Near surface spatially averaged air temperature and wind speed determined by acoustic travel time tomography

Acoustic travel time tomography is presented as a possibility for remote mo nitoring of near surface air temperature and wind fields. This technique pr ovides line-averaged effective sound speeds changing with temporally and sp atially variable air temperature and wind vector. The effective sound speed is derived from the travel times of sound signals which propagate at defin ed paths between different acoustic sources and receivers. Starting with th e travel time data a tomographic algorithm (Simultaneous iterative Reconstr uction Technique, SIRT) is used to calculate area-averaged air temperature and wind speed. The accuracy of the experimental method and the tomographic inversion algorithm is exemplarily demonstrated for one day without remark able differences in the horizontal temperature field. determined by indepen dent in situ measurements at different points within the measuring field. T he differences between the conventionally determined air temperature (point measurement) and the air temperature determined by tomography (area-averag ed measurement representative for the area of the measuring field 200 m x 3 60 m) were below 0.5 K for an average time of 10 minutes. The differences o btained between the wind speed measured at a meteorological mast and calcul ated from acoustic measurements are not higher than 0.5 m s(-1) for the sam e averaging time. The tomographically determined area-averaged distribution of air temperature (resolution 50 m x 50 m) can be used to estimate the ho rizontal gradient of air temperature as a pre-condition to detect horizonta l turbulent fluxes of sensible heat.