Mars Climate Sounder Observations of Gravity-wave Activity throughout Mars’s Lower Atmosphere

Abstract

Gravity waves are one way Mars’s lower atmospheric weather can affect the circulation and even composition of Mars’s middle and upper atmosphere. A recent study showed how on-planet observations near the center of the 15 μm CO2 band by the A3 channel (635–665 cm−1) of the Mars Climate Sounder on board the Mars Reconnaissance Orbiter could sense horizontally short, vertically broad gravity waves at ≈25 km above the surface by looking at small-scale radiance variability in temperature-sensitive channels. This approach is extended here to two additional channels closer to the wings of the 15 μm CO2 band, A1 (595–615 cm−1) and A2 (615–645 cm−1), to sense gravity waves throughout the lower atmosphere. Using information from all three channels demonstrates that gravity-wave activity in Mars’s lowermost atmosphere is dominated by orographic sources, particularly over the extremely rough terrain of Valles Marineris. Much of this orographic population is either trapped or filtered in the lowest two scale heights, such that variations in filtering and nonorographic sources shape the gravity-wave population observed at 25 km above the surface. During global dust storms, however, gravity-wave activity in the first scale height decreases by approximately a factor of 2, yet trapping/filtering of what activity remains in the tropics substantially weakens. Exceptionally high radiance variability at night in the tropics during the less dusty part of the year is the result of observing mesospheric clouds rather than gravity waves.

Gravity Waves in Different Atmospheric Layers During Martian Dust Storms

“…The reduction of GW activity in the lower atmosphere (in this work corresponding to the Lp layer) during large dust storms have been confirmed here and in previous studies that used different GW extraction methods and covered different ranges of frequency spectrum or vertical wavelength (Heavens et al., 2020, 2022; Kuroda et al., 2020). During storm time, the increased airborne aerosols are supposed to cause baroclinic and convective stabilization of the lower atmosphere (Kuroda et al., 2020).…”

The Horizontal Wavelength Spectrum of Gravity Wave Activity in Mars’s Lower Atmosphere: The Perspective from MGS–TES Nadir Observations

“…No attempt was made to filter the observations for strong dust, water ice, or mineralogical features in the spectra. Analysis of MRO-MCS observations suggests off-nadir observations of the 15 μm CO 2 band can be significantly affected by scattering by CO 2 ice, but this effect has not been noted in nadir observations, because of the different geometry and shorter path length of the observations (Heavens et al 2020(Heavens et al , 2022.…”

“…The parameters ò GW,Λ (the 1σ uncertainty in GW variance in an observation), GW, Ŵ L (the GW variance in an observation normalized by the mean of the squared temperature in each measurement within the observation), and GW, ˆL  (the 1σ uncertainty in the normalized variance), as well as the relevant time/location/elevation information, were calculated for sets of measurements in each baseline as in Heavens et al (2020Heavens et al ( , 2022, with some minor adjustments. In the case of uncertainties, the general procedure was to add the assumed instrumental noise in quadrature.…”

“…For comparison purposes, the analysis of MRO-MCS nadir observations reported by Heavens et al (2022) was obtained (Heavens 2022). This analysis is equivalent to calculating Ω GW,21 km , etc., just for MRO-MCS data.…”

“…Creasey et al (2006a) analyzed wave perturbations in radio occultation profiles from Mars Global Surveyor (MGS), broadly sampling GW activity at baselines (and approximate horizontal wavelengths) of >200 km and vertical wavelengths >10 km. More recently, Heavens et al (2020Heavens et al ( , 2022 used radiance observations of the Mars Climate Sounder (MCS) on board the Mars Reconnaissance Orbiter (MRO) in three channels near 15 μm to characterize the climatology of lower-atmospheric GW with 10-100 km horizontal wavelengths and vertical wavelengths ranging from 5 to 25 km for the off-nadir observations principally analyzed and >30-50 km for the limited number of nadir observations available. Note that the data analyzed by Heavens et al (2020Heavens et al ( , 2022 have >300×denser sampling than those analyzed by Creasey et al (2006a).…”