NO.2 FAN Qi, Wang Anyu, FAN Shaojia, LI Jiangnan, WU Dui and LEONG Ka Cheng 231 Numerical Experiment Research of a Marine Fog Event in the Pearl River Estuary Region FAN Qi 1 ( ), WANG Anyu 1 ( ), FAN Shaojia 1 ( ), LI Jiangnan 1 ( ), WU Dui 2 ( ), and LEONG Ka Cheng 3 ( ) 1 Monsoon and Environmental Research Center/Department of Atmospheric Sciences, Zhongshan University, Guangzhou 510275 2 Institute of Tropical and Marine Meteorology, CMA, Guangzhou 510080 3 Macao Meteorological and Geophysical Bureau, Macao (Received August 9, 2004; revised November 1, 2004) ABSTRACT A marine fog event that occurred near the Pearl River Estuary region on 26 March 2002 was investigated with the fifth-generation Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5). The results of numerical experiment are very consistent with the surface observations, especially in the processes of marine fog formation and evolution. Besides, a series of sensitivity numerical experiments were performed by varying the distribution of landuse type and the turbulence exchange process. It is shown that the marine fog forms along the coastal line. Tests have indicated that when the distribution of landuse type is modified, the regions where the marine fog can intrude into are obviously different. The turbulence process in the model is important for fog formation and evolution. If the influence of turbulence process is ignored in numerical simulation experiment, the simulated maximum of the cloud liquid water content and the height of fog top will be decreased greatly. Key words: the Pearl River Estuary region, marine fog, numerical experiment 1. Introduction Marine fog is a disastrous weather phenomenon. There have many marine fog events with long duration in coastal areas in South China. It causes serious hazards in all kinds of areas, and its prediction is essential for public safety and has high economic values. Many researches have been performed on the fog events with different methods. Various fog models were used to study the physical mechanism of fog formation and evolution (Ren et al., 2000; Zeng and Huang, 1998). Statistics and synoptic methods were utilized to do the prediction study of marine fog events (He, 1996; Xiong, 1997; Feng, 1995). Numerical mesoscale models can be used for fog forecasting and to investigate the relative importance of the processes affecting fog formation and evolution. With the rapid development of numerical simulation model, one has used the numerical mesoscale model to do the research on the marine fog event (Fu et al., 2004). For regional fog studies and forecasting, Ballard et al. (1991) firstly attempted fog prediction with a regional model. Experimental forecasts of fog off the coast of northern Scotland, using the United Kingdom Meteorological Office (UKMO) mesoscale model, were relatively successful and showed that results are critically dependent on the model initial conditions and the parameterization of turbulence. Kong (2002) did an experimental simulation of a marine fog-stratus case along the west coast of California by using the US Navy Coupled Ocean/Atmosphere Mesoscale Predicfion System (COAMPS) model. The purpose of his work is to show the potential usefulness of mesoscale models in forecasting this type of marine boundary weather phenomenon. He also pointed out that mesoscale models, though demanding large computing resources, have obvious advantages in providing the continuous evolution of 3-D structures for simulated fog-stratus Sponsored by the Doctoral Research Foundation of Guangdong Province and the Chinese Academy of Sciences 973 subproject Observation research on meteorological conditions and boundary characteristics in atmospheric pollution process in the Pearl River Delta region (2002CB410801).
