and −70 W m−2 of sensible heat flux during mature AFs are
characterized with stable stratification which maintains dense fog by limiting downward
mixing of dryer air. Steam fogs (SFs) develop from flow of cold air over warmer water, but are
normally of limited persistence. During the SFs, a northerly wind decreases the air temperature
below the sea surface temperature, which increases the relative humidity through evaporation
from the warmer ocean. During mature SF, 360 W m−2 of latent heat flux and 150 W m−2 of
sensible heat flux are characterized with neutral and unstable atmospheric conditions. The
increase in wind speed and wind shear mixes dry air downward to the surface and limits the
duration of the SF.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Sea fog is defined as a phenomenon with horizontal
visibility reduced by condensation to less than 1 km near the
surface over the ocean (Roach, 1994). Sea fog causes
transportation problems including automobile accidents,
aircraft takeoff and landing problems, and marine accidents
due to poor visibility. In addition, sea fog is one of the
immediate causes of property damage, public health and
financial losses (Forthun et al., 2006). Understanding the
characteristics of sea fog is hence an important societal issue.
Fog processes involve droplet microphysics, aerosol chemistry, radiation, turbulence, large/small-scale dynamics, and
surface conditions (Gultepe et al., 2007). Often, sea fog
consists of mesoscale (tens of kilometres) spatial structure
⁎ Corresponding author. Tel.: +82 51 5102177; fax: +82 51 5151689.
E-mail address: [email protected] (K.-J. Ha).
0169-8095/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.atmosres.2010.08.004
and time scales of hours. It is difficult to predict the formation
and dissipation of fog using numerical weather forecast
models because of lack of vertical and horizontal resolution
and lack of quantitative understanding of the physical
processes as turbulent mixing, radiation divergence, and
microphysics (Pagowski et al., 2004; van der Velde et al.,
2010). In addition, model skill in forecasting sea fog strongly
depends on the correct initialization of the atmospheric state
(Rémy and Bergot, 2009) as well as the correct oceanic
thermal conditions (Heo and Ha, 2010).
Fog processes involve droplet microphysics, aerosol
chemistry, radiation, turbulence, large/small-scale dynamics,
and surface conditions (Gultepe et al., 2007). Atmospheric
aerosol concentrations play an important role of fog formation under both undersaturated and slightly supersaturated
conditions (Pruppacher and Klett, 1997). Previous studies
showed the role of air quality in the fog formation. For
instance, Vautard et al. (2009) showed the trends in mist and
haze have been correlated to atmospheric aerosol trends. The
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Fig. 1. Location of Ieodo Ocean Research Station (IORS). Gray shading indicates bathymetry. The IORS is constructed on the underwater rock Ieodo in the East China
Sea (125.18°E, 32.12°N). The contours represent bathymetries of −40 m, −100 m and −200 m. The bathymetry around the IORS is about −40 m.
decline in air pollution was shown to be linked to the
visibility increase: the temporal and spatial patterns of fog
and mist declines are strongly correlated with emission
reductions. Witiw and Baars (2003) presented that some
evidence points to increasing air pollution as a possible
reason for fog increase. In addition, van Oldenborgh and van
Ulden (2003) showed that the decrease in the number of fog
and mist days is spatially and temporally correlated with the
decrease in SO2 emissions. They also showed that the
correlations are also statistically significant for dense fog.
In Korea, it has been reported that the occurrence of sea
fog in the southwestern sea or the Yellow Sea is maximum in
July, mainly due to air–sea temperature differences, as the
result of warm air advection over a cold pool (Cho et al., 2000;
Fu et al., 2006). This advection-type fog is generally
considered to be a typical sea fog in the middle latitudes
under stable conditions associated with cold sea surfaces
(Klein and Hartmann, 1993). These dense, widespread, and
long-duration fogs may begin to extend toward the coast with
sufficient advection of warmer moist air (Croft et al., 1997). In
contrast, steam fog in the Yellow Sea is associated with cold
air advection. It tends to be observed in March and April and
results from advection of cold dry air over a relatively warm
ocean. In recent years, some costly or even catastrophic
events in Korea have been caused by dense sea fog. For
instance, the number of sea fog-related ship crashes is 193
Table 1
Meteorological instruments used in the study.
