Photochemical Doppler effect

Der photochemische Doppler-Effekt (Beschreibung z.Zt. nur in englisch verfügbar)

The term “photochemical Doppler effect” was introduced by Sonnemann in 1999. This effect describes the influence of the zonal wind on the diurnal variations of the mesospheric minor constituents. The photochemistry of the mesosphere/mesopause region can be described in excellent approximation by odd oxygen-odd hydrogen chemistry. This system represents an enforced chemical oscillator driven by the diurnally-periodically solar radiation, which tends to respond in a nonlinear manner. The characteristic chemical system time ranges on the order of one day in the upper mesosphere and mesopause region, meaning it corresponds to the system driving period of radiation. As a consequence, the system shows resonance-like effects when changing the frequency of excitation. In reality, the period of radiation is fixed to one day and one has to search for the height or aeronomic condition at which chemical resonance occurs. An air parcel which moves with constant zonal wind speed along a parallel is subjected to solar radiation with a period longer or shorter than a day according to the wind direction. East winds blow against the earth’s rotation, prolonging the period. Similarly, west winds shorten the period.

The wind speeds result in shifts of the Doppler-Sonnemann period of more than +4 hours and -7 hours near the strong winter jet. The response of the minor constituents on a changing zonal wind by employing 3D-model considering the advective transport was investigated. It was found that the diurnal amplitudes and the time of maxima depends on the zonal wind.

In order to estimate the influence of the photochemical Doppler-Sonnemann effect on someone minor constituent we can calculate relative deviation that is the ralation of difference of this chemical constituent with zonal wind and without zonal wind to distribution of this constituent without zonal wind.

Figure shows relative ozone deviation for winter solstice at 62.5° N. The most pronounced features appear after sunset. The enhancement amounts up to 80% during the night. During the daytime, the values are decreased up to 40%. The altitude of both absolute maximums is 75-76 km, but region of significant changes spreads between 66-84 km. This figure demonstrate how valuable this effect on chemistry of mesosphere.

One conclusion drawn from this consideration is that models that neglect the influence of zonal winds are full of a certain error. Measurements sometimes show periods of extraordinarily high wind speeds up to more than 120 m/s even in the mesopause region. Such events can also entail episodes of unusually changed concentration of minor constituents. Therefore, the natural variability of the minor constituents is also influenced by the natural variability of the zonal wind, such as those that occur as a result of middle atmospheric warming events. The interpretation of measurements of seasonal variation of minor constituents in the upper mesosphere and mesopause region should additionally take into consideration the seasonal variation of the zonal wind. An indication for the action of the photochemical Doppler-Sonnemann effect may be the so-called winter anomaly of the night-to-day ratio (NDR) consisting of a wintertime increase of this ratio between 60 and 80 km. The calculated ozone values show the expected behavior of an increase of ozone during the night and a slight decrease during the day as exhibited by figure. [Sonnemann and Grygalashvyly, 2003]. Consequently, this effect also influences the interpretation of the middle mesospheric maximum of ozone.

relative deviation

Relative deviation for ozone at north winter solstice, 62.5° N.