THE STRUCTURE OF THE ATMOSPHERIC BOUNDARYLAYER
DURING FOGGY DAYS IN WINTER AND SPRING SEASONS AT
SOUTHERT OF BEIJING
HONGSHENG ZHANG, KAI WANG, FUYU LI, XINJIAN LIU, JIAYI CHEN
Department of Atmospheric Sciences, School of Physics, Physics，Peking University,
Key Laboratory of the Rain Storm, Drought and Waterlog of Ministry of Education ，
Beijing 100871, China
QIANG WANG
Beijing Meteorological College, China Meteorological Administrations, Beijing 100080,
China
The observational data of atmospheric boundary layer measured during foggy days in
winter and Spring seasons 1999 at the southern of Beijing are used to study the structure
of the profiles of wind velocity, temperature and humidity. The results indicate that: (1)
The attack of frontal fog at night results in quick drop of the surface temperature,
accelerated increase of the inversion height and intensity, then the humidity tends to
decrease and the wind velocity increases rapidly with height. (2) There is close relation
between the formation and dissipation of fog and the maximum and minimum wind
velocity. (3) Strong inversion zone exists in lower atmospheric layer. The inversion zone
is about 100 to 200 meters high. Equal potential temperature is observed below 100
meters. (4) After fog period, there is less surface temperature change. (5) A value of
humidity minimum exists in lower atmospheric layer. (6) Many similarities have been
seen from the obtained data and there are also some differences in different seasons.
INTRODUCTION
The planetary boundary layer (PBL) is the lower atmospheric layer and it is very
important to biosphere and human’s life (Stull, 1988). Fog is a kind of atmospheric
phenomena in PBL. It occurs when the humidity near surface atmosphere becomes
saturated and the water vapor condenses to water (or ice) on the surface of condensate
nucleus, which the visibility would be usually less than 1000 meters. Fogs contact with
the human’s life and the development of economy often, such as the high sensitivity of
ecological environment and frequent traffic accidents in foggy days. Therefore, it is
important to study of fogs’ formation and dissipation in atmospheric research and it is
also important to our social lives, the result will be helpful in setting up fogs warning
systems, estimating city environment changes and figuring out the impact of ecological
changes to climate (Huang et al., 2000).
Fogs usually occur during nighttime. The period can be divided into 4 steps: i.e.
forming, developing, maturing and dissipating (Li, 1999). In the past 20 years, there have
been a series of theoretical, experimental and numerical studies on fogs such as the
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radiative fogs, sea fog and fogs during neutral boundary layer (Jiusto and Lala, 1980; Hu
and Zhou, 1998; Duynkerke, 1999). We have gained a lot of information about some
characteristics of fogs such as its microphysical structure, development and its impacts to
aerosols (Huang et al., 2000; Li et al., 1999; Noone et al., 1992). However, there are not
sufficient studies on the structure of atmospheric boundary layer in foggy days.
The southern area of Beijing is one of the lowest areas of the city. Water vapor is
easy to gather there and fogs occurred often. The experiment was hold to study the
structure and evolvement of atmospheric boundary layer in fogs in Spring and Winter
seasons in 1999. The structure and evolvement of the profiles of wind speed, temperature
and humidity during the formation and dissipation of fogs were analysis, and compared
with spring and winter seasons in this paper.
DATA ACQUISITION AND PROCESSING
The data obtained from the tethered balloon, model AIR-3A, made by USA. The data
were automatically collected and recorded by a note-computer. Although the instrument
were checked in the laboratory before and after field experiments, but the instruments
also were testified before and after each observation. The obtained data were
preprocessed by necessary procedures with reorganizing and selecting. The data include
wind speed, wind direction, temperature and humidity profiles during April, November
and December, in 1999.
During spring season, only one typical process of fog was observed, which was
formed at 22:30 on April 22 and lasted for only 2 to 3 hours. During winter season,
totally five boundary layer observations in foggy days were carried out in November and
December, together with visibility records. They were the nights in November 5,
November 19, November 20, November22 and December 14, respectively.
RESULTS
Structure of atmospheric boundary layer during fogs in spring season
A process of fog which lasted for only 2 to 3 hours was observed in the night of April 22.
Five sets of continuous observational data were recorded during this fog period. It began
as soon as the fog formed and ended when the wind speed in the surface layer was too
strong for tethered Balloon to work smoothly. The exact observational time were 22:45,
23:38 on April 22 and 01:12, 01:46, 02:52 on April 23. The lowest visibility appeared
around one o’clock on April 23. In the five sets of obtained data, 22:45 and 23:38 on
April 22 indicated the boundary layer’s structure before the fog and 01:12 on April 23
was during the fog’s formation period. Moreover, 01:46 and 02:52 represented the
characteristics of boundary layer when fog was developing and after the fog had
disappeared, respectively. The characteristics of wind speed, wind direction, temperature
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and humidity varied with height are shown in Figure 1 to 4. In Figure 2, the coordinates
of wind direction are -60°- 450°for the aim of easy analysis.
