Microwave radiometer observations of the ozone diurnal cycle and its short-term variability over Switzerland

Abstract

Abstract. In Switzerland, two ground-based ozone microwave radiometers are operated in the vicinity of each other (ca. 40 km): the GROund-based Millimeter-wave Ozone Spectrometer (GROMOS) in Bern (Institute of Applied Physics) and the Stratospheric Ozone MOnitoring RAdiometer (SOMORA) in Payerne (MeteoSwiss). Recently, their calibration and retrieval algorithms have been fully harmonized, and updated time series are now available since 2009. Using these harmonized ozone time series, we investigate and cross-validate the strato–mesospheric ozone diurnal cycle derived from the two instruments and compare it with various model-based datasets: the dedicated GEOS-GMI Diurnal Ozone Climatology (GDOC) based on the Goddard Earth Observing System (GEOS-5) general circulation model, the Belgian Assimilation System for Chemical ObsErvations (BASCOE) – a chemical transport model driven by ERA5 dynamics, and a set of free-running simulations from the Whole Atmosphere Community Climate Model (WACCM). Overall, the two instruments show very similar ozone diurnal cycles at all seasons and pressure levels, and the models compare well with each other. There is a good agreement between the models and the measurements at most seasons and pressure levels, and the largest discrepancies can be explained by the limited vertical resolution of the microwave radiometers. However, as in a similar study over Mauna Loa, some discrepancies remain near the stratopause, at the transition region between ozone daytime accumulation and depletion. We report similar delays in the onset of the modelled ozone diurnal depletion in the lower mesosphere. Using the newly harmonized time series of GROMOS and SOMORA radiometers, we present the first observations of short-term (sub-monthly) ozone diurnal cycle variability at mid-latitudes. The short-term variability is observed in the upper stratosphere during wintertime, when the mean monthly cycle has a small amplitude and when the dynamics are more important. This is shown in the form of strong enhancements of the diurnal cycle, reaching up to 4–5 times the amplitude of the mean monthly cycle. We show that BASCOE is able to capture some of these events, and we present a case study of one such event following the minor sudden stratospheric warming of January 2015. Our analysis of this event supports the conclusions of a previous modelling study, attributing regional variability of the ozone diurnal cycle to regional anomalies in nitrogen oxide (NOx) concentrations. However, we also find periods with an enhanced diurnal cycle that do not show much change in NOx and where other processes might be dominant (e.g. atmospheric tides). Given its importance, we believe that the short-term variability of the ozone diurnal cycle should be further investigated over the globe, for instance using the BASCOE model.

Thermal tides in the middle atmosphere at mid-latitudes measured with a ground-based microwave Radiometer

“…Since heating due to the absorption of solar radiation by ozone is the main tidal forcing mechanism at these altitudes, changes in ozone VMR are expected to be reflected in our tidal amplitude measurements. A similar calculation as in Figure 11 was also shown in (Sauvageat et al, 2023). A more detailed analysis of the ozone diurnal cycle was already performed in Schranz et al (2018) and provides a pathway to future analysis.…”

Air Temperature Intermittency and Photofragment Excitation

“…One is the extent of effects from photofragment excitation on temperature intermittency, especially in the upper stratosphere where a cold bias in model-based meteorological analyses exists relative to observations. Those effects could extend upwards into the mesosphere [22,75] and also affect the area of the winter polar vortex in the Southern Hemisphere stratosphere, which has been historically problematic in free running models, with the vortex area decreasing rather than increasing with altitude [76]. The other is whether the scaling exponents for temperature and wind speed in the polar front jet stream and its 'waviness' show any changes caused by global heating since they were last measured up to three decades ago.…”

“…An outstanding problem in the stratosphere occurs far above aircraft altitudes, regarding the cold bias in meteorological analyses compared to lidar observations [22,25]. A high-altitude balloon would seem to be the only option to test the J[O 3 ] vs. C 1 (T) correlation, possibly combined with dropsondes through the depth of the stratosphere.…”

“…From a large wealth of the literature, [20] was chosen to represent the laboratory observed and modelled effects of photofragment excitation on the production of odd oxygen. The atmospheric observations of the diurnal variation are represented by [21,22]. The added effect considered here is the behaviour of ozone and temperature; the correlation of the ozone photodissociation rate with the intermittency of temperature in the lower stratosphere was originally proposed in [23] and included recently in a discussion of scale invariance from molecular to meteorological scales [24].…”

“…A consequence of the issues raised in [20][21][22][23][24][25] is the possibility of influence on isotope effects [26]; they will require further investigation if local thermodynamic equilibrium (LTE) does not pertain in the stratosphere. LTE is known not to be a universally good approximation above the stratopause, about 50 km altitude.…”