The Observational Research On The Standardization Of Wind Field In Typhoon Boundary Layer

Using observational data from three landfall typhoons obtained by the ultrasonic anemometers, which deployed at different heights of a tower located on the seaside of Chihu Town in Zhangpu County, on the basis of systematic and stringent data quality control:variations of gust factor with mean wind speed and altitude under different underlying surface were firstly studied. Effects of gust duration on gust factor were also investigated. Converting coefficients that relate maximum wind speeds with different averaging periods of Imin,2min and 10min, which characterizing typhoon intensity adopted by different institutes, were then deduced and compared with those suggested by the WMO. Then the average roughness length of nineteen fetch were calculated by two kinds of method including profile of logarithmic law and ESDU gust factor model in order to provide objective parameter that describing underlying surface for the wind field's standardized transformation of landfall typhoons. It has to be said that using method of logarithmic law profile was the premise of testing the reliability of wind speed prediction. At the meantime, both the relationship between off-sea drag coefficient and mean wind speed and wind profile discrepancy under the condition of typhoon and normal wind by mean of exponential law were also analyzed. The converting of ten minutes average wind field or Imin or 2min peak wind field at arbitrary height of the boundary layer and in any terrain to the corresponding wind field which at the standard height and under standard underlying surface were realized through the transformation model of wind field standardization afterwards. Finally, results from wind field standardization and from operational strength of typhoon were compared. The analysis results were summarized as follows:1. Roughness length under different underlying surface has a significant effect on gust factor and conversion coefficients. Gust factor of off-sea wind increased with the increase of mean wind speed while there's a decreasing trend for the offshore wind. A comparison was made between the ESDU gust fact curve of off-sea and off-sea observation, which were concordant quite well, but it's inadequate for ESDU curve to represent change rule of offshore gust factor with the mean wind speed. Converting coefficient of maximum wind speed from 2min to 10min in this paper is about 7% smaller than that suggested by the WMO, which indicates that utilizing WMO's value has a probability of producing a higher intensity estimate and with the needs of further observational verification.2. Wind speeds at 10m height forecast by logarithmic law fitting of four layers data have certain reliability. And mean roughness in each fetch which evaluated by mean of wind profile in the shape of logarithmic law and in ways of EDSU gust factor mode were reasonable. Considering drag coefficient from off-sea wind, it decreased with the increase of mean wind speed when wind speed was smaller than 11m/s, then it increased with the increase of mean wind speed when wind speed was larger than 11m/s, after reaching the maximum value when mean wind got to about 21m/s, the drag coefficient reduced again. Drag coefficients within the speed range of 11m/s?21m/s were consistent with the theoretical result of Powell.3. Most of the absolute relative errors that related standardized wind field with standard observational wind field keep within 10% although the errors were affected by altitude, underlying surface roughness and the size of average wind speed, while the absolute relative errors which related unstandardized wind field and standard observational wind field would increase to more than 50%. To sum up, both the ESDU gust factor model and the model standardization of wind field feature high reliability, and the converted results of wind speed could well represent the wind-force caused by typhoon, in which simultaneously satisfy the condition of at the standard height and under standard underlying surface.4. It's suggested that the results of wind field standardization and of typhoon operational strength are accordant by the comparison between them, which in other word, the standardization model of typhoon wind field developed in this paper can be used as a reference of determining the strength of typhoon.