The Propagation of Gravity Waves in Titan's Stratosphere.

Gravity waves (GWs) are ubiquitous and important dynamical processes in planetary atmospheres. But their properties and impact on the lower atmosphere remain unclear for most of planets due to the lack of data. The recent in situ observation from Huygens reveals GW activity in Titan's lower stratosphere. This paper investigates the upward propagation of GWs from Titan's lower atmosphere and their thermal effects using a full‐wave model. We reproduce the observed temperature perturbations with a superposition of three GW solutions with λx = 50 km and λz = 5.3, 8.9, and 28 km, with the longer wavelength one overlooked in previous studies. The simulation suggests that the propagation of GWs in Titan's lower atmosphere is almost nondissipative and thus has no wave‐induced thermal effect on the stratosphere, which is very different from the planetary thermosphere. The temperature minimum at the tropopause leads to a rapid local GW growth, which may be the primary reason for the significant GW signals above 60 km. We also find that the zonal wind may filter out the majority of GWs except for those traveling perpendicular to the wind direction, which can propagate upward to above 100 km, as observed by the Huygens probe. Plain Language Summary: Gravity waves are wave‐like air motions in the atmosphere of various planets. However, due to the lack of observations, their detailed nature and impact on the lower atmosphere are unclear for most planets, including Titan, a moon of Saturn. Fortunately, recent observations from the Huygens spacecraft obtained the temperature profile of Titan's lower atmosphere, enabling the study of gravity wave (GW) activity there. This paper aims to use a model to simulate and study the observed GWs on Titan. We reproduce the observed temperature wave structure with three kinds of GWs of different spatial scales. We found that when GWs travel in the lower atmosphere of Titan, they are not weakened by the air as that occurs in the upper atmosphere. As a result, there is also no heating effect from the waves in the lower atmosphere. Instead, a layer of cool air at around 60 km altitude may lead to a rapid growth of GWs, which may be the main reason for much larger GW signals above 60 km than below. We also found that atmospheric wind can stop most of the GWs from traveling upward into the higher atmosphere of Titan. Key Points: The temperature fluctuations observed by the Huygens probe between 10 and 140 km are well reproduced by our full‐wave modelThe propagation of gravity waves (GWs) in Titan's lower stratosphere is almost nondissipative and has limited wave‐induced thermal effectThe rapid GW growth at the tropopause is caused by the thermal structure rather than the wind effect [ABSTRACT FROM AUTHOR]