Sensor
Manufacturing company
Model
Observation elements
Range
Accuracy
Wind monitor-MA
RM YOUNG
05106
Temperature and humidity sensor
VAISALA
HMP45A
Optical precipitation sensor
3D sonic anemometer
VAISALA
Campbell scientific, Inc.
PWD-21
CSAT3
Wind direction
Wind speed
Temperature
Humidity
Visibility
Wind (ux, uy) (uz)
360°
0—60 m s−1
−40–+ 60 °C
0.8–100%
10–20,000 m
± 30 m s−1
±8 cm s−1
± 2°
± 0.3 m s−1
± 0.2 °C
± 2–±3%
± 10–± 15%
b± 4 cm s−1
b±2 cm s−1
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during the period from 2004 to 2008, and over 50% of those
occurred in the Yellow Sea. In addition, more than 20 flights
were cancelled in every year at the Incheon Airport which is
major airport in Korea located in the Yellow Sea.
Fog formation is determined not only by the meteorological conditions, but also by the oceanic conditions through
surface fluxes (He and Weisberg 2002, 2003; Virmani and
Weisberg 2003). The previous studies have suggested that
physical processes important for sea fog formation include the
turbulent transfer of heat and moisture and the cooling of air
masses that travel over colder waters. However, few studies
on the physical mechanisms of the formation and evolution of
sea fog have been conducted from direct observations over the
open sea. The Ieodo Ocean Research Station (IORS) data,
which is introduced in Section 2, has been especially useful in
formulating the synopsis for fog formation at the ocean
surface with measurements of sea temperature, wind, air
temperature, humidity, turbulent heat and momentum flux as
well as visibility. This study focuses on the comparison of
physical mechanisms between advection and steam fogs over
the Yellow Sea with the goal of better understanding the
formation, evolution and dissipation of the fogs using direct
observations of turbulent fluxes from IORS. In Section 2, a
description of IORS and instruments is summarized. Each fog
Fig. 2. Time evolution of wind speed (solid line), wind direction (wind barbs), humidity (dashed line), air temperature (dashed dot line), sea temperature (thick
solid line) and fog (shaded area, visibility b 1 km) for advection fog event (a) during 60 h from 03 UTC 1 to 15 UTC 3 June 2005 and (b) during 36 h 00 UTC 11 to 12
UTC 12 May 2005.
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event is illustrated and discussed in Section 3. Finally, we
present our conclusions for the physical explanation of the sea
fog formation, evolution and dissipation.
2. Measurement
2.1. Measurement site
The Ieodo Ocean Research Station (IORS), which was
constructed by the Korean government in 2003 in order to
observe the oceanic environment of East Asia, is an integrated
meteorological and oceanographic observation site. It is
constructed on the Ieodo underwater rock which is located
in the East China Sea (32.12oN, 125.17oE) about 149 km to the
southeast of Jeju Island (Fig. 1) while most ocean platforms
are constructed near the coastal region for convenient
management (Johansson et al., 2001). The structure is of a
fixed jacket type installed at a depth of 40 m. The facility
includes meteorological and oceanographic instruments. The
IORS has provided the oceanic–atmospheric measurement for
studying air–sea interaction due to the open-sea location.
Studies conducted at IORS have investigated various timeseries of atmospheric components; analyzed meteorological
flux data (Oh et al., 2010a,b); and validated remotely-sensed
satellite data (Heo et al., 2008).
2.2. Instrumentation
The instruments installed at IORS are as follows (Table 1);
Wind monitor-MA (RM YOUNG) and HMP45A (Vaisala), which
measure wind direction, wind speed, temperature and humidity at a height of 43 m above sea surface. A PWD-21 (Vaisala),
installed at 33 m above sea surface, is used to observe visibility.
