A study of large wind shears near the mesopause and the tropopause

Friday, March 31, 2017

The NCAR Whole Atmosphere Community Climate Model (WACCM), with a quasi-uniform horizontal resolution of $\sim$25km and a vertical resolution of 0.1 scale height, produces large horizontal winds and vertical shears maximizing in the mesosphere and lower thermosphere (MLT), similar to those found in rocket observations.

Vertical profiles of maximum wind shear image
Vertical profiles of maximum wind shear (grey) and the 99.9th percentile shear values (black) at (a) 71S (b) the equator and (c) 72N. These profiles are obtained from high-resolution WACCM simulation, and the large wind shears near the mesopause are similar to the observed wind shears. Previous global model simulations (TIME-GCM and WACCM with regular 2 degree resolution) could not reproduce the observed large wind shears.

The model also produces large vertical wind shears near the tropopause. The static stability, measured by buoyancy frequency, is maximum at the mesopause, and reaches extrema values at the tropopause. The flow can therefore attain large vertical wind shears in these regions before becoming dynamically unstable. By analyzing the probability density functions of the wind shears and their dependence on the zonal scales, it is found that smaller scale processes, most likely gravity waves, contribute most significantly to the large shears. Tidal waves generally have secondary contribution to the large winds and shears, but they can modulate wind and shear perturbations by GWs in the MLT. Implications for tracer transport and mixing in these regions are discussed by estimating diffusion coefficients based on the root mean square winds and corresponding spatial scales from model results.