232 ACTA METEOROLOGICA SINICA VOL.19 systems. However, mesoscale numerical models have not yet been used in the real-time forecast of fog phenomena. Fan et al. (2003, 2004) did several prediction experiments of radiation fog and advection fog cases in South China by using MM5 mesoscale numerical model and all achieved success. In the following, the MM5 model is utilized to simulate a marine fog event occurring in the Pearl River Estuary region. 2. Model, data and the schemes of numerical simulation To study the event, an MM5V3 model (Grell et al., 1994) is employed. In the numerical experiment, the Blackadar parameterization of the boundary layer and the Grell parameterization of deep convection are used. Explicit treatment of cloud microphysics of simple ice is employed. To account for an important role played by longwave and shortwave radiations in fog formation and evolution, the rapid radiative transfer model (RRTM) scheme is employed. In this parameterization, effects of water vapor, carbon dioxide, and liquid water on emissivity are included. Absorption and scattering by clear air, water vapor, and cloud water are accounted for in the parameterization of shortwave radiation flux. Two computational grids shown in Fig.1 have horizontal spacings of 54 and 18 km, respectively. Two-way nested interaction was employed. Twenty-three vertical levels stretched monotonically from the surface to 100 hpa. Lateral and initial conditions for numerical experiments were obtained from NCEP/NCAR reanalysis daily 2.5 2.5 grid data. The numerical experiment of the event began at 20 BT (Beijing Time) 26 March and ended at 20 BT 28 March, 48 hours in all. In order to learn the effects of different landuse type and the turbulence exchange process on the fog formation and evolution, we conducted two kinds of sensitivity experiments. In the first kind of sensitivity experiment, the distribution of landuse type is modified. There are two studies. One is to change the water landuse type south of Macao into the land type (named as Experiment B1, Fig.2) and the other is Fig.2. The modified landuse type in Experiment B1 (the pane region). Fig.1. Modeling domains. Fig.3. As in Fig.2, but for Experiment B2.
NO.2 FAN Qi, Wang Anyu, FAN Shaojia, LI Jiangnan, WU Dui and LEONG Ka Cheng 233 Fig.4. Observation(a, c, e, and g) from the surface station in the Pearl River Estuary regions and the simulated liquid cloud water content (b, d, f, and h) at 990 hpa from 20 BT 27 to 14 BT 28 March every 6 hours.
234 ACTA METEOROLOGICA SINICA VOL.19 to change the land type north of Macao into the water type (Experiment B2, Fig.3). In the second kind of sensitivity experiment, the influence of turbulence process in MM5 model is ignored (Experiment C). For comparison, the numerical experiment without any changes is named as control experiment (Experiment A). The integral time of all experiments is 48 h. 3. Numerical simulation analyses of the marine fog event occurring in the Pearl River Estuary Region There had a dense fog event with long duration in the coastal areas near the Pearl River Estuary and its adjacent regions from 26 to 28 March 2002. It was dominated by a stationary high pressure system centered at south of Macao, and the Pearl River Estuary and its surroundings were controlled by southerly flows. Influenced by the warm and damp air currents from the sea, a dense fog event appeared around the Pearl River Estuary regions. Figures 4a, 4c, 4e, and 4g show the observations from surface stations in the Pearl River Estuary regions from 20 BT 27 March to 14 BT 28 March every 6 hours. Surface observations indicated that there appeared a dense fog event in the Pearl River Estuary regions and the fog dissipated at 14 BT 28 March or so. It maintained about 18 hours. This dense fog event presented a serious hazard in areas of intense traffic, airports, harbor, and public life. In this study, an MM5 model was utilized to research this dense fog event. Marine fog can be regarded as the condensation results of the moisture or ice crystal in the lower boundary layer of sea or coastal areas (Wang, 1983). In this sense, the liquid cloud water content is often used to describe the fog area. In Figs.4b, 4d, 4f, and 4h, the simulated liquid cloud water contents at 990 hpa from 20 BT 27 to 14 BT 28 March are plotted. By comparing the simulations with the observations in Fig.4 detailedly, it can be seen that the simulated horizontal regions of the fog are almost corresponding to the observations. Comparison of Figs.4a and 4b indicates that the fog and mist appear along the coastal areas in Fig.4a, and a dense fog appears in the Pearl River Estuary regions, and that in Fig.4b, the fog and mist can be simulated along the coastal areas, and the high value region of the simulated liquid cloud water content is also located in the Pearl River Estuary and its surroundings. It is seen from Fig.4e that at that time fog only appears along some coastal areas just like Macao according to the observations. In Fig.4f, the simulation results are very similar to the observations. The fog appears only in the Pearl River Estuary regions, and no fog in other regions. In Fig.5, the time-height cross section of the simulated liquid cloud water content in Macao is displayed. It can be seen that the simulated fog event in Macao was from 19 BT 27 to 17 BT 28 March. The high values of the liquid cloud water content appeared from 20 BT 27 to 12 BT 28 March. The dense fog event maintained about 16 hours. It was very similar to the observations from the surface stations. We could not get the actual height of the fog from the surface map. Lack of sounding data in Macao also does not allow us to provide a definitive comparison of the vertical height. We can see from the vertical distribution of the liquid cloud water content in Fig.5 that the fog forms near the surface at first and the maximum of the liquid cloud water content value reaches 0.5 g kg 1. With the evolution of the fog, the fog layer became thicker and the top became high. After the sunrise, the fog began to uplift and dissipate at the surface with the warming of the ground. It is noted that the liquid cloud water content in marine fog is higher than that in radiation Fig.5. The time-height cross section of the simulated liquid cloud water content in Macao.