The turbulent fluxes have been observed by an open path eddy
covariance (OPEC) system composed of sonic anemometer
(CSAT3, Campbell Scientific Inc., USA) and an open path
infrared gas analyzer (IGRA: LI-7500, LiCor Inc., USA). The
sonic anemometer is installed on the boom of the pillar under
the deck at the heights of 16 m and 12 m from mean sea level.
The installed direction of sonic anemometer was alternated
between the northwestern (NW) and the southeastern (SE)
according to season to measure prevailing wind. The recording
speed of the fast-response data is 10 Hz. The characteristics of
the sonic anemometer and the IRGA were well documented in
previous experiments (Foken et al., 1997).
The observations started 11 February 2004 and continue
until present. Unfortunately, the moisture flux data became
erratic after fog formation apparently due to condensation on
the windows of the fast-response moisture sensor. Horizontal
and vertical velocities, temperature and moisture fluctuations
are recorded at a frequency of 10 Hz and covariances are
averaged over 30 min time periods. In order to obtain the
turbulent flux data from the IORS, Oh et al. (2010)
investigated the effect of atmospheric and oceanic conditions
on the errors in the fast-response data and evaluated different
characteristics of tilt correction methods. They mentioned
that the removal of the turbulent fluxes data generally
depend on wind speed, relative humidity, significant wave
height, visibility, and stability. According to this study, most
of the turbulent flux data are removed under any foggy
condition. To compare the turbulent fluxes between advec-
Fig. 3. Specific humidity (q) and saturated specific humidity (qs, solid line) as
functions of air temperature during (a) from 03 UTC 1 to 15 UTC 3 June 2005
and (b) from 00 UTC 11 to 12 UTC 12 May 2005 for advection fog events.
tion and steam fogs, the general quality control methods
could not be used. In general, the fluxes could be made less
noisy by more averaging. The tendencies (time derivatives)
could be made less noisy by computing over a 1 h period or
even maybe a 3 h period. However, we could not combine
into one or several hour period due to the short duration of
fogs except for the first advection fog case. The meteorological data except for turbulent fluxes measured every 10 mins
are collected to analyze the variation of ambient conditions
associated with foggy conditions.
3. Result and discussion
3.1. Fog cases
The occurrence of sea fog over the Yellow Sea tends to
increase in frequency from April to July, and in August it rapidly
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decreases (Cho et al., 2000; Zhang et al., 2009). The Yellow Sea
is a favored place for sea fog because of its relatively cold water
and location adjacent to the warm Kuroshio Current (Gao et al.,
2007; Lewis et al., 2004). The seasonal increase of frequency of
advection fogs over the Yellow Sea from June to July arises
primarily from strong warm air advection over the cooler
water, which is mainly due to the persistent southerly flow. Gao
et al. (2007) emphasized the importance of southerly winds for
the formation of advection fogs.
The IORS data capture a persistent period of advection fog
from 1 to 3 June 2005 (AF1) and on 11 May 2005 (AF2). We also
examine steam fog cases on 6 April 2004 (SF1) and on 28
February 2006 (SF2) which appear to be due to a decrease in air
temperature following an abrupt change in wind direction,
wind speed and relative humidity. Unfortunately, we have only
the sea temperature at 10 m depth and cannot estimate the
exact air–sea surface temperature difference. In the case of SF2,
the data of turbulent fluxes cannot be analyzed due to many
errors and missing values.
3.2. Advection fog
Fig. 2(a) shows the time evolution of wind direction, wind
speed, relative humidity, air temperature, sea temperature
(10-m depth) and visibility during the advection fog event
from 1600 UTC 1 June to 0230 UTC 3 June 2005. Cooling of the
air temperature caused by the cooler water, moisture
advection and evaporation from the ocean increases the
relative humidity. The air temperature decreases due to
cooling of moist air by radiative flux divergence, cooling due
to turbulent flux divergence, and cold air advection (Ha and
Mahrt, 2003). While the air temperature decreases only
slightly with time at a fixed point, stronger cooling occurs in a
Lagrangian framework following the flow. The downward
heat flux prior to fog formation is approximately 20 W m−2.
