Wind Flow Analysis Beyond Orographic Effects: A Direction Turning-Gradient Approach for Urban Impact Quantification in Germany

Wind direction is a variable that plays a secondary role in wind analyses since air flow is first studied. This paper focuses on its behaviour in ten cities and at two rural sites to investigate urban impact on wind direction. The database covered 18 years with a resolution of 1 km2, and hourly values were used. Two main procedures were followed. The first calculated the wind rose with high angular resolution from the average wind in each region studied as opposed to the usual calculations, where wind direction is measured at only one site. Two smoothing procedures were posited to acquire information about the data structure and specific details in one-degree roses. Bandwidth selection was based on the agreement with observations. Results revealed that westerly directions prevailed, although three patterns were observed. The most frequent was formed by a wide westerly sector, which may be attributed to synoptic flow, followed by wind roses with well-determined directions, such as Munich, where a mixture of relief and synoptic pattern could explain the rose shape. Finally, orographic influence is noticeable in specific roses, such as in Cologne, where the Rhine valley influence prevails. The second procedure is based on the direction turning field followed by the gradient calculation. This analysis was performed following wind direction, and two noticeable patterns of clear turning changes were occasionally found. The first pattern was formed by very close turning centres, which appear to be linked to the city, such as in Frankfurt for the easterly flow with a turning of around 65 degrees. Stuttgart, where the direction rose was barely affected by orographic features, showed a similar value of direction turning for the southerly flow, although following a different pattern formed by turning bands. Robust statistics showed the urban-rural contrast. Finally, three groups may be formed following the response against the turning field, with the weakest being for the largest cities –Berlin and Hamburg– where pollutant dispersion could prevail. An intermediate response was observed for most of the studied sites. However, Stuttgart and Frankfurt stood out due to their accused turnings that might determine complex pollution transport. Graphical Abstract