OClO as observed by TROPOMI: a comparison with meteorological parameters and polar stratospheric cloud observations

Chlorine dioxide (OClO) is a by-product of the ozone-depleting halogen chemistry in the stratosphere. Although it is rapidly photolysed at low solar zenith angles (SZAs), it plays an important role as an indicator of the chlorine activation in polar regions during polar winter and spring at twilight conditions because of the nearly linear dependence of its formation on chlorine oxide (ClO). Here, we compare slant column densities (SCDs) of chlorine dioxide (OClO) retrieved by means of differential optical absorption spectroscopy (DOAS) from spectra measured by the TROPOspheric Monitoring Instrument (TROPOMI) with meteorological data for both Antarctic and Arctic regions for the first three winters in each of the hemispheres (November 2017–October 2020). TROPOMI, a UV–Vis–NIR–SWIR instrument on board of the Sentinel-5P satellite, monitors the Earth's atmosphere in a near-polar orbit at an unprecedented spatial resolution and signal-to-noise ratio and provides daily global coverage at the Equator and thus even more frequent observations at polar regions. The observed OClO SCDs are generally well correlated with the meteorological conditions in the polar winter stratosphere; for example, the chlorine activation signal appears as a sharp gradient in the time series of the OClO SCDs once the temperature drops to values well below the nitric acid trihydrate (NAT) existence temperature (TNAT). Also a relation of enhanced OClO values at lee sides of mountains can be observed at the beginning of the winters, indicating a possible effect of lee waves on chlorine activation. The dataset is also compared with CALIPSO Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) polar stratospheric cloud (PSC) observations. In general, OClO SCDs coincide well with CALIOP measurements for which PSCs are detected. Very high OClO levels are observed for the northern hemispheric winter 2019/20, with an extraordinarily long period with a stable polar vortex being even close to the values found for southern hemispheric winters. An extraordinary winter in the Southern Hemisphere was also observed in 2019, with a minor sudden stratospheric warming at the beginning of September. In this winter, similar OClO values were measured in comparison to the previous (usual) winter till that event but with a OClO deactivation that was 1–2 weeks earlier.

Laboratory and numerical experiments on the near wake of a sphere in a stably stratified ambient

The flow around and behind a sphere in a linear density gradient has served as a model problem for both body-generated wakes in atmospheres and oceans, and as a means of generating a patch of turbulence that then decays in a stratified ambient. Here, experiments and numerical simulations are conducted for 20 values of Reynolds number, $Re$ , and internal Froude number, $Fr$ , where each is varied independently. In all cases, the early wake is affected by the background density gradient, notably in the form of the body-generated lee waves. Mean and fluctuating quantities do not reach similar states, and their subsequent evolution would not be collapsible under any universal scaling. There are five distinguishable flow regimes, which mostly overlap with previous literature based on qualitative visualisations and, in this parameter space, they maintain their distinguishing features up to and including buoyancy times of 20. The possible relation of the low $\{Re, Fr\}$ flows to their higher $\{Re, Fr\}$ counterparts is discussed.

Climatology of mountain waves over Indian region based on the study of NOAA satellite imageries

NOAA (AVHRR) imageries have been studied for four years from 1982 to 1985. Overlapping wave zones in different imageries of the same fortnight have been given successively higher weightage in the scale of 1 to 7. Different monochromatic hatching scheme for different weight has been adopted. Direction of wind at the level of wave and its wavelength has also been marked over the region. Thus, the climatology has been documented into 24 fortnights. The following two periods emerge, while waves are generally seen over the region. 16 November to 15 April- This includes Winter Season upto the beginning of pre-monsoon. 01 June to 15 October -This includes Southwest monsoon upto the third week of postmonsoon. The occurrence of lee waves during the transition period is less. They are the second fortnight of April till the end of May and the second fortnight of October till the middle of November. A scrupulous observation shows that during both the periods (i) and (ii) mentioned above there is systematic eastward shift of the leewave zones with gradual induction of different airmasses over Indian subcontinent from beginning till end. A pictorial representation of the waves over India (West of 90° E Long.) is presented here.

Internal solitary waves in the Northwest Sumatra Sea-Indonesia: from observation and modeling

>The southern Andaman waters has been well known as one of the strongest generating and propagating area of internal solitary waves (ISWs), generated by semidiurnal barotropic tidal currents that impinge submarine ridge offshore western Weh. This study aims to investigate sea surface features of internal tides and tidal current around the submarine ridge and adjacent Weh-Aceh waters, derived from satellite imagery datasets (January-May 2018) and CROCO model-output datasets. The results show that sea surface signatures of ISWs are characterized by a strong radar signal backscattering of a dense ripple package in the generating area and two groups of ISWs arch in the propagating area, where the distance of the package groups and wavelengths vary 60-80 km and 9-163 km, respectively. Observed ISWs in March 2018 was 31. The satellite and model datasets suggest that generating area of internal waves is confined over the Breuh ridge. Here, the very strong semidiurnal (M2) barotropic tidal currents of 0.5-5.0 m/s are observed. During high-tide, amplified barotropic tidal currents acrossing the ridge flow partly southeastward into the Weh-Breuh passage. The model suggests that generating internal tidal waves over the ridge are manifested by strong vertical perturbation of isopycnal and current stratifications in the Lee-waves area.