The southerly wind speed decreases from 15 m s−1 to less
than 5 m s−1 at the fog onset time. When the wind speed
increases slightly and the wind direction changes from south
to north, the supply of water vapor by advection becomes
insufficient to maintain the fog.
To identify the cause of the increase in relative humidity,
Fig. 3 shows the variation of specific humidity (q) and air
temperature (Ta) before, during and after the fog events.
Before the advection fog events, the values of Ta and q are
increasing towards saturation due to advection of warm and
moist air while those are located under a saturation line.
When saturation is almost reached during the advection
fogs, more liquid water is being produced while Ta and q
change with the saturation line and the fog is intensifying.
The Ta and q are temporally decreased during the fog. These
temporal decreases appear to parallel with saturation line.
After the AF1, the liquid water is decreasing with increasing
Ta and fixed q due to a change in wind direction which
results in the restriction on the moisture supply. After the
AF2, the liquid water is being maintained with stationary
state Ta and q. The dissipation of the fog is related to the
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decrease in the air–sea temperature difference in this case
(shown in Fig. 2b).
To recognize the variation of surface fluxes and their
relation to the evolution of advection fog, the observed data of
turbulent fluxes are analyzed before, during, and after the
weather event. In the case of AF1 shown in Fig. 4, an
important feature of the turbulent heat fluxes is a fluctuation
from positive to negative latent and sensible heat fluxes prior
to the advection fog formation, which is apparently due to
condensation in the stable surface layer and evaporation from
the ocean in the neutral surface layer. The negative latent heat
fluxes ranging from −200 W m−2 to 0 W m−2 are maintained
during the fog event (Figs. 4a and b), which suggests that
southerly wind supplies moisture to initiate and maintain the
fog event, and atmospheric cooling contributes to condensation shown in Fig. 3(a). The negative sensible heat fluxes
ranging from 0 to −60 W m−2, which imply turbulent
transport of sensible heat from atmosphere to ocean, initiate
and maintain cooling and condensation before and during the
advection fog with stable stratification (Figs. 4c and d).
Upward fluxes of fog droplets occur when the sensible heat
flux is strongly negative. The fog begins to dissipate when
latent and sensible heat fluxes converge to near zero and
atmospheric stability tends to be neutral conditions. An effect
of turbulence for vertical momentum flux was examined by
friction velocity (u*) calculated with perturbation of u, v, w.
Stable stratification develops from cooling of the lower air,
and then, the sensible heat flux is downward from atmosphere to ocean and the evaporated water is confined to a
thin layer. The dense fog occurs during the stable and neutral
stratification which limits downward mixing of dryer air.
Therefore, the advection fog is characterized by longduration, which lasts for more than 30 h in this case.
Advection fog can be persistent since the synoptic situation,
which originally created this fog, does not change very fast.
Advection fog will be dissipated through the passage of a
front, a change of air mass, increasing wind speed and change
in wind direction. In this case, the fog is dissipated through an
increasing wind speed.
Fig. 5 shows the variation of surface fluxes and their relation
to the evolution of AF2, with different behavioral patterns
compared to AF1. Prior to the fog formation, there are few
changes in the latent heat flux despite of the steadily increasing
Ta and q (Fig. 3b) by the southerly wind. The rapid decrease in Ta
just before the fog formation shown in Figs. 2(b) and 3(b)
saturates the lower atmosphere and consequently rapidly
decreases the latent heat flux from 0 to −90 W m−2. The
negative sensible heat flux ranging from 0 to −70 W m−2
during the fog is in the stable and neutral atmospheric
conditions, which induces the upward fluxes of fog droplets.
Sensible and latent heat fluxes modified by the air–sea
interaction determine the evolution of the advection fogs
together with the modification of the low-level atmospheric
stability. The relatively short duration in the AF2 resulted from
the strong turbulence caused by sudden change in wind
direction.
Fig. 4. Latent heat flux (W m−2) as a function of (a) wind speed and (b) specific humidity (q, g/kg). Sensible heat flux (W m−2) as a function of (c) wind speed and
(d) z/L. (e) friction velocity (m s−1) as a function of wind speed. Filled diamonds, circles and filled triangles are indicated before, during, and after advection fog
event from 03 UTC 1 to 15 UTC 3 June 2005.
