Observational Study On Surface-layer Turbulent Parameters Over The South China Coast

South China coastal zone is one of the most economically developed regions in China,and it’s also an area with frequent severe weather, such as typhoon, heavy rain, fog,storm surges and so on. So it’s an urgent need for economic and social development ofthis area to add observations on the coastal marine and meteorological elements, andto improve the forecasting ability, especially the numerical forecasting ability ofsevere weather. Aiming at these problems, the three-dimensional turbulence datasetswith long time, high-precision observations from Maoming Bohe marinemeteorological scientific experimental base in Guangzhou provience are used in thisthesis. The tubulent parameters of friction velocity, stability parameter, turbulenceintensity and drag coefficient are calculated directly with the eddy covariancetechnology. The variation characteristics of these surface tubulent parameters withwind speed are analyzed and compared under different conditions of onshorewind and offshore wind. The tubulent data and gradient data from the marineobservation platform are also used. They are used to study the evolution features ofthese tubulent parameters and their relations with wind speed during two typhoons,Chanthu and Hagupit. The main conclusions are as follows:During May2007to Augest2008, the overall surfcace-layer atmospheric stabilityin the South China coast was nearly neutral, and slightly tends to weak unstable. Thenear-neutral stratification takes over55%, unstable stratification31%and stablestratification14%. The prevailing wind is easterly at the observation site and thereexsit obvious seasonal variations in both wind speed and direction.Different airflows cause different effects on local turbulence characteristics, whichshow out significant differences under offshore and onshore winds. The values offriction velocity, turbulence intensity and drag coefficient under offshore wind aremuch larger than those under onshore wind, and their distribution with wind are more discreter. That means when refers to problems in the coastal boundary layer, thedirection of the airflow must be seriously considered.Nevertheless, the basic relationships between wind speed and turbulent parametersare similar under offshore wind and onshore wind. In moderate wind speed, frictionvelocity increases linearly with wind speed. The fitting equations areu*0.066U0.060in offshore wind andu*0.034U0.028in onshore wind.The stability parameter distribution is very discrete and uncertain when wind is small,and when U>8m/s it almost tends to zero. Its relationship with wind direction is notso clear. The three-dimensional turbulence intensities first decrease rapidly with windvelocity, then remain unchanged or slightly increased. The average values of threedimensions are Iu=0.245，Iv=0.199，Iw=0.125in offshore wind, and Iu=0.092，Iv=0.089，Iw=0.055in onshore wind. The drag coefficient decreases with wind speedwhen U<6m/s, then increases with wind speed slightly. These results indicate that thebasic relationships between these turbulent parameters and wind speed are decided bytheir intrinsic nature and have certain acertainty. However, specific variation formshave slightly differences due to different wind directions or underlying surfaces.Drag coefficient and atmospheric stability also has a close relationship. No matterhow the wind direction is, CDdecreases with–z/L when z/L is negative anddecreases with z/L when it is positive. The more neutral the atmospheric stability bais,the larger the drag coefficient appears, and the stronger the momentum flux transports.Typhoon systems can significantly affect the surface-layer turbulence properties.During processes of typhoon Chanthu and Hagupit, the friction velocitys are in lowvalues when typhoons’ landing, and show two peaks2hours before and after landing.The eolution curve of friction velocity has a high correlation coefficient with windspeed. The atmospheric stability is mainly neutral during influence of typhoon. It hasa sudden jump into the stable stratification when the cyclone is closest to theobservation site. During Chanthu process, the mean tubulence intensities of threedimensions are Iu=0.098，Iv=0.084，Iw=0.055, which are close to the values ofaverage results under onshore wind in this area. While in Hagupit process, the corresponding values are Iu=0.161，Iv=0.120，Iw=0.079, which are close to theoverall average results on the south China coast. The evolution of drag coefficient ismuch similar to friction velocity dring Chathu procoess. They’re both influenced bythe wind speed and the underlying surface roughness.Under both offshore and onshore winds, friction velocity and wind speed can befitted to a linear relationship, and sea surface aerodynamic roughness increase rapidlywith wind speed in e exponential form in two typhoon processes. However,the turbulence parameters under offshore and onshore winds show greater contrastdifference in Hagupit process than in Chanthu process. And the results under offshorewind are much larger than that under onshore wind in Hagupit process. While inChanthu process, the tubulence characteristics at observation site are more similar tothat over marine or mixed underlaying surface. It’s the wind direction that caused thisphenomenon. It blows easterly and northeasterly at the observation site in Chanthuprocess. There’s no obvious distinction between the two airflows. They can notrepresent the typical marine and land underlying surface, and it results in littlediffetence under offshore and onshore winds. While in Hagupit process, it blowsnorthwestly before landing and southeastly wind after landing. The characterics ofupstream underlyings are totally different, so the airflow of these two directioncarry absolutely different surface informations. The results of comparison of the twotyphoons also indicate that, only considering wind direction is not enough whenstudying on the turbulence parameters in the South China coast area boundary. If wecan track the trajectory of the particle to determine the underlying surface features thatthrough, we can understand the local tubuence better and that would be much morefavorable to determine the boundary layer tubulent parameters.