Neptune’s Vertical Wind Shear Modeled with a Generalized Thermal Wind Equation

Abstract:

We present observations of Neptune taken in the H-(1.4-1.8 μm) and K’-(2.0-2.4 μm) bands from the 10-m W.M. Keck II Telescope using NIRC2 coupled to the Adaptive Optics (AO) system. Images in both bands were taken on July 3, 2013 and August 20, 2014 over a span of 4-5 hours. We tracked the positions of dozens of bright atmospheric features over the observing nights and constructeded zonal wind profiles from changes in their longitudinal positions.

We confirm evidence of dispersion in Neptune’s zonal wind velocities about the smooth Voy- ager wind profile of Sromovsky et al. (1993) seen in Martin et al. (2012) and Fitzpatrick et al. (2014). In addition, we find significant differences between the wind speeds in the H- and K’- bands at the equatorial regions. To date, the thermal winds have been dismissed as a mechanism to explain these differences. However, these studies have relied on a simplified form of the thermal wind equation that assumes purely geostrophic flow and small Rossby numbers that are not applicable to Neptune. We present a model based on a generalized form of the thermal wind equation that explains the sign and magnitude of the observed vertical wind shear near the equator.