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Fig. 5. Same as Fig. 4 except for advection fog event from 00 UTC 11 to 12 UTC 12 May 2005.
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Fig. 6. Time evolution of wind speed (solid line, m s−1) and friction velocity (dashed line, m s−1) (a) during 60 h from 03 UTC 1 to 15 UTC 3 June 2005 (AF1) and
(b) during 36 h 00 UTC 11 to 12 UTC 12 May 2005 (AF2).
Fig. 6 shows the time evolution of wind speed and u*
during the advection fog events. A near collapse of turbulence
and downward momentum flux results from a decrease of
wind speed just prior to the fog onset. The termination of the
fog is associated with increasing wind speed and increasing
downward momentum flux, which corresponds to a negative
latent heat flux that does not exceed −100 W m−2. The u*
before the advection fogs behave apparently different from
that during and after the fogs. Stable atmospheric conditions
before the advection fogs lead to the weak u*. The time
evolutions of u* show that the momentum flux does not
govern the dissipation of advection fogs. The wind speed
seems to be modulated by mesoscale motions with a period a
few hours, which also appears in the vertical motion field.
Under a weak large-scale flow, mesoscale motions lead to a
meandering of the wind direction, and the generation of
turbulence by unresolved mesoscale motions becomes
important (Mahrt, 2007). Such mesoscale motions appear to
determine the exact time of onset and dissipation of fog.
3.3. Steam fog
Steam fog develops from flow of cold air over warmer
water, but is normally of limited spatial extent. The warmer
water generates turbulence, mixing, and significant upward
heat flux. Moisture mixes upward where the air saturates,
condenses, and forms fog (Lundquist and Bourcy, 2000). Fig. 7
depicts the time evolution of wind direction, wind speed,
relative humidity, air temperature, sea temperature and
visibility during the steam fog events. When compared to
advection fog, the steam fogs are characterized by stronger
wind speed and shorter duration. Prior to formation of the
fogs, southerly flow leads to a slight increase of air
temperature. A sudden change in wind direction from south
to north is observed at the time of the fog formation. The
northerly wind decreases the air temperature to values 9 °C
and 7 °C in SF1 and SF2, respectively, below the 10-m sea
temperature, which increases the relative humidity of the air
up to 95% due to evaporation from the warmer ocean.
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Fig. 7. Same as Fig. 2 except for steam fog event (a) during 48 h from 00 UTC 6 to 12 UTC 7 April 2004 and (b) during 36 h from 00 UTC 28 to 12 UTC 29 February
2006, respectively.
In the case of steam fog events, the increase in q (SF1) and
the decrease in Ta (SF2) are observed at the time of steam fog
formation as a result of evaporation from the sea (Fig. 8a and
b). Prior to the formation of the steam fogs, high Ta and low q
conditions are maintained in the atmosphere, and Ta and q are
gradually close to saturation. The increase in q is dominant at
the time of steam fog formation in SF1 and SF2, and the
variation of Ta and q are decreasing on the saturation line. The
decrease in Ta and the evaporation from the ocean are
persisted for hours, which continues to produce the liquid
water. In spite of the decrease in Ta, the fog is dissipated by
the decrease in q due to reduction of evaporation.
Fig. 9 shows the scatter diagram of the surface fluxes,
which is the same as Fig. 4, but for the steam fog case (SF1).
Because steam fog is formed by the advection of cold air over
warmer water, unstable stratification develops due to surface
heating. Prior to the fog formation, the change from stable to
unstable stratifications contributes to evaporation (Fig. 9b
and d), and decreasing wind speed suppresses upward
momentum flux (Fig. 9e). And then, there is oscillation
between positive and negative sensible heat flux (Fig. 9c). The
negative latent heat flux before the steam fog indicates that
the moisture accompanied by southerly winds begins to
saturate. During this period, the sensible heat flux is main
form of energy transfer from the ocean because there is
negative latent heat flux. During the fog event, latent heat flux
increases with the decrease in q, which moistens a fog layer.