Wave drag by two-dimensional mountain lee waves

lkj & Å¡pkbZ ds lkFk ok;q ds jsf[kdh; :Ik ls c<+us vkSj fLFkjrk dks vifjorZuh; ekurs gq, f}foeh; ioZrh; vojks/k esa fLFkj Lrfjr ok;q&izokg okys jsf[kdh; nzoLFkSfrd fun’kZ dk bl 'kks/k&i= esa mi;ksx fd;k x;k gSA vle&cekZ dh igkfM+;ksa ds vkSj Hkkjr ds if’peh ?kkV ds ioZrh; ok;qjks/k vkSj ioZrh; vfHkokgksa ds fo’ys"k.kkRed vk¡dM+s izkIr fd, x, gSaA vle&cekZ dh igkfM+;ksa ds nksuksa fjtksa ds ioZrh; ok;qjks/k ds lkekU; vk¡dM+s Hkh izkIr fd, x, gaSA A linear hydrostatic model of a stably stratified air-stream flow over a two-dimensional orographic barrier is considered assuming wind increases linearly with height and stability is constant. Analytical expressions for mountain drags and momentum fluxes are obtained for Assam-Burma hills as well as Western Ghats of India. The general expression for mountain drag also obtained for both the ridges of Assam-Burma hills.

A closure for lee wave drag on the large-scale ocean circulation

A new, energetically and dynamically consistent closure for the lee wave drag on the large scale circulation is developed and tested in idealized and realistic ocean model simulations. The closure is based on the radiative transfer equation for internal gravity waves, integrated over wavenumber space, and consists of two lee wave energy compartments for up-and downward propagating waves, which can be co-integrated in an ocean model. Mean parameters for vertical propagation, mean-ow interaction, and the vertical wave momentum flux are calculated assuming that the lee waves stay close to the spectral shape given by linear theory of their generation.Idealized model simulations demonstrate how lee waves are generated and interact with the mean flow and contribute to mixing, and document parameter sensitivities. A realistic eddy-permitting global model at 1/10° resolution coupled to the new closure yields a globally integrated energy flux of 0.27 TW into the lee wave field. The bottom lee wave stress on the mean flow can be locally as large as the surface wind stress and can reach into the surface layer. The interior energy transfers by the stress are directed from the mean flow to the waves, but this often reverses, for example in the Southern Ocean in case of shear reversal close to the bottom. The global integral of the interior energy transfers from mean ow to waves is 0.14 TW, while 0.04 TW is driving the mean ow, but this share depends on parameter choices for non-linear effects.

OClO as observed by TROPOMI: a comparison with meteorological parameters and PSC observations

Chlorine dioxide (OClO) is a by-product of the ozone depleting halogen chemistry in the stratosphere. Although being rapidly photolysed at low solar zenith angles (SZAs) it plays an important role as an indicator of the chlorine activation in polar regions during polar winter and spring at twilight conditions because of the nearly linear dependence of its formation to chlorine oxide (ClO). Here we compare slant column densities (SCDs) of chlorine dioxide (OClO) retrieved by means of differential optical absorption spectroscopy (DOAS) from spectra measured by the TROPOspheric Monitoring Instrument (TROPOMI) with meteorological data for both Antarctic and Arctic regions for the first three winters in each of the hemispheres (November 2017–October 2020). TROPOMI, a UV-VIS-NIR-SWIR instrument on board of the Sentinel-5P satellite monitors the Earth’s atmosphere in a near polar orbit at an unprecedented spatial resolution and signal to noise ratio and provides daily global coverage at the equator and thus even more frequent observations at polar regions. The observed OClO SCDs are generally well correlated with the meteorological conditions in the polar winter stratosphere: e.g. the chlorine activation signal appears as a sharp gradient in the time series of the OClO SCDs once the temperature drops to values well below the Nitric Acid Trihydrate (NAT) existence temperature TNAT. Also a relation of enhanced OClO values at lee sides of mountains can be observed at the beginning of the winters indicating a possible effect of occurring lee waves on chlorine activation. The dataset is also compared with CALIPSO Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) polar stratospheric cloud (PSC) observations. In general, OClO SCDs coincide well with CALIOP measurements for which PSCs are detected. Very high OClO levels are observed for the northern hemispheric winter 2019/2020 with an extraordinarly long period with a stable polar vortex being even close to the values found for Southern Hemispheric winters. Also the extraordinary winter in 2019 in the Southern Hemisphere with a minor sudden stratospheric warming at the beginning of September was observed. In this winter similar OClO values were measured in comparison to the previous (usual) winter till that event but with a 1–2 week earlier OClO deactivation.

The impact of lee waves on the Southern Ocean circulation

Lee waves play an important role in transferring energy from the geostrophic eddy field to turbulent mixing in the Southern Ocean. As such, lee waves can impact the Southern Ocean circulation and modulate its response to changing climate through their regulation on the eddy field and turbulent mixing. The drag effect of lee waves on the eddy field and the mixing effect of lee waves on the tracer field have been studied separately to show their importance. However, it remains unclear how the drag and mixing effects act together to modify the Southern Ocean circulation. In this study, a lee wave parameterization that includes both lee wave drag and its associated lee-wave-driven mixing is developed and implemented in an eddy-resolving idealized model of the Southern Ocean to simulate and quantify the impacts of lee waves on the Southern Ocean circulation. The results show that lee waves enhance the baroclinic transport of the Antarctic Circumpolar Current (ACC) and strengthen the lower overturning circulation. The impact of lee waves on the large-scale circulation are explained by the control of lee wave drag on isopycnal slopes through their effect on eddies, and by the control of lee-wave-driven mixing on deep stratification and water mass transformation. The results also show that the drag and mixing effects are coupled such that they act to weaken one another. The implication is that the future parameterization of lee waves in global ocean and climate models should take both drag and mixing effects into consideration for a more accurate representation of their impact on the ocean circulation.