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An Observational and Numerical Study on Winter and Spring Sea Fogs over the Yellow and Bohai Seas

Author: XuJie
Tutor: FuGang
School: Ocean University of China
Course: Meteorology
Keywords: sea fog numerical modeling satellite imagery atmospheric visibility
CLC: P714.2
Type: Master's thesis
Year: 2011
Downloads: 63
Quote: 1
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Abstract

In this paper, the MODIS (Moderate Resolution Imaging Spectroradiometer) satellite imagery, albedo data of MTSAT-1R(The Multi-functional Transport Satellite), weather charts from Korea Meteorological Administration, FNL (Final Analyses) re-analysis data, the RSM (Regional Spectral Model) data from Japan Meteorological Administration as well as the observational data and station sounding data along the coasts of the Yellow and Bohai Seas were used to investigate two sea fog cases occurred over the Yellow and Bohai Seas in (spring) May 2009 and (winter) February 2010, respectively. The weather situations, the sounding profiles and the related meteorological factors during the development, mature and dissipation stages of these two sea fog cases were examined. Also the WRF (Weather Research Forecast) and RAMS (Regional Atmospheric Modeling System) models were used to simulate these two sea fog cases, respectively. First, observational data was used to verify the reliability of modeling results. Then, the atmospheric visibility, liquid water path and the mixing ratio of cloud water at the height of 10 m during the different stages of these two cases were analyzed based upon modeling results. Finally, the characteristics of these two sea fog cases were compared, and the followings conclusions were made:(1) The winter sea fog case lasted for about 34 h started from 18 UTC 22 to 04 UTC 24 February 2010. It occurred over the northern part of the East China Sea, the western part of Yellow Sea and the southeastern coast of the Shandong Peninsula. Its southern boundary reached to 27oN. The fog western boundary fitted with the coast of Shandong Peninsula and southeastern part of China. In the development satge, sea fog extended into inland with its northern boundary to Dandong. The fog eastern boundary fitted with the northern coastal part of Korean Peninsula and its southern boundary reached 27oN. The spring sea fog case lasted for 36 h started from 00 UTC 03 to 12 UTC 04 May 2010. It occurred over the middle and eastern part of Yellow and Bohai Seas. Its southern boundary reached to 32oN. Its eastern boundary fitted with the western coast line of the Korean Peninsula. Its northern boundary reached to Dandong and its western boundary was far away from the Shandong Peninsula.(2) The middle and lower level weather situations of the winter sea fog case were stable during its whole period of life. The fog area was located in the front of an upper trough whose position was around 115oE. At the onset of this sea fog case, the subtropical high-pressure system was dominated over the Yellow Sea and the East China Sea. From the south of Shandong Peninsula to Sheyang there was a warm advection from the sea, which was beneficial to the development of this sea fog case. When the subtropical high-pressure system and the trough moved eastwards, the Yellow Sea was dominated by a low-pressure system. Accompanied with the intrusion of cold air from the north, the sea fog began to dissipate. For the spring sea fog case, the surface weather situations were stable during its whole period. However, the Yellow Sea was always controlled by a closed high-pressure system. At the onset of this sea fog case, the Yellow Sea and the northern part of the Korean Peninsula were influenced by a upper trough. The fog area was located in the front of this upper-trough accompanied with a warm advection, which was beneficial to the development of this sea fog case. Later, the high-pressure system moved eastwards, and the trough disappeared gradually. When the Yellow Sea was entirely controlled by the high-pressure system, the sea fog disappeared thoroughly.(3) The albedo data of satellite visible image was used to estimate the fog top height. It is shown that the fog top height was around 130 ~200 m over the Yellow and Bohai Seas, but 100 ~130 m over the East China Sea for winter case. For spring case, the fog top height was around 200~250 m over the Yellow Sea, and 170~200 m over the Bohai Sea. The analyses of sounding data showed that there were inversion layers in both cases. The difference between air temperature at 2 m altitude and dew-point temperature was small beneth the inversion layer. There existed a warm advection and great relative humidity in the foggy region. The inversion layer worked like a cover to let the water vapor concentrate below the lower level, which supplied a benifical condition for the development of sea fog.(4) The analyses of FNL and RSM data as well as the coastal station observational data showed that the relative humidity in the fog area was above 90% in both cases. In the fog area, the difference between air temperature at 2 m altitude and dew-point temperature was less than 1.5℃in both cases. The difference between air temperature at 2 m altitude and sea surface temperature in the fog area was around 1~3℃. During the development stage, the southerly were dominated in fog area in both cases. During the formation and development stages of the winter sea fog case, the lower level wind speed over the Yellow Sea was around 4~8 m/s. But at the dissipation stage, the wind speed reached to 10 ~12 m/s. The increasement of the wind speed enhanced the fog dissipation. However, during the whole stages of the spring sea fog case, the lower level wind speed remained almost the same value of 2 m/s.(5) The modeling results showed that the winter sea fog initially formed at Shandong Peninsula and the sea near Shanghai, respectively. Then two fog areas merged into a single one. The sea fog developed from 22 UTC 22 to 00 UTC 24 February 2010. Then the sea fog over the East China Sea began to disappear. During the development stage of this sea fog case, the mixing ratio of cloud water was mainly concentrated below 150 m, and the LWP (Liquid Water Path) was around 0.04~0.1 kg/m2. The mixing ratio of cloud water at the height of 10 m was around 0.4 ~0.6 g/kg. When the sea fog was at dissipation stage, the mixing ratio of cloud water at the height of 10 m was even reduced to 0.2 g/kg. While the spring sea fog case formed near the Korea Peninsula. The sea fog developed from 22 UTC 02 to 04 UTC 04 May 2010. Then, the sea fog disappeared gradually from the east to the west. During the development stage of this sea fog case, the mixing ratio of cloud water was mainly concentrated below 200 m, and the LWP was around 0.1~0.3 kg/m2. The mixing ratio of cloud water at the height of 10 m was around 0.5~0.7 g/kg. When the sea fog was at dissipation stage, the mixing ratio of cloud water at the height of 10 m reduced to 0.2 g/kg. It suggested that the LWP and the mixing ratio of cloud water at the height of 10 m of the spring sea fog case were both greater than those of the winter sea fog case.

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