Characteristics of instability of radiation-induced natural convection in shallow littoral waters

Mixing and transport of terrestrially derived solutes and particulates in littoral regions depend heavily on the characteristics of flow instability. The criteria for radiation-induced instability to occur and the scope of the unstable region in near shore waters were revealed in our previously reported scaling analysis. In the present paper, the characteristics of instability of radiation-induced natural convection in shallow littoral regions are considered. Spectral analysis is conducted to reveal the frequency modes and the power of instability at various positions for different Rayleigh numbers. Results indicate that harmonic frequency modes are present at relatively low Rayleigh numbers. Both the power of instability and the prominence of higher frequency modes increase with offshore distance and the Rayleigh number for the considered shallow region. For a given offshore distance, the frequency modes of instability are the same over the local depth, but the prominence of higher frequency modes increases with depth. It is found that the scaling relation for the stable convection region specified in the previous investigation is also applicable to the time-averaged mean flow in the unstable region.