DIAS: Differential Absorption of Sound to Obtain Temperature Profiles from Multifrequency Sodar

Abstract

The differential absorption of sound (DIAS) depends on frequency, temperature, and humidity in a known fashion. By obtaining turbulent reflections at two or more frequencies from the same target air mass, it is possible to resolve other frequency-dependent factors and obtain estimates of the absorption coefficient at the sounding frequencies. Over a restricted range of frequency and humidity, it is shown that the differential absorption is essentially a function of temperature but not of humidity. This method therefore provides the potential for temperature sounding using conventional sodars. The theory of this method is outlined, and sensitivity is estimated based on actual sodar data. As part of the Tekapo Field Experiment in the Southern Alps, two sodars were operated side by side at a range of frequencies, allowing estimates to be made of signal-to-noise ratio. Temperature retrievals using this method are discussed.