NO.2 FAN Qi, Wang Anyu, FAN Shaojia, LI Jiangnan, WU Dui and LEONG Ka Cheng 235 fog and advection fog. Because the formation of the marine fog is related with the evaporation of the liquid water in the sea, the evaporation will increase the liquid cloud water content in marine fog. The formation of this marine fog event is very representative in South China. Guo et al. (1991) made statistic analyses on the relationship between the appearance of marine fog with the synoptic situation in South China. The results have shown that there have many marine fog cases appearing when the surface is dominated by the cold high pressures and the upper layer (850 hpa) is affected by the subtropical high in South China. In Fig.6, the simulated temperature distributions at 990 hpa at 20 BT 27 and 08 BT 28 March are plotted. We can see the difference caused by the land and water landuse types. During the fog Fig.6. The simulated horizontal temperature distributions at 990 hpa at 20 BT 27 (a) and 08 BT 28 March (b). Fig.7. The simulated horizontal stream field at 850 hpa at 20 BT 27 March.
236 ACTA METEOROLOGICA SINICA VOL.19 event, the temperature of the Pearl River Estuary and its surroundings were comparatively lower than that of its south sea waters. Especially at 20 BT 27 March, the Pearl River Estuary was the center of the lower temperature. In Fig.7, the simulated horizontal stream field at 850 hpa at 20 BT 27 March is displayed. It can be seen that the Pearl River Estuary and its surroundings are dominated by the WS wind from the west of cold high pressure. Such temperature field and stream situation are all beneficial to the formation of coastal fog. 4. Sensitivity studies 4.1 The influence of landuse type on the formation of marine fog The distribution of coastal line is very important to the numerical simulation of marine fog in coastal areas. The differences in thermal and dynamical characteristics between the land and water landuse types have great effect on the advance process of marine fog. In order to discuss the influence of different landuse type on the formation of coastal marine fog, we did two sensitivity studies besides the control experiment. The first sensitivity study (Experiment B1) is to change the water landuse type south of Macao into the land type (Fig.2). In Fig.8, the time-height cross section of the simulated liquid cloud water content in Macao is displayed. Comparing the results of this sensitivity experiment with that of control experiment (Fig.5), we can see that the marine fog event from 20 BT 27 to 14 BT 28 March appearing in control experiment does not appear any more in this sensitivity experiment. In Fig.8, there just had some lower liquid cloud water content at 10 BT 28 March at 960 hpa. It suggested that this marine fog event was obviously influenced by the distribution of the coastal line. Once we change the water landuse type to the land type, the warm and damp stream from the sea cannot advance to Macao and its surroundings, that is to say, the advance process of warm air from sea is slowed down, then the marine fog cannot form in Macao and its surroundings. Fig.8. The time-height cross section of the simulated liquid cloud water content in Macao (Experiment B1). Fig.9. The simulated differences of the temperature at 990 hpa (Experiment B1 minus Experiment A). (a) 20 BT 27 March; and (b) 08 BT 28 March.
