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Register a new userCharacterising atmospheric gravity waves on the nightside lower clouds of Venus: a systematic analysis
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Added by
Jos\\'e Eduardo Silva
Publication date
2021
fields
Physics
and research's language is
English
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We present the detection and characterisation of mesoscale waves on the lower clouds of Venus using images from the Visible Infrared Thermal Imaging Spectrometer onboard the European Venus Express space mission and from the 2 $mu$m camera (IR2) instrument onboard the Japanese space mission Akatsuki. We used image navigation and processing techniques based on contrast enhancement and geometrical projections to characterise morphological properties of the detected waves, such as horizontal wavelength and the relative optical thickness drop between crests and troughs. Additionally, we performed phase velocity and trajectory tracking of wave packets. We combined these observations to derive other properties of the waves such as the vertical wavelength of detected packets. Our observations include 13 months of data from August 2007 to October 2008, and the entire available data set of IR2 from January to November 2016.We characterised almost 300 wave packets across more than 5500 images over a broad region of the globe of Venus. Our results show a wide range of properties and are not only consistent with previous observations but also expand upon them, taking advantage of two instruments that target the same cloud layer of Venus across multiple periods. In general, waves observed on the nightside lower cloud are of a larger scale than the gravity waves reported in the upper cloud. This paper is intended to provide a more in-depth view of atmospheric gravity waves on the lower cloud and enable follow-up works on their influence in the general circulation of Venus.
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Planetary-scale waves are thought to play a role in powering the yet-unexplained atmospheric superrotation of Venus. Puzzlingly, while Kelvin, Rossby and stationary waves manifest at the upper clouds (65--70 km), no planetary-scale waves or stationary patterns have been reported in the intervening level of the lower clouds (48--55 km), although the latter are probably Lee waves. Using observations by the Akatsuki orbiter and ground-based telescopes, we show that the lower clouds follow a regular cycle punctuated between 30$^{circ}$N--40$^{circ}$S by a sharp discontinuity or disruption with potential implications to Venuss general circulation and thermal structure. This disruption exhibits a westward rotation period of $sim$4.9 days faster than winds at this level ($sim$6-day period), alters clouds properties and aerosols, and remains coherent during weeks. Past observations reveal its recurrent nature since at least 1983, and numerical simulations show that a nonlinear Kelvin wave reproduces many of its properties.
We present measurements of the wind speeds at the nightside lower clouds of Venus from observations by JAXAs mission Akatsuki during 2016, complemented with new wind measurements from ground-based observations acquired with TNG/NICS in 2012 and IRTF/SpeX in 2015 and 2017. Zonal and meridional components of the winds were measured from cloud tracking on a total of 466 Akatsuki images of Venus acquired by the camera IR2 using the 2.26-$mathrm{mu m}$ filter, with spatial resolutions ranging 10--80 km per pixel and covering from 2016 March 22 to October 31. More than 149,000 wind vectors were obtained with an automatic technique of template matching, and 2,947 wind vectors were inferred with the manual procedure. The meridional profiles for both components of the winds are found to be consistent with results from the Venus Express mission during 2006--2008, although stronger wind variability is found for the zonal component at equatorial latitudes where Akatsuki observations have better viewing geometry than Venus Express. The zonal winds at low latitudes also suggest a zonal variability that could be associated with solar tides or vertically propagating orographic waves. Finally, the combination of our wind measurements from TNG/NICS, IRTF/SpeX and Akatsuki images with previously published and based in data from 1978 to 2017 suggests variations of up to 30 m s$^{-1}$ in the winds at the lower clouds of the Venus nightside.
One of the striking features about Venus atmosphere is its temporal variability and dynamics, with a chaotic polar vortex, large-scale atmospheric waves, sheared features, and variable winds that depend on local time and possibly orographic features. The aim of this research is to combine data accumulated over several years and obtain a global mean state of the atmosphere focusing in the global structure of the clouds using the cloud opacity and upper cloud temperatures. We have first produced global maps using the integrated radiance through the infrared atmospheric windows centred around 1.74{mu}m and 2.25{mu}m, that show the spatial variations of the cloud opacity in the lower clouds around 44-48 km altitude and also provide an indirect estimation of the possible particle size. We have also produced similar global maps using the brightness temperatures seen in the thermal region at 3.8{mu}m and 5.0{mu}m, which provide direct indication of the temperatures at the top of the clouds around 60-70 km altitude. These maps have been generated using the complete dataset of the Visible and InfraRed Thermal Imaging Spectrometer mapping channel (VIRTIS-M) on board Venus Express, with a wide spatial and long temporal coverage in the period from May 2006 until October 2008. Our results provide a global view of the cloud opacity, particle size and upper cloud temperatures at both hemispheres, showing the main different dynamical regions of the planet. The profiles obtained also provide the detailed dependencies with latitude, local time and longitude, diagnostic of the global circulation flow and dynamics at various altitude layers, from about 44 up to 70 km over the surface.
We first respond to two points raised by Villanueva et al. We show the JCMT discovery spectrum of PH3 can not be re-attributed to SO2, as the line width is larger than observed for SO2 features, and the required abundance would be an extreme outlier. The JCMT spectrum is also consistent with our simple model, constant PH3-abundance with altitude, with no discrepancy in line profile (within data limits); reconciliation with a full photochemical model is the subject of future work. Section 2 presents initial results from re-processed ALMA data. Villanueva et al. noted an issue with bandpass calibration. They have worked on a partially re-processed subset of the ALMA data, so we note where their conclusions, and those of Greaves et al., are now superseded. To summarise: we recover PH3 in Venus atmosphere with ALMA (~5{sigma} confidence). Localised abundance appears to peak at ~5-10 parts-per-billion (ppb), with suggestions of spatial variation. Advanced data-products suggest a planet-averaged PH3 abundance ~1-4 ppb, lower than from the earlier ALMA processing (which indicated 7+ ppb). The ALMA data are reconcilable with the JCMT detection (~20 ppb) if there is order-of-magnitude temporal variation; more advanced processing of the JCMT data is underway to check methods. Independent PH3 measurements suggest possible altitude dependence (under ~5 ppb at 60+ km, up to ~100 ppb at 50+ km; see Section 2: Conclusions.). Given that both ALMA and JCMT were working at the limit of observatory capabilities, new spectra should be obtained. The ALMA data in-hand are no longer limited by calibration, but spectral ripples still exist, probably due to size and brightness of Venus in relation to the primary beam. Further, spatial ripples are present, potentially reducing significance of real narrow spectral features.
We published spectra of phosphine molecules in Venus clouds, following open-science principles in releasing data and scripts (with community input leading to ALMA re-processing, now benefiting multiple projects). Some misconceptions about de-trending of spectral baselines have also emerged, which we address here. Using the JCMT PH3-discovery data, we show that mathematically-correct polynomial fitting of periodic ripples does not lead to fake lines (probability < ~1%). We then show that the ripples can be characterised in a non-subjective manner via Fourier transforms. A 20 ppb PH3 feature is ~5{sigma} compared to the JCMT baseline-uncertainty, and is distinctive as a narrow perturber of the periodic ripple pattern. The structure of the FT-derived baseline also shows that polynomial fitting, if unguided, can amplify artefacts and so artificially reduce significance of real lines.
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