Stratified Shear Instabilities in Diurnal Warm Layers

摘要

In low winds (less than or similar to 2 m s(-1)), diurnal warm layers form, but shear in the near-surface jet is too weak to generate shear instability and mixing. In high winds (greater than or similar to 8 m s(-1)), surface heat is rapidly mixed downward and diurnal warm layers do not form. Under moderate winds of 3-5 m s(-1), the jet persists for several hours in a state that is susceptible to shear instability. We observe low Richardson numbers of Ri approximate to 0.1 in the top 2 m between 1000 and 1600 local time (LT) (from 4 h after sunrise to 2 h before sunset). Despite Ri being well below the Ri = 1/4 threshold, instabilities do not grow quickly, nor do they overturn. The stabilizing influence of the sea surface limits growth, a result demonstrated by both linear stability analysis and two-dimensional simulations initialized from observed profiles. In some cases, growth rates are sufficiently small ( 1 h(-1)) thatmixing is not expected even though Ri 1/ 4. This changes around 1600-1700 LT. Thereafter, convective cooling causes the region of unstable flow to move downward, away from the surface. This allows shear instabilities to grow an order-of-magnitude faster and mix effectively. We corroborate the overall observed diurnal cycle of instability with a freely evolving, two-dimensional simulation that is initialized from rest before sunrise.