Abstract
Abstract. An important source of polar stratospheric clouds (PSCs), which play a crucial role in controlling polar stratospheric ozone depletion, is the temperature fluctuations induced by mountain waves. These enable stratospheric temperatures to fall below the threshold value for PSC formation in regions of negative temperature perturbations or cooling phases induced by the waves even if the synoptic-scale temperatures are too high. However, this formation mechanism is usually missing in global chemistry–climate models because these temperature fluctuations are neither resolved nor parameterised. Here, we investigate in detail the episodic and localised wintertime stratospheric cooling events produced over the Antarctic Peninsula by a parameterisation of mountain-wave-induced temperature fluctuations inserted into a 30-year run of the global chemistry–climate configuration of the UM-UKCA (Unified Model – United Kingdom Chemistry and Aerosol) model. Comparison of the probability distribution of the parameterised cooling phases with those derived from climatologies of satellite-derived AIRS brightness temperature measurements and high-resolution radiosonde temperature soundings from Rothera Research Station on the Antarctic Peninsula shows that they broadly agree with the AIRS observations and agree well with the radiosonde observations, particularly in both cases for the “cold tails” of the distributions. It is further shown that adding the parameterised cooling phase to the resolved and synoptic-scale temperatures in the UM-UKCA model results in a considerable increase in the number of instances when minimum temperatures fall below the formation temperature for PSCs made from ice water during late austral autumn and early austral winter and early austral spring, and without the additional cooling phase the temperature rarely falls below the ice frost point temperature above the Antarctic Peninsula in the model. Similarly, it was found that the formation potential for PSCs made from ice water was many times larger if the additional cooling is included. For PSCs made from nitric acid trihydrate (NAT) particles it was only during October that the additional cooling is required for temperatures to fall below the NAT formation temperature threshold (despite more NAT PSCs occurring during other months). The additional cooling phases also resulted in an increase in the surface area density of NAT particles throughout the winter and early spring, which is important for chlorine activation. The parameterisation scheme was finally shown to make substantial differences to the distribution of total column ozone during October, resulting from a shift in the position of the polar vortex.
Highlights
Polar stratospheric clouds (PSCs) are important in polar ozone chemistry as reactions on their surfaces convert reservoir species into highly reactive ozone-destroying gases containing chlorine and bromine, which contribute to the depletion of the Antarctic and Arctic stratospheric ozone layer (Solomon, 1999)
Mountain-wave-induced PSC formation, which is a significant influence on ozone chemistry, is missing from current coarse-resolution global chemistry–climate models because the small-scale temperature fluctuations associated with mountain waves are neither resolved nor parameterised – limiting our ability to make accurate predictions of stratospheric ozone
May to October over the Antarctic Peninsula at an altitude of 20.4 km for PSCs composed of (a) nitric acid trihydrate (NAT) and (b) ice particles, i.e. the difference between using T equal to either TUM−United Kingdom Chemistry and Aerosol (UKCA) + TS−SO or TUM−UKCA
Summary
Polar stratospheric clouds (PSCs) are important in polar ozone chemistry as reactions on their surfaces convert reservoir species into highly reactive ozone-destroying gases containing chlorine and bromine, which contribute to the depletion of the Antarctic and Arctic stratospheric ozone layer (Solomon, 1999). If synoptic-scale temperatures are too high for the formation of PSCs, as can occur for example over the edge region of the Antarctic stratospheric vortex ( during early winter and early spring), the addition of negative temperature anomalies induced by vertically propagating wave motion forced by stratified flow over high mountains can result in temperatures falling below the thresholds for PSC formation, i.e. the formation of PSCs due to mountain wave activity (Alexander et al, 2011, 2013; Carslaw et al, 1998; Orr et al, 2015) Hereafter, these localised negative temperature anomalies, which form in the upwelling portion of the wave through adiabatic expansion, will be referred to as the “cooling phase” of mountain waves. Regions known to be a source of remarkable mountain-wave-induced stratospheric cooling that can trigger the formation of PSCs include the Antarctic Peninsula, Scandinavia and Greenland (Dörnbrack et al, 1999, 2002; Alexander and Teitelbaum, 2007; Plougonven et al, 2008; Eckermann et al, 2009; Noel et al, 2009; Hoffmann et al, 2013, 2016, 2017)
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