The evening transition is the transition of the atmospheric boundary layer from convective to stable conditions towards the end of the day. While both the convective and the stable boundary layer are relatively well understood, much less is known about the transitions in between (i.e. the morning and evening transition). Especially over complex terrain, where processes like the valley/slope wind system additionally influences the transition periods, only few studies exist so far.
For the examination of the evening transition in the Inn Valley, data were chosen through strict criteria, where cloudy days or days with synoptic influence were excluded in order to limit the influence on the evening transition to the atmospheric boundary layer and the valley wind system. In this thesis, the evening transition period was determined through the start timing (sign change of sensible heat flux) and end timing (sensible heat flux exceeding 25% of the minimum after the evening minimum). Apart from the evening transition period itself, also certain events around that transition were examined.
In the Inn valley, the evening transition starts around 20% of the length of the day before astronomical sunset, whereas, depending on season, the evening transition mostly ends within 0-30% of the length of the day after sunset. Mostly, sensible heat flux starts to decrease at the same time when net radiation gain and latent heat flux are at their maximum. Around the start of the evening transition, net radiation balance approaches zero and wind speed increases towards the diurnal maximum. The evening transition ends after the maximum of net radiation deficit and at the time when wind direction changes from up-valley to down-valley.
A further look into the diurnal cycle was given for a case study from 16th to 21st August 2018. Although the diurnal cycle of most parameters are similar to the median of all valley wind days, a strong variability between the days was observed.
Even though the data of the i-Box sites were sufficient for examining the evening transition period at each site, there are significant differences in the timing of events between the sites due to different surface properties or different topographic profile surrounding each site. Thus, more data or measurement sites are favorable to address the transition period in a complex mountain valley (e.g. with the presence of lateral valleys) more in detail.