NO.2 FAN Qi, Wang Anyu, FAN Shaojia, LI Jiangnan, WU Dui and LEONG Ka Cheng 237
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NO.2 FAN Qi, Wang Anyu, FAN Shaojia, LI Jiangnan, WU Dui and LEONG Ka Cheng 239 In order to analyze the influence of different landuse type, the difference charts of horizontal temperature field are displayed. Figure 9 delimertes the simulated differences of temperature at 990 hpa at 20 BT 27 and 08 BT 28 March (sensitivity experiment B1 minus control experiment A). It can be seen that when the water landuse type is modified to the land type, there have great changes in the horizontal temperature field. The heat capacity of the land type is smaller than that of water type, it will be heated fast when absorbing the shortwave radiation and the temperature will be raised. The high temperature will not be of benefit to the advance process of sea fog. In Fig.10, the simulated liquid cloud water contents at 990 hpa in Experiments A and B1 from 20 BT 27 to 08 BT 28 March every 6 hours are plotted. Comparing these charts, we can see that the advance process of sea fog is faster in Experiment A. Coastal marine fog appeared in the Pearl River Estuary region at 20 BT 27 March and dissipated at 08 BT 28 March. But in Experiment B1, the advance process of sea fog slowed down when we modified the landuse type. At 20 BT 27 March the sea fog just advanced to the 18 N which was the new boundary of water and land type we modified. Coastal marine fog did not appear in the Pearl River Estuary region. Besides the temperature field, the wind field also has effect on the advance process of sea fog. Because the roughness of land type is larger than that of water type, the friction caused by the land type will slow down the advance process of sea fog. Therefore, the distribution of coastal line has much influence on the appearance of marine fog. It moves fast over the water landuse type and slows down over the land type. The second sensitivity study (Experiment B2) is to change the land type in north of Macao to the water type (Fig.3). The simulation results have shown that when we modifiy the land type to water type, the temperature decreases greatly (figures omitted). It is of benefit to the advance of sea fog. Besides the temperature field, the decrease of friction force also makes contribution to the advance process of sea fog. In Experiment B2, the sea fog can reach further north regions than that in control experiment (Fig.11). The above two sensitivity studies came to the same conclusion that the different landuse type has great influence to the advance process of sea fog. Due to the different temperature and friction force, the sea fog moves faster over the water landuse type than over the land type. The advance process of sea fog influences the formation of coastal fog directly. 4.2 The influence of turbulence process on the formation and evolution of fog The turbulence exchange process has great influence on the fog formation and evolution. Many studies (Zdunkowshi et al., 1969; Roach et al., 1976) emphasized the importance of turbulence process. In this study, we chose the Blackadar planetary boundary layer (PBL) scheme in MM5 model. The turbulence terms in this scheme are the functions of Richardson numbers. It is important to consider the turbulence exchange process in the model. Here, we did a sensitivity experiment to study the influence of this term on the formation and evolution of fog event. In this sensitivity experiment, we ignored the influence of the turbulence terms in MM5 model. The turbulence terms are changed to 0 in Blackadar PBL scheme, that is, the influence of turbulence process is ignored in the formation and evolution of fog. Figure 12 shows the time-height cross section of the simulated liquid cloud water content in Macao. We can see from this chart that the formation process of Fig.12. The time-height cross section of the simulated liquid cloud water content in Macao (Experiment C).
