Wavelet Analysis of the effects of Gravity Waves on Water Vapor in the Tropical Tropopause Layer over Darwin, Australia

The effects of inertia-gravity waves on the vertical profile of water vapor in the tropical tropopause layer (TTL) over the Dept. of Energy Atmospheric Radiation Measurements (ARM) program’s Darwin, Australia (12°S, 130°E) site is investigated using a seven year record of Vaisala RS92 radiosonde profiles of wind and temperature, taken every 12 hours from January 2006 through July 2012. The atmospheric state classification algorithm according to Evans et al. (2012) is employed to categorize different wind, temperature, and relative humidity (RH) profiles. Gravity wave characteristics are computed following the Torrence & Compo (1998) wavelet analysis methodology, where perturbation zonal wind and temperature are decomposed into time (altitude)/frequency (wavelength) space. The 2-s temporal resolution in the radiosonde vertical rise rate allows for assessment of gravity waves with vertical scales of a few hundred meters to about 10 km.

Composite analyses for each class show a robust power maximum in the TTL centered on the 2-6 km vertical wavelength band, which is strongest during wet season monsoon phases. A climatological mixing ratio minimum occurs near the tropical tropopause in the monsoon states as well, which was also shown in previous studies, but despite this, a maximum in RH and RH with respect to ice (RHI) occurs within the TTL. Profiles of perturbation temperature reveal a maximum negative perturbation for all classes within the TTL, with the strongest perturbations associated with the monsoon classes. This, coupled with the respective maximum in RH/RHI, cloud fraction and wave power, provides evidence that gravity waves aid the stratospheric dehydration process by cooling the TTL. Additionally, the direction of gravity wave energy propagation and horizontal momentum flux is estimated from radiosonde profiles of eddy horizontal winds and linked to the location of predominant convective activity.