The positive latent heat flux ranging from 70 to 360 W m−2
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wind shear. The fog is dissipated by the increasing in wind
speed and wind shear in spite of the large air–sea temperature difference, in which case increased downward mixing
of dryer air eliminates the fog.
4. Discussion and conclusion
Fig. 8. Same as Fig. 3 except for steam fog events, (a) 00 UTC 6 to 12 UTC 7
April 2004 and (b) 00 UTC 28 to 12 UTC 29 February 2006.
shows that evaporation and fog droplets are increased with
sharp variation of air temperature shown in Fig. 7(a). The
sensible heat flux oscillates between −200 W m−2 and
150 W m−2 while the atmospheric stability changes unstable
to stable conditions. The dissipation of steam fog starts when
the wind increases while the surface heat and momentum
fluxes decrease.
During the fog event, the wind speed gradually increases
and the persistent northerly wind maintains lower air
temperature. During the steam fog event, the wind speed
temporarily decreases, but the momentum flux increases due
to buoyancy generation of turbulence (Fig. 10). This mixing
apparently restricts steam fog through convective mixing of
dry air downward to the surface. Although atmospheric
conditions maintain a constant air–sea temperature difference and Ta after the steam fog, the decrease in q shown in
Fig. 8a seems to be caused by the increase in wind speed and
Advection and steam fogs are frequently observed in the
Yellow Sea from March to July. Meteorological data and
ocean data obtained from the Ieodo Ocean Research Station
(IORS), which is located in East China Sea, have been used to
study the mechanisms of advection and steam fogs over the
ocean.
The results of air–sea coupling indicate that the mechanisms of the formation, evolution and dissipation are different
for advection and steam fogs. The transport of warm moist
air by southerly flow over the cooler ocean surface forms
stable stratification. The negative latent heat fluxes ranging
from −200 W m−2 to 0 W m−2 are maintained during the
fog event when the air cools below its dew point. Weakening
winds and collapsing turbulence seem to contribute to the
onset of advection fog events. The negative sensible heat
fluxes ranging from 0 to −70 W m−2 induce the upward
fluxes of fog droplets in the stable and neutral atmospheric
conditions. Turbulent heat fluxes modified by the air–sea
interaction determine the evolution of the advection fogs
together with the modification of the low-level atmospheric
stability that confines the water vapor to a thin layer before
and during the fog events. The increasing stable stratification limits downward mixing of dryer air and produces
a dense and long-duration advection fog in defect of
turbulence.
Steam fog develops from cold air advection over a warmer
ocean surface, but is normally of limited spatial extent. Steam
fog occurs as warm air is replaced by cold air due to a rapid
change in the wind direction from north to south, leading to a
negative air–sea temperature difference. Before the change in
the wind direction, relative humidity increases due to
advection or/and evaporation. The positive latent heat flux
as much as 360 W m−2 implies that evaporation and fog
droplets are increased with sharp decrease in air temperature. And then, the sensible heat flux oscillates between
−200 W m−2 and 150 W m−2 while the atmospheric stability
changes unstable to stable conditions. The increase in wind
speed and wind shear, and consequent downward transport
of dryer air lead to dissipation of the steam fog in spite of the
large air–sea temperature difference.
Acknowledgments
This work was supported by grant of the “Eco-Technopia
21 Project” by the Korean Ministry of Environment and
Korean Ocean Research & Development Institute as "Construction of ocean research stations and their application
studies". Larry Mahrt was partly supported by Grant
N000140810409 from the U.S. Office of Naval Research. This
work was also supported by the second stage of the Brain
Korea 21 Project. The authors are very grateful to reviewers
for their constructive criticism and helpful discussions.
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Fig. 9. Same as Fig. 4 except for steam fog event from 00 UTC 6 to 12 UTC 7 April 2004.
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Fig. 10. Time evolution of wind speed (solid line, m s−1) and friction velocity (dashed line, m s−1) during 48 h from 00 UTC 6 to 12 UTC 7 April 2004 (SF1).
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