THE STRUCTURE OF THE ATMOSPHERIC BOUNDARYLAYER DURING FOGGY DAYS IN WINTER AND SPRING SEASONS AT SOUTHERT OF BEIJING

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1 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 , China QIANG WANG Beijing Meteorological College, China Meteorological Administrations, Beijing , 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 1

2 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 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 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 2

3 and humidity varied with height are shown in Figure 1 to 4. In Figure 2, the coordinates of wind direction are 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 3

4 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,

5 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. 5

6 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. REFERENCES [1] Stull, R. B.: An Introduction to Boundary Layer Meteorology. Kluwer Academic Publishers. Dordrecht,, 1-3pp (1988) [2] Huang, Y. S., Y. R. Huang, Z. H. Li, B. J. Chen, J. P. Huang, and J. X. Gu,: The Microphysical Structure and Evolution of Winter Fog in Xishuangbanna, Acta Meteorologica Sinica, Vol. 58, (2000), [3] Li, Z. H., J. P. Huang, B. Y. Sun, and H. Peng,: Burst Characteristics during the Development of Radiation Fog, Chinese Journal of Atmospheric Sciences, Vol. 23, (1999) [4] Jiusto, J. E. and G. G. Lala,: Radiation Fog Formation and Dissipation, A Case Study., J. Rech. Atmos., Vol. 14, (1980)

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