Refining valley wind days detection from in situ observations and ERA5 reanalysis

Numerous studies proposed algorithms to identify days with well-developed valley wind circulations, commonly applying thresholds based on measurements from surface weather stations and/or reanalysis datasets. In the present study, the method suggested by Lehner et al. (2019) was selected as a starting point to detect valley wind days in the Alpine Adige Valley (Italy),  based on a year-long dataset collected at an eddy covariance flux station. The method employs three fixed thresholds: two on geopotential height gradients at 700 hPa in the North-South and West-East directions (synoptic forcing), and one on longwave radiation (Clear Sky Index, local forcing), following Marty and Philipona (2000). 

To refine the procedure, in this study four geopotential pressure levels were considered, using the ERA5 reanalysis dataset covering the period 1991-2020. Additionally, the daily threshold was assessed using a n-day moving window centered on the target day. The Clear Sky Index was calculated, choosing the most suitable emissivity parameterization for the Adige Valley. Furthermore, objective adjustments to the Clear Sky Index reference limit were made. Finally, the method was tested with data from other eddy-covariance stations to verify its performance in different contexts and generalize the results.