Afternoon Effect Studies. Part 1

Afternoon effect is a type of sonar performance loss encountered during the afternoons and evenings of calm sunny days. It is caused by a temperature rise near the top of the otherwise isothermal mixed layer, increasing the sound speed there and so refracting the sound rays downwards, away from the sonar target. The temperature rise profile is governed by wind speed, solar heating and other meteorological factors, so that afternoon effect prediction should become an extension of meteorological forecasting. However there is at present no established method for computing the temperature profile from the meteorological information. A mixed-layer model is presented which appears to meet this need, subject to further testing. The influences of some key environmental factors are explored. An example of afternoon effect prediction is given, and the next steps towards a routine prediction facility are indicated. (Author).