Figure 1. The wind speed profile in
southern suburban of Beijing at the
night of April 22,1999
Figure 2. The wind direction profile in
southern suburban of Beijing at the
night of April 22,1999
Figure 3. The potential temperature
profile in southern of Beijing at the
night of April 22,1999
Figure 4. The humidity profile in
southern suburb of Beijing at the night
of April 22,1999
From the Figures 1-4, we can figure out that the wind direction was mainly north
direction during the whole fog period. Before the fog’s formation (22:45 and 23:38),
wind speed increased with the increasing height. During the fog’s developing period
(01:32), the wind speed in the surface was very small and increased very rapidly with
height, the wind direction switched largely. When the fog became mature and began to
dissipate (01:46 and 02:52), the value of wind speed in the surface layer increased to
more than 2 m/s and still kept the tendency to increase with height. The inversion zone
rose from 20 m (22:34) to 50 m (23:38) in the evening. When the fog occurred at 01:12,
there was a very strong inversion zone near surface and its thickness increased to 163 m
at 01:45 and 02:52. The rapid drop of near-surface temperature when the fog was forming
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was a premise for fogs in our general knowledge. During the developing period of the fog
01:12 - 01:45), the gradient of potential temperature became larger and the temperature
had dropped nearly 2K until 02:52 when the temperature began to rise. As for the
humidity, the characteristics were quite similar between 22:45 and 23:38. The difference
was the larger humidity at 23:38, which was reasonable for the fog’s formation. A
humidity inversion existed at the height of 40 m. It provided enough water vapor for the
fog. At 01:12, humidity reached the maximum to 10g/kg and the relative humidity almost
be 100%. The humidity at upper layer dropped quickly although the humidity inversion
still existed. At the same time, the humidity in the surface declined and fog began to
weaken till 02:52 when the fog disappeared.
During the formation and dissipation of the fog period, it accompanied with the
variation of wind speed and wind direction. During the fog’s developing period,
temperature dropped quickly and the thickness of inversion layer became larger in the
surface layer. The dissipation period of the fog in upper atmospheric layer was earlier
than that in lower layer.
Structure of atmospheric boundary layer during fogs in winter season
There were five observations for boundary layer in fogs days in winter season, the one
carried out at night on November 14 was most typical. The figure 5 shows the visibility
of the fog period. It can be seen that a heavy fog occurred in the night and two minimum
of visibility were recorded at 24 o’clock in the evening and 6 o’clock in the next
morning.
Figures 6-7 show the wind speed and wind direction profiles during the fog period in
winter season. Wind speed and wind direction fluctuated at the beginning of the
observation. Wind direction was mainly south direction in the whole night. At midnight,
the wind speed reached nearly 6 m/s, the wind speed in the surface layer was quite
smaller in the next morning. The characteristics of wind speed in the fog were quite
similar with that in spring except there was a layer where wind fluctuated within a small
range at the height of 100 m. Moreover, wind speed increased rapidly with the height
above this layer.
Figure 5. The visibility in southern suburban of Beijing at the night of Dec.14, 1999
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Figure 6. The wind speed profile in
southern suburban of Beijing at the night
of Dec.14, 1999
Figure 7. The wind direction profile in
southern suburban of Beijing at the night
of Dec.14, 1999
Figure 8. The potential temperature
profile in southern of Beijing at the night
of Dec.14, 1999
Figure 9. The humidity profile in southern
suburban of Beijing at the night of Dec.14,
1999
As for the temperature profile in Figure 8, there was an inversion layer between 100
m and 150 m at whole night. The potential temperature above 100 m increased largely at
05:00 and it increased above 120 m at 06:40. However, the value of temperature was
lower because of the influence of the fog on heating from sunshine in the evening. It
didn’t become higher until 08:30 when the fog began to dissipate.
The value of humidity near the surface was very large at 06:40, and the value of
relative humidity is about 3.5 ~ 4 g/kg with height. From 22:00 to 00:00, the value of
humidity became a little lower. The humidity rose again and kept stable afterward
accompanied with high relative humidity after midnight.
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CONCLUSION
In this paper , the observational data on atmospheric boundary layer during the fog
periods measured in Beijing southern suburban in two sample days in 1999, and the
structure and evolvement of the profiles of wind speed, temperature and humidity were
studied during the formation and dissipation of fogs in spring and winter seasons.
The results indicate that: (1) the fog in spring was a process of frontal fog. The attack
of frontal fog at night resulted in quick drop of the surface temperature, accelerated
increase of the inversion height and intensity. After frontal fog, the humidity tended to
decrease and the wind speed increased rapidly with height. (2) There were close relations
between the formation and dissipation of fog and the maximum and minimum wind
speed. (3) In winter time, there was about 100 m height where wind fluctuated within a
small range and wind speed increased rapidly. As for spring, wind speed increased with
height at the beginning of the fog and it decreased rapidly in the 200 m to 300 m height.
(4) The wind directions were different in different seasons. A tendency of south direction
of wind direction could be seen in winter, but in spring, the wind direction was northern
direction. (5) There were the strong inversion zone existed in lower atmospheric layer in
both spring and winter. The height of inversion zone was about 100 to 200 meters, and
the potential temperature was observed below 100 meters in winter season. (6) After fog
period, the surface temperature was less changing in winter than that in spring. (7) A
value of humidity minimum existed in lower atmospheric layer in winter but not in spring.
(8) In both spring and winter seasons, the humidity near the surface decreased faster than
that in higher atmospheric layer, which indicated that fogs usually dissipated near the
surface at first and then to higher layers. (9) There were shear layers of wind speed, wind
direction, potential temperature and humidity in fogs at night in both spring and winter.
The difference was the height of the shear layer was 250 m ~ 350 m in spring season, but
it rose to 400 m ~ 500 m high but in winter season.
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