240 ACTA METEOROLOGICA SINICA VOL.19 marine fog almost has not changed yet. The difference from the result of Experiment A (Fig.5) is the height of the marine fog. The turbulence exchange term has great influence on the uplift process of fog. When we ignore this exchange term, the height of fog top decreased from 900 to 950 hpa and the simulated maximum of the cloud liquid water content also decreases. The dissipation of fog near the surface is delayed in this sensitivity experiment. Without the influence of turbulence process, the fog does not develop uplift and the fog in high layer decreases very fast. It can be seen from this sensitivity experiment that the turbulence exchange process has great influence on the evolution and maintenance of fog. 5. Conclusions A three-dimensional nonhydrostatic mesoscale model MM5 was utilized to simulate a marine fog episode that occurred in the Pearl River Estuary and its surroundings on 26 March 2002. The simulation results are very consistent with the surface observations in the occurrence and maintenance of the marine fog. It is comparatively successful to simulate the fog cases using mesoscale numerical model. Besides the occurrence and dissipation time of fog, the height of fog and the liquid cloud water content values in fog are also simulated very well. We did several sensitivity experiments to study the influence of different landuse type and turbulence exchange process in MM5 model on the formation and evolution of fog. The results have shown as follows: (1) The marine fog forms almost along the coastal line. The advance speed of sea fog is faster over water landuse type than over land type. (2) The turbulence exchange process has great influence on the evolution and maintenance of fog. If we ignore the turbulence terms, the simulated maximum of the cloud liquid water content and the height of the fog top will decrease greatly. The maintenance of the fog near the surface will be lengthened. This numerical simulation work is valuable for the numerical prediction of fog. We must point out that we have not the observations on the sea, and thus we could not compare our results with such observations. REFERENCES Ballard, S. P., B. W. Go1ding, and R. N. B. Smith, 1991: Mesoscale model experimental forecasts of the haar of northeast Scotland. Mon. Wea. Rev., 119, 2107-2123. Fan Qi, Anyu Wang, et a1., 2003: Numerical prediction experiment of an advection fog in Nanling Mountain area. Acta Meteorologica Sinica, 17(3), 337-350. Fan Qi, Anyu Wang, et a1., 2004: Numerical simulation research of a radiation fog in Pearl River Delta region. Scientia Meteorologica Sinica, 24(1), 1-8. (in Chinese) Feng Junru, 1995: The analyses and prediction of dense fog in Chaozhou. Meteorology of Anhui, 1, 31-32. (in Chinese) Fu Gang, et al., 2004: An observational and numerical study of a sea fog event over the Yellow sea on 11 April, 2004. Periodical of Ocean University of China, 34(5), 720-727. (in Chinese) Grell G. A., J. Dudhia, et al., 1994: A Description of the Fifth-generation Penn State/NCAR Mesoscale Model (MM5). NCAR/TN-398+STR (NCAR Technical Note), 39-45. Guo Xiuying, et al., 1991: The statistics analyses and prediction of sea fog and the synoptic situation in spring in coastal areas in South of China. Meteorology of Guangdong, 1, 25-28. (in Chinese) He Jinde, 1996: The distribution and prediction of fog event in MeiZhou region. Meteorology of Fujian, (3), 26-27. (in Chinese) Kong Fanyou, 2002: An experimental simulation of a coastal fog-stratus case using COAMPS model. Atmospheric Research, 64, 205-215. Ren Zunhai, et al., 2000: Numerical study on the advection-radiation fog of Yangtze River, Scientia Meteorologica Sinica, 20(2), 188-193. (in Chinese) Roach, W. T., et a1., 1976: The physics of radiation fog. Part II: 2-D numerical study. Quart. J. Roy. Meteor. Soc. 102, 335-354. Wang Binhua, 1983: Sea Fog [M], Beijing, China Ocean Press. (in Chinese) Xiong Qiufen, 1997: The analyses and prediction of dense fog in Yangtse river region, Meteorology of Hubei, (2), 16-17. (in Chinese) Zdunkowski, W. B., and B. C. Nielsen, 1969: A preliminary prediction analysis of radiation fog. Pure Appl. Geophys, 75, 278-299. Zeng Xinmin and Huang Peiqiang, 1998: A numerical model of advection-radiation fog within shallowwater swamp-land area. Climate and Environmental Research, 3(3), 266-274. (in Chinese)